Addition Polymerisation or Chain Growth Polymerisation

Building a chain through free radicals

The most widely used route to addition polymers is the free-radical mechanism. Alkenes, dienes, and their derivatives are polymerised in the presence of a free-radical-generating initiator (sometimes loosely called a catalyst) — common choices are benzoyl peroxide, acetyl peroxide, and tert-butyl peroxide.

Worked case: ethene → polythene

Take the everyday example of turning ethene into polythene. A small amount of benzoyl peroxide is mixed with ethene, and the mixture is either heated or exposed to light.

The process has three distinct stages:

• Chain initiation — the peroxide initiator breaks down to give a phenyl free radical, which then adds across the ethene double bond, generating a new, larger free radical. This is the step that gets the chain started.
• Chain propagation — this new radical attacks another molecule of ethene, forming a still-bigger radical. This addition repeats over and over, with the radical getting larger and larger each time — this is what actually builds the long polymer chain. …