Polymers - Types, Classification, Properties, and Uses of Polymers - Practice Questions & MCQ

Polymers are the backbone of four major industries, i.e, plastic, fibres, elastomers and paints. Polymers are large molecules having high molecular masses formed by the chemical combination of a large number of relatively smaller molecules known as monomers. The process by which these polymers are converted into polymers is known as polymerisation.
A polymer always consists of hundreds to thousands of repeating structural units. Thus, all polymers are macromolecules but all macromolecules are not polymers.

Classification based on source
On the basis of source, polymers can be classified into three categories:

1. Natural polymers: These are substances of natural origin, i.e, these are found in nature mainly in plants and animals. The well known natural polymers are proteins, polysaccharides and rubber. Other examples include silk, wool, starch, cellulose, etc.
2. Semi-synthetic polymers: These are the substances which are obtained by using natural polymers, i.e, some modifications are made in natural polymers by artificial means. Nitrocellulose, cellulose acetate, vulcanized rubber, gun cotton, etc. are some of the important examples of this category.
3. Synthetic polymers: The man-made polymers are called synthetic polymers. Some important synthetic polymers are polyethylene, polystyrene, polyvinyl chloride(PVC), bakelite, nylon, etc.

Classification of polymers on the basis of monomers
Based on the nature of repeating structural units, polymers are divided into two categories, viz:

1. Homopolymers: Polymers whose repeating structural units are derived from only one type of monomer units are called homopolymers. For example, in the case of polyethylene polymer, which is obtained by polymerisation of ethylene molecules, the repeating structural unit, -CH₂-CH₂-, is derived from only one type of monomer, i.e, ethylene.
2. Copolymers: Polymers whose repeating structural units are derived from two or more types of monomer units are called copolymers. For example, in the case of nylon-6,6, the repeating structural unit, -NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-, is derived from two monomer units, i.e., hexamethylenediamine and adipic acid.

Classification of polymers on the basis of synthesis
Polymers can also be classified on the basis of the mode of polymerisation into two subgroups.

1. Addition polymers: In this type of polymerisation, the molecules of the same or different monomers simply add on one another leading to the formation of a macromolecule in which the molecular formula of the repeating structural unit is the same as that of the starting monomer. The polymers thus formed are called addition polymers. For example, Buna-S, Buna-N, etc.
2. Condensation polymers: In this type of polymerisation, a large number of monomer molecules combine together usually with the loss of simple molecules such as water, alcohol, ammonia, carbon dioxide, and hydrogen chloride to form a macromolecule in which the molecular formula of the repeating structural unit is generally not the same as that of the monomer.

The cationic polymerisation is initiated by the use of strong lewis acids such as BF₃, AlCl₃, SnCl_4.

On the basis of structure
There are three different types based on the structure of the polymers.

1. Linear polymers
2. Branched-chain polymers
3. Cross-linked or Network polymers

Classification based on molecular forces
On the basis of the magnitude of intermolecular forces, polymers have been divided into the following four categories:

1. Elastomers - Polymer chains are held together by weak intermolecular forces, which permits the polymer to stretch.

Eg:-   Vulcanised rubber,  Buna - S,  Buna - N, Neoprene.

2. Fibres -  Fibres are thread-like materials made up of linear chains of the polymer having H-bonding between them. They are crystalline in nature.

Eg:- Nylon 66, Terylene, Silk.

3. Thermoplastics - The intermolecular forces are intermediate between elastomers and fibres. They soften on heating and harden on cooling.

Eg:-  Polystyrene, Polyvinyls, etc.

4. Thermosetting polymers - They are highly cross-linked, hard, infusible and insoluble polymers. They cannot be reused.

Eg:-  Bakelite, Urea,  Formaldehyde, Resin, etc