... oxygen as an impurity is needed for initiation. The mechanism of the polymerisation of ethene to form ldpe, is an overall process known as 'free radical addition', which itself has three different steps, initiation, propagation, and termination. Chain initiation is the starting point, where the polymer chain is initiated by free radicals, Ra , produced between some of the ethene and the oxygen initiator. Each time a free radical hits an ethene molecule a new longer free radical is formed, this is defined as chain propagation, eventually two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed, this is known as chain termination, and as this is a random process, poly(ethene) will be made up of chains of all sorts of different lengths. The box below gives you a clearer picture of the mechanism of ethene's polymerisation to form ldpe. Taken from ...

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