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There is now a general consensus, but not unanimity, that covalent propagation does not occur. The arguments against covalent propagation and for propagation by ions (speci cally, most likely, by ion pairs) have been described [Matyjaszewski and Pugh, 1996; Matyjaszewski and Sawamoto, 1996]. (Plesch has presented the case for covalent propagation [1988, 1993, 2001, 2002].) There is no argument that the propagating species in fast cationic polymerizations (such as the fast stages described above for styrene perchloric acid) are ions (ion pairs and/or free ions). The only uncertainty is the identity of the propagating species in the slow stage. The polymerizations previously ascribed to covalent propagation may simply be due to very small concentrations of ions, concentrations that cannot be detected by conductivity or spectroscopy. Very low concentrations of ions result in observed polymerization because of their very high reactivity (Table 5-1 through 5-3). Most of the experimental observations ascribed to covalent propagation can be rationalized in terms of ionic propagation. The bimodal MWD may be the result of simultaneous propagation by ions and ion pairs with signi cantly different lifetimes such that there is little interconversion during propagation [Kunkel et al., 1992; Matyjaszewski, 2001; Matyjaszewski et al., 1994; Szwarc and Van Beylen, 1993]. Changes in polymerization rate with reaction conditions can be rationalized in terms of changes in the concentrations of free ions versus ion pairs. Finally, an interesting experiment was the addition of optically active 1-phenylethyl tri uoroacetate to styrene. Complete racemization was observed before addition was complete, consistent with the formation of carbocationic species. The addition reaction should have occurred by a stereoselective route if it proceeded by a multicenter reaction. 5-2f-2 Solvent Effects
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Polymerization rates and polymer molecular weights increase with increasing solvent polarity because there is a shift in concentrations from the unreactive (dormant) covalent species toward the ion pairs and free ions. For the perchloric acid polymerization of styrene, there is an increase in overall reaction rate by about 3 orders of magnitude when polymerization is carried out in 1,2-dichloroethane (E 9:72) compared to carbon tetrachloride (E 2:24) [Pepper and Reilly, 1962]. Table 5-7 shows data for the polymerization of p-methoxystyrene by iodine [Kanoh et al., 1965]. The apparent propagation rate constant increases by more than 2 orders of magnitude in going from carbon tetrachloride (E 2:24) to methylene chloride (E 9:08). (Although the dielectric constant E is generally used as an indication of the solvating power of a solvent, it is not necessarily a quantitative measure. Speci c solvation and polarization effects, which are not adequately described by E, may be important in a particular system.) Similar effects are observed for reaction systems where covalent species are not present. Thus, BCl3 H2 O do not polymerize neat isobutylene, but polymerization occurs for
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TABLE 5-7 Effect of Solvent on Cationic Polymerization of p-Methoxystyrene by Iodine at 30 Ca Solvent CH2 Cl2 CH2 Cl2 /CCl4 , 3/1 CH2 Cl2 /CCl4 , 1/1 CCl4
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17 1.8 0.31 0.12
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Data from Kanoh et al. [1965].
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TABLE 5-8 Effect of Solvent on kp in Radiation Polymerization of  a Isopropyl Vinyl Ether at 30 C
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Data from Def eux et al. [1983]. Calculated from E values of solvent and monomer assuming additivity of volumes and E of mixture to vary with volume fraction of solvent.
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solutions of isobutylene in solvents (e.g., CH2 Cl2 , CH3 Cl) more polar than isobutylene [Kennedy et al., 1977]. Ionic propagating species (either free ions or ion pairs) are not suf ciently stable and do not form unless the reaction medium possesses some minimum solvating ability. The preceding discussion assumes that the effect of solvent polarity on reaction rate (and polymer molecular weight) manifests itself only by altering the concentration of propagating centers. However, solvent polarity is also expected to alter propagation rate constants, kp and kp . One expects kp to decrease with increasing solvent polarity [Ledwith and Sherrington, 1974]. The transition state for propagation by a free ion involves charge dispersal, so that solvents of higher polarity stabilize the reactants more than the transition state [Reichardt, 1988]. The effect of solvent polarity on kp is more dif cult to predict. Whether increased solvent polarity increases or decreases kp depends on whether the transition state has a higher or lower dipole moment than the ion pair. There are very few data available on the effect of solvents on the different propagation rate constants. A comparison of reported kp values for styrene, ethyl vinyl ether, and isopropyl vinyl ether in different solvents shows the expected decrease in propagation rate constant with increasing solvent polarity [Def eux et al., 1983; Sawamoto an Higashimura, 1978, 1979]. Table 5-8 shows data for isopropyl vinyl ether in radiation polymerization at 30 C. A plot of ln kp versus 1=E is reasonably linear with a positive slope as expected for the reaction of an ion and neutral molecule [Reich erdt, 1988]. Data for kp as a function of solvent polarity are available for the polymerization of styrene by 1-chloro-1-(4-methylphenyl) ethane/TiCl4 at 80 C in methylcyclohexane methyl chloride [De et al., 2003]. kp increased by a factor of $3 when the solvent polarity was increased from 20% methyl chloride to 60% methyl chloride. This effect indicates that the transition state has a higher dipole moment than do the reactants, a consequence of increased charge separation in the ion pair. The need for solvation of ionic propagating species in cationic polymerization is evident when reaction is carried out in media of low dielectric constant. In addition to lowered polymerization rates in poorly solvating media, one frequently encounters increased kinetic orders in one of the reactants. The polymerization rate may show an increased order of dependence on the monomer, initiator, or coinitiator. Thus, the rate for the tin (IV) chloride polymerization of styrene depends on [M]2 in benzene solution and [M]3 in carbon tetrachloride solution [Okamura and Higashimura, 1956a,b, 1960]. Carbon tetrachloride is a poor solvating agent compared to benzene, and the higher order in styrene concentration is due to styrene taking part in solvation of propagating species. At high concentrations of styrene (and also in neat styrene) the order in styrene decreases to 2 as the reaction medium becomes equivalent to the benzene system. (That benzene is a better solvating medium than
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