The Roles of Monomer Binding and Alkoxide Nucleophilicity in Aluminum-Catalyzed Polymerization of ε-Caprolactone
Ding, K.; Miranda, M. O.; Moscato-Goodpaster, B.; Ajellal,
N.; Breyfogle, L. E.; Hermes, E. D.; Schaller, C. P.; Roe, S. E.; Cramer,
C. J.; Hillmyer, M. A.; Tolman, W. B.
Macromolecules 2012, 45, 5387 (doi:10.1021/ma301130b).
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, (LR)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 was observed using [CL]0 = 2-2.6M and [complex]0 = 9 mM, enabling independent determination of the substrate coordination (Keq) and insertion (k2) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both Keq and k2 yielded thermodynamic parameters for these steps. The rate constant k2, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter Keq 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.