ION-PAIRS, CATIONS AND ANIONS IN TERNARY SOLUTIONS OF BIS(POLYSTYRYL)MAGNESIUM AND MAGNESIUM BROMIDE IN TETRAHYDROFURAN

A conductometric method is developed for the study of the ternary solutions in tetrahydrofuran of bis(polystyryl)magnesium (PStMgPSt) containing various proportions r of magnesium bromide. The concentration of bis(polystyryl)magnesium varies from 10-(4) to 2 . 10(-3) mol . dm-3, the ratio r from 0 to 17,4 and the temperature from + 20-degrees-C down to -60-degrees-C. The addition of MgBr2 provokes the replacement of the ion pairs PStMgPSt by PStMgBr, the dismutation constant being at 20-degrees-C K(dism) = 42 +/- 2 with a molar enthalpy of -5 +/- 2 kJ . mol-1. The dismutation is chiefly ruled by a conjugated steric effect that lowers the stability of PStMgPSt. In this concentration range, aggregation of styryl ion pairs is negligible. Moreover for a given r the degree of dissociation of the polymer is independent of its concentration. This is due to the complete predominance in this concentration range of the triple anions Mg(PSt)3-, MgBr(PSt)2-and MgBr2PSt-, approximately in the proportions of the ion pairs f(pp), f(bp) and f(bb) as calculated from the dismutation constant. The triple anions MgBr3- are still completely negligible, as are also the triple cations. The single cations MgPSt+ and MgBr+ are approximately in the proportion f(pp) . K(dism)/f(bp), the latter ones becoming predominant above r = 3.