Following our continued interest in the production of bioerodible and biodegradable functional polymers for biomedical applications, we synthesized and characterized new unsaturated polyesters. The presence of functional groups in the polymer backbone provided sites for chemical modification, and through a variation in the structure, the physical properties, such as the hydrophilicity and solubility, could be affected. With 1,1-di-n-butyl-stanna-2,7-dioxacyclo-4-heptene as the initiator in the ring-opening polymerization of polyesters, a new set of functionalized polyesters was created. The polymerization of epsilon-caprolactone resulted in poly(epsilon-caprolactone) with a double bond incorporated into the structure. The polymers were obtained in a controlled manner with low molecular dispersities. The double bond was previously incorporated into L-lactide polymers, and the two reactions were compared in this study. The conversion of E-caprolactone, with a degree of polymerization of 50, was completed within 140 min, whereas for L-lactide, only a 45% conversion took place in the same period of time. The dispersities were somewhat higher with E-caprolactone because of the higher reaction rate and, therefore, lower selectivity. The incorporated C-C double bond in the polyesters provided a variety of opportunities for further modifications. In this case, the double bond of the L-lactide macromonomers was oxidized into epoxides. Epoxidation was carried out with m-chloroperoxybenzoic acid as a chemical reagent. The conversion of the double bonds into epoxides was completed, and the obtained yields were good (>95%). As a result of the mild reaction conditions, the epoxidation of the double bond was carried out quantitatively without any side reactions. (C) 2003 Wiley Periodicals, Inc.