Dual mechanism bifunctional polymers: Phosphinic acid ion-exchange/redox resins and the kinetics of metal-ion complexation

Abstract

The detailed study of bifunctional phosphinic acid resins has recently been introduced. It has been shown that they operate through the dual mechanism of ion exchange and metal-ion reduction. A key component in the synthesis is the Friedel-Crafts catalyst used for the reaction between PCl 3 and polystyrene. We now find that the order of catalyst activity is AlCl 3 > FeCl 3 > ZnCl 2 > SnCl 4 . Ferric chloride catalysis also operates by an additional redox mechanism which yields a phosphonic acid resin. The extent of loading onto phosphinic and sulfonic acid resins by zinc ions at equilibrium was studied as a function of the anion present both with and without the presence of an excess of sodium ions. Both sets of resins exchange at a comparable rate upon the introduction of macroporosity into the polystyrene support. The influence of the redox process as an additional variable in the phosphinic resin kinetics was studied with silver and mercury(II) nitrate solutions. All of the primary acid sites are oxidized after a 9-hour contact time with Ag(I) ions and a 2-hour contact time with Hg(II) ions. The mercury loading curves and the different behavior of silver, which shows more ion exchange than redox at short times while leaving a residual ion concentration on the resin irrespective of whether all of the primary acid sites have been oxidized or not, can be explained by the termolecular nature of the Ag(I) redox reaction in contrast to the bimolecular nature of the Hg(II) reaction. Studies with other ions show that the minimum metal-ion reduction potential below which no redox occurs with the phosphinic resins is probably between 0.0 and 0.3 V.