The influence of small concentrations (1—8 mmol L –1 ) of arenes ( viz ., hexafluorobenzene, chlorobenzene, benzene, toluene, and mesitylene) on the molecular weight, molecular weight distribution, and degree of functionalization by terminal olefin groups was studied for polymers prepared by low-temperature (–78 °C) isobutylene polymerization in n -hexane initiated by the MeOH—AlBr 3 and Bu t Cl—AlBr 3 systems. The criteria “extent of livingness” k el / k p were calculated, where k el and k p are the rate constants of proton elimination and chain propagation, respectively. It was established that arenes can be involved in proton elimination from the growing carbocation, and their activity in this process increases with an increase in the basicity. Arenonium ions formed by the interaction of arenes with the components of the initiating system or with the growing ionic active centers can form complexes with counteranions, thus retarding proton elimination with the transfer to the counterion.

Isobutylene polymerization in hexane at –78 °C initiated by the methanol—aluminum bromide and tert -butyl chloride—aluminum bromide systems in the presence of minor additives (to 10 mmol L –1 ) of arenes (benzene, chlorobenzene, toluene, and mesitylene) was studied. The addition of the arenes to a monomer solution has virtually no effect on the polymer yield, whereas the preliminary interaction of a concentrated solution of the Lewis acid with benzene and chlorobenzene sharply increases the monomer conversion and initiation efficiency. The results are interpreteted within a mechanism involving the participation of arene σ- and π-complexes in initiation. The concentration of the complexes is determined by the order of arene addition, and the activity and stability are related to their nature. In these experiments, hexafluorobenzene unexpectedly exhibited a pronounced inhibiting ability.

The reaction of aluminum bromide with benzene in n -hexane was studied by 27 Al NMR spectroscopy in the temperature range from –80 to +20 °C. The formation of C 6 H 6 ·Al 2 Br 6 (1 : 2) complexes is accompanied by broadening of the resonance line with δ 178. No peak splitting following a decrease in the temperature was observed but the temperature dependence of the line width passed through a maximum near –60 °C. A procedure for determination of the constant K for the formation of 1 : 2 complexes at –20, 0, and +20 °C based on the line broadening with an increase in the C 6 H 6 : Al 2 Br 6 molar ratio was proposed. The thermodynamic parameters of complex formation, Δ G , Δ H , and Δ S , were calculated.

On the basis of H-1 and C-13 spectra obtained for solutions of complexes of acyl halides with aluminium bromide RCOX . nAlBr(3) (X = Cl, Br; n =1, 2), it is shown that these adducts can exist in solution as donor-acceptor complexes, acyl salts, or combinations of both forms. The nature of the cationic initiating species for each case is predicted. The predictions coincide well with experimental results obtained by analysis of the isobutylene polymers prepared with these initiating complexes. Initiation by the 1:2 complexes allows polymeric molecules to be synthesized with an acyl group at one end. These polymerization processes have some of the characteristics of living polymerizations. The possibility for quantitative estimation of such systems using the criteria of 'approach to livingness' is demonstrated. (C) 2000 Society of Chemical Industry.

The ternary catalytic system AlBu i 3 -TiCl 4 -CCl 4 initiates the cationic polymerization of isobutylene in toluene at room temperature, whereas the binary combinations of these components do not induce isobutylene polymerization. At low CCl 4 concentrations, the polymerization rates decrease sharply with time, and the quantitative yield of the polymer is achieved at an excess of CCl 4 with respect to the titanium and aluminum components. The molecular weights of the polymers range within 1300–4000, and the index of polydispersity, as a rule, does not exceed 2.7. The influence of the conditions of component mixing (order of addition, duration of exposure prior to addition of the third component) on the yield and molecular weight of the polymerization product was found.

The ionic complex of mesitoyl bromide with aluminum bromide in a 1 : 1 composition (Mst-1) does not initiate the isobutylene polymerization in n-hexane or methylene dichloride at -78 degrees C. The corresponding ionic complex of the 1 : 2 composition (Mst-2) acts as a cationogenic initiator of the polymerization. The addition of excess Lewis acid or introduction of organic electron accepters increases the initiating activity of the Mst-1 complex and activates acyl complexes of the 1 : 2 composition including Mst-2. The results are discussed in terms of the effect of specific solvation on the nucleophilicity of counteranions, which makes the addition of the monomer to the carbocation possible.

