Effects of Solvent and Counterion

It has previously been shown that large changes can occur in the rate of a cationic polymerization by using a different solvent and/or different counterion (Sec. 5-2f). The monomer reactivity ratios are also affected by changes in the solvent or counterion. The effects are often complex and difficult to predict since changes in solvent or counterion often result in alterations in the relative amounts of the different types of propagating centers (free ion, ion pair, covalent), each of which may be differently affected by solvent. As many systems do not show an effect as do show an effect of solvent or counterion on r values [Kennedy and Marechal, 1983]. The dramatic effect that solvents can have on monomer reactivity ratios is illustrated by the data in Table 6-10 for isobutylene-p-chlorostyrene. The aluminum bromide-initiated copolymerization shows r — 1.01, r2 = 1.02 in n-hexane but 

TABLE 6-10 Effect of Solvent and Initiator on r Values in Cationic Copolymerization 

The data in Table 6-11 show the copolymer composition to be insensitive to the initiator for solvents of high polarity (1,2-dichloroethane and nitrobenzene) and also insensitive to solvent polarity for any initiator except the strongest (SbCl5). The styrene content of the copolymer decreases with increasing solvent polarity when SbCl5 is the initiator. The styrene content also decreases with decreasing initiator strength for the low-polarity solvent 

TABLE 6-11 Effects of Solvent and Counterion on Copolymer Composition in Styrene-p-Methylstyrene Cationic Copolymerization  

The effect of solvent on monomer reactivity ratios cannot be considered independent of the counterion employed. Again, the situation is difficult to predict with some comonomer systems showing altered r values for different initiators and others showing no effects. Thus the isobutylene-p-chlorostyrene system (Table 6-10) shows different ri and r-i for AlBrs and SnCU. The interdependence of the effects of solvent and counterion are shown in Table 6-11 for the copolymerization of styrene and p-methylstyrene. The initiators are listed in order of their strength as measured by their effectiveness in homopolymeiization studies. Antimony pentachloride is the strongest initiator and iodine the weakest. The order is that based on the relative concentrations of different types of propagating centers. 

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Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

Ionic ROP shows most of the characteristics described in Chap. 5. There is minimal discussion in this chapter of those characteristics that are similar to those for carbon-carbon and carbon-oxygen double-bond polymerizations. Ionic ROP shows analogous effects of solvent and counterion, propagation by different species (covalent, ion pair, free ion), and association phenomena. 

TABLE 8-9 Effect of Solvent and Counterion on Stereochemistry in Anionic Polymerization of 1,3-Dienes"... 

Jost, R, Galand, P.J.N., Schurhammer, R., Wipff, G. 2007. Supramolecular interactions of cryptates in concentrated solutions The effect of solvent and counterions investigated by MD simulations. Solvent Extr. Ion Exch. 25 (2) 257-271. 

Transfer processes can be caused by monomer, counterion, and other components of the reaction mixture (additives, solvents, impurities). The latter reactions are sometimes called spontaneous because they are zero order in monomer. However, the spontaneous elimination of /3-protons is very unlikely, and proton elimination must be assisted by some basic reagent. The ratio of the rate constants of /8-proton elimination to that of electrophilic addition depends on several factors. The relative rate of transfer decreases with temperature, and therefore polymers with higher molecular weights are formed at sufficiently low temperatures. The effect of solvent and counterion is not yet sufficiently understood. 

The foregoing discussion and the examples given show the important effects of solvents and counterions on redox equilibriums. The most dramatic effects are seen in the disproportionation equilibriums described by the following reactions ... 

To date, most computational studies of proton transfer have focused on the gas phase, but recently efforts have been made to include the effects of solvent and counterions. A brief survey of these efforts are given in Section 4.4. 

Wang, G. Cole, R. B. Effects of solvent and counterion on ion pairing and observed charge states of diquatemary ammonium salts in electrospray ionization mass spectrometry. J. Am. Soc. Mass Spectrom. 1996, 10, 1050-1058. 

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