Milled Wood Lignin: A Linear Oligomer

The degree of polymerization (DP) of softwood and hardwood milled wood lignin samples and their branching degrees were quantitatively evaluated by a novel end-group titration approach composed of QQ-HSQC, 31P NMR, and DFRC coupled with 31P NMR analysis techniques. The DP of lignin can be calculated when the C9 formula, the amounts of phenolic groups, pinoresinol (β-β), diphenylethane (β-1), and phenolic diphenyl (5-5′) lignin subunits have been determined. Data on the degree of polymerization of lignin obtained by NMR techniques were not affected by supramolecular aggregation processes. 31P NMR analysis coupled with DFRC and QQ-HSQC allowed a detailed evaluation of the occurrence of condensed units in lignin and showed the terminal nature of diphenyl ether and diphenyl subunits. The resulting data unequivocally show that milled wood lignin is a linear oligomer.