Poly(L-lactide-co-epsilon-caprolactone)-b-pOly(L-lactide) [P(LL-co-CL)-b-PLL] diblock copolyesters were synthesized in a two-step process with 1-dodecanol (DDC) and stannous octoate as the initiating system. In the first-step reaction, a 50:50 mol % amorphous Poly(L-lactide-co-epsilon-caprolactone) [P(LL-co-CL] copolyester was synthesized via the bulk copolymerization of L-lactide and F-caprolactone, which was followed by the polymerization of the PLL crystalline block at the end chain in the second-step reaction. The yielded copolyesters were characterized with dilute-solution viscometry, gel permeation chromatography, H-1- and C-13-NMR, and differential scanning calorimetry methods. The molecular weights of the P(LL-co-CL) copolyesters from the first-step reaction were controlled by the DDC concentrations, whereas in the second-step reaction, the molecular weights of the P(LL-co-CL)-b-PLL diblock copolyesters depended on the starting P(LL-co-CL) copolyester molecular weights and L-lactide/prepolymer molar ratios. The starting P(LL-co-CL) copolyester molecular weights and PLL block lengths seemed to be the main factors affecting specific thermal properties, including the melting temperature (T-m), heat of melting (Delta H-m), crystallizing temperature (T-c), and heat of crystallizing (Delta H-c) of the final P(LL-co-CL)-b-PLL diblock copolyester products. T-m, Delta H-m T-c and Delta H-c increased when the PLL block lengths increased. However, these thermal properties of the diblock copolyesters also decreased when the P(LL-co-CL) block lengths increased. (D 2007 Wiley Periodicals, Inc.