Copolyesters of ε-caprolactone, isosorbide and suberic acid by ring-opening copolymerization

Biodegradable copolyesters were prepared by copolymerization of epsilon-caprolactone (epsilon CL) with a cyclic polyester consisting of isosorbide and suberic acid (CPIS). Four initiators were compared: Sn(II)2-ethylhexanoate (SnOct(2) ) in combination with benzyl alcohol, 2,2-dimethyl-2-stanna-1,3-dioxepane (DSDOP) lanthanium and yttrium tris(isopropoxide). For SnOct 2 (combined with benzyl alcohol) and DSDPO temperature and time were varied. The highest molecular weights were obtained at the highest temperature. However, the compositions of the copolyesters varied largely. Whereas SnOct(2) favored the incorporation of CPIS, DSDOP favored incorporation of epsilon CL. A similar difference was found for La(OiPr) 3 favoring epsilon CL and Y(OiPr) favoring incorporation of CPIS. C-13 NMR spectroscopy proved that SnOct(2) yielded blocky sequences and mixtures of homo-polyesters. A greater extent of transesterification and a tendency towards random sequences was observed for DSDOP catalyzed copolymers. All reaction products contained crystallizing blocks of epsilon CL units. The melting temperatures decreased with the extent of transesterification.