Aggregation, alkyllithium-initiated

The use of aliphatic solvents causes profound changes in the observed kinetic behavior for the alkyllithium initiation reactions with styrene, butadiene, and isoprenc. i.e.. Ihe inverse correspondence between the reaction order dependence for alkyllithium and degree of organolithium aggregation is generally not observed. Also, initial rales of initiation in aliphatic solvents are several orders of magnitude less lhan those observed, under equivalent conditions, in aromatic solvents. Furthermore, pronounced induction periods are observed in aliphatic hydrocarbon solvents,... 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. A common example is n-butyllithium which is usually available as a solution in n-hexane. The C-Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. Initiation is thus fairly slow in hydrocarbon media. Addition of tetrahydrofuran to this solvent increases the concentration of unaggregated initiator (which is more active for initiation) by forming a 1 1 complex with this compound. Alkyls and aryls of the heavier alkali metals, such as Na and K, are poorly soluble in hydrocarbons because of the greater ionic character of the Na—C and K-C bonds. 

In aliphatic solvents the inverse correspondence between reaction order dependence for alkyllithium and degree of organolithium aggregation is not observed (49). In addition, the rates of initiation in aliphatic solvents are several orders of magnitude less than in aromatic solvents. Most reaction orders for alkyllithium initiators in aliphatic solvents are close to imity. These results suggest that in aliphatic solvents the initiation process may involve the direct addition of monomer with aggregated organolithium species (eq. 26) to form a cross-associated species. 

The formation of cross-associated species would be expected to complicate the kinetics and lead to variable reaction orders as a function of conversion. The observation of pronounced induction periods has been ascribed to the enhanced reactivity of the mixed (ie, cross-associated) aggregated species. The effects of cross-association provide at least a partial explanation for the discrepancies reported in the literature for the kinetic order dependencies on alkyllithium initiator concentration thus, only in the initial stages is it likely that a detailed interpretation of the mechanism is possible. 

The formation of difunctional alkyllithium initiator from the adduct of 5-BuLi and PEB in nonpolar solvent enabled the synthesis of H-shaped polymers, regular combs, centipedes, and barbwires. ° Quirk and Tsai ° prepared a trifimc-tional alkyllithium initiator based on l,3,5- ri5(l-phenylethenyl) benzene and used it for the synthesis of three-arm star polymers in benzene. Three-arm star polymers exhibiting multimodal distributions were obtained due to the intermolecular aggregation of initiator rendering initiation incomplete or slow. The addition of a polar solvent such as THF (THF/Ii = 20) to the initiator solution in benzene produced three-arm star polymer with narrow MWD. ° ... 

N,N-diethylacrylamide (DEAAm) in polar and nonpolar solvents using alkyllithium initiators is complicated due to the presence of slow aggregation dynamics of the propagating amido enolate ion pairs similar to ester enolate ion pairs in alkyl (meth) acrylate polymerization. Attempts were made to use different initiators in combination with coordinating ligands to control the polymerization, and only minimum control on molecular weight, MWD, and the stereostructure of the polymers was obtained."""-"""... 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

Simple alkyllithium compounds arc aggregated in solution, in the solid slate, and even in the gas phase. The important differences between the v arious alkyllithium compounds arc their degrees of aggregation in solution and their relative reactivity as initiators for anionic polymerization of... 

It is clear that, in general, the less associated alkyllithiums are more reactive as initiators than the more highly associated species. The effect of solvent on initiator reactivity is also consistent with the importance of association phenomena. Aromatic solvents, which tend to decrease the average degree of association and promote dissociation processes of aggregates, are reported to lead to initiation rates which are two to three powers of ten faster than in aliphatic solvents 30,40). 

Hall U3>, Hsieh 106>, Roovers and Bywater107), Tanlak and co->workers114), and Bordeianu and co-workersI1S) followed the initiation of styrene under polymerization conditions in aromatic or alkane solvents using ethyllithium, z-propyllithium, or isomers of butyllithium. Without exception, these authors found a first power dependency of initiation rate on total active center concentration. Hsieh s results106) and those of Roovers and Bywater 107, also indicate that the first order character for initiation is independent of the degree of association (4 or 6) of the alkyllithium. The first order dependence of the initiation step on total active center concentration is also maintained over the period where cross-aggregated structures, PSLi (RLi)x, are present. 

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

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