The Reaction of t-Butyllithium with Styrene and Isoprene. A Comparison of Chain Initiation with the Isomers of Butyllithium

Abstract

The use of lithium alkyls as polymerization initiators provides an important method for the production of monodisperse polymers. Surprisingly large differences in initiation rates are observed between straight and branched chain lithium alkyls. s-Butyllithium, for example, initiates several powers of ten more rapidly than n-butyllithium in benzene. Less work has been carried out on t-butyllithium. Hsieh has reported that in cyclohexane the order of efficiency (defined as more complete initiation at lower monomer consumption) is s- > t- > n-butyllithium for the dienes and s > n- > n-butyllithium for styrene. These experiments were carried out using commercial products, and it seemed of interest to repeat these experiments on purified materials as the reactions of lithium alkyls are known to be sensitive to impurities. In addition the exchange reactions of t-butyllithium with other alkyls are known to be slow whereas those of n- and s-butyllitbium are considerably faster. If, as suggested, the slow exchange rate is determined by the rate of dissociation of the predominantly tetrameric t-butyllithium, then it seemed likely that, at least in benzene, some differences in initiation behavior might be expected. In this solvent the initiation mechanism has been suggested to involve a prior dissociation of the aggregates. The rate of addition of t-butyllithium to 1,1-diphenylethylene has already been studied in benzene. At very low monomer concentration, the reaction order in initiator was found to be 0.25, which suggests the dissociative mechanism holds under these conditions. We have, therefore, studied the reaction of t-butyllithium with isoprene in benzene and hexane and with styrene in benzene and cyclohexane. The use of aliphatic hydrocarbon solvents produces quite different initiation behavior to that in aromatic solvents.