Free Radical Addition in the Polymerisation of Ethene

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

The over-all process is known as free radical addition.

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.

Questions to test your understanding

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Questions on the free radical polymerisation of ethene Answers

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