A reactive blend of poly(ethylene glycol) (PEG) and DL-oligo(lactic acid) (OLA) is obtained at high temperature to produce partial PEG/OLA multiblock copolymer without purification. The reactive blend of PEG and OLA easily reacts with poly(1,4-butylene terephthalate) (PBT) in the melt leading to the formation of high-molar mass poly(1,4-butylene terephthalate-co-ethylene oxide-co-DL-lactide) (PBTEOLA) copolymers. The analysis from H-1 NMR and solubility test reveals that the transesterification between butylene terephthalate (BT), ethyleneoxide (EO), and lactide (LA) segments during synthesis is unavoidable. The copolyesters are segmented copolyesters with certain random properties, as confirmed by their thermal behavior. The copolyesters show only one melting temperature (T-m) on the second heating run and one crystallization temperature (T-c) on the cooling cycle from differential scanning calorimetry measurement. With increase of OLA feeding composition in PEG/OLA blend or increase of content of PEG/OLA blend, the T-m and T-c of copolyesters decrease, and solubility increases. The conventional size-exclusion chromatography polystyrene calibration estimates weight-averaged molecular weight of the copolyester to be as high as 66,600 g/mol. Mechanical tests indicate that the copolyesters exhibit high Young's modulus of 50-100 MPa and good elongation at break of 32-137%. (C) 2008 Wiley Periodicals, Inc.