Synthesis and Characterization of Poly(1,5-Dioxepan-2-one-co-L-Lactic Acid) and Poly(1,5-Dioxepan-2-one-co-D,L-Lactic Acid)

Copolymers of l- or d,l-dilactide with 1,5-dioxepan-2-one (DXO) have been investigated. Statistical copolymers were obtained using stannous-2-ethylhexanoate (Sn(oct)2) as catalyst in bulk. Reactivity ratios were r (DXO) = 0.1 and r (dilactide) = 10. 13C-NMR spectroscopy indicated a more extensive block structure than expected in a totally random copolymer. Transesterification takes place and shortens the average sequence length with increased temperature. The ability of the l-lactic acid (L-LA)/DXO copolymer to crystallize seems to be retained despite substantial amounts of 1,5-dioxepan-2-one in the composition. Properties, like glass transition temperature, crystallinity, and mechanical properties, are strongly dependent on the molar composition of the copolymer. The mechanical properties of a copolymer (molar ratio: DXO/L-LA, 25/75), measured in tension, indicated a flexible, soft material. This copolymerization seems to be initiated by impurities like water present in monomers and/or Sn(oct)2, giving the tin compound the role of a catalyst.