Benzoyl peroxide is a free radical initiator, which means that it can break down to form free radicals. Free radicals are highly reactive species that can react with other molecules to start a chain reaction, which is essential for the polymerisation of ethene.

In the case of ethene polymerisation, benzoyl peroxide breaks down to form two benzoyl radicals. These radicals then react with ethene molecules to form ethyl radicals. The ethyl radicals then react with other ethene molecules to form longer and longer chains of polyethylene.

The rate of polymerisation is controlled by the concentration of benzoyl peroxide. The higher the concentration of benzoyl peroxide, the faster the polymerisation reaction will occur.

Benzoyl peroxide is also used as a cross-linking agent in the polymerisation of other types of monomers, such as styrene and butadiene. Cross-linking agents help to form stronger and more durable polymers by creating bonds between the polymer chains.