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NMR investigations of cyanate resins Niu, Junning 1993

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NMR INVESTIGATIONS OF CYANATE RESINSbyJUNNING NIUB.Sc., Peking University, 1982M.Sc., Nanjing University, 1984A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIES(DEPARTMENT OF CHEMISTRY)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAJuly 1993© Junning Niu, 1993In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)Department of L /Le nfri_s-trj,- The University of British ColumbiaVancouver, Canada"MDateDE-6 (2/88)ABSTRACTHeterogeneous polymer products are very complex systems. In manycases, the properties of these polymer systems are determined by the nature andthe degree of reaction of a small number of functional groups in the system.However, it is very difficult to study them by most analytical techniques due tothe influence of the polymer bulk and the irregularity and insolubility of thefinal product. In this thesis, both high resolution solution and solid-state NMRspectroscopies combined with specific isotopic enrichments are used to studysuch kinds of heterogeneous polymers with a particular focus on cyanate resinpolymer systems, which are newly-developed thermoset resins used forelectronic circuit boards and many structural composites.The mechanism of the curing reactions of cyanate resins based onbisphenol-A dicyanate has been investigated both in solution and in the solidstate by NMR spectroscopy. To facilitate the study, 13C and 15N isotopiclabelled cyanate resins were used. The main curing reaction was found to be theformation of triazine rings and no NMR evidence for the formation of dimeric orother intermediate species prior to triazine ring formation was found. Sideproducts are found in the solution curing due to the reaction of the cyanategroup with trace water present in the solvent. In the bulk curing, the reaction isremarkably efficient, and no detectable side reaction occurs. This can berationalized in terms of the very strong intermolecular interactions betweencyanate groups on different molecules, which is observed in the crystal structureof the bisphenol-A dicyanate monomer obtained from a single-crystal X-raydiffraction experiment.The possible cross reactions indicated by solid-state NMR spectroscopybetween cyanate and epoxy resins have been investigated by using both naturalabundance and 13C and 15N labelled monofunctional model compounds. These11soluble products were isolated and purified by adsorption chromatography andgel permeation chromatography, and were fully characterized by high resolution1H, 13C, 15N NMR spectroscopies and by mass spectrometry. The major cross-reaction product is a mixture of enantiomers which contain an oxazolidinonering formed by one cyanate molecule and two epoxy molecules. However,triazine formation from the cyanate is much faster than the two competingreactions (the cross reaction between cyanate and epoxy and the self-polymerization of epoxy) under the conditions investigated. In addition to thecross reactions of epoxy and cyanate, the reactions of epoxy with carbamatewhich is the major side product for the solution curing of cyanate resin have alsobeen investigated, and several products related to the cross reaction have beenisolated and identified. It is suggested that the reaction of epoxy and carbamateis one of the pathways in the overall cross reaction between epoxy and cyanateresins.A desired cross-linking monomer for mixed cyanate and epoxy resinsystems, the monoglycidyl ether of bisphenol-A-monocyanate, has beensynthesized and characterized. The cyanate group in the cross-linking monomeris more reactive than the epoxy group and can be cured independently underheat or by base. A more practical approach for the application of the cross-linking monomer is discussed and tested. A very tough and strong resin materialwas obtained using this approach.A bifunctional cross-linking monomer, 2-allylphenyl cyanate, for thecyanate resin (thermoset) and olefinic polymers (thermoplastic) has also beensynthesized and characterized. As a cross-linking agent, it not only reacts withitself, but also reacts with other cyanates to form hetero-triazine structures. Itcan also be copolymerized with the olefinic monomer, methyl methacrylate, toform a cross-linked polymer.111TABLE OF CONTENTSPageABSTRACT^TABLE OF CONTENTS^ ivLIST OF TABLES xLIST OF FIGURES^ xiABBREVIATIONS xxACKNOWLEDGEMENTS^ x)di1. INTRODUCTION^ 11.1. Polymer Types 11.2. Cyanate Resins^ 31.3. Epoxy Resins 91.4. Coupling Agents and Glass Reinforced Resins^ 151.5. Polymer Resin Characterizations^ 191.6. High Resolution Solid State NMR Spectroscopy of Polymers^ 221.6.1. Principles of Pulsed FT—NMR^ 231.6.2. Dipolar Coupling in Solids and High Power Proton Decoupling^ 271.6.3. Chemical Shift Anisotropy and Magic Angle Spinning^ 321.6.4. Relaxation Times^ 371.6.5. Cross Polarization 401.7. Purpose of the Thesis Research ^462. INVESTIGATIONS OF THE CURING REACTIONS OFTHE CYANATE RESIN SYSTEM^ 50iv2.1. Synthesis and Characterization of Specifically Labelled CyanateMonomers^ 512.2. Investigation of the Curing Reaction in Solution^ 562.2.1. 13C NMR Investigation of the Curing Reaction of BPADCN inSolution^ 562.2.2. '5N NMR Investigation of the Curing Reaction of BPADCN inSolution^ 592.3. Reactions of Monocyanate Model Compounds^ 612.3.1. 13C NMR Investigation of the Curing Reaction ofPTBPCN 25a in Solution^ 612.3.2. '5N NMR Investigation of the Curing Reaction ofPTBPCN 25b in Solution^ 622.4. Synthesis and Characterization of the Carbamates 28 and 29^ 642.5. Isolation and Characterization of the Triazine 31 Obtainedfrom p-tert-Butylphenyl Cyanate^ 672.6. The Mechanism of the Curing Reaction for Cyanate Resin inSolution^ 722.7. Investigation of the Curing Process in the Solid State^ 732.7.1. 13C Solid State NMR Investigations^ 74(1) BPADCN Monomer^ 74(2) Cured Resin from Solution Polymerization^ 75(3) Cured Resin from Solid State Curing 782.7.2. 15N Solid State NMR Investigations^ 78(1) BPADCN Monomer^ 78(2) Cured Resin from Solution Polymerization^ 78(3) Cured Resin from Solid State Curing 792.7.3. Quantitative Investigation of Solid State BPADCN Curing ^ 79V2.8. The Relation of the Crystal and Molecular Structure ofBPADCN to Its Curing Efficiency^ 862.9. Conclusions^ 913. INVESTIGATIONS OF THE POSSIBLE CROSS REACTIONSBETWEEN CYANATE AND EPDXY RESINS^ 933.1. Solid State NMR Investigation of the Neat Curing Reactionof the Mixed Dicyanate / Epoxy Resins^ 943.2. Neat Curing Reactions of Monofunctional Cyanate andEpoxy Compounds^ 963.2.1. Neat Curing Reactions of Unlabeled PTBPCN 25 andPTBPGE 37^ 963.2.2. Neat Curing Reactions of Using 15N labelled PTBPCN^ 1023.3. Further Characterization of the Major Cross-reaction Product^ 1043.3.1. 1D and 2D NMR Spectra^ 1043.3.2. Model Compounds for Structure 39^ 1083.3.3. Variable Temperature NMR Experiments 1113.3.4. NOE Experiments^ 1143.3.5. A Possible Mechanism for the Main Cross-linking Reaction^ 1193.4. Investigation of the Second Unidentified Product^ 1203.5. Reaction of Carbamate and Epoxy^ 1233.5.1. Reaction of 15N Enriched p-tert-Butylphenyl Carbamate 28with PTBPGE 37^ 1233.5.2. Reaction of Phenyl Carbamate with Phenyl Glycidyl Ether^ 1263.6. A Possible Mechanism for the Reaction Between Epoxy andCarbamate^ 133vi3.7. Investigation of Imidocarbonate as a Possible Cross-reactionProduct^ 1363.7.1. Reaction of Cyanate with p-tert-Butylphenol (PTBP)^ 1363.7.2. Reaction of Cyanate with Isopropanol^ 1373.8. The Mechanism of the Curing Reaction forDicyanate / Diepoxy Mixed Resins^ 1393.9. Conclusions^ 1414. SYNTHESIS AND CHARACTERIZATION OF CROSS-LINKINGAGENTS FOR MIXED CYANATE / EPDXY RESIN SYSTEMSAND FOR MIXED CYANATE / OLEFIN RESIN SYSTEMS^ 1434.1. A Cross-Linking Agent for Mixed Cyanate / Epoxy ResinSystems^ 1444.1.1. Strategy for the Synthesis of the Cross-Linking Agent,the Monoglycidyl Ether of Bisphenol-A-monocyanate 61^ 1454.1.2. Synthesis and Purification of the Monoglycidyl Ether ofBisphenol-A 62^ 1474.1.3. Synthesis of Cross Linking Monomer 61^ 1544.1.4. Curing Reaction of Cross-Linking Monomer 61 with Heat^ 1554.1.5. Curing Reaction of Cross-Linking Monomer 61 with Base^ 1584.1.6. A More Practical Approach to the Application ofCross-Linking Monomer 61^ 1634.1.7. Conclusions^ 1664.2. A Cross-Linking Agent for Mixed Cyanate / Olefin ResinSystems^ 1674.2.1. Synthesis and Characterization of the Cross-LinkingMonomer 76^ 169vii4.2.2. Self Curing Reaction of the Cross-Linking Monomer 76^1704.2.3. Curing Reaction of the Cross-Linking Monomer 76 with aCyanate Resin^ 1734.2.4 Copolymerization of the Cross-Linking Monomer 76with an Olefinic Monomer^ 1774.2.5. Conclusions^ 1785. EXPERIMENTAL^ 1805.1. High Resolution NMR Experiments^ 1805.1.1. Solution NMR Experiments 1805.1.2. Solid State NMR Experiments^ 1805.2. Mass Spectrometry Experiments 1815.3. X-ray Diffraction Experiments^ 1815.4. Syntheses^ 1825.4.1. Labelled Cyanogen Bromide^ 1825.4.2. Labelled BPADCN (2a and 2b) 1835.4.3. Labelled PTBPCN (25a and 25b)^ 1845.4.4. Triazine 31 formed from PTBPCN 1855.4.5. 2,6-Dimethy1-4-phenoxycarbonylmorpholine 46^ 1855.4.6. 5-Phenoxymethy1-2-oxazolidinone 53^ 1865.4.7. Bisphenol-A Monoglycidyl Ether 62 1875.4.8. Bisphenol-A-Monocyanate Monoglycidyl Ether 61.^ 1885.4.9. Triazine 68 Formed from the Monoglycidyl Ether ofBisphenol-A-Monocyanate 61^ 1895.4.10. A More Practical Way to Synthesize the Cross-LinkingMonomer 61^ 1905.4.11. 2-Allylphenyl Cyanate 76^ 191viii5.4.12. Triazine 77 formed from 2-Allylphenyl Cyanate 76^ 1925.5. Chromatographic Separation of the Reaction Products 1926. PROPOSALS FOR FUTURE WORK^ 1947. REFERENCES^ 1978. APPENDICES^ 205A. Crystal Structure Data for Compound 31^ 205B. Crystal Structure Data for Compound 2 215C. Crystal Structure Data for Compound 53^ 221ixLIST OF TABLESTable^ Page2.1.^Characteristic 13C and 15N Chemical Shift Valuesof the Functional Groups Derived from theCyanate Group in Solution Curing Reactions^ 742.2.^Intermolecular Distances between Cyanate Groupsin the BPADCN Crystal^  91xLIST OF FIGURESFigures^ Page1.1.^Schematic representation of different polymers.(A). Linear polymer; (B). Branched polymer; (C). Highlybranched polymer; and (D). Cross-linked polymer^ 31.2.^Vector diagrams describing the pulsed NMR experimentin the rotating frame of reference^ 261.3.^Schematic representation of dipolar interactions^ 291.4.^The effect of dipolar decoupling and magic anglespinning on' 3C solid-state NMR spectra ofpoly(butylene terephthalate)^ 311.5.^Schematic representation of chemical shift anisotropypowder patterns^  341.6.^Schematic representation of magic angle spinning^ 361.7.^Larmor frequencies for 1H and 13C in a 9.4 T magnetic field.There is no frequency overlap, and thus no overlap in the energies ^ 391.8.^Vector diagram for a 1H — 13C cross-polarization experiment^ 431.9.^Schematic representation of the 1H-13C cross-polarization pulsesequence for solid state NMR experiment^ 431.10. A more detailed representation of part C in Figure 1.8^ 441.11. A demonstration of the advantages of combining dipolardecoupling (DD), magic angle sinning (1VLkS) and crosspolarization (CP) techniques for obtaining 13C solidstate NMR spectra of poly(methyl methacrylate)^ 451.12. Schematic representation of some important active sitesin a heterogeneous polymer resin system^ 46xi1.13. The effect of selective isotopic enrichment^ 492.1.^Solution NMR spectra (1H at 300 MHz) of bisphenol-Adicyanate (BPADCN) monomer in CDC13. (A). 13C NMRspectrum of natural abundance monomer 2; (B). 13C NMRspectrum of the 13C enriched monomer 2a; (C). 15N NMRspectrum of the 15N enriched monomer 2b 542.2.^^13C solution NMR spectra (1H at 200 MHz) of (A). naturalabundance p-tert-butylphenyl cyanate (PTBPCN, 25) and(B). 13C enriched PTBPCN 25a in methyl ethyl ketone(MEK) and acetone-d6^ 552.3.^Solution NMR spectra (1H at 300 MHz) of 15N enrichedPTBPCN 25b in acetone-d6. (A). 15N spectrum with 1Hdecoupling; (B). 15N spectrum without 1H decoupling;(C). 13C spectrum with 1H decoupling^ 572.4.^13C solution NMR spectra (1H at 200 MHz) of 13C enrichedBPADCN 2a in MEK and acetone-d6 cured with 200 ppm zincoctanoate as catalyst. (A). Before heating; (B). After heatingfor 1 hour at 60 °C; (C). After 16 hours at 60 °C; (D). After 5days at 60 °C^ 582.5.^15N solution NMR spectra (1H at 300 MHz) of 15N enrichedBPADCN 2b in MEK and acetone-d6 cured with 200 ppm zincoctanoate as catalyst. (A). Before heating and without 1Hdecoupling; (B). After heating at 90 °C for 1 day and with 1Hdecoupling; (C). Same sample as in (B) without 1H decoupling^ 602.6.^13C solution NMR spectra (1H at 200 MHz) of the 13Cenriched PTBPCN 25a in MEK and acetone-d6 with200 ppm zinc octanoate added as a catalyst. (A). Beforexiithe addition of water; (B). After addition of excess waterand standing at room temperature for 24 hours; (C). Afterheating at 100 °C for 1 hour^ 63^2.7.^15N solution NMR spectra (1H at 300 MHz) of 15Nenriched PTBPCN 25b in MEK and acetone-d6 without1H decoupling. (A). After heating at 100 °C for 5 hourswithout catalyst; (B). After heating at 100 °C for 1 hourwith 200 ppm zinc octanoate as catalyst^ 652.8.^(A). 13C NMR spectrum (1H at 200 MHz) of p-tert-butylphenylcarbamate 28 in acetone-d6; (B). 15N NMR spectrum(1H at 300 MHz) without 1H decoupling of 15 % 15Nenriched 28 in acetone-d6^ 662.9.^13C solution NMR spectra (1H at 200 MHz) of the carbamate29 in DMSO-d6. (A). Before heating; (B). After heating at120 °C for 28 hours^ 682.10. (A). 13C NMR spectrum 0-H at 300 MHz) in acetone-d6 ofnatural abundance triazine 31 formed from PTBPCN 25;(B). 15N NMR spectrum 0-H at 300 MHz) in acetone-d6 of15N enriched triazine formed from PTBPCN 25b^ 702.11. Perspective view of the triazine molecule 31 formed fromPTBPCN 25. 50% probability thermal ellipsoids are shownfor the non-hydrogen atoms^ 712.12. (A). Solid state 13C CP/MAS NMR spectrum (1H at 100 MHz)of the natural abundance BPADCN 2; (B). NQS spectrum(1H at 100 MHz) of 2; (C). NQS spectrum 0-H at 100 MHz) ofthe 13C enriched BPADCN 2a; and (D) 13C CP/MAS/TOSSspectrum (1H at 400 MHz) of 2a^ 762.13. 13C solid state CP/MAS/TOSS NMR spectra (1H at 400 MHz)of (A). The solid sample obtained by evaporation of the solventafter curing the 13C enriched BPADCN 2a in MEK and acetone-d6 with 200 ppm zinc octanoate as a catalyst; (B). The samesample as in (A), NQS experiment; and (C). The solid sampleobtained from bulk curing the 13C enriched BPADCN 2a at250 °C for 15 minutes^ 772.14. 15N solid state CP/MAS/TOSS NMR spectra (1H at 400 MHz)of (A). 15N enriched BPADCN 2b; (B — D). The solid sampleobtained by evaporation of the solvent after curing the 15Nenriched BPADCN 2b in MEK and acetone-d6 with 200 ppmzinc octanoate as a catalyst, with contact times: (B). 1 ms;(C). 5 ms; (D). 1 ms with NQS pulse sequence^ 802.15. Solid state 15N CP/MAS NMR spectra (1H at 400 MHz) of theresin obtained after bulk curing the 15N enriched BPADCN2b at 250 °C for 15 minutes, with contact time 1 ms. (A).With TOSS sequence; (B). With TOSS/NQS sequence^ 812.16. Series of 13C CP/MAS NMR spectra (1-H at 400 MHz) withvariation of the contact time (CT) without sideband suppression.The sample resin was obtained by bulk curing a mixture of50% 13C enriched and 50% 15N enriched BPADCNmonomers for 15 minutes at 250 °C^ 832.17. Solid state CP/MAS/TOSS NMR spectra 0-H at 400 MHz)of the same resin sample as Figure 2.16 with CT = 5 ms.(A). 13C spectrum; (B). 15N spectrum^ 84aciv2.18. Series of 15N CP/MAS NMR spectra (1H at 400 MHz) withcontact time variation but without sideband suppression.The resin sample was the same as Figure 2.16^ 852.19. Molecular structure of BPADCN monomer 2 from thesingle crystal X-ray diffraction study showing thenumbering of the atoms (Tables in the Appendix B)^ 882.20. Perspective view of part of the unit cell contents from thecrystal structure of BPADCN monomer 2 showing theintercyanate interactions. (see Table 2.2)^ 892.21. A plane through the three-dimensional network formed by theintermolecular intercyanate interaction. The BPADCNmolecules are cyanate-connected to form parallel "strings"throughout the structure^ 90^3.1.^15N solid-state NMR spectra (1H at 400 MHz) of the resinobtained by curing EPON-825 and BPADCN (50%13Cenriched 2a and 50% 15N enriched 2b) at 180 °C for 2.5hours. (A). CP/MAS/TOSS spectrum; (B). CP/MAS/TOSSspectrum combined with the NQS technique^ 973.2. Series of 15N solid-state CP/MAS/TOSS NMR spectra(1-H at 400 MHz) of the same sample as Figure 3.1 withvariation of the contact time as indicated. The triazineresonance is indicated by T and the imidocarbonate by I^ 983.3.^NMR spectra in acetone-d6 of the crude reaction mixtureobtained by heating PTBPCN (12% 15N enriched) andPTBPGE at 180 °C for 7 hours. (A). 13C NMR spectrum(1H at 200 MHz); (B). 15N spectrum 0-H at 300 MHz)with NOE and no 1H decoupling^ 100XV^3.4.^NMR spectra of the major cross-reaction product after bothsilica gel column and Sephadex LH-20 colurnn separationsof the same reaction mixture as Figure 3.3. (A). 15N NMRspectrum (1H at 300 MHz) with NOE and no 111 decoupling;(B). 1H (500 MHz) NMR spectrum; and (C). 13C NMRspectrum (1H at 500 MHz)^ 1053.5.^(A). 1H (400 MHz) 2D COSY NMR spectrum and (B).1H/13C chemical shift correlated 2D NMR spectrum1H at 500 MHz) of the same sample as Figure 3.4^ 1063.6.^13C NMR spectrum (111 at 200 MHz) of 2,6-dimethylmorpholine 44 in acetone-d6^  1103.7.^Aliphatic regions of the 13C NMR spectra (11i at 300 MHz)of 2,6-dimethy1-4-phenoxycarbonylmorpholine 46 inacetone-d6 at the temperatures indicated^ 1123.8.^Aliphatic regions of the 13C NMR spectra (1H at 300 MHz)of the cross-reaction product (Figure 3.4), at thetemperatures indicated^ 1133.9.^1H (400 MHz) NMR spectra in aromatic regions of the majorcross-reaction products derived (A). From PTBPCN 25 andPTBPGE 37; and (B). From PTBPCN 25 and OMPGE 38^ 1163.10. 1H (400 MHz) NOE difference NMR spectra of the majorcross-reaction product derived from the reaction of PTBPCN25 and PTBPGE 37, which is the same as Figure 3.4^ 1173.11. 1H (400 MHz) NOE difference NMR spectra of themajor cross-reaction product derived from the reactionof PTBPCN 25 and OMPGE 38^  118xvi3.12. NMR spectra of the second unidentified product after bothsilica gel column and Sephadex LH-20 column separationof the crude reaction mixture obtained by heating PTBPCN(-12% 15N enriched) and PTBPGE at 180 °C for 7 hours.(A). 15N spectrum (1H at 300 MHz) without 1Hdecoupling; (B). 13C spectrum (1H at 200 MHz); and(C). 1H (200 MHz) spectrum^ 1223.13. 15N NMR spectrum (1H at 300 MHz) in acetone-d6 withNOE and no 1H decoupling of the reaction mixture obtainedby heating p-t-butylphenyl carbamate (-12%15N enriched)and PTBPGE at 180 °C for 3.5 hours^ 1243.14. NMR spectra in acetone-d6 of the second product after silicagel column separation of the same reaction mixture as inFigure 3.13. (A). 15N spectrum (1H at 300 MHz) with NOEand no 1H decoupling; (B). 13C spectrum (1H at 200 MHz);and (C). 1H (200 MHz) spectrum^ 1253.15. NMR spectra of product 53 in acetone-d6. (A). 15N spectrum(1H at 300 MHz); (B). 13C spectrum 0-H at 200 MHz); and(C). 1H (200 MHz) spectrum^ 1273.16. Molecular structure of product 53 from the single crystalX-ray diffraction experiment with the numbering of the atomsindicated. Complete structural data are given in Appendix C^ 1283.17. NMR spectra of product 55 in acetone-d6. (A). 13C spectrum(1H at 200 MHz); (B). 1H (500 MHz) spectrum^ 1313.18. (A). 1H-13C 2D heteronuclear correlation NMR spectrum0-H at 500 MHz) and (B). 1H (500 MHz) 2D COSY NMRspectrum of product 55 in acetone-d6^ 132xvii3.19. NMR spectra of product 59 in acetone-d6. (A). 13Cspectrum (1H at 200 MHz); (B). 15N spectrum(1H at 300 MHz) with no 1H decoupling^ 138^4.1.^13C NMR spectrum 0-H at 200 MHz) in acetone-d6 of thecrude product prepared by reaction at 56 °C for 90 min.and using a reactant mixture with a 1:1:1 molar ratio ofbisphenol-A, epichlorohydrin and potassium carbonate.(A). Full spectrum; (B). The expanded aromatic region 1514.2.^NMR spectrum (1H at 200 MHz) of the monoglycidylether of bisphenol-A 62 in acetone-d6.(A). 13C spectrum; (B). 1H spectrum^ 1534.3.^NMR spectrum (1H at 200 MHz) of the crude monoglycidylether of bisphenol-A-monocyanate 61 in acetone-d6.(A). 13C spectrum; (B). 1H spectrum^ 1564.4.^(A). 13C NMR spectrum (1H at 300 MHz) in acetone-d6and MEK of the crude product 68 from curing themonoglycidyl ether of bisphenol-A-monocyanate 61by heating; (B). 1H NMR spectrum (200 MHz) inacetone-d6 of the crude product 68^ 1574.5.^13C NMR spectrum (1H at 200 MHz) in acetone-d6 ofthe crude product from curing the monoglycidyl etherof bisphenol-A-monocyanate 61 at room temperaturewith diethylamine base (monomer 61 is in excess)^ 1604.6.^13C NMR spectrum (1H at 200 MHz) in acetone-d6of the crude product from curing the monoglycidylether of bisphenol-A-monocyanate 61 at roomtemperature with excess cliethylamine base^ 161xviii^4.7.^NMR spectrum (1H at 200 MHz) in acetone-d6 of thecyanate product mixture obtained from the intermediatemixture without separation. (A). 13C spectrum; (B). 1-H spectrum ^ 1654.8.^NMR spectra (1H at 200 MHz) of 2-allylphenyl cyanate 76in acetone-d6. (A). 13C spectrum; (B). 1H spectrum^ 1714.9.^NMR spectra (1H at 200 MHz) of 1,3,5-tri(2-allylphenoxy)-2,4,6-triazine 77 in acetone-d6. (A). 13C spectrum;(B). 1H spectrum^ 1724.10. NMR spectrum in acetone-d6 of the product mixture obtainedfrom curing a mixture of cross-linking monomer 76 and12% 15N enriched PTBPCN 25. (A). 13C spectrum(1H at 200 MHz); (B). 15N spectrum (1H at 300 MHz)^ 175xixABBREVIATIONSThe following abbreviations have been used throughout this thesis.a.m.u.^= atomic mass unitAr^= aromatic substituteb.p.^=^boiling pointBPADCN = bisphenol-A dicyanateBT^= bismaleimide and triazineCI^= chemical ionizationCOSY^= proton homonuclear correlation spectrumCP^= cross polarizationCT^= contact timeDGEBA^= diglycidyl ether of bisphenol-ADMMP^= dimethyl morpholineEl^= electron impactEPON-825 = low molecular weight epoxy resinFID^= free induction decayFM^= formula massFT^= Fourier transformationGPC^= gel permeation chromatographyHETCOR = heteronuclear correlation spectrumHP^= high power decouplinghr.^=^hour(s)H.T.^= high temperatureIR^= infraredL.T.^= low temperatureM+^= parent ionXXMAS^= magic angle spinningMEK^= 2-butanone (or methyl ethyl ketone)min.^= minute(s)MMA^= methyl methacrylatem.p.^= melting pointMS^= mass spectrometryms^= millisecond(s)NMR^= nuclear magnetic resonanceNOE^= nuclear overhauser effectNQS^= non quaternary suppression; pulse sequence for selecting Xnuclei with no attached protonOMPGE^= ortho-methylphenyl glycidyl etherppm^= part per millionPTBPCN = 4-tert-butylphenyl cyanatePTBPGE = 4-tert-butylphenyl glycidyl etherTg^= glass transition temperatureTMS^= tetramethyl silaneTOSS^= total suppression of spinning sidebands; pulse sequence forspinning sideband suppressionO^= chemical shiftACKNOWLEDGEMENTSFirstly, I would like to sincerely thank my supervisor Prof. C. A. Fyfe forhis guidance, encouragement, advice and support throughout the course ofthesis studies.I am also very grateful to my supervisory committee members, Prof. R. E.Pincock and Prof. F. G. Herring for their advice and proof-reading of this thesis.Many thanks are extended to the following people:All of my past and present labmates and friends for their interactionsocially and academically. In particular, Dr. N. E. Burlinson and Dr. H. Grondeyfor their valuable discussion, helpful advice and proof-reading of the thesis. Dr.G. Fu and Mr. K. Wong-Moon for their proof-reading of the thesis. Mr. K. Mokfor his valuable collaboration.Dr. S. Rettig for the determination of crystal structures from X-raydiffraction experiments. Dr. S. 0. Chan, Ms. L. Darge, and Ms. M. Austria in theNMR laboratory, Mr. T. Markus and Mr. Kam Sukul in the electronic shop fortheir frequent and kind help.Dr. D. Wang, Dr. M. Poliks, and Dr. C. Reidsema at IBM for their helpfulcollaboration.Finally, I am greatly obliged to my parents and sisters for their constantlove, encouragement and support in all aspects. I also wish to extend specialthanks to my wife, Jane and son, Simon for their love, patience and support overlast few years.CHAPTER 1.INTRODUCTIONThe work described in this thesis is the investigation of cyanate resinsand related mixed heterogeneous polymer systems primarily by high-resolutionsolution and solid-state NMR spectroscopies using specific isotopic enrichmentat the reaction sites. In the Introduction, the general features of polymer resinswill be outlined and a description of the solid-state NMR techniques will begiven to facilitate the presentation of the experimental investigations insubsequent chapters of the thesis.1.1. Polymer TypesPolymers or macromolecules are usually made up of different sequences ofrepeating chemical structural units, which may be arranged regularly orirregularly to form linear or three dimensional networks. They have very largemolecular weights, which could be 10,000 or greater. In polymer chemistry,different classifications have been used based on the polymer compositions, thepolymerization mechanisms, and the polymer structures.Polymers were originally classified by Carothers[11 in 1929 intocondensation and addition polymers on the basis of the compositional differencesbetween the polymer and the monomer from which the polymer was synthesized.Condensation polymers are those polymers which are formed frompolyfunctional monomers by various condensation reactions which involve theelimination of some small molecules such as water. Addition polymers areclassified as those which are formed from monomers without the loss of anysmall molecule. Unlike condensation polymers, the repeating unit of an additionpolymer has the same composition as the monomer.1CBFlory[21 pointed out the significant difference between the twopolymerization mechanisms, called step and chain growth polymerizations.Based on this, polymers were divided into step-growth polymers and chain-growth polymers. Step-growth polymerizations proceed by the stepwise reactionof the functional groups on different reactants. Any two molecular species canreact with each other throughout the course of the polymerization. The size ofthe polymer molecules increases at a relatively slow rate in suchpolymerizations. In chain polymerizations, the situation is quite different andfull-sized polymer molecules are produced almost immediately at the beginningof the reaction. Chain-growth polymerizations require an initiator to produce areactive center, which may be a free radical, cation or anion. A monomer canreact only with the reactive center, not directly with other monomers.Figure 1.1. Schematic representation of different structures ofpolymers. (A). Linear polymer; (B). Branched polymer;(C). Highly branched polymer; and (D). Cross-linkedpolymer.2Polymers can also be classified as linear, branched, and cross-linkedpolymers depending on their structures[3] (Figure 1.1). The linear or lightlybranched polymers are usually called thermoplastics, and they are reversiblyfusible and can be re-shaped by the application of heat or pressure. In the caseof cross-linked polymers, the molecular chains are joined together by covalentbonds and so the chains cannot slide past each other upon the application ofheat or pressure. Highly cross-linked polymers formed by the action of heat cannot be re-shaped by heating and are termed thermosets or thermoset resins. Theterm resin is generally used to indicate a precursor of a cross-linked polymericmaterial; sometimes, however, it is applied to any material whose molecules arepolymers. [4]L2. Cyanate ResinsCyanate resins, or cyanuric esters, all contain the cyanate group —0—CoNattached to a benzene ring, and the polymerization reaction proceeds by thethermally induced reaction of this group. The development of cyanate resinsstarted in the mid 1960's, with cyanate compounds first being successfullysynthesized from aromatic phenols[5,6]. A practical synthetic route formanufacturing cyanate esters was invented and developed by Bayer AG in thelater 1960's[7,8].Aliphatic cyanates readily isomerize to the corresponding isocyanates andthen subsequently can trimerize to form isocyanurates.[9] By application of heator organo-metallic compounds as catalysts, aromatic cyanates directly trimerizeto form a stable cyanurate, s-triazine ring structure. [10,11 ] Although in bothcases, based on calculations, the isocyanurate form is thermodynamically morestable, aryl cyanates and aryl cyanurates do not seem to isomerize.[12]3Cyanates can be prepared from the reaction of cyanogen halides andphenols in the presence of a hydrogen halide acceptor,[5,6] or from thiatriazolesby thermo1ysis[13,14]. For the manufacture of cyanate resins, the process usingcyanogen halides and phenols is the most important. The general method forpreparation of the monomers of cyanate resins can be represented by Equation1.1.OH XCN OCN^(CH3CH2)3NHX^[1.1]Commercially available cyanate resins are bisphenol derivativescontaining cyanate functional groups (—OCEIN). A typical monomer of thecyanate resin has the general structure 1,X - Bisphenol LinkageR - Ring SubstituentThe cyanate resin most commonly used is based on bisphenol-A dicyanate(BPADCN, 2)N=-C-0 0-CEN2derived from bisphenol A (3), which is also a raw material for epoxy resins.Bisphenol A (3) is so-called since it is formed from two phenols and one acetonemolecule as shown in Equation 1.2.42 HO9H3+ 9=0CH3H+HO OH + H2O^[1.2] The curing reaction of cyanate resins is assumed to be three cyanatefunctional groups on different monomers being cyclotrimerized to a triazine ringupon heating[- 0'-1-I [Scheme 1.1] to form the three-dimensional network of athermoset resin. Therefore chemically, this family of thermosetting monomersand their prepolymers are esters of bisphenols and cyanic acid. These threedimensional networks of oxygen linked triazine rings and bisphenol unitsshould be correctly termed polycyanurates. Since no leaving groups or volatilebyproducts are formed during the curing process, the cyanate resin producedthrough a cyclotrimerization curing reaction is classified as an addition polymer.Because of the step polymerization mechanism of the curing reaction, it can alsobe classified as a step-growth polymer.The cyclotrimerization of the cyanate resin is found to be facilitated bysoluble transition metal compounds as catalysts. The role of the solubletransition metal compounds is thought to be primarily by coordination,gathering cyanate groups in close proximity to facilitate the ringformation . [11 ,15]Curing reactions of cyanate resins have been investigated by infraredspectroscopy.[16,17] Following the disappearance of the strong cyanate —0CraNdoublets at 2240 and 2270 cm-1 during the curing process, new absorbancesappear simultaneously at 1565 cm-1 and at 1365 cm-1 (cyanurate). However,although NMR spectroscopy is a very diagnostic and reliable technique, there is5NEC-0 0-C-=NNNYN0-11-0^N2Scheme 1.16no information from NMR studies to date regarding the species involved duringthe curing reaction, or the nature and efficiency of the curing process itself.Fang has recently described the use of 13C solution NMR spectroscopy todetermine the molecular weights of bisphenol-A dicyanate (BPADCN, 2)oligomers.[18] By using modern solid-state and solution NMR techniques, thedirect detection and characterization of all the species formed during the curingprocess will be reported in this thesis.Laminating resins based on bisphenol-A dicyanate prepolymer were notcommercially available until 1975[19]. The Mitsubishi Gas Chemical companylater introduced a BT resin system, which is blends of bismaleimide and triazine(cyanate ester) resins, in the mid 1980's[20]. While BT resins solved thelaminate moisture problem, performance was compromised to some extent bytheir brittleness, and higher dielectric constants. However, pure and unblendedmaterials of cyanate resins were not available until 1985 when Interez Inc.marketed a series of resin systems for both circuit boards and advancedcomposite applications[21]. Hi-Tek Polymers Inc. introduced an improvedprocess for producing >99% pure monomers from a variety of bisphenolprecursors, leading to the introduction of three aryl dicyanates and theirprepolymers in 1985-1986. The moisture absorption problem of cyanate resinswas overcome by eliminating carbamate impurities in the monomers andavoiding use of aliphatic amines as curing catalysts. The dielectric constant ofthe cured resin decreased to 2.66 (and ultimately 2.5 with experimentalproducts), and moisture absorption at saturation was lowered to 1%. Thesedevelopments were commercialized by Hi-Tek Polymers Inc. in the 1985-4989period122,23,24]. A new cyanate resin with low temperature cure capability hasbeen recently developed125 ,26]•7The principal end uses for cyanate resins are as matrix resins for printedcircuit board laminates[22, 23, 27] and structural composites[16, 24, 28, 29].Since the 1980's, there have been major changes[30] in the requirements andperformances of high speed logic circuits, such as in RF/microwavetelecommunication switches, radar and some military devices. Theserequirements are higher circuit density, faster signal transfer speed, and greaterreliability. The recently developed cyanate resins help to meet theserequirements.In the last ten years, aerospace composites which have a high damagetolerance have been developed by utilizing a mixture of both thermoset andthermoplastic resins.