Biodegradable polyesters
2 marks · HardLactic acid is the monomer for polylactic acid (PLA). What is the displayed (structural) formula for the part of a PLA chain that is actually repeated in the polymer? Describe or draw the repeat unit of PLA, indicating the ester linkage.
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The repeat unit of polylactic acid is: –[O–CH(CH3)–CO]– Or in displayed formula: O || –O–CH(CH₃)–C– | O– where the ester linkage (–COO–) joins the monomer units.
Must show: Backbone showing –COO– linkage, Side chain CH3 shown, Hydrogen and oxygen positions clear
Also accept: Correct displayed structure; reference to the correct repeat unit and labeling of ester bond.
Common mistakes: Confusing with lactic acid monomer, showing –COOH and –OH groups instead of linked units.; Omitting ester linkage or side groups.
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Marking points
B1 · 1 mark · Draws or describes the correct backbone showing the ester (–COO–) linkage in the structure.
B2 · 1 mark · Shows correct arrangement of main atoms and side group (–CH3).
Worked solution
The repeat unit of polylactic acid (PLA) is: O || –O–CH(CH₃)–C– This shows the ester linkage formed between the carboxylic acid group (–COOH) and the alcohol group (–OH) of lactic acid, with the |–CH₃ side group attached to the central carbon. The backbone runs as –O–CH(CH₃)–CO– (with n repeats).