Carbon-lithium bond polarity

Methyllithium has a polar covalent carbon-lithium bond... 

Flowever, the electrons of a covalent bond are not necessarily shared equally by the bonded atoms, especially when the affinities of the atoms for electrons are very different. Thus, carbon-fluorine and carbon-lithium bonds, although they are not ionic, are polarized such that the electrons are associated more with the atom of higher electron affinity. This is usually the atom with the higher effective nuclear charge. 

The allylic- and benzylic-lithium active centers, which can be characterized as having polarized covalent carbon-lithium bonds in hydrocarbon solvents, have been extensively studied with regard to their structure, their kinetic behavior in the propagation event, and their association states. These latter two topics are the subject of this... 

Table 8). The same 2pz function is unoccupied in the calculation of the methyl radical in the presence of a ghost lithium atom using the geometry of CH3Li. This shows that the lithium 2pz orbital acts like a normal valence orbital in the description of the C—Li bond and not, as suggested previously,197 198 as a superposition function. The strong charge donation from Li to C is in line with the difference in electronegativity between these atoms, and with the modern picture of a strongly polar carbon-lithium bond.181-183...

In contrast to the monomer, the carbon-lithium bond in the meth-yllithium tetramer 27 is provided by two distinct orbital interactions the SOMO-ax interaction 2a, + la, and the triply degenerate SOMO-t2 interaction 3t2 lt2 (Figure 21). Strikingly, the former gives an essentially covalent electron pair bond of-85.8 kcal/mol the SOMO-a, populations are 1.02 and 0.91 e for the tetramethyl (2a2) and the tetralithium (la2) fragment, respectively (Table 8) The extremely low polarity of this C—Li electron pair bond is due to the very low energy of the (Li )4 la, orbital 3 eV below lithium 2s and only 0.8 eV above (CHj 2a, (Figure 21). As mentioned just before, this is the important... 

The present state of knowledge about the true mechanism of these polymerization reactions is not sufficiently advanced to permit a satisfactory rationalization of these effects. It should be remembered that the growing chains in these systems have been convincingly demonstrated (]J) to be associated in pairs at the site of the carbon-lithium bond. Hence it appears that the incoming monomer must react with the associated complex, which apparently can affect the mode of entry. This undoubtedly can explain the greater extent of cis-1,4 addition in the case of isoprene compared to butadiene. Furthermore, such factors as lithium concentration and presence of different solvents can be assumed to have an effect on the structure and reactivity of the associated carbon-lithium bond at the active chain end. This would certainly be expected for the highly polar carbon-lithium... 

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 problem of the real existence of the five above states of the carbon-metal bond in any specific situation will not be discussed because it is considered comprehensively in Refs. [55-57], It should only be noted that in the case of the carbon-lithium bond all five bond states appear to exist, whereas other alkali metals cannot form the slightly polar carbon-metal bond. We mean by the slightly polar bond the bond in which the electron density distribution may be assumed to be virtually symmetric (pseudosymmetric) from the viewpoint of the correspondence principle. 

It may be tentatively assumed that curve 1 in Fig. 3 does not contradict the very scarce experimental data on the anionic polymerization of symmetric vinyl monomers. For instance, it is known (bibliography see Ref. [593) that ethylene is anionically polymerized on the polarized carbon-lithium bond or the corresponding contact ion pair. However, additional experimental investigations are needed for drawing a more definite conclusion about the validity of curve 1 in Fig. 3. 

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 ... 

---