[24,31] Cyanate resins develop approximately twice thefracture toughness of multifunctional epoxies and are able to operate at 150 °C(300 °F). The unusually low capacitance properties of cyanate resins, due totheir very low dielectric constants which are in the 2.5 — 3.1 range, are alsoutilized in military aircraft which have reduced radar signatures.[17]In the electronic market, cyanate resins show some very attractivefeatures[' 7, 22, 23, 27], such as very low dielectric constant, high dimensionalstability at molten solder temperatures, and excellent adhesion to conductingmetals at temperatures up to 250 °C. Also, the processing characteristics inketone solutions and the drillability of conventional diepoxide laminates havebeen retained. Since cyanate resins exhibit good solubility in organic solvents,they can be mixed with a wide variety of thermosets such as epoxies,bismaleimides or acrylates to form compatible blends.[32, 33] Mixing withthermoplastics usually results in interpenetrating network type structures.[16,29, 34]After being fully cured, cyanate resins exhibit high glass transitiontemperatures (Tg) of at least 250 °C. Usually they are blended with epoxies up8to 60 – 70 % by weight in order to modify the Tg, increase toughness, reduce thecost and achieve non-flammability requirements. Blending with epoxies allowsBT and cyanate resins to reach their full cure at 177 °C. The laminates madefrom such blends exhibit good moisture and delamination resistance.However, the nature and the mechanism of the curing reactions whichoccur in the mixed cyanate and epoxy resins have not been clearly established todate,[30] even though the formation of oxazoline (5) and isooxazoline (6) ringstructures has been proposed[32, 35, 36].0 —..---0^,110 05 6oxazoline ring^isoxazoline ringL3. Epoxy resinsThe first patent for the synthesis of the materials designated as epoxyresins was made by Pierre Castan of Switzerland about fifty years ago[37]. Sincethat time there has been intense activity in the synthesis of new epoxycompounds with different structures[38,39,40]. In addition, investigations ofrelationships between structure and macroscopic properties have increased ourunderstanding of these polymer systems[41 ,42,43]•In a broad sense, the term epoxy refers to a chemical group consisting ofan oxygen atom bonded with two carbon atoms which are already linked in someother way. The simplest epoxy is a three-membered ring to which the term a-epoxy or 1,2–epoxy is applied. Ethylene oxide (7) is an example of this type. Theterms 1,3– and 1,4–epoxy are applied to trimethylene oxide (8) andtetrahydrofuran (9).98,0CH2 —CH279^10In general, only those epoxy compounds which contain the three-membered epoxy rings (ie. ethylene oxide derivatives) are referred to as epoxyresins. There is no universal agreement on the nomenclature of the three-membered epoxy ring[391. In fact, there is division even on the term epoxy itself;European authors generally prefer the term epoxide, while in America the termepoxy is more common. The epoxies may be designated as oxides, as in the caseof ethylene oxide (epoxyethane) (7) or cyclohexene oxide (1,2-epoxycydohexane)(10). Several of the more common monoepoxies have trivial names, such asepichlorohydrin (11), glycidic acid (12), and glycidol (13). Glycidyl (14) is used torefer to the terminal epoxy group, the name being modified by ether, ester,amine, etc., according to the nature of the group attached to the third carbon. Anepoxy resin is defined as any molecule containing more than one epoxy group(whether situated internally, terminally, or on cyclic structures) which iscapable of being converted to an useful thermoset material.[39] The term isapplied to the resins both in the thermoplastic (uncured) or thermoset (cured )state.0^0/CH2 -CHCH2CI^CH2 -CHCOOH^CH2 -CHCH2OH11^12^13/\CH2 -CHC H 2 -14A number of properties have led to the rapid development of epoxy resinsand their use in a wide range of industries. Epoxy resins belong to thethermosetting class of polymers. They offer great versatility, low shrinkage,excellent chemical resistance, outstanding adhesion, high mechanical strength,1 0NaOHOCH2CHCH2CIO15^H0CH2cH—cH2\ /16^0and very good electronic insulation. Epoxy resins can be cured quickly andeasily at practically any temperature from 5 to 150 °C, depending on the choiceof curing agents.[39]Because of their versatility, the epoxy resins are used in thousands ofindustrial applications as adhesives, caulking compounds, casting compounds,sealants, coating materials, and as laminated resins for both constructionalmaterials and electronic circuit boards[39,40,44]. These uses have stimulated anextensive research in the synthesis, structure and properties of epoxyresins. [45,46,47,48]The epoxy resin is normally synthesized by the base catalyzed reaction ofepichlorohydrin (11) with an appropriate molecule, which has several phenolichydroxyl groups.[4,39,40] The reaction proceeds in two steps as shown inScheme 1.2.OH + CICH2CH—CH2 \ /11 0NaOH Scheme 1.2First, the epoxy groups react with the phenolic hydroxyls to form a chlorohydrinintermediate (15), and then chlorine and hydrogen are stripped off to regeneratethe epoxy groups in the epoxy resin (16).The first commercial epoxy resin was made by the reaction ofepichlorohydrin (11) and bisphenol A (3), which gives the diglycidyl ether ofbisphenol-A (DGEBA) (17) and higher molecular weight species (18). The11molecular weight of the resulting DGEBA resin will depend on the ratio ofepichlorohydrin to bisphenol A employed. The greater the excess ofepichlorohydrin is used, the lower the molecular weight of the resulting resinwill be. In order to obtain high yields of the monomeric product, excessepichlorohydrin is employed, usually two or three times the stoichiometricamount. [4,391HO OH^CICH2CH—CH2\ /11 0 3NaOH [1.3]OC H2C 11 -/C H217• \C H2-CHCH20OH^ 0OCH2CHCH 0 OCH2CH CH218The commercial DGEBA epoxy resins are mixtures of high and lowmolecular weight oligomers, the distribution of molecular weights varying withthe conditions of synthesis. The general structure of an epoxy resin isrepresented as 18. The low molecular weight resins, having an n value about 1or below, are generally liquid. Above n=1, the resins are brittle solids. In fact,the pure diglycidyl ether of bisphenol-A (17), n=0, is a solid material melting atabout 45 °C. It is the impurities present in the commercial epoxy resins whichtransform them to "supercooled" liquids.[491 In the synthesis of commercial12OC H2Ctl-CH 2o'OCH2Ctl-,CH2oOCH2C,tH"CH2oCH\2 -;CHCH 0H2CCH,2 -,CHCH20H2CCH,2- C H C H2 00epoxy resins, epichlorohydrin is the principal epoxidizing reagent. Otherepihalohydrins may be used, but are not economically attractive.Epoxy resins can also be derived from different polyolefinic compounds byaddition of oxygen to the unsaturation. The direct oxidation with molecularoxygen would be an ideal synthesis process, but this is practical only in thesynthesis of ethylene oxide[50]. For other materials the oxygen is transferredfrom a source compound, either a peracid, hypochlorous acid, or hydrogenperoxide.[511 The principal industrial processes usually use the peracid method[Equation 1.4 and 1.511. Peracetic acid is more favoured for economic reasons.H202 + RCOOH ^■-- H20 + RCOOOH^[1 .4]0^"-- C--C + RCOOH^[1 .5],--^-.Higher cross-linking densities, giving increased solvent resistance andelevated glass transition temperature, Tg, can be obtained by using compound19 and especially 20.[521 A molecule of dual functionality 21, which can cross-link two types of polymer systems, may be used to give a resin system withimproved thermal and electrical properties.[53]"C=C + RCOOOH-----1319C171,2-;CHCH20CH2=CHCH221CH2CH =CH2OCH2C 1-1 -/C H20CH,2 C H C H2 0CH2 - C H C H2 0CHO ^OCH2CH-CH2/\0HC ^\CD)^OCH2C,1-10-/CH220The most valuable property of the epoxy resins is their ability totransform readily from the liquid (or thermoplastic) state to tough, hardthermoset solids. The conversion is accomplished by the addition of a chemicallyactive compound known as a curing agent (hardener, activator, or catalyst). Thismay be either a Lewis acid or base compound. Basically the cured structure maybe a homopolymer or a heteropolymer or a mixture of both types. The epoxygroup may react in one of two different ways during curing: anionically orcationically.[4,391 In the anionic mechanism, the epoxy ring may be opened byan anionic curing agent x- to produce an epoxy anion:z"--‘- \^.---X^ C—C...,_0Epoxy ring ^0.- [1.6]The epoxy anion is an active chemical species, capable of further reactionproducing additional covalent bonds and forming the resin framework. In thecationic mechanism, the epoxy group may be opened by an active hydrogen toproduce a hydroxyl group in a number of ways:14HXi"(1 )^C — C----^/ '--.L\0 +IX`..^..--^-...-+./(2) C — C C — C/ 1^10/' rHX^OH m-`.^./(3) C — C^--..^7-'\ / C — CX^0 i^i__ X OHHX-.........1^„......-C—C^+ HX^[1.7]OHX-.. 'C—C^+ HX^[1.8]--,.OH[1.9]The hydroxyl group can also react further with other epoxy groups to form a ringopened polymer chain.X+--^1^--OH.^.----n-1 ...-^\ /^-....0I^IX [ C—C-0—HI^I^_n[1 . 1 0 ]1.4. Coupling Agents and Glass Reinforced ResinsCoupling agents are defined as materials that improve the adhesive bondof a polymer to glass, mineral or metal surface, and improve the chemicalresistance (especially to water) of the bond across the interface. [54]Organofunctional silanes, which are hybrid organic-inorganic compounds, canbe used as coupling agents, or adhesion promoters, between organic polymersand inorganic mineral, or glass substrates. For example, organosilanes arewidely used as coupling agents in glass fiber-reinforced plastics{55,561.In the polymer industry, reinforced polymer systems are very importantmaterial types. Some of the advantages of the reinforcement are improved15mechanical properties, improved electrical insulation properties, increaseddimensional stability, and improved processing characteristics[57]. Thereinforcements are typically glass fibers, which are often treated withorganosilane coupling agents to promote adhesion between the glass fibers andthe polymer matrix[54]. The macroscopic properties of composites depend on theproperties of the glass fiber, the interface of coupling agent and fiber, theinterface of coupling agent and polymer, and the polymer matrix. All of thesecontributions must be optimized in order to increase the potential of using sucha polymer system[581. It is therefore necessary to investigate the nature of thesestructural components at the molecular level in order to understand theperformance properties of these reinforced polymer systems.Various theories for the adhesion promotion through a silane couplingagent have been proposed. The chemical bonding theory remains the mostimportant[591. These organosilanes used as coupling agents have the generalstructure R—Si—X3, where R is a functional alkyl group and X is a hydrolyzablegroup, such as an alkoxy group or halogen. During application, it is assumedthat (i) the X groups react with the silanol groups on the glass surface to form anether linkage [Equation 1.11]; (ii) the hydrolyzed silanes also self-condense toform polysiloxanes; and (iii) the functional groups on the R group can react withappropriate chemical groups in the resin, and thus chemically couple the resinto the glass [Equation 1.12]. The X groups serve only to provide hydrolytic sites.Their chemical nature does not affect the structures of the final products of thecoupling action. The R group is chosen for its chemical reactivity with the resincomponent. In general, the effectiveness of the silane as a coupling agent isreported to parallel the reactivity of the organofunctional group with theresin.[54,60]16---ECH—CH2IFICH— C H2 -ECH — CH2Vri--Si1024I^ OH + X—Si—RII^ 0 Si —R + HXI0 Si—CH=CH2122CH:=-CH2+23[1.12]Assuming that this theory describes the processes correctly, the couplingagent acts as a bridge to covalently bond the glass to the resin. This could beexpected to lead to the strongest interfacial bond. Indirect evidence for co-reaction of the organofunctional group with thermosetting resins has beenobtained. For example, in terms of strength properties, vinyltrichlorosilane-finished glass gives unsaturated polyester laminates with dry and wet strengthsabout 60% greater than those of laminates of ethyltrichlorosilane-finishedglass[54]. An unsaturated polyester laminating resin should be able tocopolymerize with the olefinic groups in the coupling agent, vinyltrichlorosilane,but not with ethyltrichlorosilane.However, chemical bonding at the interface is difficult to detect directly,because of the thinness of the interface. Covalent bonding between the couplingagent and glass surface was very difficult to demonstrate. Extraction studiesreveal that the silane anchored on a glass surface is stable against long timeextraction. This suggests that there is covalent chemical bonding between the17silane and the glass fiber.[611 With the development of Fourier transforminfrared spectroscopy (FT-IR), it was possible to detect the chemical reaction ofthe silane with the glass fiber.[621 It was observed that silanol condensation wasenhanced by the glass surface and cross condensation between the couplingagent and the glass occurred during drying. [63]Because of the complexity of composites and their insolubility, FT-IR hasbeen one of the few spectroscopic techniques capable of characterizing them.Consequently, a great deal of work has been done in this field[641. However,recent developments in NMR spectroscopy have made it possible to obtain high-resolution NMR spectra from solids, making it another potential spectroscopictechnique in this area.[651 The combined use of high power decoupling (HP),magic angle spinning (MAS) and cross polarization (CP) techniques makes itpossible to obtain high resolution 13C NMR spectra for insolublepolymers[66,67,68,69] as well as organosilane moieties bound to silicasurfaces[70,71,72,73]. Combined with specific isotopic enrichment, NMRspectroscopy is a much more diagnostic and selective technique than IRspectroscopy. In surface studies, 13C NMR has the particular advantage ofavoiding interference effects from the glass reinforcement matrix, which is aproblem that may arise in other surface characterization methods.The usefulness of solid state 13C CP/MAS NMR spectroscopy has beenshown for the characterization of the glass/coupling agent interface of glass fibrereinforced polymer systems. 13C NMR studies of a range of organosilanecoupling agents adsorbed on silica surfaces have been carried t[74]. Thesestudies show that it is possible to probe the glass/coupling agent interface of acomposite and obtain the information on chemical bonding between the couplingagent and the glass, as well as on the structure of the coupling agent on theglass surface. However, the investigation of the interface between the coupling18agent and polymer is much more difficult because of the strong backgroundinterferences from the polymer matrix. The proposed covalent bonding betweenthe polymer and the coupling agent anchored on the glass reinforcement hasnever been directly demonstrated.1.5. Polymer Resin CharacterizationsPolymer structure can be described at two different levels. The mostfundamental one concerns the chemical microstructure, which is defined by thechemical composition, chemical bonding and internal sequences of differentarrangements in the macromolecule, i.e. its structure, configuration andconformation. The second structural level can be termed as phase structure,which describes the intermolecular packing of the polymer molecules as crystals,semi-crystals, amorphous solids, rigid or rubbery solids or liquids. The portionthat crystallizes is a solid in the classical sense, although there may be sometype of molecular motion in the crystal and the ordering may not be perfect overa long range. The disordered portions that do not crystallize can be glassy andrigid if the temperature of observation is below the glass transition temperature,Tg, or rubbery if above Tg.The primary purpose for characterizing a polymer system is to determinethe polymerization mechanism and the structure of the polymer, and finally, torelate these to the performance properties of the polymer in its end use. If thestructure of a polymer and the polymerization mechanism are completelycharacterized and the properties of the structural components are known, thepolymerization process may possibly be optimized and controlled to get theoptimum desired properties of the polymer system. However, synthetic polymersare very complicated systems. Even for a single linear polymer chain with onlytwo different structural elements, A and B, and total n number of elements in19the polymer chain, the number of possible different chain structures is 21, whichcould easily be 10,000 or greater. To make it even more complicated, mostsynthetic polymers have a large number of different structural elements in thepolymer chains, or three dimensional networks.For many real polymers, many possible structural variables can also exist.The very important one is the chemical and stereochemical arrangement, whichincludes monomer composition, arrangement sequence, and tacticity. Branches,crosslinks, end groups, and chain defects can also be very important aspects ofpolymer structure, even if they are present in low concentration. Molecularweight and its distribution are other important factors. Morphological,conformational effects and chemical defects, which include impurities, monomerisomerizations, and side reactions, can also be considered as additionalvariables. Because a number of these structural variables can existsimultaneously in a polymer, the number of possible structures for the polymermolecule can be very large. Therefore, it is usually not possible to completelydefine the spatial coordinates of every atom in polymer molecules.The problem is further complicated by the nature of the distribution of thestructural variables along the polymer chain or on the polymer networks whichgreatly influences the polymer properties. Distributions of these structuralelements can be random, blockwise or alternating, and are determined by thenature of the polymerization process. The distribution can influence thecharacterization of the polymer in two ways. Firstly, the chain or networkstructure is highly variable because the polymerization process is a statisticalprocess. Therefore, the polymer sample is always a complex multicomponentmixture. Secondly, the detailed local structures can not be obtained because themeasurement techniques which are available today can only provide weightedaverage structural data.[75]20Ideally, spectroscopic techniques for the study of polymers should yieldnarrow linewidth, high-resolution spectra which provide diagnostic and selectivestructural information. Since polymer systems are always complex mixtures ofdifferent structural components and molecules, a suitable spectroscopic methodmust be capable of selectively monitoring more than one structural componentat one time. It must have sufficient sensitivity to detect and monitor very lowconcentrations of structural components in the polymer, since small structuralchanges may produce very large changes in the physical and mechanicalproperties. The spectroscopic technique should provide very specific information,since we will need to determine not only the structure of the single repeatingunits, but also how they are connected together and to what extent the units areordered. A non-invasive and nondestructive technique is preferable, because itallows the study of the same polymer sample by other methods. The techniqueshould also be capable of studying the polymer in its end-use form, such as afiber, film, composite, coating, or adhesive.Most spectroscopic techniques, such as UV / visible spectroscopy or massspectrometry, do not meet these requirements. However, some techniques haveevolved for polymer analysis that do, such as high-resolution solid state NMRspectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopies, andX-ray diffraction analysis. Each of them has both advantages and certainlimitations. By using them in combination, detailed structural information ofpolymers for analysis, quality control, and research can be obtained. Eventhough it is not possible to study polymers in their final engineering form,usually a solid, the contributions that high-resolution solution NMRspectroscopy of polymer studies has made to our understanding of the structureof polymers are great. For the investigation of polymerization reactions, thecorresponding monofunctional compound is frequently used as a model of the21multifunctional monomer to simulate the real polymerization process. In thiscase, the polymer chains or frameworks are not formed and all the products aresoluble.1.6. High Resolution Solid State NMR Spectroscopy of PolymersIn the last 20 years, tremendous progress has been made in the area ofNMR spectroscopy. The development of Fourier transform techniques and highmagnetic fields have changed the NMR method from essentially proton NMRspectroscopy to multinuclear spectroscopy. More recently, rapid progress hasbeen seen particularly in three areas[76]. One is the NMR imaging technique.Another is the use of multipulse sequences and multi-dimensional NMRspectroscopy to obtain additional information which was not availablepreviously. The last is the capability of obtaining high-resolution NMR spectrafrom solids. The combination of the techniques of high power decoupling, magicangle spinning, and cross polarization have opened new areas of chemistry andphysics to high resolution NMR[651• Most chemists are familiar with highresolution solution NMR for characterizing the structure of a molecule insolution. Thus, the first expectation for many polymer chemists would be thathigh resolution solid state NMR could be the counterpart of the solutiontechnique, which could provide not only the same structural information assolution NMR, but also with some advantages. These supposed advantageswould be (i) increasing sensitivity, because the sample is not diluted; (ii) theability to study insoluble materials; and (iii) the ability to observe species thatare unstable or short lived in solution because of their chemical reactivity orthermal instability. On the other hand, those interested in the physicalproperties of polymers are more concerned with the intrinsic properties of apolymer in the solid state, because polymers in the end use are often in the solid22form. High resolution solid-state NMR spectroscopy gives the possibility ofproviding a more direct link between a polymer structure and its physicalproperties in its end use.1.6.1. Principles of Pulsed FT-NMRWhen the magnetic moment m, of the nucleus interacts with a magneticfield 110, the torque exerted on the spinning nucleus causes the nuclear magneticmoment to precess about 110. The Larmor frequency COo is the frequency of thisprecession, and is proportional to the gyromagnetic ratio y of the nucleus and themagnetic field strength 1/0 as shown in equationcoo = y H0.^ (1.6.1)When a nucleus with a non-zero spin number I is placed in a magnetic field,21+1 quantized energy levels will be generated. For example, two energy levelsare generated for 1H or 13C nuclei, which have 1=1/2. The separation of theseenergy levels, AE, depends upon the magnetic field strength Ho, and can beexpressed as:AE = Y H2ic (1.6.2)More spins tend to align their magnetic moments along the field direction thanagainst it. The population of spins in each energy level (N+ and NJ is describedby the Boltzmann distribution:NJ N+N+ e —dE 1 kT^(1.6.3)23where N_ is the population of the upper energy level, and N+ is the population ofthe lower energy level. Transitions between these energy levels form the basis ofNMR spectroscopy.Transitions between the nuclear spin energy levels can be produced bybringing each chemically different nucleus in the sample into its resonancecondition. In practice, the resonance condition can be obtained by one of threemethods: Either the field is swept at a fixed frequency, a range of frequencies isswept at a fixed field, or a band of frequencies around the Larmor frequency issimultaneously excited at a fixed 1/0 field. Fourier transform (FT) NMR uses thethird method, where a short radio frequency (rf) pulse at the Larmor frequency,(Os, is applied to the system. The pulse is generally powerful enough to cover theentire frequency range of a given type of nucleus in the sample. The Boltzmanndistribution of spin populations is thus disturbed from equilibrium, and thesystem tends to reestablish its equilibrium state through transitions betweenthe energy levels. The process of returning to equilibrium state is a timedependent process involving both spin-lattice and spin-spin interactions, whichare described by the spin-lattice relaxation time (T1) and the spin-spinrelaxation time (T2) respectively. Following the rf pulse, an rf receiver is turnedon to receive the signal from the nuclei. This signal, recorded as a function oftime f(t), is called the free induction decay (FID). The time domain function f(t)and the frequency domain function F(co) are Fourier transforms of each other.Thus, the NMR spectrum, which is the frequency domain function F(co), can beobtained by a Fourier transformation of the FID.It is often convenient to describe FT NMR from the viewpoint of a rotatingframe of reference, i.e. a reference frame coincident with z axis and rotatingaround z with angular velocity coo, where x', y', z' are used to represent the threeaxes of the frame. The behavior of spins can be described by simple vector24diagrams in a coordinate system rotating at the Larmor frequency con. Theequilibrium distribution of spins leads to a net magnetization which can berepresented by the M0 vector in the field direction along z (Figure 1.2A). Figure1.2B shows the effect of an on-resonance rf field H1 which is appliedperpendicular to the magnetic field direction. The H1 field creates a torque onthe net magnetization M0, causing M0 to precess in the y'z' plane at a rate co = yH1. If the duration of the H1 pulse, t, is set to that cot = 7c/2, then themagnetization M0 is tipped by 900 into the x'y' plane (Figure 1.2C), where it isdetected. Figures 1.2D and 1.2E show how the magnetization relaxes back to itsequilibrium state by T1 and T2 relaxation processes. The decaying component ofthe magnetization in the x'y' plane is recorded as the FID signal. The return toequilibrium is shown in Figure 1.2F. The experiment can be repeated onceequilibrium is reestablished. Because T1 T2, the relaxation time T1 controls thelength of time required to obtain a FT NMR spectrum. The spin-latticerelaxation times (T1) for carbons in polymer solids are often very long (severalminutes), while the corresponding T1 values for some polymers in solution are inthe order of tenths of seconds.[76]The advantage of the pulsed Fourier transform NMR technique isprimarily its efficiency. The data can be collected all at once, rather than from aslow sweep of the field. In addition, the technique is useful for improving thesignal to noise ratio by data averaging. Many free induction decays from a weaksignal can be repeatedly co-added in the computer. The noise is random and istherefore accumulated more slowly, while the signal is coherent and continuallyadded. The signal-to-noise ratio increases as the square root of the number ofaccumulated spectra. The pulsed Fourier transform method also makesmultipulse NMR experiments for various purposes possible.25x'( D),( F ),Ho4^\,H1=0^7Hi=0( E )Figure 1.2. Vector diagrams describing the pulsed NMR experiment inthe rotating frame of reference. (x', y', z' axes are used toindicate the use of the rotating coordinate system, whichrotates around z' at the Larmor frequency.) (A). The netmagnetization M0 is aligned the magnetic field direction 1/0;(B, C). An rf field H1 is applied perpendicular to Ho and tipsthe net magnetization by 90'; (D, E). The magnetizationbegins to relax in the x'y' plane by spin-spin (T2) processesand in the z' direction by spin-lattice (T1) processes; (F). Theequilibrium magnetization is reestablished along Ho.261.6.2. Dipolar Coupling in Solids and High Power Proton DecouplingIn the NMR spectra of small molecules in solution, the dipole-dipoleinteraction usually can not be directly observed since rapid molecular tumblingaverages this interaction to zero. In solids, however, the molecules are not free totumble isotropically and the static dipole-dipole interaction generally causespeak broadening so large that all spectral details are lost. The dipole-dipoleinteraction arises from direct spin-spin coupling. For example, the magneticdipole of one spin (1H, for example) influences the magnetic dipole of anotherspin (13C, for example), which results in a splitting of 13C spectral line. For asingle crystal in which the coupled C—H pairs are faraway from each other, twopeaks will be observed, whose position and separation depend on the orientationof the two interacting nuclei in the magnetic field. For powders, all orientationsare present, and the observed pattern is a broad envelope of overlapping peaksThe dipolar interaction between two spins can be written in the generalform:XD = /1*-6/2^ (1.6.4)where xi, is the magnetic dipolar Hamiltonian, li and 12 are two spin vectors,and b is the dipolar coupling tensor.[77] The dipolar Hamiltonian for two spinspecies I and S can be written as:[781XD = XII + XSS + XIS^ (1.6.5)For abundant nuclei I, (eg. 1H), the major portion of this dipolarinteraction is that between like spins, which is XII. For dilute nuclei S (eg. 13C innatural abundance), the major dipolar interaction is usually with the abundantand nearby protons, ie. Ytis. For a normal polymer sample, the 13C atoms innatural abundance are sufficiently diluted and far enough away from each other27to make the 13C-13C dipolar interaction ;s very weak and hence negligible inmost cases.Mathematically, the heteronuclear dipole-dipole interaction, Xis, has theform[781hN1 Ns _3Xis = – YiYs ( — )2 E E r ^- 1)/iz/kz27r^k (1.6.6)Where r is the distance between the nuclei and 0 is the angle between the vectorconnecting the two interacting nuclei and the external magnetic field Ho. Thedipolar interaction strongly depends on the internuclear distance r. The dipolarcoupling for a set of magnetically isolated 13C-1H spin pairs at a certain angle,0, relative to H0 results in a splitting in the 13C spectrum (corresponding to thetwo allowable proton spin states) given by:AED Yc YH (3COS20 - 1)/r3271(1.6.7)This is shown schematically in Figure 1.3B. The variables in Equation (1.6.7)are defined in Figure 1.3A. However, for a normal powder sample, the variousC–H vectors have all possible angles with respect to the external magnetic field.For a single type of magnetically isolated C–H vector in such material, thecomplete range of dipolar couplings is expected, which produces the Pakedoublet pattern[79] (Figure 1.3C). Actually, the individual C–H vectors are notmagnetically isolated and many additional dipolar interactions can also occur.This produces an inhomogeneously broadened line with a Gaussian shape(Figure 1.3C). Therefore, the static dipole-dipole interaction in solids causes alarge line broadening, which must be removed in order to obtain high resolutionNMR spectra of solids.28AFigure 1.3. Schematic representation of dipolar interactions. (A). Dipolarinteraction between a spin pair of 13C and 1H nuclei. t arethe z components of the magnetic moments. (B). Dipolarsplitting of isolated C—H pairs at one angle relative to themagnetic field. (C). Pake pattern expected for isolated C—Hspin pairs distributed at all angles in a powder sample. (D).Approximate Gaussian line shape observed for non-isolatedC—H spin pairs, where all dipolar interactions are operative.