Ring-opening polymerization of cyclic carbonates (1,3-dioxepan-2-one and 5,5-dimethyl-1,3-dioxan-2-one)with triethyl and triisopropyl berates as initiators promoted by hydrogen chloride was carried out. The molecular weights of the obtained polymers could be controled by the amount of alkylborates, and their polydispersity ratios were small (similar to 1.15). The polymerization obeyed good first-order kinetics throughout the reaction. After the monomers were completely consumed, the polymerization proceeded again quantitatively when the same amount of monomers were introduced into the polymerization mixtures. H-1 and C-13 NMR spectroscopic studies suggested that the polymerization proceeded in a living manner via insertion of the monomer to the boron-oxygen bond.