Page 40 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

244 Biopolymers

Lignin from alkali extraction of wood is a stabilizer and
CH 3
emulsifying agent, an additive for concrete, and a filler for
natural rubber. Much less lignin is used, however, than is (CH 2 ) n
produced by the paper industry and some is simply burnt HO CH CH 2 COOH
as a fuel.
Cutin and suberin are polymers that provide physical
XIV
barriers in plants to prevent or control diffusion of water
and other small molecules. Leaves of plants are protected The monomers become linked together essentially as
by a cuticle of cutin embedded in waxes, while under- in XIII, and the bacteria use the polymers as energy re-
ground organs such as roots have a barrier layer contain- serves. The most common polymer, poly(3-hydroxy bu-
ing suberin and waxes. The monomers of both are thought tyrate)madefromXIVwheren = 0,resemblespolypropy-
to be a variety of hydroxy acids such as XII: lene in properties, but is stiffer and more brittle. A more
useful polyester is a random copolymer of 3-hydroxy bu-
CH 2 OH (CH 2 ) 7 (CHOH) 2 (CH 2 ) 7 COOH tyrate (XIV where n = 0) and 3-hydroxy valerate (XIV
where n = 1), and is tougher and more flexible. This can
XII be molded and is made into bottles and containers. Films,
coatings for paper and board, and compost bags can also
Trihydroxy acids, such as the above, are found more fre- be produced from the copolymer. Polymers of longer
quently in cutin than in suberin. In suberin, acids contain- chain acids (XIV with n = 4 for example) are rubbery
ing only one hydroxyl group at the end of the molecule and elastic, and may in future find uses as biodegrad-
or di-acids with a COOH group at each end are much able elastomers. The poly(hydroxyalkanoates) are readily
more common. Both polymers contain some monomer degraded to water and carbon dioxide, or in some cases
residues related to those of lignin (with structures like methane, by enzyme systems occurring in bacteria of soil,
XI, but having a COOH group instead of CH 2 OH at the sewage sludge, or compost. At present, however, the high
end of the side-chain); suberin can contain up to 60% of cost of production of these polyesters limits their use. Re-
these residues, while cutin contains much less. The acid search is under way to produce bacteria “engineered” (by
monomers probably become linked together by reaction transferring genes from one bacterium to another) to syn-
between the COOH of one monomer and OH of another thesize the polymers more efficiently from cheap carbon
to give ester linkages as in XIII: sources such as sugar cane and beet molasses, cheese whey
and hydrolysates of starch, and hemicellulose (see Sec-
COOH HO COO H 2 O tion II.B.3). New poly(hydroxyalkanoates), produced by
supplying bacteria with a variety of acid starting materials,
XIII are also being investigated. In addition, attempts are being
made to transfer the bacterial genes for polyester synthesis
The molecules are highly cross-linked and, like lignin, to plants, where the sun’s energy, trapped via photosynthe-
cannotbedescribedbyasinglesimplerepeatunit.Someof sis, could be utilized for poly(hydroxyalkanoate) produc-
the phenolic residues of suberin may be covalently linked tion, thus lowering costs compared to expenditure on the
to cell wall polysaccharides. While both polymers have a bacterial fermentations currently necessary for polymer
protective function, break-down products from cutin may formation.
also be important for activating defense mechanisms when
plants are invaded by pathogens.
E. Polymer-Polymer Interactions
Most commercial plastics are made from petrochemi-
cals and are nonbiodegradable. Problems of supply may For very many biopolymers, their importance in the liv-
develop in the future as oil stocks run low, and the lack of ing organism lies in the way in which they interact with
degradability can currently result in environmental pol- other polymers. In a few cases there is covalent bonding
lution when plastic objects are discarded. Thus investi- between polymers, for example, in the proteoglycans of
gations are ongoing to find materials that can be pro- animal connective tissue (see Section II.B.7), or between
duced from renewable resources and are biodegradable, lignin and polysaccharides in plant cell walls. In many
but have the properties of synthetic polymer thermoplas- more instances, however, interactions are noncovalent and
tics or elastomers. Some bacteria synthesize polyesters, involve weaker bonds, such as hydrogen or ionic bonding
the poly(hydroxyalkanoates), with these properties. The or hydrophobic associations. Although the bonding is rel-
monomers are usually of the type shown in XIV, where n atively weak, the interactions can be strong, because an
is often 0 or 1. area of the surface of one polymer molecule fits exactly