SUMMARYThe conductance behavior of the tetraphenylboride salts of the alkaline earth metals (MB,) has been studied in the temperature range from + 20°C to -70°C in tetrahydrofuran (THF). A detailed knowledge of their dissociation into free MB+ and B-ions is desirable in order to better understand their influence in anionic polymerizations of e.g. styrene with alkaline earth metals as counterions. The thermodynamic parameters as well as the X+,-values for the respective salts are taken to indicate a double solvent separated ion pair structure for the calcium and strontium salt represented as B-//M,+//B-and a mixed ion pair structure for BaBz represented as B-, Ba2+/IB-. For MgB, an ion pair structure B-,Mg2+//B-is proposed at higher temperature changing to B-//Mgz +//B-at lower temperatures.
The anionic polymerization of the strontium salt of one-ended living polystyrene (SrS,) was investigated at 20 "C in tetrahydropyran (THP) in the presence of two different concentrations of added tetraglyme. Similarly to Bas, in tetrahydrofuran (THF) and to SrS, in THF and in pure THP, the observed pseudo-first-order rate constant of propagation, /cobs, was nearly independent of the total concentration of salt, their values being 7,5. lo-, s-' and 9 . lo-, s -' , respectively, i. e. about 100 to 120 times higher than in pure THP. This indicates that the propagation occurs mainly via an increased but constant amount of free Sanions resulting from the two already known equilibria SrS, + (SrS) + + S -(KI ) and 2 SrS, + (SrS) + + (SrS,) -( K z ) and the equilibrium of glymation (SrS') + G t G , (SrS)' (Ki). A small not exactly determinable contribution of glymated ion-pairs and/or triple ions, whose rate constants would then probably be of the order of 18 1 . mol-. sand 80 1. mol-' . s -' , respectively, could not be excluded. The glymation constant K i was found to be about 3 . lo6 I ' m o l -' , i.e., approximately 17 times greater than for the Na' cation. Finally, a kinetic experiment with SrS, at 20°C in pure benzene (contaminated, however, with some remaining THP from the preparation of SrS2) indicated that propagation by ion-pairs is possible with a bimolecular apparent rate constant kapp = I , I . lo-' 1. mol-' . s -l .
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