NCERT Solutions for Class 12 Chemistry Chapter 15 Polymers

A polymer is a large molecule of high molecular mass formed by the repetitive bonding of many small molecules called monomers. The process by which the monomers are transformed into polymers is called polymerisation.

One of the common: Classifications of polymers is based on source from which polymer is derived.

1. Natural polymers
   These polymers are found in plants and animals. Examples are proteins, cellulose, starch, some resins and rubber.
2. Semi-synthetic polymers
   Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usual examples of this sub category.
3. Synthetic polymers
   A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon 6, 6) and synthetic rubbers (Buna – S) are examples of man-made polymers extensively used in daily life as well as in industry.

There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step growth polymerisation.
Addition Polymerisation or Chain Growth Polymerisation
In this type of polymerisation, the molecules of the same monomer or diferent monomers add together on a large scale to form a polymer. The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives.

Mechanism of Addition Polymerisation
A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. For example, the polymerisation of ethene to polythene consists of heating or exposing to light a mixture of ethene with a small amount of benzoyl peroxide initiator. The process starts with the addition of phenyl free radical formed by the peroxide to the ethene double bond thus generating a new and larger free radical. This step is called chain initiating step.

Polythenes are linear or slightly branched long chain molecules. These are capable of repeatedly softening on heating and hardening on cooling and are thus thermoplastic polymers.

Nylons
It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature. Nylon 6, 6 is fibre forming solid. It possesses high tensile strength. This characteristic can be attributed to the strong intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains and thus impart crystalline nature. Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.

These are the polycondensation products of dicarboxylic acids and diols. Dacron or terylene is the best known example of polyesters. It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst as per the reaction given earlier. Dacron fibre (terylene) is crease resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc.

Natural rubber
Rubber is a natural polymer and possesses elastic properties. It is also termed as elastomeric polymer. In elastomeric polymers, the polymer chains are held together by the weak intermolecular forces.

Synthetic rubbers
Synthetic rubber is any vulcanisable rubber like polymer, which is capable of getting stretched to twice its length. However, it returns to its original shape and size as soon as the external stretching force is released. Thus, synthetic rubbers are either homopolymers of 1, 3 – butadiene derivatives or copolymers of 1, 3 – butadiene or its derivatives with another unsaturated monomer.