2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP)-initiated copolymerizations of trimethylene carbonate (TMC) with β-D,L-butyrolactone (β-D,L-BL), δ-valerolactone (δ-VL), ε-caprolactone (ε-CL) or L-lactide (Lac) were performed in chlorobenzene at 80°C. When equimolar mixtures of TMC and a lactone were copolymerized, copolyesters having nearly random sequences were obtained in the case of β-D,L-BL, δ-VL and ε-CL, whereas an almost perfect homopolymerization was observed for L-lactide. Furthermore, a series of sequential copolymerizations was conducted so that the TMC was polymerized first. After removal of the Bu2Sn group with 1,2-dimercaptoethane, telechelic A-B-A triblock copolymers having free OH endgroups were obtained from all four lactones. In another series of sequential copolymerizations, the lactones were used as the first monomer. Again a telechelic triblock copolyester was obtained with β-D,L-BL, whereas copolyesters with largely randomized sequences were isolated, when δ-VL or ε-CL were used as first monomer. With L-lactide as the first monomer an almost perfect homopoly(L-lactide) was obtained and most of the TMC remained unreacted, in close analogy to the attempted random copolymerizations.
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Kricheldorf et al. (1999) studied this question.