Page 319 - Carrahers_Polymer_Chemistry,_Eighth_Edition

282 Carraher’s Polymer Chemistry

cellobiosyl fluoride monomer was subjected to polycondensation catalyzed by celulase, a hydrolysis

enzyme of cellulose. Before this, cellulose was formed only in living cells as a high-order material
with parallel-chain alignment. The synthesis of other natural-like polysaccharides followed. Chitin,
the most abundant animal-derived polysaccharide, was synthesized using a ring-opening polyaddition
of an oxazoline monomer catalyzed by the chitinase enzyme. Chondroitin and hyaluronan, both gly-
cosaminoglycan heteropolysaccharides have been synthesized using hyaluronidase enzymes. In these
reactions, the hydrolase acted to catalyze bond formation, producing polymers but not the hydrolysis
to break the bonds.
Such enzymatic catalyzed polycondensations have allowed the synthesis of a number of “natu-
ral” polysaccharide, but has also allowed the production of “non-natural” polysaccharides such as
cellulose-xylan hybrids and functionalized hyaluronan, chondroitin sulfate, and chondroitin. Such
work illustrates the ever narrowing bridge between natural and synthetic polymers and polymer
syntheses.

9.2 CELLULOSE

Cellulose was originally “discovered” by Payen in 1838. For thousands of years impure cellulose
formed the basis of much of our fuel and construction systems in the form of wood, lumber (cut
wood), and dried plant material; served as the vehicle for the retention and conveying of knowledge
and information in the form of paper; and clothing in the form of cotton, ramie, and fl ax. Much of
the earliest research was aimed at developing stronger materials with greater resistance to the nat-
ural elements (including cleaning) and to improve dyeability so that the color of choice by common
people for their clothing material could be other than a drab white. In fact, the dyeing of textile
materials, mainly cotton, was a major driving force in the expansion of the chemical industry in the
latter part of the nineteenth century.
Cellulose is a polydisperse polymer with an average degree of polymerization (DP) in the gen-
eral range of 3,500–36,000. Native cellulose is widely distributed in nature and is the principle

constituent of cotton, kapok, flax, hemp, jute, ramie, and wood. Cellulose comprises more than
one-third of all vegetable matter and is the world’s largest renewable resource. Approximately
20
50 billion tons is produced annually by land plants, which absorb 4 × 10 cal of solar energy.
Natural cotton fi bers, which are the seed hairs from Gossypium, are about 1–2 cm in length and
about 5–20 μm in diameter. The molecules in native cellulose are present in thread-like strands
or bundles called fi brils. Cellulose is not found in a pure form, but rather it is associated with
other materials such as lignin and hemicelluloses. Cotton contains the purest form of cellulose.
Wood, in its dry state, contains 40%–55% cellulose, 15%–35% lignin, and 25%–40% hemicel-
lulose. Plant pulp is the major source of commercial cellulose. The extraction of cellulose from
plants is called pulping. Pulping is achieved using thermomechanical, chemical, or mechanical
approaches. Plant pulp, from wood, is the major source of nontextile fibers while cotton is the

major source of textile fi bers.
Cellulose is used in the textile industry in cloths, cartons, carpets, blankets, and sheets. Paper

is made from cellulose. Cellulosic fibers are also used as filter materials in artificial kidneys and

reverse osmosis though today most kidney dialysis units use cuprammonium tubular fi lms derived
from cellulose rather than cellulose itself.
While the celluloses are often largely linear, they are not soluble in water because of the pres-
ence of strong intermolecular hydrogen bonding and sometimes the presence of a small amount of
cross-linking. Highly ordered crystalline cellulose has a density as high as 1.63 g/cc, while amor-
phous cellulose has a density as low as 1.47 g/cc. High molecular weight native cellulose, which is
insoluble in 17.5% aqueous sodium hydroxide, is called α-cellulose. The fraction that is soluble in
17.5% sodium hydroxide but insoluble in 8% solution is called β-cellulose, and that which is soluble
in 8% sodium hydroxide is called γ -cellulose.

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