A series of copolymers with various compositions were prepared by the ring opening copolymerization of L-lactide (L-LA) and epsilon-caprolactone (epsilon-CL) using nontoxic magnesium octoate as a catalyst in bulk. The copolymerization process and the influence of transesterification on the chain microstructure were examined by H-1 and C-13 NMR. A tapered block or gradient copolymer is expected to be formed on the basis of the reactivity difference between L-LA and epsilon-CL. Two modes of transesterification occurred and played an important role in the redistribution of comonomer sequences but not a completely random distribution. The CLC sequence formed by the second mode of transesterification was observed at the end of reaction. The coefficient of the second mode of transesterification (T-II) increased as the feed mole fraction of epsilon-CL increased. In terms of the overall feed compositions, the L-LL(Gamma) values of lactidyl sequences calculated from the reactivity ratio exceeded the L-LL(e) values determined from the product, however, the L-C(r) values of caproyl sequences were identical or shorter than the L-C(e) values. The thermal properties and crystallinities of the obtained copolymers were investigated by DSC and WAXD. The thermal properties and crystallinities depend on both the composition and the chain microstructure. The L-LA/epsilon-CL copolymer with intermediate composition exhibited some blocky character by DSC. Only single T-g was observed for each copolymer and in agreement with the calculated value from Fox equation, indicating that the amorphous region of the copolymers is miscible. The obtained copolymers can best be described as random copolymers with more or less blocky chain structure. (C) 2009 Elsevier Ltd. All rights reserved.