Chain initiation

The chain is initiated by free radicals, Ra•, produced by the reaction between some of the ethene and the oxygen initiator.

Chain propagation

Each time a free radical hits an ethene molecule a new longer free radical is formed.

Ra• + CH₂ = CH₂→ RaCH₂CH₂•

RaCH₂CH₂• + CH₂ = CH₂→ RaCH₂CH₂CH₂CH₂ •

Chain termination

Eventually two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed.

Ra(CH₂)_m• + •(CH₂)_n Ra → Ra(CH₂)_m(CH₂)_nRa

Because chain termination is a random process, polyethene will be made up of chains of all sorts of different lengths.