Free radical polymerisation

(Addition polymerisation)

Alkenes or dienes and their derivatives are polymerized in the presence of catalyst ( free radical generating initiator) like benzoyl peroxide, acetyl peroxide and tert- butyl peroxide etc.

For example the polymerisation of ethene to polyethylene is carried out by exposing to light/ heating a mixture of ethene with benzoyl peroxide. This process generally occur through three steps namely chain initiation, chain propagation and chain termination.

Mechanism

1)Initiation step

In this step phenyl radical is formed from benzoyl peroxide. The phenyl radical produced is reacted with ethene to give a new and large free radical.

Free radical polymerisation of ethene; initiation step.

2)Chain propagation step

The new and large radical formed in initiation step is reacted with another ethene to give a new and larger radical. This sequence of reaction proceed forward.

Free radical polymerisation of ethene; chain propagation step.

3)Chain termination step

In this step the radical species combined to form the polymerized product.

Free radical polymerisation of ethene; termination step.

However a fourth step called chain transfer usually occurs. In chain transfer reaction the growing polymer chain is deactivated by transferring its growing site to a previously inactive species.

Chain transfer reaction.

The species TA can be a monomer,polymer, solvent or any other molecule introduce into the mixture.

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