A series of copolymers have been synthesized by the ring-opening polymerization of glycolide, L-lactide, and epsilon-caprolactone with zirconium(IV) acetylacetonate [Zr(Acac)(4)] or stannous octoate [Sn(OCt)(2)] as the catalyst. The resulting terpolymers have been characterized by analytical techniques such as proton nuclear magnetic resonance, size exclusion chromatography, and differential scanning calorimetry. Data have confirmed that Sn(OCt)2 leads to less transesterification of polymer chains than Zr(Acac)4 under similar conditions. The various copolymers have been compression-molded and allowed to degrade in a pH 7.4 phosphate buffer at 37 degrees C. The results show that the degradation rate depends not only on the copolymer composition but also on the chain microstructure, the Sn(Oct)(2)-initiated copolymers degrading less rapidly than Zr(Acac)(4)-initiated ones with more random chain structures. The caproyl component appears the most resistant to degradation as its content increases in almost all cases. Moreover, caproyl units exhibit a protecting effect on neighboring lactyl or glycolyl units. The glycolyl content exhibits different features: it decreases because of faster degradation of glycolyl units, which are more hydrophilic than caproyl and lactyl ones, remains stable in the case of abundant C-G-C sequences, which are very resistant to degradation, or even increases because of the formation of polyglycolide crystallites. Terpolymers can crystallize during degradation if the block length of one of the components is sufficiently long, even though they are amorphous initially. (c) 2007 Wiley Periodicals, Inc.