Forming Polyesters (Edexcel International A Level (IAL) Chemistry): Revision Note

Forming 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 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 poly(ethylene terephthalate) (PET)

The monomers are ethane-1,2-diol, HOCH₂CH₂OH, and benzene-1,4-dicarboxylic acid, HOOC-C₆H₄-COOH. Each ester link forms by loss of one H₂O. The polymer is poly(ethylene terephthalate), PET. Its repeat unit is -[OCH₂CH₂OOC-C₆H₄-CO]-.

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) and a carboxylic acid group (-COOH) in the same molecule

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