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

Area of Science:

• Polymer Chemistry
• Organic Synthesis
• Materials Science

Background:

• (-)-Menthide is a seven-membered lactone derived from the natural product (-)-menthol.
• Aliphatic polyesters are a versatile class of polymers with numerous applications.
• Controlled polymerization techniques are crucial for tailoring polymer properties.

Purpose of the Study:

• To investigate the controlled polymerization of (-)-menthide.
• To characterize the resulting aliphatic polyester.
• To determine the reaction kinetics and thermodynamic parameters of the polymerization.

Main Methods:

• Polymerization of (-)-menthide using a structurally defined zinc-alkoxide catalyst.
• Characterization of the polymer by NMR spectroscopy, size exclusion chromatography, and MALDI-TOF MS.
• Monitoring polymerization rate using in situ FT-IR spectroscopy (ReactIR).
• Determination of thermodynamic parameters by measuring equilibrium monomer concentration at various temperatures.

Main Results:

• Controlled polymerization yielding aliphatic polyesters with number-average molecular weights (M(n)) up to 90 kg mol(-1).
• Polymerization rate showed a first-order dependence on (-)-menthide concentration.
• The Arrhenius equation accurately described the temperature dependence of the rate constant (E(a) = 38.4 +/- 0.9 kJ mol(-1)).
• Thermodynamic parameters determined: ΔHp° = -16.8 ± 1.6 kJ mol(-1) and ΔSp° = -27.4 ± 4.6 J mol(-1) K(-1).
• Equilibrium monomer concentrations calculated at 25°C (0.031 ± 0.018 M) and 100°C (0.120 ± 0.063 M).

Conclusions:

• The zinc-alkoxide catalyst enables controlled polymerization of (-)-menthide.
• The polymerization process is well-defined, allowing for tunable molecular weights.
• Kinetic and thermodynamic data provide fundamental insights into the ring-opening polymerization mechanism.