Chain Propagation and Termination Mechanisms for Polymerization of Conjugated Polar Alkenes by [Al]-Based Frustrated Lewis Pairs

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• 作者：Jianghua He ; Yuetao Zhang ; Laura Falivene ; Lucia Caporaso ; Luigi Cavallo ; Eugene Y.-X. Chen
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：November 25, 2014
• 年：2014
• 卷：47
• 期：22
• 页码：7765-7774
• 全文大小：511K
• ISSN：1520-5835

文摘

A combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene (NHC) and Al(C₆F₅)₃ pairs is reported. This study consists of three key parts: structural characterization of active propagating intermediates, propagation kinetics, and chain-termination pathways. Zwitterionic intermediates that simulate the active propagating species in such polymerization have been generated or isolated from the FLP activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline鈥攐ne of which, IMes⁺-CH₂C(Me)鈺?C₃H₂NO)Al(C₆F₅)₃^{鈥?/sup> (2), has been structurally characterized. Kinetics performed on the polymerization of 2-vinylpyridine by ItBu/Al(C₆F₅)₃ revealed that the polymerization follows a zero-order dependence on monomer concentration and a first-order dependence on initiator (ItBu) and activator [Al(C₆F₅)₃] concentrations, indicating a bimolecular, activated monomer propagation mechanism. The Lewis pair polymerization of conjugate polar alkenes such as methacrylates is accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (未-valerolactone)-terminated polymer chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the 尾-ketoester-terminated chain, as revealed by computational studies.}