Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Synthesis and reactivity of dinucleating di(diamino)phenolate ligands for enforcing cooperativity Zhai, Xiaofang 2017

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
24-ubc_2018_february_zhai_xiaofang.pdf [ 7.28MB ]
Metadata
JSON: 24-1.0361982.json
JSON-LD: 24-1.0361982-ld.json
RDF/XML (Pretty): 24-1.0361982-rdf.xml
RDF/JSON: 24-1.0361982-rdf.json
Turtle: 24-1.0361982-turtle.txt
N-Triples: 24-1.0361982-rdf-ntriples.txt
Original Record: 24-1.0361982-source.json
Full Text
24-1.0361982-fulltext.txt
Citation
24-1.0361982.ris

Full Text

SYNTHESIS AND REACTIVITY OF DINUCLEATING DI(DIAMINO)PHENOLATE LIGANDS FOR ENFORCING COOPERATIVITY   by Xiaofang Zhai  B.Sc., University of Waterloo, Waterloo, ON, Canada, 2015  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  December, 2017  © Xiaofang Zhai, 2017    	   ii	  Abstract   A new single-frame dinucleating di(diamino)phenolate ligand L has been synthesized and fully characterized, and its coordination chemistry with zinc precursors featuring ethyl, alkoxide, acetate and amide leaving groups has been investigated. Reaction of ligand L with diethyl zinc and Zn[N(SiMe3)2]2 led to the formation of a trinulcear zinc complex 1 (L)Zn3(CH2CH3)4 and a dinuclear zinc amide complex 4 (L)Zn2[N(SiMe3)]2, respectively. Deprotonation of ligand L followed by salt metathesis with Zn(OAc)2 gave rise to a trinuclear zinc complex 3 [(L)Zn3](µ-OAc)4. The alkyl zinc complex 1 reacted with benzyl alcohol to afford a dinuclear alkoxy zinc complex 2 [(L)Zn2](µ-OCH2Ph)2. Complexes 1-3 were fully characterized by 1H-NMR, 13C{1H}-NMR, COSY, NOESY, HSQC and HMBC NMR spectroscopy and elemental analysis. The solid-state structures of Zn complexes 1-4 were characterized by single-crystal X-ray crystallography.   The catalytic activities of complex 1 and 2 towards ring opening polymerization of racemic lactide (rac-lactide) have been studied. Complex 2 showed better control over molecular weight and dispersity than complex 1, and generated heterotactically inclined poly(lactic acid). However, complex 2 promoted extensive transesterification and depolymerization reaction. Complex 3 was found to be active to ROP of rac-lactide at high temperatures and it was active to CO2/epoxide copolymerization.  	   	  	   iii	  Lay summary 	  Plastics are playing an indispensable role in our daily life due to their good mechanical properties, lightweight and flexible shapes. However, most plastics are derived from petroleum and they are considered to be non-degradable. The accumulation of the fossil-based plastics has resulted in significant environmental pollution. The development of biodegradable polymers, such as poly(lactic acid)(PLA) has emerged as a sustainable solution to this global challenging. Herein we discuss the production and properties of PLA and other biodegradable polymers. Also, the issues with the synthesis and processing of PLA are discussed. The research presented in this thesis is focused on the design and development of highly active catalysts for the synthesis of biodegradable polymers with good physical and mechanical properties. We have successfully synthesized and characterized three zinc catalysts, and performed preliminary reactivity studies on these catalysts.          	  	   iv	  Preface  The work presented in this thesis is in collaboration with Prof. Parisa Mehrkhodavandi and Dr. Paul Kelley who was working on the project before I joined the group. Tannaz Ebrahimi and Dr. Brian O. Patrick contributed to the X-ray crystal structure solving. Jakob Marbach conducted the CO2/epoxide copolymerization reactions. Ligand design in Chapter 2 was developed by Prof. Parisa Mehrkhodavandi. Ligand L was synthesized and fully characterized by me (1H-NMR, 13C{1H}-NMR, COSY, NOESY, HSQC and HMBC NMR spectroscopy, mass spectroscopy and elemental analysis). Also, the synthetic procedures were optimized by me. The synthesis and characterization of zinc complexes 1-4 (1H-NMR, 13C{1H}-NMR, 2D NMR, X-ray crystallography, and elemental analysis) in chapter 2, and the polymerization studies in Chapter 3, were conducted by me.        	  	   v	   Table of contents Abstract .......................................................................................................................................... ii Lay summary ................................................................................................................................ iii Preface ........................................................................................................................................... iv Table of contents ........................................................................................................................... v List of tables................................................................................................................................. vii List of figures .............................................................................................................................. viii List of schemes .............................................................................................................................. xi List of abbreviations and symbols ............................................................................................. xii Acknowledgments ....................................................................................................................... xv Chapter 1: General introduction ................................................................................................. 1 1.1. Introduction to poly(lactic acid): a sustainable polymer ............................................ 1 1.2. Synthesis of PLA ............................................................................................................ 2 1.2.1. Mechanism of ring opening polymerization of lactide using metal catalysts ........... 4 1.2.2. Tacticity and microstructure of PLA ........................................................................ 8 1.3. Multi-metallic zinc catalysts for the ROP of lactide ................................................. 11 1.4. Multi-metallic complexes linked to a single-frame ligand: cooperative catalysis in ring-opening polymerization of lactide ................................................................................. 21 1.5. Cooperative catalysis in CO2/epoxide ring-opening copolymerization ................... 26 1.6. Scope of the thesis ......................................................................................................... 31 Chapter 2: Design, synthesis, and characterization of a dinucleating ligand platform and the resulting zinc complexes ....................................................................................................... 33 2.1. Introduction: ligand design for enforcing cooperative effects in catalysis ............. 33 2.2. Synthesis and characterization of ligand L ................................................................ 36 2.3. Synthesis and characterization of zinc complexes supported by chiral dinucleating ligand  ....................................................................................................................................... 39 2.3.1. Synthesis of alkyl zinc complex: a trinuclear species ............................................. 39 2.3.2. Synthesis and characterization of alkoxy zinc complex ......................................... 43 2.3.3. Synthesis and characterization of zinc acetate complex ......................................... 48 2.3.4. Attempted synthesis of dinuclear zinc complexes with bulkier functionality ........ 51 2.4. Conclusions ................................................................................................................... 54 Chapter 3: Preliminary studies of the reactivity of multi-metallic zinc complexes .............. 55  Introduction .................................................................................................................. 55  ROP of rac-lactide with alkyl zinc complexes 1 ......................................................... 57 	   vi	   ROP of rac-lactide with alkoxy zinc complex 2 ......................................................... 58  Reactivity of zinc acetate complex 3 towards lactide polymerization ..................... 63  Conclusions and perspectives ...................................................................................... 65 Experimental procedures ........................................................................................................... 69 References .................................................................................................................................... 76 Appendix A. FTIR spectrum of complex 3. .............................................................................. 81 Appendix B. Characterization of compounds in solution; 1H-NMR, 13C{1H}-NMR and 2D NMR spectra, mass spectra ........................................................................................................ 82 Appendix C. Characterization of polymers ............................................................................ 103 Appendix D. Solid state structure and crystallographic parameters ................................... 107                  	   vii	  List of tables Table 2.1. Selected bond lengths (Å) and angles (°) for complex 1. ........................................... 43	  Table 2.2. Selected bond lengths (Å) and angles (°) for complex 2. ........................................... 47	  Table 2.3. Selected bond lengths (Å) and angles (°) for complex 3. ........................................... 51	  Table 2.4. Selected bond lengths (Å) and angles (°) for complex 4. ........................................... 54	  Table 3.1. Polymerization results of rac-lactide with complex 1a ................................................ 58	  Table 3.2. Ring-opening polymerization of rac-lactide with alkoxy zinc complex 2a. ................ 59	  Table 3.3. Attempted activation of Zn acetate complex 3. .......................................................... 64	  Table 3.4. Activity tests of complex 3 towards ROP of lactide a ................................................. 64	  Table 3.5. Preliminary CO2/epoxide copolymerization test with complex 3a .............................. 68	  Table D.1. Selected bond lengths and angles for compound d. ................................................. 107	                 	  	   viii	  List of figures Figure 1.1. Molecular structure of poly(lactic acid).10 ................................................................... 2	  Figure 1.2. Synthesis of PLA from L-and D- lactic acid.12 ........................................................... 3	  Figure 1.3. Commanly used catalysts in industrial prodcution of PLA, tin(II) octanoate and aluminum isopropoxide.7 ................................................................................................................ 5	  Figure 1.4. Coordination-insertion mechanism for ROP of lactide. .............................................. 6	  Figure 1.5. Activated monomer mechanism for ROP of lactide. ................................................... 6	  Figure 1.6. Possible transesterification side reactions during ROP of lactide. .............................. 7	  Figure 1.7. PLA microstructures from the ROP of rac-lactide and meso-lactide. ......................... 9	  Figure 1.8. Schematic diagrams to show chemical shifts (in ppm) of the tetrads for PLA a) homonuclear decoupled 1H-NMR of PLA from rac-lactide; b) 13C{1H}-NMR of PLA from rac-lactide; c) homonuclear decoupled 1H-NMR of PLA from meso-lactide; d) 13C{1H}-NMR of PLA from meso-lactide.32 ............................................................................................................. 10	  Figure 1.9. Dinol supported trizinc complexes. ........................................................................... 12	  Figure 1.10. Schiff base supported dizinc complexes reported by Lin et al. ............................... 13	  Figure 1.11. Schiff base supported zinc complexes. .................................................................... 14	  Figure 1.12. b-diketiminate (BDI)-based dinuclear zinc complexes. .......................................... 15	  Figure 1.13. N-heterocyclic carbene supported zinc complex. .................................................... 16	  Figure 1.14. Zn complexes supported by diaminophenoxy ligands reported by Hillmyer and Tolman et al. ................................................................................................................................. 17	  Figure 1.15. Dizinc complexes supported by macrocyclic Schiff base ligand reported by Williams et al. ............................................................................................................................... 18	  Figure 1.16. Zn complexes supported by chiral diamino phenoxy ligand studied in Mehrkhodavandi Group. ............................................................................................................... 19	  Figure 1.17. Dinuclear and mononuclear Zn complexes supported by chiral diamino phenoxy ligand studied in Mehrkhodavandi Group .................................................................................... 20	  Figure 1.18. Dinucleating ligand developed by Hillmyer and Tolman. Dinuclear Zn complex reported by Mehrkhodavnadi. ....................................................................................................... 21	  Figure 1.19. Dinuclear Al complexes supported by single-frame imino ligand. ......................... 22	  Figure 1.20. Dimetallic Zn complex supported by a tridentate bis(pyrazolyl) amine ligand. ..... 23	  Figure 1.21. Dimetallic Al complexes supported by triaminocyclohexane ligand and their mononuclear analogues. ................................................................................................................ 23	  Figure 1.22. Mutimetallic aluminum complex and its mono- and dinuclear analogues. ............. 25	  Figure 1.23. Dinuclear aluminum and indium complexes and their mononuclear analogue. ...... 26	  Figure 1.24. Mechanism of epoxide-CO2 copolymerization and the formation of cyclic carbonates. .................................................................................................................................... 27	  Figure 1.25. N, N, O-BDI highly active dimeric Zn catalysts reported by Coates et al., and the unreactive tightly bound dimer and monomer. ............................................................................. 28	  Figure 1.26. N, N, O, O bis(anilido-aldimine)  Dizinc complex reported by Lee. ...................... 29	  Figure 1.27. Dizinc complex supported by a macrocyclic BDI ligand reported by Rieger. ........ 30	  	   ix	  Figure 1.28. Dimetallic catalysts supported by macrocyclic diphenolate ancillary ligands reported by Williams. .................................................................................................................... 31	  Figure 2.1. Highly active dinuclear indium catalyst reported by Mehrkhodavandi. ................... 34	  Figure 2.2. Design of single-frame dinucleating ligand for enforcing metal-metal cooperativity........................................................................................................................................................ 35	  Figure 2.3. Solid-state structure of compound d obtained by single crystal X-ray diffraction with ellipsoids at the 50% probability level. Most hydrogen atoms were omitted for clarity. ............. 38	  Figure 2.4. 1H-NMR spectra of free ligand L (blue) and complex 1 (red) (400 MHz, C6D6, 25 0C)....................................................................................................................................................... 41	  Figure 2.5. Solid-state structure of complex 1 with ellipsoids at the 50% probability. Hydrogen atoms were omitted for clarity. ..................................................................................................... 42	  Figure 2.6. Variable temperature NMR spectra of complex 2 (d8-toluene, 400 MHz). ............... 45	  Figure 2.7. Solid state structure of complex 2• 2benzyl alcohol with ellipsoids at the 50% probability level. Most hydrogen atoms were omitted for clarity. ............................................... 47	  Figure 2.8. 1H-NMR of Zn acetate complex 3 (400 MHz, C6D6, 25 ºC) ..................................... 49	  Figure 2.9. Solid state structure of [(L)Zn3](µ-OAc)4 complex 3 with ellipsoids at the 50% probability level. Hydrogen atoms were omitted for clarity. ........................................................ 50	  Figure 2.10. Solid-state structure of (L)Zn2[N(SiMe3)]2 complex 4 with ellipsoids at the 50% probability level. Hydrogen atoms were omitted for clarity. ........................................................ 53	  Figure 3.1. Highly active dinuclear catalysts for ROP of lactide in the literature. ...................... 56	  Figure 3.2. Zinc complexes studied in this research project. ....................................................... 57	  Figure 3.3. Plot of Ð (orange), observed Mn (blue) and theoretical Mn (green) as functions of [LA]0/[2]. The green line indicates calculated Mn values based on [LA]:[initiator] value (Assuming 4 initiators in one catalysts molecule). ....................................................................... 59	  Figure 3.4. GPC traces for depolymerisation of PLA sample with complex 2. Blue solid (PLA generated with indium catalyst) Mn=29,200 g/mol, Ð=1.02. Red solid (PLA with complex 2) Mn=17,900 g/mol, Ð=1.20. ........................................................................................................... 60	  Figure 3.5. Chain-end analysis by 1H-NMR spectrum (400 MHz, CDCl3, 25 °C), PLA generated by complex 2 and precipitated in MeOH. ..................................................................................... 61	  Figure 3.6. MALDI-TOF mass spectrum of PLA oligomer produced by complex 2. ................. 62	  Figure 3.7. Highly active dizinc acetate catalyst for CO2/epoxide copolymerization. ................ 