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 degrees C was well described by the Arrhenius equation and gave E-a = 38.4 +/- 0.9 kJ mol(-1). Thermodynamic parameters (Delta H-p(o) = - 16.8 +/- 1.6 kJ mol(-1), ASp(o) = 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 degrees C. The equilibrium monomer concentrations at 25 and 100 degrees C were calculated to be 0.031 0.018 and 0.120 +/- 0.063 M, respectively.