Aluminium bromide, initiation

Note added in proof. Marek and Chmelir [40b, c] found that the polymerization of isobutene in heptane by aluminium bromide is greatly accelerated by addition of titanium tetrachloride. They suggested that the polymerization by aluminium bromide only is initiated by a cation such as AlBr2+ which adds to the isobutene and which is formed by self-dissociation of the catalyst. The enhancement of the rate by titanium tetrachloride they attribute to an increase in the concentration of ions by the reaction... 

However, whilst our observations merely make an initiation without co-initiator appear probable for aluminium chloride in methylene dichloride, the experiments of Chmelir, Marek, and Wichterle subsequently proved this point fairly conclusively for aluminium bromide in heptane [11]. The Czech workers interpreted their findings by a scheme which is equivalent to reaction (4) or, more explicitly, to a combination of reactions (7) and (8) below. Their results occupy a central position in this field of enquiry, because they made it necessary to reassess old results and to re-examine the then current views, and therefore they were prominent among the stimuli which led to the evolution of the new theory. 

A scheme equivalent to our supposition 2 was put forward by Chmelir, Marek and Wichterle to explain the polymerisations initiated by aluminium bromide in heptane [11]. The fact that these reactions were of second order with respect to the initiator demanded an explanation in terms of a pre-initiation reaction between two molecules of initiator. 

Subsequently, Marek and Chmelir used a closely related set of ideas to explain their findings concerning two-component initiator systems, the most typical of which consists of aluminium bromide and titanium tetrachloride. In the presence of both these compounds the polymerisation of isobutene in heptane is much faster than when either of them is... 

In Experiment C9 (circular points in Figure 7) 0.75 mol of isobutylene was added at -80 °C to 3.3 mmol of aluminium bromide in 40 ml of methyl bromide over ca. 1.5 h. According to the ratio (kc - k /Kj, which was ca. 103, the initiator solution was very pure. No polymerisation took place the isobutylene was shown by GLC analysis to be unchanged and this showed no dimers. 

Aluminium bromide can polymerise isobutene via direct initiation The kinetic behaviour of this system was rationalised in terms of selfionisation of the catalyst and formation of an inactive complex between catalyst and monomer. The latter interaction was invoked to explain why the initial rate of polymerisation was directly proportional to the monomer concentration at low values of [M], but independent of it for higher values of [M]. It seems to us however that such complexation is not necessary, because the simple sequence ... 

A.3 Lewis Acid/Metal Salt Superacids. - Superacid active catalysts may be obtained by the grinding of aluminium bromide or chloride with some salts. The reaction of AlBr with titanium, copper, iron, aluminium, nickel sulphates, aluminium or titanium chlorides results in catalytic systems able to initiate n-pentane isomerization even at room temperature.The soluble complex of AlBr-j with salts is a true catalyst. Aluminium trichloride... 

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. 

The polymers from Experiments R8, R15 and R9 contained ca. 1 tritium atom, and therefore originally 1 aluminium-carbon bond, per molecule. For those experiments in which the polymer remained in solution, the final value of the conductivity suggested a minimum of 1 mol of ions for 1 mol of polymer. When aluminium chloride was used as initiator, the results obtained resembled closely those obtained with the bromide. 

The Leukart reaction has also been used in the conversion of dehydroepiandro-sterone into 17/3-formylamino-3/3-formyloxyandrost-5-ene, which on reduction with lithium aluminium hydride afforded 3/3-hydroxy-17/3-me thylaminoandrost-5-ene. Acylation with isocaproyl chloride then furnished the N-methyl-N-isocaproyl steroid (197), after selective ester hydrolysis of the initially formed ON-diacyl derivative. The amide (197) was further converted into its 3,5-cyclo-6-ketone via the 3,5-cyclo-6/3-alcohol and thence by reaction with hydrogen bromide into the corresponding 3/3-bromo-5a-6-ketone which upon dehydrobromination furnished a A2-5a-6-ketone and ultimately the 2-monoacetate of the 2/3,3/3-diol (198) after reaction with silver acetate and iodine. Hydrolysis to the 2/3,3/3-diol (198) gave a separable mixture of the 2/3,3/8-dihydroxy-5a- and -5/3-ketones.88... 

Detailed studies of systems involving aluminium-based Lewis acids and hydrogen halides are scarce. Fontana and Kidder investigated the polymerisation of propene initiated by the pair aluminium bromideTiydrt n bromide. The cocatalytic role of the latter acid was clearly proved since no polymerisation could be detected in its absence. The dependence of the rate of polymerisation upon the cocatalyst concentration and the induction periods observed make this system similar to those in which stannic chloride induces the polymerisation of olefins in the presence of variable quantities of water (see Sect. IV-C-3-b). With relatively large quantities of added hydrogen bromide, addition of this acid to the mcmomer to give fso-propyl bromide must have constituted an important side reaction. 

The treatment of aluminium(III) bromide with an excess of phosgene at salt-ice temperatures gives an initial lively reaction which, upon warming to 15 "C, proceeds in a vigorous second stage to give COClBr [2127] ... 

In the photopolymerisation of methyl methacrylate induced by mixtures of chloroacetic acid derivatives and dimethylaniline the kinetics were found to be influenced by the degree of substitution of the chlorine atoms while the initiation occurred through a complex S. Ethyl aluminium sesquichloride has been found to induce the photocopolymerisation of 9-vinylanthracene with methyl methacrylate at -20°c while photochromic polymers have been made by reacting para-bromoazobenzene with ethylene gas O. Surface active N-cetylpyridinium bromide and chloride have been found to... 

Model experiments showed that initiation is quantitative only in some systems. Thus, benzyl bromide reacts with AgSbF6 with 100% 140) yield, while AgPF6 gives only 75 % yield 136). Mercury-, lead-, ferric halides and aluminium perchlorate were also used and generally gave lower initiation efficiencies and produced some homopolymer. This may be due to direct initiation by the added salts, in agreement with the recent data of Gandini et al., who observed initiation of vinyl and heterocyclic cationic polymerization by aluminium triflate 141). 

The observed stereochemistry is accounted for by initial attack from the outside followed by nucleophilic attack from the inside upon a carbonium ion stabilized by delocalization into the double bond. Hence bromide attack is anti to the double bond. Reduction of (64) with lithium aluminium hydride gives (65). 

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