Macromolecular engineering of polylactones and polylactides. 22. Copolymerization of ε-caprolactone and 1,4,8-trioxaspiro[4.6]-9-undecanone initiated by aluminum isopropoxide

Novel biodegradable and biocompatible polyesters have been prepared by random copolymerization of ε-caprolactone (ε-CL) and 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO) initiated by Al(OiPr)3. Copolyesters of molecular weight (Mn) up to 104 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(ε-caprolactone) (PCL) with well-defined content of ketone or hydroxyl pendent groups have been made available. The ε-CL and TOSUO comonomer units are randomly distributed within the copolymers, as assessed by the 13C-NMR sequence analysis. Indeed, the reactivity ratios (rC and rT) have been calculated from the average length of the ε-CL sequences (LC) and the TOSUO sequences (LT) and found to be 1.3 (rC) and 1.0 (rT), respectively. There is an interesting parallelism between the crystallinity and the average length of the ε-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 (Tg), melting temperature (Tm), and melting enthalpy (ΔHm). The thermal stability depends on the copolyester composition, since it decreases as the molar fraction of TOSUO units is increased.