The effect of the counterion composition on the chain restriction in the cationic polymerization of isobutylene

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.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1184–1187, May, 1996.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids 5. Polymerization of isobutylene in the presence of the complexes of benzoyl chloride with aluminum bromide

Polymerization of isobutylene inn-hexane at -78 °C in the presence of the complex of benzoyl chloride with AIBr₃ (1 : 2) was investigated. The results were compared to those obtained previously for the polymerization of this monomer induced by the complex of acetyl bromide with AlBr₃. Both complexes initiate the polymerization only by acyl cations. The number average molecular weight (M _n ) of the polymer linearly increases as the degree of isobutylene conversion increases. The polymerization restarts after repeated addition of the monomer, andM _n continues to increase linearly. The efficiency of the initiaton by the benzoyl chloride complex does not exceed 6.2 %; the reaction has the second order with respect to the initiator in the case of PhCOCI · A1₂Br₆; and the chain-propagation rate constant is 13.9 L mol^–1 s t. The use of PhCOCI Al₂Br₆ as the initator of the polymerization of isobutylene allows one to prepare macromolecules with very low contents of the terminal C=C double bonds and with narrow molecular weight distributions. Unlike the MeCOBr·AlBr₃ complex, PhCOCl · AlBr₃ does not initiate polymerization of isobutylene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1175–1179, May, 1996.     

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

The ionic complex of mesitoyl bromide with aluminum bromide in a 1∶1 composition (Mst-1) does not initiate the isobutylene polymerization inn-hexane or methylene dichloride at −78 °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 acceptors 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. For Part 9, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 52–56, January, 1998.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acycl halides with lewis acids

The block polymerization of isobutylene with α-methylstyrene induced by acyl initiators was investigated. Thek _el/k _p values (the criterion for “closeness to the living state,”), wherek _el is the rate constant of proton elimination from a growing carbocation andk _p is the rate constant of chain growth, were analyzed. The minimumk _el/k _p values are characteristic of processes occurring in the presence of PhCOCl·2AlBr₃ and an equimolar mixture of MeCOBr·AlBr₃ with PhCOCl·AlBr₃. It was concluded that these complexes are efficient initators for the synthesis of block copolymers with a relatively narrow molecular-weight distribution and a low content of homopolymers. For Part 10, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1944–1948, October, 1999.     

Cationic polymerization of hydrocarbon monomers under the action of acyl halide complexes with Lewis acids

Polymerization of isobutylene in hexane at –78 °C under the action of the complex AcBr · 2AlBr₃ (Ac-2) affords polyisobutylene having C=O groups at the head and C-Br or C=C groups at the tail of all the molecules. The presence of the latter indicates that there occurs proton elimination from the growing carbocation with the formation of a superacid HBr · 2AlBr₃ which is unable to initiate the polymerization repeatedly under given contitions. This makes it possible to consider proton elimination as the reaction of the decay of active centers with the rate constantk _d. This value has been calculated from the rate of accumulation of the polymeric molecules having terminal C=C bonds:k _d=3.5 · 10^–4 s^–1. The rate constant of chain growthk _g has been determined from polymerization kinetics and from the content of active centers:k _g=6.2 L mol^–1 s^–1.For part 3, see ref. 4.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 883–886, May, 1993.     

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

The polymerization of isobutylene inn-hexane at −78°C under the action of the superacid HBr·2A1Br₃ as well as acetyl complexes MeCOBr·AlBr₃ and MeCOBr·2AlBr₃ in the presence of HBr·AlBr₃ and HBr·2AlBr₃, respectively, was studied. Unlike the superacid providing a quantitative yield of polyisobutylene (PIB) due to protonogenic initiation, the acetyl complexes suppress the proton initiation. In the presence of a mixture of both complexes with the superacid, only macromolecules with the head acetyl fragments MeC(O) are formed, which is evidence for a carbocationic initiation. The data obtained are explained by trapping of protons by the carbonyl groups to form ionic structures of the type (where PIB is polyisobutylene) and to suppress the ionization of the superacids due to the common ion effect. For Part 7, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 974–978, May, 1997.     

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

The polymerization of isobutylene in hexane at −78 °C in the presence of the MeCOBr·AlBr₃+PhCOCl·AlBr₃ and MeCOBr·2AlBr₃+PhCOCl·2AlBr₃ mixtures was investigated. It was established that only acetyl bromide shows an initiating activity, while benzoyl chloride seems to be part of a counterion, which markedly increases the “viability” of growing carbocationic centers. Possible mechanisms for the formation of initiating centers in mixed complexes are discussed.     

Cationic polymerization of hydrocarbon monomers under the effect of complexes of acyl halides with lewis acids 3. Polymer chain initiation and breaking in hexane at -78‡C

Polymerization of isobutylene under the effect of the AcBr·2AlBr₃ (Ac-2) complex in hexane at –78C has some characteristics of living polymerization; however, in contrast to truly living polymerization, the process is characterized by high values of M_w/M_n and continuously occurring chain breaking. The steady-state concentration of active polymerization sites is established at the very beginning of the process; the dependence of the number of active sites on the starting concentration of Ac-2 is in agreement with the hypothesis concerning initiation by free acyl cations. The features of polymerization under the effect of the AcBr-AlBr₃ (Ac-1) complex are characteristic of cationic polymerization processes.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2506–2512, November, 1990.     

Cationic polymerization of hydrocarbon monomers under the effect of complexes of acyl halides with lewis acids. 2. Effect of the polarity of the solvent on the mechanism of initiation

The polarity of the solvent significantly affects polymerization of isobutylene under the effect of complexes of acetyl bromide with aluminum bromide of 12 (Ac-2) and 11 (Ac-1) composition. Going from hexane to the more polar CH₂Cl₂ is accompanied by an increase in the polymerization rates in both cases, a decrease in the molecular weight and molecular-weight distribution, and an increase in the ratio of C-Br/C=C groups in polyisobutylenes in both cases. In contrast to hexane, in CH₂Cl₂ the mechanism of initiation of the Ac-2 complex becomes nonselective, and only 42% of all of the polyisobutylenes contain head CO groups. The degree of efficiency of initiation by Ac-2 in CH₂Cl₂ is 100% (in comparison to 8.5% in hexane). The mechanism of initiation of polymerization by the Ac-1 complex is the same in hexane and in CH₂Cl₂. Polyisobutylenes containing no head carbonyl groups are obtained in both cases.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1496–1499, July, 1990.