Cationic polymerization of isobuten

Fig. 3.54. Carbenium ion additions to isobutene as key steps in the cationic polymerization of isobutene. The dashed arrow corresponds to the overall reaction.

Intermolecular additions of carbenium ions to olefins give polymers. Such a reaction is used in industry, for example, in the cationic polymerization of isobutene (Figure 3.43). One of the rare cases of an intermolecular carbenium ion addition to an olefin without polymer formation occurs in the industrial synthesis of isooctane (Figure 3.44). 

An artificial rubber may be made by cationic polymerization of isobutene using acid initiation with BF3 and water. What is the mechanism of the polymerization, and what is the structure of the polymer ... 

In 1986 Faust and Kennedy reported the first example of controlled/living cationic polymerization of isobutene, which was initiated by a cumyl ace-... 

Some recent syntheses employ the first method (A), where, for example, living cationic polymerizations of isobutene [222], (f-butyl)dimethylsilyl vinyl ether [223,224], and 2-methyloxazoline [225] are initiated from appropriate pendant functional groups. 

Both methods spread the charge on the growing macrocation and render the /9-proton less likely to transfer to monomer (as in reaction 9-41). The first method is typified by initiation with HI/I2 in which the nucleophilic counterion is the Ie//2 anion. The second primarily involves combinations of a cation-generator, like a tertiary alkyl halide, with a Lewis acid, such as EtAlCl2- A number of initiator combinations of the latter type have been reported for living cationic polymerization of isobutene [S). 

Both the monomer and polymer are soluble in the solvent in these reactions. Fairly high polymer concentrations can be obtained by judicious choice of solvent. Solution processes are used in the production of c(5-polybutadiene with butyl lithium catalyst in hexane solvent (Section 9.2.7). The cationic polymerization of isobutene in methyl chloride (Section 9.4.4) is initiated as a homogeneous reaction, but the polymer precipitates as it is formed. Diluents are necessary in these reactions to control the ionic polymerizations. Their use is avoided where possible in free-radical chain growth or in step-growth polymerizations because of the added costs involved in handling and recovering the solvents. 

Phosgene can be used as an initiator for the living cationic polymerization of isobutene or styrene. Aluminium(III) chloride was used as the co-initiator, but the proposed mechanisms for its involvement are "imaginative", and do not take into account the known chemistry of the AlClj-COClj system (see Section 9.1.2.5) [2255aj. 

The first hint toward a living cationic polymerization of isobutene was provided by Nuyken and coworkers in 1982 when they reported that the cationic polymerization of isobutene could be initiated by 2-chloro-2,4,4-trimethylpentane (Scheme 8.6), which is a low molecular weight analog of the covalent chloride-terminated PIB, in a polar solvent at —85°C [26]. This work indicated that the irreversible termination reaction in the inifer polymerization mechanism could become reversible in a more polar medium, which was indeed demonstrated by Kennedy who... 

Scheme 8.6 Structures of (a) 2-chloro-2,4,4-trimethylpentane, (b) cumyl esters, and (c) cumyl ethers used as initiators for the cationic polymerization of isobutene.

Figure 8.1 The first example of a living cationic polymerization of isobutene using cumyl acetate as initiator and BCI3 as activator in dichloromethane at —30°C. Number-average molar mass, polydispersity index (numbers in the plot), and number of polymer chains (inset) are reported as a function of the mass of PIB obtained. Source Reprinted with permission from Faust R, Kennedy JP. J Polym Sci A Polym Chem 1987 25 1847 [28]. Copyright 1987 John Wiley and Sons, Inc.

Heat removal and viscosity increases during polymerization are facilitated by using a diluent solvent. However recovery and repurification of the solvent, together with flammability hazards, have limited this technique. Heat removal can be conveniently carried out using the latent heat of vaporization, e.g. the cationic polymerization of isobutene to make polyisobutylene (butyl rubber) is maintained at — 100°C by the refluxing ethylene solvent. 

The cationic polymerization of isobutene, styrene, and alkyl vinyl ethers is strongly influenced by Lewis acids such as AICI3, BF3, TiCl4, ZnCl2. 

Subsequent polymerization studies on related aluminocenium systems, such as the Cp 2AC and Cp 2AC cations, showed that both the sterics of the Cp-type ligand and the nature of the counterion may greatly affect the polymerization activity [17, 18]. For instance, it was recently reported that Cp2Al is more active in isobutylene polymerization when associated with the Al(OR )4 vs. MeB(C6F5>3 anion, which was attributed to weaker Cp2AF/Al(OR )4 interactions [R = C(CF3>3] [18]. Other representative discrete aluminum cations (compounds 3—5, Fig. 1) found to readily undergo the direct cationic polymerization of isobutene or/and isoprene are depicted above [15, 19]. 

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