The influence of π-complexing additives on the dissociation of organolithium compounds in non-polar medium: a long time unexplored area in anionic polymerization

In 1965 0ʹDriscoll et al. showed that the addition of a π-complexing agent such as 1,2,4,5-tetramethylbenzene (durene) has a marked effect on the anionic propagation of polystyryllithium (PStLi) in benzene, comparable to but much weaker than the effect of tetrahydrofuran (THF) on this polymerization described by Worsfold and Bywater. Indeed much higher quantities of durene than of THF were necessary to bring about a similar effect i.e. an initial increase in the rate of propagation, passing then through a maximum to decrease afterwards with increasing amounts of durene or THF. In analogy with THF, dissociating the ionic aggregates through formation of a σ-complex with the Li+-cation, this behavior was attributed to a similar dissociating effect of durene, forming a π-complex with the Li+-cation. A similar behavior was found for PStLi in cyclohexane, requiring however, even higher quantities of durene than in benzene. In order to further substantiate this dissociative capacity of π-complexing agents in general and of durene in particular, a preliminary study of the kinetics of propagation of PStLi in cyclohexane in the presence of 0.27 M durene was carried out. We could show that in these circumstances the dissociation constant (Kd) of the dimers of PStLi, which is estimated to be around 10−7 M in pure cyclohexane, was equal to about 4×10−3 M when 0.27 M durene was added. This means that at a formal concentration of PStLi of 10−3 M, 75% of the dimeric aggregates are disrupted. Also other π-complexing agents are studied in this work, such as benzene, hexamethylbenzene (HMB), tertamethylethylene (TME) and tetraphenylethylene (TPhE).