Incorporation of p-cresol into lignins via peroxidase-catalysed copolymerization in nonaqueous media

Lignin, the second most abundant biopolymer after cellulose, is a low value by-product of agricultural and wood conversion processes, including wood pulp manufacture. Copolymerization with phenols has the potential to convert by-product lignins to higher-value phenolic resins. In this initial investigation, we have studied the use of horseradish peroxidase (HRP) in aqueous dioxane to catalyse the grafting of p-cresol (p-methylphenol) onto milled wood lignin, kraft lignin, and a lignin selectively o-demethylated by a brown-rot fungus. Advantages of this system are (1) the mild reaction conditions employed and (2) the ability of HRP to function in the dioxane: water solutions which solubilize lignin. The reaction is monitored by gel permeation chromatography using a reaction system of [¹⁴C]-p-cresol with unlabeled lignins. We have found that optimal incorporation of cresol into high-molecular-weight polymer occurs at 50-70% dioxane in water under the conditions used; a maximum incorporation of ca. 4 mol% (e.g., p-cresol incorporated per C₉ lignin unit) was obtained. Blocking the phenolic hydroxyl groups of the lignin inhibits copolymerization, consistent with the proposed mechanism of phenoxy radical copolymerization for this reaction.