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(O i -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.
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Ding et al. (2012) studied this question.