The Transition Rates Between the Different Ionic States of Polystyrylsodium in Polar Organic Solvents

Abstract

AbstractBased on the fact that in polar organic solvents polystyrylsodium exists in three thermodynamicly and kinetically distinguishable ionic states (the contact ion pair, the solvent separated ion pair, the free carbanion), the rate constants for the transitions between these three states were investigated by a novel method in various solvents over a broad range of temperature. This method is based on the analysis of the molecular weight distribution of polystyrene samples produced under kinetically controlled conditions. The formal relations and the experimental methods are described in detail.The results obtained in the solvents tetrahydropyrane (THP), tetrahydrofuran (THF) and dimethoxiethane (DME), reveal that the dissociation of polystyrylsodium proceeds via two clearly distinguishable steps: The first step, the transition of the contact ion pair into the solvent separated ion pair proceeds relatively slowly and is characterized by a small activation energy and an extraordinarily high negative activation entropy. In the sequence THP, THF and DME the corresponding transition rate constants increase each by a factor of some 50, whereas the rate constant for the reverse reaction appear to be almost unaffected in these solvents. – The second step, the dissociation of the solvent separated ion pair into its free ions proceeds rapidly; the association of the ions is found to be almost diffusion controlled in agreement with theoretical calculations.