Understanding the Mechanism of Polymerization of 蔚-Caprolactone Catalyzed by Aluminum Salen Complexes

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• 作者：Maria O. Miranda ; Yvonne DePorre ; Hugo Vazquez-Lima ; Michelle A. Johnson ; Daniel J. Marell ; Christopher J. Cramer ; William B. Tolman
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：December 2, 2013
• 年：2013
• 卷：52
• 期：23
• 页码：13692-13701
• 全文大小：460K
• 年卷期：v.52,no.23(December 2, 2013)
• ISSN：1520-510X

文摘

Studies of the kinetics of polymerization of 蔚-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 掳C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (K_eq) and catalytic rate constants (k₂) as a function of R and temperature. While K_eq varied only slightly with the electron donating properties of R (Hammett 蟻 = +0.16(8)), k₂ showed a more significant dependence reflected by 蟻 = +1.4(1). Thermodynamic parameters 螖G掳 (associated with K_eq) and 螖G (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 K_eq being associated with coordination of monomer according to the typical coordination鈥搃nsertion mechanism. An alternative hypothesis is presented associating K_eq with 鈥渘onproductive鈥?coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.