An investigation of the factors controlling the chemical structures of lignin dehydrogenation polymers

Abstract

Abstract Dehydrogenation polymer (DHP) is widely used as a model polymer of lignin. The chemical structure of DHP is highly affected by the synthesis conditions; however, the factors affecting DHP structure are not comprehensively understood. In this study, guaiacyl DHPs were synthesized to investigate the effects of reaction scale, pH, polymerization-enzyme activity, reaction media containing organic solvent, and differences between Zutropf (ZT) and Zulauf (ZL) modes on DHP properties. The DHPs were structurally characterized by size exclusion chromatography, 1H-13C HSQC NMR, and thioacidolysis with and without Raney nickel desulfuration. In ZT mode, smaller reaction scale significantly increased β-O-4 content, and β-O-4 formation was negatively correlated with the dose of polymerization-enzyme, horseradish peroxidase. Acidic condition (pH 4.0) in succinate buffer also increased the β-O-4 content of the DHP, although the α position of the DHP was acylated by the incorporation of succinic acid. DHPs prepared at pH 9.0 had high β-1 contents and low β-5 contents, while the reaction in 20% 1,4-dioxane markedly increased the molecular weight of the DHP. A systematic approach controlling the molecular structure of DHPs would increase their value as models for native and isolated lignins.