Understanding the mechanism of polymerization of ε-caprolactone catalyzed by aluminum salen complexes

Maria O. Miranda, Yvonne Deporre, Hugo Vazquez-Lima, Michelle A. Johnson, Daniel J. Marell, Christopher J. Cramer, William B. Tolman

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Abstract

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M < [CL] 0 < 2.6 M) in toluene-d8 at temperatures ranging from 20 to 90 C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k 2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG (associated with Keq) and ΔG⧧ (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.

Original languageEnglish (US)
Pages (from-to)13692-13701
Number of pages10
JournalInorganic chemistry
Volume52
Issue number23
DOIs
StatePublished - Dec 2 2013

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