Ethylene oligomerization mechanism

Scheme 6.16.5 Relevance of the insertion step for the different ethylene oligomerization mechanisms (a) and (b) showing ethylene insertion into a Ni-hydride and a Ni—all

Figure 5.12. Postulated mechanism for ethylene oligomerization according to Shell s SHOP process with a P,0-...

It is largely accepted that the active species in ethylene oligomerization is a nickel hydride species like 8. The mechanism for the hydride formation is supported by the reactions depicted in eqs. (7)-(9). Complex 9 eliminates butadiene at low temperatures and becomes active at 40 °C. Insertion of ethylene and elimination of styrene from structure 4 at 70 °C causes the complex to become active, while the more strongly bound cyclopentadienyl ligand in structure 10 needs 130 °C [49]. The elimination products of these reactions could be detected by GLC. 

Scheme 1. Postulated mechanism for ethylene oligomerization via a P n 0-stabilized nickel hydride species pi, p2... Pn = propagation steps Ci, C2.. . e = elimination steps.

Chapter 22 presents the different types of polymers produced from ethylene, propylene, and copolymers of these olefins with other monomers, along with some basic principles that apply to polyolefin synthesis and characterization. The basic features of the mechanism of the polymerization are presented first to provide a framework for the description of different catalysts and materials. After presenting different types of catalysts that have been used for the polymerization of these monomers, a more detailed description of several features of the mechanism is presented. Finally, an overview of ethylene oligomerization, as well as diene polymerization and oligomerization, is presented. The primary journal literature and patent literature on olefin polymerization is immense. Fortunately, many reviews of olefin polymerization have been published. The coverage of olefin polymerization and oligomerization in this chapter is selective, and the reader is directed to review articles for more comprehensive coverage of these topics. " ... 

Ethylene oligomerization proceeds in accordance with two mechanisms of C-chain growth, namely, catalytic or stoichiometric [14-16]. 

Schrock has found an ethylene oligomerization catalyst, Ta(=CH/-Bu)-Hl2(PMe3)3, which does appear to go via metalcycles (Eq. 11.61). After 20-50 ethylene units have been inserted, the chain -eliminates to give a 1-alkene. Since the alkyl form of the catalyst is d, the unmodified Green-Rooney mechanism is allowed. 

The mechanism of Ni-catalyzed ethylene oligomerization involves both nickel hydride and nickel alkyl species. The mechanism is known in the literature as the metal-hydride mechanism, Cossee-Arlman mechanism, or ethylene insertion - -hydride elimination mechanism and results in a Schulz-Flory distribution of the oligomerization products. The mechanism is depicted in Figure 6.16.4. Note that two other coordination sites at the nickel are occupied by one bidentate ligand or two monodentate ligands (see Section 2.4 for details) that have been omitted in Figure 6.16.4 for clarity. 

In ethylene oligomerization, oxidative addition plays no role. In the insertion-elimination mechanism, the metal oxidation state is constant throughout the catalytic cycle. In the metallacycle mechanism, the oxidative step is an oxidative coupling reaction (see below). 

Extrusion is the reverse reaction of insertion (CN +1, VE +2, ON unchanged). The reaction plays a very important role in ethylene oligomerization according to the insertion/elimination mechanism as the so-called P-H-elimination. Scheme 6.16.6 illustrates this elementary step for the extrusion of a 1-hexene product from a Ni-hexyl complex. The extrusion step is followed by 1-hexene dissociation from the complex (see above) to finally liberate the 1-hexene product. Below, in Scheme 6.16.8, the extrusion step is shown as part of a more complex reaction sequence for the liberation of a 1-alkene product from a chromium metallacyclic intermediate. 

Oxidative coupling is the decisive step in the Cr-based ethylene oligomerization according to the metallacycle mechanism. As shown in Scheme 6.16.7, two ethylene units of the ligand sphere of the Crmetallacyclic intermediate. Assuming a cationic Cr-complex in the first place (with the MAO anion as very weakly coordinating counter-ion), the oxidation state of chromium increases in the process from +I to +III. 

Ethylene oligomerization catalyzed by AI-, Zr-, and Ni-complexes follows a so-called insertion/elimination mechanism that results in the production of 1-alkene mixtures of different chain lengths. The mechanistic reason for this product distribution is the fact that each metal-alkyl complex shows the same probability of chain growth independent of the chain length of the attached alkyl group. 

The mechanism of ethylene oligomerization by SHOP and related catalysts, such as phosphinophenolate complexes, has been the subject of intense investigation (see COMC (1982), Chapter 52, references cited by Heinicke et al. and Pietsch et air It is generally agreed that the actual catalytic species are nickel hydride complexes, generated by ethylene insertion into the Ni-aryl bond of the precursor followed by /3-H elimination reaction (Scheme 50). Styrene or styrene derivatives can be detected in the reaction medium as a product of this activation process. In the case of the salicylaldiminate and anilinotropolone catalysts, styrene elimination is not required,... 

The olefin oligomerization mechanism on a bimetallic center proposed by Rodriguez, van Looy, and Gabant [37-41] for ethylene oligomerization on the (MeOl Ti/EtjAl system is as follows ... 

Various zeolites, for example, HZSM-5, omega zeolites, ZSM-25, HZSM-11, and HZSM-48, have been investigated as ethylene oligomerization catalysts [265— 277]. These catalysts yielded gasoline and diesel fuels. The process occurred not by the classical carbonium ion mechanism, but by the formation of alkoxy structures as intermediate products. Not all of the acid OH groups (—20%) were active in this reaction, due to the structural heterogeneity caused by the presence of nonequivalent Si atoms. 

Pino, E, Mechanism of ethylene oligomerization by Ziegler Natta catalysts, Adv. Polym. Set, 4, 394, 1965. 

Mel nikov, V N., Matkovskyi, P. E., Russiyan, L. N., and Bucheva, Z. G., Mechanism of interaction between zirconium isobutyrate and sesquiethylaluminum chloride in catalysts for ethylene oligomerization, Kinet. Kat., 29, 124, 1988. 

Borovkov, V. Yu., and Kazanska, V B., Mechanism of ethylene oligomerization on ZSM-type zeolites, Dokl Akad. Nauk SSSR, 286, 914, 1986 (Russian). 

R.R. Schrock - Rapid Selective Dimerization of Ethylene to 1-Butene by a Tantalum Catalyst and a New Mechanism for Ethylene Oligomerization,/. Am. Chem. Soc. 101, 5099, 1979 S.J. McLain, J. Sancho, R.R. Schrock - Metallacyclopentane to Metallacyclobutane Ring Contraction, J. Am. Chem. Soc. 101,5451,1979 ... 

Explain why (a) Cossee mechanism is not the only possible mechanism for chain propagation in ethylene oligomerization reaction (b) selectivity of the oligomerization of ethylene is expected to be a function of the electronic and steric properties of the P, O ligands (c) conversion of 6.46 to 6.47 is accompanied by the formation of styrene (d) the reaction of Ni(COD)j with appropriate phosphonium ylide and PPhj gives 6.46 (e) in ethylene oligomerization, product formation may take place by chain transfer (f) on replacement of PPhj in 6.46 by PMOj, there is an almost complete inhibition of the catalytic reaction. 

Oligomerization of ethylene using a Ziegler catalyst produces unbranched alpha olefins in the C12-C16 range by an insertion mechanism. A similar reaction using triethylaluminum produces linear alcohols for the production of biodegradable detergents. 

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