COMPARATIVE KINETICS OF INDUCED RADICAL AUTOCONDENSATION OF POLYFLAVONOID TANNINS .1. MODIFIED AND NONMODIFIED TANNINS

Comparative kinetics of the radical autocondensation induced by SiO2 o f a polyflavonoid tannin, namely quebracho tannin a mostly profisetini din/prorobinetinidin tannin, in its natural extract state, sulfited, c arbohydrate free, and in its adhesive intermediate form were carried o ut by electron spin resonance (ESR). The results obtained not only con firmed the existence of strong radical mechanisms of tannin autoconden sation in the presence of dissolved SiO2, but also pointed out new eff ects of interest in such a reaction. The SiO2 induced autocondensation proceeds at a faster rate, and radical surge and concentration decay appear to be more marked and more rapid the more colloidal the tannin solution is. This appears to indicate that a colloid-induced intramice llar radical mechanism of the reaction is at work. As a consequence th e intensity and rate of the radicals surge and decay decreases passing from more colloidal to less colloidal tannin solutions. Thus, the dec ay rate decreases passing from natural tannin to adhesive intermediate , to carbohydrate-free tannin, to almost disappear for sulfited tannin . The colloidal state of the solution appears to depend mostly, but no t only, on the presence of the polymeric carbohydrates in the extract. (C) 1996 John Wiley & Sons, Inc.