A urethane is a molecule in which a carbonyl group is part of an ester and an amide (it is an amide in one direction, an ester in the other direction). It is formed from the reaction of an isocyanate with an alcohol. A polyurethane is a polymer formed between a diisocyanate and a diol. Draw curved arrows to show the movement of electrons in this step of the mechanism of the formation of a polyurethane.