Mechanism of ethylene oligomerizations

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,... 

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

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

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. " ... 

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 ... 

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. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Variations of this mechanism included the suggestion of a partially bonded alkene molecule,299 the participation of a titanium-aluminum ion pair,300 and a concerted alkene insertion.301 The development of the activator-alkyl mechanism was probably strongly influenced by the Aufbau reaction, studied originally by Ziegle.102 He observed that Group I—III alkyl compounds such as Et3Al catalyzed the oligomerization of ethylene to terminal alkenes. Additional evidence of such mechanism comes from the fact that alkylaluminum compounds exist in dimeric... 

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

All this has been experimentally proved (in agreement with the proposed kinetic models though they cannot all be referred to the same mechanism) both in ethylene and propylene oligomerization (homophase system) and in some cases of ethylene and propylene polymerization, provided homogeneous catalytic systems are used. 

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. 

The data on ethylene transformations on SAPO-5 could be e lained taking into consideration the concerted mechanism of oligomerization proposed earlier [8]. According to this mechanism, first molecule of ethylene forms an H-complex with a bridged OH-group and after that it could be transformed into ethoxy group and further into oligomer chains ... 

Competing oligomerization and dimerization reactions are observed in accord with the work of Vives et al. (1965). The main polymerization product is the dimer [62], demonstrated by comparison with an independently synthesized sample (from condensation of ethylene phosphate and methyl hydroxyethyl phosphate Kluger and Thatcher, 1985). Of the three mechanisms proposed by Vives et al. the most likely is reaction of the initial exocyclic cleavage product, ethylene phosphate, with ring-opened methyl hydroxyethyl phosphate (Scheme 21b). 

---