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carbon tetrachloride is not entirely clear from a consideration of dielectric constants since the values are quite close 2.28 and 2.24 for benzene and carbon tetrachloride, respectively. N However, other measures of solvating power such as the ET scale show the difference N between benzene and CCl4 . The ET value for a solvent is derived from the energy of UV excitation for a pyridinium N-phenoxide betaine dye in the solvent. Benzene and CCl4 N have ET values of 0.111 and 0.052, respectively, with larger values being indicative of higher solvating power [Reichardt, 1988].) The polymerization of styrene by trichloroacetic acid without solvent and in 1,2-dichloroethane and nitroethane solutions illustrates the situation where the initiator solvates ionic propagating species [Brown and Mathieson, 1958]. The kinetic order in the concentration of trichloroacetic acid increases from 1 in the highly polar nitroethane to 2 in the less polar 1,2-dichloroethane to 3 in neat styrene. Another effect of reaction medium on polymerization is the observation that the polymerization rate in some systems (e.g., styrene tri ic acid and N-vinylcarbazole n-butylmagnesium bromide) becomes independent of or inversely proportional to the monomer concentration at higher monomer concentrations [Biswas and John, 1978; Chmelir and Schulz, 1979; Gandini and Cheradame, 1980; Hatada et al., 1980; Sawamoto et al., 1977, 1978]. The mechanism for this behavior may be different depending on the reaction system. Monomer may form an unreactive complex with initiator or coinitiator. Varying the monomer concentration may alter concentrations of ionic propagating species through a change in solvent polarity [Roth et al., 1997]. Plesch [1993, 2001, 2002] cautioned that propagation rate constants may be low by an order of magnitude if calculated on the assumption of a rstorder dependence on monomer. However, data from isobutylene polymerization indicate that this complication is not present, propagation rate constants are independent of monomer concentration [Mayr et al., 2002]. 5-2f-3 Counterion Effects
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It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed; see Sec. 5-3e-2.) 5-2g Living Cationic Polymerization
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Living polymerizations are polymerizations without chain-breaking reactions (Sec. 3-15). For a highly puri ed reaction system devoid of impurities, the concentration of propagating species remains constant throughout the polymerization and even after 100% conversion. A plot of M n versus conversion is linear. Further, after 100% conversion, additional polymerization takes place by adding more monomer to the reaction system. The molecular weight of the polymer increases further since the propagating centers remain intact. The utility of living polymers for producing polymeric materials with different architectures and well-de ned end groups was discussed in Sec. 3-15. The rst living polymerizations were achieved in anionic polymerizations of styrenes and 1,3-dienes in the mid-1950s (Sec. 5-3b-1). Some of the potential of living anionic polymerization has been realized with commerical processes for block copolymers of styrene with butadiene and isoprene.
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Living anionic polymerization is relatively easy to achieve if one has a well-puri ed system without water or other adventitious agents that terminate anionic propagating centers by proton transfer. Anionic propagating systems do not have a built-in termination reaction, anionic centers are reasonably stable under a wide range of polymerization conditions. It has been much more dif cult to achieve living cationic polymerizations (LCPs), even in well-puri ed systems and even when the components of the reaction system have been carefully chosen so that nucleophiles are not present to terminate the cationic centers. Cationic propagating centers have a facile built-in termination reaction transfer of b-protons to monomer, counterion or some other basic species present. In spite of this formidable problem, LCP has been achieved starting with major advances in the early 1980s [Faust and Kennedy, 1987; Faust et al., 1986; Higashimura et al., 1988; Higashimura and Sawamoto, 1985; Kaszas et al., 1990, Kennedy, 1990, 1999; Kennedy and Ivan, 1992; Matyjaszewski and Sawamoto, 1996; Sawamoto, 1991; Sawamoto and Higashimura, 1986, 1990; Sawamoto et al., 1987]. To achieve LCP, one needs to start by choosing the initiator, coinitiator, and other components of a reaction so that there is no nucleophile present that can irreversibly terminate the propagating cationic species. Basic components also need to be avoided to minimize b-proton transfer. However, even with the most judicious choice of reaction system, b-proton transfer is still present because monomer itself is a base. One needs to minimize b-proton transfer to monomer to achieve LCP. Both free ions and ion pairs are highly reactive species with very short lifetimes (less than several seconds), and both propagation and transfer are very fast. Very fast reactions result in uncontrolled exotherms, the reaction temperature increases, and transfer increases relative to propagation because the activation energy for transfer is greater than that for propagation. The result is a lack of control and a nonliving polymerization. LCP is achieved by slowing down the reaction, speci cally, extending the lifetime of the propagating centers, so that signi cant propagation occurs prior to b-proton transfer to monomer. Lower temperature is one reaction parameter that is used to achieve this objective. The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems; most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene
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