63	  Figure A.1. FTIR spectrum of Zn acetate complex 3 (peaks at 1431 and 1583 cm-1 indicating bridging acetate groups).112 ........................................................................................................... 81	  Figure B.1. 1H-NMR spectrum of compound d (300 MHz, CDCl3, 25 ºC) ................................. 82	  Figure B.2. 1H-NMR spectrum of ligand L’ (300 MHz, CDCl3, 25 ºC) ...................................... 82	  Figure B.3. 1H-NMR spectrum of  L (400 MHz, CDCl3, 25 ºC) ................................................. 83	  Figure B.4. 13C{1H}-NMR spectrum of  L (400 MHz, CDCl3, 25 ºC). ........................................ 84	  Figure B.5. COSY NMR spectrum of L (400 MHz, CDCl3, 25 ºC) ............................................ 85	  Figure B.6. NOESY NMR spectrum of L (400 MHz, CDCl3, 25 ºC) ......................................... 86	  Figure B.7. HSQC NMR spectrum of L (400 MHz, CDCl3, 25 ºC) ............................................ 87	  	   x	  Figure B.8. HMBC NMR spectrum of  L (400 MHz, CDCl3, 25 ºC) .......................................... 88	  Figure B.9. Mass spectrum of L .................................................................................................. 88	  Figure B.10. 1H-NMR spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) .................... 89	  Figure B.11. COSY spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) ........................ 90	  Figure B.12. NOESY NMR spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) ........... 90	  Figure B.13. 13C{1H}-NMR of alkyl Zn complex 1 (400 MHz, C6D6, 25 ºC) ............................. 91	  Figure B.14. HSQC of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) ....................................... 92	  Figure B.15. HMBC of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) ...................................... 93	  Figure B.16. 1H-NMR of alkoxy Zn complex 2 (d8-toluene ,400 MHz, 25 ºC) ........................... 94	  Figure B.17. 13C{1H}-NMR alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) .............................. 94	  Figure B.18. COSY of alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) ....................................... 95	  Figure B.19. NOESY of alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) .................................... 96	  Figure B.20. ROESY of alkoxy complex 2 (400 MHz, d8-tol, 25 ºC) ......................................... 97	  Figure B.21. HMQC (Sanyal,  #212) HMBC (bottom) spectra of complex 2 (1H-NMR spectra obtained in d-THF, 400 MHz, 25 ºC) ........................................................................................... 99	  Figure B.22. 1H-NMR of Zn acetate complex 3 (400 MHz, C6D6, 25 ºC) ................................. 100	  Figure B.23. 13C{1H}-NMR of complex 3 (400 MHz, C6D6, 25 ºC) ......................................... 100	  Figure B.24.  HSQC NMR spectrum of complex 3. (400 MHz, C6D6, 25 ºC) .......................... 101	  Figure B.25. 1H-NMR of Zn amide complex 4 (400 MHz, C6D6, 25 ºC) .................................. 102	  Figure C.1. Chain end analysis by 1H-NMR spectrum for polymers precipitated with wet hexane (400 MHz, CDCl3, 25 ºC) ........................................................................................................... 103	  Figure C.2. COSY-NMR spectrum showing correlation of –CH- and –OH group in the chain end. (400 MHz, CDCl3, 25 ºC) ................................................................................................... 104	  Figure C.3.1H-NMR spectrum of PLA generated with complex 2 (400 MHz, CDCl3, 25 ºC) .. 105	  Figure C.4. MALDI-TOF mass spectrum of PLA sample generated with complex 2 .............. 105	  Figure C.5. Homonuclear decoupled 1H{1H} NMR spectrum obtained from rac-lactide with complex 2 (Pr=0.61) (600 MHz, CDCl3, 25 °C) ......................................................................... 106	  	  	  	  	  	  	  	  	  	   xi	  	  List of schemes Scheme 2.1. Synthesis of (R,R)-N,N-dimethyl-trans-1,2-diaminocyclohexane. ......................... 36	  Scheme 2.2. Synthesis of ligand L. .............................................................................................. 37	  Scheme 2.3. Synthesis of trinuclear alkyl Zn complex 1 (L)Zn3(CH2CH3)4. ............................... 40	  Scheme 2.4. Synthesis of dinuclear Zn complex 2. ...................................................................... 44	  Scheme 2.5. Synthesis of trinuclear Zn acetate complex 3. ......................................................... 48	  Scheme 2.6. Attempted synthesis of dinuclear Zn species with increased steric bulk. ................ 52	  Scheme 3.1.  Proposed synthetic route for ligands. ...................................................................... 67	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	   xii	  	  List of abbreviations and symbols 	  (+)                        dextrorotatory (-)                         levorotatory (±)                        racemic Ac acetate Binap 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl Bn benzyl tBu tert-butyl CEM                      chain end control mechanism 13C{1H}  CHO proton decoupled carbon  cyclohexene oxide Cm cumyl COSY  correlation spectroscopy Cy cyclohexyl Ð dispersity  DME DMF Dimethylether N,N-dimethylformamide DMSO dimethylsulfoxide ee                         enantiomeric excess Et ethyl i                            iso linkage ( = meso linkage) 1H{1H} HMBC HSQC proton decoupled proton  heteronuclear multiple bond correlation heteronuclear single quantum correlation ki initiation rate 	   xiii	  kp propagation rate kt chain transfer rate LA                         lactide D-LA                     D-lactide ( = (R,R)-lactide) L-LA                      L-lactide ( = (S,S)-lactide) rac-LA                  50:50 racemic mixture of D-lactide and L-lactide m                          meso linkage m- meta substituent MALDI-TOF matrix-assisted laser desorption time of flight mass spectroscopy Me methyl NMR NOESY nuclear magnetic resonance spectroscopy nuclear overhauser effect spectroscopy o- ortho substituent p- para substituent  Ph phenyl PLA                        poly(lactic acid) PLLA – PLDA         poly(L-lactide) and poly(D-lactide) Pm probability of finding a meso linkage in a PLA chain Pr probability of finding a racemic linkage in a PLA chain iPr iso-propyl nPr propyl  PS                          poly(styrene) r                            racemic linkage ROP    ROCOP ROESY                    ring opening polymerization   ring opening copolymerization rotating-frame nuclear overhauser effect correlation spectroscopy       RT room temperature 	   xiv	  s                           syndio linkage ( = racemic linkage) SCM                      site control mechanism                   Tg Tm glass transition temperature melting point THF tetrahydrofuran UV ultraviolet VT  variable temperature k Related to the coordination mode μ Indicate a bridging moiety                 	   	   xv	  Acknowledgments  My deep thanks go to my supervisor Prof. Parisa Mehrkhodavandi who is a passionate scientist and a demanding but encouraging mentor. Without her guidance and help I could not finish my Master’s study. She made me the chemist I am today and I am truly honoured that I had the chance to work with her. Her advices, encouragements and passion will always stay with me. I am very grateful for my labmates and my friends, with a special dedication to my mentors Dr. Paul Kelley, Dr. Love-Ese Chile and Dr. Nicholas Hein who gave me so much help with my chemistry and also with my life in graduate school. Thanks to all the MerCats, Tannaz Ebrahibi, Alex Kremer, Steve Chang, Carlos Andres Diaz Lopez, Hyuk Joon Jung for their support in lab. I would not achieve my degree without their patience, suggestions and friendship. I am luck to work with a group of fascinating people. I would like to thank my family. Thanks to my grandparents and parents who raised me up to a right person. It is impossible to pursue a degree on the other side of the world without their emotional and financial support. A dedication to my dear sister and brother-in-law. Finally, I want to thank every person I have met in the chemistry department and in the CREATE SusSyn program. They made my life in graduate school unique and enjoyable.	  	  	  	  	  	  	   xvi	                                                                                                                                           To my parents,        	   1	   Chapter 1: General introduction  	  Strong, lightweight, flexible and versatile plastics play an indispensable role in our life. They are widely used in packaging, electronics, automotive, construction, and agriculture.1 Plastics production has been growing exponentially since the 1960s and is expected to double by 2036.2 Although there are over 1 000 types of plastic, 90 % of plastics are derived from fossil fuels, a non-renewable resource,1 and the accumulation of petrochemical plastics has resulted in significant environmental pollution. Millions of tons of plastic waste are discarded every year, polluting our lands, rivers and oceans, and the treatment of these oil-based plastics has remains a global challenge. The recent advance in bio-based and biodegradable polymers has emerged as a powerful and sustainable solution to plastic disposal problems. 1.1.    Introduction to poly(lactic acid): a sustainable polymer 	  Poly(lactic acid) (Figure 1.1) is one of the most promising alternatives to petroleum-based polymers and has drawn considerable attention in recent years due to its excellent renewability, biodegradability and good biocompatibility.3 The global production capacity of PLA is about 180,000 tons in 2012, and it is expected to reach 800,000 tons/year by 2020.1 The production of PLA is considered to be a sustainable process because PLA is derived from renewable sources such as corn, potato or sugarcane, and PLA production is more energetically efficient than most fossil-based polymers.4 In addition, the production of PLA has much less carbon footprint than other fossil-based plastics as a significant amount of carbon dioxide is fixed during the growth of corn, potato and sugarcane plants.5 However, these plants are the 	   2	  main agricultural products in many developing countries. New ideas for decreasing PLA price and reducing the ethical drawback of PLA production, includes the usage of second generation feedstocks such as bagasse and crop residues (stems, strews and leaves).6 The end of life options of PLA is superior to most petroleum-based polymers because PLA is able to degrade into smaller molecules such as lactic acid, water, and carbon dioxide in natural environments.4 The biodegradation of PLA occurred by a two-step process. First, the high molecular weight polymer hydrolyzes into low molecular weight oligomers in presence of moisture and acid or base. The complete environmental degradation of PLA can take up to 2 years, generating water and carbon dioxide.7  PLA has reasonably good optical, physical, mechanical, and barrier properties compared to existing petroleum-based polymers.8 Originally PLA was only being used in medical implants, drug delivery, orthopedic fixation, and sutures because of its high cost and limited low molecular weight.9 However, recent improvements in polymerization allows industrial production of high molecular weight PLA that has expanded its application in various areas, such as food packaging, agriculture, and electronics.5   	  Figure 1.1. Molecular structure of poly(lactic acid).10  	  1.2.    Synthesis of PLA 	  Lactic acid, the monomer of PLA, is produced by fermentation on an industrial scale.6 Lactic acid has two stereoisomers L(+) and D (-) that are derived from bacteria homofermative OOOOn	   3	  fermentation and heterofermative fermentation of carbohydrates.5 Two methods are commonly used in industrial production of PLA (Figure 1.2). One is the direct condensation polymerization of lactic acid. Solvents and water are removed under high vacuum and temperatures in condensation process, making this method less attractive.6,7 Also, the uncontrolled polymerization results in low molecular weight polymers with poor mechanical properties.11 Direct polymerization by polycondenstaion in an azeotropic solution is more practicable. The azeotropic solution has a lower boiling point than water, which makes it easier to separate PLA from solvents.14 In industry, PLA is predominantly produced from ring opening polymerization of lactide, which is more efficient in making high molecular weight PLA.12  Lactide is a cyclic dimer made from oligomerization of lactic acid by removing water under mild conditions and without solvent.12 The low molecular weight pre-polymer is depolymerized into a mixture of lactide stereoisomers: L-LA, D-LA and meso-LA.  The mixture is purified by vacuum distillation and undergoes controlled ring opening polymerization to produce high molecular weight PLA.12  Figure 1.2. Synthesis of PLA from L-and D- lactic acid.12 OHHOH3C HOOHHOH CH3OHOOOOOOOOpolyL-Lactic acidD-Lactic acidHOOOOOOOOOHHOOOOOOOOpolyOOOODirect condensationAzeotropic dehydration condensation-H2OPolymerization through lactideformationLow MW pre-polymerLow MW prepolymerRing openingpolymerizationHigh MW polymernnnChain coupling agentsLactide	   4	  The methods of processing for biopolymers are typically the same as the established polymer manufacturing techniques, but the control over processing temperature is more demanding for biopolymers. Semicrystalline PLA has a relatively high Tg (58 °C) and low Tm (175 °C) as compared to other thermoplastics.13 In addition, PLA is sensitive to water and heat due to its biodegradable nature. Therefore, the processing of PLA is difficult and costly, and that is the main reason for the high price of PLA, making it less competitive to petrochemical plastics.14 1.2.1.   Mechanism of ring opening polymerization of lactide using metal catalysts 	  The study of metal-mediated ring opening polymerization is of great interest because it allows for the preparation of PLAs in a reproducible and controlled fashion, and hundreds of catalysts have been reported for being active to lactide polymerization including main group, transition and rare earth metals.15-18  With well-defined metal catalysts, the activity and selectivity are tunable, affording a better understanding of mechanism of polymerization. The most widely used catalyst for industrial production of PLA is tin(II) bis(2-ethylhexanoate) (Figure 1.3).17  It is easy to handle, commercially available and highly active.17 However, the toxicity of tin as a metal has limited its use in the synthesis of polymers used in food packaging and biomedical applications, which leads to more research focused on biocompatible metals such as K, Mg, Ca, Ti, Zn, Fe, and Al as catalysts.10 Although, there are various active metal catalysts, this thesis will be focus on Zn-based catalysts and many distinguished zinc complexes are discussed in the following sections. 	   5	        Figure 1.3. Commanly used catalysts in industrial prodcution of PLA, tin(II) octanoate and aluminum isopropoxide.7 	  A large amount of research has been conducted towards metal mediated ring opening polymerization of lactide. In general, there are two mechanisms: coordination-insertion and activated monomer. The coordination insertion mechanism was first reported by Dittrich and Schulz in 1971.19 This mechanism has been further supported by experimental and theoretical studies.20-22 There are three steps in this mechanism as described in Figure 1.4: 1) Coordination of monomer to the Lewis- acidic metal center, which activates the monomer. This is of primary importance to promote the reaction; 2) Through the formation of a four-membered ring intermediate, insertion of monomer into metal-alkoxide bond by nucleophilic attack of alkoxy group on the carbonyl carbon occurs; 3) Ring opening of lactide through the cleavage of acyl-oxygen bond, and the initiating group forms the resulting alkyl ester end group which can be confirmed by 1H-NMR analysis. Subsequently, the polymerization can be quenched by water or alcohol to remove the residue organometallic moiety. OOOOSnOAlOO	   6	   Figure 1.4. Coordination-insertion mechanism for ROP of lactide. 	  The activated mechanism is similar to the coordination-insertion mechanism except that the initiating group is from an external nucleophile instead of the metal-alkoxide bond.23 After the coordination of monomer to the Lewis-acidic metal, the ester bond is polarized and activated. The external nucleophile, usually alcohol, ring opens the monomer, generating a new polymeryl nucleophile for the next monomer addition.  Figure 1.5. Activated monomer mechanism for ROP of lactide. 	  There are two possible transesterification side reactions in this coordination-insertion mechanism (Figure 1.6): intermolecular and intramolecular transesterification. Polymer ROMLn OOOOOOOOLnMROOOOORO MLnROOOOOMLnR'OHROOOOOOR'TerminationOOOOMOOOOMROHROOOOOOHOOOOnROOOOOOHn	   7	  molecular weight depends on kpropagation/kinitiation and these side reactions.17 Intramolecular transesterification, also known as “backbiting” reaction occurs when metal alkoxy group attacks one carbonyl group on the propagating chain, leading to macrocyclic structures and shorter chains. Intermolecular transesterification also results from the nucleophilic attack of metal alkoxy group on a carbonyl group, but the carbonyl group is from another polymer chain, resulting in chain redistribution. These side reactions cause large Đ and irreproducible polymer molecular weight. It is reported that the extent of transesterification is highly dependent on the identity of the metallic initiators, with aluminum alkoxides having a lower transesterification activity compared to tin alkoxides.24 Transesterification can be characterized by matrix assisted laser desorption or ionization coupled with a time of flight analyzer mass spectroscopy (MALDI-TOF). If there is no transesterification, the polymer chains are distributed evenly with 144 m/z separation corresponding to one unit of lactide. If there are peaks separated by 72 m/z which is the molar mass of a lactic acid unit, it indicates transesterification occurred. In addition, chain end analysis by 1H-NMR spectroscopy can be used to confirm transesterification, giving that a cyclic polymer has no chain end.15  	  Figure 1.6. Possible transesterification side reactions during ROP of lactide. O[M]OOOOROOn[M]OOROOOOO nIntramolecular[M]OOOORO[M]OOOROO [M]OORO[M]OOOOOOROIntermolecular	   8	  	  Although ROP of lactide is an efficient way to prepare PLA with high degree of control in terms of molecular weight and polydispersity, it has non-negligible limitations: low catalytic productivity and potential contamination from catalyst residues.25 Immortal ROP (iROP) is a powerful alternative to “classical living” ROP.25 In this process, a nucleophile acts as an initiator and chain transfer agent (CTA). In iROP, the chain transfer rate is much faster than the initiation rate and propagation rate, resulting in the formation of one polymer chain per chain transfer agent over the initiator. Thus, the number of growing polymer chains is much larger than the number of catalyst molecules, significantly decreasing catalyst loading.26 These features make iROP more sustainable and attractive for metal catalyzed polymerization.   