A new polymerization system is reported that provides aliphatic polyurethanes with novel structures from the anionic ring-opening polymerization of five-membered carbamates. By use of a bifunctional initiating system, the anionic ring-opening polymerization of cyclic carbamates gives unimodal polymers with D values generally narrow. A series of PUs with controlled molecular weights were synthesized by changing the reaction conditions. Detailed analysis of the reaction demonstrates that repeated acylations of in situ formed urethane anions lead to the formation of the polyurethane.