Page 350 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Plants 313

Lignin has a complex structure that varies with the source, growing conditions, and so on. This
complex and varied structure is typical of many plant-derived macromolecules. Lignin is generally
considered as being formed from three different phenylpropanoid alcohols—coniferyl, coumaryl,
and sinapyl alcohols, which are synthesized from phenylalanine via various cinnamic acid derivatives
and is sometimes commercially treated as being composed of a C repeat unit where the superstruc-
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ture contains aromatic and aliphatic alcohols and ethers, and aliphatic aldehydes and vinyl units.
Lignin is found in plant cell walls of supporting and conducting tissue, mostly the tracheids and
vessel parts of the xylem. It is largely found in the thickened secondary wall but can occur elsewhere
close to the celluloses and hemicelluloses.
The presence of rigid aromatic groups and hydrogen bonding by the alcohol, aldehyde, and ether
groups give a fairly rigid material that strengthens stems and vascular tissue in plants allowing
upward growth. It also allows water and minerals to be conducted through the xylem under negative

pressure without collapse of the plant. This structure can be flexibilized through introduction of a
plasticizer, in nature mainly water. The presence of the hydrophillic aromatic groups helps ward off
excessive amounts of water, allowing the material to have a variable flexibility but to maintain some

strength. This type of balance between flexibility and strength is also utilized by polymer chemists

as they work to modify lignin as well as synthetic polymers to give strong, semifl exible materials.
Without the presence of water, lignin is brittle, but with the presence of water, the tough lignin pro-
vides plants with some degree of protection from animals.
Its chemical durability also makes it indigestible to plant eaters and its bonding to cellulose and
protein material in the plant also discourages plant eaters from eating it. Formation of lignin also
helps block the growth of pathogens and is often the response to partial plant destruction.
The role of transferring water by lignin makes the ability to produce lignin critical to any plants
survival and permits even primitive plants to colonize dry land.
During the synthesis of plant cell walls, polysaccharides are generally initially laid down.

This is followed by the biosynthesis of lignin that fills the spaces between the polysaccharide
fibers acting to cement them together. The filling of cell wall spaces results in at least some of

the lignin having a somewhat two-dimensional structure similar to a sheet of paper rather than
the typical three- dimensional structure for most polysaccharides and other natural macromol-
ecules. The lignin sheets act as a barrier toward the outside elements, including marauding pests
as noted above.
The lignin is generally considered as being insoluble but it can be solubilized utilizing special
systems such as strong basic solutions that disrupt the internal hydrogen bonding. It is not clear
even now if the undegraded lignin is solubilized or if degradations accompany lignin solubiliza-
tion. Weight-average molecular weights for some lignin samples vary depending on the type of sol-
vent employed consistent with degradation occurring for at least some of these systems. Molecular
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weight values to about 10 have been reported for some alkali-extracted fractions.
Solubilized lignin solutions are easily oxidized and the presence of the aromatic unit containing
electron-donating ether and alcohol moieties makes it available for electrophilic substitution reac-
tions such as nitration, halogenation, hydroxylation, and so on.
Since its removal is the major step in the production of paper pulp, vast amounts of lignin are
available as a byproduct of paper manufacture. Its industrial use is just beginning and it remains
one of the major underused materials. Its sulfonic acid derivative is used as an extender for phenolic
resins, as a wetting agent for oil drilling muds, and for the production of vanillin. Combined, this
accounts for less than 1% of this important “byproduct.” In the enlightened age of green chemistry,
greater emphasis on the use of lignin itself and in products derived from it must occur.

9.14 MELANINS

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Light is continuous ranging from wavelengths smaller than 10 cm (Gamma radiation) to greater
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than 10 cm. Radiation serves as the basis for the synthesis of many natural macromolecules via

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