Carbon-lithium bond solvation effects

Ihe r values show that the relative rate uf entry of the diene and styrene monomers is apparently controlled very closely by the nature of the carbon-lithium bond. Thus, in hydrocarbons the preference is very strong for the dienes, whereas, in the presence of a highly solvating medium such as H -furan, the exact reverse is true. Solvents of intermediate polarity show a lesser effect. Apparently, the effect of the solvent in influencing the charge separation at the carbon-lithium bond profoundly influences the kinetics of the copolymerization. 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

Because of the complicating effects of counterion and solvent associated with anionic polymerization, relatively few reactivity ratios have been determined for anionic systems. Typical reactivity ratios for the anionic copolymerization of styrene and a few other monomers are shown in Table 8.3. Most of the values were determined from the copolymer composition equation [Eq. (7.11) or (7.18)]. A dramatic effect of solvent is seen with styrene-butadiene copolymerization, where a change from the nonpolar hexane to the highly solvating THF reverses the order of reactivity. Again in the case of hydrocarbon solvent, the reaction temperature shows a minimal in uence on reactivity ratios, while in the case of polar solvents, such as THF, the reactivity ratios vary considerably, which has been rationalized by considering the solvation of carbon-lithium bond. Thus as the temperature is increased (from -78°C to 25°C), the extent of solvation by THF is expected to decrease, resulting in more covalent carbon-lithium bond. 

A fairly consistent relationship is found in these and related data. Conditions of kinetic control usually favor the less-substituted enolate. The principal reason for this result is that removal of the less hindered hydrogen is more rapid, for steric reasons, than removal of more hindered protons, and this more rapid reaction leads to the less substituted enolate. Similar results are obtained using either lithium diisopropylamide or triphenylmethyllithium. On the other hand, at equilibrium it is the more substituted enolate that is usually the dominant species. The stability of carbon-carbon double bonds increases with increasing substitution, and it is this substituent effect that leads to the greater stability of the more substituted enolate. Highly substituted enolates, especially if the substituents are bulky, are not solvated effectively, however, and may be present in only minor amounts at equilibrium. 

A more recent study of ethynyllithium included the effect of lithium solvation on dimerization. At the 6-3H-G level successive coordinations of water to form HCCLi(OH2), HCCLi(OH2)2, and HCCLi(OH2)3 have AE of —21, —10, and —5 kcal mol , respectively. As solvation of lithium increases, the dimerization to the corresponding solvated ethynyllithium dimer becomes less exothermic. An interesting further feature is that the dimer with two waters per lithium has D2, symmetry, with each tetracoordinated carbon associated with two terminal ethynyl carbons and two waters. With fewer waters of solvation the coordination number of the lithiums is increased by association with the r-bond of the acetylene. This feature... 

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