Alkyllithium, anionic initiators

Polymerization of butadiene using anionic initiators (alkyllithium) in a nonpolar solvent produces a polymer with a high cis configuration. A high cis-polybutadiene is also obtained when coordination catalysts are used. 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

Anionic guest inclusion compounds, 24 170 Anionic halide complexes, 24 540 Anionic initiators, 24 244-265 alkali metals, 24 245-248 alkyllithium compounds, 24 248-255 1,1-diphenylmethylcarbanions as,... 

In solution-based polymerization, use of the initiating anionic species allows control over the trans/cis nricrostructure of the diene portion of the copolymer. In solution SBR, the alkyllithium catalyst allows the 1,2 content to be changed with certain modifying agents such as ethers or amines. Anionic initiators are used to control the molecular weight, molecular weight distribution, and the microstructure of the copolymer... 

Measurement of reactivity ratios under normal free-radical and CCT polymerization conditions indicates that CCT is a modified free-radical polymerization as expected.434 The reactivity ratios for MMA and butyl methacrylate were used as a mechanistic probe. Reactivity ratios were 1.04 and 0.81 for classical anionic polymerization, 1.10 and 0.72 for alkyllithium/trialkylaluminum initiated polymerization, 1.76 and 0.67 for group transfer polymerization, 0.98 and 1.26 for atom transfer radical polymerization, 0.75 and 0.98 for CCT, and 0.93 and 1.22 for classical free-radical polymerization. These ratios suggest that ATRP and CCT proceed via radical propagation. 

In conclusion, FMC has developed a viable, commercial synthesis of a family of omega-(/-butyldimethylsilyloxy)-l-alkyllithiums that are valuable anionic initiators. A variety of chain lengths are available between the protected hydroxyl ftmction and the carbon-lithium bond. These hydrocarbon soluble initiators afford very high 1,4-microstructure in the polymerization of polydienes, such as... 

Bywater, S., Worsfold, D.J., 1967a. Alkyllithium anionic polymerization initiators in hydrocarbon solvents. J. Organomet. Chem. 10 (1), 1-6. 

Cationic initiation and Ziegler-Natta methods have also been employed successfully in order to obtain poly(vinylferrocene) [14]. Due to the electron-donating nature of a ferrocene substituent, it was initially believed that anionic initiators would not be able to induce the polymerization of vinylferrocene. However, in the early 1990s, living anionic polymerization of vinylferrocene in solution was achieved at low temperatures (-70°C to -30°C) in THE using alkyllithium initiators [15]. Block copolymers of poly(vinylferrocene) with poly(methyl methacrylate), PVEc-b-PMMA (2.5) or polystyrene, PVFc-h-PS, as coblocks were also reported (Scheme 2.1) [15]. 

In this work, a series of diblock polymers were used as dispersing agents to stabilize the polystyrene particles. The method of polymerization is by anionic techniques using alkyllithiums as initiators and alkanes as... 

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. 

The polyaminophosphazene base t-BuP4 was used in combination with alkyllithium as initiator for the anionic polymerization of EO (Scheme 15(a)). The space inside the molecule is sufficient to host the compact lithium cation and the base works as a cryptand for Li ions with the polar amino and imino groups located inside the globular molecule and the outer shell formed by alkyl substituents. The equilibrium between complexed lithium alkoxide ion pairs and reactive free anions is thus shifted allowing polymerization. 

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. 

Commercially, the poly(styrene-Aelastomer-Astyrene) materials are made by anionic polymerization (7,45—47). An alkyllithium initiator (RLi) first reacts with styrene [100-42-5] monomer ... 

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. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

The polymerization was carried out in THF under the conditions of high vacuum or argon atmosphere with a catalytic amount of alkyllithium as an initiator. Anionic polymerization of 3a with n-BuLi in THF followed by quenching with ethanol afforded polymer 6 in 56 % yield. The molecular weight distribution of the polymer was determined by gel permeation chromatography (GPC), calibrated by polystyrene standards, with chlorofrom as eluent Mn = 6.1xl0"4, Mw/Mn = 1.3. 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

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). 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. Alkyls or aryls of the heavier alkali metals are poorly soluble in hydrocarbons, a consequence of their more ionic nature. The heavier alkali metal compounds, as well as alkyllithiums, are soluble in more polar solvents such as ethers. The use of most of the alkali metal compounds, especially, the more ionic ones, in ether solvents is somewhat limited by their reactivity toward ethers. The problem is overcome by working below ambient temperatures and/or using less reactive (i.e., resonance-stabilized) anions as in benzylpotassium, cumylcesium and diphenylmethyllithium. 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

Nucleophilic substitution of the chlorine atom present in 2-chlorothiepane (132) using Grignard reagents has provided a synthetic route to 2-methyl- (133) or 2-phenyl- (134) thiepanes (equation 27) (69JHC115). An a-sulfinyl carbanion (114), generated by alkyllithium attack on thiepane 1-oxide (115), was found to act as nucleophile in the synthesis of cis and trans sulfoxides of thiepane (133 equation 28) (78TL5239). Polymerization of 2-thiepanone (135) has been initiated by attack of the f-butoxide anion and concomitant liberation of a thiolate anion (Scheme 26) (64MI51700). 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

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. 

The latter three are obtained by solution polymerization technique with alkyllithium initiator through the anionic mechanism. For these materials, the analysis of block sequences is also an interesting subject in the area of TLC application. However, because a somewhat different principle has to be applied to achieve separation by the difference in block sequences, this subject will be discussed in a subsequent section (cf. Section IV.2.). 

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 ... 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

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... 

The use of alkyllithium initiators which contain functional groups provides a versatile method for the preparation of end functionalized polymers and macromonomers. For a living anionic polymerization, each functionalized initiator molecule produces one macromolecule with the functional group from the initiator residue at one chain end and the active carbanionic propagating species at the other chain end. 

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). 

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