Chemistry Reference
In-Depth Information
the rubber tree Hevea braziliensis
[5,7]
. The other 3% contains various proteins
that sometimes trigger allergies. Guayule, a plant shrub, also grows rubber that
does not trigger allergic reactions. Natural rubber is obtained by tapping the bark
of a tree that grows wild in South America and is cultivated in other parts of the
world. Since natural rubber has a glass transition temperature of
70
C and very
high abrasion resistance, it has been extensively used to make tires. However, to
turn natural rubber (a soft material at room temperature) into a tire, cross-linking
of the cis-1,4-polyisoprene molecules, a process called vulcanization, is needed. It
is worth noting that natural rubber is very reactive due to the presence of many
double bonds in its structure. And such a chemical moiety is vulnerable to free-
radical and oxidation reactions. It is because natural rubber is used in its cross-
linked form that it is not biodegradable and not easily recycled.
2
13.2.2
Polysaccharides
Polysaccharides, otherwise known as carbohydrates, are polymeric structures in
biological systems. Polysaccharides are made of many monosaccharides, which
are composed primarily of carbon, oxygen, and hydrogen. The most common
types of monosaccharide units contain either five or six carbon atoms. Due to the
presence of several hydroxyl groups within the monosaccharide structure, hydro-
gen bonds are formed between polymer chains. Glucose is the most abundant six-
carbon sugar and is the monomeric unit of both cellulose and starch.
Two types of linkages are possible from the polymerization of glucose: alpha
or beta acetal linkages. These different linkages are possible since glucose primar-
ily exists in alpha or beta anomers. An anomer is one of two stereoisomers of a
cyclic sugar that differ only in configuration at the anomeric carbon of the hemi-
acetal
[8]
. In a glucose ring, when the OH group of the hemiacetal is in the down
position it is called
α
-glucose, compared to when the OH group is in the up posi-
tion it is called
-glucose anomer is the most stable because all of
the substituents larger than hydrogen are in the more spacious equatorial position,
including the OH group
[8]
. The same naming applies to polymerized forms of
the glucose molecules. When the bond between two glucose molecules at the
anomeric carbon is up, it is a
β
-glucose. The
β
β
linkage, as illustrated with cellulose compared to
the
linkage of amylose in
Fig. 13.1
. The bond angles influence the structure of
the polymer. For example, cellulose is an almost linear chain, whereas amylose
forms a spiral structure.
α
13.2.2.1
Starch
Starch is an abundant polymer stored in plants for energy consumption in the
form of granules. Although glucose is the monomer in both starch and cellulose,
starch differs from cellulose in bonding by two ways: (1) the glucose rings are
connected through carbons 1 and 4 by
linkages and
(2) frequent branching occurs at carbon 6. Starch consists of two polysaccharides,
amylose and amylopectin, illustrated in
Fig. 13.2
, which can be separated due to
α
linkages rather than
β
