Influence of the Substitution Pattern of Terpene-Based Seven-Membered Lactones on Yttrium-Mediated Ring-Opening...

Terpenes exhibit a wide range of structures, thus enabling diverse applications. Herein, (-)-menthide and trans (+)-carvomenthide were synthesized from terpenoids (-)-menthone and (+)-dihydrocarvone, differing solely in their substitution pattern. (-)-Menthide features a methyl group at position 4 and an isopropyl group at position 7, whereas (+)-carvomenthide shows the inverse arrangement. Both lactones were transformed into polyesters via ring-opening polymerization (ROP) using an amino-alkoxy-bis(phenolate) yttrium catalyst. Polymerization kinetics revealed first-order behavior, with (+)-carvomenthide polymerizing significantly faster than (-)-menthide, while (-)-menthide demonstrated a more controlled polymerization. Activation energies were 36.3 kJ mol^-1 for (+)-carvomenthide and 40.8 kJ mol^-1 for (-)-menthide. The Gibbs free energy of activation confirmed the lower energy barrier for (+)-carvomenthide polymerization, with experimental values of 81.9 kJ mol^-1 compared to 89.1 kJ mol^-1 for (-)-menthide. Density functional theory (DFT) calculations supported the experimental results, with computed Gibbs free energy barriers of 84.5 kJ mol^-1 for trans (+)-carvomenthide and 82.3 kJ mol^-1 for (-)-menthide and mechanistic differences. Refined free energy barriers were determined to be 102.1 kJ mol^⁻1 for (-)-menthide and 84.9 kJ mol^⁻1 for trans (+)-carvomenthide, agreeing with the experimental trend. These findings highlight the critical role of molecular structure and substituent position on polymerization kinetics and energy barriers in metal-catalyzed polymerization.