Systems organolithium

In solution, dipole-dipole interactions constitute a relaxation mechanism, and the dipolar relaxation which is the basis for the well-known nuclear Overhauser effect (NOE), mostly used in the homonuclear H, H case. The 2D HOESY method between H and Li has been used to obtain structural information of many organolithium systems in solution and this field was reviewed in 1995. Li is commonly used as the relaxation is dominated by the dipole-dipole mechanism and the relaxation time is relatively long. Knowledge of the proximity of the lithium cation relative to protons in the substrate is used to derive information about the structure and aggregation of organolithium systems in solution. In a few cases quantitative investigations have been made °. An average error of the lithium position of ca 0.2 A was reported. 

It was not until the study of methyllithium and dilithiomethane by Yannoni, Lagow and coworkers that solid state C CP/MAS NMR was applied to organolithium systems (Figure 6) . However, as mentioned above, heteronuclear dipolar Li, C coupling unfortunately complicated the observation of the single C resonance in this particular case, even if Li decoupling in a Li-enriched compound was used. The need for... 

Wen and Grutzner used, among other NMR parameters, the QSC of the lithium enolate of acetaldehyde to deduce that it exists as tetramers of different solvation in THF and THF/n-hexane solvent systems . However, the most thorough study of Li QSC and the most interesting in the present context was reported by Jackman and coworkers in 1987167 -pjjg effects on the QSC values of both aggregation and solvation in a number of organolithium systems was studied in this paper, i.e. different arylamides, phenolates, enolates, substituted phenyllithium complexes and lithium phenylacetylide. 

Kinetics in Non-Polar Media. Polymerization of vinyl monomers in non-polar solvents, i.e., hydrocarbon media, has been almost entirely restricted to the organolithium systems (7), since the latter yield homogeneous solutions. In addition, there has been a particularly strong interest in the polymerization of the 1,3-dienes, e.g., isoprene and butadiene, because these systems lead to high 1,4 chain structures, which yield rubbery polymers. In the case of isoprene, especially, it is possible to actually obtain a polymer with more than 90% of the eis-1,4 chain structure (7, 8, 9), closely resembling the microstructure of the natural rubber molecule. 

On the basis of these studies, we can conclude that while both types of exchanges outlined above can occur in organolithium systems, the specific details and relative rates of reactions are very dependent on the solvent and the alkyl group involved. 

Table II. Absolute Propagation Rate Constants in Organolithium Systems...

Most of the data referred to above were obtained in earlier work, and were based on infrared spectroscopy. In recent years, more reliable data were obtained by means of NMR spectroscopy, using both and resonances (9-14). Some of these investigations suggested that, aside from the dramatic effects of polar solvents on the chain structure in organolithium systems, there were some subtle effects even in non-polar media, e.g., caused by initiator concentration and type of non-polar solvent. Sinn and coworkers ( ), for example, used infrared spectroscopy to show an effect of butyl lithium concentration on the chain structure of polyisoprene and polybutadiene. Hence an extensive study was carried out recently (, M) on the influence of reaction parameters on the chain structure of polybutadiene and polyisoprene prepared in non-polar media. 

In addition, the organoalkali initiators only work effectively with the conjugated monomers. They are ineffective with the olefins, or even with ethylene. (Some success has been reported (A8) in the polymerization of ethylene to a reasonably high molecular weight in highly chelated organolithium systems. However, these polymerizations required relatively higher temperatures and showed much evidence of termination reactions.)... 

Unlike sodium naphthalene, which requires the presence of highly solvating solvents, such as tetrahydrofuran (THF), the organolithium systems can operate in various polar and nonpolar solvents such as ethers or hydrocarbons. However, the rates are much slower in the latter than in the former solvents. Hence, if the initiation reaction (Eq. (2.67)) is very much slower than the propagation reaction, the molecular weight distribution may be considerably broadened (Hsieh and McKinney, 1966). This does not, of course, vitiate the living polymer aspect of the polymerization, which has been shown (Morton et al., 1963) to operate in these systems, regardless of type of solvent, if side reactions do not intervene. 

Unhke sodium naphthalene, which requires the presence of highly solvating solvents, such as tetrahydrofuran (THF), the organolithium systems can operate in various polar and nonpolar solvents such as ethers or hydrocarbons. However, the rates are much slower in the latter than in the former solvents. 

As is well known, homogeneous anionic polymerization is especially well suited to the synthesis of various and unique polymers, because of the non-terminating character of the polymerization. The two main avenues that have been explored with organolithium systems in our laboratories are l) block copolymerization, and 2) synthesis of polymers having functional end groups. 

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