Proton NMR chemical shift induced by ionic association on a poly(ethylene oxide) chain

Abstract

When a polymer strongly binds metal ions in solution, a substantial number of counterions can also be bound to it, if the polarity of the solvent is not too high. This is actually obeerved for poly(ethy1ene oxide) (PEO) and sodium thiocyanate in nitromethanel and PEO and barium salts in methanol.2 The phenomenon may be regarded as an ionic association between the cation of the PEO-metal complex and counteranion and resembles the ion condeneation of polyelectrolytes. Although our previous conductivity studies have disclosed directly the presence of ionic association, the analytical calculation is quite intricate because of the strong interactions between metal cations that were bound to the same PEO chain.t2 Thus the simultaneous use of a more straightforward analytical method may be desirable.1H and 1% NMR spectroscopy have frequently been applied to the investigation of ionic association, especially ion pair formation between alkali metal ions and carbanions.3~4 Downfield shifts of 1H and l3C signals were induced by the shift of the ion pair equilibrium toward tight ion pairs from free ions or solvent separated ion pairs. Here we wish to report the poseibility of estimating by lH NMR the ionic association between alkaline earth metal cations and counteranions on the PEO chain. It has been well established that PEO forms complexes with alkali and alkaline-earth metals in several organic solvents? A lH NMR study of PEO oligomers in methanol showed that a downfield shift of the methylene protons was induced by the complexation with potassium ion, although the absolute values of the chemical shift differences were small.6 On the other hand, a relatively large chemical shift was observed in the case of the complexation with alkaline-earth metal ions in the same s01vent.~ This was also considered to be induced by metal complexation, although the absolute chemical shift values were much different from those observed for alkali metalcations.Commercial PEO samples with an average molecular weight of 2 X 104 were used. lH NMR spectra were measured on a JEOL JNM-PMXGOSI spectrometer in deuterated methanol. The methylene protons of dioxane in benzene solution, which was added as an external standard, were taken as a reference. Chemical shift values were measured downfield with respect to the methylene signal of PEO in the absence of salt. Figure 1 shows a plot of the l H NMR chemical shift as a function of the [Ba2+1/[PE01 ratio for barium thiocyanate and barium chloride in deuterated methanol. As shown in the f i i e , the addition of barium salts causes a downfield shift of the methylene protons of PEO. Both curves seem to saturate at a certain value of the ratio [Ba2+]/[PEO]. These curves correspond well to the conductivity curves of our previous study,2 since the ratio [Ba2+]/[PE0] at which the chemical shift or conductivity approaches an asymptotic value is very similar. Thus it is natural to consider that the chemical shift is induced by the complexation of PEO with Ba2+ ion. However, it should be noted that the asymp...

Binding of Monovalent and Multivalent Metal Cations to Polyethylene Oxide in Methanol Probed by Electrophoretic and Diffusion NMR

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Block copolymer-mediated synthesis of silver nanoparticles from silver ions in aqueous media

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Characterization of multicomponent crystal formed between indomethacin and lidocaine

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