Formation of Polyesters

Specification Point 9.22C:
  • Explain:
    a) why polyesters are condensation polymers
    b) how a polyester is formed when a monomer molecule containing two carboxylic acid groups is reacted with a monomer molecule containing two alcohol groups
    c) how a molecule of water is formed each time an ester link is formed

Polyesters

  • Condensation polymers are formed when two different monomers are linked together with the removal of a small molecule, usually water.
  • This is a key difference between condensation polymers and addition polymers:
    • Addition polymerisation forms the polymer molecule only.
    • Condensation polymerisation forms the polymer molecule and one water molecule per linkage.
  • The monomers have two identical functional groups present, one on each end.
  • The functional groups at the ends of one monomer react with the functional group on the end of the other monomer, in so doing creating long chains of alternating monomers, forming the polymer.
  • Polyesters are formed from two different monomers and produce water, hence they are condensation polymers.

Formation of Polyesters

  • Polyesters are polymers that are held together by ester links.
  • An ester link is formed when an alcohol and an acid react:

R-OH + R-COOH → R-COO-R + H2O

(R represents a side chain)

Ester Link, Edexcel GCSE Chemistry

The ester ethyl propanoate is made from reacting propanoic acid and ethanol

  • An example is polyethylene terephthalate (PET) which is a polyester used widely in fabric for clothing and sheets due to its resistance to creasing.
  • It is made from terephthalic acid and ethane-1,2-diol.
  • The acid is dicarboxylic (a carboxylic acid with a -COOH group at either end) and the alcohol is diol (an alcohol with an -OH group at either end).
  • Each -COOH group reacts with a -OH group on the adjacent monomer.
  • An ester linkage is formed with the subsequent loss of one water molecule per link.

Making PET, Edexcel GCSE Chemistry

Diagram showing the condensation polymerisation reaction in which polyethylene terephthalate is produced

Formation of Water 

  • For every ester linkage formed in condensation polymerisation, one molecule of water is formed from the combination of a proton (H+) and a hydroxyl ion (OH).
  • Curiously the proton comes from the alcohol monomer rather than the acidic one.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.