(Reproduced from reference [78])29Equation 1.6.6 for is shows the spatial and spin parts separately. Ifeither of these parts can be averaged to zero by some coherent process, the effectof Xls can be removed from the spectrum. One way to remove the dipolarcoupling involves modulation of the spatial part in Equation 1.6.6 bymechanically spinning the sample at 6 = 54.74°, the magic angle, which reducesthe (1-3cos2e) term to zero[80] (see on). The other option is to average /igkz thespin part of Ytis, to zero. This can be achieved by high power dipolar decoupling.Dipolar decoupling is accomplished by applying an additional rf field tothe solid sample at the proton Larmor frequency, as is done to remove the scalarJ-coupling in solution. For solids, however, the amplitude of the decoupling fieldmust be larger compared to the static dipole-dipole interaction. Instead of the 1gauss field used in solution NMR spectroscopy, an approximately 12 gauss(corresponding to approximately 50 kHz) field is required to remove the effect ofthe C—H dipolar interaction from solid state 13C NMR spectra. Thus, highresolution solid state NMR spectroscopy requires amplifiers capable of providingsufficient power for dipolar decoupling, and rf probes capable of withstandingthese high power levels.The effect of dipolar decoupling on solid state NMR spectra is illustratedin Figure 1.4.[81] All three 13C NMR spectra are of bulk poly(butyleneterephthalate). The top spectrum is obtained under the conditions used for asolution NMR experiment, whereas in the middle spectrum the dipolarinteractions with the protons are decoupled. The spectrum in Figure 1.4B stilldoes not exhibit the high resolution associated with solution state 13C NMRspectra. Although the static dipole-dipole interactions with the protons havebeen removed in this spectrum, the lines are still broad, primarily because ofchemical shift anisotropy. The bottom spectrum (Figure 1.4C) combines magicangle spinning (MAS) with high power proton decoupling. (MM averages all the30 iepsoonouniesi Ole \wwwi werf Ii004,001^ 1^1^1300 100 -100ppm FROM TMSFigure 1.4. The effects of dipolar decoupling and magic angle spinningon 13C solid state NMR spectra of poly(butyleneterephthalate). (A). Spectrum obtained using low powerdecoupling. (B). Spectrum obtained using high power(dipolar) decoupling. The primary source of line broadeningin this spectrum arises from the chemical shift anisotropy.(C). Spectrum obtained using dipolar (high power)decoupling in addition to magic angle spinning (MAS).(Reproduced from reference [81]).31overlapping chemical shift anisotropies to their isotropic values, as will bediscussed in the following section.)1.6.3. Chemical Shift Anisotropy and Magic Angle SpinningThe chemical shift arises because the electrons in a particular atominteract with the magnetic field, Ho. The external magnetic field induces electriccurrents in the molecule, and these currents produce a local magnetic field HL atthe nucleus. Thus, the nucleus no longer experiences only the external field 110,but Ho+HL, which alters the resonance frequency of the nucleus. The chemicalshift tensor a describes the orientation and the magnitude of this threedimensional local field.In general, the chemical shift interaction has the formrn5€0- = y —h IAA)2TE(1.6.8)where 6 is the chemical shift tensor and Yea is the chemical shift Hamiltonian.Equation 1.6.8 can also be simplified as[78]h,ea = Y — azz 1z Ho211(1.6.9)where azz is the projection of the chemical shift tensor onto the z axis, whichcan be expressed in terms of the diagonal elements of a, aii, and the respectivedirection cosines, Xii, with respect to Ho:2^2^2azz = a11X11+ a22A22+ a33X33^(1.6.10)The principal values of the chemical shift tensor describe the magnitude of thetensor in three mutually perpendicular directions in the molecule, and the32direction cosines specify the orientation of the tensor with respect to the externalfield. In solution, molecules tumble freely and isotropically, averaging thechemical shift tensor to its isotropic value ciso. Thus, the isotropic chemicalshift is the only part of the chemical shift tensor observed in solution state NMR.However, motion in solids is limited, and a broad anisotropic chemical shiftpattern is usually observed for a solid sample. The chemical shift observed insolution state NMR, or the isotropic chemical shift, is one-third of the sum of thediagonal elements of the tensor:1 ,cis° = 5 kan + a22 + a33) (1.6.11)Equations (1.6.9) and (1.6.10) indicate that the chemical shift of aparticular nucleus in the solid state depends upon the orientation of themolecule with respect to the magnetic field. For a single crystal in absence ofother interactions, a single and relatively narrow signal is observed whoseposition depends on the orientation of the crystal with respect to the magneticfield 110. This can be seen schematically in Figure 1.5A. For a powder sample,rather than a single crystal, all crystallite orientations are present and theresultant NMR spectrum shows a characteristic chemical shift anisotropypowder pattern. Figures 1.5B and C illustrate two theoretical chemical shiftanisotropy patterns. For carbon monoxide, the axial symmetry of the molecule isreflected in a pattern as shown in Figure 1.5B.[82] The position of all or alrepresents the observed resonance frequency when the principal axis system isparallel or perpendicular to the field direction. Figure 1.5C shows a generalizedchemical shift anisotropy powder pattern for the more common nonsymmetricalcase. The isotropic chemical shifts ais0=1(1 / )(,--, 3, ,a11 +sr:322+033)i are indicated bythe dotted lines in Figure 1.5.33I 0HICHoCEOIA^1 ^1 ,^1I1BI111I1I1I1CFigure 1.5. Schematic representation of chemical shift anisotropy powderpatterns. (A). Chemical shifts observed for two orientations of acarbon monoxide single crystal relative to the static magneticfield. (B). Axially symmetrical chemical shift anisotropy powderpattern would be observed for axially symmetrical moleculessuch as polycrystalline carbon monoxide. (C). Generalanisotropic chemical shift anisotropy powder pattern fornonsymmetrical molecules. The dotted lines represent theisotropic chemical shifts, oiso. (Reproduced from reference [78])34Molecular motion causes a partial narrowing of the chemical shift tensorpowder pattern. The exact shape of the motionally narrowed chemical shiftpattern contains information concerning the axis and angular range of themotion. In general, carboxyl, carbonyl, and aromatic carbons have the largestanisotropies I ail — a33I, or powder pattern widths (approximately 180 — 250ppm). The anisotropies for methyl, methylene, and methine carbons are usuallyless and are in the order of 30 —60 ppm.The chemical shift anisotropy contributes a line broadening in solid stateNMR spectra that often obscures the structural information available from theisotropic chemical shift. In addition, powder data give only the principalcomponents not their orientations. For example, less structural information canbe deduced from the spectrum in Figure 1.4B than from the spectrum in Figure1.4C. For this reason, high resolution solid state NMR spectra are usuallyobtained using magic angle spinning to reduce the chemical shift anisotropy andobtain the isotropic value.Magic angle spinning (MAS) is carried out by mechanically spinning asample about an axis making the "magic angle" (0 = 54.7°) with respect to themagnetic field direction (Figure 1.6). MAS is not a new technique. In 1959Andrew et al.[831 used it to narrow the 23Na line in NaC1, and in the same yearLowe independently used MAS to narrow dipolar broadened 19F lines in CaF2and Teflon[84]. These early attempts to remove dipolar interactions with MASwere not totally successful as it was impossible to spin the sample rapidlyenough to remove the large homonuclear dipole-dipole interactions. However,Schaefer and Stejskal (1976) showed that if one removed the static dipolarinteraction by high power proton decoupling, the remaining chemical shiftanisotropy could be averaged to its isotropic value using mAs.[66c]35A54°44'\ 90°^180°9Figure 1.6. Schematic representation of magic angle spinning. (A). Thegeometric arrangement for mechanical sample spinning. (B).Variation of the term (3cos20-1) as a function of 0. The curvecrosses the axis at 0=54°44', the "magic angle". (Reproducedfrom reference [65])Under rapid mechanical sample rotation about an angle 0 with respect tothe magnetic field direction (Figure 1.6), the direction cosines in Equation(1.6.10) become time dependent. Therefore, the chemical shift Hamiltonian can„avgbe divided into a time-independent part, it cy , and a time-dependent part,0t): [78]vgXcy^a + Za(t) (1.6.12)Taking the time average under fast sample rotation, only the time-independent, Avgpart, if^, will be left.1^0 1evg = y 11— /z1/0 [ — sin443 (011 +022+c733) + (3cos20 —1)271^2x(functions of direction cosines)]^(1.6.13)36The angle 0 in Equation (1.6.13) is the angle that the rotation axis makeswith the static magnetic field direction (Figure 1.6). For 0 = 54.7° (the magicangle), the (3cos20 — 1) term becomes zero and sin20 is 2/3. The first term inEquation (1.6.13) becomes one-third of the trace of the tensor, ie. the isotropicchemical shift. Using MAS, the chemical shift powder pattern is thus reduced toits isotropic average value. As mentioned in the previous section, MAS can alsoreduce the dipole-dipole interaction. The effect of MAS on the 13C NMRspectrum of the solid, poly(butylene terephthalate), is shown in Figure 1.4C. Inthis spectrum the broad overlapping carbonyl and aromatic resonances in Figure1.4B have been reduced to their isotropic averages, and a truly high resolutionspectrum is obtained.1.6.4. Relaxation Times As in conventional FT-NMR spectroscopy of liquids, spin relaxation timesalso play an important role in obtaining high resolution NMR spectra of solids.The most fundamental relaxation times, which are important for both solutionand solid state NMR spectroscopy, are the spin-lattice relaxation time (T1) andspin-spin relaxation time (T2). T1 characterizes the regrowth of themagnetization back to its equilibrium value along the direction of the static fieldafter it has been perturbed by a radio frequency pulse. It dictates the repeattime which may be used for signal collection. The spin system must couple to itssurroundings, or lattice, in order for T1 relaxation to occur. In this process thespin system gives its excess energy to its surroundings, or lattice. Thisnonradiative process of spin relaxation occurs via a modulation of local magneticfields by molecular motion at the proper frequency. In polymer systems, the localfields usually arise from the proton magnetic moments. Many importantmotional processes in solids, particularly in polymers, have characteristic37frequencies in the range of tens of kHz, which are not high enough to produceefficient T1 relaxation under normal circumstances. The spin-lattice relaxationis most efficient when the correlation frequencies of these motions are near theLarmor frequency, typically in several to several hundreds of MHz range. SinceLarmor frequencies which are effective for T1 relaxation are determined by thestrength of the static magnetic field, one way to match the Larmor frequencies tothe frequency of molecular motion in tens of kHz range is to perform the NMRexperiment in a very low magnetic field. However, this approach is not practicalbecause NMR sensitivity and resolution decrease as the magnetic field islowered.In general, each type of nucleus has its own T1 (protons are an exception,see later). T1 values are different for different nuclei because of differences inLarmor frequencies, motions and available relaxation mechanisms. Becausedifferent nuclei can be coupled by various interactions, their relaxation timesmay not be independent. As described previously, the isotopically abundant andnonisolated nuclei (eg. 1H) are strongly coupled in solids. The relaxation timesof different chemical types of these nuclei are often averaged by spin diffusion,or mutual spin flips among strongly coupled nuclei. Therefore, the protons in asolid polymer usually all have the same T1 value. In addition, the proton T1 isgenerally shorter than most of the carbon T1 values.[851 Nonequilibriummagnetization in any part of the proton spin system is transferred tosurrounding protons in times on the order of 100 !Is by spin diffusion.[781As expected, solids, with less molecular motion in the megahertzfrequency range, often have long 13C T1 times. Even though the 13C in a methylgroup which generally rotates rapidly often has relatively short T1 time, the T1times for other carbons can be extremely long. The 13C in the rotating methylgroup rarely exchanges its short T1 time with the other carbons in the sample38via spin diffusion, because the natural abundance of 13C is very low and thedistance between the 13C nuclei is very long on average, and thus, the couplingbetween the 13C nuclei is very weak. In addition, the methyl carbon cannottransfer its short T1 to its nearby protons, and then from the protons back toother carbons, because the carbon and proton Larmor frequencies are very farapart and thus there is no overlap in the carbon and proton energies (Figure1.7). On the other hand, the protons on the methyl group also have short T1values, and they can transfer this short T1 to all the other protons via spindiffusion, as the protons are essentially 100% naturally abundant and closeenough to strongly couple to each other.1H13c _ A0^200^400MHzFigure 1.7. Larmor frequencies for 1Hand 13C in a 9.4 T magneticfield. There is no frequencyoverlap, and thus no overlapin the energies.Spin diffusion is not an actual molecular diffusion but the transportationof spin energy within the spin system by mutual, energy conserving spin flips.Two adjacent protons in a solid strongly coupled by the dipolar interactionessentially have the same resonance frequency even if they are chemicallydifferent because of their large line widths caused by strong dipolar coupling. Ifthe two have antiparallel magnetic moments to each other, it is an energeticallyfavorable process for both moments to change orientation simultaneously or"flip", again yielding an antiparallel pair but with magnetic moments reversed.Such flips can occur rapidly among neighboring, coupled protons in a solid and39serve to distribute excess energy or magnetization among all the coupledprotons. This process, like any diffusion process, is driven by a concentrationgradient, which is the spatial gradient of the magnetization in this case. It isspin diffusion that ensures that the 1H linewidth is homogeneous. Because ofspin diffusion, the whole proton spin system is coupled to the lattice via the mostefficiently relaxing portions of the system. Energy transfer to the lattice is veryefficient near paramagnetic impurities, lattice defects, and molecular segmentsin rapid motion, such as methyl side chains or possibly end groups.When x and y components of g magnetization are created by a rf pulse,the decay of these components is characterized by T2. There is no energyexchange associated with this decay and it is a pure entropy process. This decayoccurs because all the spins do not precess at the exactly same rate and tend toget out of phase. The linewidth of a NMR spectrum, v112, is related to T2 by:1V1/2 -TCT2(1.6.14)for a Lorentzian line. Molecular motion at the Larmor frequency affects T2; lowfrequency motion, on the order of 100 — 1000 Hz, can also affect T2.[78]Inhomogeneity in the static field also appears to affect T2, because the spins arein different fields and hence get out of phase more rapidly. However, for highresolution solid state NMR this last factor is usually negligible in a wellshimmed magnet. For polymers in both solution and solid state, Ti. is usuallymuch greater than T2, T1 » T2.1.6.5. Cross Polarization (CP)In addition to removing the strong dipolar interaction and averagingchemical shift anisotropy, one other difficulty must be overcome before high-40resolution solid state 13C NMR spectra can be obtained in an efficient way. Asdescribed previously, T1 characterizes the return of the spin system to itsequilibrium state after being perturbed by a rf pulse, and controls the rate atwhich the experiment can be repeated. For rare nuclei, such as 13C, whichrequire more signal accumulating to obtain their spectra, the repetition ratebecomes very important.Since the relaxation times of rare nuclei 13C are normally not short, thesignal accumulation time can be very long. However, this problem can becircumvented by transferring magnetization from the abundant 1H nuclearspins to the rare 13C nuclei under observation. Through this process, therepetition rate for 13C signal collecting is now determined by the shorter T1 ofthe 1H nuclei. The process of magnetization transfer from abundant spins torare spins is termed cross polarization (CP) and was first introduced by Pines etal.(861Although 13C and 1H have Larmor frequencies different by factor of four(Figure 1.7), Hartmann and Hahn in 1962[87] demonstrated that energy may betransferred between them in the rotating frame. Thus, energy transfer betweennuclei with very different Larmor frequencies such as 1H and 13C can beachieved if the following "Hartmann-Hahn condition" is matched:7C H1C = YH H1H (1.6.15)Equation 1.6.15 results in a "match" of the rotating frame energies for 1H and13C. Since yif is four times of 'ye, it is necessary for the match that the strengthof the applied carbon field (Hic) is four times the strength of the applied protonfield (HIH). Cross polarization gives increased S/N (the signal to noise ratio) in agiven time period.41The vector diagrams for this experiment are shown in Figure 1.8, and thepulse sequence is shown in Figure 1.9. The vector diagrams in Figure 1.8A showthe proton and carbon spin systems equilibrated in the magnetic field. A 90° rfPulse (Hm) is applied to the proton along the x' axis and brings the protonmagnetization along the y' axis (Figure 1.8B). Then, a long proton pulse(HiH)with its phase shifted by 900 locks the proton magnetization along the y' axis(Figure 1.8C). The strong Ili ji spin-locking pulse force the proton spins toprecess around the y' axis of their rotating frame with a frequency co-H = Yll H1H •Magnetization decay processes occur during this time[66e1. Meanwhile, thecarbons are exposed to a long carbon pulse I/1c along the direction of the spin-lock field (y' axis), which causes the carbon magnetization to precess along theI/1c field with the frequency coc = Tc Hic. If coH and (pc are equal, an energyexchange between both nuclei becomes possible, which causes the 13Cmagnetization to grow along the Hic field (Figures 1.8C and 1.9).At this point, the cross polarization process can be described in moredetail in Figure 1.10. When the Hartmann-Hahn condition (Th Iiiii = 7c Hid ismatched by adjusting the power levels of the Hai and H1c fields, the z-components of both the 1H and the 13C magnetizations have the same timedependence (Figure 1.10) as the two precession frequencies are the same, con =O. Because the z-component time dependence is common to both spin systems,mutual spin flips can occur between the 1H spins and the 13C spins. Thisprocess can be visualized as a "flow" of magnetization from the abundant protonspins to the rare 1-3C spins.An alternative way to visualize polarization transfer is from a spintemperature or thermodynamic point of view. This concept has been described indetail by Pines et al.[881421H^z'^z'90,(1)spin lock^z'protonsalong y axis(2)protons xprecess aroundy' axis c°H =z'90° (SPIN LOCK)y, DECOUPLE ALLOW PROTONS TORE-EQUILIBRATE1H13C^z'^ (1) apply rfpower /-iicalong y axis>^x'(2)13C magnetization^_grows up^(pc — Yc Hicalong y' axis(3) 13C spinsprecessA^B^around y' axisFigure 1.8. Vector diagram for a 1H — 13C cross-polarization experiment.The carbon reference frame and the proton frame arerotating at different frequencies wc, coH. (Reproduced fromreference [81])lacCONTACTI^TIMEOBSERVE FID WidT TIME^a. TIMEFigure 1.9. Schematic representation of the 1H--13C cross-polarizationpulse sequence for solid state NMR experiments.43Figure 1.10. A more detailed representation of part C in Figure 1.8. The13C spins and the 1H spins are precessing about thedirection of the spin-lock field with frequency coc=ycHic, andcoH=ThHill respectively. When the Hartmann-Hahn conditionis matched (coc = o3H), the two spin systems have z-components with the same frequency dependence. Thusmutual spin flips can occur between 1H and 13C.(Reproduced from reference [81])The advantages of cross polarization are twofold. Firstly, as mentionedpreviously, it circumvents the problem of the long carbon Ti. values normallyfound in solids. The 13C nuclei obtain their magnetization from the protons, andthus it is the proton Ti. which controls the cross polarization experimentrepetition rate. Secondly, the 13C shows an enhancement in its signal intensity,which can be as large as the ratio of yH / yc, (a factor of nearly four). Thus, crosspolarization saves experimental time by reducing the waiting period as well asby improving the signal to noise ratio. Figure 1.11 demonstrates the advantage44BCD I;H-3-1CH2- C=0OCH3CH3Figure 1.11. A demonstration of the advantages of combining DD, MAS, andCP techniques for obtaining 13C solid-state NMR spectra ofpoly(methyl methacrylate). (A). Stationary sample; no crosspolarization; low power decoupling. (B). Stationary sample;cross polarization; high power decoupling. The spectrum showsthe effects of chemical shift anisotropy. (C). Magic anglespinning; no cross polarization; high power decoupling. Thespectrum shows "high-resolution" isotropic shifts but the signalto noise ratio is low as cross polarization is not used. (D).Complete experiment: cross polarization; magic angle spinning;and high power decoupling. The spectrum shows similarresolution as (C) but the signal to noise ratio is greatlyenhanced by the use of the cross polarization technique.(Reproduced from reference [891)45CrossReactionSitesZzGlass/SurfaceEnd Groups ICouplingAgentsResin Reaction Sites ILCross Linking Agentsof combining dipolar decoupling (DD), magic angle spinning (MAS) and crosspolarization (CP) techniques for a typical glassy polymer, poly(methylmethacrylate) (PMMA). Comparison of C and D clearly illustrates the value ofcross polarization. [89]1.7. Purpose of this Thesis ResearchAs briefly described above, polymer resin systems are normally complexheterogeneous materials. The final resins are usually multiphase, amorphousand insoluble solids.Figure 1.12. Schematic representation of some important activesites in a heterogeneous polymer resin systemFigure 1.12 shows a typical heterogeneous polymer resin system. Thehigher the degree of cross-linking in the polymer system, the more the46individual molecules are immobilized, and thus, the greater the strength andstability of the polymer matrix. In many cases, the mechanical, physical andchemical properties of a polymer are determined by the nature and the degree ofthe reaction for a very small number of functional groups in the polymer systemor in added molecules. The interference from the bulk polymer matrix makes itvery difficult for most of analytical techniques to study the reaction of thesesmall number of functional groups (eg. the different reaction sites, the endgroups, the coupling agent, the cross linking agent or the curing agent in thepolymer system, as shown in the Figure 1.12).The NMR signal of an isotopically enriched element will be many timeshigher than the signal of the corresponding low natural abundance element. For13C NMR, the signal enhancement from isotopic enrichment can be up to 99 fold(the natural abundance of 13C is 1.1%). For 15N NMR, even higher increases insignal intensity may be obtained, since the natural abundance of 15N is only0.37%. By using specific isotopic enrichment in a functional group, e.g. in acoupling agent or a cross-linking agent, we can greatly reduce the contributionsfrom the nuclei in the sample bulk. By subtracting the spectrum of acorresponding sample with no isotopic enrichment, the spectrum of only theenriched group may be obtained since the NMR spectrum has greatly enhancedintensity for the enriched nucleus in the functional group. It is thus possible tounambiguously trace the enriched functional group during a polymerizationprocess and characterize its product species.The effect of isotopic enrichment can be demonstrated in Figure 1.13,which shows 13C CP/MAS spectra for a cured phenolic resin.[901 The topspectrum was obtained from a sample with natural abundance. The importanceof using isotopic enrichment can be seen by comparison of both spectra. In47spectrum B, using only about 5% 13C enriched formaldehyde makes the reactionproducts from the formaldehyde much more clearly distinguishable.By the use of solid state NMR spectroscopy combined with specific isotopicenrichment, it should be possible to efficiently study the properties and thereaction mechanisms of the small number of very important chemical groupswhich determine the physical, mechanical, and chemical properties of a polymer.Several projects are included in this thesis. Each of them corresponds to theinvestigation of one of the different reaction sites or different agents inheterogeneous polymer systems as shown in Figure 1.12 with particularemphasis on the case of cyanate resin related system. They will be discussed indetail in the following chapters.48CH2200 160 0 PPM from TMSFigure 1.13. The effect of selective isotopic enrichment. (A). Solid-state13C NMR spectra obtained at 22.6 MHz from a curedphenolic resin (phenol/formaldehyde/sodium hydroxide1/2/0.01, cured at 110 °C for 24 hours), magic-angle spinningat 3.6 kHz; (B). conditions as in (A) except that the samplewas prepared by using formaldehyde 13C enriched to —5%.The small peaks marked s denote spinning sidebands.(Reproduced from reference [90])49CHAPTER 2.INVESTIGATIONS OF THE CURING REACTIONSOF THE CYANATE RESIN SYSTEMConventional electronic circuit boards are made from glass-fibrereinforced epoxy resins. However, they often fall short of the thermal andelectrical performance demands of many modern high speed devices. Withcurrent trends toward increased circuit densities, shorter propagation delays,elevated operating temperatures, and higher reliability, new advanced materialsare being developed to satisfy these demands. Among these materials, cyanateresins are considered to be very promising systems. As described in the previouschapter, the cyanate resins are commonly derived from the bisphenol type ofmonomer. The cured resins exhibit good thermal, mechanical and insulatingcharacteristics and have been considered for many electronic packaging andstructural materials applications. [17,27,30]Bisphenol A dicyanate (BPADCN, 2) is one of the cyanate resin monomersmost often used. The curing reaction is postulated to proceed by reaction of threecyanate (—OCN) groups on different molecules to form a triazine ring as shownin Scheme 1.1. Although the reactions of some model systems have been studiedand IR investigations have been carried out to identify some of the functionalgroups during curing, [16,22] there is little direct evidence to date regarding thespecies involved, or the nature and efficiency of the curing process itself.The purpose of this project was to carry out an investigation of the curingreactions of these resins by high resolution 13C and 15N NMR spectroscopy bothin solution and in the solid state. To facilitate the study and to clearly monitorthe reactions of the functional groups during the curing process, not only thenatural abundance dicyanate monomer 2, but also the 13C and 15N enriched50dicyanate monomers, 2a and 2b, were used. This greatly enhances thecontribution of the cyanate group and its reaction products to the NMR spectra.In addition, to provide reference spectra for the identification of reactionproducts, the reactions of the analogous monocyanates, p-tert-butylphenylcyanate (PTBPCN) in natural abundance (25) and with specific isotopicenrichment (25a) and (25b), were also studied. Since the monocyanates can notform a cross-linked network during triazine formation, the better solution NMRspectra can be obtained.NCO OCNN13C02013CN^15NC0 0C15N 2a^ 2bOCN 013CN 0C15N25^ 25a^ 25b2.1. Syntheses and Characterizations of Specifically Labeled CyanateMonomersThe 13C and 15N enriched cyanate monomers were synthesized by a two-step procedure. First, the labeled cyanogen bromide (Br13CN or BrC15N) wasmade by using either K13CN or KC15N as the labeled starting material[Equation 2.1], following a similar procedure to that of Hartman and Dreger[91].51K13CN^+^Br2 ^..- Br13CN + KBrKC15N^+^Br2^BrC15N +^KBr^[2.1]The labeled cyanogen bromides were then reacted with the appropriate phenolto produce the corresponding labeled cyanate compounds [Equation 2.2].OH + BrC*N + Et3 N ^MM.OC*N + Et3NHBr [2.2]BrC*N = Br13CN or BrC15NThe 13C and 15N enriched dicyanate monomers 2a and 2b were preparedfrom bisphenol-A by reaction with labeled cyanogen bromide as indicated inEquation 2.3. However, before using labeled materials, the preparations werecarried out with unlabeled reagents, and a small scale reaction suitable forproducing appropriate quantities for NMR studies was optimized. The synthesisprocedures are modifications of literature methods[602] and are given in detailin the Experimental chapter at the end of the thesis.N13 COBr13Cyl013CN2a HO OH^ [2.3]30C15N2b52Figures 2.1A and B show the 13C solution NMR spectra (reference: TMS)with assignments for natural abundance and 13C enriched BPADCN monomers,2 and 2a, respectively. The large increase in the intensity of the peak at — 109ppm due to the —OCN carbon indicates that it is possible to follow its reaction inthe curing process by comparison of spectra from the labeled and the unlabeledmaterials.Figure 2.1C shows the 15N solution NMR spectrum of the 15N enrichedBPADCN monomer 2b. The resonance for the nitrogen in the —OCN group isclearly observed at — 53 ppm (reference: neat formamide). The naturalabundance signal is not detectable at all under these conditions. The 15N spectraduring curing will thus show signals only from the cyanate group and itssubsequent reaction products.The labeled monocyanates, 25a and 25b, were synthesized by the reactionof p-tert-butylphenol 26 with labeled cyanogen bromide according to Equation2.4. This reaction is a modified literature procedure{931 and is described indetail in the Experimental chapter. The corresponding unlabeled compound 25was also synthesized by the same method in larger quantities. Brl 3CNBrC15N01 3C NOH 25a [2.4]260C1 5 N25bFigure 2.2A shows the 13C NMR spectrum of the natural abundancemonocyanate PTBPCN 25 together with the assignment. The correspondingspectrum of the 13C labeled material 25a, given in Figure 2.2B, indicates the53c7OCN1150^100i^T50 PPMc2 c6 c3 c2c6c7I^Icl c4IIc5B c7—0C*NCDC13C ..r...••••••wirsomeaLawa..............eorolmarroauqm.slr^80^I40^1-10^-40 PPM1Figure 2.1. Solution NMR spectra 0-H at 300 MHz) of bisphenol Adicyanate (BPADCN) monomer in CDC13. (A). 13C NMRspectrum of natural abundance monomer 2; (B). 13C NMRspectrum of the 13C enriched monomer 2a; (C). 15N NMRspectrum of the 15N enriched monomer 2b.54c6 c3 c2 m2 ml m3 m4C1-13COCH2CH3c3 c2a'2 a'lCD3C0CD3ml a'l^cl c4li Ic5m2^m4m3a'2c6^Ac7c6 c3 c2c7OC*Nc7^ BJ200^160^120^80^ao^0 PPMFigure 2.2. 13C solution NMR spectra (1H at 200 MHz) of (A). naturalabundance p-tert-butylphenyl cyanate (PTBPCN, 25) and(B). 