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“Copolymerisation has been used by nature in polypeptides which may contain as many as 20 different amino acids.
Chemists are still far behind”.
Do you think that daily life would have been easier and colourful without the discovery and varied applications of polymers? The use of polymers in the manufacture of plastic buckets, cups and saucers, children’s toys, packaging bags, synthetic clothing materials, automobile tyres, gears and seals, electrical insulating materials and machine parts has completely revolutionised the daily life as well as the industrial scenario. Indeed, the polymers are the backbone of four major industries viz. plastics, elastomers, fibres and paints and varnishes.
The word ‘polymer’ is coined from two Greek words: poly means many and mer means unit or part. The term polymer is defined as very large molecules having high molecular mass (103-107u). These are also referred to as macromolecules, which are formed by joining of repeating structural units on a large scale. The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds. The process of formation of polymers from respective monomers is called polymerisation.
There are several ways of classification of polymers based on some special considerations.
One of the common classifications of polymers is based on source from which polymer is derived.
Classification of Polymers
Under this type of classification, there are three sub categories.
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.
Polymers can also be classified on the basis of their structure, molecular forces or modes of polymerisation.
Types of Polymerisation Reactions
There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step 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. This mode of polymerisation leads to an increase in chain length and chain growth can take place through the formation of either free radicals or ionic species. However, the free radical governed addition or chain growth polymerisation is the most common mode.
Free radical mechanism
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. As this radical reacts with another molecule of ethene, another bigger sized radical is formed. The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step. Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product. This step is called the chain terminating step. The sequence of steps involved in the formation of polythene are depicted as follows:
Chain terminating step
For termination of the long chain, these free radicals can combine in different ways to form polythene. One mode of termination of chain is shown as under:
The addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers, for example polythene discussed above is a homopolymer. The polymers made by addition polymerisation from two different monomers are termed as copolymers. Buna-S, which is formed by polymerisation of buta–1, 3–diene and styrene is an example of copolymer formed by addition polymerisation.
NCERT Class 12 Books Chemistry Chapter 15- Polymers- PDF Download
Unit 15: Polymers
अध्याय 15: बहुलक
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