Chemistry Reference
In-Depth Information
Chapter 8
Naturally Occurring Polymers
8.1 Naturally Occurring Polymers
There are many naturally occurring polymeric materials. Many are quite complex. It is possible,
however, to apply an arbitrary classification and to divide them into six main categories. These are:
1. Polysaccharides. This category includes starch, cellulose, chitin, pectin, alginic acid, natural gums,
and others.
2. Proteins or naturally occurring polyamides found in animal and vegetable sources.
3. Polyisoprenes or natural rubbers and similar materials that are isolated from saps of plants.
4. Polynucleotides include all the deoxyribonucleic acid (DNAs) and all the ribonucleic acids
(RNAs) found in all living organisms.
5. Lignin or polymeric materials of coniferyl alcohol and related substances.
6. Naturally occurring miscellaneous polymers, like, for instance, shellac, a resin secreted by the Lac
insect. This is a complex cross-linked polyester of 9,10,16-trihydroxy-exadecanoic acid (aleuritic
acid). The structure also includes some unsaturated long-chain aliphatic acids together with other
compounds [
1
].
8.2 Polysaccharides
Fischer [
2
] carried out some of the original investigations of the monomeric species of many
polysaccharides during the last century. He was able to demonstrate the configurational relationships
within some monosaccharides.
The monomers in these naturally occurring polymers are five- or six-carbon sugars. There is
considerable variety among the polysaccharides and the polymers generally tend to be polydisperse,
depending upon the source.
8.2.1 Hemicelluloses
Hydrolyses of hemicelluloses yield mixtures of glucose, glucuronic acid, xylose, arabinose, galactose,
galacturonic acid, mannose, and rhamnose. Some common polymers of pentoses, also known as
pentosans, are xylan, galactan, araban, and others. Pentosans are found in large amounts (20-40%)
