Anionic polymerization isoprene, with butyllithium

Synthetic cA-l,4-polyisoprene (structure 5.42) is produced at an annual rate of about 100,000 t by the anionic polymerization of isoprene when a low dielectric solvent, such as hexane, and K-butyllithium are used. But, when a stronger dielectric solvent, such as diethy-lether, is used along with w-butyllithium, equal molar amount of tra i -l,4-polyisoprene and cA-3,4-polyisoprene units is produced. It is believed that an intermediate cisoid conformation assures the formation of a cis product. An outline describing the formation of cA-1,4-polyisoprene is given in structure 5.42. 

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 polymerization of thietane and various 2-, 3- and polysubstituted thietanes has been achieved with alkali metals (Li, Na, K, Cs, Rb), naphthyl sodiumn-butyllithium, °" 1,4-dilithio-l, 1,4,4-tetraphenylbutane, and a thiolate anion.Treatment of 3-chlorothietane with aqueous sodium thiocyanate is said to give polymeric material.Polymerization of thietane has been effected with Grignard reagents.Thietane and substituted thietanes have been polymerized with dialkyl zinc reagents.A copolymer has been obtained by treating 2-methylthietane and styrene with -butyllithium a block copolymer has been derived from thietane and isoprene. " Copolymers of thietane and 3,3-dimethylthietane with pivalolactone have been reported. ... 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Nonpolar hydrocarbon monomers such as styrene, isoprene, and butadiene are polymerized in hydrocarbon solvents such as benzene or cyclohexane. Initiation is achieved with the use of alkyllithiums such as sec-butyllithium and molecular mass is controlled by the ratio of initiator to monomer. The living nature of anionic polymerization allows the syntheses of block copolymers by sequential addition of the monomers. After one monomer is exhausted, the chain remains reactive, or living. The addition of the second monomer then continues the polymerization to form a block copolymer. Such techniques are used to synthesize polystyrene-polyisoprene or polystyrene-polybutadiene copolymers (PS-PI or PS-PB, respectively). 

This polymer can then be isolated, redissolved in a suitable solvent and reacted with butyllithium (reaction 15) in order to try to create a terminal secondary anion which may be used to initiate the polymerization of further monomers, such as styrene or isoprene, to form block copolymers. An efficiency of only about 20% is achieved. ... 

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. 

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). 

Burchard [65] was the pioneer for the preparation of functional starshaped pol)miers in nonpolar solvents by an anionic core-first method. To build the cores pure / -DVB was reacted with /i-butyllithium in dilute cyclohexane solution. Suspensions of small crosslinked poly(DVB) noduli were obtained that contained numerous lithium organic sites. In a second step, styrene (or isoprene) was added to the living cores and polymerized. The polymeric species obtained exhibit huge molar mass distribution and rather large polydispersity indices. Even if these star-shaped polymers could exhibit active sites at the outer end of the branches, the efficiency of initiation of a second generation of monomers or of hmctionalization was never given by the authors. 

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