Quantitative evaluation of the ion-binding characteristics of poly(4′-vinylbenzo-18-crown-6)

We have carried out an in-depth study of the alkali metal ion binding characteristics of a model ionophoric macromolecule, poly(4′-vinylbenzo 18-crown-6), P18C6. Ion selective electrode potentiometry has been utilized to determine stability constants for complexstion of K⁺, Na⁺, and Cs⁺ to P18C6 in methanol/water (80/20)_w. Our results show that (he intrinsic binding constants, K_n, of P18C6 towards K⁺C1⁻ and Na⁺Cl⁻ are identical to that of the monomeric analog, benzo-18-crown-6.

In the absence of supporting electrolyte, the apparent binding constant of benzo-18-crown-6 is greater than (hat of P18C6. At low tonic strength the binding of K⁺ to P18C6 depends strongly on the fraction of crown moieties bound. P18C6 binds K⁺C1⁻ and Na⁺Cl⁻ with a stoichiometry of 2 mol of crown per mole of M⁺ . This binding, however, is not sandwich mode. The binding of K⁺Cl⁻, K⁺NO₃ and K⁺ClO₄⁻ to P18C6 in methanol/water differs significantly, with K_N(K⁺ClO₄⁻1 being nearly an order of magnitude greater than K_N(K⁺Cl⁻). Cs⁺ is bound to P1SC6 with a stoichiometry of ∼3 mol of crown per mole of Cs⁺, and the ion is bound cooperatively in a sandwich geometry.