Styrene alkyl lithium polymerization

The relative ionic nature of the catalyst required for these monomers has been determined. Spirin, Pres-Yakubovich, Polyakov, Gant-makher and Medvedev (57) studied the alkyl lithium polymerization of styrene, isoprene and butadiene. At high alkyllithium concentrations, styrene polymerized more rapidly than either isoprene or butadiene. As the ionicity was decreased by reducing the alkyllithium concentrations to about 10 moles per liter, the rates of polymerizations of the monomers were nearly the same. 

Dr. N.R. Legge, 1987 Charles Goodyear medalist and, at that time, director of the synthetic polyisoprene program at Shell Development Company, recalled that it was in attempting to provide a solution to this problem of poor "green strength in alkyl lithium polymerized polyisoprene that the styrenic block copolymers were first synthesized. The functional use of the first block copolymers was not as an identifiable monolithic rubber structure but provided a vital function to another identifiable material and lost its identity in this process. 

Fig. 9. The rate of chain propagation in the lithium alkyl initiated polymerization of styrene as a function of concentration of growing chains. The lines drawn have a slope of 0.50 corresponding to a reaction order of one half. (O) Cyclohexane 30 C [62] (A) benzene 30°C [17] ( ) cyclohexane 40°C [61]. The broken line indicates the rates expected in benzene at 40 C from the activation energy determined in ref. 17.

Initiation of anionic polymerization of styrene, dienes and their derivatives by alkyl lithium in hydrocarbon solvents was extensively studied by Ziegler163) and thereafter by many other workers. Since the rates of initiation are often comparable to those of propagation, both processes occur simultaneously and then, while the monomer is quantitatively polymerized, an appreciable fraction of the initiator remains unutilized in the system. Hence, it is advantageous to use fast alkyl lithiums as initiators, especially when a polymer of a narrow molecular weight distribution is the desired product. 

Different mechanisms govern the initiation of polymerization by alkyl lithiums depending upon whether the reaction takes place in aromatic hydrocarbons, like benzene or toluene, or whether it proceeds in aliphatic ones, like cydo-hexane or n-hexane. The following discussion is restricted to polymerization of styrene and the dienes, and the initiation in aromatic hydrocarbons is considered first. 

Kinetics of addition of the tetrameric t-BuLi to 1,1-diphenyl ethylene in benzene was investigated by Evans et al.169. This reaction was found to be first order in the ethylene but, significantly, Zt order in /-butyl lithium. The Zt order dependence of the initiation induced by the tetrameric sec-butyl lithium was observed in the polymerization of styrene or isoprene proceeding in benzene159,164. This is shown in Fig. 25. Both observations lend further support to the schemes involving monomeric alkyl lithiums as the active, initiating species. 

The alkyl-lithium initiated, living anionic polymerization of elastomers was described in 1928 by Ziegler. To polymerize styrene-isoprene block copolymers Szwarc et al., [1956] used sodium naphthalene as an anion-radical di-initiator, while Shell used an organolithium initiator. The polymerization mechanism was described by By water [1965]. 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

The preparations by anionic mechanism of A——A type block copolymers of styrene and butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of c/s-1,4 microstructure [346]. This is discussed in Chap. 4. It is more difficult, however, to form block copolymers from methyl methacrylate and styrene, because living methyl methacrylate polymers fail to initiate polymerizations of styrene [347]. The poly(methyl methacrylate) anions may not be sufficiently basic to initiate styrene polymerizations [345]. 

It should be noted that the preparation of n-type (reduced) polyacetylene using strong organic bases (e.g., alkyl lithium compounds) or more commonly electron transfer reagents (e.g., sodium naphthalide radical anion) employs the two major classes of initiators used in anionic polymerization of monomers such as styrene and butadiene. Reductive doping can also be accomplished by deprotonation of, for example, acetylene/butadiene copolymers and related phenylenepentadienylenes." ... 

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. 

The mechanism of anionic polymerization of styrene and its derivatives is well known and documented, and does not require reviewing. Polymerization initiated in hydrocarbon solvents by lithium alkyls yields dimeric dormant polymers, (P, Li)2, in equilibrium with the active monomeric chains, P, Li, i.e. 

Some new initiators soluble in hydrocarbons were described during the last few years. Organo-lithium compounds form 1 1 complexes with alkyls of Mg 134,135), Zn 136) or Cd l36), and their usefulness as initiators of anionic polymerization of styrene and the dienes was established 137). 

Organolithum compounds (lithium alkyls) are the most valuable initiators in anionic polymerization.120168 169172-175 Since living anionic polymerization requires the fastest possible initiation, sec- or ferf-butyllithium is usually used. Lithium alkyls add readily to the double bond of styrene [Eq. (13.32)] or conjugated dienes and form free ions or an ion pair depending on the solvent ... 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

Sinn, Lundborg and Kirchner (61) have reported that the homogeneous polymerization of styrene in benzene using lithium alkyls at 50° produces a relatively low molecular weight polymer a portion of which is crystallizable by treatment with boiling heptane. 

The radical model cannot be applied for ionic and coordination polymerizations. With a few exceptions, termination by mutual combination of active centres does not occur. The only possibility is to measure the rate of each copolymerization independently. The situation can be greatly simplified for copolymerizations in living systems. The constants ku and k22 can usually be measured easily in homopolymerizations. Also, the coaddition constants fc12 or k2] are often directly accessible when the M] and M2 active centres can be differentiated spectroscopically or when the rate of monomer M2 (M[) consumption at M] M 2 centres can be measured. Ionic equibria, association, polarity of medium and solvation must be respected, even when their quantitative effect is not known exactly. The unusual situations confronting macromolecular chemistry will be demonstrated by the example of the anionic copolymerization of styrene with butadiene initiated by lithium alkyls in hydrocarbon medium. 

Polymers with even narrower mass distributions, e.g. with PDI values close to 1, arise in living polymerization systems, in which no chain termination processes can occur at all, such that all chains remain bound to the metal centre from which they have started to grow at the same time. Living polymerizations, which offer useful opportunities, e.g. with regard to the production of block copolymers by exchange of one monomer for another, occur in anionic polymerizations of styrenes or butadienes such as are induced by simple lithium alkyls. For a-olefin polymerization catalysts of the type discussed above, living polymerizations are rare. These more elaborate catalysts can thus release a newly formed polymer chain within a time interval of typically less than one... 

Since shortly after its discovery by Szwarc et al. [5] in the mid-1950s, living anionic polymerization has been recognized as an ideal route to styrenic block copolymers [6]. To date, living anionic polymerization remains the only commercially important technology for SBC synthesis. The anionic polymerization of styrene and common dienes such as butadiene and isoprene satisfies the criteria outlined above, particularly when carried out in a hydrocarbon solvent and initiated by an appropriate lithium alkyl. 

The kinetics and mechanistic details of the lithium alkyl-initiated anionic polymerization of styrene and diene monomers in hydrocarbon solvents have been the subject of numerous investigations [15]. Some of the first investigations revealed that the propagation reaction was first order in monomer, as might be expected, but followed a fractional order in the lithium alkyl [16]. Most investigators have observed a 0.5 order for the polymerization of styrene. Values have been quoted for the polymerization of butadiene and isoprene ranging from about 0.17 to 0.5, with 0.25 being the most commonly quoted value for both monomers. There is some evidence that the order in lithium for diene polymerization... 

In initiation by an alkali-metal alkyl, the alkyl links up with the monomer to form a carbanion, leaving the metal as a cation to compensate the negative charge. An example is the initiation of styrene polymerization by butyl lithium [70-72] ... 

---