Additional Polymers

Under the right conditions, Alkenes can undergo polymerisation.
The small unsaturated starting molecules are referred to as monomers, and they join together to form a long chain saturated polymer.
CH₂=CH₂ + CH₂=CH₂ + CH₂=CH₂ -CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-
The reaction requires a catalyst; originally the catalyst was O₂, but an organic peroxide can be used.
The mechanism involves radicals; the catalyst provides the radicals to get the reaction started.
As the reaction involves radicals, it is made up of three stages; initiation, propagation, termination.

Initiation

This involves creating a radical using the catalyst.
If the catalyst is an organic peroxide, the weak O-O bond must first be broken to form a radical:
R-O-O-R R-O^. + R-O^.
(R represents an alkyl group)

The radical now goes on to react with the alkene. The radical combines with one of the double bonds in the alkene.
The new radical goes on to react with more alkenes:
R-O-CH₂-CH₂^. + CH₂=CH₂ R-O-CH₂-CH₂- CH₂-CH₂^.
The reaction is repeated many times, and so the chain grows longer and longer.
The reaction is very rapid, chains of up to ten thousand monomers can be formed in seconds.
As the chains grow, they flail around; sometimes “backbiting” can occur.
The result of “backbiting” is to create a new growing point in the middle part of the chain. A branched chain will now grow; this is what occurs in the process used to make low-density polythene: there is around a branch per every 50 atoms (average).

The reaction ends when all of the radicals are used up, one way this can happen is by radicals simply joining together.