Catalytic polymerization of a cyclic ester derived from a "cool" natural precursor

(-)-Menthide, a seven-membered lactone derived from the natural product (-)-menthol, was polymerized using a structurally defined zinc-alkoxide catalyst to form an aliphatic polyester. The polymer was fully characterized by NMR spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. The polymerization reaction occurred in a controlled fashion and polymer samples with M_n values up to 90 kg mol^-1 were obtained by varying the catalyst loading. Monitoring of the rate of polymerization by in situ FT-IR spectroscopy (ReactIR) revealed a first order dependence on (-)-menthide. The temperature dependence of the observed rate constant between 30 and 90 °C was well described by the Arrhenius equation and gave E_a = 38.4 ± 0.9 kJ mol^-1. Thermodynamic parameters (ΔH_p° = -16.8 ± 1.6 kJ mol^-1, ΔS_p° = -27.4 ± 4.6 J mol^-1 K^-1) for the polymerization of (-)-menthide were also determined by measuring the equilibrium monomer concentration at different temperatures ranging from 40 to 100 °C. The equilibrium monomer concentrations at 25 and 100 °C were calculated to be 0.031 ± 0.018 and 0.120 ± 0.063 M, respectively.