ChemSusChem | 2021
Identifying the interunit linkages connecting free phenolic terminal units in lignin.
Abstract
Understanding the linkages connecting phenolic terminal and non-phenolic units in complex branched polymer, lignin, is crucial to facilitate efficient and selective valorization of lignin. Herein, we identify the interunit linkages connecting phenolic units by premethylation of the phenolic hydroxy groups and thioacidolysis-desulfuration. Interestingly, the phenolic units were connected by only β-5, β-1 and β-O-4 linkages. The phenolic unit abundance was approximately 20%. The result revealed 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 were not detected, indicating that these units are further elongated to form non-phenolic units. This study revealed the linkage types connecting phenolic and non-phenolic units and their elongation mechanisms.