Two series of poly(epsilon-caprolactone)-b-poly(ethylene glycol)-b-poly(epsilon-caprolactone) triblock copolymers were prepared by the ring opening polymerization of epsilon-caprolactone in the presence of poly(ethylene glycol) and dibutylmagnesium in 1,4-dioxane solution at 70 degrees C. The triblock structure and molecular weight of the copolymers were analyzed and confirmed by H-1 NMR, C-13 NMR, FTIR, and gel permeation chromatography. The crystallization and thermal properties of the copolymers were investigated by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The results illustrated that the crystallization and melting behaviors of the copolymers were depended on the copolymer composition and the relative length of each block in copolymers. Crystallization exothermal peaks (T-c) and melting endothermic peaks (T-m) of PEG block were significantly influenced by the relative length of PCL blocks, due to the hindrance of the lateral PCL blocks. With increasing of the length of PCL blocks, the diffraction and the melting peak of PEG block disappeared gradually in the WAXD patterns and DSC curves, respectively. In contrast, the crystallization of PCL blocks was not suppressed by the middle PEG block. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 429-436, 2009