Understanding the Mechanism of Polymerization of ε‑Caprolactone Catalyzed by Aluminum Salen Complexes

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M 0 d8 at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG° (associated with Keq) and ΔG⧧ (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination–insertion mechanism. An alternative hypothesis is presented associating Keq with “nonproductive” coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.