Of the three major biopolymers that constitute wood, lignin is distinctly different from the other macromolecular polymers [34]. Lignin is an amorphous, cross-linked, and three-dimensional polyphenolic polymer that is synthesized by enzymatic de- hydrogenative polymerization of 4-hydroxyphenyl propanoid units [35, 36]. The biosynthesis of lignin stems from the polymerization of three types of phenylpropane units as monolignols: coniferyl, sinapyl, andp-coumaryl alcohol [37, 38]. Figure 8.4 depicts these three structures. It has been identified that lignin from softwood is made up of more than 90 % of coniferyl alcohol with the remaining being mainly p-coumaryl alcohol units. Contrary to SW, lignin contained in hardwood is made up of varying ratios of coniferyl, sinapyl, and typically lesser amounts of p-coumaryl alcohol type of units.

The polymerization process is initiated by an enzyme-catalyzed oxidation of the monolignol phenolic hydroxyl groups to yield free radicals. A monolignol free radical can then couple with another monolignol to generate a dilignol. Subsequent nucle­ophilic attack by water, alcohols, or phenolic hydroxyl groups on the benzyl carbon of the quinone methide intermediate, restores the aromaticity of the benzene ring. The generated dilignols then undergo further polymerization to form protolignin.

Although the exact structure of protolignin is unknown, improvements in meth­ods for identifying lignin-degradation products and advancements in spectroscopic methods have enabled scientists to elucidate the predominant structural features of lignin. Table 8.3 showed the typical abundance of common linkages and functional groups found in softwood lignin [39, 40].

The property of polydispersity, just as with hemicellulose, characterizes lignin as well. The DP for softwood lignin is approximately 60-100 and the molecular weight is in excess of 10,000 [41, 42].

R,= OMe, R2 = H Ri= R2 = OMe R,= R2 =H

Lignin in wood behaves as an insoluble three-dimensional network. It plays an important role in the cell’s endurance and development, as it affects the transport of water, nutrients, and metabolites in the plant cell. It acts as binder between cells cre­ating a composite material that has a remarkable resistance to impact, compression, and bending [26].

Lignin is much less hydrophilic than either cellulose or hemicelluloses, and it has a general effect of inhibiting water adsorption and fiber swelling. Solvents that have been identified to significantly dissolve lignin include low molecular alcohols, dioxane, acetone, pyridine, dimethyl sulfoxide and select ionic liquids. Further­more, it has been observed that at elevated temperatures, thermal softening of lignin takes place, which allows depolymerization reactions of acidic or alkaline nature to accelerate [43].