13C enriched PTBPCN 25a in methyl ethyl ketone(MEK) and acetone-d6.55successful introduction of 13C into the cyanate group which gives the resonanceline at 8 = 109 ppm. Figure 2.3A shows the 15N spectrum of the 15N labeledcyanate 25b. There is a single sharp resonance at - 53 ppm consistent with theBPADCN spectra. The spectrum obtained without decoupling shows that thereare no coupled protons (Figure 2.3B). The 13C spectrum (Figure 2.3C) shows asplitting of the -OCN resonance, C7, into a doublet due to the coupling to the15N nucleus which is consistent with the 15N spectrum and confirms theintroduction of the 15N nucleus into the cyanate group. The coupling constant inthe cyanate group is JC-N = 11.6 Hz.2.2. Investigation of the Curing Reaction in SolutionBecause of its inherent higher resolution, solution NMR was first used tocharacterize the species involved in the curing reaction process, includingreactants, main products, side products and any possible intermediates.2.2.1. 13C NMR Investigation of the Curing Reaction of BPADCN in Solution The progress of the curing reaction in solution was investigated for the13C enriched BPADCN monomer by high-resolution 13C NMR at 50 MHz (protonfrequency of 200 MHz) with proton decoupling. The solvent used was 2-butanone(also called methyl ethyl ketone, MEK) with a small amount of acetone-d6 addedto provide a deuterium lock signal. Zinc octanoate (200 ppm) was added as acatalyst, and in different experiments the solution was heated at 60 °C, 90 °Cand 100 °C for various times in sealed glass tubes. Representative spectra aregiven in Figure 2.4. Figure 2.4A shows the 13C spectrum of the BPADCNmonomer before reaction. The three large signals at high-field are due to theMEK solvent. The intense signal at 8 = 109 ppm is due to the enriched -013CNgroup. This resonance and those derived from it are used to monitor the progress561^,^-,-^,^/^,^-,^.^.-^i^r^-'- ^T^T^I^I200 150 100 50 PPMFigure 2.3. Solution NMR spectra (1H at 300 MHz) of 15N enrichedPTBPCN 25b in acetone-d6. (A). 15N spectrum with 1Hdecoupling; (B). 15N spectrum without 1H decoupling; (C).13C spectrum with 1H decoupling.57Figure 2.4. 13C solution NMR spectra (1H at 200 MHz) of 13C enrichedBPADCN 2a in MEK and acetone-d6 cured with 200 ppmzinc octanoate as catalyst. (A). Before heating; (B). Afterheating for 1 hour at 60 °C; (C). After 16 hours at 60 °C; (D).After 5 days at 60 °C.58of the curing reaction. The other signals in the spectrum are relatively small andagree with those of the unlabeled monomer previously obtained (Figure 2.1B).After just one hour heating at 60 °C, two new resonances at 156 and 174ppm (labeled a7 and t7, respectively) are observed (Figure 2.4B) which grow inintensity with time (Figure 2.4C and D). These are assigned to the expectedtriazine (8 = 174 ppm, t7) and one other species (8 = 156 ppm, a7). The nature ofthis second species will be discussed in more detail later. Subsequent spectrarecorded after longer heating periods show the same two major resonances buttheir intensities diminish relative to those of the solvent signals, although thecyanate resonance of the monomer disappears at, the same time. This is becausehigh-resolution NMR spectroscopy only detects those species present insolution. With the curing process taking place while the solution spectra ofFigure 2.4 were obtained, considerable quantities of solid material hadprecipitated from solution. The precipitates are considered to be cured cyanateresin which contains mainly the triazine moiety (8 = 174 ppm) and somecarbamate moiety (8 = 156 ppm) which will be discussed later.2.2.2. 15N NMR Investigation of the Curing Reaction of BPADCN in SolutionA series of 15N spectra 0-H, 300 MHz) were obtained under identicalconditions to those of the 13C spectra discussed above and are presented inFigure 2.5. These spectra are particularly informative as the only signalsobserved are from the -0C15N group and its reaction products, with nointerference at all from solvent or unlabeled monomer resonances. Figure 2.5Bshows that a single sharp signal at - 53 ppm as expected and two additionalresonances are observed at -40 ppm and 87 ppm during curing, in generalagreement with the results from the 13C NMR spectra. The signal at 87 ppm can59. -40 PPM1 180 i 1 O 1B-0--C-N*H2I IoFigure 2.5. 15N solution NMR spectra (1H at 300 MHz) of 15N enrichedBPADCN 2b in MEK and acetone-d6 with 200 ppm zincoctanoate as catalyst. (A). Before heating and without 1Hdecoupling; (B). After heating at 90 °C for 1 day and with 1Hdecoupling; (C). Same sample as in (B) without 1Hdecoupling.60be assigned to the expected triazine ring. Figure 2.5C was obtained withoutproton decoupling during acquisition. The signal at —40 ppm shows a tripletstructure indicating that it is coupled to two protons while the others areunaffected as expected. These general characteristics are maintained duringfurther curing (spectra not shown), although again a considerable amount ofsolid material has precipitated and only soluble species are detected in thesolution NMR spectra.The second product species is postulated to be the carbamate compound 27formed by the addition of water to the cyanate group as shown in Equation2•5.[6b]OC*N + H20 90_C^H2 [2.5]27This structure fits all of the characteristics of the 13C and 15N NMR spectra,particularly the coupling of the 15N nucleus to two protons. Furtherconfirmation of the structure of this species is given below. One very importantfeature of the spectra is that they rule out the formation of substantial amountsof any long lived intermediate "dimer" species on the route to triazine ringformation. These species would all have shown two resonances in both their 13Cand 15N spectra. Thus the curing reaction appears to be remarkably clean!2.3. Reactions of Monocyanate Model Compounds2.3.1. 13C NMR Investigation of the Curing Reaction of PTBPCN 25a inSolution As a complement to the solution NMR studies of BPADCN discussedabove, the solution reactions of the analogous monocyanate were investigated,61with particular emphasis on the identification of the reaction products and theeffect of added water. As indicated previously, in the case of the monocyanate,no cross-linking polymerization can occur and there will be no precipitation fromthe reaction mixture.Figure 2.6A shows the 13C spectrum of the 13C enriched monomerPTBPCN 25a in MEK solvent, together with its assignment. The largeresonance at 109 ppm is due to the cyanate group (c7). An excess of water and200 ppm zinc octanoate were added to the system and it was allowed to stand for24 hours. The —OCN resonance is reduced and a new signal appears at 156 ppm(Figure 2.6B), which is consistent with the behavior of the dicyanate andassigned to p-tert-butylphenyl carbamate 28 in the present instance [Equation2.6].[2.6]On heating the sample at 100 °C for 1 hour (Figure 2.6C), the intensity of thissignal increases and small peaks appear at ö = 174 ppm (triazine, t7) and 171ppm (unassigned). After prolonged heating at 100 °C, the triazine resonance isthe major component in the spectrum. Further information on the nature of theother reaction products comes from the 15N spectra (see on).2.3.2. 15N NMR Investigation of the Curing Reaction of PTBPCN 25b inSolution An investigation of 15N spectra of PTBPCN in MEK and acetone-d6 gaveresults in agreement with the 13C data of the previous section. The experimentswere carried out with the 15N enriched PTBPCN 25b whose spectrum was62Figure 2.6. 13C solution NMR spectra (1H at 200 MHz) of the 13Cenriched PTBPCN 25a in MEK and acetone-d6 with 200 ppmzinc octanoate added as a catalyst. (A). Before the addition ofwater; (B). After addition of excess water and standing atroom temperature for 24 hours; (C). After heating at 100 °Cfor 1 hour.63c6 c3 c2c7OC*Na'2 a'lCD3C0CD3c7Am2 ml m3 m4CH3COCH2CH3m4m2c6m3c3m 1 c2I,' a'2c5cl c4c7 Ba7—o-C-NH2IIoa7I Aic7 Ca7t7II H, tlI i200^160^120^80^40^0 PPM'presented previously in Figure 2.3. Figure 2.7A shows that there is no reactionin MEK and acetone without catalyst on heating at 100 °C for up to five hours,even in the presence of added water. The corresponding 13C spectra which arenot shown give the same result. It would appear that trace amounts of catalystare needed at least at this temperature to induce the reaction. Just as expected,reactions were observed after adding 200 ppm zinc octanoate as catalyst andheating the sample at 100 °C for only one hour (Figure 2.7B). These reactionscause the appearance of two main resonances in the 15N spectrum, one at 87ppm due to the formation of triazine 31 and the second one at —41 ppm which issplit into a triplet due to coupling to two protons. This is consistent withformation of 28 by the addition of water to the cyanate group as described in theprevious section. Two small singlet resonances are also observed due to verysmall amounts of other side products.2.4. Synthesis and Characterization of the Carbamates 28 and 29In order to confirm that the species with 8 = — 156 ppm in the 13C NMRspectrum and 8 = — —40 ppm in the 15N NMR spectrum is indeed a carbamate,both carbamates, 28 and 29, were synthesized by reaction of the appropriatecyanates (BPADCN and PTBPCN) with water using zinc octanoate or an acid[9]as a catalyst and were purified by recrystallization in acetone. Details of theprocedures are given in the Experimental chapter later. The 13C and 15N NMRspectra of p-tert-butylphenyl carbamate 28 show resonances at 8 = 156 ppm forthe carbonyl carbon (Figure 2.8A) and 8 = —41 ppm for the nitrogen which iscoupled to two hydrogens giving a triplet resonance (Figure 2.8B). These spectraas well as the spectra of the carbamate 29 (not shown) are consistent with theprevious results, and indicate that the carbamate is indeed formed in the curingprocess of the cyanate resin.64I80 0 -40 1TM'Figure 2.7. 15N solution NMR spectra (1H at 300 MHz) of 15N enrichedPTBPCN 25b in MEK and acetone-d6 without 1Hdecoupling. (A). After heating at 100 °C for 5 hours withoutcatalyst; (B). After heating at 100 °C for 1 hour with 200 ppmzinc octanoate as catalyst.65C6ACS C4^CIC6 C3 C2C7N H20C3C2CI C4C7140^100^60Err r-r7-r- If-20^-30^-40^-50^PpmFigure 2.8. (A). 13C NMR spectrum (1H at 200 MHz) of p-tert-butylphenyl carbamate 28 in acetone-d6; (B). 15N NMRspectrum (1H at 300 MHz) without 1H decoupling of 15 %15N enriched 28 in acetone-d6.CS66A00NH 2o3 H2N0,002930H'N^N-HOH + 2 ONOH303 HO [2.7]To monitor the possible involvement of the carbamate in further curingprocesses, the isolated carbamate 29 was heated in acetone and MEK at 120 °Cin a sealed tube. After heating for 24 hours, a white precipitate appeared, whichwas not soluble in many solvents, such as ketone, chloroform, ethyl acetate,toluene, ether etc., and only dissolved sparingly in dimethyl sulfoxide (DMSO).This white precipitate was separated and identified as isocyanuric acidC31{3N303, 30. It shows signals at .5 = 151 ppm in its 13C NMR spectrum andat ö = 22 ppm in its 15N NMR spectrum. Its MS spectrum shows a parent ion atM+ = 129 a.m.u. as expected.Figure 2.9A shows the 13C solution NMR spectrum with assignments forthe sample of carbamate 29 dissolved in DMSO-d6. Figure 2.9B gives the 13CNMR spectrum for the same sample after heating for 28 hours at 120 °C. Thespectrum shows that the carbamate decomposed to yield two compounds,bisphenol A (3) and isocyanuric acid (30), as shown in Equation 2.7. It should bementioned that the decomposition is favoured at high temperature.2.5. Isolation and Characterization of the Triazine (31) obtained fromp-tert-Butylphenyl CyanateBecause of the critical importance of triazine ring formation to thecyanate curing process, it was decided to isolate the anticipated triazine product67Figure 2.9. 13C solution NMR spectra (1H at 200 MHz) of the carbamate29 in DMSO-d6. (A). Before heating; (B). After heating at120 °C for 28 hours.68from the reaction of p-tert-butylphenyl cyanate and thoroughly characterize it toprove that it did indeed have the expected structure 31, 1,3,5-p-tert-butylphenoxy-2,4,6-triazine.0 N 0YflyNN031As indicated in the Experimental chapter, the crystals obtained weresuitable for a single crystal X-ray structure determination. Both NMR (seeFigure 2.10) and single crystal X-ray diffraction experiments (see Figure 2.11)were carried out on this material. All of the spectral characteristics were inagreement both with the postulated structure 31 and with the 13C and 15Nspectra recorded during the curing reaction in solution.The crystallographic data, the atomic coordinates, bond lengths andangles are given in the Appendix A. A perspective view of the molecularstructure is shown in Figure 2.11. The triazine ring is clearly visible in thecentre of the molecule and appears undistorted. The symmetrical 1,3,5-substitution of the triazine ring appears to minimize intramolecular stericinteractions between the substituent groups. The NMR spectra (Figure 2.10) oftriazine 31 show chemical shifts 5 = 174 ppm for the carbons on the triazinerings and 5 = 87 ppm for the nitrogens on the triazine rings as expected.Therefore, the equivalence of the crystalline triazine product and the majorreaction product from the curing reactions in solution, which has characteristic69t2 t3 t6t2^a'2 a' 1CD3 COCD3t3ti t4t7a' 1Aa'2T90 80 710 60,^tPPMFigure 2.10. (A). 13C NMR spectrum (1H at 300 MHz) in acetone-d6 ofnatural abundance triazine 31 formed from PTBPCN 25; (B).15N NMR spectrum (I-H at 300 MHz) in acetone-d6 of 15Nenriched triazine formed from PTBPCN 25b.70C28^ C27CZ 7AC24Figure 2.11. Perspective view of the triazine molecule 31 formed fromPTBPCN 25. 50% probability thermal ellipsoids are shownfor the non-hydrogen atoms.7113C and 15N chemical shifts of 8 = 174 ppm and 8 = 87 ppm respectively, isclearly established. The postulation that the curing reactions of bothmonocyanate and dicyanate monomers in solution lead to analogous triazineproducts is verified.2.6. The Mechanism of the Curing Reaction for Cyanate Resin inSolutionThe results of the investigations in previous sections therefore suggestthat the main curing reaction for the cyanate resin in solution is to form triazinerings 33. There is no evidence for the formation of any long lived dimericintermediates during the course of triazine formation. However, a side reactiondoes occur due to trace water present in the solvent used (MEK in the presentinstance). The reaction of the cyanate and water in the presence of catalyst andunder heating up to 100 °C for a relatively short time gives a carbamate sideproduct 27. With prolonged heating at high temperature, the carbamate 27tends to decompose to the phenol 34 and the isocyanic acid (35) which is notstable and is immediately converted to its trimeric form, isocyanuric acid (30).According to the literature[9], the phenol can react with unreacted cyanate toform an imidocarbonate 36. However, this occurs only at low temperature. Athigh temperature, the reverse reaction is preferable. The synthesis andcharacterization of the imidocarbonate will be discussed in the next chapter.Therefore, the general reaction scheme for the curing of cyanate resin in solutioncan be given as in Scheme 2.1.Side reactions which involve the reaction of the cyanate group with waterimpurities in the MEK solvent could complicate the curing process in solutionand make the final resin susceptible to attack by water. They also weaken thestrength of the final resin because they decrease the number of cross-linking7232vs.A,^0-0-0-NH202734[0=C=N-H]350H^H30O'N'OOCNH .T .sites by the loss of cyanate groups. Thus, bulk curing would be preferable. In thefollowing sections this is discussed in detail.Scheme 2.12.7.^Investigations of the Curing Process in the Solid StateIn order to probe the curing reactions in the solid state, 13C and 15N solid-state NMR investigations[94] were carried out on the 13C and 15N enrichedBPADCN materials previously described. The chemical shift informationobtained from the solution NMR experiments described above were used asreference data for structural assignments. The characteristic chemical shifts ofthe different functionalities determined in these studies are summarized inTable 2.1.73Table 2.1. Characteristic 13C and 15N Chemical Shift Values of theFunctional Groups Derived from the Cyanate Group inSolution Curing Reactions.Functionalities 13cppm from TMS15Nppm from neatformamide109 53OCN0^w 0\^N-402-0,pN.-( 0 izz174 87• 0-C_NH26 156 —400H^,4^HN^*N-0^N . 0H151 22w+0-0, *^*(o)-- o - 0 = N H159 432.7.1. 13C Solid State NMR Investigations (1) BPADCN MonomerFigure 2.12A shows the 13C CP MAS[66] spectrum of natural abundanceBPADCN monomer 2 at 25 MHz 0-H frequency 100 MHz) together with theassignment in terms of the molecular structure. The NQS[951 (non-Quaternarysuppression) experiment on the same sample is shown in Figure 2.12B. TheNQS technique only detects the carbons with no attached protons. Exceptions74are methyl carbon resonances which are only partly eliminated due to theirreduced dipolar interaction caused by the methyl group rotational motion.[95]The carbon in the —OCN group gives rise to three resonances due to residualdipolar coupling to the directly bonded 14N (I = 1) quadrupolar nucleus (theseare reduced but not eliminated by MAS).[961 Figure 2.12C is the correspondingdipolar-dephased spectrum of the 13C enriched BPADCN monomer 2a andshows only these three signals as anticipated. Figure 2.12D shows the CP/MASspectrum, with sidebands removed by the TOSS (total suppression of spinningsidebands) pulse sequence,[971 of the 13C enriched monomer obtained at 100MHz (400 MHz for 1H). The spectrum is simplified as the three resonances havebecome almost degenerate because the dipolar coupling is independent of themagnetic field. The residual dipolar coupling is thus relatively reduced at thehigher magnetic field while the quadrupolar coupling is reduced. Further 13Cspectra of this system were therefore obtained mainly at high field (1H at 400MHz).(2) Cured Resin from Solution PolymerizationFigures 2.13A and B show the 13C CP MAS TOSS spectra of the solidobtained by evaporation of the solvent after the solution polymerizationdescribed previously. There is an intense signal at approximately 174 ppmassigned to the triazine ring carbons. The second broad signal at approximately155 ppm is a composite signal corresponding roughly to the solution signalassigned to the carbamate compound 27 and its decomposed derivativeisocyanuric acid, 30. It should be noted that these spectra are not quantitativeand that the signals due to species such as carbamate 27 and isocyanuric acid30 would be greatly enhanced by the cross-polarization process compared to thetriazine signal as there are two protons only two bonds away from the 13Cenriched carbons in their molecular structures.75Figure 2.12. (A). Solid state 13C CP/MAS NMR spectrum (3-H at 100 MHz)of the natural abundance BPADCN 2; (B). NQS spectrum0-H at 100 MHz) of 2; (C). NQS spectrum 0-H at 100 MHz) ofthe 13C enriched BPADCN 2a; and (D) 13C CP/MAS/TOSSspectrum 0-H at 400 MHz) of 2a.76 c6 c3 c2Acl & c4cl & c4NC.c6 c3 c2• *c5 c4.c7c OCNc 1 & c4c7—0C.NL•••••••■•■••••150^100^50^ppmFigure 2.13. 13C solid state CP/MAS/TOSS NMR spectra (1-H at 400 MHz)of (A). The solid sample obtained by evaporation of thesolvent after curing the 13C enriched BPADCN 2a in MEKand acetone-d6 with 200 ppm zinc octanoate as a catalyst;(B). The same sample as in (A), NQS experiment; and (C).The solid sample obtained from bulk curing the 13C enrichedBPADCN 2a at 250 °C for 15 minutes.77a7t7 A0H ,J,^H'N * II 't7 zz......„..... 0 t7*)--N *N a-0),--,-NC,-t0^ILA_150^100^50^Ppm(3) Cured Resin from Solid State CuringFigure 2.13C shows the 13C solid-state spectrum of the product from thebulk curing (no catalyst) of the 13C labelled BPADCN monomer 2a sample for 15minutes at 250 °C. The spectrum is remarkably clean, showing a single majorresonance at 174 ppm with no indication of substantial amounts of unreactedmonomer or carbamate side product 27. This would appear to be a very viablepolymerization process, and remarkably efficient. It further confirms that thecarbamate side reaction in solution is caused by water impurities in the ketonicsolvents used.2.7.2. 15N Solid State NMR Investigations (1) BPADCN MonomerFigure 2.14A shows the high-resolution solid state 15N NMR spectrum ofthe labeled BPADCN monomer 2b at 40.6 MHz (1H at 400 MHz; reference: neatformamide). There are two sharp resonances, indicating that either the sitesymmetry is lower than the symmetry of the isolated gas phase molecule or thatthere are two nonequivalent molecules in one unit cell. This is also reflected inthe two methyl resonances in the solid state 13C spectrum of BPADCNmonomer, Figure 2.12A. The small peaks denoted ss are spinning sidebands dueto incomplete sideband suppression.(2) Cured Resin from Solution PolymerizationFigures 2.14B — D show 15N spectra as a function of the contact time usedin 111/15N cross-polarization experiments for the solid sample, which wasobtained from the solution polymerization by evaporation of the solvent. Theresonance at lower field can be assigned to the triazine ring nitrogens, but thereis substantial intensity at higher field due to other species, carbamate 27 andisocyanuric acid, 30. However, as before, these will be greatly enhanced by the78cross-polarization process, because there are directly attached protons coupled tothe 15N nucleus in the two cases. This dipolar coupling interaction depends on1/r3 ( where r is the distance between 1H and 15N nuclei), and this will have anextreme effect on the cross-polarization process. The relative intensities inspectrum 2.14B bear little relation to the actual concentrations of the differentspecies. At longer contact times, this effect is less important and Figure 2.14Cbetter reflects the concentrations of the different species. Figure 2.14D whichonly shows the signals from 15N nuclei without attached protons confirms thatthe higher field components all have directly bonded protons. Only one peak isobserved here, corresponding to the triazine ring nitrogens.(3) Cured Resin from Solid State CuringFigure 2.15A shows the high-resolution solid-state 15N spectrum of theproduct obtained from a bulk curing of the 15N labelled BPADCN monomer 2bunder identical conditions to those used for the 13C labeled BPADCN monomer2a previously discussed (bulk curing at 250 °C for 16 minutes). The spectrum ismuch cleaner than those in Figure 2.14 obtained from the solution curing. Also,as before, the "non-triazine" signals are greatly enhanced. Figure 2.15B, whichshows only non-proton bearing nitrogens, indicates as before that the otherspecies have directly attached protons.2.7.3. Quantitative Investigation of Solid State BPADCN CuringAccording to the previous results, the curing of BPADCN appears to be avery efficient process, especially when carried out on the neat material whereside reactions with water are minimized. An attempt was made to quantify thecuring reaction and also to relate the 13C and 15N spectra directly to each otherby investigating a mixed BPADCN resin made up of 50%13C enriched monomer79Figure 2.14. 15N solid state CP/MAS/TOSS NMR spectra (1H at 400 MHz)of (A). 15N enriched BPADCN 2b; (B — D). The solid sampleobtained by evaporation of the solvent after curing the 15Nenriched BPADCN 2b in MEK and acetone-d6 with 200 ppmzinc octanoate as a catalyst, with contact times: (B). 1 ms;(C). 5 ms; (D). 1 ms with NQS pulse sequence.80B- 0- C-N*1-12II0CD100^0^-100^ppmA^I 1 00^0^-100^ppmFigure 2.15. Solid state 15N CP/MAS NMR spectra (1-H at 400 MHz) ofthe resin obtained after bulk curing the 15N enrichedBPADCN 2b at 250 °C for 15 minutes, with contact time 1ms. (A). with TOSS sequence; (B). with TOSS/NQS sequence.2a and 50% 15N enriched monomer 2b. This mixed resin was cured for 15minutes at 250 °C.Both 13C and 15N CP/MAS spectra were obtained in a high magnetic field(1H, 400 MHz). The disadvantage of a high magnetic field is that the largechemical shift anisotropies, which increase with the strength of the magneticfield, give rise to large numbers of spinning sidebands. In the more qualitativestudies presented in the previous sections, these were eliminated using the81TOSS pulse sequence. However, for quantitative data it is important that theintensities of the spinning sidebands are taken into account. From a knowledgeof where other resonances could occur, a spinning rate was chosen to avoidoverlap of spinning sidebands and isotropic resonances. As noted previously,because the side products contain protons in close proximity to the labellednucleus under observation, the spectra are very sensitive to the contact timeused for cross polarization. A complete variation of contact times was thereforecarried out in the experiments on both nuclei.The series of spectra shown in Figure 2.16 were obtained by the variationof contact time in the 13C CP/MAS NMR experiments for the labeled and curedBPADCN resin mixture. Only at very short contact times is there any indicationof resonances due to species other than triazine. The contributions are small andit was very difficult to try to estimate them due to contributions from naturalabundance resonances. Quantitation will be more reliable from 15N data. Themaximum intensity of the 13C spectra occurs at about 5 ms of contact time, afterwhich all of the signals decay due to the proton T1 p relaxation process. Aspectrum obtained at this contact time using the TOSS sequence (Figure 2.17A)indicates that there are negligible contributions to the spectrum from the speciesother than triazine.Figures 2.17B, and 2.18 show an analogous series of 15N CP/MAS NMRspectra obtained from the same labelled sample. In the case of 15N spectra, theside products have protons directly attached to the 15N nucleus, greatlyenhancing their contribution to the spectra. However, there is no naturalabundance contribution to the spectrum. From an analysis of these data, thetotal contribution of species other than triazine to the spectrum is concluded tobe less than five percent. Thus, the very high efficiency of the solid state curingreaction is verified.82300.^.^JPpm.^1^.200.^1^.100CT(ms)20.015.010.05.02.01.00.50.20.1Figure 2.16. Series of 13C CP/MAS NMR spectra (1H at 400 MHz) withvariation of the contact time (CT) without sidebandsuppression. The sample resin was obtained by bulk curing amixture of 50% 13C enriched and 50% 15N enrichedBPADCN monomers for 15 minutes at 250 °C.83A i .^.^t^.^.150.^.^1^.50.^1Ppm100Figure 2.17. Solid state CP/MAS/TOSS NMR spectra (1H at 400 MHz) ofthe same resin sample as Figure 2.16 with CT = 5 ms. (A).13C spectrum; (B). 15N spectrum.84300^200^100 0^-100 -200 Ppm15.010.05.01^.^1^•^1^.0.20.1Figure 2.18. Series of 15N CP/MAS NMR spectra (1H at 400 MHz) withcontact time variation but without sideband suppression.The resin sample was the same as Figure 2.16.85The solid-state NMR spectra thus indicate that the cured resin solid iscomposed mainly of triazine ring linkages, whether the curing reaction is carriedout "neat" or in a solution. A catalyst is not necessary for the curing reaction ifthe temperature is high enough, and side-products come mainly from reactionwith water present in the solvent. The reaction is very efficient, especially whenthe neat monomer is cured, more than 95% of the cyanate functionalities beingconverted to triazine rings in these experiments.2.8. The Relation of the Crystal and Molecular Structure of BPADCNto Its Curing EfficiencyFrom the results above, the cyanate to triazine conversion is confirmed asthe basic curing reaction. It is demonstrated that triazine ring formationproceeds without the accumulation of substantial amounts of intermediatedimeric species, and that the small amounts of side products come from thereaction of the cyanate group with water present in the solvent. In the case ofthermal curing of the neat resin, the reaction is found to be remarkably efficient,over 95% of the cyanate groups present being converted to triazine rings. Inmany ways, this high efficiency of the curing reaction of neat BPADCN issurprising because the Bisphenol-A moiety is quite rigid and tends to form avery rigid cross-linked framework, which one would intuitively feel it would bedifficult to cure the resin near completion.To try to better understand the factors involved in the curing process,crystals of BPADCN suitable for single crystal X-ray diffraction studies wereobtained by recrystallization from cyclohexane solution, and the crystal andmolecular structures were determined. The crystallographic data are given inTable V in the Appendix B. The final positional parameters from the refinementare presented in Tables VI, VII, and VIII in the Appendix B with the numbering86of the atoms in the molecular structure shown in Figure 2.19. In the crystal, thetwo pseudo planes of symmetry through Cl, which exist in solution due torotation about the C1—C4, Cl—C10, C13-02 and C7-01 bonds, are removed. Allof the atoms in the molecule are now unique and in principle, all should giveseparate signals in the high-resolution solid state NMR experiments, althoughin practice not all differences are large enough to be resolved. This lack ofmolecular symmetry in the crystal explains the two resonances for the twomethyl groups in the solid-state 13C NMR spectra (Figure 2.12) and the twonitrogen resonances for the two cyanate groups (—OCN) in the 15N spectra(Figure 2.14A) as described above.More interesting in the context of the curing reactions of this monomer isthe crystal structure depicted in Figures 2.20 and 2.21. Figure 2.20 shows thearrangement of the molecules in the unit cell and indicates clearly thatinteractions between cyanate groups on different molecules are the dominantfactors controlling the formation of the lattice structure. The interactions allinvolve four cyanate groups on different molecules. Two of these molecules forma four-membered ring by strong C17---N2 interactions and the remaining twocyanate groups each interact via an N1---02 interaction with one of the twooxygens in this four-membered ring. These intermolecular contacts are given inTable 2.2. Thus, in the solid-state, the functional groups needed for the curingreaction represented in Scheme 1.1 are all in very close proximity and stronglyinteracting. Figure 2.21 illustrates how these inter-cyanate group interactionsform a complete three-dimensional structure, the lattice being made up ofparallel "strings" of cyanate-bonded molecules.87Figure 2.19. Molecular structure of BPADCN monomer 2 from the singlecrystal X-ray diffraction study showing the numbering of theatoms (Tables in the Appendix B).88C11C13^)Cl2rit'iy cioC15C1402C17C14^02Cl5r1 C13C10.cf3C12 N1C11^C1601C8C9Figure 2.20. Perspective view of part of the unit cell contents from thecrystal structure of BPADCN monomer 2 showing theintercyanate interactions. (see Table 2.2).N189Figure 2.21. A plane through the three-dimensional network formed bythe intermolecular intercyanate interaction. The BPADCNmolecules are cyanate-connected to form parallel "strings"throughout the structure.90Table 2.2. Intermolecular Distances between CyanateGrou s in the BPADCN CrystalAtom Atom* Distance (A)0(2) N(1)' 3.318(4)N(2) C(17)" 3.484(4)3.546(6)N(2) N(2)"* The symbols 'and "refer to different moleculesAlthough melting the sample will destroy the perfection of this orderingpattern, it is expected that strong intercyanate group interactions will still occurand substantial local ordering may persist in the melt. These results form aguide for modelling the curing process, and the high efficiency of the neat curingreaction becomes more understandable. The reduced curing efficiency in solutioncan be explained as due to the presence of solvents with strongly polar groupswhere competitive interactions with the cyanate groups can occur. When otherdicyanates or derivatives are considered as potential resin systems in the future,it would be worthwhile to carry out a single crystal X-ray structuralinvestigation to check whether the changes in molecular structure havedestroyed the very strong intercyanate interactions which are present in thecase of BPADCN.2.9. ConclusionsThe mechanism of the curing reactions of cyanate polymer resins based onbisphenol A dicyanate (BPADCN) has been investigated both in solution and inthe solid state by NMR spectroscopic techniques. To increase the signal to noiseratio (S/N) and to unambiguously characterize the reactions of the cyanate91functional groups, 13C and 15N enriched cyanate resins and monocyanatemodel compounds were used, the latter yielding soluble and isolable analogs. Insolution, the main reaction is formation of triazine rings as identified by NMRand MS techniques and characterized by single crystal X-ray diffraction on anisolated crystalline material from the monocyanate model compound. Sideproducts are