1.2.2.   Tacticity and microstructure of PLA 	  Lactide has two stereocenters and therefore three isomers: DL- (meso-), L- and D- (racemic mixture, rac-lactide). The ROP of meso- and rac-lactide results in a wide range of polymer microstructures. (Figure 1.7).16 The microstructure of PLA has significant effects on the physical properties of polymer bulk.27 Stereoselctive catalysts are required in order to tune the properties of final product. Three different microstructures can be obtained from rac-lactide: atactic, isotactic, and heterotactic. Atactic PLA arises from ROP with no stereocontrol to yield a polymer chain with random stereocenters. Heterotactic PLA is afforded from alternating enchainment of D-LA and L-LA. Isotactic PLA has uniform stereocenters, resulting from polymerization of only L-LA or D-LA. It should be noted that isotactic stereoblocks can be formed during the polymerization of 	   9	  rac-LA where one monomer is preferential over the other. ROP of meso-lactide can lead to syndiotactic PLA with alternating S and R stereocenters (-SRSRSR-) or heterotactic PLA with doubly alternating centers (-SSRRSSRR-).    Figure 1.7. PLA microstructures from the ROP of rac-lactide and meso-lactide. 	  The identification of polymer tacticity had been challenging. Munson et al. have conclusively reported that 13C{1H}-NMR and homonuclear decoupled 1H-NMR spectroscopy can be used in the characterization of PLA tacticity (Figure 1.8).28 Homonuclear decoupling of methyl signals is utilizd to eliminate the effect of splitting between the methyl and methine protons. As a result, the methine protons display a singlet resonance at 5.15-5.25 ppm.28 Adapted OOOOOOOOOOOOL-LAD-LAmeso-LArac-LAmeso-LAheterotacticsyndiotacticstereoblockisotactic O OOORRPDLAOOOOSSPLLAOOOOORR OOOm nPDLA-PLLAOOOOOOn(-RR-SS-)nRRSSSSOOOOOOnRSRS(-RS-RS-)nn nOOOOnatactic	   10	  from the Bovey formalism, “i” describes an iso and ‘s’ describes a syndio relationship between adjacent stereocentres.28 Stereoregularity can be quantified by the Pm and Pr values, and they represent the probability of meso or racemic enchainment (the probability of forming a syndiotactic or isotactic diad) respectively.29 These values can be calculated from homonuclear decoupled 1H NMR spectra directly (see Appendix A for calculation).30 For rac-lactide, Pr=1.00 (Pm=0.00), representing perfect heterotactic polymer, and Pr=0.00 (Pm=1.00) representing perfect isotactic polymer. Pr=Pm=0.5 describes complete atactic polymers.  Figure 1.8. Schematic diagrams to show chemical shifts (in ppm) of the tetrads for PLA a) homonuclear decoupled 1H-NMR of PLA from rac-lactide; b) 13C{1H}-NMR of PLA from rac-lactide; c) homonuclear decoupled 1H-NMR of PLA from meso-lactide; d) 13C{1H}-NMR of PLA from meso-lactide.32  Several mechanisms have been proposed for the ROP of lactide including anionic, pseudo-anionic (general base catalysis), coordination–insertion ROP and a monomer-activated mechanism.30 In all cases, stereocontrol can be realised by two different mechanisms, chain end control and enantiomorphic site control.28-29 In a chain end controlled mechanism, the chirality of the propagating chain end bound to the catalyst determines the chirality of the next monomer to 	   11	  be inserted; this is generally associated with hindered but achiral catalyst systems. Enantiomorphic site control, however, is demonstrated when the chirality of the catalyst, and not the chain end, dictates the chirality of the next insertion.28  1.3.   Multi-metallic zinc catalysts for the ROP of lactide  	  Much effort has been devoted to utilizing zinc derivatives as potential catalysts for ROP of lactide due to its low price, wide availability and non-toxic nature.31  Zinc powder itself is an industrial catalyst with relatively good polymerization activity.17 With reaction time of several days at 140 °C in bulk, it is roughly as active as Al(Oi-Pr)3.17 Numerous zinc salts have also been investigated.32-34 So far, the best heterogeneous zinc catalyst with regards to lactide conversion and degree of polymerization were observed with zinc(II) lactate, Zn(Lact)2, which is commercially available and can be readily obtained from ZnO and ethyl lactate or lactide.19  Multi-metallic homogeneous catalysts exhibit potential in catalysis due to their intriguing synergistic cooperation between metal centers.35-38 Synergistic catalysis is found in enzymes, which are highly active and selective catalysts developed by nature.39-41 Inspired by tandem catalysis in enzymes, multi-metallic systems have received extensive attention in polymerization catalysis.35-36, 39 To date, some of the best catalysts for ROP of lactide are homogeneous multi-metallic zinc complexes supported by electron-donating ligands, such as binolate, Schiff base, b-diiminate (BDI), N-heterocyclic carbenes (NHC) and bis-(amino)phenolates. In this section, the recent advances in multi-metallic zinc catalysts for lactide polymerization are reviewed. Zinc catalysts supported by bulky racemic binolate ligand for ROP of lactide were first reported by Chisholm and coworkers (Figure 1.9).42 At room temperature, polymerization of rac-	   12	  lacide with A1 reached 96% conversion in 40 h and when the temperature was raised to 80 0C, 99% conversion was reached in 4 h. Although the chiral binolate ligand with lithium and aluminum complexes did not show stereoselectivity in the polymerization of rac-lactide, the dizinc catalysts afforded heterotactically enriched PLA. More recently, Ko et al. reported a binolate containing trizinc complex A2 which is active to ROP of e-caprolactone and b-butyrolactone in the presence of alcohol.43  Figure 1.9. Dinol supported trizinc complexes. 	  Schiff base tridentate N, N, O motifs are widely used in ligand design due to its synthetic ease, as well as featuring tunable steric and electronic properties. Lin et al. reported a series of Schiff based supported dinuclear Zn and Mg benzoxide complexes for lactide polymerization.44 Dizinc complex A3 and its magnesium analogue are efficient initiators for ROP of L-lactide and rac-lactide in a controlled fashion.  With A3, 90% conversion was achieved in 3.5h at 60 °C yielding heterotactic-enriched polymer (Pr = 0.75) with a low polydispersity index (Đ = 1.05). Solvents are shown to affect stereoselectivity of the catalysts. Kinetic studies revealed interesting mechanistic insights: they show first order dependency on lactide and catalyst A3, indicating A3 undergoes dissociation in solution and the active intermediate is a monomeric species; While the kinetic studies showed a secondary dependency on lactide and first order on the magnesium analogue, suggesting a dimeric Mg propagating species.44 A series of zinc analogues to A3 were OZnOZnOOZnEtOOEtOZnOZnOOZn EtEtt-BuOHOHt-BuOHOH=A1 A2	   13	  also generated.45-46 It is reported that the substituents on Schiff base have a great effect on the reactivity of Zn and Mg complexes. With a more electron-withdrawing group, the catalyst showed decreasing activity due to the increasing bond strength of metal-alkoxide bond. However, A4 showed much higher activity than A3 towards lactide ROP. It reached 92% conversion in 30mins at room temperature with the same catalyst loading.46 Complex A4 and rac-lactide also generated hetero-enriched PLA (Pr = 0.59-0.74). The kinetic studies also indicate a monomeric propagating species.   Figure 1.10. Schiff base supported dizinc complexes reported by Lin et al. 	  More recently, Darensbourg et al. reported a series of Schiff base ligand derived from natural amino acid.47 The dizinc complexes A5 coordinated by these ligands show comparable activity to A4, but have better stereoselectivity compared to A3 and A4 due to its steric substituents on the Schiff base ligand. (Pr = 0.81-0.89).  t-But-Bu OZnN N Ot-But-BuOZnNNOOOBnBnNNO ZnO OBnXNN OZnXBnA3 A4X=4-H,Cl, Br,4,6-di-tBu	   14	   Figure 1.11. Schiff base supported zinc complexes.  Bulky b-diketiminate (BDI) ligands are widely investigated in catalysis for ROP of lactide. Coates et al. reported a series of Zn(II) and Mg(II) alkoxides based on BDI ligand framework (Figure 1.12).48 It was found that  Zn complexes with an -OiPr initiator (A6-A8) displays higher activity than their –OAc analogue (A9). Also, it was reported that -Et, -N(SiMe3)2, and -OAc are inferior initiating groups, and this is attributed to the reactions between these groups and impurities such as lactic acid. The BDI-ZnOiPr catalysts (A6-A8) exhibit high activity and stereoselectivity (Pr = 0.76-0.94), and the substituents on BDI ligand have significant effects on polymerization rate and selectivity (iPr > Et > nPr). Catalyst A6, was the most active catalyst, producing highly heterotactic PLA (Pr = 0.9) with a narrow dispersity (Đ = 1.10). More recently, Schaper et al. further studied BDI ligand based zinc catalysts for lactide ROP.49 They synthesized a series of zinc complexes that are active initiators for the polymerization of rac-lactide. Zinc complex A10 is the most active catalyst among these catalysts reported by Schaper. and it produces heterotactic PLA (Pr = 0.84-0.87) with rac-lactide, but it is less active than A6-A8.  t-But-BuNZnNOROO t-But-BuNZnNORArArA5R=benzyl, isobutyl, 2-(methylthio)ethyl, HAr=F	   15	   Figure 1.12. b-diketiminate (BDI)-based dinuclear zinc complexes.  N-heterocyclic carbenes (NHC) are well studied in metal-free polymerization catalysis.50 NHC-based metal catalysts also show great potential in lactide polymerization due to their readily tuned steric properties,  which are important for stereoselectivity.51 Hillmyer et al. reported a NHC-dizinc complex A11. (Figure 1.13)52 Complex A11 is an efficient catalyst for the polymerization of D-LA and L-LA, yielding heterotactic enriched PLA (Pr = 0.6) with good control over molecular weight and polydispersity (Đ = 1.2). A11 is dimeric in the solid state but the solution structure of A11 was not revealed. Kinetic studies show a first order dependence on monomer concentration indicating a monomeric active species. However, the dissociation of NHC from the metal is possible and the catalysis of free carbene cannot be ruled out. Free carbene with D-LA and L-LA also shows high activity and has better stereoselectivity, generating isotactic enriched polymer (Pm = 0.75). In the past decade, efforts were made to limit the potential for Zn-NHC dissociation but the issue continues to remain a challenge.53-54 NZnNNZnNPhPhOONZnNPhPhi-Pri-PrA10RRRROONZnNRRRRiPriPr NZnNiPr iPriPriPrNZnNiPriPriPrO OiPrO OR=iPr,  A6R=Et,  A7R=nPr, A8A9	   16	   Figure 1.13. N-heterocyclic carbene supported zinc complex. 	  In 2002, Hillmyer, Tolman and coworkers reported a novel dizinc-monoalkoxide complex A12 supported by a dinucleating  diaminophenoxy ligand (Figure 1.14).55 It rapidly polymerized D-LA and L-LA at room temperature (90% conversion in 30 min at 0.3 mol% catalyst loading) with good control over molecular weight and narrow Đ. Complex A12 did not show stereoselectivity, and atactic PLA was produced from rac-LA. The polymerization was living, which was proved by the addition of monomers to the reaction mixture already at full conversion. Kinetic studies (first order dependency on monomer and catalyst), and end group analysis by 1H-NMR confirmed the bimetallic coordination-insertion mechanism. Hillmyer and Tolman later reported another dizinc alkoxide complex A14 coordinated by the same dinucleating ligand as complex A12.56 Complex A14 showed high activity in lactide polymerization and it is one of the most active Zn-containing catalysts for ROP of lactide.57 Complex A13, a monomeric Zn ethyl compound, is the precursor of complex A14 and shows no activity towards lactide polymerization. Complex A14 is a dimer in the solid-state and cleaves into monomers in solution. Polymerization of D-LA and L-LA with complex A14 reached 93% conversion in 18 min at a very low level of catalyst loading (<0.1 mol%), and yielded Mn as high as 130 kg mol-1. The polymerization followed a coordination-insertion mechanism and displayed good control of molecular weight and narrow Đ (~1.4). Though the Mn had a linear dependence NNRRZnOPhNNRRZnOPhOOPhPh1/2 R= 2,4,6-trimethylphenylA11	   17	  on LA conversion, the experimental molecular weight is lower than the theoretical values due to impurities or/and exchange agents. Similar to complex A12, complex A14 did not show any stereoselectivity.   Figure 1.14. Zn complexes supported by diaminophenoxy ligands reported by Hillmyer and Tolman et al.  More recently, Williams et al. reported a dizinc complex supported by a bis(imino)diphenylamido macrocyclic Schiff base ligand.58 They synthesized two Zn-HMDS (bis(trimethylsilyl)amido ) complexes A15 and A16, and prepared the monozinc analogue A17  in order to compare their catalytic performance and confirm the cooperative activity between two zinc centers. Complex A15 and A16 are all highly efficient catalysts for rac-LA ROP, and A15, the most active catalyst, showed better rates than the previous reported Zn-containing catalysts for lactide polymerization in the literature.57  Complex A15 polymerized 1000 equiv. of monomer within one minute with TOF values up to 60,000 h-1 (0.1 mol% loading, 298 K, [LA] = 1 M).  The activity of dizinc complex A15 is three times greater than that of the monozinc complex A17, indicating dinuclear cooperativity between the two zinc centers. Kinetic studies revealed a zero-order dependency on monomer concentration, which is unexpected, and it is attributed to monomer saturation due to the steric hindrance of HMDS group and the high rates NNNZnOZnNOClClRNNOZnRN NO ZnOORNNOZn1/2R=tBu, MeA12 A13 A14	   18	  of initiation and propagation.59 Polymerization is well controlled in all cases, producing PLA with predicable molecular weight and narrow dispersities. However, the polymerization is not living (Mn only up to 57 kg mol-1), and transesterification is observed in the MALDY-TOF spectrum. The analogous zinc alkoxide initiators A18 and A19 were prepared to understand the high activity of zinc-HMDS complexes. Both are efficient catalysts for rac-lactide polymerization with good control over molecular weight and dispersity. However, they are much slower than their HMDS analogues. Furthermore, complexes A18 and A19 show first-order dependence on monomer concentration in kinetic studies. The dramatic differences in performance of Zn-HMDS complex A15 and its alkoxide analogue A18 are rationalized by structural data. The high activities displayed by complex A15 is rationalized by the macrocyclic ligand adopts a “folded” conformation due to the steric hindrances from –TMS groups. In this conformation, the ligand provides strong electron donation and open coordinate sites at metal centres. Also, the distance between metals are shortened in the “folded” conformation, resulting in enhanced metal-metal cooperativity.     Figure 1.15. Dizinc complexes supported by macrocyclic Schiff base ligand reported by Williams et al. NNNNNNZn X ZnXnnX=N(SiMe3)2n=1, A15n=2, A16NNNNNNZn X ZnXnnX=OiPrn=1, A18n=2, A19X=N(SiMe3)2n=1,  A15n=2, A16NNNZn XX=N(SiMe3)2, A17X=OiPr, A20	   19	  	  Inspired by the work of Hillmyer and Tolman, the Mehrkhodavandi Group reported a chiral version of complex A13 in an attempt to enhance the stereoselectivity of this family of complexes.60 In contrast to complex A13, complex A21 is unreactive to methanol, ethanol and isopropanol. However, treatment of complex A21 with phenol resulted in the formation of complex A22 due to the higher acidity of phenol in organic solvents. Polymerization studies showed that complex A22 is significantly slower than A14, which is explained by the lability of the terminal secondary amine. In complex A14, the ethylene diamine dissociates from the metal center, forming an unsaturated zinc center. However, the terminal amine group in complex A22 did not undergo dissociation due to the rigid diaminocyclohexane backbone of the ligand. Polymerization of rac-LA with complex A22 generated atactic PLA with poor molecular weight control and broad dispersity, which is attributed to incomplete initiation.   Figure 1.16. Zn complexes supported by chiral diamino phenoxy ligand studied in Mehrkhodavandi Group. 	  Based on the structures of A21 and A22, a series of dimeric zinc catalysts A23-A25 supported by similar diamino phenolates and Schiff bases were reported and the role of the central nitrogen donor was revisited.61 In our previous publication, it was reported that the nature RRONNR'ZnRRONNZnO PhR=t-Bu,HR'=Me, HA21 A22	   20	  of the central amine donor is essential to catalyst reactivity in a family of dinuclear indium complexes bearing the diamino phenolate ligands.62 It was shown that the rate of polymerization is much faster when the central amine donor was a secondary amine.62-63 Similarly, the structure and reactivity of the dimeric zinc complexes with secondary amine (A23), imine (A24, A25) and tertiary amine (A26) were different. Complex A23-A25 were dimeric in solution and solid-state, and display ligand lability of the terminal amine donor, exhibiting an equilibrium between k 3-and k 2-. They are highly active for ROP of lactide with the reaction rates comparable to the some of the mostly highly active zinc catalysts in the literature,61 and show good control over molecular weight and dispersity. Complexes A24 and A25 generate heterotactically enriched PLA with Pr values of 0.80 and 0.68. In contrast, complex A26 is monoculcear in solution and the solid-state, and is much less active and produces atactic PLA. Minor transesterification occurs during polymerization with A24 and A25, while polymerization with A23 undergoes extensive transesterification and depolymerisation.   