Polyurethanes with allyl side chains were synthesized by the simultaneous acid-catalyzed reaction of dialdehydes (1), alkylene N,N'-bis(trimethylsilyl) carbamates (4), and allyltrimethylsilane (5). When 5 was added to a mixture of 1, 4, and the catalyst, a low molecular weight polymer was formed, as well as a large amount of an insoluble gel. However, when a mixture of 1, 4, and 5 was added to the catalyst, the formation of gel was depressed, and the desired polyurethanes, consisting of 1, 4, and 5 in a molar ratio of 1/1/2, were obtained in good yields. This polyurethane synthesis is unusual in that it concurrently constructs both the polymer backbone and the functional side chains from three starting compounds.