Polyisoprene initiated with alkyllithium

Figure 2. Microstructure of polyisoprene initiated with alkyllithium in cyclohexane...

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. 

High cis-1,4 polyisoprene forms with lithium or alkyllithium initiators in nonpolar solvents, because propagation takes place through essentially covalent or intimate ion-pair lithium to carix)n bonds. An intermediate pseudo-six-membered ring is believed to form during the addition of the diene ... 

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. 

Alkali Metals The direct use alkali metals and alkaline-earth metals as initiators for anionic polymerization of diene monomers as first reported in 1910 is primarily of historical interest because these are uncontrolled, heterogeneous processes [4]. One of the most significant developments in anionic vinyl polymerization was the discovery reported in 1956 by Stavely and coworkers at Firestone Tire and Rubber Company that polymerization of neat isoprene with lithium dispersion produced high di-l,4-polyisoprene, similar in structure and properties to Hevea natural rubber [47]. This discovery led to development of commercial anionic solution polymerization processes using alkyllithium initiators. 

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). 

Ceresa drew attention to the fact that out of 1400 copolymers only 5% of the products had been isolated with a reasonable degree of purity and only 20 species had been properly characterized. Exploitation of anionic polymerization, however, soon improved the situation markedly. The use of alkyllithium initiators, which were initially evaluated by Stavely and co-workers at Firestone for the synthesis of high-cis polyisoprene, in the preparation of block copolymers from styrene and butadiene, and styrene and isoprene, is particularly noteworthy. 

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