Initiator alkyllithium

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. 

Table 2. Association Numbers and Fractional Kinetic Orders for Alkyllithium Initiators...

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 ... 

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. 

The association phenomena occurring with alkyllithium initiators in nonpolar solvents results in very low polymerization rates. A typical styrene or isoprene polymerization by... 

Another complication introduced by the associative properties of organolithium solutions in non-polar solvents is the fact that the alkyllithium initiators are themselves associated and can be expected to "cross associate" with the active polymer chain ends. Thus some of our studies (26) on the effect of added ethyl lithium on the viscosity o -polyisoprenyl lithium solutions in n-hexane support the following association equilibrium... 

Lewis bases effect dramatic changes in microstructure, initiation rates, propagation rates, and monomer reactivity ratios for alkyllithium—initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents... 

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. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

EVALUATION OF ALKYLLITHIUM INITIATED BUTADIENE-STYRENE RANDOM COPOLYMER (SOLPREN 1204) IN COMPOUNDED STOCK... 

Copolymerization. The copolymerization of butadiene-styrene with alkyllithium initiator has drawn considerable attention in the last decade because of the inversion phenomenon (12) and commercial importance (13). It has been known that the rate of styrene homopolymerization with alkyllithium is more rapid than butadiene homopolymerization in hydrocarbon solvent. However, the story is different when a mixture of butadiene and styrene is used. The propagating polymer chains are rich in butadiene until late in reaction when styrene content suddenly increases. This phenomenon is called inversion because of the rate of butadiene polymerization is now faster than the styrene. As a result, a block copolymer is obtained in this system. However, the copolymerization characteristic is changed if a small amount of polar solvent... 

Since the reversal of activity of butadiene with respect to styrene in alkyllithium system has been observed (12), it would be of interest to find out whether the inversion phenomenon still holds in the case of the lithium morgholinide system. Four temperatures, namely 30, 40, 50 and 60 C were chosen for this study. At 30°C polymerization temperature the curve is characteristic of block copolymerization when one plots percent bound styrene vs percent conversion (Fig. 1). Initially, a small amount (/>/3%) of styrene is polymerized. This is followed by a block of butadiene. The remaining styrene is then polymerized after all the butadiene is consumed. This result is identical to the alkyllithium initiated copolymerization. 

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.). 

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,... 

Quantitative Analysis of Alkyllithium initiator Solutions. The amount of carbon-bound lithium is calculated from the difference between the tolal amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, ally I chloride, or ethylene dibromide. 

Copolymerization Initiators. The copolyineri/ution of styrene and dienes in hydrocarbon solution wilh alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios lor styrene (r, < 0.11 and dienes (rj > 10). In urder to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such us tetrahydrofuran or an alkali metal alkoxide (MtOR. where Ml = Na, K. Rb, or Cs>. 

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. 

Quantitative Analysis of Alkyllithium Initiator Solutions. Solutions of alkyllithiiim compounds frequendy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allylic and benzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

Relative Reactivity of Alkyllithium Initiators Styrene menthyllithium(2) > sec-BuLi(4) > /-PrLi(4-6) >... 

In the alkyllithium initiated polymerizations of vinyl monomers, Lewis bases such as ethers and amines alter the kinetics, stereochemistry, and monomer reactivity ratios for copolymerization. In general, the magnitude of these effects has been directly or indirectly attributed to the extent or nature of the interaction of the Lewis base with the organolithium initiator or with the organolithium chain end of the growing polymer. Unfortunately, all of these observed effects are kinetic in nature, and therefore the observed effects of solvent represent a composite effect on the transition-state versus the ground state as shown below in Eq. (6), where 5 represents the differential... 

Polymers. The polymers used in the blending experiments were prepared by anionic polymerization using an alkyllithium initiator and a chemical randomizing agent to control monomer sequence, in the manner described by Hsieh and Wofford (3). Randomness was checked in each case by measuring the styrene content as a function of conversion. Table I gives descriptive data for these polymers. 

The historical development of alkyllithium-initiated polymerization of olefins and diolefins for the synthesis of elastomeric materials is of interest not only because of its scientific and technological significance but also because of the insight it provides into the thinking and methodology of polymer (elastomer) researchers. 

The lithium and alkyllithium initiation of diene polymerization has, from the earliest times, remained in the shadow of other, apparently more important, initiator systems. However, it has now become clear that the alkyllithium catalyst is the most efficient, initiator system at present available for diene polymerization. That organolithium initiators are not used much more widely is due largely to economic considerations,... 

Alkyllithium initiation does not have the specificity of the coordination... 

It is now apparent that the alkyllithium initiators actually have some very important advantages over both free-radical and coordination catalysts ... 

Polybutadiene made by alkyllithium initiators is essentially free of the gel and color sometimes associated with the transition-metal catalysts and thus occupies a large market as an impact modifier in plastics. 

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