Addition polymerisation is the reaction that forms many of the polymers.

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Addition polymerisation is the reaction that forms many of the polymers (a long molecule made up from lots of small molecules called monomers). Usually there is a C=C double bond as there is in alkenes. Addition polymers were found by accident and are know as A-A polymers because all the monomer molecules are the same.

Polymerisation is a three-stage process. Below shows the polymerisation of ethane to low density poly(ethene) (ldpe)

Stage 1 : INITIATION This is when there is a creation of a radical from a catalyst

Stage 2 : PROPAGATION A radical now combines with one of the electrons from the double bond in the alkene:

New radical then reacts with more alkene:

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The chain grows longer and eventually ends up attacking itself, back-biting. This creates a new growing point in the middle of the chain and a branched chain now grows.

Stage 3 : TERMINATION The reaction ends when the radicals are used up. This can happen by the radicals joining together.

The reasons for the low density of the poly(ethene) is due to its branched chains. They cannot fit closely together which results in a low density.

There are different forms of poly(ethene) which have different properties. ...

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