Figure 1.17. Dinuclear and mononuclear Zn complexes supported by chiral diamino phenoxy ligand studied in Mehrkhodavandi Group 	  In an attempt to further investigate the cooperative catalysis in lactide polymerization, further research on dinucleating ligands were conducted by our group. Based on the dinucleating tButBuONNHZn O2RRONNZn O2tButBuONNZn OR=tBu, A24R=Cm,  A25A26A23	   21	  bis(diamino)-phenolate ligand in complex A12 developed by Hillmyer and Tolman,55, 64 a chiral version ligand platform was designed and synthesized.63 A dinuclear Zn complex A27 supported by this ligand was prepared and studied for ROP of lactide. However, complex A27 showed much lower activity compared to complex A12. Polymerization of rac-lactide with complex A27 reached 95% conversion in 5 days, generating PLA with slightly heterotactic bias (Pr = 0.64). The dramatic decrease in activity is due to the non-labile ancillary ligand. The ligand could not decoordinate from the metal center during polymerization, making unavailable coordination site for monomer. This research suggests that the design of new dinuclear catalysts will require a delicate steric balance to facilitate monomer coordination and to allow the isolation of thermodynamically stable complexes.  Figure 1.18. Dinucleating ligand developed by Hillmyer and Tolman. Dinuclear Zn complex reported by Mehrkhodavnadi.  1.4.   Multi-metallic complexes linked to a single-frame ligand: cooperative catalysis in ring-opening polymerization of lactide 	  As discussed in the previous section, most of the highly active catalysts for lactide ROP are dinuclear species, and their high activity depends on the dinuclear nature.48, 56, 61, 65-66 However, most of these catalysts are supported by a mononucleating ligand and dissociation into NNNOHNNNNH HZnOZnNOClClA27	   22	  monomeric species could occur during polymerization. Dinucleating ligands with a single-frame scaffold are able to overcome the dissociation problem and induce cooperative reaction pathways, leading to enhanced activity and selectivity.67 Numerous studies on dinuclear catalysts for ROP of lactide are reported, but detailed studies on kinetics and comparisons of dinuclear to pertinent mononuclear species are very limited.68 The recent studies on multi-metallic complexes linked to a single-frame ligand for cooperative catalysis in lactide ROP are reviewed in the following section, and it is organized based on two ligand types: N-donor ligands and N,O-donor ligands. Two skeletons based on N-donor ligands are reported in the literature, and they are all imino-derivatives.67 Shaver et al. reported a series of bimetallic dimethyl Al anilido-aldimine complexes A28.69 This ligand family arises from the salen ligand, but with more steric bulk. The increased steric bulk was designed to improve the isospecificity of lactide polymerization by chain-end control mechanism. Solution polymerization of rac-lactide with A28 at 70 ºC showed moderate activity with good control over molecular weight and polydispersity, but does not afford stereoselectivity. Kinetic studies showed a first-order dependence on lactide, and no metal-metal cooperativity was revealed. The attempted synthesis of a monometallic analogue failed.  Figure 1.19. Dinuclear Al complexes supported by single-frame imino ligand.   NNAlArN NAl Arnn=2, 3A28	   23	  Carpentier et al.  reported a dimetallic Zn complex supported by a tridentate bis(pyrazolyl) amine ligand A29.70 It is active towards the polymerization of rac-lactide, reaching >99% conversion in 30h at room temperature with good control over molecular weight. It displayed better activity and control than its mononuclear alkyl Zn and cationic Zn analogues, but it also produced atactic PLA and there was no metal-metal cooperativity, as determined by kinetic studies.  Figure 1.20. Dimetallic Zn complex supported by a tridentate bis(pyrazolyl) amine ligand. 	  Fontaine et al. reported a ligand platform supported by a triaminocyclohexane with pendant soft donor.71 The bimetallic Al complex A30 is active to e-lactone and rac-lactide polymerization while the monometallic analogue is not. However, no cooperative behavior is observed. It should be noted that no activator, such as alcohol is needed for the polymerization with the bimetallic complex.             Figure 1.21. Dimetallic Al complexes supported by triaminocyclohexane ligand and their mononuclear analogues. NN N NNZnZnOA29NNNAlAlRRRR=PPh2, SPhA30NNNAlRRR	   24	  	  As mentioned previously, Williams et al. reported a macrocyclic Schiff base ligand and a series of highly active dizinc catalysts A15-A19 supported by this scaffold. 58 These complexes show clear cooperative catalysis in rac-lactide polymerization, and the ligand conformation is essential for the metal-metal cooperativity. N, O-mutidentate ligands are the most studied ligands in ROP of lactide due to their readily tunable electronic and steric properties.67 It had been reported that possible cooperative catalysis could occur between metal centers linked by M-O-M’ bonds.55, 72  In 2008, Redshaw et al. reported a series of multinuclear alkylaluminum complexes supported by a macrocyclic Schiff base platform.73 The macrocyclic ligands were designed to provide a suitable catalytic environment that could enforce cooperativity between metal centers and to prevent complex aggregation in solution. The aluminum complex A31 is active for ROP of e-caprolactone polymerization and shows good control over molecular mass and polydispersity. The mono-and dinuclear analogue of complex A31 are also studied for e-caprolactone polymerization, and they show reduced reactivity, suggesting a cooperative effect exists in complex A31. It is proposed the Al…Al distance may favor the coordination of lactide monomer to both catalytic centers: one Al being used as a Lewis acid and the other Al using Al-R functionality to attack the carbonyl group. 	   25	   Figure 1.22. Mutimetallic aluminum complex and its mono- and dinuclear analogues.  Dinuclear aluminum complex A32 and indium complex A33 coordinated by a single-frame bis(phenoxy imine) platform were first investigated by Kirillov et al.68 These catalysts showed clear metal-metal cooperativity. Kinetic studies demonstrated that the dinuclear Al complex has lower activation free energy than that of directly related mononuclear species. Also, complex A32 exhibits 5-to-10-fold enhanced reactivity in ROP of rac-lactide when compared to its mononuclear analogue. It is proposed that the enhanced activity may stem from beneficial cooperative effects of the metal centers. It is suggested that the distance between metal centers could approach as close as to 2.8 to 3.0 Å due to the low rotation barrier of aryl-aryl bond, which would induce metal-metal cooperativity. NtBuO NAl AlNtBuO NAl AliPrNHtBuO NiPrAliPr iPriPrNtBuO NiPrAliPr iPrAlA31	   26	   Figure 1.23. Dinuclear aluminum and indium complexes and their mononuclear analogue.  1.5.   Cooperative catalysis in CO2/epoxide ring-opening copolymerization 	  Another area where cooperative catalysis is studied extensively is with the ring opening copolymerization (ROCOP) of CO2 and epoxide. The reaction is a sustainable method to capture carbon, as well as to synthesize a wide range of aliphatic polycarbonates which have potential applications as thermoplastics, resins, packaging materials.74  The polymerization followed a two-step process: 1) insertion of CO2 into a metal alkoxide, and 2) insertion of epoxide into a metal carbonate. Therefore, most catalysts for this reaction feature alkoxide or carboxylate groups which are similar to the proposed catalytic intermediates.74 Cyclic carbonates are a common by-product of the reaction, resulting from degradation of the propagating polycarbonate chain by depolymerization or backbiting.75 NNO MBnRRR2NNR1O MBnXXNNOMBnRRA32:  M= Al, X=Me, R1=SiPh3 ,Ph   R2=Me, HA33:  M=In,  X=CH2SiMe3, R1=SiPh3, R2=Me	   27	   Figure 1.24. Mechanism of epoxide-CO2 copolymerization and the formation of cyclic carbonates. 	  The most commonly used metal catalysts in this reaction include Zn(II), Co(III) and Cr(III).76 It is common in this area of catalysis that dinuclear catalysts show great performances and it is proposed bimetallic pathways could accelerate epoxide ring opening.76-77 In the following section, the recent progress in the development of bimetallic catalysts in ROCOP of CO2 and epoxides is reviewed. One of the most important developments in the last decades is reported by Coates et al., regarding a series of highly active bimetallic zinc BDI catalysts A34.77-80 These catalysts all displayed high activity (TOF = 193~247 h-1, at 50 ºC and 7 bar pressure of CO2) and high selectivity (carbonate linkages>94% ). They also examined ligand substituents effects on reactivity for these catalysts.80 Later, in their landmark paper investigating the mechanism behind this reaction, they demonstrated clear evidences for cooperative catalysis.77 It was found that the reaction followed a bimetallic epoxide enchainment process, and the most active species existed OLnM OOOOOPR R'O OOOPR R'LnMCO2n nORR'metal alkoxidemetal carbonateOOORR'polymerization manifold cyclizationmanifold	   28	  as “loosely associated” dimers (Zn…Zn separation 4.0-4.1 Å ).77 It was confirmed by kinetic studies that the order on zinc varies from 1.0 to 1.8.    Figure 1.25. N, N, O-BDI highly active dimeric Zn catalysts reported by Coates et al., and the unreactive tightly bound dimer and monomer.  Inspired by Coates’s study, many other research was focused on the development of dinuclear catalysts for ROCOP of CO2/epoxide.  Lee et al. reported a series of bis(anilido-aldimine) Zn(II) complexes A35.81 They show high activities (TOF= 106-312 h-1, at 80 ºC, 12 bar of CO2) and good selectivity (carbonate linkages 91-94%) at low catalyst loading ([Zn]:[epoxide]=1:5600), while the mononuclear species only displayed negligible activity, indicating cooperative catalysis occurred  in the polymerization. Their work also strongly supported the bimetallic mechanism proposed by Coates and co-workers.77 NZnNRRRRNZnNRRRO ORO ONZnNRRRRNZnNRRRO ORO ONZnNRRRROOTightly bound dimerUnreactiveLoosely bound dimerHighly activeMonomer incapable of dimerizationUnreactiveA34	   29	   Figure 1.26. N, N, O, O bis(anilido-aldimine)  Dizinc complex reported by Lee. 	  More recently, many efforts were made towards the investigation of “tethered” Zn(II) BDI catalysts. Among those, the best system for ROCOP of CO2/epoxide was reported by Rieger et al., with their dinuclear Zn(II) catalyst A36 (Figure 1.27).82 This tethered catalyst showed TOF of 155, 000 h-1, at 100 ºC and 30 bar CO2. It also showed high activity at 1 bar CO2 with TOF of 940 h-1. Unlike other reported systems where the rate-determining step is the ring-opening of epoxide, the activity of this catalyst showed a strong dependency on CO2 pressure, suggesting a significantly accelerated cooperative ring-opening step to an extent that pressure dependency on the rate equation occurred. Later,  they reported mechanistic studies on this dinuclear Zn system, and confirmed their hypothesis that the cooperative catalysis induced by the flexible tethered structure is essential for the observed high activities.83-84 N NHiPriPrZnO OMeHS SHMeFFFN NHiPriPrZnO OFFFA35	   30	   Figure 1.27. Dizinc complex supported by a macrocyclic BDI ligand reported by Rieger.  Williams and co-workers investigated a range of dimetallic catalysts A37 supported by macrocyclic diphenolate ancillary ligands for CO2/CHO ROCOP. A di-zinc catalyst was reported as the first catalyst showing promising activity under 1 bar pressure of CO2. (TOF=25 h-1, 100 ºC)85 They attributed the high activity to the dimetallic core by comparison to a mononuclear analogue complex that was inactive in this reaction. Based on DFT calculations and experimental studies, they proposed a chain shuttling mechanism in which one metal center coordinates the epoxide and the other center inserts carbon dioxide.86-87 They also explored a variety of ligands, metals and co-ligands, and found that changing the metal center has a significant effect on the reactivity and a heterodinulcear Mg/Zn complex is more active than the homodinuclear complexes.86, 88-89 Interestingly, it is also found complex A37 (M=Zn, X=OAc) is active towards e-caprolactone polymerization in presence of chain transfer agents or epoxide. 90 NNF3CZnN(TMS)2 NNCF3Zn(TMS)2NA36	   31	   Figure 1.28. Dimetallic catalysts supported by macrocyclic diphenolate ancillary ligands reported by Williams. 	  1.6.   Scope of the thesis 	  Based on the previous studies on dinuclear indium catalysts in our group, it was determined that the dinuclear nature of the catalysts is essential to the activity and selectivity.66, 91 In order to enforce the cooperativity between metal centers, it was of great interest to design a new dinucleating ligand platform with single-framework that would prevent dissociation of two metal centers during polymerization.  1) Development of a new dinucleating ligand platform. A novel dinucleating ligand L was synthesized and fully characterized (1H-NMR, 13C{1H}-NMR, COSY, NOESY, HSQC, HMBC NMR, EA). Also, the synthetic procedures were optimized. 2) Development of new zinc complexes supported by ligand L. Attempts at complexation of L’ with zinc failed. Zinc complexes (1-4) supported by L are successfully synthesized, and tBuNHNHOtBuNHNHOM MXXM=Mg, Zn, Co(II/III), Fe(III)X= OAc, carboxylate, alkoxide, halideA37	   32	  complexes 1-3 are fully characterized (1H-NMR, 13C{1H}-NMR, 2D NMR, EA, X-ray crystallography).  3) Preliminary reactivity studies of complexes 1-3 as catalyst for the polymerization of lactide and CO2/epoxide copolymerization.              	   33	   Chapter 2: Design, synthesis, and characterization of a dinucleating ligand platform and the resulting zinc complexes 	  2.1.   Introduction: ligand design for enforcing cooperative effects in catalysis	  	  Catalysis is the key to convert simple molecules into highly functionalized organic compounds in a sustainable way. Nature has created enzymes to perform chemical transformations in a highly efficient and selective manner.92 In many cases, at the enzyme active sites where the catalytic environment is strictly controlled, multiple metal ions are working synergistically to activate the challenging substrates.92 Mimicking nature has led to the development of polymetallic catalytic systems to investigate the cooperative effects between metal centers.93-96 Enzymes moderate activity and selectivity by adopting particular protein conformations.67 Therefore, when developing cooperative catalysts, the construction of a suitable ligand is essential for achieving efficient synergistic action of metal centers. The design of ligands requires combining multiple catalytic sites into a single-frame structure and optimizing metal-metal separation.67 It is proposed that the optimal separation between two metal centers is within 3.5- 6Å  other than a direct interaction.93 Therefore, a delicate control over metal-metal proximity and ligand conformation is highly important for efficient cooperative catalytic effects. As discussed in Chapter 1, our group has focused on ligand design and catalysts development for ROP of cyclic esters. Our first generation catalyst (Figure 2.1), an asymmetrically bridged dinuclear indium complex supported by chiral phenoxy cyclohexyldiamine ligand shows extremely high activity and moderate stereoselectivity towards lactide polymerization. However, when steric bulk was introduced to the ligand in order to 	   34	  improve its selectivity, the catalyst was found to dissociate and all selectivity was lost, indicating that the dinuclear nature is essential in its reactivity. To avoid catalyst dissociation, a single-frame ligand backbone is needed to enforce the close proximity between metal centers. In addition, like other asymmetric catalysis, to achieve enantioselective ring opening polymerization of cyclic esters, ligands with stereoelectronic properties and modular donors are required to alter steric bulk and electrophilicity of the metal center.97  Figure 2.1. Highly active dinuclear indium catalyst reported by Mehrkhodavandi.  In an attempt to enforce metal-metal cooperativity during polymerization, our group had reported a chiral dinucleating ligand based on the dinucleating bis(diamino)-phenolate ligand in complex A12 developed by Hillmyer and Tolman.55, 64 However, the zinc complex A27  based on the ligand exhibited much lower activity than A12 due to the significant steric hindrance around the zinc centers.63 In this project, a chiral dinucleating phenoxydiamino ligand L bearing a more flexible backbone was designed to vary the proximity of metal centers (Figure 2.2). We chose biphenol as the dinucleating scaffold because it possesses multiple binding sites with a well-defined structure, and a relatively low barrier of rotation around its aryl-aryl bond.98 In addition, ligand L bears a bulky chiral cyclohexyldiamine group which is designed  to enforce enantioselectivity.  NNOtBu tBuInHNNOtButBuIn HClOClCl	   35	   Figure 2.2. Design of single-frame dinucleating ligand for enforcing metal-metal cooperativity.  We are particularly interested in exploring metal complexes derived from L’ and L because the 2,2’-biphenol motif has a lower barrier of rotation around the aryl-aryl bond,99 which could bring metal centers into close proximity, allowing for cooperative catalysis. The precursor imine compound L’ was successfully synthesized, but attempts to generate zinc complexes with L’ all failed. Therefore, this thesis focuses on the synthesis and reactivity studies of ligand L, a novel chiral dinucleating di(diamino)phenolate ligand. 	  	  	  	  	  NNNOHNNNNH HOHNHillmyer and Tolman, 2003 Mehrkhodavandi, 2016OHNNHONNL'OHNHNHOHNNL	   36	  2.2.   