The Role of Metallacycles in the Chromium-Catalyzed Trimerization of Ethylene

Emrich, R.; Heinemann, Oliver; Jolly, P. W.; Krüger, Carl; Verhovnik, Glenn P. J.

doi:10.1021/om961044c

Cited by 273 publications

(188 citation statements)

References 7 publications

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“…Data of thermal analysis of the polymers obtained at high pressures converge with the XRD findings: high melting temperatures (132-135 • C), high heats of melting (169-199 J/g) and high melt indexes (0.95 g/cm 3 ). Polymers poor-solubility in NMR and GPC solvents can be considered as an additional evidence for the compact structures and relatively high molecular weights of these solid products.…”

Section: Solid Products Obtained At Higher Ethylene Pressuressupporting

confidence: 71%

“…The linear CH 2 backbone is evidenced by the signal at 1.3 ppm, while the doublet at 0.86-0.87 ppm indicates the presence of the isopropyl group. Isopropyl and isobutyl terminations as well as traces of the oxygenated compounds observed in 1 H NMR spectrum appear as a consequence of the use of the co-catalyst Al(iBu) 3 .…”

Section: Trimerisation Versus Polymerisation Activitymentioning

confidence: 97%

“…In fact, the Phillips trimerisation technology is the sole ethylene trimerisation-based process, which is industrially applied. It uses mild process conditions (115 • C and ambient pressure) and produces 45-50 kg/cm 3 trimer. Small amounts of 1-octene and 1-decene are also formed, while solid product formation is negligible.…”

Section: Introductionmentioning

confidence: 99%

“…Cp Cr-based catalysts were reported as very active ethylene polymerisation catalysts [12,13]. Recently, it was shown that, when bulky aromatic substituents are part of the Cp ring, the polymerisation activity is inhibited and the trimerisation activity of the catalyst develops [3,14,15].…”

Section: Introductionmentioning

confidence: 99%

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Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

Nenu

Bodart

Weckhuysen

2007

Journal of Molecular Catalysis A: Chemical

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A Phillips Cr/SiO 2 polymerisation catalyst was converted into an ethylene trimerisation catalyst after assembling new active sites on the silica surface in the presence of TAC (1,3,5-triphenylhexahydro 1,3,5-triazacyclohexane) as ligand and CH 2 Cl 2 as solvent. The reaction conditions play a role in tuning the catalytic activity of this system. It is a true trimerisation catalyst at low ethylene pressures, while, at high pressures, the polymerisation activity becomes dominant. Moreover, at high ethylene pressures, polyethylene characterised by a high crystallinity degree is obtained. Instead, a highly branched heavy oligomer of ethylene characterised by a very narrow molecular weight distribution is obtained at 1 bar. 1-Hexene, formed as the main product in the ethylene trimerisation reaction at low pressures, is partially incorporated into this growing oligomer making redundant the use of a second comonomer feedstock and an expensive catalyst.

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Section: Solid Products Obtained At Higher Ethylene Pressuressupporting

confidence: 71%

Section: Trimerisation Versus Polymerisation Activitymentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

Nenu

Bodart

Weckhuysen

2007

Journal of Molecular Catalysis A: Chemical

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“…The mechanism is based on the shuttling of the Cr species between Cr n and Cr n?2 oxidation states. The metallacyclic intermediates have been supported by the well-characterized g 5 -cyclopentadienylstabilized chromacyclopentane and chromacycloheptane complexes in 1997 [69]. However, a logical mechanistic speculation concerning the oxidation states of chromium in the Phillips trimerization catalytic system came 11 years later in 2008, when Gambarotta and coworkers isolated a single component trimerization and polymerization chromium catalyst.…”

Section: Ethylene Oligomerizationmentioning

confidence: 99%

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

et al. 2015

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In the context of homogeneous catalysis, openshell systems are often quite challenging to characterize. Nuclear magnetic resonance (NMR) spectroscopy is the most frequently applied tool to characterize organometallic compounds, but NMR spectra are usually broad, difficult to interpret and often futile for the study of paramagnetic compounds. As such, electron paramagnetic resonance (EPR) has proven itself as a useful spectroscopic technique to characterize paramagnetic complexes and reactive intermediates. EPR spectroscopy is a particularly useful tool to investigate their electronic structures, which is fundamental to understand their reactivity. This paper describes some selected examples of studies where EPR spectroscopy has been useful for the characterization of open-shell organometallic complexes. The paper concentrates in particular on systems where EPR spectroscopy has proven useful to understand catalytic reaction mechanisms involving paramagnetic organometallic catalysts. The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect. 1. Section 2 of the review focusses on examples of C-C and C-N bond formation reactions, with an emphasis on catalytic reactions where ligand/substrate non-innocence plays an important role. Both carbon and nitrogen centered radicals have been shown to play an important role in these reactions. A few selected examples of catalytic alcohol oxidation proceeding via related N-centered ligand radicals are included in this section as well. Section 3 covers examples of the use of EPR spectroscopy to study important commercial ethylene oligomerization and polymerization processes. In Sect. 4 the use of EPR spectroscopy to understand the mechanisms of Atom Transfer Radical Polymerization is discussed. While this review focusses predominantly on the application of EPR spectroscopy in mechanistic studies of C-C and C-N bond formation reactions mediated by organometallic catalysts, a few selected examples describing the application of EPR spectroscopy in other catalytic reactions such as water splitting, photo-catalysis, photo-redox-catalysis and related reactions in which metal initiated (free) radical formation plays a role are included as well. EPR spectroscopic investigation in this area of research are dominated by EPR spectroscopic studies in isotropic solution, including spin trapping experiments. These reactions are highlighted in Sect. 5. EPR spectroscopic studies have proven useful to discern the correct oxidation states of the active catalysts and also to determine the effective concentrations of the active species. EPR is definitely a spectroscopic technique that is indispensable in understanding the reactivity of paramagnetic complexes and in conjunction with other advanced techniques such as X-ray absorption spectroscopy and pulsed laser polymerization it will continue to be a very practical tool.

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