Studies of the kinetics of polymerization of epsilon-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-d(8) at temperatures ranging from 20 to 90 degrees 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 rho = +0.16(8)), k(2) showed a more significant dependence reflected by rho = +1.4(1). Thermodynamic parameters Delta G degrees (associated with K-eq) and Delta G(double dagger) (associated with k(2)) were determined, with the former being similar to 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.