condenastion polymerisation

Chemistry higher tier


Addition polymers such as polythene and PVC use small unsaturated monomers called alkenes joining or adding together to make long chain polymers. All the monomer is used up and no waste products are produced in this addition polymerisation reaction. However this is not the only way in which polymers can be made. Condensation polymerisation does not use unsaturated monomers (monomers with double bonds) instead it uses monomers with reactive end groups.

Note - before you read this section it might be a good idea to revise functional groups. Make sure you know what carboxylic acids, alcohols, amino-acids, esters, amides and peptides are.

Polyester

Polyester is perhaps the best known condensation polymer. But what exactly is a condensation polymer? Condensation polymerisation or step-growth polymerisation as it is sometimes known, is the reaction of two different monomers each of which has reactive end groups (functional groups). During a condensation reaction a small molecule is normally given off, this can be water (H2O), or hydrogen chloride gas (HCl) or even methanol (CH3OH).
The monomer for condensatio reactions have reactive ends, that is functional groups on both ends of the monomers. e.g. You should be familiar with the structure of alcohols. Alcohols have a hydroxyl group (-OH) bonded to a carbon atom e.g. the first two members of the alcohol homologous series are shown in the table below.
alcohol displayed formula formula
methanol CH3OH
ethanol C2H5OH
However it would not be possible to make a polymer using any of these alcohols because the reactive hydroxyl group ,-OH, is only on one end of the molecule. What we need is another homologous series with the reactive alcohol hydroxyl group on both ends of the molecule. Diols (remember di =2, -ol is ending for alcohol name) are a homologous series with the reactive hydroxyl group on both ends of the molecule. The first member of this series is ethane diol or ethane-1,2-diol. The numbers simply tell us the reactive hydroxyl group is on carbon atoms 1 and 2. The hydroxyl groups could both be on carbon atom 1 which is not what we want to make a polymer, they must be on the ends of the molecule. The structure of ethane-1,2-diol and propane-1,3-diol are shown below.
alcohol displayed formula formula
ethane-1,2-diol HOCH2CH2OH
propane-1,3-diol HOCH2CH2CH2OH

Carboxylic acids

The first two carboxylic acids are shown in the table below.

carboxylic acid displayed formula formula
methanoic acid CHCOOH
ethanoic acid CH3COOH

However none of these carboxylic acids can be used as monomers for the same reason we met for the alcohols, we need the reactive functional group, the carboxyl group, on both ends of the molecule. This leads us to another homologous series of carboxylic acids, the diacids. These are similar to the carboxylic acids you have previously met but have the carboxyl group on both ends of the molecule. The first two members of this homologous series of diacids is shown below.

carboxylic acid displayed formula formula
ethane-1,2-dioic acid HOOCCHCOOH
propane-1,3-dioic acid HOOCCH2COOH

Making esters

Esters are made in a condensation reaction between an alcohol and a carboxylic acid, as shown below, here the alcohol methanol undergoes a condensation reaction with ethanoic acid:

ester mechanism

However no polymerisation reaction is possible because once the ester is formed since reaction is effectively finished as the alcohol and acid cannot react further. However if we replace the alcohol with a diol and the acid with a diacid then once the ester is formed the reaction can still carry on as the alcohol and acid still have reactive functional groups on the end of the molecules, this is shown below:

polyester mechanism

This reaction can effectively carry on as the ester molecule has a reactive hydroxyl group on one end and a reactive carboxyl on the other, each of these can react with other molecules to build up a large polymer molecule made up of lots of ester linkages, a polyester!
We can summarise this as: polyester mechanism The 2 water molecules in the equation come from: one from the formation of the ester linkage and one from the removal of an -OH from the carboxyl group on the acid and the loss of a H from the diol molecule.

Key Points


Practice questions

Check your understanding - Questions on condensation polymers

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