7.7.1 Formation of Polyesters

• Addition polymerisation has been covered in reactions of alkenes
  • They are made using monomers that have C-C double bonds joined together to form polymers such as (poly)ethene

• Condensation polymerisation is another type of reaction and is used in the making of polyesters
  • A small molecule (eg. a water molecule) is lost when the monomers join together to form a polyester
  • Polyesters contain ester linkages

This polymer structure shows an ester functional group linking monomers together

Formation of polyesters

• A diol and a dicarboxylic acid are required to form a polyester
  • A diol contains 2 -OH groups
  • A dicarboxylic acid contains 2 COOH groups

The position of the functional groups on both of these molecules allows condensation polymerisation to take place effectively

• When the polyester is formed, one of the -OH groups on the diol and the hydrogen atom of the -COOH are expelled as a water molecule (H₂O)
• The resulting polymer is a polyester

Expulsion of a water molecule in this condensation polymerisation forms the polyester called Terylene (PET)

Hydroxycarboxylic acids

• So far the examples of making polyesters have focused on using 2 separate monomers for the polymerisation
• There is another route to making polyesters
• A single monomer containing both of the key functional groups can also be used
• These monomers are called hydroxycarboxylic acids
  • They contain an alcohol group (-OH) at one end of the molecule while the other end is capped by a carboxylic acid group (-COOH)

Both functional groups are needed to make a polyester are from the same monomer