Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone

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• 作者：Daniel E. Stasiw ; Mukunda Mandal ; Benjamin D. Neisen ; Lauren A. Mitchell ; Christopher J. CramerWilliam B. Tolman
• 刊名：Inorganic Chemistry
• 出版年：2017
• 出版时间：January 17, 2017
• 年：2017
• 卷：56
• 期：2
• 页码：725-728
• 全文大小：306K
• ISSN：1520-510X

文摘

Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d₈ by ¹H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t_1/2 for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local minimum. Nor were local minima for cis-bound precomplexes found, suggesting a concerted coordination–insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the “resting” ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymerization catalysts.