Biphenyl structure and its impact on the macromolecular structure of lignin: A critical review

Biphenyl linkage is the second most abundant linkage in softwood lignin; the β-O-4′ linkage being the most abundant linkage. That about 20–28% of monolignols are linked by biphenyl linkage has been demonstrated by UV spectroscopy, permanganate oxidation, 13C NMR, and thioacidolysis followed by 31P NMR. Most, if not all, of the biphenyl structures in softwood lignin are etherified, indicating that biphenyl structures are formed while monolignols are being transported to the lignifying zone during lignin biosynthesis. Along with 4-O-5′ (∼3/100 C9 units), β-6′/α-6′ (∼3/100 C9) and noncyclic α-O-4′ structures (4/100 C9), biphenyl structures are the major branching points of lignin structure. Thus, at least one out of every three C9 units in softwood lignin are branching units. This fact indicates that softwood lignin is most likely a network polymer. Many alkyl-aryl ether bonds are cleaved during ball milling for the isolation of Milled Wood Lignin (MWL), but the number of branching points remains the same. Therefore, softwood milled wood lignin, with a number average DP of 20, exists most likely as at least a highly branched polymer and unlikely as a linear polymer. Harwood lignin has less biphenyl structures (∼9%), but has more 4-O-5′ (∼7%), noncyclic α-O-4′ (6%) and similar β-6′/α-6′ (∼2%) linkages as compared with softwood lignin. Thus, only one out of every four C9 units is a branching point, indicating that hardwood lignin is most likely a highly branched polymer. Hardwood MWL is most likely a branched polymer, but one cannot rule out the chance of finding a few linear oligomers in MWL. One potential branching point is not considered in the above discussion. The lignin-carbohydrate complexes are also branching points in lignin. The number of LCC linkages in native lignin cannot be determined precisely.