Synthesis and characterization of ligand L 	  The precursor compound L’ and ligand L supported by 2,2’-biphenol backbone were synthesized based on published procedures with modifications.65, 100-101 The synthesis of (R,R)-N,N-dimethyl-trans-1,2-diaminocyclohexane was performed according to the literature100 from commercially available (±)trans-diaminocyclohexane (Scheme 2.1). A chiral resolution was performed using L-tartaric acid to obtain enantiopure diamine. One amine group was protected by tert-butoxycarbonyl(Boc) group to prevent it from undergoing subsequent reaction. Formation of alkylated amine was achieved by reductive amination in the presence of formaldehyde and sodium cyanoborohydride in acidic condition. The asymmetrically methylated cyclohexyldiamine was obtained after deprotection of the Boc group.  Scheme 2.1. Synthesis of (R,R)-N,N-dimethyl-trans-1,2-diaminocyclohexane.   The synthesis of the bis-aldehyde biphenol d (Scheme 2.2) followed a modified procedure from the literature,101 and we have successfully optimized the reaction to obtain a higher yield. In the literature procedure, sodium hydride102 was used to synthesize compound b from the starting material 2,2’-biphenol, but the reaction suffered from low yield. (50%-60% yield in literature).98, 101, 103 The resulting overall yield for the multistep synthesis of d was only NH2NH2racemicchiral resolutionNH2NH21. HCl2.(Boc)2OHNNH2OO1.HCHO2.NaBH3CN, CH3COOHHNNOONH2NHCl60 0C MeOHCH3CNMeOH	   37	  around 23-27%. The low yield was attributed to the poor solubility of NaH in organic solvents, leading to incomplete deprotonation of 2, 2’-biphenol. With NaH, a significant amount of byproduct mono-(methyoxymethyoxy)biphenyl was formed, resulting in the low yield of compound b. In order to obtain complete deprotonation, a series of strong bases (tert-BuOK, KH, CaH2, trimethylamine, n-BuLi) were tested. Although both tert-BuOK and n-BuLi are soluble in THF, reaction with n-BuLi afforded better yield. After the optimization of reaction conditions (see Experimental procedures), the yield of compound b was significantly increased to 85%. The two methoxymethyl(MOM) groups of b directed ortho metalation in the presence of n-BuLi, and compound c was formed after the addition of dimethylformamide. Hydrolysis of the MOM protecting groups in c afforded the desired biphenol bis-aldehyde compound d. Recrystallization of d from hexane/ ethyl acetate afforded needle crystals suitable for X-ray diffraction analysis. Scheme 2.2.	  Synthesis of ligand L.   OH nBuLiCH3OCH2ClHOOMOMMOMOnBuLi, DMFEt2O, 0 oCOMOMMOMOOHCCHODCM, EtOH, 60 oCOHHOOHCCHONNH2MeOHNaBH4MeOHTHFHClOHNNHONNOHNHNHOHNNL' La b c d	   38	   Figure 2.3. Solid-state structure of compound d obtained by single crystal X-ray diffraction with ellipsoids at the 50% probability level. Most hydrogen atoms were omitted for clarity.  Condensation of compound d with 2.1 equivalent of the chiral N,N-dimethyl-trans-1,2-diaminocyclohexane at room temperature in methanol led to the formation of the imine precursor L’ with high yield (90%-95%). Compound L’ is characterized by 1H-NMR spectroscopy with appearance of imine resonances at 8.35 ppm (Figure B.2). Dry methanol and excess of N,N-dimethyl-trans-1,2-diaminocyclohexane is essential in this reaction to avoid hydrolysis of imine product and the formation of mono-substituted imine. However, when the condensation reaction was scaled up to 0.5 g (2.1 mmol), multiple species were formed evidenced by multiple peaks in imine resonance area in the 1H-NMR spectrum and all attempts of further purification were unsuccessful. Desired diaminophenol ligand L was obtained from the reduction reaction of L’ with sodium borohydride.  Ligand L was characterized by 1H-NMR, 13C{1H}-NMR, 2D NMR spectroscopy (COSY, NOESY, HSQC and HMBC), mass spectroscopy and elemental analysis. The 1H-NMR spectrum of L (Figure B.3) displays characteristic doublets at 3.87 ppm and 4.15 ppm corresponding to a set of diastereotopic protons from NH-CH2-Ar, and three sets of aromatic protons (two doublets 	   39	  and one triplet). The 1H-NMR spectrum of L is more simplified compared to that of L’. COSY NMR spectrum (Figure B.5) confirmed the structure of L by showing the correlation between two diastereotopic protons and correlation between aromatic protons. In addition, in NOESY NMR spectrum (Figure B.6), a correlation between the methyl protons on amine and the diastereotopic protons from NH-CH2-Ar was observed, indicating a close spatial distance between the amine arm and biphenol backbone. Furthermore, the 13C{1H}-NMR spectrum (Figure B.4) combined with HSQC and HMBC spectra (Figure B.7 and Figure B.8) provided additional insights into the structure of proligand L. All the protons and carbon atoms are assigned, and even the aromatic protons and carbons can be distinguished according to HSQC and HMBC spectra. The structure of L was further verified by mass spectroscopy with a characteristic peak at 495.3 m/z (Figure B.9).  2.3.   Synthesis and characterization of zinc complexes supported by chiral dinucleating ligand  	  2.3.1.   Synthesis of alkyl zinc complex: a trinuclear species 	  Treatment of ligand L with 2.1 equiv. of diethyl zinc at low temperature afforded alkyl zinc complex via a protonolysis reaction with the elimination of ethane. Surprisingly, a trinuclear alkyl zinc complex (L)Zn3(CH2CH3)4 (1) was isolated as the main product instead of the expected dinuclear species. In order to obtain a dinuclear zinc species, different amounts of diethyl zinc, ranging from 0.5 to 2 equivalents, were added to solutions of L under various temperatures and concentrations.  However, all reactions resulted in the formation of the trinuclear zinc complex, suggesting the trinuclear zinc complex is more thermodynamically favored and stabilized by the 	   40	  ligand framework.  The complex was characterized by 1H-NMR, 13C{1H}-NMR, 2D-NMR (COSY, NOESY, HSQC, HMBC) spectroscopy, and X-ray crystallography.  Scheme 2.3.	  Synthesis of trinuclear alkyl Zn complex 1 (L)Zn3(CH2CH3)4.  The 1H-NMR spectrum (Figure 2.4) of the alkyl zinc complex (L)Zn3Et3 (1) displays a lower degree of symmetry compared to the free ligand L. The methyl groups on terminal amine give rise to two distinct singlets at 1.49 ppm and 1.74 ppm, indicating a desymmetrized ligand. Complex 1 exhibits two doublets at 3.27 ppm and 4.67 ppm corresponding to the diastereotopic protons from NH-CH2-Ar, while in the free ligand the doublets are closer observed at 3.78 ppm and 3.96 ppm, respectively. The ethyl group protons on Zn1 and Zn2 exhibit the same chemical shifts, which are different from those on Zn3, but they all shift downfield compared to the ethyl protons of pure ZnEt2. The terminal methyl protons CH3-CH2-Zn1(Zn2) are observed as a triplet at 1.69 ppm, which is further confirmed by NOESY-NMR (Figure B.12), where it displays spatial correlation with the methyl groups on terminal amine. A quartet of doublets corresponding to the methylene protons CH3-CH2-Zn1(Zn2) is found at 0.13-0.25 ppm, and it shows a strong correlation with the methyl group CH3-CH2-Zn1(Zn2) in the COSY-NMR spectrum (Figure B.11). The methyl protons from CH3-CH2-Zn3 appears at 0.88 ppm and the methylene protons are observed as a multiplet at 0.59-0.63 ppm. The ethyl peaks on Zn3 are obscured by ligand peaks, but can be distinguished in COSY spectrum. These observations are toluene,rt, 16h-C2H6ONHNZn1 Zn3OZn2NNHZn(CH2CH3)2OHNNMe2HONNMe2HH1	   41	  supported by 13C{1H}-NMR, HSQC and HMBC spectroscopy (Figure B.13-15). In addition, the previously reported alkyl zinc complexes by our group showed similar NMR features.60-61   Figure 2.4. 1H-NMR spectra of free ligand L (blue) and complex 1 (red) (400 MHz, C6D6, 25 0C) 	  Single crystals suitable for X-ray diffraction were grown from a diethyl ether and THF solution at room temperature. The molecule is trinuclear with the three four-coordinate Zn(II) ion adopting a distorted tetrahedral geometry. The differences between bond angles O1-Zn1-C37 (127.5(3)°) and N1-Zn1-N2 (82.8(2)°) are illustrative of the distortion from ideal tetrahedral geometry. The bond length of Zn3-O1(2.180 Å) and Zn3-O2(2.197 Å) are significantly longer than Zn1-O1(1.985 Å) and Zn2-O2(1.971 Å), indicating Zn3 is more weakly bonded to the ligand, and has different electronic properties when compared to Zn1 and Zn2. The Zn1-C37 distance is 1.974(8) Å, and similarly Zn2-C31 distance is 1.984(8) Å which is consistent with the usually seen Zn(II)-alkyl bond distance (1.93-1.98 Å).104-106 However, the bond lengths of Zn3-	   42	  C33 ( 2.017(9) Å) and Zn3-C35 (2.044(9) Å) are slightly longer than  Zn1-C37 and Zn2-C31. The above analysis suggests Zn1 and Zn2 possesses similar electronic properties and are very different from Zn3. The Zn-N length of the central secondary amine is shorter than the terminal tertiary amine, which is evidence that N2 and N4 are more weakly coordinated to Zn1 and Zn2, respectively.  The bond length of Zn1-N1(2.115(6) Å) and Zn2-N3 (2.121(6) Å) is close to the Zn-N length of a central tertiary amine donor in a mononuclear alkyl zinc complex with a similar ligand motif.60. The distance between Zn1 and Zn3 is 3.600 Å, slightly shorter than the distance between Zn2 and Zn3 (3.616 Å).  Figure 2.5. Solid-state structure of complex 1 with ellipsoids at the 50% probability. Hydrogen atoms were omitted for clarity. 	  	  	  	   43	  Table 2.1. Selected bond lengths (Å) and angles (°) for complex 1. Bond lengths (Å) Zn1-N1 2.115(6) Zn2-N3 2.121(6) Zn3-O1 2.180(5) Zn1-N2 2.185(6) Zn2-N4 2.198(6) Zn3-O2 2.197(5) Zn1-O1 1.985(5) Zn2-O2 1.971(5) Zn3-C33 2.017(9) Zn1-C37 1.974(8) Zn2-C31 1.984(8) Zn3-C35 2.044(9) Bond angles (°) N1-Zn1-N2 82.8(2) N3-Zn2-N4 82.7(2) O1-Zn3-C33 106.4(3) O1-Zn1-C37 127.5(3) O2-Zn2-C31 128.0(3) O2-Zn2-C35 105.8(3) 	  2.3.2.   Synthesis and characterization of alkoxy zinc complex   Unlike the Hillmyer and Tolman complex A14, which is generated from the reaction of alkyl zinc complex with ethanol, reaction of complex 1 with ethanol only resulted in intractable mixtures or decomposition. This was also observed in our previously reported alkyl zinc complex and is explained by steric arguments and the acidity of the alcohol.60 Benzyl alcohol (pKa=15.4 in DMSO) which is more acidic than ethanol (pKa=29.8 in DMSO),107 was chosen to synthesize the alkoxy zinc complex starting from complex 1. Treatment of complex 1 with 4 equiv. of benzyl alcohol in toluene at 100 °C for 48 h afforded alkoxy zinc (L)Zn2(µ-OCH2Ph)2 complex 2 with only 25% yield. The low yield is resulted from the complicated purification of complex 2 because it is soluble in most organic solvents. The prolonged reaction time and high reaction temperature indicate low reactivity of complex 1 towards benzyl alcohol. The low reactivity of complex 1 can be attributed to steric hindrance because there is no significant electronic difference between complex 1 and A14 based on the bond lengths from solid-state structure.    	   44	  Scheme 2.4.	  Synthesis of dinuclear Zn complex 2.   Complex 2 is characterized by 1H-NMR, 13C{1H}-NMR, 2D-NMR (COSY, NOESY, HSQC, HMBC) spectroscopy, X-ray crystallography, and elemental analysis. The 1H-NMR spectrum (Figure 2.6) at room temperature displays a quartet for the Zn-OCH2-Ph methylene protons at 4.86 ppm, suggesting no free rotation along the alkoxide O-C bridge. In contrast, the methylene protons of Zn-OCH2-Ph are observed as a broad multiplet at room temperature in another dinuclear benzyloxy zinc system, which indicates a dynamic rotation along the O-C bond.61 These observations imply complex 2 has a more sterically encumbered environment around the zinc centers. This was also confirmed by the NOSEY-NMR (Figure B.19) spectrum, where the methylene protons of Zn-OCH2-Ph showed spatial interactions with the N(CH3)2 protons and cyclohexyl protons. Protons from N(CH3)2 exhibit two sharp singlets at 1.95 and 2.55 ppm, a downfield shift when compared to N(CH3)2 in the alkyl zinc complex 1. These two methyl groups can be distinguished from the NOESY NMR spectra (Figure B.19), where only one methyl group shows spatial interactions with the methylene protons of Zn-OCH2-Ph, and they all show correlation with the aromatic protons of the O-CH2Ph group. The diastereotopic protons from NH-CH2-Ar show up as a doublet of doublet at 3.45 ppm and a pseudo-triplet at 3.81 ppm. It should be noted that there are two H-bonded benzyl alcohol molecules observed in 1H-NMR spectra. The H-bonded benzyl alcohol –OH resonance is found at 3.90 ppm as a broad ONHNZn ZnOZnNNHOHToluene, 100 0C, 48h4ONHNZnOZnNNHOO2OHOH	   45	  singlet. The methylene protons in Ar-CH2-OH appear at 4.52 ppm also as a broad singlet, suggesting there is rotation along C-O bond. In addition, it is found that the H-bonded benzyl alcohol molecules undergo exchange with the bridging bezyloxy groups, which is evidenced by ROESY-NMR spectrum (Figure B.20). These peaks become more broad and coalescence at higher temperatures (Figure 2.6) due to the rapid molecule exchange.  In order to understand if there is any possible rotation of the alkoxide O-C at high temperatures, variable temperature NMR experiments were performed in d8-toluene (Figure 2.6). It was found that the quartet at 4.86 ppm of benzyl alkoxy (-CH2) at room temperature appeared as a singlet at high temperatures, indicating that rotation around the bridging alkoxide O-C does occur at higher temperatures.   Figure 2.6. Variable temperature NMR spectra of complex 2 (d8-toluene, 400 MHz). 	  	   46	  X-ray quality crystals of complex 2 were obtained from a diethyl ether/THF solution. The solid-state structure of complex 2 is a dinuclear species with two bridging benzyloxy groups. It is proposed the third zinc center Zn3 in complex 1 decoordinated from ligand during the protonolysis reaction as it is weakly bonded to O1 and O2 in complex 1, as evidenced by the unusually long Zn-O bond length. Both Zn centers adopt a distorted trigonal-bipyramidal geometry. The Zn-O and Zn-N distances in complex 2 are shorter than those in complex 1, indicating a more compacted structure. The most notable feature of the structure is that the Zn1 and Zn2 separation (2.911(6) Å) is shorter than the Zn…Zn separation in most alkoxy bridged dizinc systems.40, 56, 61, 108-110 The Zn2O2 bridging core is asymmetric with two different Zn-O distances, Zn1-O4 being longer than Zn1-O3. A significant structural difference between complex 1 and complex 2 is that complex 2 has a longer Zn1-N1(the central amine donor) bond length than that of Zn1-N2 (the terminal amine), indicating that the terminal tertiary amines are coordinated to the zinc centers more strongly. Another feature of the solid-state structure is that there are two benzyl alcohol molecules H-bonded to the phenoxy oxygen of the ligand backbone (O1-H6A 1.810 Å and O2-H5A 1.890 Å), which is consistent with the structure in solution observed from 1H-NMR spectroscopy.  	   47	   Figure 2.7. Solid state structure of complex 2• 2benzyl alcohol with ellipsoids at the 50% probability level. Most hydrogen atoms were omitted for clarity. 	  	  Table 2.2.	  Selected bond lengths (Å) and angles (°) for complex 2. Bond lengths (Å) Zn1-Zn2 2.911(6) Zn2-N3 2.248(3) Zn1-N1 2.207(3) Zn2-N4 2.148(3) Zn1-N2 2.153(3) Zn2-O2 1.980(3) Zn1-O1 2.010(3) Zn2-O3 2.080(2) Zn1-O3 1.990(2) Zn2-O4 2.004(2) Zn1-O4 2.093(2)   Bond angles (⁰) Zn1-O4-Zn2 90.54(10) O1-Zn1-O3 111.46(10) O2-Zn2-O4 121.29(11) N1-Zn1-N2 81.87(13) N3-Zn2-N4 80.58(11) N2-Zn1-O4 93.71(11) N4-Zn2-O3 94.55(10)   	   48	  2.3.3.   Synthesis and characterization of zinc acetate complex 	  In an attempt to synthesize dinuclear zinc species and upon consideration of the structure of complex 2, we decided to choose a zinc reagent with bridging groups. Zinc (II) acetate proved to be a suitable candidate because anhydrous zinc acetate features a zinc center coordinated to four oxygen atoms with bridging bidentate acetate groups.111  Also, zinc acetate complexes are commonly used in epoxide/CO2 copolymerization.27, 74 Deprotonation of the amine proligand L with potassium tert-butoxide followed by salt metathesis with 2 equiv of anhydrous Zn(OAc)2 in toluene at 100 °C for 16 h afforded a unexpected trizinc tetra-acetate complex (3) in 53% yield. Similar to the syntheses of alkyl Zn complex 1, various equivalents of Zn(OAc)2 (1equiv, 1.5 equiv) were used in an attempt to obtain dinuclear complex, but all reactions gave rise to trinuclear species, indicating the trinuclear structure is more thermodynamically stable. Scheme 2.5. Synthesis of trinuclear Zn acetate complex 3.   OHNNMe2HONNMe2HHt-BuOKtoluene, rtOKNNMe2KONNMe2HH-KOActoluene, 100 0CONHNZn ZnOZnNNHOO O OO OO O Zn(OAc)23	   49	  	  Complex 3 was characterized by 1H-NMR, 13C{1H}-NMR, 2D-NMR, X-ray crystallography, IR spectroscopy and elemental analysis. In the 1H-NMR spectrum (Figure 2.8), the four acetate groups show a sharp singlet at 2.12 ppm, which indicates these four acetate groups are all bidentate bridging groups. This is confirmed by IR spectroscopy (Figure A.1), where only bridging acetate (C=O, 1585 and 1429 cm-1) coordination mode 112 is observed. The methyl groups from–N(CH3)2 split into two singlets at 1.55 ppm and 2.34 ppm. The diastereotopic protons from NH-CH2-Ar show up as two doublet of doublet at 5.34 ppm and 3.55 ppm. It is noticeable that complex 3 is air-stable, and no decomposition was observed via 1H-NMR spectroscopy over several weeks.   Figure 2.8. 1H-NMR of Zn acetate complex 3 (400 MHz, C6D6, 25 ºC) 	  Single crystals of complex 3 were obtained from a diethyl ether and hexane solution. X-ray diffraction analysis revealed a molecular formula [(L)Zn3](µ-OAc)4, a trizinc structure with four bridging acetate groups. The molecule contains two five-coordinated Zn2+ (Zn1 and Zn2) centers with distorted trigonal bi-pyramidal geometries, and one six-coordinated Zn2+ (Zn3) 	   50	  center between Zn1 and Zn2 with distorted octahedral geometry. Zn1 and Zn3, Zn2 and Zn3 are bridged by two bidentate acetate groups, respectively. The O-C-O moieties of bridging acetate groups and two Zn atoms formed an eight-membered bi-metallacycle, presenting a unique trinuclear paddle-wheel conformation. The paddle-wheel is usually found as dinuclear Zn carboxylate clusters in metal organic frameworks,113 and it is rarely reported for trinuclear species.114   The Zn-O bond lengths are in the range of 1.954-2.119 Å, and Zn-N length is between 2.137-2.191 Å, comparable to other known Zn(II) acetate complexes.115-116 41, 117-118 Also, it should be noted that Zn-N distance between terminal amine and Zn center is shorter than that between the central amine donor and Zn center, which is also observed in complex 2. The Zn1-Zn3 distance is 3.264 Å and Zn2-Zn3 distance is 3.258 Å, consistent with other reported dinuclear Zn acetate complexes.117-118  Figure 2.9. Solid state structure of [(L)Zn3](µ-OAc)4 complex 3 with ellipsoids at the 50% probability level. Hydrogen atoms were omitted for clarity. 	   