formed by the reaction of the cyanate functionalities with tracewater present in the ketonic solvent, but there is no NMR evidence for theformation of dimeric or other intermediate species prior to triazine ringformation.The resins from the solution curing and also those formed directly bycuring of the neat resin were characterized by high resolution solid-state NMR.In the former case triazine ring formation and the presence of side productswere confirmed by both 13C and 15N solid-state NMR. In the case of curing theneat resin, the reaction is very clean and very efficient. It is shown to be almostquantitative. The efficiency of this process is rationalized in terms of the verystrong intermolecular intercyanate bonding interactions which are observed inthe crystal structure of the BPADCN monomer obtained from a single crystal X-ray diffraction experiment.92CHAPTER 3.INVESTIGATIONS OF THE POSSIBLE CROSSREACTIONS BETWEEN CYANATE AND EPDXY RESINSA detailed investigation of the mechanism of the curing reaction of thecyanate resin both in solution and in the bulk was reported in the previouschapter.Cyanate resins have many excellent properties as mentioned previously.They can be mixed with many kinds of thermosetting resins to form compatibleformulation blends. In commercial applications, they are usually modified withepoxy resins giving complete curing at temperatures as low as 177 °C.E3°1However, the nature and even the existence of any cross-linking reactionsbetween the cyanate and epoxy functional groups are not yet clear. Five-membered oxazole 5 and isooxazole 6 rings have been proposed as cross-reactionproducts[32,35,361, but there is no general evidence or incisive spectroscopicinformation (such as from NMR, mass spectrometry, or X-ray diffractiontechniques) to confirm or refute these proposed structures.In the present chapter, an investigation of possible cross-curing reactionsbetween cyanate and epoxy resins is reported. In order to obtain soluble andisolable cross-reaction products, the monofunctional cyanate, p-tert-butylphenylcyanate (PTBPCN, 25), and the monofunctional epoxides, p-tert-butylphenylglycidyl ether (PTBPGE, 37) and ortho-methylphenyl glycidyl ether (OMPGE,38), were used as model compounds for the two resins to prevent formation of across-linked network. The reaction products were separated by absorptionchromatography (silica gel column) and gel permeation chromatography(Sephadex LH-20 column), and fully characterized by NMR and massspectrometry. The results demonstrate that the major cross-reaction product93OCH2 C H — CH2\o/37OCH2CH\ ---/CH20CH3380 C N25contains a five-member oxazolidinone ring and is composed of one cyanate andtwo epoxy monomers. It is not an oxazole ring structure as proposed in theliterature132,35,361. The reaction between the epoxy and the carbamate which isthe side product produced during the cyanate resin curing process discussed inChapter 2 was also investigated. Several related cross-reaction products havealso been identified. Finally, the mechanism of the curing reaction for thecyanate/epoxy mixed resins will be discussed.3.1. Solid State NMR Investigation of the Neat Curing Reaction of theMixed Dicyanate / Epoxy ResinsA neat mixture of the two resins [the mixed 13C and 15N enrichedBPADCN resin (50% 2a and 50% 2b) and the natural abundance epoxy resin(EPON-825, 18, n=0.2), 1:1 molar ratio] was cured at 180 °C for 2.5 hours in air.Solid-state 13C and 15N spectra were obtained both on the crude products andalso on the materials after exhaustive extraction (3 days) with MEK solvent. Inaddition, a sample cured under nitrogen protection which prevents the resinfrom reacting with moisture or oxygen in the air was also prepared. It gave thesame results as the sample cured in air.0^ OH,^\ 1CH2-CHCH 0^ OCH2CHCH 0o/ \OCH2CH CH2189413C NMR Spectra The most intense resonance is due to triazine ring carbons (-173 ppm).The next largest peak is found at -155 ppm which is consistent with the0upresence of carbamate 0 G-N— functional groups. Unreacted epoxy groupsare present and their intensities are reduced somewhat after MEK extractionsince unreacted EPON-825 dissolves in the MEK (spectra are not shown).15N NMR SpectraThese spectra are particularly important in this investigation becauseonly the cyanate functional group and its derivatives give signals. As can beseen in Figure 3.1A, the spectrum shows several signals. The major signal at 87ppm can be assigned to triazine. The cyanate signal at 52 ppm is very small,which means that most of the cyanate groups have reacted. Three other smallsignals appear at the higher field side of the cyanate. Two of them (at -42 ppm—o,C=NHand -23 ppm) can be assigned to be imidocarbonate^o^andisocyanurate groups. The imidocarbonate species can be considered as an adductof the cyanate and a hydroxyl group on the epoxy resin. Hydroxyl groups exist inthe cured or partially cured epoxy resin as shown in Equations 1.7 - 1.9 andstructure 18. Even EPON-825, which is a reasonably pure monomeric form ofthe epoxy resin, does contain a small number of hydroxyl groups (see structure18). In addition, as will be shown later, some phenol moieties can be produced inthe curing process itself.Besides these signals, there is also a very intense resonance at - -36 ppm.0From previous work, this could be assigned to carbamate -0-C-NH2 moieties.0However, the carbamate species, — 0-C-NH2, were not observed at all in theneat curing of BPADCN resin reported in the previous chapter due to the95absence of water, although in the current case a small number of them might beexpected from any entrained water in the epoxy resin. However, this signalgrows relatively slowly as a function of contact time (Figure 3.2) whereas the0^I ^Initrogen in the — 0- C - NH2 functional group should cross-polarize veryefficiently due to having two directly bonded protons. Most importantly, in thenon-proton-attached nitrogen selection experiment (Figure 3.1B), substantialspectral intensity remains. Thus it would appear that this resonance is due to adifferent nitrogen species in which the nitrogen has a similar local chemical0^environment as in the^NE12 functional group, but has no directly0 ,attached protons, i.e.^. It could thus be due to products from a crossreaction between the dicyanate and the epoxy resins. Based on theseobservations it was assumed that some cross reactions occurred, giving rise tothe —36 ppm signal. Thus, attempts were made to investigate it further usingmonocyanate and monoepoxy analogues to obtain soluble and isolable productswhich could be more easily characterized.3.2. Neat Curing Reactions of Monofunctional Cyanate and EpoxyCompoundsAn investigation of the curing reactions of a mixture of the twomonofunctional compounds was carried out using unlabeled materials first toascertain whether any cross reaction occurred between them.3.2.1. Neat Curing Reactions of Unlabeled PTBPCN 25 and PTBPGE 37In the 13C spectrum (not shown) of the unreacted mixture of the tworeactants PTBPCN 25 and PTBPGE 37, the resonance at 109 ppm due to thecyanate group and the two resonances at 44 and 55 ppm due to the two carbons96zi •4L'1",....."•—••","."A"."."""r's.."0-•,0>.-%-. NN** yz...)._ •N^11^\A^o^ii^II^o i/ 00■ 0/ iII-0- C-N*/1 11,11■1/11111TISIVIT100r •^•01^1^,-100 ppmFigure 3.1. 15N solid-state NMR spectra (1H at 400 MHz) of the resinobtained by curing EPON-825 and BPADCN (50% 13Cenriched 2a and 50% 15N enriched 2b) at 180 °C for 2.5hours. (A). CP/MAS/TOSS spectrum; (B). CP/MAS/TOSSspectrum combined with the NQS technique.97ICA CT (ms)5.04.03.02.01.51.00.50.20.10.05200 100 -100 ppmFigure 3.2. Series of 15N solid-state CP/MAS/TOSS NMR spectra (1Hat 400 MHz) of the same sample as Figure 3.1 withvariation of the contact time as indicated. The triazineresonance is indicated by T and the imidocarbonate by I.98(Figure 3.2 continued)CT (ms)30.025.020.017.5.^. .^. -1 00 ppmioo^I^. .^I15.012.510.09.08.07.06.098-1in the epoxy ring are particularly important, as reactions of the two functionalgroups should be reflected by changes in these resonances.After heating the mixture of the two monofunctional compounds, 25 and37 (in an approximately 1:1 molar ratio), for 12 hours at 100 °C, there is nochange in the epoxy resonances while some of the cyanates (resonance at 109ppm) have been converted to the triazine (resonance at 174 ppm) as would beanticipated from the results of Chapter 2. Heating the sample further for twomore days at a higher temperature (125 °C) induces further reaction and theformation of a solid precipitate. The solid material was separated from the crudereaction mixture and identified as the triazine compound 31 by NMR. Theunchanged epoxy carbon resonances indicates that the epoxy compound remainsunreacted. Therefore, it is concluded that under these conditions, the epoxygroups do not react either with each other or with the cyanate groups and theonly reaction which takes place is triazine formation by the cyanate groups. Inorder to observe the cross reaction between two resins, the temperature has to beraised even higher.The two monofunctional compounds, 25 and 37, were reacted neat in anapproximate 1:1 molar ratio at 180 °C for 7 hours. The crude reaction mixturewas dissolved in acetone-d6 and the 13C NMR spectrum obtained is shown inFigure 3.3A. The most important features of the spectrum are that the cyanategroup in PTBPCN 25 has completely reacted and has been converted mainly totriazine and that the most of PTBPGE 37 remains unreacted as shown by thethree characteristic high field resonances in the aliphatic region. Thus, at leastunder the conditions of this reaction, the amount of cross reaction between thetwo compounds must be relatively limited. However, more careful inspection ofthe spectrum indicates that some of the epoxy groups have indeed reacted asindicated by the series of small resonances in the 45 — 80 ppm region which are99Figure 3.3. NMR spectra in acetone-d6 of the crude reaction mixtureobtained by heating PTBPCN (12% 15N enriched) andPTBPGE at 180 °C for 7 hours. (A). 13C NMR spectrum 0-Hat 200 MHz); (B). 15N spectrum (1H at 300 MHz) with NOEand no 1H decoupling.1000/ \- OCH2- CH- CH200I^I11()/ •=z r"......14•■••■•■1,Ano -OCNI I 10,^I^,80i^1^I^i401^I^I^I^I^I^I^I-40 IDIDni160^' 120 ppm . 80^40B^z^\I..------- II /0 —0—C—V/\ ^II^\00 0I^Icharacteristic of products from an epoxy ring opening reaction. However, it is notclear from the spectrum whether they are due to the cross reaction betweencyanate and epoxy or due to self polymerization of epoxy groups.Separation and purification of the reaction products from the cyanate/epoxy reaction was achieved using a combination of adsorption chromatographyand gel permeation chromatography. A general protocol was developed for theseparation of the reaction products, consisting of:1) Extraction with pentane in which the triazine product is only slightlysoluble and is thus largely removed from the reaction mixture.2) Adsorption chromatographic separation on a silica gel (230-400 mesh,BDH No9385-48) column (2.5 cm x 17 cm) using eluants with graduallyincreasing polarity from 100% pentane to 100% diethyl ether. Finally thecolumn was stripped clean using ethyl acetate.3) When necessary, gel permeation chromatography (GPC) was carriedout on a Lipophilic Sephadex LH-20 (Sigma) column (2.0 cm x 55 cm) withacetone as eluant.The reaction mixture was separated using the methods described abovewith most of the triazine product being removed by pentane extraction beforethe chromatographic separation. The approximate weight percentages of thecomponents after the silica gel column separation of the reaction mixture are:triazine -27%PTBPGE and minor unknowns -38%imidocarbonate and phenol^ -10%0ll-0-C-NH2 and minor unknownsmajor unidentified cross-reaction product fractionsecond unidentified reaction product fraction101others^ -5%The structure of the second unidentified product will be discussed in detail laterin a subsequent section.The major cross-reaction product fraction was further purified by GPC.El and CI mass spectra both show a parent ion peak at 587 a.m.u., which fitswith a formula of C37H49N05. Such a molecule could be derived from onecyanate and two epoxy monomers. In the HPLC experiment on a Waters 945spectrometer using 50% diethyl ether and 50% pentane as an eluant, the majorcross-reaction product gives two incompletely separated fractions. This suggeststhat it is probably a mixture of isomers with the same molecular mass and verysimilar structures.3.2.2. Neat Curing Reactions Using  15N Labeled PTBPCNIn order to better characterize the species in the product mixture, -12%15N labelled PTBPCN (25 and 25b) was reacted with PTBPGE 37 at 180 °C for7 hours as before. The use of 15N enrichment means that the much morediagnostic 15N spectra can be obtained with no natural background, whilelimiting the enrichment to a level of -12% means that a large enough quantityfor chromatographic separation could be processed.Figure 3.3B shows the 15N spectra of the crude reaction mixture obtainedwith NOE and without proton decoupling. There are clearly three major speciesin the mixture although there may be small amounts of others present whichmight be discriminated against by the experimental conditions or by their lackof directly bonded protons.The three major species, as indicated in the spectra, are:Triazine^ 87 ppmImidocarbonate^43 ppm102Unknown species^—36 ppmThere is no indication of carbamate resonance signals which would beexpected at —41 ppm in 15N NMR spectrum (Figure 3.3B). This is because thecarbamate functional groups react with PTBPGE 37 as will be discussed in moredetail later. However, they were found after the chromatographic separation(spectra are not shown) and could be formed by hydrolysis of —OCN orimidocarbonate groupings on the silica gel column during the separation. Itshould also be mentioned that the spectral contribution from the imidocarbonatecomponent is greatly enhanced by NOE in this spectrum. The signals of theunknown species at —36 ppm consist of two 15N resonances in the range ofcarbamates. The nitrogens have no directly attached protons, but have similarlocal environments to that of a carbamate (Figure 3.3B). They appear in thesame chemical shift range where the non-protonated resonance was observed inthe solid-state 15N spectra (Figure 3.1) from the mixed BPADCN / EPDXY resincuring. It is considered that these resonances are due to the same species in bothpreparations.Chromatographic separation of the mixture was first carried out on asilica gel column. After further purification of the cross-reaction product by GPC,a pure sample was obtained. The El mass spectrum of the purified major cross-reaction product is in complete agreement with the previous mass spectral data.Most importantly, the parent ion peak mass of 587 a.m.u. confirms thecompound is formed by the combination of one cyanate and two epoxy molecules.The composition of the major cross-reaction product was also confirmed bythe synthesis, isolation and purification of the corresponding reaction productfrom PTBPCN 25 (FM = 175) and ortho-methylphenyl glycidyl ether (OMPGE,38) (FM = 164). The El mass spectrum of the major cross-reaction product fromthis reaction shows the parent ion mass is now 503, again in exact agreement103with the proposed composition being one cyanate and two epoxy monomer units.Both of these compounds were subsequently investigated to unambiguouslydetermine their structures.3.3. Further Characterization of the Major Cross-Reaction Product3.3.1. 1D and 2D NMR spectraFigures 3.4A, B and C show, respectively, the 15N, 1-H and 13C spectra ofthe major cross-reaction product from the reaction of PTBPCN 25 and PTBPGE37 dissolved in acetone-d6. The 15N spectrum (Figure 3.4A) indicates that twosimilar nitrogen environments are present, both of which are non-protonated. Inaddition, since the MS indicates that there is only one cyanate moiety present,the molecule must exist in two, very similar isomeric forms. The 1H spectrum(Figure 3.4B) gives the expected ratios of methyl, aliphatic and aromatic protonsfor the proposed composition, but the numbers and relative intensities of themultiplets in the aliphatic region (see Figure 3.5A) again suggest the presence ofisomers. This is confirmed by the 13C NMR spectrum (Figure 3.4C) whereeleven signals (probably twelve signals with two degenerate) are observed in thealiphatic region. Since the MS shows that only two epoxy groups are present,this again infers the presence of two isomers. The 1H-coupled 13C spectrum (notshown) shows that the aliphatic carbons in the sample are present as four0N-CH2-6H-CH2- 0 groupings as indicated.From this information, the 1H 2D COSY NMR spectrum (Figure 3.5A)and the 1H/13C heteronuclear shift correlation 2D NMR experiment (Figure3.5B) make it possible to assign the 13C and 1H resonances in the aliphaticregion of the spectra as shown. These two experiments better define the system,104Figure 3.4. NMR spectra of the major cross-reaction product after bothsilica gel column and Sephadex LH-20 column separations ofthe same reaction mixture as Figure 3.3. (A). 15N NMRspectrum (1H at 300 MHz) with NOE and no 1H decoupling;(B). 1H (500 MHz) NMR spectrum; and (C). 13C NMRspectrum (1H at 500 MHz).105/—0—C— NII^\0-CH3aromatic protonsaliphatic protonsH20i^i^I7 5aromatic carbons1(-----A---Th 'CH3C^"amem.W.N.■•••■■■^i 1i1401.r^I100 60 PPMepoxy ring has openedill.■••■■/"..•■•■•■••,ti^14.4 4.04.8 3.6 PmAFigure 3.5. (A). 1H (400 MHz) 2D COSY NMR spectrum and (B). 1H/13Cchemical shift correlated 2D NMR spectrum (1H at 500 MHz)of the same sample as Figure 3.4.106106-1CH -/ x0-C-N^00H^\^,r_CH —39but as will be seen, they are not sufficient to make a definite assignment of thestructure.From the 1D and 2D NMR experiments, two possible structures (39 and40) which are in agreement with the general features of the NMR and MS dataare proposed as shown below.0-CH2Although the proposed structures 39 and 40 are quite different and itwould appear at first sight that they should be easily distinguishable from the1H and 13C NMR experiments, closer inspection reveals that both of them aremade up from combinations of the exact same groupings of local environments.For example, the nitrogen local environment is 41, that of the carbonyl carbon is42 and the moiety from the epoxy ring opening is 43 in both cases. Thus, bothstructures are in good general agreement with the observed resonances andchemical shift values of the 13C, 15N, and 1H NMR spectra and also showexactly the same local connectivities deduced from the 2D NMR experiments.107/^\-0 - C-N N-CH2 -CH -CH2-0 -II^\ /0 0' 142^ 43Furthermore, there are opportunities in both structures for the formationof closely related stereoisomers which would explain the multiplicity of signalsobserved in the spectra. As indicated, both structures have two chiral centers.Compound 39 is expected to exist in two isomeric forms (cis and trans), and thus0CN-CH2 -H -CH2 -0yield two groupings (six 13C signals) and two 15N signals.However, the restricted rotation about the partially double bond of CO—N in 39could double the number of aliphatic carbon resonances without affecting the15N spectrum.0In the case of 40, the two N-CH2 - CH - CH2 -0 groupings are notequivalent in the structure, giving rise to six aliphatic carbon signals. Inaddition, the presence of two chiral centres as before will give rise to RR(SS) andRS(SR) isomers, doubling the number of carbon resonances to twelve, asobserved.As will be described, various other experiments have been carried out inan attempt to make an assignment of structure. Although no one experiment isdefinitive, the balance of evidences favors the second structure 40.3.3.2. Model Compounds for Structure 39In order to probe the influence of ring configuration and inversion on thechemical shift differences in 39, the model compound 2,6-dimethylmorpholine(DMMP, 44) shown below was studied.108CH3/ *(HN^0*(CH3The 1H spectrum of DMMP 44 gives the correct relative intensities for theprotons in different environments (spectrum not shown). The two stereoisomers(cis and trans) are present in different amounts and can be clearly identified.The 13C spectrum of DMMP 44 has much better separation of the differentsignals which can be assigned as indicated (Figure 3.6). However, it is clear thatonly six aliphatic carbon signals can be produced by the basic ring system of thereference compound 44 and the doubling of the number of environments wouldhave to be due to a restricted rotation about the CO—N bond as previouslyindicated.In order to test this latter possibility, a suitable model compound wassynthesized. The compound chosen was the derivative 46 formed by the reactionof compound 44 with phenyl chloroformate 45 as in equation [3.1].44CH3/ x HCI(0)^0 C CI + HN016 ^.-\ x45^44CH3/K^0- -N^0IF^\ *^ KCH346CH3[3 .1 ]The linkages of 46 are identical to those of compound 39. The referencecompound has a bulky phenyl substituent and contains the essential CO—N unitwhere a restricted rotation about the CO—N bond would occur even though thering substituents are different in two cases.109Figure 3.6. 13C NMR spectrum ( 1 H at 200 MHz) of 2,6-dimethylmorpholine 44 in acetone-d6.The effect of the restricted rotation about the CO—N bond can be seen inthe 13C spectrum at room temperature (20 °C) in Figure 3.7. The carbonresonances in the aliphatic region almost double in number and become tensignals (twelve signals with two degenerates). In particular, the CH2 carbon110signals in the two isomers are furthest apart because the CH2 carbons are closerto the CO—N bond than all of the others.3.3.3. Variable Temperature NMR Experiments To confirm that restricted rotation exists in the model compound 46,variable temperature 13C NMR experiments were carried out. The spectra attemperatures from 20 — 60 °C are shown in Figure 3.7. The line coalescence atelevated temperature is very obvious and occurs very easily (just slightly aboveroom temperature). Similar changes are observed in the proton NMR spectra(not shown).In order to probe possible effects from restricted rotation around the CO -Nbond in the major cross reaction product, a variable temperature 13C NMRstudy was also carried out on this material (Figure 3.8). However, in contrast tothe results from model compound 46, there is no clear evidence for linecoalescence at higher temperatures. There is some broadening of the lines aboveroom temperature which can be ascribed to temperature gradients within thesample. It is possible that the energy barrier for rotation could be too large to beaveraged over the accessible temperature range but this is considered unlikelydue to the similarity of the N substituent groups. Based on the results on themodel compound 46, if structure 39 were the correct structure of the cross-reaction product, averaging should occur just above room temperature in thecorresponding 1H NMR spectra due to the smaller frequency separations of theresonances. However, here as well no indication of averaging was observed,suggesting that restricted bond rotation is not present in the major cross-reaction product.Furthermore, unlike the major cross-reaction product, the two isomers ofcompound 46 (trans and cis) could be easily separated using a silica gel column,111Figure 3.7. Aliphatic regions of the 13C NMR spectra (1H at 300 MHz)of 2,6-dimethy1-4-phenoxycarbonylmorpholine 46 in acetone-d6 at the temperatures indicated.112CH3CH3''4Avr#4.*""%wefiL400.440302070^65 PPrn 50 15CHCH'0—C—N 0\^(0Cis^and^Trans'40 3020N-CH25N-C H2o^ T ( °C)60_ jt,_____,,Lili ^50CH'0 0CH r.)^rv—T—I—Fir I tilt75^701.--^f^I^I —150I I T-145 ppmFigure 3.8. Aliphatic regions of the 13C NMR spectra (1H at 300 MHz)of the cross-reaction product (Figure 3.4), at thetemperatures indicated.113because the physical properties of the two model isomers are more different thanthose of the isomers in the major cross-reaction product mixture. This againsuggests that the major cross-reaction product does not have a similar structureto compound 46. Therefore, structure 39 can be ruled out.3.3.4. NOE ExperimentsAlthough, as indicated previously, the local environments in structures 39and 40 are identical, there are some differences when longer range effects areconsidered. In particular, the structural unit 47, as shown below, is unique tostructure 40. In the proton NMR spectrum, it is possible to identify theresonances due to the aliphatic methine (CH) proton and the resonances of theortho protons on the aromatic ring which are clearly separated from thealiphatic signals. In structural unit 47, an NOE effect should be observablebetween the resonances of the CH proton and those of the ortho protons on thearomatic ring. CH2-047Particularly importantly, the resonances due to the ortho protons on thearomatic ring which is derived from the cyanate monomer and not fromPTBPGE can also be identified by the comparison of the spectrum of the majorcross-reaction product made from PTBPCN 25 and PTBPGE 37 (Figure 3.9A)with that of the analog made from PTBPCN 25 and OMPGE 38 (Figure 3.9B).114Figure 3.10 presents the results of 1H NOE difference experiments on the majorcross reaction product made from PTBPCN 25 and PTBPGE 37. The spectra01CH2-CH-CH2-0-confirm that there are two separated N^ structural units withinone molecule. The assignments of the 1H resonances deduced from the 1H 2DCOSY experiment (Figure 3.5A) are also confirmed by these experiments. It is01very clear that in both N-CH2-CH-CH2- Ounits, NOEs exist between the CH (orCH') proton and the N—CH2 (or N-CH2') protons, between the CH (or CH')proton and the 0—CH2 (or 0—CH2') protons, and between the 0—CH2 (or 0—CH2') protons and the ortho protons on the aromatic ring. However, there is only01N-CH2-CH-CH2-0one^ unit which shows NOEs between the CH proton and theortho protons on an aromatic ring (see the spectrum in which the CH proton at4.95 ppm was irradiated in Figure 3.10). Most importantly, it can be identifiedthat this aromatic ring is derived from the cyanate monomer as shown in Figure3.9, assuming that they have structures 40 and 48 as indicated. The NOEdifference experiments for compound 48 (Figure 3.11) show the same results asdescribed above.An NOE is seen clearly between the CH proton and the ortho protons onthe cyanate-derived aromatic ring, providing the strongest evidence in favor ofstructures 40 and 48 for the cross-reaction products. The other NOE effects are0shown within the N-CH2-6H-CH2-0- unit or between the 0—CH2 (or 0—CH2')and ortho protons on aromatic ring, which do not distinguish between 39 and40. They do, however, demonstrate the reliability of the experiments and areconsistent with the connectivities deduced from the 2D NMR spectra (Figure3.5).115ortho H on cyanatederived ringortho H on cyanatederived ringI^I7.5^IDPm^6.5Figure 3.9. 1H (400 MHz) NMR spectra in aromatic regions of the majorcross-reaction products derived (A). From PTBPCN 25 andPTBPGE 37; and (B). From PTBPCN 25 and OMPGE 38.116Figure 3.10. 1 H (400 MHz) NOE difference NMR spectra of the majorcross-reaction product derived from the reaction of PTBPCN25 and PTBPGE 37, which is the same as Figure 3.4.117CH O-CH24.85 (CH')N-CH2'CH' O-CH2' IO-CH2'CH'4ring ( 6.86 ppm )4.27 (0-CH2)°Rho H on epoxy derivedring ( 6.82 ppm ) O-CH2' Irradiated at(ppm)4.06 (0-M2')(Mho H on epoxyi derived ring( 6.89 ppm )4.08 (0-CH2')ortho H on epoxy derivedortho H on cyanate/ derived ringO-CH2CH^N-CH2ortho H on cyanatederived ring4.95 (CH)zo z 66 6..F,Fn^I -101, hkilo6.0^-^5.0^4.0^ppm7.0Figure 3.11.1H (400 MHz) NOE difference NMR spectra of the majorcross-reaction product derived from the reaction of PTBPCN25 and OMPGE 38.118.....A4vi r Heow•-.-..0"—^N 4.13 (0-CH2')4.24 (0-CH2)CH I4.86 (CH')ortho H on cyanate/ derived ringohho H on cyanatei derived ring00= =7.0CH4.97 (CH)118-10-CH2'0-CH2'/ortho H on epoxyderived ringtortho H on epoxyderived ringIrradiated at(Ppm)4.10 (0-CH2')Mho H on epoxyderived ring0-CH2CH30- CH3.3.5. A Possible Mechanism for the Main Cross-linking ReactionBased on the above information, a possible mechanism for the formationof the major cross-reaction product 40 (and analog 48) can be proposed. Theaddition of two epoxy functional groups to a cyanate group gives the unstableintermediate species 49 which then rearranges by migration of the phenoxygroup from cyanate to the CH carbon to give compound 48 [Equation 3.21. Thisis in agreement with the placement of the cyanate phenyl ring in the positionindicated. Strong NOEs between the CH proton and the ortho protons on thearomatic ring which is derived from the cyanate are observed in both compounds40 and 48 as discussed above.119C H 3 CH30-CH249OCN +25CH 30-CH2OCH CH-CH2 \ ,^20CH3^38[3.2]3.4. Investigation of the Second Unidentified ProductThe second unidentified product fraction was also further purified by GPC(Sephadex LH-20 column). The 15N spectrum of the main component is shownin Figure 3.12A, and indicates that there is a single nitrogen present with oneattached proton. The chemical shift at - -40 ppm is in the range previously0IIfound to be characteristic of a carbamate -°-C- NH2 environment. The 13C120spectrum (Figure 3.12B) indicates three carbon signals in the opened epoxy ringregion. In addition to the major peaks, there are a number of very smalladditional signals in the spectrum indicating that the sample is not completelypure. The 1H NMR spectrum (Figure 3.12C) also shows some minor peaks, butthose due to the major component are clearly identifiable as indicated. Thechemical shifts and intensities of the signals in the 13C and 1H spectra reveal0the presence of a p-t-butylphenyl group, a N -CH2-CH-CH2 -0 moiety from theopening of the epoxy ring and also a carbonyl group. The 1H 2D COSY NMRspectrum (not shown) shows the expected connectivities within the0N-CH2-CH-CH2 -0 moiety and makes an assignment of this portion possible. Inaddition, the nitrogen atom is known from the 15N spectrum (Figure 3.12A) tohave a single attached hydrogen and a carbamate environment, eg.0I^I-0-C-NH- . Furthermore, the NMR spectra show that the ratios of thedifferent components: phenyl ring, opened glycidyl unit, and nitrogen are 1:1:1(or 2:2:2 etc.).The mass spectrum gives information regarding the molecular weight, butthere is some ambiguity because of the presence of the minor impurities.However, the major peak in the spectrum is at M+.249 a.m.u. which wouldcorrespond to a 1:1:1 ratio of the components if it were the true parent ion. Fromall of these information, a possible structure for the second unidentified productcan be proposed as 50. To confirm this and to make complete assignments inFigure 3.12 further experiments were carried out (see on).0-CH2-5 0121Figure 3.12. NMR spectra of the second unidentified product after bothsilica gel column and Sephadex LH-20 column separation ofthe crude reaction mixture obtained by heating PTBPCN(-12% 15N enriched) and PTBPGE at 180 °C for 7 hours. (A).15N spectrum (1H at 300 MHz) without 1H decoupling; (B).13C spectrum (1H at 200 MHz); and (C). 1H (200 MHz)spectrum.1229 8 4„CH2 30-CH2-CH\ 2 flo-0• , " • -• i •^i^i^...,„„,6.0 ppm 4.0 2.0I " " I^I8.03.5. Reaction of Carbamate and EpoxyCarbamate is formed from the reaction of cyanate with water, and is oneof the side products of the curing reaction of cyanate in solution. To determinewhether a cross reaction product could be formed between the epoxy monomerand the carbamate, both 15N labelled and natural abundance model carbamatecompounds were used in neat reactions with epoxy compounds.3.5.1. Reaction of  15N Enriched p-tert-Butylphenyl Carbamate 28 withPTBPGE 37 A mixture of 12% 15N enriched p-t-butylphenyl carbamate 28 (made fromhydrolysis of 12% 15N labelled PTBPCN 25) and PTBPGE 37 (1:2 mole ratio)was heated at 180 °C for 3.5 hours. The 15N NMR spectrum of the reactionmixture (Figure 3.13) shows that all the carbamate has reacted and there areonly two major nitrogen environments, one of them with a single attachedproton (species A). The reaction mixture was separated using a silica gel columnand a similar procedure described above. Two major reaction products, each ofthem with one single nitrogen environment, were isolated.One of the products has identical 15N, 13C, and 1H NMR spectra to thoseof the second unidentified reaction product obtained from the reaction ofPTBPCN 25 and PTBPGE 37. It also shows same parent ion peak (M+.249a.m.u.) in the mass spectrum. Therefore, they are considered to be the samecompound with the carbamate derived sample being of high purity.The second compound shows a single nitrogen resonance at 23 ppm withno proton coupling (Figure 3.14A). The 13C NMR spectrum (Figure 3.14B) alsoindicates three resonances in the region corresponding to a ring-opened epoxy123•^N-CH2-CH-CH2moiety -(:). The 1H NMR spectrum (Figure 3.14C) gives theratios of methyl protons, aliphatic protons and aromatic protons as 9:6:4. Thestructure of this product can be seen by comparison with the data from theproduct formed from the reaction of phenyl carbamate with phenyl glycidyl etherdescribed in the next section.A8:01^I^I40^0^-40^PpmFigure 3.13. 