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.
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