Addition (chain reaction) polymerisation

In addition polymerisation, the monomers which are unsaturated compounds eg;alkenes, alkadienes and their derivatives combine to form a polymer. Addition polymerisation lead to increase in chain length through formation of free radical or ionic species. Thus it is of type free radical, anionic, cationic and coordination on the basis of reactive centre. But the addition polymerisation with free radical mechanism is most common.

The characteristics of chain polymerisation is that only monomer react with the reactive centre to give polymers. Monomer doesn't react with monomer. The different sized species such as dimer, trimer, tetramer,n - mer does not react with each other.

Mechanism

In chain growth polymerisation an intiator is used to produce an initiator species (R*) with a reactive centre. The reactive centre may be a cation, anion or a free radical. Each monomer added to the reactive centre regenerate a reactive species. Then successive addition of 100 or 1000 or more monomer to the reactive centre gives a polymer. The process can be ceased by destroying the reactive centre by a termination step. The most common example of chain polymerisation is that of vinyl monomers.

Mechanism of addition polymerisation.

Types of addition polymers

1) Free radical polymerisation

Here the initiator is a free radical and the propagating site of reactivity (*) is a carbon radical.

2) Anionic polymerisation

Here the initiator is a nucleophile and the propagating site of reactivity (*) is a carboanion.

3) Cationic polymerisation

Here the initiator is an acid and the propagating site of reactivity (*) is a carbocation.

4) Coordination catalytic polymerisation

Here the initiator is a transition metal complex and the propagating site of reactivity (*) is a terminal catalytic complex.

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