Cationic polymerization combination with counterion

Chain transfer is the most common chain terminating reaction in cationic polymerization and can include transfer to monomer, solvent, and polymer. Termination by combination with the counterion can also occur in some systems. In some cases, cationic polymerization may be used to prepare stereoregular polymers. Although the exact mechanism is unclear, it is known that stereoregularity varies with initiator and solvent.18 Lower temperatures also tend to favor more stereoregular polymers. 

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). 

For obtaining a cationic polymerization, the new carbocation generated between R and the monomer should have enough stability to be relatively easily formed and to continue the polymerization (for example, CH2=CH2 is not polymerized using a cationic initiator, while (CH3)2C=CH2 can be polymerized because the species RCH2-C(CH3)2 is stable enough to be formed). The stability of the carbocation increases as the chain length increases. Chain transfer reactions are common in carbocation polymerization. The termination reactions typically occur because of the combination of the cationic component with a counterion. 

Figure 8.6 illustrates, for example, the complicated equilibria [19] in initiation by alkyl halides which are widely used as initiators in combination with coinitiator such as aluminum alkyl halides or aluminum halide Lewis acids. Each carbocation can initiate polymerization or remove an alkyl (ethyl) group from the counterion to produce a saturated hydrocarbon, REt, and a more acidic Lewis acid. The propagating cation can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the coinitiator used is diethylaluminum chloride there may be major contributions to the polymerization from ethylaluminum dichloride or aluminum chloride. 

Another type of chain reaction is possible by ionic initiation. Both, positive and negative ions may start polymerization reactions (cationic and anionic polymerization). The existence of ions in solution is illustrated in Fig. 3.24 (B = base, M = metal). The interaction with the solvent plays an important role in the activity of the catalyst. In the covalent combination, no reactivity is expected. As the ions separate, they become increasingly available for reaction. While one of the ions supports the reaction, the other, the counterion, is in the vicinity and may also affect the rate of reaction. Since cationic initiators accept a pair of electrons, one finds these among the Lewis acids. Anionic initiators donate an electron pair, so they are Lewis bases. 

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