Formation of Polyesters (Cambridge International A-Level Chemistry)

Spec point — 35.1.1

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 (e.g. a water molecule) is lost when the monomers join together to form a polyester
  • Polyesters contain ester linkages

Example of a polyester

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

Diol and dicarboxylic acid examples

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 dicarboxylic acid and the hydrogen atom of the diol are expelled as a water molecule (H₂O)
• The resulting polymer is a polyester

Forming polyethylene terephthalate (PET)

Expulsion of a water molecule in this condensation polymerisation forms the polyester called Polyethylene terephthalate also known as Terylene or 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)

Using hydroxycarboxylic acids to form condensation polymers

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

Exam Hint

• Polyesters can be made using condensation polymerisation
• The monomers needed are diols and dicarboxylic acids / dioyl chlorides or a single hydroxycarboxylic acid monomer