Characteristic behaviour of the complexation of copper(II) with polymer-bound vinylimidazole ligands

Some polymer ligands of N-vinylimidazole-co-N-vinylpyrrolidone (Plm) having different mole fractions of N-vinylpyrrolidone have been prepared. Successive formation constants for the complexation of Cu^II with these Plms in aqueous solutions have been determined by means of spectrum deconvolution at pH 3.5. The present results demonstrate that the successive formation constants are given by K₁ = [CuL]/[Cu][L], K₂ = [CuL₂]/[CuL], K₃ = [CuL₃]/[CuL₂] and K₄ = [CuL₄]/[CuL₃] (charge omitted, L = imidazole residue on Plm). The results clearly indicate that the concentration ratio, [CuL]:[CuL₂]:[CuL₃]:[CuL₄], is largely constant for a given Plm when the concentration of imidazole residues is varied. The values of K₂, K₃ and K₄ decrease with increasing mole fraction of vinylpyrrolidone residues (VPRo) in the Plms. The successive formation constants increase in the order K₂ < K₃ < K₄ for poly(N-vinylimidazole), whereas the reverse (i.e. K₂ > K₃ > K₄) is found for Plm with a VPRo mole fraction of 0.8. Above this VPRo mole fraction, CuL₄ species are rarely formed. These results also suggest that the distributions of CuL_n species in Cu^II-Plm solutions can be controlled by adjusting the content of VPRo in the Plm.