MECHANISM OF POLYPHENOLIC TANNIN RESIN HARDENING BY HEXAMETHYLENETETRAMINE - CP-MAS C-13-NMR

Polyflavonoid tannins have been shown by C-13-NMR to react with hexame thylenetetramine (hexamine) at considerably higher rates than with phe nol with the formation of both benzylamine and methylene crosslinks to form hardened resins. Predominantly prodelphinidin-type tannins appea r to present a much higher proportion of benzylamine bridges than of m ethylene bridges, while in procyanidin-type tannins the proportion of the two types of crosslinks appear to be comparable. The greater the n ucleophilicity of the flavonoid tannin A-rings, the greater is the pro portion of benzylamine bridges which appear to form. At parity of the type of tannin, the faster the reaction with hexamine, the higher the proportion of benzylamines which appear to form. (C) 1995 John Wiley a nd Sons, Inc.