Roles of Monomer Binding and Alkoxide Nucleophilicity in Aluminum-Catalyzed Polymerization of ε-Caprolactone

The kinetics of polymerization of epsilon-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, NO2) 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.