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NMR investigations of cyanate resins Niu, Junning

Abstract

Heterogeneous polymer products are very complex systems. In many cases, the properties of these polymer systems are determined by the nature and the degree of reaction of a small number of functional groups in the system. However, it is very difficult to study them by most analytical techniques due to the influence of the polymer bulk and the irregularity and insolubility of the final product. In this thesis, both high resolution solution and solid-state NMR spectroscopies combined with specific isotopic enrichments are used to study such kinds of heterogeneous polymers with a particular focus on cyanate resin polymer systems, which are newly-developed thermo set resins used for electronic circuit boards and many structural composites. The mechanism of the curing reactions of cyanate resins based on bisphenol-A dicyanate has been investigated both in solution and in the solidstate by NMR spectroscopy. To facilitate the study, 13C and 15N isotopic labelled cyanate resins were used. The main curing reaction was found to be the formation of triazine rings and no NMR evidence for the formation of dimeric or other intermediate species prior to triazine ring formation was found. Side products are found in the solution curing due to the reaction of the cyanate group with trace water present in the solvent. In the bulk curing, the reaction is remarkably efficient, and no detectable side reaction occurs. This can be rationalized in terms of the very strong intermolecular interactions between cyanate groups on different molecules, which is observed in the crystal structure of the bisphenol-A dicyanate monomer obtained from a single-crystal X-ray diffraction experiment. The possible cross reactions indicated by solid-state NMR spectroscopy between cyanate and epoxy resins have been investigated by using both natural abundance and 13C and 15N labelled monofunctional model compounds. These soluble products were isolated and purified by adsorption chromatography and gel permeation chromatography, and were fully characterized by high resolution1H, 13C, 15N NMR spectroscopies and by mass spectrometry. The major cross-reaction product is a mixture of enantiomers which contain an oxazolidinone ring formed by one cyanate molecule and two epoxy molecules. However, triazine formation from the cyanate is much faster than the two competing reactions (the cross reaction between cyanate and epoxy and the self-polymerization of epoxy) under the conditions investigated. In addition to the cross reactions of epoxy and cyanate, the reactions of epoxy with carbamat ewhich is the major side product for the solution curing of cyanate resin have also been investigated, and several products related to the cross reaction have been isolated and identified. It is suggested that the reaction of epoxy and carbamateis one of the pathways in the overall cross reaction between epoxy and cyanateresins. A desired cross-linking monomer for mixed cyanate and epoxy resinsystems, the monoglycidyl ether of bisphenol-A-monocyanate, has been synthesized and characterized. The cyanate group in the cross-linking monomeris more reactive than the epoxy group and can be cured independently under heat or by base. A more practical approach for the application of the cross-linking monomer is discussed and tested. A very tough and strong resin material was obtained using this approach. A bifunctional cross-linking monomer, 2-allylphenyl cyanate, for the cyanate resin (thermo set) and olefinic polymers (thermoplastic) has also been synthesized and characterized. As a cross-linking agent, it not only reacts with itself, but also reacts with other cyanates to form hetero-triazine structures. Itcan also be copolymerized with the olefinic monomer, methyl methacrylate, to form a cross-linked polymer.

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