New ionic liquids with hydrolytically stable anions as alternatives to hexafluorophosphate and tetrafluoroborate salts in the free radical polymerization and preparation of ion-conducting composites

Abstract

The main objective of this study was to replace the traditionally used hydrolytically unstable PF6 and BF4 ionic solvents with new, stable ionic liquids (ILs) and to investigate the potential of ILs in formulation of the polymers based on methyl methacrylate and acrylonitrile with narrow polydispersities, high yields and molecular weights. Various ILs with four anions, namely, tris(pentafluoroethyl)trifluorophosphate, trifluoromethanesulfonate, trifluoroacetate and tetracyanoborate, were tested. The influence of the IL's characteristics on the free radical polymerization process was found to be significant. Trifluoromethanesulfonate- and trifluoroacetate-based ionic solvents provide the formation of polymers with the highest molecular weight (Mr≈8.8 × 105 for polyacrylonitrile (PAN) and Mw≈18.9 × 105 for polymethylmethacrylate (PMMA)) in quantitative yield (PAN, 96%; PMMA, 92%). Taking into account, the high ionic conductivity and satisfactory electrochemical stability of the tetracyanoborate-based ILs, composite films derived from PAN and B(CN)4 ILs were prepared and their physico–chemical properties were investigated. The nature of the cation strongly influences the properties of the resulting composite films, namely, their flexibility, transparency, surface morphology and ionic conductivity (the maximum conductivity was found to be 1.0 × 10−2 S cm−1 at 25 °C). Ionic Liquids (ILs) are design solvents for free radical polymerization and preparation of composite materials! There is NO general rule for choosing of ionic solvent, but by means of design it is possible to select the best one for a concrete process. CF3SO3 and CF3COO based ILs provide the formation of highest molecular weight polyacrylonitrile (PAN, Mn≈8.8 × 105) and polymethylmethacrylate (PMMA, Mw≈18.9 × 105) in a quantitative yield (PAN – 96%, PMMA – 92%). (CF3CF2)3PF3 ILs promote the formation of high molecular weight PMMA even upon the polymerization under the atmospheric pressure with the presence of air oxygen. Nature of the IL stongly influences the properties of the resulted composites films, namely, their flexibility, transparency and surface morphology. Composite films derived from PAN and B(CN)4 ionic liquids possess ionic conductivity as high as 1.0 × 10−2 S cm−1 at 25°C.