Alkyllithium, anionic initiators reaction

As mentioned, LiAlH4 in refluxing THF was the initial system introduced to reduce preformed tosyl-hydrazones to hydrocarbons and a number of successful conversions have been reported, representative examples of which are presented in Table 5. Alkene side products often accompany the hydrocarbon products,a result attributed to proton abstraction from the a-carbon of intermediate (59), leading to a vinyldiimide anion (64), followed by N2 expulsion and protonation during work-up (Scheme 3). With certain ketones, including 17-keto steroids, alkenes are the major s or sole product (entries 7-9, Table 5). This side reaction mimics the elimination obtained upon treatment of to-sylhydrazones with other strong bases (i.e. alkyllithiums, the Shapiro reaction 29). Note that use of LiAlD4 introduces one deuterium (with H2O work-up) or two deuteriums (with D2O work-up entries 5 and 6, Table 5, respectively). 

It is desirable to develop anionic functionalization reactions using alkyllithium initiators that proceed in hydrocarbon solution at room temperature and above. These conditions maintain the uniqueness of alkyllithium-initiated anionic polymerization, in particular the ability to prepare polydienes with high 1,4-microstructure, and these conditions are also analogous and relevant to commercial anionic polymerization processes. In contrast, functionalizations that proceed efficiently only at -78 °C in tetrahydroftiran (THF) are of more limited utility. 

Copolymers of 1,3-butadiene and styrene (SBR) are elastomers of great technical importance that are used for automobile tires [465-474]. In addition to a free-radical process, they can be made by anionic initiation with alkyllithium compounds. In polar solvents the reaction rate of styrene anions with 1,3-butadiene is greater than with styrene, whereas in polar solvents this is just the other way around. The copolymerization parameter rj for styrene-butadiene is 0.03 in hexane and 8 in THF r2 is calculated as 12.5 in hexane and 0.2 in THF [465]. Therefore, a strong dependence of the styrene content of the polymers on the degree of conversion is observed in discontinuous polymerizations. 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

However, the decrease in rate was initially explained by the difference in reactivity of the alkyllithiums present, i.e., whether or not a primary or secondary anion was involved. Yet, the propagation reaction was quite different because the structure of the active species did not vary after the addition of the first ethylene unit. The fact that the constant rate (the rate after 1 mole of ethylene is consumed per mole of BuLi) varied linearly with initiator concentration at TMEDA /-BuLi = 1.0 indicated that a monomeric complex was responsible for the propagation reaction (PELi-TMEDA) [Eqs. (13), (14)]. 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

Coupled (star branched) and end-functional PBs are possible using alkyllithium technology because of the presence of a living anion on the chain end. This anion is available for further reactions, which is discussed later. Upon the addition of polar agents (modifiers), such as ethers or amines, the alkyllithium initiators can produce PBs with vinyl contents up to lOO /o. The vinyl content can be controlled by the ratio of... 

Anionic polymerisation employs nucleophiles such as alkyllithiums, alkoxides or hydroxide as the initiator. Hydroxide, for example, adds to an electron-deficient alkene to form the most stable carbanion (in a Michael-type reaction). The carbanion then adds to another electron-deficient alkene to build the polymer chain. The polymerisation is terminated by, for example, protonation. 

The methodology of living anionic polymerization, especially alkyllithium-initiated polymerization, is very useful for the preparation of chain-end functionalized polymers with well-defined structures (9,10). Since these living polymerizations generate stable, anionic polymer chain ends (P Li ) when all of the monomer has been consumed, post-polymerization reactions with a variety of electrophilic species can be used to generate a diverse array of chain-end functional groups as shown in eq. 1,... 

Side reactions like the one shown above can be minimized by using less nucleophilic initiators and low temperatures. This can yield living polymerizations of aciylic and methacrylic monomers. In addition, it is possible to add conunon ions like LiCl to alkyllithium to tighten the ion pairs of the propagating anion-counterion species. That also increases the tendency to form living poly-mers. This approach, however, offers only limited success. 

Alkyllithium Compounds. Although anionic polymerization of vinyl monomers can be effected with a variety of organometallic compounds, alkyllithium compoimds are the most useful class of initiators (1,21,24,33). A variety of simple alkyllithium compounds are readily available commercially in hydrocarbon solvents such as hexane and cyclohexane. They can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. 

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