Thermal latency of novel chelating borate catalysts as latent catalysts for epoxy-phenolic resins

Novel quaternary ammonium bis(2-oxybenzoyloxy)borate salts (1a-1c) or quaternary ammonium bis(1,2-benzenedioxy)borate salts (2a and 2b) with tetra-n-butylammonium (TBA(+)), tetra-n-octylammonium (TOA(+)), or bis(triphenylphosphoranylidene)ammonium (PNP+) cations were synthesized as latent catalysts of epoxy/phenol-novolac resins by the complexation between boric acid and salicylic acid or catechol, followed by neutralization with quaternary ammonium hydroxide. Polyaddition reactions of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-bisphenol F (44BPF) or bisphenol F (BPF-D) with the ammonium borates were investigated as model reactions of epoxy/phenol-novolac resin systems with respect to the thermal latency and storage stability of the catalyst. The polyaddition of DGEBA/44BPF with 1a-1c in diglyme at 150 degreesC for 6 h proceeded up to 85-96% conversions and gave polymers with number-average molecular weights of 4180-10,500, whereas the polyaddition at 80 degreesC for 6 h gave less than 8% conversions. However, the polyaddition with 2a containing TBA(+) cation proceeded to only a 32% conversion at 150 degreesC for 6 If in diglyme and to a 64% conversion even at 180 degreesC for 6 h in triglyme and only gave low molecular weight oligomers, and no reaction proceeded in the polyaddition at 80 degreesC. However, polyaddition with 2b containing PNP+ cation proceeded up to a 96% conversion at 150 degreesC for 6 h in diglyme and gave a higher molecular weight polymer with a number-average molecular weight of 8050, whereas the polyaddition at 80 degreesC for 6 h gave only a 5% conversion. The catalytic activity of ammonium borates 1a-1c and 2a and 2b depended on the borate anion structure: la and le with bis(2-oxybenzoyloxy)borate anion revealed higher activity than 2a and 2b with bis(1,2-benzenedioxy)borate anion, respectively. In comparison with tetra-n-butylammonium bromide (TBAB) as a conventional ammonium salt or tetra-n-butylammonium tetrakis(benzoyloxy)borate (TBA-TBB), 1a-1c and 2b revealed better thermal latency. The catalytic activity of ammonium borates also depended on the bulkiness of the ammonium cation, and the order of activity was 1c (PNP+) > 1b (TOA(+)) greater than or equal to 1a (TBA(+)) and 2b (PNP+) > 2a (TBA(+)). The storage stability of DGEBA/BPF-D with the ammonium borate catalysts 1a-1c and 2a and 2b in bulk at 40 degreesC was much better than that with TBAB and TBA-TBB. (C) 2002 Wiley Periodicals, Inc.