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Polymerisation is the process in which the small monomer units join together to form a large polymer molecule.

Addition polymerisation

Addition polymerisation is a process involving many small, unsaturated monomers combining to form one large polymer molecule.

The alkenes ethene and propene are two important feedstocks in the petrochemical industry which can also be used to make addition polymers.

How to draw structural formulae for polymers

The formation of poly(ethene) from ethene is an example of addition polymerisation. Small unsaturated ethene monomers join up by the opening of the double bond allowing them to join up to form a long carbon chain.

Polymers made in this way are called addition polymers.

The monomers used to make other addition polymers are drawn in a similar shape to ethene, for example, propene.

Although this is the usual way to draw the structural formula for propene, for the purposes of showing how the molecule acts as a monomer and can form a polymer it should be drawn in a different way.

Examples of other monomers are given below.

Given the structure of the monomer the structural formula of the addition polymer can be drawn.

For example

Remember to show only six carbon atoms in the chain if three monomers are used. Also, remember to show the 'open' bonds at each end of the polymer section.

If the structure of the polymer is given then the structure of the monomer can be worked out.

The example below shows the repeating unit, ie the part of the molecule that is repeated.