Forming Polyesters (Edexcel International A Level Chemistry)

• 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 polyethene

• Condensation polymerisation is another type of reaction whereby a polymer is produced by repeated condensation reactions between monomers
• Natural condensation polymers are all formed by elimination of water
  • Although the process of condensation polymerisation involves the elimination of a small molecule

• Condensation polymers can be identified because the monomers are linked by ester or amide bonds

Polyester

• Is formed by the reaction between dicarboxylic acid monomers and diol monomers
• Polyester is produced by linking these monomers with ester bonds / links

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, the H atom on the diol and the -OH group of the -COOH are expelled as a water molecule (H₂O)
• The resulting polymer is a polyester
  • In this example, the polyester is poly(ethylene terephthalate) or PET, which is sometimes known by its brand names of Terylene or Dacron

 

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

Formation of polyesters - 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 that are needed to make the polyester come from the same monomer