A short fragment of a polymer chain (three repeat units, end groups shown) is drawn. Which functional group links the repeat units?

AUrethane
BEster
CAmide
DEther

Answer: B

The units are joined by -CO-O- groups, i.e. esters, so the polymer is a polyester: poly(3-hydroxybutanoate), PHB, a biodegradable polymer. An amide would contain N and an ether has no C=O.

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