Aluminium alkyl halides

Alkyl aluminium halides are used in many ways as coinitiators for the cationic polymerization. Due to presence of alkyl groups, which have the characteristics of potential carbanions, the alkyl aluminium halides and the counterions formed from them cause the following irreversible competing reactions whereby hydrocarbons are released — Termination by interaction of the cation with the alkyl group of the counterion, e-g-... 

The self-ionisation of aluminium chloride and bromide in nitrobenzene has been studied in great detail [15], and the rates of the forward and back reactions have been determined so that all the relevant equilibrium constants are known. The whole body of evidence available shows that self-ionisation of the initiator, with or without other ionogenic reactions in the initiator solutions, can be regarded as well established for all aluminium halides and as highly probable for the alkyl aluminium halides. Moreover, the ionogenic reactions are relatively slow and - except under the dirtiest conditions - the concentration of ions in the initiator solution will be very much less than [A1X3]0. 

Doi et al. studied the effect of different aluminium alkyls on the polydispersity of syndiotactic polypropylene obtained with the V(acac)3-alkyl aluminium halide soluble catalytic system, at temperatures below —65 °C. These authors found that, by varying the type of aluminum alkyl not only the propagation and transfer rates are changed, but also the polymer polydispersity index decreases in the following order ... 

Research initiated at the CSIRO Division of Materials Science, Melbourne and now being continued at the University of Tasmania, has led to the development of a group of highly active and extremely versatile catalyst systems (refs. 6-8). The systems comprise nickel dithio-e-diketonate phosphine complexes (I) and (II) activated by a suitable cocatalyst such as an alkyl aluminium halide complex. 

Reactions between A1 and alkyl halides yield alkyl aluminium halides (equation 18.28) note that 18.7 is in equilibrium with [R2A1(ij,-X)2A1R2] and [RXAl(p-X)2AlRX] via a redistribution reaction, but 18.7 predominates in the mixture. 

A series of papers presents a detailed account of the hydrocyanation of ajS-unsaturated ketones with either (a) mixtures of trialkylaluminium, or an alkyl-aluminium halide, with HCN or h) a dialkylaluminium cyanide. Cholest-4-en-3-one gives the 5a- and 5 -cyano-3-ketones almost quantitatively. A A -ll-oxo-steroid gave the 8)3-cyano-l 1-ketone, despite being unreactive by older methods. Mechanistic features of these processes have been largely elucidated conditions can be chosen to give products of either kinetic or thermodynamic control. Kinetic data are analysed for hydrocyanation of cholest-4-en-3-one and b-norandrost-4-en-3-one. ... 

The most interesting series of observations in this field have been made on the catalytic action of Lewis acids, which are also effective catalysts of Diels-Alder reactions (Section 4.1.6). In the presence of BF3 or its derivatives it has been possible to obtain 1,3-cycloadditions that do not proceed under uncatalysed conditions, e.g. those of benzonitrile oxide to aliphatic nitriles and carbonylic compounds and to oximes . The method has been applied also to the cycloaddition of diazomethane to CN multiple bonds, by using alkyl aluminium halides . Lewis acid catalysts probably act by making the dipolarophiles more polar, through complexing. 

Furukawa and co-workers have developed catalysts from alkyl aluminium halides and transition metal halides, such as VCI3 or TiC, which permit the production of equimolar butadiene-acrylonitrile copolymers (containing 48 % acrylonitrile). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Kennedy, J. P. and Trivedi, P. D. Cationic Olefin Polymerization Using Alkyl Halide — Alkyl-Aluminium Initiator Systems. I. Reactivity Studies. II. Molecular Weight Studies. Vol. 28, pp. 83-151. 

Detailed studies showed that the concentration of ions was much smaller than c0, and therefore that of ion-pairs was negligible. I had thus been reminded of the BIE and could respond appropriately when shortly afterwards Grattan at the other end of the same laboratory found linear K - cQ plots in his work with A1X3 in alkyl halides RX [35, 36]. This indicated that in adequately pure systems the aluminium halides ionise thus ... 

D. W. Grattan, P. H. Plesch, Ionisation of Aluminium Halides in Alkyl Halides, J. Chem. Soc. Dalton Trans., 1977,1734. 

It is known that the conducting species in solutions of aluminium halides in alkyl halides are complex for instance solutions of aluminium bromide in ethyl bromide [70] contain the ions Al2Br5+ and Al2Br7 ... 

In the field of cationic polymerisation a notoriously intractable problem is the mechanism of initiation by aluminium halides. Despite much work on the polymerisation itself, there are few studies of the initiation mechanism. Existing theories are shown to be inadequate to explain the most characteristic features of the reactions when a solution of an aluminium halide in an alkyl halide is introduced into a solution of isobutene, there ensues a fast polymerisation which generally stops at incomplete conversion, and the number of moles of polymer formed is much smaller than the number of moles of initiator these features are found over a very wide range of conditions. 

On the other hand, an investigator interested in the fundamental chemistry rather than in the optimisation of the reaction conditions (from the commercial point of view) would naturally refrain from preparing, and then using after a variable storage-time, a solution of aluminium halide in alkyl halide, because the rapid reactions which can occur in such systems at all but the lowest temperatures would obscure largely the true nature of the initiator. For such studies it is necessary to introduce the monomer into a freshly prepared initiator solution, or to introduce into a solution of the monomer the solid initiator or a freshly prepared solution of initiator in a very pure and inert solvent. 

The question of purity becomes of paramount importance when one studies the details of initiation by metal halides. A study of aluminium halide solutions in alkyl halide solvents shows that provided the impurity level is less than about 10"7 mol l 1, the halides ionise simply by binary disproportionation ... 

Proposition 1 The initiator solutions of aluminium halide, A1X3, in alkyl halide RX contain ions formed by the self-ionization of the aluminium halide ... 

Figure 7 The change of conductivity when isobutylene is added very slowly to solutions of aluminium halide in alkyl halide at ca. -78 °C. No polymerisation occurred in any of these experiments. The experimental conditions are specified in Table 3. The point marks the conductivity of mixture C7 when it was warmed to 0 °C...

Horie, T. et al., Studies of the selective 0-alkylation and dealkylation of flavonoids. X. Selective demethylation of 7-OH-3,5,8-triOMe flavones with anhydrous aluminium halide in acetonitrile or ether, J. Org. Chem., 52, 4702, 1987. 

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