Question

What is the role of benzoyl peroxide in the addition polymerisation of alkenes? Explain its mode of action with the help of an example.

Answer 1

Free radical polymerisation

• Benzoyl peroxide acts as a chain initiator in the addition polymerisation process.
• It is used to initiate the polymerisation reaction by generating free radicals.
• In the formation of polythene, a mixture of ethene and a small amount of benzoyl peroxide is heated or exposed to light.

With the help of the energy provided, benzoyl peroxide dissociates into free radicals.

• The process starts with the addition of phenyl free radical, formed by peroxide, to the ethene double bond, thus generating another but larger free radical. This is the chain initiation step. It is depicted as follow:
  

• This radical reacts with another ethene molecule, and another longer chain radical is formed. The repetition of this sequence with new and bigger radicals carries the reaction forward. This is the chain propagation step.

• At some stage, the product radical reacts with another free radical to stop the polymerisation process. This is the chain-terminating step.

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