Polymer International Polym Int 57:637–643 (2008) Spectroscopic and thermal studies of poly[(N-vinylimidazole)-co-(maleic acid)] hydrogel and its quaternized form Nursel Pekel ∗ and Olgun G ¨ uven Department of Chemistry, Hacettepe University, 06800 Beytepe, Ankara, Turkey Abstract BACKGROUND: In this study, poly[(N-vinylimidazole)-co-(maleic acid)] (poly(VIm/MA)) hydrogels were prepared by γ -irradiation of ternary mixtures of N-vinylimidazole–maleic acid–water using a 60 Co γ -source. Spectroscopic and thermal analyses of these hydrogels as a function of protonation showed that the results are consistent with the existence of an H-bridged complex when the imidazole rings are partially protonated. Finally, the efficiency and binding trends of Cu 2+ , Co 2+ , Cd 2+ and Pb 2+ ions with both protonated and unprotonated poly(VIm/MA) hydrogels were determined. RESULTS: Gelation of 90% was reached at around 180 kGy dose at the end of irradiation. The poly(VIm/MA) hydrogels synthesized were further protonated in HCl solutions with different concentrations. Hydrogels originally showed 450% volumetric swelling; this ratio reached 1900% after protonation at pH = 5.0. Fourier transform infrared spectral changes in the + N–H stretching region (3200–3600 and 1173 cm −1 ) and the ring mode deformation at 915 cm −1 are consistent with the formation of an H-bridged complex between the protonated and unprotonated imidazole rings upon partial protonation. Similar changes were obtained from NMR spectra of both the protonated and unprotonated forms of the hydrogels. CONCLUSION: Protonated and unprotonated hydrogels have been used in heavy metal ion adsorption studies for environmental purposes. Adsorption decreased with decreasing pH value due to the protonation of the VIm ring. The adsorption of Me 2+ ions decreased in the order Cu 2+ > Co 2+ > Cd 2+ > Pb 2+ , which is related to the complexation stability as well as the ionic radius of the metal ions. These results show that P(VIm/MA) hydrogels can be used efficiently to remove heavy metal ions from aqueous solutions. However, the protonated form is a bad choice for heavy metal ion adsorption due to electrostatic repulsion forces; it can nevertheless be assumed to be a good choice for anion adsorption from environmental waste water systems.  2007 Society of Chemical Industry Supplementary electronic material for this paper is available in Wiley InterScience at http://www.interscience.wiley.com/jpages/ 0959-8103/suppmat/ Keywords: poly(N-vinylimidazole); maleic acid; complexation; protonation; NMR INTRODUCTION Polymer structures are usually protonated or quater- nized to permit electrostatic binding of anionic metal complexes (such as Fe(CN) 3−/4− 6 ) or deprotonated to permit incorporation of cationic complexes (such as Ru(NH 3 ) 3+/2+ 6 ). Because the capacity of these kinds of ion-exchange polymers is ultimately determined by the density of the charged sites, it is important to determine the extent of protonation, quaternization or deprotonation. 1–3 The solution behaviour of poly(N-vinylimidazole) (PVIm) has been well characterized and reported. 4–6 These studies show that the conformation of the poly- mer chain is sensitive to its degree of protonation, its degree of quaternization, the nature of the solvent and the type of added salts. It is believed that both overall and local chain conformations of the polymer are important in its interaction with transition metal ions and its catalytic properties. The ability to form complexes with transition metal ions also presumably depends on the ability of the polymer to assume a conformation which facilitates chelation. A conforma- tional transition of the polymer upon protonation was demonstrated by potentiometry, viscosity and NMR studies. 7 The polymer coil first collapses upon par- tial protonation but expands with further protonation. The contraction of the coil during the initial stages of neutralization was interpreted as the result of hydrogen bonding between protonated and unprotonated non- nearest neighbouring imidazole rings. PVIm possesses electronegative nitrogen atoms in the imidazole rings that can be protonated in the presence of an acid to form polycationic polymers. Anions in the electrolyte can then bind electrostatically to these protonated sites. 8 ∗ Correspondence to: Nursel Pekel, Department of Chemistry, Hacettepe University, 06800 Beytepe, Ankara, Turkey E-mail: nursel@hacettepe.edu.tr (Received 28 March 2007; revised version received 29 June 2007; accepted 20 August 2007) Published online 13 December 2007; DOI: 10.1002/pi.2389  2007 Society of Chemical Industry. Polym Int 0959–8103/2007/$30.00