15N NMR spectrum (1H at 300 MHz) in acetone-d6 withNOE and no 1H decoupling of the reaction mixture obtainedby heating p-t-butylphenyl carbamate (-12% 15N enriched)and PTBPGE at 180 °C for 3.5 hours.124Figure 3.14. NMR spectra in acetone-d6 of the second product after silicagel column separation of the same reaction mixture as inFigure 3.13. (A). 15N spectrum (1H at 300 MHz) with NOEand no 1H decoupling; (B). 13C spectrum (1H at 200 MHz);and (C). 1H (200 MHz) spectrum.125A7^-I--40 PpmT-T-T--1- 1--- r-80^40epoxy ring has opened0160 410pp^s1 tz:1^lm^ro-C H3aliphatic protonsaromatic protons0-CH250^0^ 53NH0-C H23.5.2. Reaction of Phenyl Carbamate with Phenyl Glycidyl EtherAs described later in the Experimental chapter, the product 53 wasproduced by the reaction of phenyl carbamate 51 with phenyl glycidyl ether 52at 180 °C for 3.5 hours. It corresponds to the second unknown product of thereaction of PTBPCN 25 and PTBPGE 37 according to the similarities betweentheir 1H and 13C NMR spectra. Since it can be isolated and well purified, thestructure of this product can be well determined.In this case, the interpretation of the mass spectrum is unambiguousbecause the sample is pure, and the parent ion mass at M+.193 a.m.u. clearlyidentifies the ratio of phenyl ring to ring-opened epoxy moiety to nitrogen asbeing 1:1:1, indicating the loss of one phenol during the reaction. The 15N, 13Cand 1H spectra shown in Figures 3.15 and the 1H 2D COSY NMR spectrum (notshown) are in complete agreement with the proposed structure 53. Furthermore,a good quality crystal of this sample was obtained from recrystallization inacetone and was investigated by single crystal X-ray diffraction yielding themolecular structure shown in Figure 3.16. It indeed contains an oxazolidinonering confirming the conclusions from the NMR and mass spectra. Moreimportantly, with the exception of the nuclei in the phenyl group which areaffected by the substitution of H for t-butyl, the 13C and 1H spectra areidentical with those of the second unknown product obtained from reaction ofPTBPCN 25 and PTBPGE 37 . Therefore, the second unknown product can beassigned the corresponding structure 50.126Figure 3.15. NMR spectra of product 53 in acetone-d6. (A). 15N spectrum(1H at 300 MHz); (B). 13C spectrum (1H at 200 MHz); and(C). 1H (200 MHz) spectrum.1274.0 ppm7.0^6.0 5.0A 3-408-44^ppm,^,^,-32 -3694r^,CF12 30-CH2-CH98H1Figure 3.16. Molecular structure of product 53 from the single crystal X-ray diffraction experiment with the numbering of the atomsindicated. Complete structural data are given in Appendix C.128Besides product 53, the reaction of phenyl carbamate and phenyl glycidylether also produced two other products and some polymerized materials (mostlypolyether polymer). The reaction products were isolated and purified byabsorption chromatography (silica gel column) and gel permeationchromatography (Sephadex LH-20 column). The approximate weightpercentages of the components in this reaction mixture were:product 54, phenol and unreacted phenyl glycidyl ether —35%polymerized materials —25%product 55 fraction —15%product 53 fraction —25%On standing for several days, a product identified as 54 crystallized andwas isolated from the first fraction. After further purification byrecrystallization from acetone, MS analysis yielded a molecular weight of 244,and elemental analysis showed composition of C 72.8% H 6.6% 0 20.5%corresponding to the formula C15111603. From the 13C, 1H NMR spectra (notshown), the product can be identified to be 2-hydroxyl-1,3-diphenoxy propane 54(shown in Equation 3.3 below).Another product fraction identified as 55 (shown in Equation 3.3 below)can be further purified using GPC (Sephadex LH-20 column) with acetone as theeluant. From the similarities between their 13C and 1H NMR spectra, itcorresponds to the compound with a single 15N resonance at 23 ppm (Figure3.14A) in the mixture from the reaction of —12 % 15N enriched p-tert-butylphenyl carbamate with PTBPGE. In the CI mass spectrum, it shows aparent ion peak at 579 mass units [ (M+1)+=580 a.m.u.].Figure 3.17 shows the 13C and 1H NMR spectra of this product. The 13CNMR spectrum (Figure 3.17A) shows three resonances in the aliphatic regionwhich indicate that there are one or more identical ring-opened glycidyl units12901N—CH2—CH—CH2-0. From the 1H NMR spectrum (Figure 3.17B), the ratio ofaromatic protons to aliphatic protons is approximately 5:6. This means that the012ratio of phenyl groups to the structural units N-CH2 -CH -CH -0 could be either5:6 or more likely 1:1 with a contribution of one additional hydrogen from a01hydroxyl group on the N-CH2 -CH- CH2 - o unit. By comparison with the protonratio found for the corresponding product made from the reaction of p-t-butylphenyl carbamate and PTBPGE (Figure 3.14C), the 1:1 ratio of phenyl and01the structure unit N -CH2 -CH - CH2 —0can be established. The 1H-13C 2Dheteronuclear shift correlation and the 1H 2D COSY NMR spectra (Figures013.18A and B) confirm the existence of the structural unitand a hydroxyl group which is connected to the -CH- carbon. The 1H resonanceof the hydroxyl group is split into a doublet by coupling to the —CH— proton. Bycombining all of the information from the mass spectrum (M+ = 579) and the01NMR spectra, the ratio of phenyl, the structural unit N-CH2-CH-CH2 -0 andnitrogen should be 3:3:3 and the structure of this product is deduced to be 55.Therefore, the reaction of phenyl carbamate 51 and phenyl glycidyl ether 52can be represented as in Equation [3.3].N — CH2 — CH — C H2 — 0130Figure 3.17. NMR spectra of product 55 in acetone-d6. (A). 13C spectrum(1H at 200 MHz); (B). 1H (500 MHz) spectrum.131670ii^2 3 4^5CI-W1-1 CHpOH7 06A4 3^2815.^i^I^'^1160 120 ppm^80B^ aliphatic protonsaromatic protons1^I40„PP.^7.0^6.5^ppmI^T^V^llll^I4.5 4.0ASP^41:;=)Ca1Z413P41=21> 50607014.5I i4.01 PpmFigure 3.18. (A). 1H-13C 2D heteronuclear correlation NMR spectrum(1H at 500 MHz) and (B). 1H (500 MHz) 2D COSY NMRspectrum of product 55 in acetone-d6.13214.0I4.5 IDPmO-CH2N-CH2/\4.04.5BCHOH132-10-0—NH2i0510- CH 2- C C H2520-CH2530-CH2-CH-CH2-OH54[3.3]00-CH2-CH-CH2-NN-CH2-CH-CH2-OH^ OH0^N^0CH2-CH-CH2-55^OHPhenol is necessary for the formation of 54 [Equation 3.4]. This isproduced during the formation of compounds 53 and 55. (see Scheme 3.1).0—CH2-0070H2052HO0-CH2-CH-CH2-OH54[3.4]3.6. A Possible Mechanism for the Reaction Between Epoxy andCarbamateAs will be described in the Experimental chapter, during the reaction ofphenyl carbamate and phenyl glycidyl ether, some white solid formed at the133beginning and then disappeared at longer reaction times. The reaction wasstopped after the white solid formed and this material was isolated by filtrationand then washed with pentane, diethyl ether, and acetone. The 13C NMRspectrum which shows only a single resonance at 151 ppm and the massspectrum which shows a parent ion peak at M+.129 a.m.u., corresponding to aformula C3H3N303 indicate that it is isocyanuric acid 30. This is thedecomposition product of the carbamate as described in Chapter 2. Isocyanuricacid is obtained from three phenyl carbamates by elimination of three phenolmoieties. In addition, the formation of product 53 also involves elimination of aphenol moiety. Thus, a possible mechanism for the overall reaction can be given,as shown in Scheme 3.1.At high temperatures, the carbamate 51 may first decompose to phenoland isocyanic acid (0=C=NH) which is a very unstable intermediate under theseconditions. Isocyanic acid will immediately react either with phenyl glycidylether 52 to form the product 53 or with other isocyanic acid molecules to form atrimer, isocyanuric acid 30. Isocyanuric acid 30 is the intermediate for product55 and only exists at the beginning of the reaction. The phenol produced in thedecomposition of the carbamate can react with phenyl glycidyl ether 52 to yieldproduct 54. All of these reactions are shown in Scheme 3.1 below.134(0)^0 CH2-CH-CH -OH540-CH2-CH-,CH2^52 ^0^(0)-0-CH2^NH53 o0-cH2-CH-,cH2520-C-NH2510(0)^ OH^+^[0=C=NH0HN^NH300 N^0o o-cH2-cti-,cH252^000-CH2-CH-CH2-N^—CH2-CH-CHOH^ OH0 N 055CH2-CH-CH2-OHScheme 3.1The intermediate 30, which is produced by decomposition of thecarbamate, was also observed in the curing reaction of dicyanate resin insolution described in Chapter 2. This can be seen in the 15N solid-state NMRspectra of the resin cured in solution (Figure 2.14) and the 13C spectrum of135Figure 2.9. The peak at —22 ppm (Figures 2.14B and C) disappears in the non-protonated nitrogen selection experiment (Figure 2.14D).3.7. Investigation of Imidocarbonate as a Possible Cross-reactionProductSince some hydroxyl groups are always present in the commercial epoxyresins, it was thought that the reaction between cyanate and these hydroxylgroups might act as another way to cross-link the two resin systems. To provethis, p-tert-butylphenol (PTBP, 56) was first reacted with the cyanate PTBPCN25 to provide a suitable reference compound (59), and then isopropanol (57) wasused as a model compound for the secondary hydroxyl groups in the polyetherunits of the epoxy resin (58).CH3-CH-CH3^mAO-CH2-CH-CH2-ONAA,1OH OH57^ 583.7.1. Reaction of Cyanate with p-tert-Butylphenol (PTBP) 13C NMR spectra (not shown) show that heating the reaction mixture ofthe PTBPCN 25 and PTBP 56 in MEK/acetone-d6 solvent for up to six days at100 °C gives complete conversion of the cyanate (8 = 109 ppm) to triazine (8 =174 ppm), but there is no reaction at all for cyanate and the correspondingphenol under these conditions (spectra not shown). After a trace of sodiumhydroxide has been added as a catalyst to the mixture, and the solution hasstood for 10 hours at room temperature, the 13C NMR spectrum (not shown)shows that there is efficient conversion of the cyanate group to another speciescharacterized by a 13C resonance at 159 ppm as well as the production of a1360)—OH base25^56a-ONC=NH59OCN [3.5]small amount of triazine. This major species is thought to have animidocarbonate structure 59.A very clean conversion to the same product species is also observed atroom temperature in acetone solvent with triethylamine as a catalyst. Theseconditions were used to prepare and isolate this compound as described in theExperimental chapter. The 13C and 15N NMR spectra of this purified compoundare shown in Figure 3.19A and B together with the complete assignments. The15N spectrum shows a resonance at 43 ppm due to a nitrogen with a singleattached hydrogen. Combining this information with the data from the massspectrum, which yields parent ion M+ = 325 a.m.u., confirms that it is theimidocarbonate 59. Therefore, basic conditions are necessary for the formation ofthe imidocarbonate, which can be represented as in Equation 3.5.3.7.2. Reaction of Cyanate with Isopropanol After heating PTBPCN 25 in an excess of isopropanol 57 at 100 °C for onehour with triethylamine as a catalyst, the 13C NMR spectrum shows that thereis complete conversion of the cyanate group to a new species characterized by aresonance at 159 ppm together with a small amount of triazine (spectrum notshown). From the similarity of this shift to that of the reaction product withPTBP 56, it is thought that the product of the reaction of cyanate 25 withisopropanol 57 has structure 60 and that reactions of this general type couldform a viable attractive route to the cross reaction between cyanate and epoxyresins. This is indeed observed in curing the mixed dicyanate (BPADCN) and137Figure 3.19. NMR spectra of product 59 in acetone-d6. (A). 13C spectrum(1H at 200 MHz); (B). 15N spectrum 0-H at 300 MHz) withno 1H decoupling.1381^810 907 .C=N H0596 5 43 21312BIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII60^50^40^30 ppm131 I1^1^f^11401^I^I1001^i^160fPpm138-1epoxy (EPON-825) resins, which gives a small peak at 43 ppm in the 15N solid-state NMR spectrum (Figure 3.1A) and disappears in the non-protonatednitrogen selection experiment (Figure 3.1B). However, this contribution to thecross reaction is quite limited compared to the major cross reaction which formsthe oxazoliclinone ring basic structure 40.CH3 -CH -CH3I0\C=NH(o o)- 13.8. The Mechanism of the Curing Reaction for Dicyanate / DiepoxyMixed ResinsAs discussed above, the reactions of monofunctional model compoundsyield several quite interesting products which can be isolated and characterized.All of these products are directly related to the curing reaction of the dicyanateand diepoxy resin mixture. Each of them corresponds to a specific cross reactionbetween the two resins, and can be easily identified by comparison of the highresolution solution NMR spectra for the low molecular weight products and thesolid-state NMR spectra for the cross-linked resins.Among these products, the most important one is the major cross reactionadduct 40 derived from monocyanate 25 and monoepoxy 37. The possiblemechanism for this cross reaction is as proposed in Equation 3.2. Because itcomes from the direct reaction between one cyanate and two epoxy molecules, itcorresponds to a cross-reaction product between the two resins and makes amajor contribution to the large peak at —36 ppm in Figure 3.1. From both theproduct percentage yields for the model compound reactions and the 15N solid60139state NMR spectrum of the bulk cured resin mixture, it can be concluded thatthis cross reaction is the major one between the two resins, even though it islimited to —12%.Cross-linking between the two resins can also occur through the reactionof the cyanate with the hydroxyl groups in the epoxy resin, which gives theproduct with the imidocarbonate type of structure 60. However, its contributionto the overall cross reaction is much less than that of the major cross reactionmentioned above.The reactions of monocarbamate 51 and monoepoxy 52 give products 53,54, and 55. Product 53 shows a resonance at —40 ppm in its 15N NMR spectrumand corresponds to the product in the cured mixed resins which makes partialcontribution to the peak at —36 ppm in Figure 3.1. However, this contribution isvery limited in comparison to that of product with the structure similar to 40from the major cross reaction. Its presence can be detected in the variablecontact time experiment (Figure 3.2) and the non-protonated nitrogen selectionexperiment (Figure 3.1B). In the non-protonated nitrogen selection experiment,the intensity of the resonance at —36 ppm remains almost constant. Althoughproduct 53 is not formed from the direct reaction between epoxy and cyanate, itis related to the cross-reaction between the two resins. During its formation[Scheme 3.1], the phenol moiety is cleaved from the cyanate, and can continue toreact with epoxy to form compound 54 [Equation 3.4]. Since this phenol moietycomes from the cyanate resin, it can be considered that this reaction [Equation3.4] is a cross-reaction between the two resins and that product 54 is also across-reaction product.Product 55 has a resonance at 23 ppm in the 15N NMR spectrum andcorresponds to the species in the cured mixed resins which yields a small peak at23 ppm in the 15N solid-state NMR spectrum (Figure 3.1). As in the case of140product 53, the formation of product 55 also involves elimination of a phenolmoiety from the cyanate (Scheme 3.1), and then reaction of this phenol withepoxy to form the cross-reaction product 54. Intermediate 30 is a trimer ofmoieties originally in the cyanate, with loss of the phenol part from the cyanateresin. It only links epoxy monomers, and thus acts as a curing agent for theepoxy resin. Therefore, compound 55 is not a cross-reaction product between theepoxy and cyanate monomers but the product of a curing reaction for the epoxyresin with intermediate 30 as the curing agent.3.9. ConclusionsThe possible cross reactions which solid-state NMR indicates occurbetween cyanate-functionalized and epoxy-functionalized resins have beeninvestigated using both natural abundance and labelled monofuctional modelcompounds. These soluble products were isolated and purified by adsorptionchromatography and gel permeation chromatography, and then fullycharacterized by high resolution 1H, 13C, 15N NMR spectroscopy and by massspectrometry.The major cross-reaction product between cyanate and epoxy resinmonomers contains one cyanate monomer and two epoxy monomers, and veryclearly indicates that cross reaction between the cyanate and the epoxy canoccur during the mixed resin curing process. The balance of all information todate is considered to favor an oxazolidinone structure 40 for the major cross-reaction product. The reaction between the cyanate and the hydroxy groups inthe epoxy resin is also a cross-reaction pathway even though it is very limited.Besides the direct reaction of epoxy and cyanate, cross reactions for the epoxyand cyanate mixed resins can also occur through the reaction of epoxy and thecarbamate derived from the cyanate group.141However, epoxy consumption lags cyanate consumption in the overallreaction as triazine formation from the cyanate is much faster than the twocompeting reactions, the cross reaction between cyanate and epoxy, and the self-polymerization of epoxy, under the conditions investigated. Thus, cross reactionbetween cyanate and epoxy is limited; approximately 12% cross reactionbetween cyanate and epoxy was found in the overall reaction under the curingconditions used in the present study.142CHAPTER 4.SYNTHESIS AND CHARACTERIZATION OFCROSS-LINKING AGENTSFOR MIXED CYANATE / EPDXY RESIN SYSTEMS ANDFOR MIXED CYANATE / OLEFIN RESIN SYSTEMSThe end use of a polymer system is often decided from engineeringrequirements. The structure at the molecular level is not the only importantfactor when an application requires a bulk property to be within a specificrange. Thus, many synthetic polymers are designed such that the physicalproperties will be optimum for a particular end use. In the past this meantdeveloping completely new polymers, but in the last fifteen years, there has beenmuch interest in the properties and applications of multicomponent polymersystems.[98] It is often found that mixing two polymer systems together canproduce a synergistic effect combining both desirable advantages of the twosystems. In this way, the desired mechanical properties and processingcapabilities can often be obtained. Thus, a specialized polymer system can beobtained by combining two or more different but known polymers.Multicomponent polymer materials are defined as mixtures of two or morestructurally different polymeric species, such as polymer blends, blocks, grafts,cross-linked polymers, or interpenetrating polymer networks (IPN's). They canbe classified into two groups in terms of the type of bonding between differentcomponents.[991 Polymer blends and IPN's do not have chemical bonds betweenthe different components, while block copolymers, graft copolymers and cross-linked polymers contain intermolecular covalent bonds which hold the differentspecies together.143The cross-linking bonds in cross-linked polymers are formed by crossreactions which can proceed directly through the side groups on differentcomponents or through a cross-linking agent which can react with the twodifferent components and hold them together. In the latter case, the desireddegree of cross-linking can be exactly controlled by controlling the amount ofcross-linking agent added.4.1. A Cross-Linking Agent for Mixed Cyanate / Epoxy Resin SystemsAs described previously, the advantages of using epoxy resins for printedcircuit boards are that the high mechanical performance and low shrinkageproperties of these resins provide toughness and mechanical strength for thecircuit boards. In addition, epoxy resins can be cured at any temperaturebetween 5 °C to 150 °C depending on the choice of curing agents. They can alsobe modified in many ways for various specific applications or properties.[39]However, compared to the cyanate resins, epoxy resins have the disadvantage ofrelatively high dielectric constants, which make the signal transfer speedslower. [17,30] Furthermore, epoxy resins show high moisture absorption andlow heat insulation relative to the cyanate resins. As a result, the performanceof printed circuit boards made from epoxy resins may not be reliable when theyare operated at elevated temperature or in humid environments.In general, cyanate resins have the advantages of low dielectric constants,low moisture absorption, and high thermal stability.[17,301 However, a seriousdisadvantage of cyanate resins is the lack of mechanical strength. A circuitboard made from cyanate resin alone can be easily broken in use.Thus it can be seen that both cyanate and epoxy resins show somedesirable properties for printed circuit boards but neither of them has all of theideal characteristics. Therefore, cyanate resins are usually cured together with144epoxy resins to try to get the desirable properties from both. However, as seen inthe previous chapter, although cross reaction between the cyanate resin and theepoxy resin can occur during the curing process, it is of limited efficiency, and iscomplex and difficult to control. Thus, obtaining an alternative to the directcross reaction for cross-linking the two resins will be very important for bettercontrol of the properties of the final resin. This might be done by the addition ofa cross-linking agent which is a "mixed monomer", ie which contains bothcyanate and epoxy functional groups. This would give a predictable degree ofcross-linking under controlled conditions which could be varied by changing theproportion of the cross-linking agent. In addition, since the curing reaction of acyanate resin can be initiated thermally while a base is needed for curing anepoxy resin, a two stage curing process could be carried out. This is the mainidea behind the research in the present chapter.4.1.1. Strategy for the Synthesis of the Cross-Linking Agent, the MonoglycidylEther of Bisphenol-A-monocyanate 61NCO OCH2CH —CH2\/o61The mixed functional target molecule, the monoglycidyl ether ofbisphenol-A-monocyanate 61 or 2-(4-cyanatopheny1)-244-(2,3-epoxypropoxy)-phenylipropane, was chosen as the cross-linking agent to be synthesized. Thetwo possible routes considered for the synthesis are shown as A and B in Scheme4.1. They are different only in the order in which the functionalities are added to145the starting material, bisphenol A (3). Each route includes two reactions and oneintermediate species.HO OCH2CH-,CH2oRoute A 62HO OH NCO OCH2Ctl-ICH2oRoute B3 61{4}NCO OH63Scheme 4.1By examination of the conditions for the four different reactions, it wasconsidered that route A was more likely to be successful than route B for severalreasons. Although the addition reactions of the cyanate and the epoxy functionalgroups both involve base, heating is required for the addition of the epoxy group,whereas the formation of the cyanate proceeds at a much lower temperature (0°C). From the curing mechanism for a cyanate resin discussed in Chapter two,the cyanate functional group formed in the reaction of step {3} in route B mightbe trimerized or destroyed by the high temperature used in the reaction of step{4} in route B. Furthermore, due to the presence of bases in both reaction steps{3} and {4}, the intermediate product of route B, bisphenol-A monocyanate or 2-(4-cyanatopheny1)-2-(4-hydroxyphenyl)propane 63, may react with itself to formpoly(bisphenol-A imidocarbonate) 64 as shown below. In contrast, in route A of146the synthesis, the epoxide intermediate, the monoglycidyl ether of bisphenol-Aor 244-(2,3-epoxypropoxy)pheny1]-2-(4-hydroxyphenyl)propane 62, will remainunreacted under the low temperature (0 °C) conditions for the formation of thecyanate in step {2} in Scheme 4.1.HO OCN64Consequently, route A in Scheme 4.1 was chosen for the preparation ofthe target cross-linking monomer, the monoglycidyl ether of bisphenol-A-monocyanate 61. The procedure for the synthesis was first to synthesize, purify,and characterize the intermediate, the monoglycidyl ether of bisphenol-A 62 andthen to prepare the target monomer 61 from this intermediate. After thesuccessful synthesis of the intermediate 62 and the target cross-linkingmonomer 61, the independent curing reaction of the cyanate functionality in thecross-linking monomer 61 was studied by application of heat. The curing ofepoxy functionality was also studied using a base as a curing agent. It is shownthat the curing reaction of the epoxy group in the cross-linking monomer 61 isnot independent of the curing of the cyanate group. Characterizations werecarried out by NMR and MS experiments as described in the Experimentalchapter. In the following sections, the investigations of these reactions and thecharacterization of the product species will be discussed in detail.4.1.2. Synthesis and Purification of the Monoglycidyl Ether of Bisphenol-A, 62 In general, the formation of epoxy resins is described by the followingscheme (Scheme 4.2):[100]147OHOCH2CHCH 066 OHCH2 -CHCH 0/0012-/CHCH OCH2CHCH 0-^OH18OCH2CH - CH2\o/HO OH CI -CH2R1-/C H23HO OCH2C1-,1---,CH262CI-CH2CH-/CH20OCH2Ctl-/CH217HO OHCI-CH2Cti-/CH20OC H2 H C H2 0OHOCH2CH- C H2\o/67HO OH^01-CH201-1-CH20Scheme 4.2148Structure 18 represents the general formula of epoxy resins based onbisphenol A and epichlorohydrin. Like compound 17, such compounds are alsotermed diglycidyl ethers since all of them contain two glycidyl ether groups,0/\0-CH2CH—CH2 , per molecule. In order for the final products to becommercially useful as epoxy resins, it is necessary that the polymers areterminated by epoxy groups, through which they may be subsequently cross-linked. This is achieved by carrying out the polymerization with an excess ofepichlorohydrin. In a typical process for the preparation of a epoxy resin, amixture of bisphenol A and epichlorohydrin (about 1:4 molar ratio) is heated toabout 60 °C with stirring. Solid sodium hydroxide (2 moles per mole of bisphenolA) is added slowly. The unreacted epichlorohydrin is then removed bydistillation under reduced pressure.[- 00]As seen in the scheme, the monoglycidyl ether of bisphenol-A 62 exists asan intermediate at the beginning of the course of an epoxy resin synthesis eventhough it has never been isolated as a synthetic target product. Thus, it wasthought that it should be possible to synthesize the intermediate product 62 byreacting bisphenol A and epichlorohydrin with a suitable base, but that a 1:1:1of molar ratio of bisphenol A, epichlorohydrin, and base should be used. Themajor product expected from this reaction should be the desired intermediate 62.However, since the two hydroxyl groups in bisphenol-A are identical, theselective epoxidation of one of them is very difficult. The product from overreaction, the diglycidyl ether of bisphenol-A, and some unreacted bisphenol-Aare also to be expected as side products. Therefore, purification of the desiredintermediate 62 must be conducted after the preparation. This wasaccomplished by extraction followed by adsorption chromatography in thefollowing experiments.149A moderate reaction temperature of 56 °C and relatively short reactiontime of 90 minutes were chosen for the initial reaction conditions.[1011 Theresulting crude product was identified by NMR spectroscopy to be a mixture of61, 17, 3 and some unknown side products as shown in Figure 4.1. As aconsequence, purification of this crude mixture was carried out to obtain pureintermediate product 62. The sequence chosen for the purification was firstextraction of the crude product using different organic solvent with increasingpolarities, followed by separation of each component using a silica gel column.The details of this will be presented later. However, it was found that theintermediate 62 could not be isolated as a single pure component by thispurification procedure. The best fraction obtained from the silica gel column wasa mixture of intermediate 62 and bisphenol A (3). Thus, in order to obtain pure62 from the silica gel column, there should be no bisphenol A present in thecrude mixture. This means that all of bisphenol A must react completely at thesynthesis step. Although other methods (such as using a combination of differentchromatographic sequences) might also yield pure 62, varying the reactionconditions to eliminate bisphenol A at the synthesis step was thought to be themost straightforward approach. On the other hand, variations of the reactionconditions in the preparation step might not only allow to complete theconsumption of bisphenol A, but also could produce a higher yield of the desiredproduct 62.For investigations of the efficiency and selectivity of the epwddationprocess, a number of reactions were carried out under different conditions withvariation of the reaction temperatures and times and the choice of a base used.The optimum reaction conditions for the preparation of intermediate 62 werefound to be using potassium carbonate as the base and keeping the reactiontemperature at 80 °C and the reaction time at 150 minutes (see Equation 4.1),150Figure 4.1. 13C NMR spectrum (1-H at 200 MHz) in acetone-d6 ofthe crude product prepared by reaction at 56 °C for 90min. and using a reactant mixture with a 1:1:1 molarratio of bisphenol A, epichlorohydrin and potassiumcarbonate. (A). Full spectrum; (B). The expandedaromatic region.151OHlbHO1pHO3b 4b 7b12g 13g 14gCH2CH—CH2\o/ip 12p 13p 14pOCH2CH-CH2\o/12g12p3p3b lOg10p 7g7p7b14g14p13g 6g13p 6p6bT^7^T120 40 ppm80160747 5b^5p144.5 144.0 143.5 143.0 142.5 ppmllp--2(r/fr--I 1 g157.5^157.08p156.5^156.0 ppm2P 2bA9g9p4p4b8g15 1-1HO 0 0K2 CO3OH 4. CI-CH2C\H-,CH20^80°C HO150 min.C• 0 OCH2CH-CH2'0/^[4 .1]which will be given in detail in the Experimental chapter. Under theseconditions, the major component is the desired intermediate product 62. Minorside products of this reaction are compounds 17 and 66 and some unidentifiedcompounds. The most important feature is that there is no bisphenol A in thecrude product mixture formed under these reaction conditions and pureintermediate 62 should be obtained using the same purification procedure asmentioned above, which are:Purification of the desired intermediate 62 was accomplished first byextraction using solvents of increasing polarities, then followed bychromatographic separation of the proper fraction from the extraction using asilica gel column. The purpose of using the extraction as a preliminarypurification step is to remove any polymeric species formed during the reaction.Due to their adhesive properties, these species could damage the silica gel bedand affect the resolution of the column. Therefore, prior removal of these speciesgives better column separations.As a result, pure monoglycidyl ether of bisphenol-A 62 was obtainedthrough this purification sequence as a crystalline material (m.p. = 86 — 88 °C).The overall yield is about 20 %. Both MS and NMR experiments are consistentwith the structure. The mass spectrum shows M+ = 284 a.m.u.. Its 13C and 1HNMR spectra are shown in Figure 4.2. Since the monoglycidyl ether ofbisphenol-A 62 is composed of half moieties of bisphenol-A (3) and bisphenol-Adiglycidyl ether (17), compound 62 gives very similar NMR spectra to those of a152Figure 4.2. NMR spectrum (1H at 200 MHz) of the monoglycidylether of bisphenol-A, 62, in acetone-d6. (A). 13Cspectrum; (B). 1H spectrum.153A3p 4p 7P 9p 10p2^ 11p 12p 13p 14p1pHO OCH2CH-CH23p^\0/10p9p4p627p14p12p 13p6p2p451)5lip^•■.0kMOMMT■IMWOI^I^I160^120^80^40 Ppm7p12p 14p9.0 7.0 5.0 3.0 ppm153-1oHO-(0)--^-(0)-0CH2CH-CH2\ /0NCO62+ BrCN 4. (CH3CH2)3N< 0 °COcH2o\H-/CH2 + (CH3CH2)3NHBrmixture of compounds 3 and 17. The assignments in Figure 4.2 are made fromthe known spectra of bisphenol A (3) and the diglycidyl ether of bisphenol-A(17). [102]4.1.3. Synthesis of Cross-Linking Monomer 61After successful synthesis of the intermediate compound 62, the targetcross-linking monomer 61 was synthesized by using 1:1:1 molar ratio of purified62, cyanogen bromide, and triethylamine as a base as shown in Equation 4.2.The detailed procedure for the preparation is given in the Experimental chapter.