Methods of Polymerisation

There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step growth polymerisation.

Addition Polymerization

Ø In this type of polymerisation, the molecules of the same monomer or different monomers add together on a large scale to form a polymer.

Ø The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives.

Ø This mode of polymerisation leading to an increase in chain length or chain growth can take place through the formation of either free radicals or ionic species.

Ø However, the free radical governed addition or chain growth polymerisation is the most common mode.

Free Radical Mechanism

Ø 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.

Ø 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.

Ø This step is called chain initiating step.

Ø As this radical reacts with another molecule of ethene, another bigger sized radical is formed.

Ø The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step.

Ø Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product.

Ø This step is called the chain terminating step.

The sequence of steps may be depicted as follows:

Chain initiation steps

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:

Condensation Polymerization

Ø This type of polymerisation generally involves a repetitive condensation reaction between two bi-functional monomers.

Ø These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers.

Ø In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on.

Ø Since, each step produces a distinct functionalised species and is independent of each other; this process is also called as step growth polymerisation.

Ø The formation of terylene or dacron by the interaction of ethylene glycol and terephthalic acid is an example of this type of polymerisation.

Copolymerisation

Ø Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer.

Ø The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also.

Ø It contains multiple units of each monomer used in the same polymeric chain.

Ø For example, a mixture of 1, 3 – butadiene and styrene can form a copolymer.

Ø Copolymers have properties quite different from homopolymers.

Ø For example, butadiene - styrene copolymer is quite tough and is a good substitute for natural rubber.

Ø It is used for the manufacture of autotyres, floortiles, footwear components, cable insulation, etc.