Theoretical Mechanistic Studies on the trans-1,4-Specific Polymerization of Isoprene Catalyzed by a Cationic La鈥揂l Binuclear Complex

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• 作者：Xiaohui Kang ; Yi Luo ; Guangli Zhou ; Xingbao Wang ; Xuerong Yu ; Zhaomin Hou ; Jingping Qu
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：July 22, 2014
• 年：2014
• 卷：47
• 期：14
• 页码：4596-4606
• 全文大小：582K
• ISSN：1520-5835

文摘

This paper reports a DFT study on trans-1,4-specific polymerization of isoprene catalyzed by the cationic heterobimetallic half-sandwich complex [(C₅Me₅)La(AlMe₄)]⁺. The possible structures of the active species, viz., [(C₅Me₅)La(渭₂-Me)₃AlMe]⁺ (A), [(C₅Me₅)La(渭₂-Me)₂AlMe₂]⁺ (B), and [(C₅Me₅)La(Me)(渭₂-Me)AlMe₂]⁺ (C), have been investigated. On the basis of the chain initiation and the structure transformations among these three species, C has been proposed to be the true active species smoothly producing trans-1,4-polyisoprene observed experimentally. Both La/Al bimetal-cooperating monomer insertion and La-center-based insertion pathways have been calculated, and the latter is found to be more favorable, where the AlMe₃ moiety serves as a ligand coordinating to the La center via a methyl group. In contrast to this, in the Y analogous system, the AlMe₃ ligand is proposed to leave away from the Y center during the chain propagation and the cis-1,4-selectivity is preferred, showing a consistence with experimental results. Such a situation could be ascribed to the smaller ionic radius of Y and thermodynamically favorable dissociation of AlMe₃ from Y center in comparison with the La system. These results suggest that such an alkylaluminum compound plays a crucial role in the regulation of selectivity in the polymerization system investigated.