Biodegradable polyesters

3 marks · Normal

Lactic acid has the formula . When making a biodegradable polyester such as polylactic acid (PLA), lactic acid monomers react with each other to form a long polymer chain.

Explain, using your knowledge of its structure, how lactic acid can form a biodegradable polyester like PLA from a single type of monomer.

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.

Show answer

Lactic acid contains both a carboxylic acid group (–COOH) and an alcohol group (–OH) in the same molecule. During polymerisation, the –OH group of one lactic acid molecule reacts with the –COOH group of another, eliminating water (a condensation reaction) and forming an ester linkage. As this repeats, a long chain is formed, resulting in a biodegradable polyester (polylactic acid, PLA) from only lactic acid monomers.

Also accept: Reference to lactic acid self-polymerising due to both functional groups; Idea that each lactic acid molecule provides both groups needed to form ester links in the chain

Common mistakes: Saying two different monomers are needed; Not mentioning both functional groups; Describing addition (not condensation) polymerisation; Missing out elimination of water

Show mark scheme and worked solution

Marking points

M1 · 1 mark · States or clearly implies that lactic acid has both a carboxylic acid (–COOH) and an alcohol (–OH) group in the same molecule.

M2 · 1 mark · Describes that lactic acid monomers react together by a condensation reaction (elimination of water).

A1 · 1 mark · Correctly explains that an ester linkage is formed when the –OH of one molecule joins with the –COOH of another, producing polylactic acid.

Worked solution

Lactic acid has the structure , which contains both a carboxylic acid group (–COOH) and an alcohol group (–OH) in the same molecule.

During the formation of PLA, the –OH group from one lactic acid reacts with the –COOH group of another lactic acid molecule. This eliminates water (H2O), which is the condensation step.

This process repeats, so more lactic acid monomers join, forming a long polymer chain with ester linkages between the monomers. The result is a biodegradable polyester called polylactic acid (PLA), and only lactic acid is required as the monomer.