Ethylene zirconium alkyls

Polymerization of Ethylene by Zirconium Alkyl Halides in Toluene at 80°C. Concentration 3.00 X 10 3 mole liter-1 Ethylene Partial Pressure 10 atm.Hydrogen Partial Pressure 10 atm (9, 16)... 

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]. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Ziegler-Naita caialysts consist of a combination of alkyls or hydrides of Group I-III metals with salts of the Group IV-VHI metals. The most generally efficient catalyst combinations are those in which an aluminum alkyl derivative is interacted with titanium, vanadium, chromium or zirconium salts. The most important application of these catalysts is in the polymerization of olefins and conjugated dienes. Not every catalyst combination is equally effective in such polymerizations. As a general rule, Ziegler-Natta combinations that will polymerize 1-olefins will also polymerize ethylene, but the reverse is not true. 

Titanium and zirconium tetrabenzyl and the mixed metal—benzyl halides are soluble in hydrocarbon solvents and will polymerize ethylene and a-olefins, the latter to stereo-specific polymers [64], The structures of the true initiators are not known but they are unlikely to be the simple organo-metal compounds. Catalysts of higher activity are obtained when they are used in combination with aluminium alkyls. It is of interest to note that titanium tetra(dimethyl amide) reacts with acrylonitrile to form an active species, which then forms high molecular weight polymer by coordination polymerization [65]. 

Salt metathesis was employed to synthesize half-sandwich zirconium and hafnium dichloride complexes 331 incorporating the bidentate, mono-anionic benzamidinate ligand (Equation (26)). The corresponding zirconium dimethyl and dibenzyl complexes have also been prepared using appropriate alkylating reagents.260 The zirconium dichloride complex (R= H), upon activation with MAO, are active for both polymerizations of ethylene... 

At this point Ziegler and his coworkers carried out experiments on the effects of adding various other metal compounds to triethylaluminum. In one of these experiments with zirconium acetylacetonate, ethylene, and triethylaluminum, they found, to their surprise, an autoclave filled with a solid cake of snow-white polyethylene (1. ) Further work revealed that aluminum alkyls in conjunction with certain transition metal compounds of Groups IV-VI, as well as uranium and thorium, were active ethylene polymerization catalysts. Ultimately, Ziegler catalysts were described to be the product of reaction of metal alkyls, aryls, or hydrides of Groups I-IV and certain transition metal compounds of Groups IV-VIII (Reaction 4). The choice of a particular catalyst and experimental conditions is dictated by the structure of the monomer to be polymerized. 

Alkylation of isobutane with ethylene in the presence of zirconium chloride took place at 100 under 15 atmospheres pressure (Ipatieff, 1, p. 682). The product was completely paraffinic and consisted chiefly of hexanes, octanes, and decanes. The catalyst was converted to a dark, pasty mass which was still catalytically active as was shown by its re-use in a second experiment with isobutane and ethylene. 

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