Biodegradable polyesters
5 marks · HardPolyesters, such as polylactic acid (PLA), often biodegrade more rapidly than addition polymers like poly(ethene).
Answer the following questions about their biodegradability.
Explain what is meant by 'biodegradable polyester'.
State what is meant by an 'ester linkage' in the context of polyester structure.
Compare the biodegradability of polyesters (such as PLA) and addition polymers (such as poly(ethene)).
Explain why polyesters with ester linkages are more biodegradable, referring to the chemical structure and environmental breakdown processes.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A polyester that can be broken down by natural processes (by microorganisms or enzymes) into smaller, harmless substances.
(b) An ester linkage is a bond formed between a carboxylic acid group (–COOH) and an alcohol group (–OH), producing the –COO– bond that connects monomer units in polyesters.
(c) Polyesters like PLA are more biodegradable than addition polymers because the ester linkages in their backbone can be broken down by hydrolysis, especially by the enzymes of microorganisms found in the environment. Addition polymers such as poly(ethene) lack these reactive bonds and are more resistant to natural breakdown, making them much less biodegradable.
Common mistakes: Describing all polymers as biodegradable.; Confusing ester linkage with other bonds (e.g., amide).; Not referring to the chemical reason for faster breakdown.
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Marking points
B1 (a) · 1 mark · Defines biodegradable polyester as one that can be broken down naturally or by microorganisms/enzymes.
B1 (b) · 1 mark · States correctly what an ester linkage is (bond formed between carboxylic acid and alcohol producing –COO– in the polymer chain).
M1 (c) · 1 mark · States polyesters have ester linkages while addition polymers do not, or compares their chemical structures reasonably.
M2 (c) · 1 mark · Explains that ester linkages can be hydrolysed/broken down by microorganisms (or environmental processes).
A1 (c) · 1 mark · Links these points to the reason for greater biodegradability of polyesters (clear comparison and reason).
Worked solution
a) A biodegradable polyester is one that can be broken down by microorganisms or natural processes into harmless substances.
b) An ester linkage in a polyester is a bond (–COO–) formed when the carboxylic acid group of one monomer reacts with the alcohol group of another.
c) Polyesters like PLA are more biodegradable than addition polymers such as poly(ethene) because their ester linkages can be broken down by hydrolysis in the presence of water, enzymes, or bacteria. Addition polymers do not contain such reactive bonds and are more resistant to environmental breakdown, leading to slower degradation.