Summary:The ring-opening polymerization (ROP) of 1,5-dioxepan-2-one (DXO) catalyzed by diphenyl phosphate (DPP) using 3-phenyl-1-propanol (PPA) as an initiator has been studied. The polymerization in toluene at room temperature produced poly(1,5-dioxepan-2-one) (PDXO) with a narrow polydispersity index. The obtained PDXO possessing a PPA residue at the a-chain end was confirmed by the 1 H NMR and MALDI-TOF MS measurements. The livingness of the ROP of DXO was supported by the kinetic and chain-extension experiments. The diblock and random copolymerizations of DXO and e-caprolactone (CL) afforded respective copolymers. The monomer reactivity ratios were obtained as r DXO ¼ 0.59 and r CL ¼ 1.19 using the Kelen-Tüdös method, and the random copolymers showed different thermal properties corresponding to copolymer compositions.