Thioacidolysis of enzymatic dehydrogenation polymers from p-hydroxyphenyl, guaiacyl, and syringyl precursors

Lignin polymer models (DHPs) were obtained from p-coumaryl, coniferyl, and/or sinapyl alcohols under conditions simulating the end-wise or the bulk polymerization modes. Their structures were characterized by thioacidolysis followed by Raney nickel desulfuration in order to investigate the correlative variations of DHP structures with polymerization mode and precursor type. Whatever the precursors, addition rates and final concentrations of monolignols in the dehydrogenative medium appeared discriminatory enough to give rise to end-wise and bulk type DHPs, the former mimicking less imperfectly native lignins. The behavior of the p-hydroxyphenyl precursor, less easily oxidized, is influenced by the presence of other types of precursors. Polymerized alone, it gives rise to a very condensed polymer. In contrast, when copolymerized with G and/or S precursors, it is efficiently involved in labile bonds. H units of the copolymers are mainly terminal units with free phenolic groups, in synthetic DHPs as well as in native lignins.