Lipase-Catalyzed Copolymerization of ω-Pentadecalactone with p-Dioxanone and Characterization of Copolymer Thermal and Crystalline Properties

Candida antarctica Lipase B (CALB), a metal-free enzyme, was successfully employed as catalyst for ring-opening copolymerization of ω–pentadecalactone (PDL) with p-dioxanone (DO) under mild reaction conditions (< 80ºC, atmospheric pressure). Poly(PDL-co-DO) with high molecular weight (Mw > 30 000) and a wide range of comonomer contents were synthesized using various PDL/DO feed ratios. During the copolymerization reaction, large ring PDL was found to be more reactive than its smaller counterpart DO, resulting in higher PDL/DO unit ratios in polymer chains than the corresponding PDL/DO monomer ratios in the feed. The copolymers were typically isolated in 50-90 wt% yields as the monomer conversion was limited by the equilibrium between monomers and copolymer. 1H- and 13C-NMR analysis on poly(PDL-co-DO) formed by CALB showed that the copolymers contain nearly random sequences of PDL and DO units with a slight tendency towards alternating arrangements. Copolymerization with PDL was found to remarkably enhance PDO thermal stability. DSC and WAXS results demonstrate high crystallinity in all copolymers over the whole range of compositions. Depending on copolymer composition, the crystal lattice of either PDO or PPDL hosts units of the other comonomer, a behavior typical of an isodimorphic system. In poly(PDL-co-DO), both melting temperature and melting enthalpy display a minimum at 70 mol% DO, i.e. at the pseudoeutectic composition. WAXS diffractograms show a single crystal phase (that of either PPDL or PDO) on either side of the pseudoeutectic, and coexistence of PPDL and PDO crystals at the pseudoeutectic.