Effect of lithium butoxide additives on the anionic propagation of polystyryllithium in ethereal solvents

Abstract

The addition of tertiary butoxide lithium ( t-BuOLi) was found to slow down the anionic propagation of polystyryllithium (PStLi) in tetrahydropyran (THP), whereas, the addition of normal butoxide lithium ( n-BuOLi) increases the rate over the investigated ranges of PStLi concentrations. Both lithium butoxides, which are tetrameric in ethereal solvents, also increase the conductance of PStLi in THP solution much more than expected on the basis of the separate conductances of PStLi and the lithium alkoxides and the increase is more important in the case of the addition of n-BuOLi than in that of t-BuOLi. These phenomena are fully accounted for by a similar mechanism as that invoked to explain the influence of the addition of lithium chloride (LiCl), which contrasts at first sight with the lithium alkoxides by displaying both a retarding and an accelerating effect at the lower and the higher concentrations respectively of the investigated PStLi concentration domain. This common cause of the observed phenomena consists of the dissociation of the dimers of LiCl or the tetramers of the lithium alkoxides into the free Li +-ion and the multiple anions (ClLiCl − in the case of LiCl and (BuOLi) 3BuO − in the case of the lithium alkoxides) but also of the often overlooked Li +-ion scavenging reaction by the LiCl dimers or the lithium alkoxide tetramers producing multiple cations. The first reaction providing Li +-ions represses the ionic dissociation of PStLi by a common ion effect reducing thereby the amount of free PSt −-anions which are the main contributors to the rate of propagation and resulting therefore, in retardation. The second one scavenging Li +-ions reduces the concentrations of free Li +-ions and increases therefore, the concentration of reactive free polystyryl anions and as a consequence accelerates the propagation reaction. The concentration of PStLi at which a crossover occurs from retardation to acceleration lies for the addition of LiCl in the investigated range of PStLi concentrations. For the addition of t-BuOLi calculations show that this crossover concentrations lies above the highest investigated PStLi concentration, whereas, for the addition of n-BuOLi it lies below the lowest investigated PStLi concentration giving the impression that t-BuOLi only retards the propagation and that n-BuOLi only accelerates but in actual fact the same mechanism is operating in all three cases.