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This page gives you the facts and a simple, uncluttered mechanism for the polymerisation of ethene by a free radical addition reaction. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.

### The Facts

An addition reaction is one in which two or more molecules join together to give a single product. During the polymerisation of ethene, thousands of ethene molecules join together to make poly(ethene) – commonly called polythene.

n~CH_2{=}CH_2 \longrightarrow {{[\hspace{1.5mu}}\mathllap{-}CH_2{-}CH_2{-}\mathllap{]\hspace{2.5mu}}}_n

The number of molecules joining up is very variable, but is in the region of 2000 to 20000.

#### Conditions

 Temperature: about 200°C Pressure: about 2000 atmospheres Initiator: a small amount of oxygen as an impurity

Note: The oxygen is sometimes described as a catalyst for the reaction. That's not strictly true. A catalyst can be recovered unchanged at the end of a reaction, but in this case the oxygen is used up. It gets incorporated into the polymer molecules – as you will see shortly.

### The Mechanism

#### Chain initiation

The chain is initiated by free radicals, Ra•, produced by 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.

\begin{aligned} \text{Ra}{\bullet} + CH_2{=}CH_2 &\longrightarrow \text{Ra}CH_2CH_2{\bullet} \\ \\ \text{Ra}CH_2CH_2{\bullet} + CH_2{=}CH_2 &\longrightarrow \text{Ra}CH_2CH_2CH_2CH_2{\bullet} \\ \\ {}&\text{etc.} \end{aligned}

#### Chain termination

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

\text{Ra}(CH_2)_m{\bullet} + {\bullet}(CH_2)_n\text{Ra} \longrightarrow \text{Ra}(CH_2)_{m+n}\text{Ra}

Because chain termination is a random process, poly(ethene) will be made up of chains of all sorts of different lengths.