An AB diblock copolymer of poly(L-lactide) (PLLA) and poly(oxyethylene) (PEG) with a cinnamate terminal in the PEG block was prepared by the copolymerization of L-lactide and partially end-modified PEG followed by fractionation. The first step was the terminal modification of PEG with cinnamoyl chloride (CC), in which the degree of cinnamoylation of the hydroxyl terminals of PEG was roughly controlled by the feed ratio of both reactants, The resultant PEG cinnamate was subjected to copolymerization with L-lactide to produce a mixture of unreacted PEG dicinnamate (C-PEG-C), the diblock copolymer (PLLA-PEG-C), and the triblock copolymer (PLLA-PEG-PLLA) corresponding to the three components of the PEG cinnamate. This mixture was separated by phase fluctuation chromatography (PFC) to obtain PLLA-PEG-C in sufficient purity. This process, involving the stoichiometric control of the terminal reaction of telechelic oligomers and the utilization of PFC for fractionation, can be an efficient method for synthesizing end-functionalized diblock copolymers from readily available telechelic oligomers.