Formation and Stability of Polymer Chelates

Formation and Stability of Intra-Polymer Chelates of CutII) 

Studies on the formation of polymer chelates have been almost exclusively concerned with the Cu(II) ion. The reasons are (i) the stability of Cu(II) chelates in an aqueous solution is sufficient to make them difficult to hydrolyze under experimental conditions (ii) Cu(II) ions can be readily determined by chelate titration, atomic absorption spectrophotometry, etc. (iii) Cu(II) chelates are active in both visible and ESR spectra. 

In order to study the shape of a polymer chelate, viscometric measurements of a homogeneous solution of PVP were carried out46. The reduction in viscosity of the PVP solution with increasing concentration of metal ions is shown in Fig. 8(a). At constant PVP concentration, an increase in the amount of metal ions added causes a decrease in viscosity, which reveals that the polymer-ligand chain is markedly contracted due to intra-polymer chelation. An intra-polymer chelate takes a very compact form and the metal ions are crowded within the contracted polymer chain, as illustrated in Fig. 8(b). The contracted shape of an intra-polymer chelate was also reported for PAA47, poly(vinylamine)48, and poly(vinylalcohol)49.  

PVP were calculated by computer, using the modified method of Bjenum, and the constants obtained are plotted in Fig. 9 together with those of a pyridine-Cu systemS5 In the monomeric pyridine system, the stepwise formation constant decreases with the number of coordinated ligands because the number of vacant sites on the Cu ion, or the ligand-accepting ability of the Cu ion, decreases with succes- 

An enhancement of stability due to the contracted conformation of a polymer-ligand chain may also be deduced from Table 5. The (34 value of a PAA-Cu chelate is larger in the system with higher ionic strength. The interpretation is that the local concentration of ligands is increased by the contraction of the PAA chain on the addition of the neutral salts°). 

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