Mechanism of polyphenolic tannin resin hardening by hexamethylenetetramine: CP–MAS 13C‐NMR

Polyflavonoid tannins have been shown by 13C‐NMR to react with hexamethylenetetramine (hexamine) at considerably higher rates than with phenol with the formation of both benzylamine and methylene crosslinks to form hardened resins. Predominantly prodelphinidintype tannins appear to present a much higher proportion of benzylamine bridges than of methylene bridges, while in procyanidin‐type tannins the proportion of the two types of crosslinks appear to be comparable. The greater the nucleophilicity of the flavonoid tannin A‐rings, the greater is the proportion of benzylamine bridges which appear to form. At parity of the type of tannins, the faster the reaction with hexamine, the higher the proportion of benzylamines which appear to form.