Role of 1,2-Dimethoxyethane in the Transformation from Ethylene Polymerization to Trimerization Using Chromium Tris(2-ethylhexanoate)-Based Catalyst System: A DFT Study

Abstract

Selective ethylene trimerization into 1-hexene, which is an important comonomer for production of copolymers with high performance, was first commercialized in 2003 using a chromium tris(2ethylhexanoate) (Cr(EH) 3 )-based catalyst. One milestone during the development of this catalyst is the addition of 1,2-dimethoxyethane (DME) to Cr(EH) 3 /partially hydrolyzed tri-isobutyl aluminum (PIBAO) catalyst, resulting in a transformation from ethylene polymerization into trimerization. However, the mechanism of this transformation is unknown. In this work, the role of DME in the switching mechanism from ethylene polymerization to trimerization was investigated by a density functional theory (DFT) method based on the 10 most plausible molecular models for the active species of Cr(EH) 3 /PIBAO catalyst with or without DME coordination. For neutral models (Cr(I)OR; Cr(I)OR/DME; Cr(I)R; Cr(I)R/DME; R refers to isobutyl ligand), those without DME coordination were found to be dominantly ethylene trimerization sites. After DME coordination, both the weak electron-donating effect and steric effect of DME increased the total energy barriers of metallocycle growth, leading to ethylene trimerization with dimerization as a side reaction. So neutral models mainly presented ethylene trimerization with or without DME coordination and were not consistent with experimental evidence of transformation. For cationic models (Cr(I) þ ; Cr(I) þ /DME; Cr(II) þ OR; Cr(II) þ OR/DME; Cr(II) þ R; Cr(II) þ R/DME), metallocycle growth leading to ethylene polymerization occurred on those models without DME. After DME coordination, the strong electron-donating effect of DME was found to facilitate metallocycle expansion, while the dominant role of the steric effect of DME made nine-membered ring formation unfavorable, and thus 1-hexene was liberated from the seven-membered ring. So all the cationic models presented a transformation from ethylene polymerization to trimerization after DME coordination and might be the most plausible active sites for the Cr(EH) 3 /PIBAO catalyst system. The results provided a much deeper understanding of the highly selective trimerization mechanism and for further development of new catalysts with high performance as well.

Effect of Donors on the Activation Mechanism in Ziegler–Natta Catalysis: A Computational Study

“…To overcome this problem, all of the reaction pathways have been determined starting with the reactants (the MgCl 2 -supported titanium species and the AlEt 3 moiety) in the vicinity of each other,aprocedure that has also been followed by other groups. [70][71][72][73][74][75][76][77][78] Such an approach is particularly correct for the current systems, in which the substrate, the AlEt 3 moiety,i sarelatively small entity in comparison to the significantly larger MgCl 2supported titanium active species.…”

Mechanistic Studies of Ethylene and α-Olefin Co-Oligomerization Catalyzed by Chromium–PNP Complexes

“…5 These reactions most likely proceed through a metallacycle ring expansion mechanism, in which oxidative coupling of two ethylene molecules at an electrophilic metal center affords a metallacyclopentane intermediate, which undergoes ethylene insertion to give a metallacycloheptane that eliminates 1-hexene (Scheme 1). 11–16 The elimination step may involve either a β -hydride elimination/reduction elimination sequence or a concerted 3,7-hydride shift; it is difficult to distinguish between these alternatives, 13 but the latter is favored by recent theoretical studies. 15–17 Whichever mechanism operates, the elimination is expected on geometric grounds to be much more favorable, relative to further insertion, for the seven-membered rather than for the five-membered metallacycle, accounting for the observed selectivity.…”

Exploring Basic Components Effect on the Catalytic Efficiency of Chevron-Phillips Catalyst in Ethylene Trimerization

“…Concluding this remark, according to Table 3, by increasing the number of chlorides 242 substituted with the methyl group in the composition of the Al-compound, which indicates 243 an increase in the halocarbon/chromium molar ratio, the trimerization requires to overcome 244 lower energy barriers [14]. In other words, an increase in halocarbons leads to an increase on 245 the 1-hexene selectivity [22].…”