Polymerisation of alkenes
These reactions are of very considerable comercial importance. As they form the basis for the manufacture of a whole array of plastics.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.
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.
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 |
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.
etc
Chain termination
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.
© LEV