SPECIFIC SOLVENT EFFECTS OF DIOXANE AND N,N,N',N'-TETRAMETHYLETHYLENEDIAMINE IN THE ANIONIC-POLYMERIZATION OF STYRENE WITH LI+ AS COUNTERION

The anionic propagation of living polystyryllithium shows a distinctly different behavior depending on the nature of the solvent used (polar or non-polar). To demonstrate the transition of one type of behavior to the other, studies have been carried out in the past in mixtures of benzene and tetrahydrofuran (THF) and mixtures of dioxane and THF. Th e results found in mixtures of benzene and THF could entirely be accou nted for in terms of classical dielectric theory. In contrast to this the reactivity of the ion pair in dioxane -THF mixtures cannot be expl ained in these simple terms. In order to elucidate this controversy, s imilar experiments were carried out in this work in mixtures of N,N,N' ,N'-tetramethylethylene diamine (TMEDA) and THF and the results are co mpared with those previously found in the other mixtures. From this co mparison it can be concluded that in all cases the specific solvation of Li+ is of utmost importance and that it is the solvent-separated io n pairs, the concentration of which is always small compared to that o f the externally solvated contact ion pairs, which are mainly responsi ble for the observed propagation. A direct comparison between dioxane and TMEDA does not seem to hold. Indeed, whereas the bidentate TMEDA r eplaces equally well two THF molecules in the externally solvated cont act ion pairs as in the solvent-separated ion pairs, one must conclude from the comparison of the results that dioxane cannot enter the solv ation shell of the solvent-separated ion pair without destroying it. O n top of that, recent results show that unlace TMEDA, which can act as a bidentate ligand, dioxane, even in its boat-form, cannot do this, s ince the free electron pairs on the oxygen atoms remain too far away f rom the Li+-cation.