We report spirocyclic imidazolidines derived from N-heterocyclic carbenes and beta-lactones. These spirocycles are initiators for the zwitterionic ring-expansion polymerization of beta-lactones to generate cyclic polyesters of well-defined molecular weight. The zwitterionic strategy based on the equilibrium between the zwitterionic intermediates and spiro macrocyclics enables the controlled synthesis of cyclic poly(beta-lactone)s