Anionic polymerization alkyllithium compounds

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). 

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. 

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

Anionic polymerization of vinyl monomers can be effected with a variety of organomciallic compounds alkyllithium compounds are the most useful class. A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds ate soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. 

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... 

Alkyllithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. -Butyllithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization ofbutadiene, isoprene, and styrene with linear and branched structures. Because of the high degree of association (hexameric), w-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

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]. 

Also, addition of small quantities of Lewis bases such as amines to alkyllithium reagents in hydrocarbons markedly affects reactivity, especially in connection with various anionic polymerization reactions. Findings such as these prompted a number of research groups in the early 1950 s to study in detail the role of Lewis bases in the structures of organolithium compounds (4, 5). In each case it was concluded that coordination complexes form when amines are added to organolithium reagents in hydrocarbons. 

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

Initiation The mechanism of initiation of anionic polymerization of vinyl monomers with alkyllithium compounds and other organometallic compounds is complicated by association and cross-association phenomena in hydrocarbon solvents and by the presence of a variety of ionic species in polar media [3, 4, 45, 48, 55, 56]. The kinetics of initiation is complicated by competing propagation and the occurrence of cross-association of the alkyllithium initiator with the propagating organolithium [55]. Thus, only the initial rates provide reliable kinetic data. 

Elimination of the proton from 1, 4-diene or 1, 3-diene compounds is often used for the preparation of metal dienyl complexes. The following compounds are utilized as deprotonating reagents alkyllithium compounds, (trimethylsilylmethyl )potassium, alkali metal amides, etc. Also useful are alkali metals as sands in the presence of triethylamine, which prevents anionic polymerization of dienes. 

Another possibility is anionic polymerization with alkyllithium in combination with barium compounds such as barium 2,4-pentanedionate [192-194, 203-205]. Also, cobalt(II) chloride in combination with diethylaluminum chloride and triethylamine is used, yielding a polymer with 91% trans-l A and 9% 1,2 structures. 

Natural rubber (hevea) is 98% c/ -l,4-polyisoprene with 2% 3,4-structure. It can be synthesized by anionic polymerization with alkyllithium compounds or with Ziegler-Natta catalysts [220-225]. The polymerization is carried out in solvents. Impurities such as acetylenes, carbonyl compounds, hydrogen sulfide, and water have to be removed [217,226-228]. 

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. 

Syndiotactic 1.2 polybutadiene has also been made by Longiave and Castelli (49) using an anionic cobalt catalyst made from oxygenated aluminum compounds. Less amounts of 1.2-structure were found in polymerizations in hydrocarbon media. Alkyllithium produced only 6.8% 1.2-structure with the remainder being 1.4 cis and trans. 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

Commercial anionically prepared, random SBR polymers (solution SBR) prepared by alkyllithium-initiated polymerization typically have 23% ci s-1,4, 49% trans-1,4, and 28% vinyl microstructure compared to 10% cis-1,4, 70% trans-1,4, and 20% vinyl microstructure for emulsion SBR with the same comonomer composition. Solution SBRs typically have branched architectures to eliminate cold flow. Compared to emulsion SBR, solution random SBRs require less accelerator and give higher compounded Mooney viscosity, lower heat buildup, increased resilience, and better retread abrasion index (179). Terpolymers of styrene, isoprene, and butadiene have been prepared using a chain of single-stirred reactors, whereby the steady-state concentration of each monomer and Lewis base... 

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