1,3-Dioxepane was polymerized with triflic acid as an initiator in the presence of acetic acid (AA) and hexane diacid. The structure of the poly(1,3-dioxepane) (polyDOP) obtained was characterized by H-1 NMR spectra and gel permeation chromatography. The molecular weights (MWs) were determined by vapor pressure osmometry. The results obtained in both systems were completely different from those in which low-MW polyols were used as chain-transfer agents. When the molar ratio of carboxylic acid to triflic acid was low, high-MW polyDOP with a controlled MW and narrow MW distribution was obtained. The content of the ester group in the final product depended greatly on the molar ratio of AA to triflic acid. The polymerization mechanism is discussed.