Roles of monomer binding and alkoxide nucleophilicity in aluminum-catalyzed polymerization of ε - Caprolactone

The kinetics of polymerization of ε-caprolactone (CL) initiated by aluminum-alkoxide complexes supported by the dianionic forms of N,N-bis[methyl-(2-hydroxy-3-tert-butyl-5-R-phenyl)]-N,N- dimethylethylenediamines, (L ^R)Al(Oi-Pr) (R = OMe, Br, NO ₂) were studied. The ligands are sterically similar but have variable electron donating characteristics due to the differing remote (para) ligand substituents R. Saturation kinetics were observed using [CL] ₀ = 2-2.6 M and [complex] ₀ = 7 mM, enabling independent determination of the substrate coordination (K _eq) and insertion (k ₂) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both K _eq and k ₂ yielded thermodynamic parameters for these steps. The rate constant k ₂, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter K _eq is invariant as a function of ligand electronic properties. Density functional calculations provide atomic-level detail for the structures of key reaction intermediates and their associated thermochemistries.