A short fragment of a polymer chain (end groups shown) is drawn. Which linkage joins the diol-derived and diisocyanate-derived units?

AUrea
BEster
CAmide
DUrethane

Answer: D

The linking group is -NH-CO-O-, a urethane (carbamate), formed when an isocyanate reacts with an alcohol. The polymer is a polyurethane. An amide lacks the extra oxygen and an ester lacks nitrogen.

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