Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. I. 13C-NMR, 29Si-NMR, X-ray, and polarimetry studies and mechanism

Polyflavonoid tannins have been found to autocondense and harden when in presence of small amounts of SiO 2 at high pH. Small amounts of boric acid and AICI 3 were found to have the same effect. The mechanism of tannin autocondensation to hardening was found to depend on the Lewis acid behaviour of the additives used. Such mechanism involves Lewis acid acceptance of electrons from the ether oxygen of the flavonoid unit pyran ring with subsequent...