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 |
| alcohol | displayed formula | formula |
|---|---|---|
| ethane-1,2-diol |  |
HOCH2CH2OH |
| propane-1,3-diol |  |
HOCH2CH2CH2OH |
| 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 |
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:
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:
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:
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.