Aluminum Effects in the Syndiospecific Copolymerization of Styrene with Ethylene by Cationic Fluorenyl Scandium Alkyl Catalysts

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• 作者：Xiaofang Li ; Xiaoying Wang ; Xin Tong ; Hongxia Zhang ; Yuanyuan Chen ; Ying Liu ; Hui Liu ; Xiaojie Wang ; Masayoshi Nishiura ; Huan He ; Zhenguo Lin ; Shaowen Zhang ; Zhaomin Hou
• 刊名：Organometallics
• 出版年：2013
• 出版时间：March 11, 2013
• 年：2013
• 卷：32
• 期：5
• 页码：1445-1458
• 全文大小：616K
• 年卷期：v.32,no.5(March 11, 2013)
• ISSN：1520-6041

文摘

A series of half-sandwich fluorenyl (Flu鈥? scandium dialkyl complexes Flu鈥睸c(CH₂SiMe₃)₂(THF)_n (1, Flu鈥?= C₁₃H₉, n = 1; 2, Flu鈥?= 2,7-^tBu₂C₁₃H₇, n = 1; 3, Flu鈥?= 9-SiMe₃C₁₃H₈, n = 1; 4, Flu鈥?= 2,7-^tBu₂-9-SiMe₃C₁₃H₆, n = 1; 5, Flu鈥?= 9-CH₂CH₂NMe₂C₁₃H₈, n = 0; 6, Flu鈥?= 2,7-^tBu₂-9-CH₂CH₂NMe₂C₁₃H₆, n = 0) have been synthesized and structurally characterized. In comparison with the well-known cyclopentadienyl-ligated scandium catalyst system [(C₅Me₄SiMe₃)Sc(CH₂SiMe₃)₂(THF)]/[Ph₃C][B(C₆F₅)₄], the analogous combinations of the fluorenyl-ligated, THF-containing complexes 1鈥?b>4 with [Ph₃C][B(C₆F₅)₄] show relatively low activities, albeit with similar syndioselectivities for styrene polymerization and styrene鈥揺thylene copolymerization. However, on treatment with 15 equiv of AlⁱBu₃, the 1鈥?b>4/[Ph₃C][B(C₆F₅)₄] combinations show a dramatic increase in catalytic activity without changes in the stereoselectivity. In contrast, the combinations of complexes 5 and 6, which have an amino group attached to the fluorenyl ring and intramolecularly bonded to the metal center, exhibit very low activity, no matter whether or not AlⁱBu₃ is present, affording syndiotactic polystyrenes with broad molecular weight distributions. The DFT calculations of the activation mechanism by using the representative catalysts suggest that AlⁱBu₃ can capture the THF molecule from the catalyst precursors 1鈥?b>4 at first,and then the new, THF-free cationic half-sandwich scandium active species [Flu鈥睸c(CH₂SiMe₃)][B(C₆F₅)₄] with less steric hindrance around the metal center is generated in the presence of an activator such as [Ph₃C][B(C₆F₅)₄]. The DFT calculations on the syndioselectivity of styrene (co)polymerization catalyzed by [Flu鈥睸c(CH₂SiMe₃)][B(C₆F₅)₄] have also been carried out, thus shedding new light on the mechanistic aspects of the (co)polymerization processes.