Abstract
Zirconocene-catalyzed coordination oligomerization of higher α-olefins is of theoretical and practical interest. In this paper, we present the results of experimental and theoretical study of α-olefin oligomerization, catalyzed by (η5-C5H5)]2ZrX2 1/1′ and O[SiMe2(η5-C5H4)]2ZrX2 2/2′ (X = Cl, Me) with the activation by modified methylalymoxane MMAO-12 or by perfluoroalkyl borate [PhNMe2H][B(C6F5)4] (NBF) in the presence and in the absence of organoaluminium compounds, Al(CH2CHMe2)3 (TIBA) and/or Et2AlCl. Under the conditions providing a conventional mononuclear reaction mechanism, 1′ catalyzed dimerization with low selectivity, while 2′ initiated the formation of oligomers in equal mass ratio. The presence of TIBA and especially Et2AlCl resulted in an increase of the selectivity of dimerization. Quantum chemical simulations of the main and side processes performed at the M-06x/ DGDZVP level of the density functional theory (DFT) allowed to explain experimental results involving traditional mononuclear and novel Zr-Al1 and Zr-Al2 mechanistic concepts.
Highlights
Zirconocenes hold a significant position among single-site catalysts of α-olefin polymerization due to high catalytic activity, excellent copolymerization homogeneity, and wide boundaries of regioand sterecontrol [1,2,3,4,5,6]
It is known that zirconocene dichloride (η5 -C5 H5 )2 ZrCl2 (1, Scheme 1B) [27,28,29,30], disubstituted zirconocenes (η5 -RC5 H4 )2 ZrCl2 [31,32,33,34,35], ansa-zirconocenes Z(η5 -C5 H4 )2 ZrCl2 [36,37], and thiapentalene zirconium complex [38] catalyze the oligomerization of α-olefins RCH=CH2 with a formation of the mixtures of methylenealkanes and higher oligomers (Scheme 1B)
We report the results of the experimental study of 1-octene oligomerization using zirconocenes 1 and 2 and their dimethyl derivatives 10 and 20 (Scheme 1B), activated by modified zirconocenes 1 and 2 and their dimethyl derivatives 1′ and 2′ (Scheme 1B), activated by modified methylalymoxane MMAO-12 and perfluoroalkyl borate [PhNMe2 H][B(C6 F5 )4 ] (NBF ) with and without methylalymoxane MMAO-12 and perfluoroalkyl borate [PhNMe2H][B(C6F5)4] (NBF) with and organoaluminium components and molecular hydrogen
Summary
Zirconocenes hold a significant position among single-site catalysts of α-olefin polymerization due to high catalytic activity, excellent copolymerization homogeneity, and wide boundaries of regioand sterecontrol [1,2,3,4,5,6]. For 1, this reaction results in selective formation of methylenealkanes (DPn = 2, Scheme 1B) in the presence of a minor excess of methylaluminoxane (MAO). Christoffers and Bergman [28,29] demonstrated that chloride is an indispensable component of the catalyst system that provides the high selectivity of dimerization. Their observation was confirmed experimentally by an increase of the selectivity of zirconocene/MAO catalyzed dimerization in the Polymers 2020, 12, 1590; doi:10.3390/polym12071590 www.mdpi.com/journal/polymers
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