Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 176

by the subsequent radical coupling eventually resulting in polymerisation to form lignin
(Chakar and Ragauskas 2004). Whilst this basic mechanism for the formation of the
phenolic radicals can be agreed upon, the actual processes involved in the polymeri-
sation of these coupled dimers and oligomers, whether random or controlled, is the
subject of ongoing investigation and debate. The coupling of the radicals to the grow-
ing lignin chain proceeds in an essentially combinatorial fashion and, depending on the
species (and hence the prevalence of monolignol building blocks), the chemical linkages
between the monomer radicals vary significantly. In both hardwood and softwood, the
highest proportion of inter-unit bonds occur through coupling of the phenolic radicals of
the β carbon with the methoxyl group at the 4 position of the phenylpropane structure
as shown in Table 7.2. This β -O-4 bond typically accounts for approximately half of

Table7.2 Commonlinkagesbetweenmonolignolsinlignin.AdaptedfromSjostrom(1998).

Bond Chemical structure Percentage of total bond linkages

Softwood Hardwood

β -O-4 50 60

MeO

HO

O

OMe

OH

α -O-4 2-8 7

OH

MeO

O

MeO

OH

β -5 9-12 6

HO

OMe

O

OMe

OH

5-5 10-11 5

OH

MeO OMe

O

OH

( continued overleaf )