Novel biodegradable and biocompatible polyesters have been prepared by random copolymerization of-epsilon-caprolactone (epsilon-CL) and 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO) initiated by Al((OPr)-Pr-i)(3). Copolyesters of molecular weight (M-n) up to 10(4) and containing up to 90 mol % TOSUO repeating units have been prepared. The copolymer molecular weight is controlled by the molar ratio of the consumed comonomers and the initiator. The TOSUO constitutive units have been quantitatively deacetalized into ketones, which have also been efficiently reduced into hydroxyl groups without any apparent chain degradation. Therefore, poly(epsilon-caprolactone) (PCL) with well-defined content of ketone or hydroxyl pendent groups have been made available. The epsilon-CL and TOSUO comonomer units are randomly distributed within the copolymers, as assessed by the C-13-NMR sequence analysis. Indeed, the reactivity ratios (r(C) and r(T)) have been calculated from the average length of the epsilon-CL sequences (L-C) and the TOSUO sequences (L-T) and found to be 1.3 (r(C)) and 1.0 (r(T)), respectively. There is an interesting parallelism between the crystallinity and the average length of the epsilon-CL sequences. Copolyesters containing a molar fraction of TOSUO units smaller than 0.15 are semicrystalline, whereas the incorporation of larger amounts of TOSUO units results in amorphous copolyesters. Conversion of the ethylene acetal pendent groups into ketones and ultimately into hydroxyl pendent groups results in increasing glass transition temperature (T-g), melting temperature (T-m), and melting enthalpy (Delta H-m). The thermal stability depends on the copolyester composition, since it decreases as the molar fraction of TOSUO units is increased.