Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of 未-Valerolactone with N-Heterocyclic Carbenes

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• 作者：Ashwin K. Acharya ; Young A. Chang ; Gavin O. Jones ; Julia E. Rice ; James L. Hedrick ; Hans W. Horn ; Robert M. Waymouth
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：June 19, 2014
• 年：2014
• 卷：118
• 期：24
• 页码：6553-6560
• 全文大小：401K
• 年卷期：v.118,no.24(June 19, 2014)
• ISSN：1520-5207

文摘

Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of 未-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) were carried out. The ZROP of 未-valerolactone generates cyclic poly(valerolactone)s whose molecular weights are higher than predicted from [VL]₀/[NHC]_0. Kinetic studies reveal the rate of polymerization is first order in [VL] and first order in [NHC]. Density functional theory (DFT) calculations were carried out to elucidate the key steps involved in the ring-opening of 未-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on 未-valerolactone to form a zwitterionic tetrahedral intermediate. DFT calculations indicate that the highest activation barrier of the entire mechanism is associated with the ring-opening of the tetrahedral intermediate formed from the NHC and 未-valerolactone, a result consistent with inefficient initiation to generate reactive zwitterions. The large barrier in this step is due to the fact that ring-opening requires a partial positive charge to develop next to the directly attached NHC moiety which already bears a delocalized positive charge.