Mechanism of Addition Polymerisation

The three-step radical mechanism, in full

1. Chain initiation

Benzoyl peroxide, $C_6H_5{-}C(=O){-}O{-}O{-}C(=O){-}C_6H_5$, splits homolytically on heating/light into two benzoyloxy radicals, which quickly lose $CO_2$ to give two phenyl radicals, $\cdot C_6H_5$.

Each phenyl radical then adds across the double bond of an ethene molecule:

2. Chain propagation

The new radical keeps attacking fresh ethene molecules, one after another, so the chain — and the radical sitting at its tip — keeps growing:

3. Chain termination

The chain finally stops growing when two such long radical chains collide and combine directly, pairing up their unpaired electrons and forming a single, much longer, non-radical molecule — polythene:

(this is only one of the possible ways two radical ends can combine to terminate the chain.)

Homopolymers vs copolymers

• An addition polymer built from one single kind of monomer is a homopolymer — polythene, made only from ethene, is a homopolymer. …