The interaction oftert-butyl chloride with aluminum bromide in methylene dibromide at −30°C leads to the formation of two types of adducts, which give signals with δ 2.4 and 3.2 in the1H NMR spectra in addition to that of free alkyl halide. these signals are attributed to a polarized complex (PC) and ion pair (IP), respectively. An excess of AlBr3 shifts the equilibria toward IP. The latter contains more AlBr3 than the polarized complex. Based on the spectral data, we calculated the limiting values of some equilibrium constants. The ability of AlBr3 to solvate counterions is consistent with the results of isobutylene polymerization under the action of the initiating ButCl−AlBr3 system at different ratios of the starting concentrations [AlBr3]0/[ButCl]0. An increase in this ratio results in both the acceleration of polymerization and an increase in the relative role of chain transfer reactions.

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Polymerization of isobutylene in n-hexane at -78 degrees C initiated by MeCOBr . AlBr3 was studied. The results obtained were compared with the corresponding data for RCOX . 2AIBr(3) complexes (R = Me or Ph, X = Cl or Br). The main peculiarities of the polymerization mechanism under the action of MeCOBr . AlBr3 were established. The rate constants of proton elimination and of chain termination and chain growth were determined experimentally.

The effects of the nature of halogens in the initiatingtert-butyl halide-aluminum-containing Lewis acid system on the number average molecular weightM n and the structure of end groups of polyisobutylene macromolecules obtained in the cationic polymerization of isobutylene in hexane at -78 °C were studied. An increase inM n is observed in the transition from chlorine to bromine and iodine, accompanied by a decrease in the fraction of end C=C groups and an increase in the relative content of C-Hal groups (Hal = Cl, Br, and I). When atoms of different halogens are present in the counterion, more bulky atoms preferentially participate in the formation of the end groups. The results are interpreted within the framework of the principle of hard and soft acids and bases.

The effect of 2,6-dimethylpyridine on the cationic polymerization of isobutylene inn-hexane and dichloromethane at -78 °C under the action of complexes of acetyl bromide with AlBr3 of the compositions 1 : 1 and 1 : 2 was investigated. 2,6-Dimethylpyridine significantly depresses the initiation and chain transfer processes involving free protons and also retards the proton elimination from growing carbocations.

Dependences of the initial rates of cationic polymerization of isobutylene on the monomer concentration observed in hexane at -78 degrees C for different initiating systems show a complicated character, and in most cases, pass through a maximum. Possible ways to explain the effect of monomer on the formation of active centers of polymerization, and on their stability, are discussed.

Viscometry was used to ascertain the hypothesis on association of the active centers in cationic polymerization of isobutylene in n-hexane at -78 degrees C initiated by the tert-butyl chloride-ethylaluminum sesquichloride system. It is shown that the efflux time of nondeactivated polymer solution measured after polymerization is complete is considerably higher than the efflux time of the solution of this polymer with the same concentration after deactivation. The degree of association calculated as a ratio of the apparent weight-average molecular mass of nondeactivated polymer M(eta) to that of deactivated polymer was found to be 1.36. This corresponds to 50-60% content of associated macromolecules in the solution of nondeactivated polymer.

Influence of solvation on the mechanism of polymerization of isobutylene in the presence of catalytic systems on the base of aluminum halides is described. Polymerization of isobutylene under the action of complexes of aluminium bromide with halogen anhydrides of organic acids has been studied in details. The mechanism of initiation and rates of chain limitation reactions are shown to depend on the nature of halogen anhydride and on the complex components ratio. The solvation of the active centre of polymerization with aluminium bromide complexes is concluded to take place in <<quasiliving>> processes, while the solvation of the anion with free aluminium halide promotes the reaction of chain limitation.

The polymerization of butadiene in n-hexane on the catalytic system diododichlorotitanium-triisobutylaluminum was studied. The catalyst was prepared by mixing the components in a solution of the monomer (method I) or was preliminarily formed before the introduction of the monomer (method II). In the first case the activity of the system passes through a maximum when the initial monomer concentration [M]0 is increased; in the second case it is a first-order function with respect to the monomer. The concentration of paramagnetic Ti3+ ions, calculated from the ESR spectra, and the activity are directly dependent on [M]0. The data are discussed from the standpoint of the topochemical characteristics of the polymerization process and the influence of the monomer on titanium reduction.

The influence of solvation processes on the mechanism of polymerization of isobutylene in the presence of catalytic systems based on aluminium halides is reviewed. Polymerization of isobutylene on exposure to aluminium bromide complexes with halogen anhydrides of organic acids is explored in detail. It is shown that the mechanism of initiation and the rate of the chain limitation reactions depend on the nature of the halogen anhydride and the ratio of the components of the complex. It is concluded that in “quasi-living” processes there is solvation of the active centre of polymerization by the aluminium bromide complexes, while solvation of the anion by the free aluminium halide promotes the chain propagation limitation reaction.