Biodegradable polyesters
4 marks · NormalBiodegradable polyesters such as PLA can be broken down by environmental processes more easily than many other polymers.
Explain what is meant by the term 'ester linkage' in a polyester.
Describe why polyesters with ester linkages are more biodegradable than addition polymers.
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(a) An ester linkage is the group –COO– formed when a carboxylic acid and an alcohol react and join together, releasing water.
(b) Polyesters have ester linkages in their backbone, which can be broken down (hydrolysed) by water and enzymes, so microorganisms can break them down. Addition polymers do not have such reactive bonds and are harder to degrade.
Common mistakes: Vague explanation of biodegradation; Not mentioning the bond responsible for hydrolysis
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Marking points
B1 (a) · 1 mark · Defines ester linkage as –COO– group formed between a carboxylic acid and an alcohol.
M1 (b) · 1 mark · States that ester linkages can be broken down by hydrolysis/water/enzymes.
A1 (b) · 1 mark · References microorganisms or biological agents in breaking down polyesters.
A2 (b) · 1 mark · Compares to addition polymers as less biodegradable due to absence of ester linkages.
Worked solution
a) An ester linkage is the –COO– bond formed between the carboxylic acid group of one monomer and the alcohol group of another during polyester formation.
b) Polyesters are more biodegradable because their ester linkages can be hydrolysed by water or enzymes, allowing bacteria and other microorganisms to break the polymer chains. Addition polymers do not have these reactive bonds and are much less susceptible to degradation.