Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids

Polymerization of isobutylene inn-hexane at −78°C initiated by MeCOBr·AlBr₃ was studied. The results obtained were compared with the corresponding data for RCOX·2AlBr₃ 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. For Part 8, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1147–1151, June, 1997.     

The influence of small additives of arenes on low-temperature isobutylene polymerization in an aliphatic solvent

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 - and -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.     

Effect of minor additives of arenes on chain termination in low-temperature cationic polymerization of isobutylene in aliphatic solvents

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—AlBr₃ and Bu^tCl—AlBr₃ 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.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids

Two criteria for the quantitative estimation of “closeness to the living state” for polymerization systems in which an, important role belongs to elimination of a proton from the growing carbocation during cationic polymerization are proposed. The first criterion, (C=C)_rel, is the proportion of units with C=C bonds in the polymer chains. The second criterion,k _cl/k _gr, is the ratio of the rate constants for proton elimination and chain growth. The criteria are used in experiments on the polymerization of isobutene in hexane and dichloromethane induced by complexes of acyl halides with aluminum bromide. Limitations and fields of application, of these criteria are examined. For part 6, see Ref. 1 Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 745–749, April, 1997.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids 6. The effect of 2,6-dimethylpyridine on the polymerization of isobutylene

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.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1180–1183, May, 1996.     

1H NMR spectroscopic study of theert-butyl chloride—aluminum bromide cationic initiating system

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 Bu^tCl−AlBr₃ system at different ratios of the starting concentrations [AlBr₃]₀/[Bu^tCl]₀. An increase in this ratio results in both the acceleration of polymerization and an increase in the relative role of chain transfer reactions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2217–2222, November, 1998.     

Ziegler catalytic systems in cationic polymerization

The ternary catalytic system AlBuⁱ ₃-TiCl₄-CCl₄ 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 CCl₄ concentrations, the polymerization rates decrease sharply with time, and the quantitative yield of the polymer is achieved at an excess of CCl₄ 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. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 711–714, April, 1999.     

Study of complex formation between aluminum bromide and benzene by 27Al NMR spectroscopy

The reaction of aluminum bromide with benzene in n-hexane was studied by ²⁷Al NMR spectroscopy in the temperature range from –80 to +20 °C. The formation of C₆H₆·Al₂Br₆ (1 : 2) complexes is accompanied by broadening of the resonance line with 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 C₆H₆ : Al₂Br₆ molar ratio was proposed. The thermodynamic parameters of complex formation, G, H, and S, were calculated.