Lignin, the second major component of wood, is produced in large quantities as an underutilized by-product during chemical pulping. In its natural form lignin is a three-dimensional polymer constituted of random polymerized phenylpropane (C9) units (Fig. 6.1) [1]. During pulping the lignin macromolecule is degraded and modified (see below). The polymeric nature of lignin nevertheless prevails after pulping. Consequently, the development of uses in which the polymeric nature of the by-product lignin is exploited has received widespread attention [2]. Phenyl propanoid units of lignin: R<sub>1,</sub> R<sub>2</sub> = H, OCH<sub>3</sub>, or / and (§) = possible linkage to other phenyl propanoid units. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203733721/133de2cc-80dd-40dd-adae-6aa51983d908/content/fig6_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>