High polymer chelates. I. Structure and stability of polymethacroylacetone chelates

Abstract

AbstractThe chelates formed by polymethacroylactone (PMA) with several divalent cations were studied by spectrophotometric and potentiometric methods, and their behavior was compared to that of chelates formed by a low molecular weight diketone of similar structure, pivaloylacetone. The structure of the chelates formed by both types of diketones is identical, as indicated by the elementary analyses as well as the absorption spectra in crystalline state and in solution (these spectra are further indicative of resonance phenomena and orbital distribution in the chelate rings). The stability of these chelates was also investigated. A calculation based on the usual two‐step chelation process seems to indicate a higher stability of the high polymeric chelates over the low molecular weight ones, but the values of the constants become identical when taking in account the specific structure of the polymeric ligand: the metal ion, when fixed on a diketo group of the chain, is practically always in the nearest vicinity of a second chelating group, reacting immediately with it to form the chelate species MA2. The process will accordingly be dependent on the molar concentrations in the ligand only for the first step, but not for the second one as for monofunctional diketones. Finally, a linear relationship was found between the logarithms of the stability constants of the various chelates and the electronegativities of the corresponding metals. The behavior of both types of diketones is again identical. On the basis of this parallel in the behavior of low and high molecular weigh ligands of similar structure, a method is proposed to predict the selectivity of synthetic selective ion exchangers, and its validity is confirmed by absorption experiments in aqueous media.