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 < [CL] ₀ < 2.6 M) in toluene-d₈ at temperatures ranging from 20 to 90 C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (K_eq) and catalytic rate constants (k ₂) as a function of R and temperature. While K_eq 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 K_eq) 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 K_eq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating K_eq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.