pubs.acs.org/cm Published on Web 04/09/2010 r 2010 American Chemical Society 2730 Chem. Mater. 2010, 22, 2730–2740 DOI:10.1021/cm9030655 Thermally-Induced Acid Generation from 18-Molybdodiphosphate and 18-Tungstodiphosphate within Poly(2-Hydroxyethyl Methacrylate) Films Antonios M. Douvas,* ,† Konstantina Yannakopoulou, ‡ and Panagiotis Argitis* ,† † Institute of Microelectronics and ‡ Institute of Physical Chemistry, National Center for Scientific Research “Demokritos”, 15310 Aghia Paraskevi, Athens, Greece Received October 2, 2009. Revised Manuscript Received March 18, 2010 The thermally induced acid generation by two Dawson-type polyoxometalates (POMs), namely, the ammonium 18-molybdodiphosphate, (NH 4 ) 6 P 2 Mo 18 O 62 , (Mo 18 6- ) and the ammonium 18-tung- stodiphosphate, (NH 4 ) 6 P 2 W 18 O 62 , (W 18 6- ) within poly(2-hydroxyethyl methacrylate) (PHEMA) films is reported. The acid is generated by the simultaneous thermal reduction of POMs and oxidation of a small percentage of PHEMA hydroxyl groups, and it subsequently catalyzes the cross-linking of the polymer. The generated protons are detected by introducing a known acid indicator (methylene blue, MB) within the films and monitoring the indicator’s protonation with UV spectroscopy. The acid-catalyzed cross-linking of PHEMA is studied by dissolution studies sup- ported with FTIR and NMR spectroscopy. From the combination of those two spectroscopic studies it is concluded that PHEMA cross-linking within POM-PHEMA films is an acid-catalyzed reaction involving elimination of hydroxyl groups (possibly transesterification) accompanied by the side reaction of acid-catalyzed dehydration of PHEMA that leads to the formation of soluble products. Both POMs investigated can be completely removed, if desired, from the thermally cross-linked PHEMA films at the end of the process, by incubation in an aqueous solution of base, allowing therefore the use of POMs as acid generating agents in applications where the removal of the acid generator from the polymer films is beneficial. Introduction During the past few years a significant increase in the research activity regarding polyoxometalates (POMs) is observed, mostly because of the high industrial and academic interest in those compounds. 1 That trend has been considerably supported by the synthesis of new, more advanced POM structures, 2 the improvement in their characterization methods, and the contemporary theoretical investigations of POMs, 3 as they contributed to the deeper understanding of their structure, electronic properties, and chemical reactivity. 4 In general, POMs are metal-oxygen anionic clusters with well-defined mo- lecular structure, 5 which are well-known for, among others, the following properties: (a) they can thermally or photochemically oxidize a vast number of organic compounds, because of their capability to accept a certain number of electrons without substantial change of their structure; 6-8 and (b) they are very weak Brønsted bases, and thus their conjugate acids (the heteropoly acids) are very strong Brønsted acids with an acidity approaching the super acids’ region. 9,10 Because of those two fundamental properties, POMs are used in a broad field of applications in both oxidation and acid catalysis, 6,9,11 and recently have been considered in additional applications as well, such as fuel cells, 12 molecular conductors, 13 proton memory devices, 14 and solid-state electronic devices. 15 *To whom correspondence should be addressed. E-mail: adouvas@imel. demokritos.gr (A.M.D.), argitis@imel.demokritos.gr (P.A.). Fax: þ30-210- 6511723. Phone: þ30-210-6503231 (A.M.D.), þ30-210-6503114 (P.A.). (1) (a) Long, D.-L.; Tsunashima, R.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 2. (b) Long, D.-L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105. (2) (a) M€ uller, A.; Roy, S. Coord. Chem. Rev. 2003, 245, 153. (b) M€ uller, A.; Beckmann, E.; B€ ogge, H.; Schmidtmann, M.; Dress, A. Angew. Chem., Int. Ed. 2002, 41, 1162. (3) (a) Poblet, J. M.; L opez, X.; Bo, C. Chem. Soc. Rev. 2003, 32, 297. (b) L opez, X.; Bo, C.; Poblet, J. M. J. Am. Chem. Soc. 2002, 124, 12574. (4) (a) Mizuno, N.; Yamaguchi, K.; Kamata, K. Coord. Chem. Rev. 2005, 249, 1944. (b) Geletii, Y. V.; Botar, B.; K€ ogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem., Int. Ed. 2008, 47, 3896. (5) (a) Pope, M. T.; M€ uller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (b) Pope, M. T. 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