Abstract
Zirconocene-mediated selective dimerization of α-olefins usually occurs when precatalyst (η5-C5H5)2ZrCl2 is activated by minimal excess of methylalumoxane (MAO). In this paper, we present the results of density functional theory (DFT) simulation of the initiation, propagation, and termination stages of dimerization and oligomerization of propylene within the framework of Zr-Al binuclear mechanism at M-06x/DGDZVP level of theory. The results of the analysis of the reaction profiles allow to explain experimental facts such as oligomerization of α-olefins at high MAO/(η5-C5H5)2ZrCl2 ratios and increase of the selectivity of dimerization in the presence of R2AlCl. The results of DFT simulations confirm the crucial role of the presence of chloride in the selectivity of dimerization. The molecular hydrogen was found in silico and proven experimentally as an effective agent that increases the rate and selectivity of dimerization.
Highlights
Despite half-century of the fruitful study of zirconocene-catalyzed polymerization of α-olefins, some aspects of the reaction mechanism are unclear
Given that the reaction of LZrCl2 with TIBA results in the formation of Zr-Al hydride complexes [14,15,16,17], we proposed that the addition of minimal amounts of MAO results in the formation of cationic Zr-Al hydride species that catalyze dimerization of α-olefins
We report the results of density functional theory (DFT) modeling of initiation, propagation, and termination stages of α-olefin dimerization and oligomerization catalyzed by cationic (η5-C5H5)2Zr-Al species derived from R2AlX (R = Me, iBu; X = H, Cl, Me, Scheme 2) in comparison with traditional mechanism
Summary
Despite half-century of the fruitful study of zirconocene-catalyzed polymerization of α-olefins, some aspects of the reaction mechanism are unclear. Cosse-Arlman cationic coordination-insertion mechanism combined with few termination event pathways [1,2,3,4,5,6,7] (Scheme 1) is a highly selective dimerization of α-olefins with a formation of methylenealkanes [8,9,10,11,12,13]. Given that the reaction of LZrCl2 with TIBA results in the formation of Zr-Al hydride complexes [14,15,16,17], we proposed that the addition of minimal amounts of MAO results in the formation of cationic Zr-Al hydride species that catalyze dimerization of α-olefins. We recently proposed a binuclear Zr-Al model (Scheme 2)
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