THE POLYMERISATION OF ETHENE
A Free Radical Addition Reaction
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.
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.
The number of molecules joining up is very variable, but is in the region of 2000 to 20000.
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 over-all process is known as free radical addition.
The chain is initiated by free radicals, Ra, produced by reaction 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.
Eventually two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed.
Because chain termination is a random process, poly(ethene) will be made up of chains of all sorts of different lengths.