51	  Table 2.3. Selected bond lengths (Å) and angles (°) for complex 3. Bond lengths (Å) Zn1-O1 1.957(4) Zn2-O2 1.953(5) Zn3-O1 2.096(5) Zn1-O5 1.974(5) Zn2-O8 1.974(5) Zn3-O2 2.076(4) Zn1-O3 2.099(4) Zn2-O9 2.086(4) Zn3-O4 2.109(5) Zn1-N1 2.191(5) Zn2-N3 2.176(5) Zn3-O6 2.119(5) Zn1-N2 2.137(6) Zn2-N4 2.143(6) Zn3-O7 2.098(5) O3-Zn1-O5 98.5(2) O8-Zn2-O9 99.1(2) Zn3-O10 2.083(4) Bond angles (⁰) N1-Zn1-O5 92.5(2) N3-Zn2-O8 90.8(2) O2-Zn3-O7 94.7(18) O1-Zn1-O5 118.9(2) O2-Zn2-O8 118.6(2) O7-Zn3-O10 98.0(2) 	  	  2.3.4.   Attempted synthesis of dinuclear zinc complexes with bulkier functionality 	  Based on the trinuclear structure of complex 1 and 3, we revisited the structure and electronic nature of the ligand L. It was hypothesized that ligand template effects and the unexpected high electron-donating ability of the phenoxyl oxygen lead to the formation of trinuclear complexes. However, in order to study metal-metal cooperativity in catalysis, it is preferential to build a bimetallic system rather than a more complicated multi-metallic system. It was proposed that the introduction of more steric constraints would force the ligand to give rise to dinuclear species. In this scenario, it proved challenging to modify steric properties of the ligand due to the synthetic difficulty, thus we chose a more sterically hindered Zn reagent-zinc bis(bistrimethylsilyl)amide Zn[N(SiMe3)]2. The synthesis of Zn amide complex 4 followed a literature procedure with modifications.82 Treatment of ligand L with 2 equiv. of Zn[N(SiMe3)2]2 in toluene at room temperature for 16 h led to the formation of dinuclear zinc amide species evidenced by 1H-NMR spectroscopy and X-ray diffraction analysis. However, all the attempts to purify the product 	   52	  resulted in intractable mixtures because the non-polar trimethylsilyl (-TMS) group is soluble in most organic solvents. After multiple washes with cold hexane, a relatively clean 1H-NMR spectrum (Figure B.25) was obtained. The methyl protons from HN(SiMe3)2 is observed at 0.37 ppm and they had a upfield shift to 0.19 ppm in complex 4. The –N(CH3)2 resonances split into two singlets at 2.03 and 1.59 ppm. The diastereotopic protons from NH-CH2-Ar show up as two doublets at 4.36 ppm and 3.29 ppm. Scheme 2.6. Attempted synthesis of dinuclear Zn species with increased steric bulk. 	     The X-ray quality crystal was obtained from a hexane solution at room temperature. From XRD analysis, the formula of the molecule is (L)Zn2[N(SiMe3)]2. The most notable feature of this structure is the wide separation of the two Zn centers (Zn-Zn distance 7.6 Å), much longer than the Zn-Zn separation in complexes 1-3. The two Zn centers are in distorted tetrahedral geometries, bound to three N atoms (two amine and one amide) and one O atom from the phenoxy ligand. Although the zinc centers in complex 1 also adopt distorted tetrahedral geometries, the bond lengths of Zn-N and Zn-O in complex 4 are much shorter than those in complex 1. This is attributed to the bulkiness of –N(SiMe3)2 group which induced steric pressure and enhanced Zn-N and Zn-O bond, and this is also seen in other reported zinc trimethylsilyl Me3SiNZnNSiMe3SiMe3 SiMe3OHNNMe2HONNMe2HHONNMe2ONMe2NHHZnNSiMe3Me3Si ZnNSiMe3Me3Si2rt, toluene, 16hrs-2HN(SiMe3)24	   53	  amide complexes. 119-120The bond length of Zn1-N3 (amide) in complex 4 is much longer than Zn-N bond length (1.833(11) Å) in solid-state Zn[N(SiMe3)2]2.120 The lengthening of Zn-N bond is probably due to the electron donating properties from the phenoxy ligand backbone.  From a structural point of view, metal-metal cooperativity is expected in complex 4 because the wide separation of zinc centers provides an adequate accessibility of the catalytic centers, and the low barrier rotation along aryl-aryl bond in the ligand framework gives rise to flexibility and varying conformations. Also, in a reported dizinc system that demonstrated unprecedented activity in ROCOP of CO2 and epoxide, the Zn-Zn separation was calculated as 7.77 Å, and with a tethered ligand backbone the metal-metal distance varies from 4.50 Å to 5.66 Å during the catalysis.82   Figure 2.10. Solid-state structure of (L)Zn2[N(SiMe3)]2 complex 4 with ellipsoids at the 50% probability level. Hydrogen atoms were omitted for clarity. 	   54	  Table 2.4. Selected bond lengths (Å) and angles (°) for complex 4. Bond lengths (Å) Zn1-O1 1.932(2) Zn2-O2 1.925(2) Zn1-N1 2.085(3) Zn2-N4 2.079(3) Zn1-N2 2.167(3) Zn2-N5 2.140(3) Zn1-N3 1.921(3) Zn2-N6 1.918(3) Bond angles (⁰) O1-Zn1-N1 95.26(11) O2-Zn2-N4 93.96(10) N3-Zn1-N2 113.86(13) N5-Zn2-N6 117.19(12)  2.4.   Conclusions 	  A novel single-frame dinucleating ligand L based on 2,2’-biphenol motif was successfully synthesized and fully characterized. We have optimized the synthesis of 3,3’-diformyl-2,2’-dihydroxy-1,1’-biphenyl to afford a higher yield. However, scale-up of the synthesis of L is still problematic, and the overall yield is relatively low due to the multiple-step synthesis. The coordination chemistry of ligand L was studied with various zinc precursors. A series of multi-metallic zinc complexes 1-4 were synthesized based on ligand L.   Trinuclear alkyl zinc complex 1 was synthesized by direct reaction of proligand L with ZnEt2. Dinuclear alkoxy zinc complex 2 was synthesized via the reaction of complex 1 with benzyl alcohol. Trinuclear zinc acetate complex 3 was synthesized via deprotonation of proligand L followed by salt metathesis with Zn(OAc)2. Complexes 1-3 were fully characterized by 1H-NMR, 13C{1H}-NMR, 2D-NMR spectroscopy, and single crystal X-ray crystallography. The structure of complex 2 in solution was further studied by various temperature 1H-NMR spectroscopy. Attempted synthesis of dinuclear zinc species with bulkier functionality resulted in a dizinc amide complex 4, but the isolation of the desired species failed. From the solid-state structure analysis of complexes 1-4, Zn-Zn separation varies from 2.9 to 7.6 Å. 	   55	  Chapter 3: Preliminary studies of the reactivity of multi-metallic zinc complexes     Introduction 	  As discussed in Chapter 1, there are a significant amount of multimetallic zinc catalysts that exhibit interesting cooperativity during ROP of lactide and ROCOP of CO2 and epoxides. Cooperative operations enhance activity and selectivity significantly, but many dinuclear systems suffer from dissociation in solution, which reduced their performances.29, 66, 74 To date, most of the highly active catalysts for lactide polymerization are dimeric species arising from the aggregation of two discrete metal centers, or dinucleating species supported by a tethered ligand (Figure 3.1). The bulky dimeric b-diketiminate zinc complex A6 reported by Coates et al., is highly active and selective (Pr=0.90) for ROP of rac-lactide, and kinetic studies revealed an order of 1.56±0.06 on [Zn], indicating a dinuclear active species in the polymerization medium.29 The achiral zinc catalyst A14 reported by Hillmyer and Tolman displayed remarkably rapid rate of LA polymerization and produced atactic PLA.56 The kinetic studies indicate a possible fractional dependence (1.33 at 0 °C or 1.75 at 25 °C) on catalyst, although plots of kobs versus [A14/2]0 is linear. The dizinc-monoalkoxide complex A12 supported by a tethered dinucleating ligand also showed high activity and no stereocontrol, and kinetic studies found a first order dependence on [A12].55 Our group reported a highly active chiral dinuclear indium complex, and later in the detailed mechanistic studies it was found that the catalyst retains its dinuclear structure in solution, and the dinuclear nature is essential for its activity and selectivity.65-66  	   56	    Figure 3.1. Highly active dinuclear catalysts for ROP of lactide in the literature. 	  All these studies have led us to the design of a novel single-frame dinucleating ligand family to prevent dissociation and enforce metal-metal proximity during polymerization. Preliminary studies on the reactivity of multimetallic zinc complexes 1-3 supported by ligand L are discussed in this chapter. In this research project, trinuclear alkyl zinc complex 1 and dinuclear alkoxy zinc complex 2 were studied in their reactivity towards ROP of rac-lactide. Reactivity of trinuclear zinc acetate complex 3 was tested towards ROP of rac-lactide and copolymerization of CO2/epoxides. NZnNOONZnNiPriPrA6Coates, 2001RN NO ZnOORNNOZn1/2R=tBu, MeA14Hillmyer and Tolman, 2003NNOtBu tBuInHNNOtButBuIn HClOClClNNNZnOZnNOClClA12Hillmyer and Tolman, 2002 Mehrkhodavandi, 2008	   57	   Figure 3.2. Zinc complexes studied in this research project. 	     ROP of rac-lactide with alkyl zinc complexes 1  Alkyl groups are not known to be good initiators for ring opening polymerization of cyclic esters.48, 70 Zn-alkyl bond is not as nucleophilic as alkoxides, so the initiating step is slower than propagating step, and extensive transesterification side reactions could occur. Therefore, alkyl zinc complexes usually have poor control over molecular weight and generate PLA with large dispersities. Preliminary studies were conducted on the reactivity of trinuclear alkyl zinc complex 1 towards rac-lactide polymerization. Complex 1 showed significantly good activity compared to similar alkyl zinc systems. 61, 70, 121 Ring opening polymerization of 200 equiv of rac-lactide with complex 1 reached 96% conversion in 5 h at room temperature. However, the experimental molecular weights of the resulting polymers are not reproducible and the polymers had relatively broad dispersities (Table 3.1), which suggests the active species are generated from trace impurities in monomer (water, lactic acid or hydrolyzed lactide).    ONHNZnOZnNNHOO2ONHNZn1 Zn3OZn2NNH1ONHNZn ZnOZnNNHOO O OO OO O3OHOH	   58	  Table 3.1. Polymerization results of rac-lactide with complex 1a Entry [LA]0/[1] Conv.%a Mn,expb(kg/mol) Mw,expb(kg/mol) Ðb 1 200 98 13.7 17.9 1.30 2 200 96 18.6 23.2 1.24 3 400 98 15.3 20.4 1.33 4 400 98 13.6 18.9 1.39 5 600 95 15.9 20.6 1.30 6 600 96 12.0 15.3 1.27 7 800 95 11.0 13.7 1.25 8 800 97 19.2 27.3 1.42 	  a All reactions carried out in CH2Cl2 at 25 °C with rac-lactide; [1]=0.5 mM. Polymer samples are obtained at 98% conversion after 16 h as determined by 1H-NMR spectroscopy. b Determined by GPC (dn/dc=0.044 for PLA).     ROP of rac-lactide with alkoxy zinc complex 2 	  Alkoxy zinc complex 2 showed good activity towards rac-lactide polymerization at room temperature, converting 1000 equiv. of rac-lactide in 5 h. Representative results are summarized in Table 3.2. Complex 2 is much more active than the previously reported mononuclear zinc complex A22 and the dinuclear alkoxy zinc catalyst A27 supported by a tethered ligand,63,60 but it is less active than the dimeric zinc species A23-A25 with two bridging alkoxide groups.61          	   59	  Table 3.2. Ring-opening polymerization of rac-lactide with alkoxy zinc complex 2a. Entry [LA]0/[2] Mn,theob(kg/mol) Mn,expc(kg/mol) Mw,expc(kg/mol) Ðc Prd 1 200 7.60 11.2 13.2 1.19 0.63 2 400 13.9 16.8 21.5 1.28 0.61 3 600 21.2 18.0 21.0 1.17 0.63 4 800 28.2 21.6 27.2 1.26 0.65 5 1000 35.5 23.0 30.5 1.32 0.64 	  a All reactions carried out in CH2Cl2 at 25 °C with rac-lactide; [2]=0.5 mM;  Polymer samples are obtained at 98% conversion after 6 h as determined by 1H-NMR spectroscopy. b Calculated from [LA]0/[−OBn]× monomer conversion × MLA (144.13 g/mol) + MBnOH (108.14 g/mol)(Assuming 4 initiators in one catalyst molecule) c Determined by GPC (dn/dc=0.044 for PLA). d Calculated from 1H{1H} NMR spectra. 	   Figure 3.3. Plot of Ð (orange), observed Mn (blue) and theoretical Mn (green) as functions of [LA]0/[2]. The green line indicates calculated Mn values based on [LA]:[initiator] value (Assuming 4 initiators in one catalysts molecule). 	  1.001.201.401.601.802.002.202.402.602.803.000.005.0010.0015.0020.0025.0030.0035.0040.000 200 400 600 800 1000 1200Ð Mn(kg/mol)[LA]0/[2]ROP of rac-lactide with complex 2	   60	  The experimental molecular weights of the obtained polymer with complex 2 were lower than the theoretical values with Ð ranging from 1.17-1.32. (Table 3.2, Figure 3.3). Based on our previous studies on a dimeric alkoxy zinc catalyst,61 it is suspected that the lower experimental molecular weight is resulted from depolymerization reaction of formed PLA with complex 2, which also could lead to lightly broad Ð. In order to test the hypothesis, a perfectly monodispersed PLA (Mn=29,200 g/mol, Ð=1.02) synthesized using our reported dinuclear indium system122 was exposed to complex 2 under the same reaction condition  for 16 h, and it was found the molecular weight decreased from 29,200 g/mol to 17,900 g/mol and the dispersity increased from 1.02 to 1.20, indicating extensive transesterification and depolymerization reactions occurred during the polymerization with complex 2.  Figure 3.4. GPC traces for depolymerisation of PLA sample with complex 2. Blue solid (PLA generated with indium catalyst) Mn=29,200 g/mol, Ð=1.02. Red solid (PLA with complex 2) Mn=17,900 g/mol, Ð=1.20. 05010015020025030015 20 25 30Refractive Index (mV)Retention volumn (mL)Depolymerization studies with Zn alkoxyl complex 2PLAPLA+2	   61	  	  To characterize the polymers generated with complex 2, polymerization with 20 equiv of lactide was performed and the oligomers are analyzed by 1H-NMR spectroscopy and MALDI-TOF mass spectroscopy. The 1H-NMR spectrum (Figure 3.5) clearly shows peaks at 2.71 and 7.36 ppm corresponding to the hydroxyl and benzyl alkoxyl chain end of the polymer.  It also shows a peak at 3.72 ppm which is assigned to the methoxy group. It is proposed the methoxy group replaces the organometallic moiety during the work-up when cold methanol is used to precipitate the polymer. This is confirmed by the absence of the methoxy peak in the 1H-NMR spectrum of another batch of polymer when cold wet hexane was used to quench the reaction (Figure C.1). The multiplet peak at 4.35 ppm is assigned to the methylene proton at the chain end, and this is confirmed by COSY NMR spectrum (Figure C.2) where a correlation of methylene -CH- proton and the –OH proton was observed.                        Figure 3.5. Chain-end analysis by 1H-NMR spectrum (400 MHz, CDCl3, 25 °C), PLA generated by complex 2 and precipitated in MeOH. 	  !CH!HOOOOOOOCH3nHO O O OOOOn!OCH3!OHAr!H	   62	  The MALDI-TOF mass spectrum also displays the benzyl alkoxyl chain end. (Figure 3.6) The major peaks are separated by 72 m/z, indicating the presence of intermolecular transesterification reactions. This is also seen in the MALDI-TOF mass spectrum (Figure C.4) of other polymer samples generated with complex 2. Both 1H-NMR spectrum and MALDI-TOF analysis suggest the chain end is the initiating benzyloxy group. Therefore, it is proved that the polymerization followed a coordination-insertion mechanism.  Figure 3.6. MALDI-TOF mass spectrum of PLA oligomer produced by complex 2.   Tacticity of PLA generated with complex 2 is characterized by 1H{1H} NMR spectroscopy. The 1H{1H} NMR spectra of PLA generated from rac-lactide with complex 2 show moderate heterotactic bias with Pr value of 0.61-0.65, which is comparable to other similar zinc systems (Figure C.5).61, 123  M=72×n+23+108.14 	   63	     Reactivity of zinc acetate complex 3 towards lactide polymerization  	  It is known that acetate group is an inferior initiating group for lactone polymerization because it is a weak nucleophile.48 Examples of zinc complexes with acetate initiating groups that are active for lactone polymerization without a co-catalyst are very limited, and none of them showed good activity at room temperature or good control over molecular weight and dispersity.48, 124-125 In contrast, zinc acetate complexes have shown great potential in ROCOP of CO2/epoxide.74, 79, 85, 126-127 128-129 Williams et al. reported a dizinc acetate complex A37 which showed interesting reactivity towards ROCOP of CO2/epoxide, ROP of lactone,  ROCOP of epoxide/anhydride.85, 90, 102, 126 The promising results of activity studies on dizinc acetate complex A37 and related tri-zinc acetate systems have encouraged us to explore the reactivity of trizinc acetate complex 3 towards e-caprolactone (e-CL) polymerization, cyclohexene oxide (CHO)/  e-CL polymerization, lactide polymerization and CO2/epoxide copolymerization. It was found complex 3 is inactive for e-caprolactone polymerization, CHO /e-CL polymerization. Efforts were made to activate it by utilizing co-catalysts and conducting the reactions at elevated temperatures in various solvents, but all the attempts failed (Table 3.3).   Figure 3.7. Highly active dizinc acetate catalyst for CO2/epoxide copolymerization. tBuNHNHOtBuNHNHOZn ZnXXA37X=OAc	   64	  Table 3.3. Attempted activation of Zn acetate complex 3. Entry Monomer Co-catalyst [M]0/[Co-cat.]/[3] Solvent Temp. (⁰C) Time (day) Conv.% 1 e-CL - 80/1 CD2Cl2 25 5  0 2 e-CL - 80/1 d8-toluene 100  5  0 3 CHO/ e-CL - 60/80/1 d8-toluene 100  5  0 4 CHO/ e-CL - 60/80/1 THF 65  7  0 5 CHO/ e-CL PPNCl a 60/80/1/1 d8-toluene 100  5  0 6 CHO/ e-CL [HNMe2Ph]+[BArF]-  60/80/4/1 d8-toluene 100  5  0 7 CHO/ e-CL Benzyl alcohol 60/80/4/1 d8-toluene 100  5  0 a PPNCl: bis(triphenylphosphine)iminium chloride 	  Complex 3 is active to ROP of lactide at room temperature, reaching 87% conversion in 5 days. Efforts were made to optimize the reaction conditions to obtain the highest catalytic activity of complex 3, and further investigation is still ongoing. Table 3.4. Activity tests of complex 3 towards ROP of lactide a Entry Cat. Loading (mol%) Temp.( °C) Solvent Time (day) Conv. (%)b 1 2.0  25 CD2Cl2 5  87% 2 2.0  40  CD2Cl2 2  93% 3 1.5  100 d8-tol 3  0% 4 2.0  100 d8-tol 3  12% aAll reactions were conducted in vacuum-sealed NMR tubes with rac-lactide and [3]= 2mM. b Determined by 1H-NMR spectroscopy. 	   