Initiators in hydrocarbon solutions

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. 

ABSTRACT. A new class of protected hydroxyl containing functionalized initiators were recently disclosed by the Defense Evaluation and Research Agency (DERA). These novel initiators have the general structure TBS-0-(CH2)n-Li. Excellent solubility in hydrocarbon solvents was exhibited by these materials which allowed the preparation of telechelic, high 1,4-microstructure polybutadienes. The two-step synthesis of these functionalized initiators from commercially available raw materials will be presented in detail. The first step involved reaction of an omega-haloalcohol with /-butyldimethylsilyl chloride, in the presence of an acid acceptor, to form the precursor. This precursor was then reacted with lithium metal in a hydrocarbon solvent to afford a solution of the functionalized initiator. The thermal stability of these initiators in hydrocarbon solution will also be presented. The application of the precursors and functionalized initiators in anionic polymerization of dienes will be briefly discussed. 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

When a mixture of styrene and 1,3-butadiene (or isoprene) undergoes lithium-initiated anionic polymerization in hydrocarbon solution, the diene polymerizes first. It is unexpected, since styrene when polymerized alone, is more reactive than, for example, 1,3-butadiene. The explanation is based on the differences of the rates of the four possible propagation reactions the rate of the reaction of the styryl chain end with butadiene (crossover rate) is much faster than the those of the other three reactions484,485 (styryl with styrene, butadienyl with butadiene or styrene). This means that the styryl chain end reacts preferentially with butadiene. 

The relative reactivities of alkyllithiums as polymerization initiators are intimately linked to their degree of association. In the following the average degree of association in hydrocarbon solution, where known, is indicated in brackets after the alkyllithium. For sryrene polymerization,... 

In benzene solution, measurements have been made of the rate of reaction of butyllithium with styrene (27), l l-diphenylethylene (6), and with fluorene (7). In each case the reaction was first order in olefin and close to one-sixth order in butyllithium. This latter, fractional order has been attributed to the sixfold association of lithium alkyls in hydrocarbon solution. The actual species active in initiation is the monomeric butyllithium in equilibrium with the hexamer. 

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]. 

The title compound was initially synthesized by the pyrolysis of Re3H3(CO)i2 at 190° in hydrocarbon solution. Treatment of the complex with carbon monoxide at atmospheric pressure gradually converts it into higher carbonyls, as indicated in the reaction sequence below. At slightly elevated temperatures, the reaction is much faster and hydrogen is evolved. The suggestion that the reverse transformation might be possible led to the current synthesis. The direct hydro-... 

Alkyllithium initiators are primarily used as initiators for polymerizations of styrenes and dienes. They effect quantitative living polymerization of styrenes and dienes in hydrocarbon solution. In general, these alkyllithium initiators are too reactive for alkyl methacrylates and vinylpyridines. n-Butyllithium is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched structures. Because of its high degree of association (hexameric), n-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50 °C) and in the presence of small amounts of Lewis base to increase the rate of initiation relative to propagation and thus obtain polymers with narrower molecular weight distributions [55, 57]. 

Scheme 7.8 Mechanism of styrene initiation with n-butyllithium in hydrocarbon solution.

Since n-butyllithium is aggregated predominantly into hexamers in hydrocarbon solution [44], the fractional kinetic order dependency of the initiation process on the total concentration of initiator has been rationalized on the basis that unassociated n-butyllithium is the initiating species and that it is formed by the equilibrium dissociation of the hexamer as shown in Scheme 7.8. 

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

TABLE 7.6 Anionic Copolymerization Parameters in Hydrocarbon Solution with Alkyllithium Initiators [45, 56, 199-205]... 

---