Comparative kinetics of the induced radical autocondensation of polyflavonoid tannins. II. Flavonoid units effects

Abstract

ABSTRACT:Comparative kinetics of the radical autocondensation induced by SiO 2 on a series of polyflavonoid tannins, namely, pine, pecan, mimosa, quebracho, gambier, sumach, and on the catechin monomer as a model compound were carried out by electron spin resonance. The induced radical autocondensation appeared to be independently catalyzed by the known base mechanism, as well as SiO 2 and Lewis acid attack directly at the heterocycle oxygen. The reaction occurs in two definite steps: the first, the radical-anion formation, the second, the condensation proper with other flavonoid units of the reactive sites formed. The rate determining step depends on both the main flavonoid unit structure of each tannin and particularly on the level of colloidal state of the tannin solution and the number-average degree of polymerization (DP n ), with the latter two parameters being the main determining ones for the second reaction step and the first two for the first reaction step. It is, however, the combination of the three parameters that determines the total observable effect for each of the flavonoid tannins. The SiO 2 attack at the heterocycle ether oxygen is of such an intensity that the A-rings' phenoxide radicals, which drive the reaction, surge very rapidly to such a higher proportion than the B-rings phenoxide radicals that the B-rings also start to surge later by shifting to the left of the • B _ A• equilibrium. There are also indications that ionic mechanisms might be more important for the second step of the reaction. Different radical-anion species and the relative movements of the relevant equilibria involved can be clearly identified from the spectra peaks. The initial, maximum intensity of the peaks has been shown to be the parameter defining the first step of the reaction, while the radical decay rate has been shown to refer to the second step of the reaction. Hydrolyzable tannins have been shown not to undergo neither any silicainduced radical surge nor autocondensation as predictable from their structures.

Tannins: Major Sources, Properties and Applications

“…In recent years, the importance of the marked colloidal nature of tannin extract solutions has come to the fore [42,43,[52][53][54][55][56][57][58][59][60] . It is the presence of both polymeric carbohydrates in the extract, as well as of the higher molecular fraction of the polyphenolic tannins, which determines the colloidal state of tannin extract solutions in water [42,52] .…”

Effect of poly(ethylene glycol) on the hydrophobic interactions and rheology of proanthocyanidin biopolymers from Pinus radiata

“…The tannin content (%) and the Stiasny value illustrated the extent and subsequent reactivity of tannin extraction under these HWE conditions, whereas the high sugar fraction indicated the importance of the molecular association of some of the tannins with complex carbohydrates 7, 10, 23…”

“…Pizzi and Stephaou6 studied many of the condensation mechanisms, including those of Pinus radiata , which frequently lead to processing difficulties at the higher concentrations required for adhesive formulations. Interactions due to colloidal self‐associations7–9 and secondary interactions10 have been shown to dictate their detailed rheological response. Strong nucleophiles such as phloroglucinol, m ‐phenylene diamine, and urea have been shown to block the mechanisms of self‐condensation8, 11 and colloidal or molecular association12, 13 in solution.…”

Applications of Tannin Resin Adhesives in the Wood Industry

“…Nevertheless, hardening through tannin autocondensation without any aldehyde addition is also possible. The mechanism of polyflavonoid autocondensation has been examined using carbon-13 nuclear magnetic resonance and electron-spin resonance spectroscopy, among others [19][20][21].…”