65	  	     Conclusions and perspectives 	  A novel single-frame dinucleating ligand L based on 2,2’-biphenol motif was successfully synthesized and fully characterized. Ligand L demonstrated interesting coordination chemistry with various zinc precursors, leading to a series of multinuclear zinc complexes 1-4. However, ligand framework L tended to form trinuclear species that possessed heavily congested active sites and significant steric hindrance. Trinuclear alkyl zinc complex 1 and dinuclear zinc amide complex 4 were synthesized by direct reaction of proligand L with ZnEt2 and Zn[N(SiMe3)]2, respectively. Dinuclear alkoxy zinc complex 2 was synthesized via the reaction of complex 1 with benzyl alcohol. Trinuclear zinc acetate complex 3 was synthesized via deprotonation of ligand L followed by salt metathesis with Zn(OAc)2. Complex 1-3 were fully characterized by 1H-NMR, 13C{1H}-NMR, 2D-NMR spectroscopy, single crystal X-ray crystallography and elemental analysis.   Preliminary reactivity studies on complex 1-3 were performed. Alkyl zinc complex 1 was active to rac-lactide polymerization, but it generated polymers with uncontrolled molecular weight and larger polydispersity, which is attributed to impurities in the polymerization system. Alkoxy zinc complex 2 showed better control on lactide polymerization, affording moderate heterotactic-enriched polymers with rac-lactide. However it is much less active than other similar dimeric zinc species,56, 61 and it promoted depolymerisation reactions. Zinc acetate complex 3 was active to lactide polymerization at high catalyst loadings and it showed catalytic activity to CO2/epoxide copolymerization under high CO2 pressure.  	   66	  Complexes 1-3 were designed to perform cooperative catalysis during polymerization due to the fluxional rotation along aryl-aryl bond in ligand L. However, they all possess a rigid structure and the rotation is significantly hindered. The low activity of complex 2 and 3 may be attributed to the high energy barrier to lactide coordination. The zinc centers in complex 2 and complex 3 are heavily congested, and the great steric hindrance prevents coordination from monomers. These results suggest that the design of new dinucleating ligands should balance steric hindrance to allow the formation of thermodynamically stable dimetallic complexes and to create a favorable coordination site for monomers. Future directions for this project include: 1)  Optimization of ligand synthesis and modification of ligand. The Betti reaction, a three-component condensation reaction, is used to synthesize aminophenoxyl compounds via a one-pot reaction.130 It would be interesting to develop a new synthetic route for L’ and L based on the Betti reaction. Also, we can substitute the rigid cyclohexyl diamine by a more flexible diamine and the subsequent synthesis of ligand can be realized by a one-pot reaction.131-132 2) Modification of ligand with an electron-withdrawing group on the para-position of the phenol backbone to decease the electron-donating ability of the phenol oxygen, thus to facilitate the formation of dinuclear species. 3) Synthesis of dinuclear species starting from complex 4, such as dizinc-ethoxide complex, or dizinc-OiPr complex. 4) Synthesis of heterobimetallic complex supported by L, such as Al-Zn, Mg-Zn, Ca-Zn complexes.53      	   67	    Scheme 3.1. Proposed synthetic route for ligands. 	   Based on our preliminary studies, complex 3 exhibited catalytic activities towards ROCOP of CO2/epoxide under harsh conditions (30 atm CO2 pressure, 80 °C) (Table 3.5), but further investigations are required to confirm the results. It is reported tri-zinc complexes are less active than mono- or di-zinc catalysts due to their hindered steric coordination environments.79, 127, 129 In complex 3, the three zinc centers are coordinately saturated and there is no free coordination site for monomers, resulting in the observed low reactivity. Complex 3 can be used for bulk polymerization of lactones. Reactivity tests on complex 4 include lactone polymerization and CO2/epoxide copolymerization. It is notable that complex 4 has a flexible structure and the wide Zn-Zn separation would allow every transition state to adopt optimal structures and thus improve catalytic activity.  OH CHONH3 EtOH, rt OHNH2+ +NH2NOHHO+ + ?CHHHO O HnNH2NH3CCH3OHHOH HO OHHONHNHNN+ +OHNHNHOHNNL2	   68	  Table 3.5. Preliminary CO2/epoxide copolymerization test with complex 3a	  Entry Temp.(°C) CO2 pressure (atm) Time (h) Conv.% Carbonate linkagesd(%) TONb TOFc/h-1 1 80 30 24 4.2% 84 21.2 2.3 2 80 30 24 4.7% 88 20.3 2.1 3 100 30 24 7.2% 92 95.1 4.0 4e 100 5 20 25% 88 146 7.3 5f 80 1 24 29% >99% 4 96 aAll reactions were performed in neat CHO with [CHO]:[3]=1300:1.bMoles of CHO consumed per mole of zinc. cMoles of CHO consumed per mole of zinc per hour. d Calculated by integration of methine resonances in 1H-NMR product (CDCl3, 400 MHz, 25 °C). e Catalyst as prepared by Meng et al. Reaction performed in neat CHO with [mono]:[cat]=1000:1.129 f Catalyst as prepared by Williams et al. Reaction performed in neat CHO with [mono]:[cat]=1000:1127          	   69	  Experimental procedures                                                                                                                               General considerations. All air and/or water sensitive reactions were carried out under N2 in an MBraun glovebox.  Bruker Avance 600 MHz, 400 MHz or 300 MHz spectrometers were used to record the 1H NMR, 13C{1H} NMR spectra, 2D NMR spectra and 1H{1H} NMR spectra. 1H NMR chemical shifts are given in ppm versus residual protons in deuterated solvents as follows: d 7.27 for CDCl3, d 7.16 for C6D6, d 2.08 for d8-tol. 13C{1H} NMR chemical shifts are given in ppm versus residual 13C in solvents as follows: d 77.00 for CDCl3. Diffraction measurements for X-ray crystallography were made on Bruker X8 APEX II and Bruker APEX DUO diffractometers with graphite monochromated Mo-Kα radiation. The structures were solved by direct methods and refined by full-matrix least-squares using the SHELXTL crystallographic software of the Bruker-AXS.  Unless specified, all non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were constrained to geometrically calculated positions but were not refined. EA CHN analysis was performed using a Carlo Erba EA1108 elemental analyzer.  The elemental composition of an unknown sample was determined by using a calibration factor. The calibration factor was determined by analyzing a suitable certified organic standard (OAS) of a known elemental composition. Molecular weights were determined by GPC-LLS using an Agilent liquid chromatograph equipped with an Agilent 1200 series pump and autosampler, three Phenogel 5 μm Narrow Bore columns (4.6 × 300 mm with 500 Å, 103 Å and 104 Å pore size), a Wyatt Optilab differential refractometer, Wyatt tristar miniDAWN (laser light scattering detector) and a Wyatt ViscoStar viscometer. The column temperature was set at 40 °C. A flow rate of 0.5 mL/min was used and samples were dissolved in THF (ca. 2 mg/mL), and a dn/dc value of 0.042 mL/g was used. Narrow molecular weight polystyrene standards were used for system calibration purposes.	   	   70	     Materials. Toluene, diethyl ether, hexane, and tetrahydrofuran were degassed and dried using alumina columns in a solvent purification system. THF was further dried over sodium/benzophenone and vacuum transferred to a Straus flask where it was degassed prior to use. In addition, CH3CN, CHCl3 and CH2Cl2 were dried over CaH2 and vacuum transferred to a Straus flask where they were degassed prior to use. Deuterated chloroform (CDCl3) was dried over CaH2 and vacuum transferred to a Straus flask and then degassed through a series of freeze-pump-thaw cycles. Diethyl Zn solution and ZnCl2 from Sigma Aldrich and used without further purification. Benzyl alcohol was dried over CaH2 and distilled to a Straus flask prior to use. Potassium tert-butoxide, (±)-trans-diaminocyclohexane, were obtained from Alfa Aesar and used without further purification.  Lactide samples were obtained from Purac Biomaterials and recrystallized several times from hot, dry toluene and dried under vacuum prior to use. Synthesis of (R,R)-N,N-dimethyl-trans-1,2-diaminocyclohexane was performed according to literature procedures133-134 from (±)trans-diaminocyclohexane and distilled under vacuum at 70 °C prior to use. Synthesis of 2,2’-bis-(methoxymethoxy)biphenyl b. The preparation of compound b, c and d was adapted from literature procedures101 with modifications. Under nitrogen, 2,2’-biphenol (2.10 g, 11.32 mmol) was dissolved in THF (40 ml). After the solution was cooled to    -78 0C in dry ice/acetone cooling bath, n-butyllithium (9.51 mL, 23.76 mmol, 2.5M in hexane) was added to the solution. The resulting solution was allowed to warm to room temperature and stirred for 2 h, and then methyoxymethyl chloride (1.81 mL, 23.76 mmol) was added slowly. The reaction mixture was left to stir overnight to afford a milky cloudy mixture. THF was removed in vacuo at room temperature. Water was added to quench the reaction, and the aqueous layer was extracted with dichloromethane (30 mL × 3). The combined organic extracts were dried over 	   71	  anhydrous Na2SO4. After the removal of solvent, the residue was purified by column chromatography on silica gel eluted with hexane/ethyl acetate. Compound b was obtained as a colorless oil in 83% yield (2.57 g): Rf =0.7(hexane/ethyl acetate=5:1); 1H NMR (300 MHz, CDCl3, 25 ºC): 𝛿 7.28-7.33 (6H, m, Ar-H), 7.08 (2H, t, Ar-H), 5.08 (4H, s, OCH2-), 3.35 (6H, s, OCH3)  Synthesis of 3,3’-Diformyl-2,2’-bis(methyoxymethyloxy)biphenyl c. Under nitrogen, the solution of compound b (2.42 g, 8.82 mmol) in diethyl ether was treated with n-butyllithium (2.5 M in hexane, 11.32 mL, 28.23 mmol) at room temperature. The reaction mixture was stirred for 2 h until it turned to a dark orange suspension. After the mixture was cooled to 0 0C, dimethylformamide (2.1 mL, 28.23 mmol) was added. The reaction was allowed to warm to room temperature and stirred overnight to give a yellowish milky suspension. Saturated NH4Cl (40 mL) was added to quench the reaction. The aqueous layer was extracted with ethyl acetate (40 mL × 3) and the combined organic layer was washed with water and brine and dried over Na2SO4. After the removal of solvent, the product was purified by column chromatography on silica gel with hexane/ethyl acetate elution. Compound c was obtained as light yellowish solid in 55% yield (1.60 g): Rf=0.4 (hexane/ethyl acetate=4:1); 1H NMR (300MHz, CDCl3, 25 ºC) 𝛿 10.44 (2H, s, CHO), 7.94 (2H, dd, Ar-H), 7.68 (2H, dd, Ar-H), 7.38 (2H, t, Ar-H), 4.82 (4H, s, OCH2-), 3.16 (6H, s, OCH3). Synthesis of 3,3’-Diformyl-2,2’-dihydroxy-1,1’-biphenyl d. After compound c (2.00 g, 6.05 mmol) was dissolved in minimum amount of DCM, HCl (6M, 30 ml) and ethanol (30ml) were added successively. The reaction mixture was heated to 60 0C and stirred overnight. Solvent was removed in vacuo and the residue was washed with water. The aqueous layer was extracted with DCM (40 mL ×3). The organic extracts were combined and dried over Na2SO4 to 	   72	  give a pale brownish solid in 95% yield (1.40 g). No further purification is required. 1H NMR (300MHz, CDCl3, 25 ºC) 𝛿 11.44 (2H, s, -CHO), 9.96 (2H, s, -OH), 7.64 (4H, m, Ar-H), 7.13 (2H, t, Ar-H). Synthesis of (RR/RR)-L’. Fresh distilled (R,R)-N,N-dimethyl-trans-1,2-diaminocyclohexane(0.29g, 2.08 mmol) was added to a solution of d (0.24g, 0.10 mmol) in dry methanol( 40 mL). A bright yellow solution was obtained and it was stirred at room temperature overnight. The solvent was removed in vacuo at room temperature to afford a dark yellow crystalline product in 90.5% yield (0.44 g). Attempts at further purification was not successful. 1H NMR (300MHz, CDCl3, 25 ºC) 𝛿 8.35 (2H, s, N=CH-), 7.47 (2H, d, Ar-H), 7.27 (2H, d, Ar-H), 6.95 (2H, t, Ar-H), 3.23 (2H, br, m, -CH2 of DACH), 2.53 (2H, br, m, -CH2 of DACH), 2.26 (12H, s, -N(CH3)2), 1.82 (8H, br, m, -CH2 of DACH), 1.61 (2H, br, m, -CH2 of DACH), 2.18 (6H, m, -CH2 of DACH) Synthesis of (RR/RR)-L. L’ (0.44 g, 0.90 mmol) was dissolved in methanol (30 mL) and after the solution was cooled to 0 0C, sodium borohydride (0.17 g, 4.48 mmol) was added. The reaction mixture was allowed to warm up to room temperature and stirred overnight. The methanol was removed in vacuo and the resulting yellowish residue was washed with water (30 mL). The aqueous layer was extracted with DCM (20 mL ×3) and the organic extracts were combined and dried over Na2SO4. After the removal of DCM, the resulting mixture was washed with acetonitrile (10 mL ×3) and filtered. Pale yellowish solid was collected after filtration in 23% yield (0.098 g). The compound was used in complexation without further purification. 1H NMR (300MHz, CDCl3, 25 ºC) 𝛿 7.28 (2H, d, Ar-H), 7.03(2H, d, Ar-H), 6.84 (2H, t, Ar-H), 4.13 (2H, d, -NH-CH2), 3.85 (2H, d, -NH-CH2), 2.25 (2H, m, N-CH of DACH), 2.21 (4H, m, -CH2 of DACH), 2.16 (12H, s, N(CH3)2), 1.77 (4H, m, -CH2 of DACH), 1.69 (2H, m, -CH2 of 	   73	  DACH),1.18 (8H, m, -CH2 of DACH). EI-LRMS: cal. [M]+ 495.70, found [M]+ 495.30 Anal. Cal. For C30H46N4O2: C, 72.83; N, 11.33; H, 9.37. Synthesis of alkyl Zn complex 1. Proligand RR/RR L (184 mg, 0.37 mmol) was dissolved in toluene (5 mL), and diethyl zinc (1M in hexanes, 0.78 mL, 0.78 mmol) was dissolved in toluene (5 mL).  After the solutions were cooled to -30 0C, the proligand solution was added to diethyl zinc solution. The reaction mixture was allowed to warm to room temperature and stirred overnight. The solvent was removed in vacuo and the resulting residue was washed with hexane (5 mL × 3). The product was isolated as an off-white powder in 78% yield (214 mg) without further purification. Single crystal of 1 was grown from a saturated solution of diethyl ether with several drops of THF at room temperature for single-crystal X-ray analysis. 1H-NMR (400MHz, C6D6, 25 ºC) 𝛿 7.18 (2H, d, Ar-H), 6.91(2H, d, Ar-H), 6.83 (2H, t, Ar-H), 4.67 (2H, d, -NH-CH2-Ar), 3.27 (2H, d, -NH-CH2-Ar), 2.00 (2H, br, m, -CH2 of DACH), 1.77 (4H, overlapping mutiplets, -CH2 of DACH), 1.74 (6H, s, N(CH3)2), 1.69 (6H, t, Zn1/Zn2-CH2-CH3), 1.49 (6H, s, N(CH3)2), 1.22 (8H, br, m, -CH2 of DACH), 1.05 (4H, br, m, CH2 of DACH), 0.88 (6H, t, Zn3-CH2-CH3), 0.59-0.63 (4H, m, Zn3-CH2-CH3), 0.49 (2H, br, m, -CH2 of DACH), 0.13-0.25 (4H, br, m, Zn1/Zn2-CH2-CH3). Synthesis of alkoxy RR/RR-[(L) Zn2](µ-OBn)2complex 2•2 benzyl alcohol. Benzyl alcohol (119 mg, 1.098 mmol) was added to a solution of alkyl Zn complex 1 (221 mg, 0.275 mmol) in toluene (15 mL) at room temperature. The reaction mixture was stirred for 48 h at 100 0C, and a yellow precipitate was formed. The resulting mixture was filtered through celite and the filtrate was collected. The solvent was removed in vacuo, and the residue was washed with hexane (10 mL × 3) and diethyl ether (10 mL ×3) subsequently. The product was isolated as yellow powder. Yield: 56.50 mg (25%). Single crystal was obtained from a saturated solution of diethyl ether 	   74	  and THF. 1H-NMR (400MHz, d8-tol, 25 ºC) 𝛿 7.58 (2H, d, Ar-H), 7.28 (8H, br, d, Ar-H), 7.13 (14H, overlapping multiplets, Ar-H), 6.85 (2H, t, Ar-H), 4.86 (4H, q, µO-CH2-Ph), 4.52 (4H, br, s, HO-CH2-Ph), 3.90 (2H, br, s, HO-CH2-Ph), 3.81 (2H, t, -NH-CH2-Ar), 3.45(2H, dd, -NH-CH2-Ar),  2.55 (6H, s, N(CH3)2), 1.95 (6H, s, N(CH3)2), 1.73 (2H, m, -CH2 of DACH), 1.61 (2H, m, -CH2 of DACH), 1.37 (8H, m, -CH2 of DACH), 0.67 (4H, m, -CH2 of DACH), 0.52 (2H, m, -CH2 of DACH), 0.32 (2H, q, -CH2 of DACH). Anal. Calc. for C58H74N4O6Zn2: C, 66.09; H, 7.08; N, 5.32. Found: C, 66.39; H, 6.98; N, 5.17 Synthesis of Zn acetate [(L)Zn3](µ-OAc)4  complex 3. Proligand RR/RR L (179.3 mg, 0.362 mmol) was dissolved in toluene (15 mL), and potassium tert-butoxide(81.2 mg, 0.724 mmol) was added. The reaction mixture was left to stir for 3 h until a clear yellow solution was obtained. Anhydrous Zinc acetate (132.8 mg, 0.724 mmol) was added to the reaction and the reaction mixture was stirred at 100 0C for 16 h. The resulting solution was filtered through celite and solvent was removed in vacuo. The residue was washed with hexanes (10 mL × 3) to afford an off-white solid in 53 % yield (169.2 mg). Single crystal was obtained from a saturated solution of diethyl ether and THF. 1H-NMR (400 MHz, C6D6, 25 ºC) 𝛿 7.49 (2H, d, Ar-H), 6.97 (2H, d, Ar-H), 6.87 (2H, t, Ar-H), 5.34 (2H, d, -NH-CH2-Ar), 3.55 (2H, d, -NH-CH2-Ar), 2.34 (6H, s, -N(CH3)2), 2.12 (12H, overlapping singlet, -OAc), 2.03 (2H, overlapping multiplet, -CH2 of DACH), 1.77 (2H, m, -CH2 of DACH), 1.55 (6H, s, -N(CH3)2), 1.23 (6H, m, -CH2 of DACH), 0.60 (4H, m, -CH2 of DACH), 0.25 (4H, m, -CH2 of DACH) Anal. Calc. for C38H60N4O10Zn3: C, 49.13; N, 6.03; O, 17.22. Found: C, 47.28; N, 6.12; O, 17.54 Attempted Synthesis of Zn amide complex 4. Zn[N(SiMe3)2]2 was synthesized according to literature procedures,135 and was distilled before use. Proligand RR/RR L (132mg, 0.267 mmol) 	   75	  was dissolved in toluene (10 mL), and fresh distilled Zn[N(SiMe3)2]2 (0.2060 g, 0.534 mmol) was added to the solution. The reaction was left to stir overnight. The solvent was removed in vacuo , resulting in a yellowish solid which is further washed with cold hexanes (10 mL × 3) to afford a light yellowish solid. Single crystal was obtained from a saturated hexane solution at room temperature. The product was found to be a mixture by 1H-NMR, probably mononuclear species. All attempts to purify the desired dinuclear complex failed.  Representative ring opening polymerization of rac-lactide catalyzed by complex 2. A solution of complex 2(8.6 mg, 0.0104 mmol) in DCM (1 mL) was added to a solution of rac-lactide (315.6 mg, 2.08 mmol) in DCM (1 mL) to obtain a 5 mM concentration of catalyst. The mixture was allowed to stir at room temperature for 16 h. A portion of the reaction mixture was dissolved in CDCl3 to prepare a sample for 1H-NMR spectroscopy to check the conversion. The resulting solution was concentrated under vacuum and the crude polymeric material was redissolved in a minimum amount of DCM. Cold methanol was then added to the solution, leading to the precipitation of the polymer and then the supernatant was removed. This procedure was repeated for 3 times, and the resulting polymer was dried under vacuum. CO2/CHO copolymerization conditions. Cyclohexene oxide (15 mL, 148.2 mmol) and complex 3 (0.113 mmol) was added to the reactor and the reaction mixture was left to stir for 24 h under 30 atm CO2 at 80 °C. The crude reaction mixture was washed with cold hexane, and the polymer was dried under vacuum for 16 h. The product was analyzed by 1H-NMR spectroscopy (400 MHz, CD3Cl, 25 °C), where the protons adjacent to the carbonate linkage resonated at 4.6 ppm, while the absence of a peak at 3.45 ppm showed that there were no polyether linkages.  	  	   