[4.2]61The crude product 61 is a viscous light yellow liquid. An attempt to purifythis crude product was made using distillation under reduced pressure, but wasnot successful due to the high boiling point of compound 61 and its trimerizationat elevated temperatures. However, the NMR spectra of the product 61 in Figure4.3 show that there are only very minor impurities present (resonance b is dueto the residual benzene solvent). Most importantly, the 13C spectrum (Figure4.3A) shows that the cyanate functional group which gives a resonance at 109ppm is indeed formed in the reaction. In addition, the 1H spectrum (Figure4.3B) shows that only a very small phenolic proton signal at 8.2 ppm is left,154which means most of the hydroxyl groups are converted to cyanate groups.Meanwhile, the three characteristic peaks at high field, which come from theglycidyl ether group, remain unchanged. This proves that the formation of thecyanate at step {2} in Scheme 4.1 does not affect the glycidyl ether functionalgroup formed in the previous step {1}. The mass spectrum shows the parent ionpeak of product 61 at 309 a.m.u., which is consistent with the NMR experimentsand confirms the formation of the cyanate, Therefore, the desired cross-linkingmonomer 61 has been successfully synthesized and characterized.4.1.4. Curing of the Cross-Linking Monomer 61 with HeatSince the cross-linking monomer 61 has two different functional groups,cyanate and epoxy, and each of them can be cured by different mechanisms, itwas thought that monomer 61 could react independently with the cyanate or theepoxy functional groups on other molecules. The first attempt was to cure thecyanate functional group of the monomer alone. The monomer 61 was simplyheated at 180 °C for 15 minutes. A brown-yellow solid was produced which wasidentified as compound 68 from MS and NMR experiments (Figure 4.4).The mass spectrum gives the parent ion M+.927 for the solid product,which is three times the mass of the cross-linking monomer 61 and correspondsto the trimer structure 68. The 13C NMR spectrum (Figure 4.4A) shows that allof the cyanate groups have reacted (the peak at 109 ppm disappears) andtriazine rings have formed (the resonance at 174 ppm appear). Particularlyimportantly, the 13C NMR spectrum shows the epoxy groups remain unreactedduring this curing process as reflected in the three characteristic peaks at highfield for the glycidyl ether moiety. The 1H spectrum (Figure 4.4B) is similar tothat of the monomer 61 (Figure 4.3B), which also means the glycidyl ether155Figure 4.3. NMR spectrum (1 H at 200 MHz) of the crudemonoglycidyl ether of bisphenol-A-monocyanate 61 inacetone-d6. (A). 13C spectrum; (B). 1 H spectrum.15610c7c13c 14c12c6clclcNCO 2c3c 4c^7c^gc 10c161lc 12c 13c 14cOCH2CH—CH2\01Ab4c^3c9c2c 5c11c 8cI6...irmpod!^I^I160 8011111]^-1120t 1401^1ppmBPc4cJill.^910cI^' -• -I ' -` -.- -' T '9.0 7.0 5.0 PPIn156-13.0Figure 4.4. (A). 13C NMR spectrum (111 at 300 MHz) in acetone-d6 and MEK of the crude product 68 from curing themonoglycidyl ether of bisphenol-A-monocyanate 61 byheating; (B). 1H NMR spectrum (200 MHz) in acetone-d6 of the crude product 68.1579t 10t12t 13t 14t11t00H20F1-0H2\/0at 4t 7toseA9t4t 10t 13t3t 1 t 14t 7t6titlit^8t2t 5t44.40,400.0•0_00,,i^,^1Ppm1^i^1^1^ 1160 120 80^40 " I ' , ,9.0 7.0 5.0 3.0 1.0 PpmOCH2 CH - C H2\ o /68OCH2CF\I / CH2functional groups are unchanged. Therefore, it is confirmed that the cyanatefunctional group in the cross-linking monomer 61 can be cured independentlywithout affecting the epoxy functional groups.4.1.5. Curing of the Cross-Linking Monomer 61 with Base The independent reaction of the cyanate functional group in the cross-linking monomer 61 by heat has been verified. The next step was to try to find away to independently cure the other functional group, the epoxy, in themonomer 61. Because cyanate groups are more reactive than epoxy groupsunder heating, the consumption of the cyanate resin is faster than that of theepoxy resin in a mixed resin system during a heat-curing process as shown inChapter 3. For this reason, heat should be avoided if it is desired that only theepoxy functional group in monomer 61 be cured.From the curing mechanism for epoxy resin mentioned before, [39,1 00] aamine with an active hydrogen [diethylamine, (CH3CH2)2NH] was used as acuring agent to try to react only with the epoxy group in monomer 61 at room158temperature. A mixture of monomer 61 (0.26 mmole) and diethylamine (0.15mmole) was stood at room temperature for one day. Its 13C NMR spectrum isshown in Figure 4.5. The spectrum shows that the reaction of the cyanate groupis still faster than that of the epoxy group under these basic conditions since thecyanate resonance at 109 ppm disappears completely while the threecharacteristic glycidyl resonances at higher field are still present. Meanwhile,resonances derived from the cyanate group at 167 ppm and 174 ppm for anunknown are produced during the reaction, which indicates that most of thecyanate groups are transformed to this unknown. According to the literature,[9]aryl cyanates react with secondary amine to form N,N-disubstituted isoureas 69.If aryl cyanate is excess, the isoureas 69 can continue to react with aryl cyanatesand form triazine 70 (see Equation 4.3). Therefore, the unknown productobtained from the reaction of monomer 61 and diethylamine can becharacterized as the compound with the triazine structure 72. The MS spectrumshows a parent ion at 716 a.m.u., confirming the identification. The resonancesat 167 ppm and 174 ppm in the 13C NMR spectrum (Figure 4.5) are due to thetwo different carbons on the triazine ring of 72. The small resonance at 158 ppmis assigned to the carbon in isourea 71, which is derived from the cyanate carbonin monomer 61. Furthermore, this can be verified by a reaction of monomer 61with excess of diethylamine, which leads to formation of the isourea 71 onlywithout further reaction to form the triazine 72. A mixture of monomer 61 (0.28mmole) and diethylamine (0.43 mmole) was stood at room temperature for oneday. Its 13C NMR spectrum (Figure 4.6) shows there is no triazine 72 formed.All of the cyanate carbons are converted to the isourea carbons of 71, showing aresonance at 158 ppm. The MS spectrum is consistent with the result, showing aparent ion at 382 a.m.u.. The overall reaction can be represented as in Equation4.4.159Figure 4.5. 13C NMR spectrum (1 H at 200 MHz) in acetone-d6 ofthe crude product from curing the monoglycidyl etherof bisphenol-A-monocyanate 61 at room temperaturewith diethylamine base (monomer 61 is in excess).160116 117CH3CH2N,,CH2CH310114^ m_r,ts..............a,v 12 vi 12 wt111115^114113^,,,N,....CH2 —CHCH20 112\&0OCH2C Il'-'-/C H 2o111 1151 511414181131117116I 51I Ii 1T160i^•Ppm80120 40114^113 112^111C4--ICHCH 0110 19^17^14^i3.80115^116,CH2CH30—C—N\IINH CH,CHQ—i4i911 1i3110112113115114171611611^15^18i2‘60,411.••■•~100..~4.ftiverav~oiemmillin1^'^1^I^I^I^1^-,^1^1^.^1BO 160 140 120 100 80 60 40 20PPMFigure 4.6. 13C NMR spectrum (1H at 200 MHz) in acetone-d6 ofthe crude product from curing the monoglycidyl etherof bisphenol-A monocyanate 61 at room temperaturewith excess diethylamine base.,161R/Ar-O-C-Nr R 2 ArOCNii^\,,NH rl - ArOH69Ar-O-CEN + HN\ R'R^R''N'Ar0 OAr[4.3]72 o/\0-CH2 CH —CH20/\CH2 —CHCH -700—C—N + HN,CH2CH3‘CH2CH3610/\CH2 —CHCH 0/ CH 2 CH 30-C-NII^\NH CH2ru"[4.4] 71excess monomer 61CH 3CH2,N,CH2CH3From these experiments, it can be concluded that the epoxy functionalgroup in the cross-linking monomer 61 can not be cured independently withoutaffecting the cyanate functional group. However, it should be emphasized thateven though the epoxy group could not be cured independently, monomer 61 stillcan be used as a cross-linking agent. It should be possible to use it in a mixedcyanate and epoxy resin system either by first curing monomer 61 with the162cyanate resin under mild heating and then curing it with the epoxy resin byaddition of base, or by curing all of them together under the action of both heatand base curing agent.4.1.6. A More Practical Approach to the Application of the Cross-LinkingMonomer, 61As seen previously, the most difficult part in the synthesis of the cross-linking monomer 61 is to obtain the pure intermediate compound 62, andconsiderable effort was expended for this purpose. Even though it wassuccessfully obtained in pure form, its yield is perhaps too low (onlyapproximately 20 %) to be used in practical applications. In addition, thechromatographic separation is not an efficient process for economic reasons.Fortunately, these procedures are necessary only for characterization purposes;it is not necessary to get the purified intermediate compound 62 in a realsynthetic procedure for production of final mixed resins.From the results obtained, the major side products in the synthetic step{1} presented in Scheme 4.1 are the overreacted the cliglycidyl ether ofbisphenol-A (17 or 18) and unreacted bisphenol A (3). The first one(17 or 18) is actually an epoxy resin monomer or precursor, one of thecomponents in the mixed cyanate/epoxy resin system. The second one (3) wouldbe transformed in the next synthetic step {2} to bisphenol-A dicyanate 2, whichis also one of the components in the mixed resin system. Therefore, withoutseparation after step {1}, the final product after step {2} will be a mixture of thecross-linking monomer 61, the cyanate resin and the epoxy resin, which is justthe desired mixed resin system. Thus, the final major side products arecompatible with the real application system and do not affect the application ofthe cross-linking monomer 61 in any way.163To check these conclusions, a real mixture of the cross-linking monomer61, the cyanate resin and the epoxy resin was synthesized and tested. A mixtureof bisphenol-A, epichlorohydrin, and potassium carbonate with 1:1:1 molar ratiowas reacted at 80 °C for 90 minutes. The NMR spectra show the crudeintermediate product to be a mixture of three major components: bisphenol-A 3,the monoglycidyl ether of bisphenol-A 62, and the diglycidyl ether of bisphenol-A 17 plus some high molecular weight species. The 13C spectrum also gives theapproximate percentages of each component in this intermediate mixture: about28% for bisphenol A, 38% for the monoglycidyl ether of bisphenol-A and 34% forthe diglycidyl ether of bisphenol-A and some high molecular weight species.From these data, the amounts of cyanogen bromide and triethylamine base foradding the cyanate functional groups into the intermediate mixture werecalculated. The details of the preparation procedure for both synthetic steps aregiven in the Experimental chapter.NMR spectra of the final product mixture containing the cross-linkingmonomer 61 were obtained (Figure 4.7). Particularly importantly, the 13Cspectrum (Figure 4.7A) not only shows that the cyanate functional groups areformed, but also that there are two main kinds of cyanate group in the finalcross-linking product mixture, as reflected in the two very similar peaks at thecyanate region (-109 ppm). In comparison with the cyanate spectra obtainedbefore, the one at higher field can be assigned to bisphenol-A dicyanate 2 andthe one at lower field can be assigned to the monoglycidyl ether of bisphenol-A-monocyanate 61. On careful examination, one or more additional small cyanatepeaks are also found nearby at lower field, which are perhaps due to somehigher molecular weight species. The 1H spectrum (Figure 4.7B) shows that thepeak at — 8.2 ppm, which belongs to the hydroxyl groups on a phenol ring,almost completely disappears. This means that all of the hydroxyl groups on the164Figure 4.7. NMR spectrum (1H at 200 MHz) in acetone-d6 of thecyanate product mixture obtained from theintermediate mixture without separation. (A). 13Cspectrum; (B). 1H spectrum.165ANCO OCN2ba NCOb OCH2CH-CH,\ i^..o61 IIIIIIIIIIIIIIIIII110^109I.a...............LIL......1J^....1._..I.A.^1 160^120^80^40 ppmr8.0 6.0 4.0 2.0 Ppm165-1different phenols have been converted to cyanate groups, and thus thispreparative procedure is very efficient for practical applications of the mixedresin system.This final product mixture containing the cross-linking monomer 61 wascured using diethylamine as a curing agent at room temperature for one day.The results are similar to that of a relative pure monomer 61 discussed above.The final product of the curing reaction also depends on the molar ratio of themonomer 61 to diethylamine. 13C NMR spectra (not shown) also show that thecyanate functional groups react faster than the epoxy groups as before since allof the cyanate peaks at — 109 ppm have disappeared while the majority of thethree high field peaks for the epoxy groups are still left.A very important goal for use of this mixed product system is to get atough and strong cured mixed resin material. The product mixture containingthe cross-linking monomer 61 was cured at 180 °C for 3 hours. A very tough andstrong resin material was obtained, which is much tougher and stronger thanthe cured resin obtained by curing the cyanate resin and the epoxy resintogether as described in Chapter 3. Further investigations of the preparation ona large and more practical scale under industrial conditions are warranted.4.1.7. Conclusions The desired cross-linking monomer for a mixed cyanate resin and epoxyresin system, the monoglycidyl ether of bisphenol-A-monocyanate 61, has beensynthesized and characterized. The intermediate compound, the monoglycidylether of bisphenol-A 62, was also synthesized and purified by extraction andchromatographic separation using a silica gel column. The cyanate functionalgroup in the cross-linking monomer 61 can be cured independently by heat toform the triazine structure 68, but the epoxy functional group in the cross-166linking monomer 61 can not be cured independently of the cyanate groupbecause the latter is more reactive than the epoxy group under both heat andbasic conditions. By using a secondary amine, diethylamine, as a curing agent,the cyanate groups in the cross-linking monomer 61 react with diethylamine toform the types of structure 71 or 72, depending on the molar ratio of monomer61 to diethylamine. A more practical approach for the application of the cross-linking monomer 61 has been discussed and tested. Most interestingly, underheat curing, a very tough and strong resin material was produced from thiscross-linking mixed resin mixture.4.2. A Cross-Linking Agent for Mixed Cyanate / Olefin Resin SystemsAn interpenetrating polymer network (IPN) is a particular kind ofmulticomponent polymer system. The original definition of INP requires bothchemical species to be self cross-linked.[99] However, if one component is alinear thermoplastic polymer while the other is cross-linked thermoset polymerthen the final system will be only partially, but selectively cross-linked, and isreferred to as a Semi Interpenetrating Polymer Network (SIPN)[16].The SIPN system is used to combine the advantages of both thethermoplastics and the thermosets. Thermoplastics are the dominant polymersin low temperature engineering applications because they are tough and easy toprocess. However, most thermoplastics have relative low Tg values, and will losetheir hardness and mechanical strength at high temperatures. In addition, mostthermoplastics must be heated to 150 °C or more above their highest end usetemperature before their viscosity is low enough to allow for processing, whilemost organic molecules decompose at a significant rate at 350 °C or above.Therefore, very few thermoplastics have end use temperatures of 200 °C orabove.167Unlike thermoplastics, thermosets are cross-linked polymers and usuallyhave high Tg values. They can be processed at temperatures not far above theirhighest use temperature. Thus, the end use temperature of a thermoset can beclose to its decomposition temperature. However, high temperature thermosetsare generally brittle and thus lack the toughness of the thermoplastics.The concept behind SIPN is again to obtain the most attractive features ofboth materials, the toughness and the high end use temperature[l 6]• In a SIPNsystem, the linear polymer is not formally cross-linked, but it will be highlyentangled. The final material should be less rigid than a full IPN when usedabove the glass transition temperature of the linear polymer, but it should betougher since the linear molecules may be free to move.As a result of SIPN technology, materials, such as cyanate resins,conventionally regarded as being too brittle for certain applications, can now beused. Dicyanate Semi Interpenetrating Polymer Networks (SIPNs) are veryuseful as matrix materials.[16,29,34] They can be made by dissolving athermoplastic in cross-linking dicyanates and then curing the resulting mixture.The SIPNs produced in this way could be very strong, with tensile strengths of10,000 to 12,000 psi, and flexible, with elongations to break of 10 to 17 percent.Dicyanate SIPNs also have good thermal stability. The softening temperaturesof the SIPNs are significantly higher than those of the correspondingthermoplastics. [34]However, there is no cross reaction between the cyanate resin and thethermoplastics, such as polyolefins, in the dicyanate SIPNs. It was thought thatthe addition of a certain amount of cross-linking agent, which could link the twopolymers together, should make the SIPNs even stronger and tougher. Thus, thepurpose of this portion of the research was to design and synthesize a cross-168linking agent for mixed cyanate and olefin resin systems as a complement to theSIPN technique.4.2.1. Synthesis and Characterization of the Cross-Linking Monomer 76 For the cyanate resin and olefin resin the cross-linking agent shouldcontain both cyanate —OCN and olefinic ^CC^ groups. This can beachieved by reaction of cyanogen bromide BrCN with an appropriate phenolwith an attached olefinic group.2-allylphenol 75 was chosen for use in this project as it is easily producedby a reaction of phenol and allyl chloride (or 3-chloro-1 -propene, 73) with astrong base, followed by Claisen rearrangement as shown in Equation 4.5,[103]and it is inexpensive and commercially available. OHCH2=CHCH2CI +73NaOH74200 °C^[4.5]OHCH 2C H ----CH 275The target monomer 2-allylphenyl cyanate 76 was obtained by reaction of2-allylphenol 75 and cyanogen bromide in the presence of a base, triethylamine,at low temperature (molar ratio 1:1:1) as shown in Equation 4.6. The details ofthe preparative procedure will be given in the Experimental chapter.169OHCH2CH=CH2+ BrCN + (CH3CH2)3N75< 0 °COCNCH2CH=CH276(CH3CH2)3NHBr^[4.6]The final product is a colorless liquid which can be purified by distillationunder reduced pressure (1.4 mmHg) at 55 °C. The mass spectrum shows theparent peak at M+ = 159 a.m.u.. Its NMR spectra are given in Figure 4.8together with the complete assignments. The —OCN group gives a 13C resonanceat 109 ppm which is consistent with the other cyanates previously studied.4.2.2. Self Curing Reaction of the Cross-Linking Monomer 76 On heating 2-allylphenyl cyanate at 180 °C for 2 hours, a solid compoundis produced. This solid product was purified by recrystallization from acetone,and had m.p. = 110 — 111 °C. Its mass spectrum gives M+ = 477 a.m.u.. TheNMR spectra are shown in Figure 4.9. The 13C NMR spectrum shows aresonance at 174 ppm which is a characteristic of triazine ring carbons.Therefore, both MS and NMR experiments indicate the solid product obtainedfrom curing 2-allylphenyl cyanate 76 is 1,3,5-tri(2-allylphenoxy)-2,4,6-triazine77. This is consistent with the curing reactions of the other cyanates discussedpreviously and can be represented as in Equation 4.7.170Figure 4.8. NMR spectra (1H at 200 MHz) of 2-allylphenylcyanate 76 in acetone-d6. (A). 13C spectrum; (B). 1Hspectrum.17111 I 1410 ppm'810,7.0 5.0•^I^'^•^•^•^I I3.0 Ppm54 9  63 7A10OCN 7 8 96 (.....\ 2 CH2CH=CH2x .._.)'^3 76482109.0B8/Figure 4.9. NMR spectra (1 H at 200 MHz) of 1,3,5-tri(2-allylphenoxy)-2,4,6-triazine 77 in acetone-d6. (A). 13Cspectrum; (B). 1 H spectrum.172TrI•1171,111■VTT^T/T7711-TIVITY^vlilir7111,1•8.0^6.0 4.0 Ppm793 54 69 87CH2=CHCH28A110160^10^80^Ppm79r-8•■•••CH2CH =CH276CH2CH=CH20 N 0YOYN^NY0C H 2 C II = C H 2 [4.7]OCN3 ACH2=CHCH2 774.2.3. Curing Reaction of the Cross-Linking Monomer 76 with a Cyanate ResinAs a cross-linking agent, 2-allylphenyl cyanate 76 should not only reactwith itself. In order to link the cyanate resin, it must react with the cyanategroup of the cyanate resin on which the cross-linking monomer 76 is desired tobe anchored. This is a concern because the structure of the cross-linkingmonomer 76 is quite different from the cyanates studied previously which haveno ortho substituents.To test this, a mixture of 2-allylphenyl cyanate 76 and p-tert-butylphenylcyanate (PTBPCN, 25) was cured at 180 °C for 3 hours. As previously, thePTBPCN 25 was used as a model compound of the real cyanate resin to ensurethat the final products were soluble and could be characterized by highresolution solution NMR spectroscopy.The 13C NMR spectrum (Figure 4.10A) of the final product shows thetriazine ring resonance (— 174 ppm) as a group of several peaks, indicating thatit is a mixture of several products with different triazine rings. Both El and CIMS experiments also confirm the NMR result and detect different triazineproducts with parent peaks at M+ = 477, M+ = 493, M+ = 509, and M+ = 525173a.m.u. respectively. From the MS experiments the approximate percentages ofeach of the components are as follows:M+ = 477 13%M+ = 493 36%M= 509 40%M+ = 525 11%In order to obtain the 15N NMR spectrum for the final products, 12% of15N enriched p-tert-butylphenyl cyanate (PTBPCN) was reacted with 2-allylphenyl cyanate 76. The 15N NMR spectrum (Figure 4.10B) also shows fourpeaks in the range of triazine ring nitrogen at — 87 ppm. Since only nitrogensderived from the enriched PTBPCN can give signals in the 15N NMR spectrum,the 15N spectrum should be cleaner and more easily interpretable than the 13Cspectrum. The 13C spectrum gives all of the signals for all different carbons ondifferent triazine rings. Some of them are degenerate and not distinguishable asseen in Figure 4.10A. A total of six carbon signals for the four different triazinesshould be obtained, but only four resolved signals are observed. Therefore,quantitative data can not be obtained from the 13C spectrum.As expected from Equation 4.8, there are only four different kinds ofenriched nitrogens on three different triazines (31, 78, 79,) which should give15N signals. This is exactly what is seen in the 15N spectrum in Figure 4.10B.All four signals are well resolved and distinguishable. Thus, quantitative datacan be obtained from the 15N spectrum. From the 15N resonance in the 15Nspectrum of triazine 31 obtained in Chapter 2, the peak at lowest field in Figure4.10B corresponds to this compound. The peak at highest field in Figure 4.10B isthought to correspond to the enriched nitrogen on the triazine ring in 78. Thetwo close peaks of equal intensity in the middle represent the two differentenriched nitrogen positions on triazine 79. From 15N NMR spectrum, the molar174Figure 4.10. NMR spectrum in acetone-d6 of the product mixtureobtained from curing a mixture of cross-linkingmonomer 76 and 12% 15N enriched PTBPCN 25. (A).13C spectrum (1 H at 200 MHz); (B). 15N spectrum0-H at 300 MHz).175CH2CH=CHe0 NOYOYNyN`d^780arCH2CH=CH2a89.0^88.0^87.0^86.0^ppmA175.0^174.5I'160^120^80^40 Ppm175-1N* 0YOY*N^N*Y031CH2CH=CH278OCNCH2CH=CH276+CH2CH=CH20 N 0Y1(5rN^N*-.'1"-.079CH2CH=C H277C H2=CHCH20 N* 0YO'rN^N*-r0CH2CH=CH20 N 0YOYNN,r0CH2CH=C H2ratios of triazines 78: 79 : 31 are estimated to be approximately 1 : 3 : 3. This isconsistent with the previous results obtained from the MS experiments.OCN25[4.8]There are three different units on each triazine ring. Each unit is fromone cyanate group. If 2-allylphenyl cyanate 76 is designated to be A andPTBPCN 25 is designated to be B, the combinations for formation of a triazinering should have eight different ways:176AAB^ABBAAA^ABA^BAB^BBBBAA BBAThe combinations of AAA and BBB form triazines 77 and 31 respectively. Allcombinations in the second column form triazine 78 and all combinations in thethird column form triazine 79. If each of the combinations have equalprobability of formation, the ratios of the four different triazines 77 : 78 : 79 : 33will be 1 : 3 : 3 : 1. This fits perfectly the results from both the NMR and the MSexperiments.Thus, it has been shown that the copolymerization between the cross-linking monomer 76 and the cyanate 25 is an ideally random case. There is nopreference at all between them for triazine ring formation. This is an idealproperty for a cross-linking agent.4.2.4. Copolymerization of the Cross-Linking Monomer 76 with an OlefinicMonomerA preliminary investigation of the copolymerization of the cross-linkingmonomer 76 with an olefinic monomer was carried out. Firstly, the possiblepolymerization for the cross-linking monomer 76 alone under free radicalinitiation was tested to see whether it can be self polymerized through its ally'double bond without affecting the cyanate group. 1 g of cross-linking monomer76 was mixed with 1 % benzoyl peroxide (BPO) and heated to 95 °C for one day.The 13C INTMR spectrum (not shown) shows that all of the monomer 76 hastrimerized to form 77 without reaction of the allyl double bond. Subsequently, amuch lower temperature (45 °C) was used to polymerize the monomer 76 with 1% azobisisobutyronitrile (AIBN) as initiator. This polymerization process wascontinued for ten days. The 13C NMR spectrum (not shown) still shows that all177of the monomer 76 has been converted to 77, but there is no any indication ofreaction of the allyl double bond. These results suggest that the cross-linkingmonomer 76 can not be self polymerized through the allyl double bond withoutaffecting the cyanate functional group (similar to the mixed cyanate/ epoxysystem).The polymerization of triazine 77 alone was also tested at 120 °C for 5days using 1 % BP0 as initiator. There is still no allyl double bond opening.However, it can be copolymerized with other olefinic monomer. This was verifiedby copolymerization of 0.2 g triazine 77 with 0.8 g methyl methacrylate (MMA)at 95 °C for one day using 1 % BP0 as initiator. The final polymer product wasextracted by boiling benzene for 1 day using a soxhlet extractor. More than 60 %of the polymer product was left after the extraction. When poly(methylmethacrylate) (PMMA) is formed under the exactly same condition as above,nothing was left after extracting for only 16 hours. The difference between theseindicates the final polymer product in the first case is cross-linked by the cross-linking agent 77 and therefore is not soluble in benzene. Further investigationsof this reaction should be carried out in future work.4.2.5. Conclusions As a complementary approach to the SIPN multicomponent polymersystem, a bifunctional cross-linking agent for the cyanate resin (thermoset) andpolyolefine (thermoplastic) mixed system, 2-allylphenyl cyanate 76, has beensynthesized and characterized. Like the other cyanates as previously described,2-allylphenyl cyanate 76 easily forms the cross-linking triazine compound 77upon heating. 77 is a crystalline solid with m.p. = 110 — 111 °C. As a cross-linking agent, 2-allylphenyl cyanate 76 not only reacts with itself, but alsoreacts with another cyanate to form heterogeneous triazine rings, such as178triazines 78 and 79. Even though it can not polymerize with its own monomerthrough the allyl double bond, it can copolymerize with an other olefinicmonomer, such as methyl methacrylate, to form a cross-linked and insolublepolymer.179CHAPTER 5.EXPERIMENTAL5.1. High Resolution NMR ExperimentsIn this section, the general conditions employed for the NMR experimentsare described. The detailed conditions of specific experiments and the proceduresused to prepare the NMR samples have been given in earlier sections.5.1.1. Solution NMR Experiments Conventional 1H, 13C, and 15N solution NMR spectra were obtainedusing Bruker ACE 200 and Varian XL-300 spectrometers. The variabletemperature experiments were carried out on the Varian XL-300 spectrometer.The NOE difference NMR spectra and 1H COSY 2D NMR spectra were obtainedon Bruker WH-400 and ANIX-500 spectrometers. 1H-13C heteronuclearchemical shift correlation (HETCOR) 2D NMR spectra were obtained using aBruker AMX-500 spectrometer with an inverse detection pulse sequence. Allsolution NMR spectra were obtained using 5 mm tubes. Deuterated solventsused were from Cambridge Isotope Laboratories. 13C and 1H chemical shifts aregiven with respect to TMS and the 15N chemical shifts are given with respect toneat formamide.5.1.2. Solid State NMR Experiments 13C and 15N CP MAS solid-state NMR spectra were obtained usingBruker CXP-100 and MSL-400 spectrometers with commercial doubleresonance probes and with the magic angle set using the 79Br resonance ofKBrE1041. All solid state NMR spectra were obtained using 7 mm od samplerotors. The 13C and 15N chemical shifts of the solid state NMR spectra are18031given with respect to TMS and neat formamide, using adamantane and15NH4C1 as the intermediate external references, respectively.5.2. Mass Spectrometry ExperimentsElectron impact mass spectra (EIMS) and desorption chemical ionizationmass spectra (DCIMS) were obtained using Kratos MS 50 (70 eV) andDelsiNermag R10-10B mass spectrometers. The ionizing gas in the latterexperiments was NH3.5.3. X-ray Diffraction ExperimentsAll measurements were made on a Rigaku AFC6S cliffractometer withgraphite monochromated CuKoc radiation.5.3.1. Triazine 3 1Crystallographic data appear in Table I in Appendix A. Final atomiccoordinates, bond lengths and bond angles are given in Tables II—IV inAppendix A, respectively.1815.3.2. Bisphenol-A Dicyanate 2NCO OCN2Crystallographic data appear in Table V in Appendix B. Final atomiccoordinates, bond lengths and bond angles are given in Tables VI—VIII inAppendix B, respectively.5.3.3. 5-Phenoxymethy1-2-oxazolidinone 53NH\53^0Crystallographic data appear in Table IX in Appendix C. Final atomiccoordinates, bond lengths and bond angles are given in Tables X—XII inAppendix C, respectively.5.4. SynthesesAll chemical reagents used in the syntheses were supplied by AldrichChemical Co.and BDH unless otherwise indicated.5.4.1. Labelled Cyanogen Bromide BrC*N (Br13CN or BrC15N)[91]0.50 ml Br2 (0.01 mol) and 0.5 ml H20 were added to a 25 ml r.b. flaskfitted with a magnetic stirring bar, sitting in a salted ice-water bath in a fumehood. To the stirred mixture, a solution of 0.65 g (0.01 mol) labelled KC*N(K13CN with 99% 13C or KC15N with 99% 15N) dissolved in 2 ml H20 wasadded dropwise from a pipet over a 20 minute period [Note: The rate of adding0-CH2182KCN and the speed of stirring should be controlled properly to avoid KC*Nbeing in excess in any local portion of the solution. Otherwise, the reactionmixture will turn to black, which is probably caused by the formation of (CN)x].The correct color change should be from dark-red to fresh-red to orange to yellowto colorless. The exact amount of KC*N solution, which is needed to titrate thesystem just to a colorless or light yellow end point, was added. The purificationof the formed cyanogen bromide can be performed by distillation immediatelyafter reaching the end point.. The distillation was conducted at roomtemperature under vacuum with a dry ice acetone trap as product collector. Thefinal BrC*N product is a colorless crystalline solid with m.p. = 48 — 50 °C(reported m.p. = 49 —51 °C for BrCN[91]), yield = 70 — 75 %.5.4.2. Labelled Bisphenol-A Dicyanate (BPADCN 2a or 2b)[6a,92]N1 3 CO 01 3 C N^1 5 NCO 0C1 5N2a 2b1.10 g (0.0105 ml) of BrC*N (Brl 3CN or BrC1 5N) and 1.14 g (0.005 mol)of Bisphenol A previously dissolved together in 10 ml of acetone were added to a50 ml r.b. flask fitted with an equalizing pressure dropping funnel and amagnetic stirring bar, sitting in a salted ice-water bath in a fume hood. Themixture was stirred rapidly with cooling in the salted ice-water bath while 1.01g (0.01 mol) of (CH3CH2)3N was added dropwise over a 20 minute periodthrough the dropping funnel. A white solid, (CH3CH2)3NHBr, appeared afteradding (CH3CH2)3N. Stirring was continued for 30 minutes while the mixturewarmed to room temperature. The product was isolated by slowly pouring themixture into 50 ml of ice-cooled water with vigorous stirring. In this step, the183(CH3CH2)3NHBr solid dissolved and the crude cyanate product precipitated.The precipitate was then isolated by filtration and washed with water until aneutral eluate was obtained. After vacuum drying, the crude cyanate productwas recrystallized from cyclohexane, m.p. = 80 — 81 °C (80 °C in lit .