Identifying the Interunit Linkages Connecting Free Phenolic Terminal Units in Lignin

Understanding the linkages connecting phenolic terminal and nonphenolic units in complex branched polymer, lignin, is crucial to facilitate efficient and selective valorization of lignin. In this study, the interunit linkages connecting phenolic units are identified by premethylation of the phenolic hydroxy groups and thioacidolysis-desulfuration. Interestingly, the phenolic units are found to be connected by only β-5, β-1, and β-O-4 linkages. The phenolic unit abundance is approximately 20 %. The result reveals that lignin polymerization terminates with the three linkages by a coupling between a monomer and the polymer terminus, which is reasonably explained by the radical coupling mechanism. Unexpectedly, 5-5, 4-O-5, and β-β linkages connecting the phenolic units are not detected, indicating that these units are further elongated to form nonphenolic units. This study reveals the linkage types connecting phenolic and nonphenolic units and their elongation mechanisms.