The Modular Structure of Lignin

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<jats:p> A kinetic investigation of the polymerization of lignin-like polymers of coniferyl alcohol (DHP) using horseradish peroxidase as the phenol oxidase revealed the product to have a bimodal distribution of molecular weight. A peak representing material of low molecular weight M<jats:sub>w</jats:sub> 3000–4000 was first formed, followed by the formation of a second peak in the molecular weight range 80 000–100 000 whose growth appeared to proceed at the expense of the first.Alkaline degradation of lignins isolated from spruce and hemlock also gave products having a bimodal distribution of molecular weight, with peaks at the same values as found for the synthetic lignin. Lignin isolated during alkaline pulping of finely divided wood showed the presence of the same two components of high and low molecular weight.From these results, we have constructed models for the formation of lignin and for its degradation during an alkaline pulping of wood. The lignin macromolecule as it occurs in wood may well be material of narrow molecular weight distribution with d.p. about 500. It is proposed that this macromolecule is an assembly of subassemblies, or modules, each of d.p. about 20. Lignin formation is seen to proceed by enzyme dehydrogenation of the monomers first producing free radicals as described by Freudenberg, the free radicals reacting by a step condensation to form the modules, followed by intermodular addition polymerization to form the lignin macromolecules. Lignin degradation in pulping reactions involves first the nonrandom homolytic chain depolymerization of the macromolecule to yield free-radical carrying modules, which, after stabilization against recondensation by the pulping chemicals, break down to lower oligomers and monomers by a step depolymerization reaction. </jats:p>

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