Controlled ring-opening polymerization of a seven-membered cyclic carbonate, 1,3-dioxepan-2-one (7CC), with H2O/HCl.Et2O initiator system was investigated. The molecular weight (M-n) of the obtained polymer could be controlled by the amount of H2O in the range of 10(3)-10(4) maintaining a low polydispersity ratio (M-w/M-n = 1.11-1.17). The second portion of the monomer after completion of the first polymerization was converted quantitatively to give the corresponding polymer with a low M-w/M-n. The kinetic and H-1 NMR spectroscopic studies suggested that the nucleophilic attack of H2O to the monomer activated with hydrogen chloride afforded alpha-hydroxyl-omega-carbonic acid, followed by decarboxylation reaction to give alpha,omega-dihydroxyl compound. It was suggested that the chain growth in this system was the attack of the terminal hydroxyl group to the monomer activated with hydrogen chloride.