76	  References 1. Schwede, K.(2016, Nov 30) Global bioplastics production capacities continue to grow despite low oil price. Retrieved from http://www.european-bioplastics.org/market-data-update-2016/ 2. Bourguignon, D. Briefing: Plastics in a circular economy: Opportunities and challenges European Parliamentary Research Service. 2017. 3. Chen, Y. Y.; Geever, L. M.; Killion, J. A.; Lyons, J. G.; Higginbotham, C. L.; Devine, D. M., Polym-Plast Technol 2016, 55 (10), 1057-1075. 4. Nagarajan, V.; Mohanty, A. K.; Misratt, M. Acs. Sustain. Chem. Eng. 2016, 4 (6), 2899-2916. 5. Jamshidian, M.; Tehrany, E. A.; Imran, M.; Jacquot, M.; Desobry, S. Compr. Rev.  Food Sci. Food Saf. 2010, 9 (5), 552-571. 6. Lovett, J.(2015, Sep 30) Corbion Purac successfully develops PLA resin from second generation  feedstocks. Retrieved from http://www.corbion.com/media/pressreleases?newsId=1955535 7. Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12 (23), 1841-1846. 8. Auras, R. A.; Harte, B.; Selke, S.; Hernandez, R.  J. Plast. Film. Sheet. 2003, 19 (2), 123-135. 9. Garlotta, D. J. Polym. Environ. 2001, 9 (2), 63-84. 10. Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym. Rev. 2008, 48 (1), 11-63. 11. Li, C.-Y.; Chen, P.-S.; Hsu, S.-J.; Lin, C.-H.; Huang, H.-Y.; Ko, B.-T. J. Organomet. Chem. 2012, 716, 175-181. 12. Achmad, F.; Yamane, K.; Quan, S.; Kokugan, T. Chem. Eng. J. 2009, 151 (1-3), 342-350. 13. Auras, R. A. L., L.-T.; Selke, S. E.; Tsuji, H. Poly (lactic acid): synthesis, structures, properties, processing, and applications. John Wiley & Sons: 2011; Vol. 10. 14. Lim, L. T.; Auras, R.; Rubino, M. Progress in Polymer Science. 2008, 33 (8), 820-852. 15. Baker, G. L.; Vogel, E. B.; Smith, M. R. Polym. Rev. 2008, 48 (1), 64-84. 16. Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F.; Spassky, N.; LeBorgne, A.; Wisniewski, M. Macromolecules. 1996, 29 (20), 6461-6465. 17. Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39 (2), 486-494. 18. Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104 (12), 6147-6176. 19. Thomas, C. M. Chem. Soc. Rev. 2010, 39 (1), 165-173. 20. Dittrich, W.; Schulz, R. C. Angew. Makromol. Chem. 1971, 15, 109. 21. Eguiburu, J. L.; Fernandez-Berridi, M. J.; Cossio, F. P.; San Roman, J. Macromolecules 1999, 32 (25), 8252-8258. 22. Degee, P.; Dubois, P.; Jerome, R. Macromol Symp 1997, 123, 67-84. 23. Degee, P.; Dubois, P.; Jerome, R.; Jacobsen, S.; Fritz, H. G. Macromol Symp. 1999, 144, 289-302. 24. Brunelle, D. J.	   Functional Polymers: Design, Synthesis, and Applications. Hanser Publishers: 1993; Vol. 361. 25. Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules. 1988, 21 (2), 286-293. 26. Ajellal, N.; Carpentier, J. F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39 (36), 8363-8376. 27. Inoue, S. J. Polym. Sci. Pol. Chem. 2000, 38 (16), 2861-2871. 	   77	  28. Williams, C. K.; Hillmyer, M. A. Polym. Rev. 2008, 48 (1), 1-10. 29. Zell, M. T.; Padden, B. E.; Paterick, A. J.; Thakur, K. A. M.; Kean, R. T.; Hillmyer, M. A.; Munson, E. J. Macromolecules 2002, 35 (20), 7700-7707. 30. Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124 (7), 1316-1326. 31. Coudane, J.; UstarizPeyret, C.; Schwach, G.; Vert, M. J. Polym. Sci. Pol. Chem. 1997, 35 (9), 1651-1658. 32. Inkinen, S.; Hakkarainen, M.; Albertsson, A. C.; Sodergard, A. Biomacromolecules. 2011, 12 (3), 523-532. 33. Kricheldorf, H. R.; Damrau, D. O. Macromol. Chem. Physic. 1997, 198 (6), 1753-1766. 34. Kreiser-Saunders, I.; Kricheldorf, H. R. Macromol. Chem. Physic. 1998, 199 (6), 1081-1087. 35. Schwach, G.; Coudane, J.; Engel, R.; Vert, M. Polym. Int. 1998, 46 (3), 177-182. 36. Radlauer, M. R.; Buckley, A. K.; Henling, L. M.; Agapie, T. J. Am. Chem. Soc. 2013, 135 (10), 3784-3787. 37. Nakano, K.; Hashimoto, S.; Nozaki, K. Chem. Sci. 2010, 1 (3), 369-373. 38. Matsunaga, S.; Shibasaki, M. Chem. Commun. 2014, 50 (9), 1044-1057. 39. Liu, Y.; Ren, W. M.; He, K. K.; Lu, X. B. Nat. Commun. 2014, 5. 5687. 40. Jedrzejas, M. J.; Setlow, P. Chem. Rev. 2001, 101 (3), 607-618. 41. Bosch, S.; Comba, P.; Gahan, L. R.; Schenk, G. Inorg. Chem. 2014, 53 (17), 9036-9051. 42. Parkin, G. Chem. Rev. 2004, 104 (2), 699-767. 43. Chisholm, M. H.; Lin, C. C.; Gallucci, J. C.; Ko, B. T. Dalton Trans. 2003,  (3), 406-412. 44. Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C. Polymer. 2005, 46 (23), 9784-9792. 45. Chen, H. Y. T., H. Y.; Lin, C. C. Macromolecules. 2006,  (39), 3745-3752. 46. Hung, W.-C.; Huang, Y.; Lin, C.-C. J. Polym. Sci. A. 2008, 46 (19), 6466-6476. 47. Darensbourg, D. J.; Karroonnirun, O. Inorg. Chem. 2010, 49 (5), 2360-71. 48. Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001,  (123), 3229-3238. 49. Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.; Prud'homme, R. E.; Schaper, F. Organometallics. 2010, 29 (9), 2139-2147. 50. Naumann, S.; Dove, A. P. Polym. Int. 2016, 65 (1), 16-27. 51. Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem. Commun. 2004,  (1), 112-113. 52. Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B. Chem. Commun. 2004,  (21), 2504-2505. 53. Arnold, P. L.; Casely, I. J.; Turner, Z. R.; Bellabarba, R.; Tooze, R. B. Dalton Trans. 2009,  (35), 7236-7247. 54. Fliedel, C.; Mameri, S.; Dagorne, S.; Aviles, T. Appl. Organomet. Chem. 2014, 28 (7), 504-511. 55. Williams, C. K.; Brooks, N. R.; Hillmyer, M. A.; Tolman, W. B. Chem. Commun. 2002,  (18), 2132-2133. 56. Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125 (37), 11350-11359. 57. Poirier, V.; Roisnel, T.; Carpentier, J. F.; Sarazin, Y. Dalton Trans. 2009,  (44), 9820-9827. 	   78	  58. Thevenon, A.; Romain, C.; Bennington, M. S.; White, A. J. P.; Davidson, H. J.; Brooker, S.; Williams, C. K. Angew.Chem. Int. Ed. 2016, 55 (30), 8680-8685. 59. Ding, K. Y.; Miranda, M. O.; Moscato-Goodpaster, B.; Ajellal, N.; Breyfogle, L. E.; Hermes, E. D.; Schaller, C. P.; Roe, S. E.; Cramer, C. J.; Hillmyer, M. A.; Tolman, W. B. Macromolecules. 2012, 45 (13), 5387-5396. 60. Labourdette, G.; Lee, D. J.; Patrick, B. O.; Ezhova, M. B.; Mehrkhodavandi, P. Organometallics. 2009, 28 (5), 1309-1319. 61. Ebrahimi, T.; Mamleeva, E.; Yu, I.; Hatzikiriakos, S. G.; Mehrkhodavandi, P. Inorg. Chem. 2016, 55 (18), 9445-9453. 62. Osten, K. M.; Aluthge, D. C.; Patrick, B. O.; Mehrkhodavandi, P. Inorg. Chem. 2014, 53 (18), 9897-9906. 63. Kremer, A. B.; Osten, K. M.; Yu, I.; Ebrahimi, T.; Aluthge, D. C.; Mehrkhodavandi, P. Inorg. Chem. 2016, 55 (11), 5365-5374. 64. Breyfogle, L. E.; Williams, C. K.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans. 2006,  (7), 928-936. 65. Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P. Angew.Chem. Int. Ed. 2008, 47 (12), 2290-3. 66. Yu, I.; Acosta-Ramirez, A.; Mehrkhodavandi, P. J. Am. Chem. Soc. 2012, 134 (30), 12758-12773. 67. Bratko, I.; Gomez, M. Dalton Trans. 2013, 42 (30), 10664-10681. 68. Normand, M.; Roisnel, T.; Carpentier, J. F.; Kirillov, E. Chem. Commun. (Camb) 2013, 49 (99), 11692-4. 69. Allan, L. E. N.; Bélanger, J. A.; Callaghan, L. M.; Cameron, D. J. A.; Decken, A.; Shaver, M. P. J. Organomet. Chem. 2012, 706-707, 106-112. 70. Lian, B.; Thomas, C. M.; Casagrande, O. L.; Lehmann, C. W.; Roisnel, T.; Carpentier, J. F. Inorg. Chem. 2007, 46 (1), 328-340. 71. Thibault, M. H.; Fontaine, F. G. Dalton Trans. 2010, 39 (24), 5688-97. 72. Singh, S.; Roesky, H. W. Dalton Trans. 2007,  (14), 1360-70. 73. Arbaoui, A.; Redshaw, C.; Hughes, D. L. Chem. Commun. (Camb) 2008,  (39), 4717-9. 74. Coates, G. W.; Moore, D. R. Angew.Chem. Int. Ed. 2004, 43 (48), 6618-6639. 75. Gorecki, P.; Kuran, W. J. Polym. Sci. Pol. Lett. 1985, 23 (6), 299-304. 76. Trott, G.; Saini, P. K.; Williams, C. K. Philos T R Soc A 2015,  (374). 77. Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.2003, 125 (39), 11911-11924. 78. Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120 (42), 11018-11019. 79. Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123 (36), 8738-8749. 80. Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew. Chem. Int. Ed. 2002, 41 (14), 2599. 81. Lee, B. Y.; Kwon, H. Y.; Lee, S. Y.; Young, U. G. Abstr Pap Am Chem S. 2005, 229, U996-U996. 82. Lehenmeier, M. W.; Kissling, S.; Altenbuchner, P. T.; Bruckmeier, C.; Deglmann, P.; Brym, A. K.; Rieger, B. Angew. Chem. Int. Ed. 2013, 52 (37), 9821-9826. 83. Kissling, S.; Altenbuchner, P. T.; Lehenmeier, M. W.; Herdtweck, E.; Deglmann, P.; Seemann, U. B.; Rieger, B. Chem-Eur J. 2015, 21 (22), 8148-8157. 	   79	  84. Kissling, S.; Lehenmeier, M. W.; Altenbuchner, P. T.; Kronast, A.; Reiter, M.; Deglmann, P.; Seemann, U. B.; Rieger, B. Chem. Commun. 2015, 51 (22), 4579-4582. 85. Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K. Angew. Chem. Int. Ed. 2009, 48 (5), 931-933. 86. Buchard, A.; Jutz, F.; Kember, M. R.; White, A. J. P.; Rzepa, H. S.; Williams, C. K. Macromolecules. 2012, 45 (17), 6781-6795. 87. Kember, M. R.; Jutz, F.; Buchard, A.; White, A. J. P.; Williams, C. K. Chem. Sci. 2012, 3 (4), 1245-1255. 88. Kember, M. R.; Williams, C. K. J. Am. Chem. Soc. 2012, 134 (38), 15676-15679. 89. Saini, P. K.; Romain, C.; Williams, C. K. Abstr Pap Am Chem S. 2014, 248. 90. Zhu, Y. Q.; Romain, C.; Williams, C. K. J. Am. Chem. Soc. 2015, 137 (38), 12179-12182. 91. Aluthge, D. C.; Ahn, J. M.; Mehrkhodavandi, P. Chem. Sci. 2015, 6 (9), 5284-5292. 92. Strater, N.; Lipscomb, W. N.; Klabunde, T.; Krebs, B. Angew. Chem. Int. Ed. 1996, 35 (18), 2024-2055. 93. van den Beuken, E. K.; Feringa, B. L. Tetrahedron. 1998, 54 (43), 12985-13011. 94. Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102 (6), 2187-2209. 95. Park, J.; Hong, S. Chem. Soc. Rev. 2012, 41 (21), 6931-6943. 96. van der Vlugt, J. I. Eur. J. Inorg. Chem. 2012,  (3), 363-375. 97. Lundgren, M. S. R. J. Ligand design in metal chemistry: Reactivity and catalysis. Wiley & Sons, Ltd. : 2016; Vol. 10, p 274. 98. Moneta, W. B., P.; Pierre, J.-L. J. Chem. Soc., Chem. Commun. 1985,  (13), 899. 99. Sternhell, G. B. L. D. F. S. J. Am. Chem. Soc. 1980, 102, 5618. 100. Mitchell, J. M. F., N. S. Tet. Lett. 2000, 41, 8431. 101. Zhang, H. C.; Huang, W. S.; Pu, L. J. Org. Chem. 2001, 66 (2), 481-7. 102. Nahil, M. A.; Wu, C. F.; Williams, P. T. Fuel Process Technol. 2015, 130, 46-53. 103. Jiang, H. L.; He, F.; Lu, J. S.; Xing, Y. P. J. Mol. Catal a-Chem. 2013, 380, 139-143. 104. Corey, E. J.; Yuen, P. W.; Hannon, F. J.; Wierda, D. A. J. Org. Chem. 1990, 55 (3), 784-786. 105. Hammes, B. S.; Carrano, C. J. Inorg. Chem. 1999, 38 (20), 4593-4600. 106. Looney, A.; Han, R.; Gorrell, I. B.; Cornebise, M.; Yoon, K.; Parkin, G.; Rheingold, A. L. Organometallics 1995, 14 (1), 274-288. 107. Bordwell, F. G. Acc. Chem. Res. 1988, 21 (12), 456-463. 108. Sanyal, R.; Zhang, X. P.; Kundu, P.; Chattopadhyay, T.; Zhao, C. Y.; Mautner, F. A.; Das, D. Inorg. Chem. 2015, 54 (5), 2315-2324. 109. Kundu, P.; Zangrando, E.; Chakraborty, P. Polyhedron. 2015, 101, 6-13. 110. Ghosh, K.; Kumar, P.; Tyagi, N.; Singh, U. P. Inorg. Chem. 2010, 49 (17), 7614-7616. 111. Clegg, W.; Little, I. R.; Straughan, B. P. Acta Crystallogr C. 1986, 42, 1701-1703. 112. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co- ordination Compounds/Part B, Applications in Coordination, Organometallic, and Bioinorganic Chemistry, 5th ed. Wiley: New York, 1997. 113. Vagin, S. I.; Ott, A. K.; Rieger, B. Chem-Ing-Tech. 2007, 79 (6), 767-780. 114. Jones, M. D.; Davidson, M. G.; Keir, C. G.; Hughes, L. M.; Mahon, M. F.; Apperley, D. C. Eur. J. Inorg. Chem. 2009,  (5), 635-642. 115. Akine, S.; Taniguchi, T.; Nabeshima, T. Inorg. Chem. 2004, 43 (20), 6142-6144. 	   80	  116. Zhang, G. Q.; Wang, S. Q.; Gan, Q.; Zhang, Y. F.; Yang, G. Q.; Ma, J. S.; Xu, H. J. Eur. J. Inorg. Chem. 2005,  (20), 4186-4192. 117. Akpan, E. D.; Ojwach, S. O.; Omondi, B.; Nyamori, V. O. New. J. Chem. 2016, 40 (4), 3499-3510. 118. Escobedo-Martinez, C.; Lozada, M. C.; Gnecco, D.; Enriquez, R. G.; Soriano-Garcia, M.; Reynolds, W. F. J. Chem. Crystallogr. 2012, 42 (8), 794-802. 119. Bakker, J. M.; Deacon, G. B.; Junk, P. C.; Moxey, G. J.; Turner, D. R. Z Anorg Allg Chem. 2007, 633 (2), 251-255. 120. Kloubert, T.; Gorls, H.; Westerhausen, M. Z Anorg Allg Chem. 2010, 636 (13-14), 2405-2408. 121. Al-Khafaji, Y. F.; Elsegood, M. R. J.; Frese, J. W. A.; Redshaw, C. Rsc. Adv. 2017, 7 (8), 4510-4517. 122. Ebrahimi, T.; Aluthge, D. C.; Patrick, B. O.; Hatzikiriakos, S. G.; Mehrkhodavandi, P. Acs. Catal. 2017, 7 (10), 6413-6418. 123. Knight, P. D.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2008, 47 (24), 11711-11719. 124. Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton Trans. 2009,  (25), 4832-4846. 125. Borner, J.; Herres-Pawlis, S.; Floke, U.; Huber, K. Eur. J. Inorg. Chem. 2007,  (36), 5645-5651. 126. Jutz, F.; Buchard, A.; Kember, M. R.; Fredriksen, S. B.; Williams, C. K. J. Am. Chem. Soc. 2011, 133 (43), 17395-17405. 127. Kember, M. R.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2009, 48 (19), 9535-9542. 128. Xu, Y. H.; Wang, S. J.; Lin, L. M.; Xiao, M.; Meng, Y. Z. Polym Chem-Uk. 2015, 6 (9), 1533-1540. 129. Xu, Y. H.; Xiao, M.; Wang, S. J.; Pan, M.; Meng, Y. Z. Polym. Chem-Uk. 2014, 5 (12), 3838-3846. 130. Cardellicchio, C.; Capozzi, M. A. M.; Naso, F. Tetrahedron-Asymmetr. 2010, 21 (5), 507-517. 131. Xu, L.; Lowe, M. P.; Rettig, S. J.; Orvig, C. Inorg. Chem. 2001, 40 (18), 4623-4626. 132. Sopo, H.; Vaisanen, A.; Sillanpaa, R. Polyhedron. 2007, 26 (1), 184-196. 133. Lee, D. W.; Ha, H. J. Synth. Commun. 2007, 37 (4-6), 737-742. 134. Larrow, J. F.; Jacobsen, E. N.; Gao, Y.; Hong, Y. P.; Nie, X. Y.; Zepp, C. M. J. Org.  Chem. 1994, 59 (7), 1939-1942. 135. Lee, D. Y.; Hartwig, J. F. Org. Lett. 2005, 7 (6), 1169-1172.    	  	  	  	   81	  Appendix A. FTIR spectrum of complex 3. 	  	  Figure A.1. FTIR spectrum of Zn acetate complex 3 (peaks at 1431 and 1583 cm-1 indicating bridging acetate groups).112 32422934158314311398405060708090100650 1150 1650 2150 2650 3150 3650%"Tcm&1	   82	   Appendix B. Characterization of compounds in solution; 1H-NMR, 13C{1H}-NMR and 2D NMR spectra, mass spectra  Figure B.1. 1H-NMR spectrum of compound d (300 MHz, CDCl3, 25 ºC)  Figure B.2.	  1H-NMR spectrum of ligand L’ (300 MHz, CDCl3, 25 ºC) !CHO!OHAr!H!N=CH Ar!H!N(CH3)2N!CH N!CHCyclohexyl	   83	  	  Figure B.3. 1H-NMR spectrum of  L (400 MHz, CDCl3, 25 ºC) 	  	  	  	  	  	  	  	  	  	  Ar#HNH#CH2#Ar#N(CH3)2Cyclohexyl	   84	  	   Figure B.4. 13C{1H}-NMR spectrum of  L (400 MHz, CDCl3, 25 ºC). 	  	  OHHN HO NHN N12345 677891011 1213141516	   85	   Figure B.5.	  COSY NMR spectrum of L (400 MHz, CDCl3, 25 ºC) 	   86	   Figure B.6. NOESY NMR spectrum of L (400 MHz, CDCl3, 25 ºC) 	   87	   Figure B.7. HSQC NMR spectrum of L (400 MHz, CDCl3, 25 ºC) 	   88	   Figure B.8. HMBC NMR spectrum of  L (400 MHz, CDCl3, 25 ºC)   Figure B.9. Mass spectrum of L   	   89	   Figure B.10. 1H-NMR spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) Ar#H NH#CH2#Ar#N(CH3)2#Zn1/Zn2#CH2#CH3#N(CH3)2#Zn3#CH2#CH3#Zn3#CH2#CH3#Zn1/Zn2#CH2#CH3Cyclohexyl	   90	   Figure B.11. COSY spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC)  Figure B.12. NOESY NMR spectrum of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) !2.0!1.5!1.0!0.50.00.51.01.52.02.53.03.5!10123456f1 (ppm)f2 (ppm)H16H15O91011 121314NHN77'123456 Zn18Zn3OZn2NNH8'15161718H18H17!1.0!0.50.00.51.01.52.02.53.03.54.04.55.0!1.5!1.0!0.50.00.51.01.52.02.53.03.54.04.55.05.5f1 (ppm)f2 (ppm)Zn1$CH2$CH3Zn1$CH2$CH3	   91	   Figure B.13. 13C{1H}-NMR of alkyl Zn complex 1 (400 MHz, C6D6, 25 ºC) C9C6 C8C7C7’C16,16’C17C18C15 C15’O91011 121314NHN77'123456 Zn18Zn3OZn2NNH8'1516171815'16'C10C12C14 C13C11C1	   92	   Figure B.14. HSQC of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) 	   93	   Figure B.15. HMBC of alkyl zinc complex 1 (400 MHz, C6D6, 25 ºC) 	  	  	   94	   Figure B.16. 1H-NMR of alkoxy Zn complex 2 (d8-toluene ,400 MHz, 25 ºC)  Figure B.17. 13C{1H}-NMR alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) Ar#HNH#CH2#Ar#N(CH3)2#N(CH3)2Cyclohexyl#OCH2#PhHOCH2#PhHOCH2#PhC7 and&C7’c15ONHNZnOZnNNHoO12345677' 1515'8 8' 91011 121314161718192021C11C9C14C10C12C13	   95	   Figure B.18. COSY of alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) 	   96	   Figure B.19. NOESY of alkoxy Zn complex 2 (400 MHz, C6D6, 25 ºC) 	   97	   Figure B.20. ROESY of alkoxy complex 2 (400 MHz, d8-tol, 25 ºC) 	   98	   	   99	   Figure B.21. HMQC (Sanyal,  #212) HMBC (bottom) spectra of complex 2 (1H-NMR spectra obtained in d-THF, 400 MHz, 25 ºC) 	   100	   Figure B.22. 1H-NMR of Zn acetate complex 3 (400 MHz, C6D6, 25 ºC)  Figure B.23. 13C{1H}-NMR of complex 3 (400 MHz, C6D6, 25 ºC) Ar#HNH#CH2#Ar#N(CH3)2 #N(CH3)2#OAcCyclohexyl!OAc!NMe2!CH2!VONHNZn ZnOZnNNHOO O OO OO O	   101	   Figure B.24.  HSQC NMR spectrum of complex 3. (400 MHz, C6D6, 25 ºC) 	   102	    Figure B.25. 1H-NMR of Zn amide complex 4 (400 MHz, C6D6, 25 ºC)       	  	   103	  Appendix C. Characterization of polymers   	  	  	  Figure C.1. Chain end analysis by 1H-NMR spectrum for polymers precipitated with wet hexane (400 MHz, CDCl3, 25 ºC) 	   104	   Figure C.2. COSY-NMR spectrum showing correlation of –CH- and –OH group in the chain end. (400 MHz, CDCl3, 25 ºC)   HO O O OOOOn	   105	  	  Figure C.3.1H-NMR spectrum of PLA generated with complex 2 (400 MHz, CDCl3, 25 ºC) 	  Figure C.4. MALDI-TOF mass spectrum of PLA sample generated with complex 2 HO O O OOOOnWith%400%eq.%lactideHOOOOOOOCH3n	   106	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	   	  Figure C.5. Homonuclear decoupled 1H{1H} NMR spectrum obtained from rac-lactide with complex 2 (Pr=0.61) (600 MHz, CDCl3, 25 °C)  	  	  	  	  	  	  	  	  	  	  	  	  	   107	  Appendix D. Solid state structure and crystallographic parameters 	  Table D.1. Selected bond lengths and angles for compound d. 	  	   Atom-Atom Bond length (Å)/Angle (°)   Bond length O1-H2 1.916 O2-C3 1.356 O1-C1 1.245 C1-C2 1.442 O3-H4 1.890 O4-C13 1.359 C14-C12 1.440 O3-C14 1.236 Bond Angles H4-O3-C14 52.566 H4-O4-C13 109.409 O3-C14-C12 125.585 H2-O2-C3 109.392 H2-O1-C1 96.531 C2-C2-O1 124.456      

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.24.1-0361982/manifest

Comment

Related Items