Biodegradable polyesters

2 marks · Easy

Polyesters such as polylactic acid (PLA) are more biodegradable than most addition polymers.

Calculate the number of ester linkages in a PLA chain formed from 10 lactic acid monomers. Show your working.

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9 ester linkages

Also accept: There are 9 ester groups in a chain of 10 monomers

Common mistakes: Incorrectly stating 10 instead of 9

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Marking points

M1 · 1 mark · Considers that each monomer (after the first) forms one new ester linkage (number of ester linkages = number of monomers - 1)

A1 · 1 mark · Correct final answer: 9 ester linkages (for 10 monomers)

Worked solution

Each time a new lactic acid monomer is joined to the chain, an ester linkage forms between the –OH and –COOH groups.

For a chain of 10 monomers, the number of ester linkages is 10 − 1 = 9.

Answer: 9 ester linkages.