HIGH-RESOLUTION C-13 AND N-15 NMR INVESTIGATIONS OF THE MECHANISM OF THE CURING REACTIONS OF CYANATE-BASED POLYMER RESINS IN SOLUTION AND THE SOLID-STATE

The mechanism of the curing reactions of cyanate polymer resins based on Bisphenol A dicyanate has been investigated both in solution and in the solid state by NMR spectroscopic techniques. To increase S/N and to unambiguously characterize the reactions of the cyanate functionalities, C-13- and N-15-enriched cyanate resins and monocyanate model compounds were used, the latter yielding soluble and isolable analogs. In solution, the main reaction is formation of triazine rings as identified by NMR and MS techniques and characterized by single-crystal X-ray diffraction on an isolated crystalline compound from the monocyanate model compound. Side products are formed by the reaction of the cyanate functionalities with trace water present in the solvent but there is no NMR evidence for the formation of dimeric or other intermediate species prior to triazine ring formation. The isolated resins from the solution curing and also those formed directly by heat curing of the neat resin were characterized by high-resolution solid-state NMR. In the former case triazine ring formation and the presence of side products were confirmed by both C-13 and N-15 solid-state NMR and their proportions quantified. In the case of the neat resin, the curing reaction is shown to be almost quantitative. This is rationalized in terms of the very strong intermolecular intercyanate bonding interactions observed in the crystal structure of the Bisphenol A dicyanate monomer obtained from a single-crystal X-ray diffraction experiment.