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

• Published in: Macromolecules Vol. 45; no. 13; pp. 5387 - 5396
• Main Authors: Ding, Keying, Miranda, Maria O, Moscato-Goodpaster, Beth, Ajellal, Noureddine, Breyfogle, Laurie E, Hermes, Eric D, Schaller, Chris P, Roe, Stephanie E, Cramer, Christopher J, Hillmyer, Marc A, Tolman, William B
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 10-07-2012
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts; Ring opening polymerization; Aryloxy complex; Aluminium complex; Complex catalyst polymerization; Experimental study; Lactone polymer; Chemical reaction kinetics; Solution polymerization; Alkoxy complex; Reaction mechanism; Rate constant; Polycaprolactone; Activation energy
• ISSN: 0024-9297; 1520-5835
• 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 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.