[61), yield =80— 85%. The 13C and 15N NMR spectra are shown in Figure 2.1.5.4.3. Labelled p-tert-Butylphenyl Cyanate (PTBPCN 25a or 25b)[93}013 ON 0C15N25a 25bTo a 50 ml r.b. flask equipped with a magnetic stirrer and a pressureequalized dropping funnel, sitting in a salted ice-water bath in a fume hood, asolution of 1.10 g (0.0105 mol) of BrC*N (Br13CN or BrC15N) and 1.5 g (0.01mol) of p-tert-butylphenol in 10 ml acetone was added. The mixture was stirredrapidly and 1.01 g ( 1.39 ml, 0.01 mol) of (CH3CH2)3N was added dropwise overa 20 minute period through a dropping funnel. (CH3CH2)3NHBr appeared as awhite solid during the addition of (CH3CH2)3N. After an additional 15 minutesof stirring, the mixture was warmed up to room temperature. The whiteprecipitate of (CH3CH2)3NHBr was removed by filtration and the solvent thenremoved by evaporation using a rotary evaporator under reduced pressure atroom temperature. The final mixture was distilled under a vacuum of 0.5 mmHgat 75 °C. The final product is a colorless liquid, yield 76 — 87%. The 13C and 15NNMR spectra are shown in Figures 2.2 and 2.3.184315.4.4. Triazine 31 Formed from p-tert-Butylphenyl CyanateThe triazine was prepared by heating a sample of p-tert-butylphenylcyanate in a sealed tube for two hours at 150 °C. When the sample turned solid,it was recrystallized from acetone, yielding needle shaped colorless crystals, m.p.= 193.5 °C (Cal. for C33H39N303: C, 75.4; H, 7.5; N, 8.0 %. Found: C, 75.4; H,7.5; N, 7.9 %). The 1H and 13C NMR spectra in deuterated chloroform andacetone (Figure 2.10) were in complete agreement with the postulated structure,as is the MS spectrum showing the parent ion M+ = 525 a.m.u.A small quantity of 15N labelled material was also synthesized andrecrystallized from acetone. The 15N spectrum (Figure 2.10) was again inagreement with the proposed structure and showed no coupling to protons asexpected.5.4.5. 2,6-Dimethy1-4-phenoxycarbonylmorpholine 46CH3^/ ^<0-C-N^0^O ` ^<cH346185A solution of phenyl chloroformate (0.02 mole, 3.13 g) in 10 ml of driedtoluene was added dropwise with stirring, through a dropping funnel equippedwith a drying tube, into a mixture of 2,6-dimethylmorpholine (0.02 mole, 2.30 g)and pyridine (0.02 mole, 1.58 g), which had been pre-cooled in an ice bath. Thesolid formed during the reaction was removed by filtration. The toluene phasewas decolorized by decolorizing neutral carbon, (Norit, Fisher ScientificCompany) and was dried over MgSO4. The toluene solvent was then removed byevaporation. The crude liquid product (3.84 g, yield 81 %) solidified after sittingat room temperature overnight. The pure product was obtained byrecrystallization from acetone as colorless crystals (mp. 68 - 73 °C). It is amixture of cis and trans isomers (Cal. for C131-117NO3: C, 66.4; H, 7.3; N, 6.0 %.Found: C, 66.5; H, 7.3; N, 6.0 %). Two isomers were separated bychromatography on a silica gel (230-400 mesh, BDH No9385-48) column (2.5 cmx 17 cm) using eluants with gradually increasing polarity from 100% pentane to100% diethyl ether. The cis isomer (mp. 77.5 — 78.2 °C) and the trans isomer(mp. 87.5 — 88.0 °C) show different NMR spectra (Figure 3.7).5.4.6. 5-Phenoxymethy1-2-oxazolidinone 53^0—CH2 0-453^oPhenyl carbamate (0.01 mole, 1.37 g) and phenyl glycidyl ether (0.02mole, 3.00 g) were mixed in a r.b. flask equipped with a condenser. The mixturewas heated with stirring at 180 °C for 3.5 hours. Some white solid appearedafter all the phenyl carbamate had dissolved at the beginning of the reactionand disappeared at the end of the reaction. (This white solid can be separated186from the reaction mixture at this stage by filtration. It was found to be cyanuricacid. Further details are given in Chapter 4.) The final brown sticky mixturewas dissolved in diethyl ether leaving some residual solids. The solid wasfiltered and washed with Et20, and then recrystallized from acetone. It is acolorless crystalline material, 12% yield, mp. 120 —121 °C (Cal. for Ci °Hi iNO3:C, 62.2; H, 5.7; N, 7.3 %. Found: C, 62.4; H, 5.9; N, 7.2 %).5.4.7. Bisphenol-A Monoglycidyl Ether 62HO 0 C H2 C ---/C H262Bisphenol A (0.02 mole, 4.56 g), anhydrous potassium carbonate (0.02mole, 2.76 g) and 10 ml of methyl ethyl ketone (MEK) were mixed in a r.b. flaskequipped with a condenser and a magnetic stirrer. The mixture was heated and3 ml of H20 was added to dissolve the solid producing a clear solution. Then, 1 -chloro-2,3-epoxypropane ( 0.02 mole, 1.85 g) was then added dropwise to themixture with stirring through a dropping funnel. The reaction mixture washeated to reflux for 150 minutes with continuous stirring. Stirring wascontinued for another 45 minutes while the mixture cooled down to roomtemperature. The white precipitate formed in the reaction was removed byfiltration. The clear product mixture was extracted by a mixture of 30 ml ofdichloromethane and 20 ml of H20. After extraction and separation, theaqueous phase was washed twice with 10 ml of dichloromethane. The combinedorganic phases were washed twice with 15 ml of 1 M sodium hydroxide solutionfollowed by four 25 ml portions of water until neutrality was obtained. Theorganic phase was dried overnight over anhydrous magnesium sulfate. After187filtration and evaporation, a light yellow viscous liquid product (3.50 g, 55 %crude yield) was obtained.Purification of the crude product was carried out by extraction andchromatography. The crude product mixture was extracted with the followingsolvents (the weight and yield of each extraction are given in parenthesesrespectively): (i) 25 ml n-pentane (36.7 mg, 2.0 %); (ii) 20 ml n-pentane (10.6 mg,0.6 %); (iii) 30 ml 50 % n-pentane/ 50 % diethyl ether (1.01g, 60.3 %); (iv) 20 ml50 % n-pentane/ 50 % diethyl ether (370 mg, 20.3 %); (v) 25 ml diethyl ether(307.7 mg, 16.7 %). Further purification was done by adsorptionchromatography on silica gel (230 — 400 mesh, MERCK 9385) column (2.5 x 17.8cm) using 50 % n-pentane/ 50 % diethyl ether as an eluant. 96.3 mg of purecrystalline product were obtained from a loading of 216.6 mg of sample onto thecolumn, which was from the extraction fraction by 30 ml 50 % n-pentane/ 50 %diethyl ether above. This pure crystalline product was obtained in 44.5 % yieldof the loading sample and in 18.9 % overall yield, m.p. = 86 — 88 °C (Cal. forC181-12003: C, 76.0; H, 7.1; 0, 16.9 %. Found: C, 75.9; H, 7.0 %). The structure ofthis product was completely characterized as 62 by NMR (see Figure 4.2) andMS experiments (parent ion M+ = 284 a.m.u.).5.4.8. The Monoglycid_v1 Ether of Bisphenol-A-Monocyanate 61NCO OC H 2 C H —ICH 2o61A 50 ml r.b. flask fitted with an equalizing pressure dropping funnel andmagnetic stirring bar, sitting in a salted ice-water bath in a fume hood, wascharged with a solution of 0.60 g (0.0057 mole) of BrCN and 1.42 g (0.0050 mole)188OCH2C1-\1-/ C I-12o68of bisphenol A monoglycidyl ether in 10 ml acetone. The mixture was stirredrapidly with cooling in the bath while 0.70 ml (0.0050 mole) of (CH3CH2)3N wasadded dropwise over a 20 minute period through the dropping funnel.(CH3CH2)3NHBr appeared as a white solid after adding (CH3CH2)3N. Stirringwas continued for 15 minutes while the mixture warmed to room temperature.The solid was removed by filtration and then by extraction with mixture of 20 mlof benzene and 20 ml of H20. The organic phase was collected, washed with twoportions of 20 ml of H20 and dried over anhydrous MgSO4 for 2 hours. Afterfiltration and evaporation of the solvent under vacuum, a clear light yellowviscous liquid product (1.36g, 88 % yield) was obtained. It was characterized byNMR (Figure 4.3) and MS experiment (parent ion M+=309 a.m.u.) as thecyanate 61 with minor impurities (Cal. for C191119NO3: C, 73.7; H, 6.2; N, 4.5%. Found: C, 73.2; H, 6.3; N, 4.2 %).5.4.9. Triazine 68 Formed from the Monoglycidyl Ether of Bisphenol-A-Monocyanate 611892.0 g of the monoglycidyl ether of bisphenol-A-monocyanate 61 werecharged into a r.b. flask and heated at 180 °C in vacuum (1.5 mmHg) for 15minutes. A bright brown color cured solid was obtained. It was characterized byNMR (Figure 4.4) and MS (parent ion M+.927 a.m.u.) as the triazine product 68with minor impurities (Cal. for C57H57N309: C, 73.7; H, 6.2; N, 4.5 %. Found:C, 72.8; H, 6.2; N, 4.2 %).5.4.10. A More Practical Way to Synthesize the Cross-Linking Monomer 61A mixture of bisphenol-A (0.05 mole) and potassium carbonate (0.05 mole)in 20 ml MEK and 6 ml H20 was heated to boiling for 10 minutes and all of thesolid was dissolved. Then, 0.05 mole of epichlorohydrin was added to themixture through a dropping funnel at a rate of 0.5 ml per minutes. After that,the mixture was kept refluxing and stirring for 90 minutes (at about 80 °C), andthen cooled down to the room temperature with continuous stirring. The crudeproduct mixture was extracted twice with 20 ml H20 and the dried over Mg504overnight. After the drying agent was removed by filtration, the solvents wereremoved through a rotavapor at 80 °C under reduced pressure for 10 minutes.The '3C NMR spectrum of the product mixture shows that themonoglycidyl ether of bisphenol-A is about 32%, the bisphenol-A is about 36%,and the diglycidyl ether species are approximately 32%. From these estimateddata, approximate 10% of excess amounts of cyanogen bromide andtriethylamine were used to convert phenol moieties to cyanate groups.3.963 g of the product mixture obtained in the previous step and 2.242 gof cyanogen bromide were dissolved together in 20 ml acetone and sat in an ice-water-bath with stirring. 1.800 g of triethylamine was slowly added in through adropping funnel in a 30 minute period. The white precipitate produced in thereaction was removed by filtration and then by extraction with 30 ml H20. The190crude products were dissolved in 30 ml acetone and dried over MgSO4overnight. After filtration, the solvents were removed by a rotavapor at 70 °C forless than 5 minutes under reduced pressure. The final product mixture wasobtained as a viscous liquid. The NMR spectrum shows that it contains threemajor products: bisphenol-A dicyanate 2, the diglycidyl ether of bisphenol-A 17,and the monoglycidyl ether of bisphenol-A-monocyanate 61, which still can beused as the cross-linking agent for the cyanate and the epoxy mixed resins.5.4.11. 2-Allylphenyl Cyanate 76OCNCH2CH=CH276To a 50 ml r.b. flask equipped with a magnetic stirrer and a pressureequalized dropping funnel, sitting in a salted ice-water bath in a fume hood, asolution of 3.15 g (0.0300 mole) of BrCN and 3.5 g (0.0261 mole) of 2-allylphenolin 15 ml acetone was added. The mixture was stirred rapidly with cooling in thebath while 2.64 g (3.64 ml, 0.0261 mole) of (CH3CH2)3N was added through adropping funnel over a 20 minute period. (CH3CH2)3NHBr appeared as whitesolid while adding (CH3CH2)3N. After an additional 30 minutes of stirring, themixture was warmed up to room temperature. The (CH3CH2)3NHBr wasremoved by filtration and the solvent was then removed using a rotaryevaporator under reduced pressure at room temperature. The final mixture waspurified by distillation under a vacuum of 1.5 mmHg at 70 °C. The final productis a colorless liquid, yield 85 %. The NMR spectra are shown in Figure 4.8. TheMS experiment gives the parent ion at M+.159 a.m.u.. (Cal. for Ci 0119NO: C,75.5; H, 5.7; N, 8.8 %. Found: C, 75.6; H, 5.8; N, 8.7 %).191CH2CH=CH20CH2CH=CH2CH2=CHCH2 775.4.12. Triazine 77 Formed from 2-Allylphenyl CyanateThe liquid sample of 2-allylphenyl cyanate 76 changed to a whitecrystalline solid after sitting at room temperature for two months. (Thistrimerization process can be shortened to 2 hours if the sample is heated to 150°C.) It was recrystallized from acetone yielding long needle shaped crystals, m.p.= 110 —111 °C (Cal. for C301127N303: C, 75.5; H, 5.7; N, 8.8 %. Found: C, 75.6;H, 5.8; N, 8.7 %). The 1H and 13C NMR spectra in deuterated chloroform andacetone are in complete agreement with the postulated structure (Figure 4.9).MS spectrum shows the parent ion at M+ = 477 a.m.u..5.5. Chromatographic Separation of the Reaction ProductsThin-layer chromatographic (TLC) separations were done on commercialaluminum-backed silica gel plates (E. Merck, Type 5554). Preparative Thin-layer chromatography was done on 20 cm x 20 cm plates coated with 2 mm ofsilica gel (E. Merck, Silica Gel 60). Visualization was accomplished withultraviolet light. Adsorption chromatographic separation was done using a silicagel (230-400 mesh, BDH No9385-48) column (2.5 cm x 17 cm) and using eluantswith gradually increasing polarity from 100% pentane to 100% diethyl ether.Preparative column chromatography was done on a silica gel (230 — 400 mesh,192BDH No9385-48) column (7 cm x 17 cm). Gel permeation chromatography wascarried out using a Lipophilic Sephadex LH-20 (Sigma) column (2.0 cm x 55 cm)and acetone as eluant.193CHAPTER 6.PROPOSALS FOR FUTURE WORKThe cyanate resin and its related heterogeneous polymer systems arenewly developed materials and have many potentially useful features. From theresults in the thesis, it is thought some work should be continued in the future.(1) Application of the Cross-Linking Monomer 61As described above, one of the major applications for cyanate resin is formaking electronic circuit boards mixed with epoxy resin. The monomer 61 canbe used as a cross-linking agent for this mixed resin system. Particularlyimportantly as shown in Chapter 4, the cross-linking agent 61 can be obtainedin a very straightforward manner which is compatible with the normal mixedresin composition.The effect on the properties of the end product by using the cross-linkingmonomer 61 should be further studied on a large scale and in a boardfabrication. The physical, mechanical, thermal and electrical properties of themixed resin obtained from the practical approach as described in Section 4.1.6should be tested under real application conditions. This should be done incollaboration with some industrial agency.(2) Application of the Cross-Linking Monomer 76As discussed in Section 4.2.5, the triazine 77 obtained from 76 can becopolymerized with methyl methacrylate monomers to form a cross-linked andinsoluble copolymer. Further investigations should be carried out using otherolefinic monomers and also using the monomer 76 directly. In the latter case,even though the cyanate group is quite active, it may still be possible to let the194allyl double bond in 76 react with olefinic monomers first without affecting thecyanate group, because the presence large number of olefinic monomers mightsufficiently dilute the cyanate monomers to make the trimerization of thecyanate groups slower than the double bond opening of the allyl groups.If this prediction is correct, the copolymer formed in this way will be alinear molecule with some free cyanate groups attached. It will still be solublesince no cross-link or very few cross-links will have formed. This kind ofcopolymer is ready to form an insoluble cross-linked polymer network throughthe free cyanate groups on the polymer chain just by application of heat. Itscuring behavior should be similar to a thermoset resin, but the properties of theend polymer product should be different.Again, the physical, mechanical and thermal properties of the polymerobtained by using this cross-linking agent should be tested under realapplication conditions.(3) A Possible Coupling Agent for the Glass-Reinforced Cyanate ResinComposites During these thesis studies, some work was done on the characterizationof a silane coupling agent, triethoxyvinylsilane (TEVS). They show that TEVScan be firmly anchored on a silica gel surface by reaction of the ethoxy group inthe TEVS with the silanol group on the silica gel surface. The Si-0---Si bondsformed show strong stability against long time extraction and acidic hydrolysis.NMR spectra reveal that the TEVS on the silica gel can be copolymerized withan unsaturated monomer, such as styrene and methyl methacrylate. Thechemical bonding between the anchored coupling agent TEVS and the polymermatrix is directly observed by solid state NMR spectroscopy. These illustrate195that silane coupling agents can be useful in the formation of glass-reinforcedpolymer composites.However, there is currently no coupling agent for cyanate groups. 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J.; "Organic Polymer Chemistry", Chapman and Hall,London, New York, 1988.[101] Mok, K; B.Sc. Thesis, Department of chemistry at the University ofBritish Columbia, 1992.[102] Jagannathan, N. R.; and Herring, F. G.; J. Polym. Sci., Part A, 26, 1(1988).[103] Matin, H; et al.; "Polyepoxy-substituted Aromatic Compounds, andPolymers", U.S. 2,938,875, (1960).[104] Frye, J. S.; Maciel, G.E.; J. Mag. Res., 48, 125 (1982).204APPEDICESA. Crystal Structure Data for Compound 81Table I. Crystal DataEmpirical Formula^ C33H39N303Formula Weight 525.69Crystal Color, Habit^ colorless, prismCrystal Dimensions (mm) 0.250 X 0.300 X 0.450Crystal System^ monoclinicNo. Reflections Used for UnitCell Determination (29 range)^25 ( 59.3 - 96.0°)Omega Scan Peak Widthat Half-height^ 0.37Lattice Parameters:a= 12.214 (2)Ab . 6.556 (2)Ac . 37.981 (2)A8 - 90.644 (7)°Space GroupZ valueDcalcF000P(CuKa)V . 3041 (1)A3P21 /c (#14)41.148 g/cm 311285.50 cm-1205atomTable II.^Final Atomic CoordinatesY zx0(1) 0.1993(1) 0.3243(2) 0.25210(3)0(2) 0.4550(1) -0.1540(2) 0.27712(3)0(3) 0.3737(1) -0.0339(2) 0.16511(3)N(1) 0.3240(1) 0.0899(2) 0.26633(4)N(2) 0.4168(1) -0.0878(2) 0.22098(4)N(3) 0.2800(1) 0.1557(2) 0.20631(3)C(1) 0.2704(1) 0.1849(3) 0.24057(4)C(2) 0.3953(1) -0.0437(3) 0.25435(4)C(3) 0.3547(1) 0.0175(3) 0.19880(4)C(4) 0.1360(1) 0.4320(3) 0.22687(4)C(5) 0.1815(1) 0.5856(3) 0.20775(5)C(6) 0.1168(2) 0.6921(3) 0.18434(5)C(7) 0.0062(1) 0.6497(3) 0.17949(4)C(8) -0.0370(1) 0.4956(3) 0.20016(5)C(9) 0.0270(2) 0.3867(3) 0.22388(5)C(10) 0.4337(1) -0.1405(3) 0.31349(5)C(11) 0.3899(2) -0.3072(3) 0.32910(5)C(12) 0.3773(2) -0.3066(3) 0.36537(6)C(13) 0.4075(1) -0.1428(3) 0.38586(5)C(14) 0.4507(2) 0.0240(3) 0.36860(5)C(15) 0.4648(2) 0.0258(3) 0.33247(5)C(16) 0.3168(1) 0.0469(3) 0.13610(4)0(17) 0.3032(2) -0.0876(3) 0.10899(5)C(18) 0.2542(2) -0.0245(3) 0.07794(5)206(Table II continued)atomC(19) 0.2166(2) 0.1728(3) 0.07331(4)C(20) 0.2325(2) 0.3041(3) 0.10126(5)C(21) 0.2834(2) 0.2453(3) 0.13246(5)C(22) -0.0613(2) 0.7663(3) 0.15224(5)C(23) -0.1810(2) 0.6995(4) 0.15149(6)C(24) -0.0580(3) 0.9928(4) 0.16063(9)C(25) -0.0152(2) 0.7255(5) 0.11593(6)C(26) 0.3925(2) -0.1487(4) 0.42583(5)C(27) 0.4561(5) -0.325(1) 0.4418(1)C(27A) 0.377(3) -0.366(5) 0.4372(5)C(28) 0.2714(4) -0.180(1) 0.4341(1)C(28A) 0.306(1) 0.003(4) 0.4336(3)C(29) 0.4302(7) 0.045(1) 0.4442(1)C(29A) 0.499(2) -0.080(5) 0.4413(4)C(30) 0.1577(2) 0.2382(3) 0.03936(5)C(31) 0.1681(3) 0.4650(4) 0.03286(7)C(32) 0.0371(2) 0.1866(6) 0.04286(8)C(33) 0.2034(3) 0.1278(5) 0.00744(6)207LOE6'0 ZESti'0- EZEP*0 (EZ)H8LZt7'0 ELTP*0- OLOE*0 (YZZ)H1/.917'0 LTEE*0- SZWO (ZZ)HZSTT*0 ZZLL'O 60900 (IZ)HZ860'0 066C0 L8500- (0Z)HOTTT*0 88L5'0 T810'0- (6T)HTb8T'0 1910'T 58800- (8T)HOEPT*0 9L901 tT0T*0- (L.T)HZO9U0 8060T 6L10'0 (91)H65T*0 LES5'0 EG8T*0- (ST)HSEET'0 SLLL'O OTZZ'O - (ti)H9PLT*0 EtZL'O SETZ'O- (EI)HSTST*0 8EtE*0 ES6Z*0 (zT)H06600 IStt'0 890Z*0 (TT)H9850'0 tZZT*0- 88tZ*0 (01)H9TTT*0 06ZZ'0- T8ZE*0 (6)H80ZE'0 617PT*0 9966'0 (8)14EZ8E*0 SttT'0 OZLI7'0 (L)HL9LE'0 69Zt'0- 8SPE*0 (9)1405TE*0 85P0- 5L90 (5)HE8EZ*0 88L0 E5000- (t7)H6L6T*0 0E900 05TT*0- (E)H9OLT*0 0Z08'0 8010 (Z)HLOTZ*0 G6T9'0 E650 (T)Hz^A^X^1.1101e80Z(panupuoo II am')(Table II continued)atom x Y zH(23A) 0.4368 -0.4498 0.4282H(24) 0.5345 -0.3055 0.4378H(24A) 0.3763 -0.3731 0.462911(25) 0.2292 -0.0627 0.425314(25A) 0.2980 0.0171 0.4591H(26) 0.2451 -0.3044 0.4225H(26A) 0.3260 0.1357 0.4235H(27) 0.2624 -0.1920 0.4596H(27A) 0.2361 -0.0419 0.4232H(28) 0.5086 0.0644 0.4402H(28A) 0.4956 -0.0869 0.4670H(29) 0.3895 0.1615 0.4346H(29A) 0.5577 -0.1678 0.4329H(30) 0.4168 0.0333 0.4695H(30A) 0.5129 0.0614 0.4340H(31) 0.2455 0.5003 0.0301H(32) 0.1274 0.5016 0.0114H(33) 0.1383 0.5400 0.0529H(34) 0.0065 0.2624 0.0627H(35) -0.0019 0.2240 0.021111(36) 0.0290 0.0399 0.047011(37) 0.1970 -0.0199 0.010913(38) 0.1620 0.1680 -0.013713(39) 0.2807 0.1641 0.0046209atom atomTable III.distanceBond Lengths (A)atom atom distance0(1) C(1) 1.338(2) C(13) C(14) 1.383(3)0(1) C(4) 1.414(2) C(13) C(26) 1.532(3)0(2) C(2) 1.337(2) C(14) C(15) 1.385(3)0(2) C(10) 1.411(2) C(16) C(17) 1.365(2)0(3) C(3) 1.346(2) C(16) C(21) 1.370(3)0(3) C(16) 1.400(2) C(17) C(18) 1.379(3)N(1) C(1) 1.326(2) C(18) C(19) 1.383(3)N(1) C(2) 1.320(2) C(19) C(20) 1.379(3)N(2) C(2) 1.329(2) C(19) C(30) 1.531(3)N(2) C(3) 1.321(2) C(20) C(21) 1.387(3)N(3) C(1) 1.322(2) C(22) C(23) 1.525(3)N(3) C(3) 1.319(2) C(22) C(24) 1.519(3)C(4) C(5) 1.364(3) C(22) C(25) 1.519(3)C(4) C(9) 1.367(2) C(26) C(27) 1.517(6)C(5) C(6) 1.374(3) C(26) C(27A) 1.50(3)C(6) C(7) 1.389(2) C(26) C(28) 1.529(5)C(7) C(8) 1.387(2) C(26) C(28A) 1.49(2)C(7) C(22) 1.523(2) C(26) C(29) 1.516(6)C(8) C(9) 1.385(2) C(26) C(29A) 1.49(2)C(10) C(11) 1.357(3) C(30) C(31) 1.513(4)C(10) C(15) 1.359(3) C(30) C(32) 1.519(4)C(11) C(12) 1.388(3) C(30) C(33) 1.523(3)C(12) C(13) 1.374(3)210atom atomTable IV.^Bond Anglesatom^angle^atom atom atom angleC(1) 0(1) C(4) 118.1(1) 0(2) C(10) C(15) 121.1(2)C(2) 0(2) C(10) 119.6(1) C(11) C(10) C(15) 121.6(2)C(3) 0(3) C(16) 124.3(1) C(10) C(11) C(12) 118.7(2)C(1) N(1) C(2) 112.3(1) C(11) C(12) C(13) 122.2(2)C(2) N(2) C(3) 112.1(1) C(12) C(13) C(14) 116.8(2)C(1) N(3) C(3) 112.4(1) C(12) C(13) C(26) 120.5(2)0(1) C(1) N(1) 113.3(1) C(14) C(13) C(26) 122.7(2)0(1) C(1) N(3) 119.1(1) C(13) C(14) C(15) 121.9(2)N(1) C(1) N(3) 127.6(2) C(10) C(15) C(14) 118.8(2)0(2) C(2) N(1) 119.6(2) 0(3) C(16) C(17) 113.9(2)0(2) C(2) N(2) 112.8(2) 0(3) C(16) C(21) 125.7(2)N(1) C(2) N(2) 127.7(2) C(17) C(16) C(21) 1'20.3(2)0(3) C(3) N(2) 111.8(1) C(16) C(17) C(18) 120.0(2)0(3) C(3) N(3) 120.3(1) C(17) C(18) C(19) 122.0(2)N(2) C(3) N(3) 127.9(2) C(18) C(19) C(20) 116.2(2)0(1) C(4) C(5) 120.4(2) C(18) C(19) C(30) 121.4(2)0(1) C(4) C(9) 118.2(2) C(20) C(19) C(30) 122.3(2)C(5) C(4) C(9) 121.2(2) C(19) C(20) C(21) 122.9(2)C(4) C(5) C(6) 119.0(2) C(16) C(21) C(20) 118.7(2)C(5) C(6) C(7) 122.4(2) C(7) C(22) C(23) 112.3(2)C(6) C(7) C(8) 116.5(2) C(7) C(22) C(24) 109.6(2)C(6) C(7) C(22) 120.5(2) C(7) C(22) C(25) 108.9(2)C(8) C(7) C(22) 122.9(2) C(23) C(22) C(24) 107.9(2)C(7) C(8) C(9) 121.9(2) C(23) C(22) C(25) 107.3(2)C(4) C(9) C(8) 119.0(2) C(24) C(22) C(25) 110.7(2)0(2) C(10) C(11) 117.1(2) C(13) C(26) C(27) 110.4(3)211(Table IV continued)atom atom atom angleC(13) C(26) C(27A) 109(1)C(13) C(26) C(28) 109.4(2)C(13) C(26) C(28A) 105.8(6)C(13) C(26) C(29) 113.3(3)C(13) C(26) C(29A) 105.6(6)C(27) C(26) C(28) 108.0(4)C(27) C(26) C(29) 107.7(4)C(27A) C(26) C(28A) 119(2)C(27A) C(26) C(29A) 107(2)C(28) C(26) C(29) 107.9(4)C(28A) C(26) C(29A) 110(1)C(27A) C(27) C(29A) 123(2)C(19) C(30) C(31) 111.9(2)C(19) C(30) C(32) 108.1(2)C(19) C(30) C(33) 111.3(2)C(31) C(30) C(32) 108.5(3)C(31) C(30) C(33) 107.7(2)C(32) C(30) C(33) 109.2(2)212(Table IV continued)atom atom atom angle atom atom atom angleC(4) C(5) H(1) 120.50 C(22) C(23) H(15) 109.47C(6) C(5) H(1) 120.50 H(13) C(23) H(14) 109.47C(5) C(6) H(2) 118.81 H(13) C(23) H(15) 109.47C(7) C(6) H(2) 118.81 H(14) C(23) H(15) 109.47C(7) c(8) H(3) 119.05 C(22) C(24) H(16) 109.47C(9) C(8) H(3) 119.05 C(22) C(24) H(17) 109.47C(4) C(9) H(4) 120.53 C(22) C(24) 11(18) 109.47C(8) C(9) H(4) 120.52 13(16) C(24) H(17) 109.47C(10) C(11) H(5) 120.66 11(16) C(24) H(18) 109.47C(12) C(11) H(5) 120.65 H(17) C(24) H(16) 109.48C(11) C(12) H(6) 118.92 C(22) C(25) H(19) 109.47C(13) C(12) H(6) 118.91 C(22) C(25) H(20) 109.47C(13) C(14) H(7) 119.04 C(22) C(25) H(21) 109.47C(15) C(14) H(7) 119.04 H(19) C(25) H(20) 109.47C(10) C(15) H(8) 120.61 H(19) C(25) H(21) 109.47C(14) C(15) H(8) 120.61 H(20) C(25) H(21) 109.48C(16) C(17) H(9) 120.01 C(26) C(27) H(22) 109.49C(18) C(17) H(9) 120.02 C(26) C(27) H(23) 109.47C(17) C(18) 11(10) 119.02 C(26) C(27) H(24) 109.50C(19) C(18) 13(10) 119.03 H(22) C(27) H(23) 109.44C(19) C(20) 13(11) 118.57 11(22) C(27) 13(24) 109.48C(21) C(20) H(11) 118.57 13(23) C(27) 11(24) 109.45C(16) C(21) 13(12) 120.65 C(26) C(27A) H(22A) 109.51C(20) C(21) H(12) 120.65 C(26) C(27A) H(23A) 109.38C(22) C(23) H(13) 109.47 C(26) C(27A) H(24A) 109.51C(22) C(23) H(14) 109.47 H(22A) C(27A) H(23A) 109.42213(Table IV continued)atom atom atom angle atom atom atom angleH(22A) C(27A) H(24A) 109.60 C(30) C(31) 13(31) 109.47H(23A) C(27A) H(24A) 109.42 C(30) C(31) 14(32) 109.50C(26) C(28) H(25) 109.45 C(30) C(31) 14(33) 109.47C(26) C(28) H(26) 109.45 14(31) C(31) 14(32) 109.47C(26) C(28) H(27) 109.44 14(31) C(31) 14(33) 109.4313(25) C(28) H(26) 109.51 14(32) C(31) 14(33) 109.4811(25) C(28) H(27) 109.49 C(30) C(32) H(34) 109.46H(26) C(28) H(27) 109.49 C(30) C(32) 13(35) 109.47C(26) C(28A) 13(25A) 109.57 C(30) C(32) 14(36) 109.44C(26) C(28A) H(26A) 109.51 H(34) C(32) 14(35) 109.52C(26) C(28A) H(27A) 109.51 14(34) C(32) 11(36) 109.46H(25A) C(28A) H(26A) 109.44 13(35) C(32) 14(36) 109.48H(25A) C(28A) H(27A) 109.44 C(30) C(33) H(37) 109.45H(26A) C(28A) H(27A) 109.35 C(30) C(33) 14(38) 109.49C(26) C(29) 13(28) 109.43 C(30) C(33) 11(39) 109.47C(26) C(29) 13(29) 109.47 14(37) C(33) H(38) 109.48C(26) C(29) H(30) 109.45 H(37) C(33) 14(39) 109.44H(28) C(29) H(29) 109.49 14(38) C(33) 13(39) 109.5113(28) C(29) 11(30) 109.47H(29) C(29) H(30) 109.52C(26) C(29A) H(28A) 109.62C(26) C(29A) H(29A) 109.53C(26) C(29A) H(30A) 109.54H(28A) C(29A) H(29A) 109.42H(28A) C(29A) H(30A) 109.43H(29A) C(29A) H(30A) 109.30214B. Crystal Structure Data for Compound 2Table V. Crystal DataEmpirical Formula^ C17H14N2 02Formula Weight 278.31Crystal Color, Habit^ colorless, prismCrystal Dimensions^(mm)Crystal SystemNo.^Reflections Used for Unit0.250 X 0.400 X 0.400monoclinicCell Determination (28 range) 25^( 20.0 - 26.10)Omega Scan Peak Widthat Half-height 0.35Lattice^Parameters:a 10.072 (2)Ab 11.410 (2)Ac 13.351 (3)A0 = 108.49 (2)eSpace GroupZ valueDcalc000P(MoKa)V . 1455 (1)A3P2 1/a (#14)41.270 g/cm 35840.79 cm-1215Table VI. Final Atomic Coordinatesatom x Y z0(1) 0.6290(2) 0.7298(2) 0.5398(1)0(2) 0.0664(2) 0.0994(2) 0.1896(2)N(1) 0.6805(3) 0.9327(3) 0.5819(3)N(2) 0.1602(3) -0.0415(2) 0.0952(3)C(1) 0.1644(2) 0.5929(2) 0.1848(2)C(2) 0.0261(3) 0.6547(2) 0.1776(2)C(3) 0.2080(3) 0.6329(2) 0.0898(2)C(4) 0.2827(2) 0.6291(2) 0.2843(2)C(S) 0.4001(3) 0.5586(2) 0.3247(2)C(6) 0.5141(3) 0.5933(2) 0.4078(2)C(7) 0.5076(3) 0.6996(2) 0.4518(2)C(8) 0.3965(3) 0.7725(2) 0.4177(2)C(9) 0.2838(3) 0.7364(2) 0.3327(2)C(10) 0.1424(2) 0.4601(2) 0.1847(2)C(11) 0.0766(3) 0.4101(2) 0.2512(2)C(12) 0.0537(3) 0.2909(3) 0.2520(2)C(13) 0.0970(3) 0.2214(2) 0.1858(2)C(14) 0.1627(3) 0.2648(2) 0.1205(2)C(15) 0.1850(2) 0.3849(2) 0.1201(2)C(16) 0.6532(3) 0.8390(3) 0.5601(3)C(17) 0.1193(3) 0.0261(3) 0.1389(3)216(Table VI continued)atomH(1) 0.0026 0.6400 0.2423H(4) 0.3024 0.6041 0.0976H(2) 0.0363 0.7393 0.1692H(3) -0.0487 0.6245 0.1167H(5) 0.1421 0.6014 0.0246H(6) 0.2072 0.7187 0.0865H(7) 0.4018 0.4814 0.2930H(8) 0.5969 0.5432 0.4341H(9) 0.3956 0.8484 0.4518H(10) 0.2028 0.7885 0.3063H(11) 0.0457 0.4607 0.2988H(12) 0.0071 0.2570 0.2995H(13) 0.1941 0.2127 0.0741H(14) 0.2325 0.4171 0.0725217Table VII. Bond Lengths (A)atom atom distance atom atom distance0(1) C(7) 1.443(3) C(4) C(9) 1.383(3)0(1) C(16) 1.281(3) C(5) C(6) 1.377(3)0(2) C(13) 1.430(3) C(6) C(7) 1.359(3)0(2) C(17) 1.292(4) C(7) C(8) 1.352(3)N(1) C(16) 1.120(4) C(8) C(9) 1.389(4)N(2) C(17) 1.122(4) C(10) C(11) 1.388(3)C(1) C(2) 1.537(3) C(10) C(15) 1.378(3)C(1) C(3) 1.538(3) C(11) C(12) 1.380(4)C(1) C(4) 1.533(3) C(12) C(13) 1.359(4)C(1) C(10) 1.531(3) C(13) C(14) 1.345(3)C(4) C(5) 1.390(3) C(14) C(15) 1.389(3)218Table VIII. Bond Anglesatom atom atom angle atom atom atom angleC(7) 0(1) C(16) 117.4(2) C(6) C(7) C(8) 123.5(2)C(13) 0(2) C(17) 118.8(2) C(7) C(8) C(9) 117.9(2)C(2) C(1) C(3) 107.6(2) C(4) C(9) C(8) 121.9(2)C(2) C(1) C(4) 111.7(2) C(1) C(10) C(11) 120.3(2)C(2) C(1) C(10) 109.0(2) C(1) C(10) C(15) 122.8(2)C(3) C(1) C(4) 106.8(2) C(11) C(10) C(15) 116.8(2)C(3) C(1) C(10) 111.9(2) C(10) C(11) C(12) 121.7(2)C(4) C(1) C(10) 109.9(2) C(11) C(12) C(13) 118.7(2)C(1) C(4) C(5) 120.8(2) 0(2) C(13) C(12) 115.3(2)C(1) C(4) C(9) 122.2(2) 0(2) C(13) C(14) 122.5(2)C(5) C(4) C(9) 116.8(2) C(12) C(13) C(14) 122.2(2)C(4) C(5) C(6) 122.4(2) C(13) C(14) C(15) 118.7(2)C(5) C(6) C(7) 117.6(2) C(10) C(15) C(14) 121.9(2)0(1) C(7) C(6) 114.9(2) 0(1) C(16) N(1) 176.4(4)0(1) C(7) C(8) 121.6(2) 0(2) C(17) N(2) 176.5(3)(Table VIII continued)atom atom atom angle atom atom atom angleC(1) C(2) H(1) 109.47 C(5) C(6) H(8) 121.21C(1) C(2) H(2) 109.47 C(7) C(6) H(8) 121.21C(1) C(2) H(3) 109.47 C(7) C(8) H(9) 121.06H(1) C(2) H(2) 109.47 C(9) C(8) H(9) 121.06H(1) C(2) H(3) 109.47 C(4) C(9) H(10) 119.08H(2) C(2) H(3) 109.47 C(8) C(9) H(10) 119.07C(1) C(3) H(4) 109.47 C(10) C(11) H(11) 119.15C(1) C(3) H(5) 109.47 C(12) C(11) H(11) 119.15C(1) C(3) H(6) 109.47 C(11) C(12) H(12) 120.63H(4) C(3) H(5) 109.47 C(13) C(12) H(12) 120.63H(4) C(3) H(6) 109.47 C(13) C(14) H(13) 120.67H(5) C(3) H(6) 109.47 C(15) C(14) H(13) 120.67C(4) C(5) H(7) 118.81 C(10) C(15) H(14) 119.05C(6) C(5) H(7) 118.81 C(14) C(15) H(14) 119.04220C. Crystal Structure Data for Compound 53Table DC. Crystal DataEmpirical Formula^ C10 H11NO 3Formula Weight 193.20Crystal Color, Habit^ colorless, irregularCrystal Dimensions (mm) 0.150 X 0.350 X 0.450Crystal System^ triclinicNo. Reflections Used for UnitCell Determination (28 range)^25 (104.2 - 122.9°)Omega Scan Peak Widthat Half-height^ 0.20Lattice Parameters:a -b =c =^8.1373^(6)A10.489^(1)A6.0086^(6)Aa 103.552 (7)°0^"' 97.495 (8)°Y m 72.003 (7)0V = 473.25 (8)A3Space Group PI^(#2)Z value 2Dcalc 1.356 g/cm 3F000 204P(CuKa) 8.00 cm-1221atomTable X.^Final Atomic CoordinatesY zx0(1) 0.0999(1) 0.67281(9) 0.0362(2)0(2) 0.1227(1) 0.8322(1) -0.1370(2)0(3) 0.2053(1) 0.4590(1) 0.2836(2)N(3) -0.0537(2) 0.8848(1) 0.1611(2)C(2) 0.0593(2) 0.8029(1) 0.0097(2)C(4) -0.0895(2) 0.8153(1) 0.3200(3)C(5) 0.0058(2) 0.6662(1) 0.2206(2)C(6) 0.1307(2) 0.5976(1) 0.3935(2)C(7) 0.3233(2) 0.3755(1) 0.4113(2)C(8) 0.3691(2) 0.4153(2) 0.6412(3)C(9) 0.4890(2) 0.3195(2) 0.7522(3)C(1O) 0.5613(2) 0.1886(2) 0.6378(4)C(11) 0.5153(2) 0.1507(2) 0.4087(4)C(12) 0.3959(2) 0.2430(1) 0.2941(3)H(1) -0.087(2) 0.977(2) 0.163(3)H(2) -0.038(2) 0.845(2) 0.470(3)H(3) -0.214(2) 0.832(2) 0.330(3)H(4) -0.076(2) 0.612(1) 0.151(2)H(5) 0.068(2) 0.602(1) 0.529(3)H(6) 0.223(2) 0.646(2) 0.449(3)H(7) 0.320(2) 0.509(2) 0.725(3)H(8) 0.519(2) 0.347(2) 0.913(3)H(9) 0.646(2) 0.120(2) 0.719(3)H(10) 0.563(3) 0.060(2) 0.323(3)H(11) 0.366(2) 0.217(2) 0.134(3)222atom atomTable XL^Bond Lengths (A)distance^atom atom distance0(1) C(2) 1.344(1)' C(5) C(6) 1.501(2)0(1) C(5) 1.452(1) C(7) C(8) 1.383(2)0(2) C(2) 1.217(2) C(7) C(12) 1.386(2)0(3) C(6) 1.424(2) C(8) C(9) 1.395(2)0(3) C(7) 1.371(2) C(9) C(10) 1.367(3)N(3) C(2) 1.332(2) C(10) C(11) 1.377(3)N(3) C(4) 1.434(2) C(11) C(12) 1.383(2)C(4) C(5) 1.529(2) N(3) H(1) 0.92(2)C(8) H(7) 0.98(2) C(4) H(2) 0.97(2)C(9) H(8) 0.96(2) C(4) H(3) 0.98(2)C(10) H(9) 1.00(2) C(5) H(4) 1.00(1)C(11) H(10) 0.96(2) C(6) H(5) 1.00(1)C(12) H(11) 0.96(2) C(6) H(6) 1.01(1)223atom atomTable XII.^Bond Anglesatom^angle^atom atom atom angleC(2) 0(1) C(5) 109.74(9) 0(3) C(6) C(5) 107.2(1)C(6) 0(3) C(7) 117.5(1) 0(3) C(7) C(8) 124.5(1)C(2) N(3) C(4) 113.2(1) 0(3) C(7) C(12) 115.1(1)0(1) C(2) 0(2) 121.0(1) C(8) C(7) C(12) 120.4(1)0(1) C(2) N(3) 110.1(1) C(7) C(8) C(9) 118.7(1)0(2) C(2) N(3) 128.9(1) C(8) C(9) C(10) 121.2(2)N(3) C(4) C(5) 101.5(1) C(9) C(10) C(11) 119.4(1)0(1) C(5) C(4) 104.83(9) C(10) C(11) C(12) 120.8(2)0(1) C(5) C(6) 109.6(1) C(7) C(12) C(11) 119.4(2)C(4) C(5) C(6) 112.6(1) C(5) C(6) H(6) 110.3(9)C(2) N(3) H(1) 118(1) H(5) C(6) H(6) 108(1)C(4) N(3) H(1) 129(1) C(7) C(8) H(7) 121(1)N(3) C(4) H(2) 109(1) C(9) C(8) H(7) 120(1)N(3) C(4) H(3) 112(1) C(8) C(9) H(8) 119(1)C(5) C(4) H(2) 110(1) C(10) C(9) H(8) 120(1)C(5) C(4) H(3) 114(1) C(9) C(10) H(9) 121(1)H(2) C(4) H(3) 110(2) C(11) C(10) H(9) 120(1)0(1) C(5) H(4) 107.2(8) C(10) C(11) H(10) 122(1)C(4) C(5) H(4) 111.9(8) C(12) C(11) H(10) 117(1)C(6) C(5) H(4) 110.3(8) C(7) C(12) H(11) 120(1)0(3) C(6) H(5) 110.7(8) C(11) C(12) H(11) 121(1)0(3) C(6) H(6) 110.7(8) N(3) H(1) 0(2) 170(2)C(5) C(6) H(5) 109.8(8)224

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