POLYMERS

Polymers are macro-sized molecules with relatively high molecular mass. They are formed by joining together large number of small molecules. The process of formation of polymers from their starting materials is called polymerisation and the small molecules which combine with each other are termed as monomers.

Polymers formed from one kind of monomers is called homopolymer while those formed from more than one kind of monomer units is called copolymer or mixed polymer.

Ethylene or ethene is the monomer unit of polyethylene. It is a homopolymer.
n CH2= CH2 = (-CH2CH2-)n

Nylon-66 is a copolymer made from Hexamethylene diamine and Adipic acid.
nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH =
- [NH-(CH2)6-NH-CO-(CH2)4-CO-]n + nH2O


Classification of Polymers.
Polymers are classified in different ways.

1. Classification based on source.On the basis of source polymers are classified into two types. They are natural polymers and synthetic polymers.

Natural polymers are obtained from nature. They are cellulose, starch, natural rubbers, proteins etc. Natural rubber is a polymer of isoprene. It is obtained from the latex of rubber tree. Polymers such as polysaccharides (starch, cellulose), proteins and nucleic acid which control various life processes in plants and animals are also called as biopolymers.

Synthetic polymers are artificially prepared in the laboratory. They are man-made polymers. Examples are polyethene, polyvinyl chloride (PVC), nylon, terylene, bakelite etc. They find diverse applications in man's daily life.

2. Classification based on Structure. Bases on structure, polymers are classified as:

Linear polymers.In these, the monomeric units are linked together to form long straight chains. They are closely packed and hence have high densities, high melting point and high tensile strength. eg; nylon, polyethene.

Branched chain polymers. The monomer units are linked together to form a main chain. Side chains of different lengths arises from the main chain thus forming branches. These polymers are irregularly packed and have low tensile strength and melting points. eg; amylopectin, glycogen.

Cross Linked polymers.In these the monomer units are linked together to form a three dimensional network. These cross-linked polymers are hard, rigid and brittle. eg; melamine, bakelite.

3. Classification Based on Synthesis. On this basis, polymers can be classified as addition polymers and condensation polymers.

Addition polymers.It is formed by the direct addition of repeated monomers. eg; polyethylene, polypropylene, PVC etc.

Condensation polymers.They are formed from two or more kind of monomers. Each monomer usually contains two functional groups. During the formation of condensation polymers simple molecules such as H2O is lost. eg; Nylon 66, Dacron.