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 (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.