Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. III. CP‐MAS 13C‐NMR of different tannins and models

Induced Accelerated Autocondensation of Polyflavonoid Tannins for Phenolic Polycondensates. 111. CP-MAS 13C-NMR of Different Tannins and ModelsThe autocondensation of polyflavonoid tannins induced by weak Lewis acids has recently been described.'It opens the possibility of new applications of polymeric tannins as adhesives.2~~Foremost could be the use of tannins as wood adhesives without the use of any aldehyde hardener, leading to environmentally friendly, zero-aldehyde-emission natural binders.Traces of small amounts of weak Lewis acids such as silica, silicic acid, boric acid, and aluminum trichloride induce rapid autocondensation of concentrated tannin so- lutions to hardened resins.lS3The mechanism of autocondensation was found to be based on Lewis acid acceptance of electrons from the ether oxygen of the flavonoid unit pyran ring with concurrent and consequent facilitation of base-induced heterocycle opening.The reactive site so formed (C2) is blocked, by the charge of the Lewis acid counterion, from undergoing the intramolecular bonds rotations associated with the catechinic acid4 and phlobatannin rearrangements observed on monomeric model compounds.'These then do not occur, or occur to a much reduced extent on the polymeric natural tannin.'The effect hence addresses the subsequent condensation of the reactive site formed, away from internal rearrangements but toward condensation with a flavonoid unit in another polymer chain.'*3This leads to crosslinking and networking of the material to a hardened state.'*3The presence of this mechanism was also shown to reverse the relative ease of cleavage between interflavonoid bond and heterocycle opening in procyanidin tannins.' An important aspect remains to be investigated the extent and type of reactions occurring when using different tannins in relation to what is observed for monomeric flavonoid model compounds. EXPERIMENTAL 13C-NMRSolid-state (CP-MAS) 13C-NMR spectra of gelled concentrated water solutions of (+)-catechin at pH 12 with