Aluminum alkyl-electron

A further improvement in performanee was achieved by the addition of electron donors (Lewis bases), such as esters, ethers, or amines, to the catalyst, which can form complexes with aluminum alkyls. Electron donors could be added either with the co-catalyst, or during the y-titanium trichloride milling stage, depending on the compound used or the result required. In addition to... 

Other organometallic compounds of aluminum include the alkyl hydrides, R2A1H. Molecular association of these compounds leads to cyclic tetramers. When the dimeric and trimeric compounds are dissolved in a basic aprotic solvent, the aggregates separate as a result of formation of bonds between A1 and the unshared pair of electrons on the solvent molecule. Toward Lewis bases such as trimeth-ylamine, aluminum alkyls are strong Lewis acids (as are aluminum halides). 

Snell et al, "Colorimetric Methods of Analysis," vol IIA, Van Nostrand(1959), 156-87 Aluminum Alkyls were prepd in 1865 hy the action of aluminum on mercury alkyls(Refs 1 6)(see also Note below). Later they were made by the action of "electron metal (alloy of Al and Mg) on a sola of the alkyl halide in ether (Refs 2 6). The Al trialkyU are volatile liquids, violently attacked by air or water. Following are examples trimethylaluminum A1(CHS), d 0.752 at 20°/4°, mp 15.0°, bp 126. la, tri-ethyl-, Al(CaH5) d 0.837 at 20°/4°, mp -52.5°, bp 185.6a, tri-n-propyl-, Al(n-CjH,),. d 0.823 at 20°/4°, mp -107°, bp ca 250° (Refs 3,4,5 6). These three compds are inflammable in air and for this reason may be of interest as components of liquid propellants for rockets Note The prepn of a compd called "Aethyl-aluminium" was claimed by W.Hallwachs A.Schaffarik, Ann 109, 207(1859) hut it was not properly identified and its props (except that it is violently decomp by water) were not detd... 

Snell et al, "Colorimetric Methods of Analysis," vol II A, Van Nostrand(1959), 156-87 Aluminum Alkyls were prepd in 1865 by tbe action of aluminum on mercury alkyls(Refs 1 6)(see also Note below). Later they were made by the action of "electron metal (alloy of Al and Mg) on a soln of the alkyl halide in ether (Refs 2 6). The Al trialkyls are volatile liquids, violently attacked by air or water. 

A similar reaction scheme which accounts for the coordination mode of diene monomers was already suggested by Witte in 1981 [49]. Though DIBAH or TIBA are known as Lewis acids, in an anhydrous hydrocarbon environment aluminum alkyls can act as electron donors which are capable of coordinating to vacant Nd sites. This is evidenced as DIBAH forms oligomers [461]. Thus, DIBAH acts as an electron donor as well as an electron acceptor. In addition, complexes of aluminum alkyls with various Nd precursors were isolated and characterized, e.g. [184-186,234]. 

Metallocenes (Fig. 2) are sandwich structures, typically incorporating a transition metal such as titanium, zirconium, or hafnium in the center. The metal atom is linked to two aromatic rings with five carbon atoms and to two other groups—often chlorine or alkyl. The rings play a key role in the polymerization activity (23-27). Electrons associated with the rings influence the metal, modifying its propensity to attack carbon-carbon double bonds of the olefins. The activities of these metallocenes combined by aluminum alkyls, however, are too low to be of commercial interest. Activation with methylaluminoxane, however, causes them to become 10-100 times more active than Ziegler-Natta catalysts. 

Aluminum alkyl halides of the types R2A1X and RA1X2 are halide-bridged dimers in which, unlike Al2Me6, Al is surrounded by an electron octet. They are stronger Lewis acids than the trialkyls. The sterically very crowded compounds mes MX2 are monomeric they fail to give complexes with Et20 (M = Al, Ga, In X = Cl, Br).27... 

The sttuctures of organoaluminum halides R AlXs are dominated by dimeric structures with AI2X2 four-membered rings for X = Cl, Br, and I. Unlike the aluminum alkyls, these hahde bridges are electron-precise and result from intermolecular Lewis acid-base complexation. These dimers do dissociate and readily react with Lewis bases, but the enthalpies of dissociation are higher than those for the aluminum alkyls. In many cases, the strength of the association is sufficient that dimers are observed in the gas phase. 

Aluminum alkyls are stronger Lewis acids than the trialkyls of boron and galUum, and complex formation in the presence of Lewis bases is an important aspect of the reaction chemistry of organoaluminum compounds. The Lewis acidity of organoaluminum compounds accounts for the strong association of aluminum alkyls and hydrides by three-center two-electron bonds, as well as association of organoaluminum... 

The phenomena observed were attributed to the different electron acceptor power of the aluminum compounds in agreement with the hypotheses of Henrici-Olive and Olive It was also observed examining the isotactic fraction of polypropylene prepared with S-TiClj and various aluminum alkyls, that M WD width depends on the nature of the ligand and specifically that it decreases in this order ... 

In conclusion, electronic density of the transition metal may be influenced, case by case, by the effect of the reaction with aluminum alkyl and, as a result, the carbon-transition metal bond stability, olefin coordination and insertion capacity, stereochemical control of active centre and chain transfer and propagation processes, hence polymer MWD, may also be affected. This is particulary true for soluble catalytic systems for which the existence of active centres as bimetallic complexes is likely. 

Properties of the metal alkyl usually determine whether olefin insertion occurs, but trends are observed with various olefin substituents. For alkyl-substituted olefins, the rate of insertion parallels the coordinating ability of the olefin (C2H4 > CH2CHR > CH2=CR2 > RCH=CHR) smaller olefins react most rapidly Electron-withdrawing substituents enhance olefin reactivity toward insertion for transition metal systems. Since the activated olefins also exhibit better coordinating ability than unactivated ones, it is not clear whether the effect arises from enhanced precoordination of olefin or accelerated migratory insertion. For aluminum alkyls. 

The coordinate mechanism is based on earlier proposals that describe the organometallic growth reactions of ethylene with aluminum alkyls alone. The reaction is considered anionic because the negative end of the olefin coordinates with an organometallic complex in the surface. Olefin molecules are inserted one at a time between the metal ions in the complex and the all l chain to extend the chain by two carbon atoms (see Fig. 9.11). This mechanism is more satisfying in that the ion pair never becomes widely separated. Addition at an electron-deficient bond bridging the metals in the organometallic complex has also been proposed. 

The rate of polymerization of polar monomers, for example, maleic anhydride, acrylonitrile, or methyl methacrylate, can be enhanced by coraplexing them with a metal halide (zinc or vanadium chloride) or an organoaluminum halide (ethyl aluminum sesqui-chloride). These complexed monomers participate in a one-electron transfer reaction with either an uncomplexed monomer or another electron-donor monomer, for example, olefin, diene, or styrene, and thus form alternating copolymers (11) with free-radical initiators. An alternating styrene/acrylonitrile copolymer (12) has been prepared by free-radical initiation of equimolar mixtures of the monomers in the presence of nitrile-coraplexing agents such as aluminum alkyls. 

The nickel trimer forms both 1 1 and 1 2 adducts with aluminum alkyls depending on the Lewis acid concentration. The formation constants differ only by the statistical factor so the electronic effects of one COA1 on the other are negligible. 

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