Making a polyester
3 marks · NormalExplain why a polyester molecule contains ester groups and describe how these groups are formed during polymerisation.
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Polyester molecules contain ester groups because, during condensation polymerisation, a diol and a dicarboxylic acid react to form an ester linkage, releasing water. The –OH group from the acid and an –H from the alcohol join to form water, leaving an ester bond between the two monomer units.
Must show: Reference to loss of water, Joining of –OH from acid and –H from alcohol
Common mistakes: Not mentioning loss of water; Describing only addition polymerisation
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Marking points
M1 · 1 mark · Mentions that polyester contains ester groups.
M2 · 1 mark · States that the ester group is formed by a reaction between a diol and a dicarboxylic acid.
A1 · 1 mark · Describes how water is lost when the –OH group of acid reacts with the –H of the alcohol.
Worked solution
Polyesters have ester groups formed by the reaction of a –COOH group on the dicarboxylic acid with a –OH group on the diol. This reaction releases a molecule of water and forms an ester linkage (–CO–O–) between the two monomers.