Solvation by monomer

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Although, as has been pointed out, many authors adopted the idea that the carbenium ions can be solvated by monomer, no one before the present author has attempted to explain why in certain situations a monomer molecule could form a stable complex with the carbenium ion instead of reacting with it [12], In other words, the conditions under which a monomer molecule can act as a solvator to a carbenium ion instead of reacting with it have not been explained hitherto. From a different point of view, we need to find out what it is that may inhibit the addition of such a cation to an alkene when they first meet. 

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. 

Ion pairs and free ions are specifically solvated by monomer. This is the reason for the small difference in the monomer addition rates to ion pairs or free ions [135, 136]. 

Controlled/living systems can be usually obtained when the polymerization is sufficiently slow and when either nucleophilic anions or additives are present (Sections IV and V). This means that the proportion of carbenium ions should be low and conversion to dormant species, fast. Nevertheless, under such conditions cationic species can be detected by dynamic NMR, by ligand exchange, salt, and solvent effects, and by other methods discussed in Chapters 2, 3, and in this section. Under typical controlled/living conditions, dormant species such as onium ions and covalent esters predominate. It is possible that the active species are strongly solvated by monomer and by some additives. These interactions may lead to a stabilization of the carbocations. However, in the most general case, this stabilization has a dynamic sense and can be described by the reversible exchange between carbocations and dormant species. 

Solvation by monomer itself was postulated by two of us in the polymerization of B-propiolactone. This is highly polar and powerfully solvating compound. It could be assumed that due to its dipolar feature 6-propiolacto-ne could solvate not only cation but also the growing anion. 

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

For the sake of simplicity in this discussion, Bronsted acids have been referred to as discrete species. Given the extreme acidity of such acids as HSbFs, it must be recognized that they are dissociated in solution and exist as their ions and SbFg strongly solvated by monomer, solvent and initiator fragments. Moreover, in some cases, these ions may be further aggregated into more complex ionic species by homoconjugation... 

---