Condensation polymers form in a different way to addition polymers like poly(ethene). Instead of forming just the polymer molecule as the only product, two products form:
- a polymer molecule
- a small molecule, often water
One type of condensation polymers are polyesters.
Forming esters
An ester forms when an alcohol reacts with a carboxylic acid. For example:
Ethanol + ethanoic acid → ethyl ethanoate + water
CH3CH2OH + CH3COOH → CH3COOCH2CH3 + H2O
Ethyl ethanoate is an ester. Notice that one water molecule forms for each ester molecule made.
Forming polyesters
A polyester forms from two different monomers:
- a ‘dicarboxylic acid’ which contains two carboxylic acid groups, –COOH
- a ‘diol’ which contains two alcohol groups, –OH
For example, PET is a polyester made from terephthalic acid and ethane-1,2-diol. It is used to make drinks bottles and fleece clothing.
Notice that:
- both ends of each monomer molecule have a functional group, so can react with another monomer molecule
- one molecule of water is formed every time an ester link is formed
Key fact
Addition polymerization needs one type of monomer and forms one product. To form polyesters, condensation polymerization needs two types of monomer and forms two products.
