Temperature-heavy metal- and temperature-anion/molecule-responsive systems based on PEG acrylate copolymers containing dipyridyl ligands

Abstract

Thermoresponsive copolymers carrying di(2-pyridyl)methyl ligands are shown to respond sensitively and selectively to the presence of heavy metal cations, while their metal complexes respond in a likewise selective and sensitive manner to the presence of anions or molecules with higher metal affinity. A set of well-defined copolymers of poly(ethylene glycol) methyl/phenyl ether acrylate, and N-di(2-pyridyl)methylacrylamide was prepared through a combination of RAFT radical polymerization and postpolymerization modification of activated esters. Products were characterised by 1 H and 19F NMR spectroscopy, size exclusion chromatography, FT-IR spectroscopy, and turbidity measurements. Ligand–metal complexation, as observed by UV–vis spectroscopy, was found to increase lower critical solution temperature (LCST) transitions in water drastically (e.g. up to 22 C) for addition of small amounts (e.g. 0.4 mM) of Cu(II), Co(II), Fe(II) and Ag(I) salts, attributed to a tethering of charge to the polymer. Conversely, salts of Mn(II) and Gd(III) did not affect copolymer solubility. Observed LCST transitions of polymer–metal complexes decreased with the addition of anions or molecules which formed more stable complexes, poorly soluble compounds, or underwent redox reactions with the metal cation. Selectivity toward specific anion or molecule analytes could be tuned though the choice of metal. An isothermal phase separation of a polymer–Cu(II) solution (5 g/L) in response to the addition of as little as 0.4 mM sodium cyanide is demonstrated while the addition of an equal amount of sodium azide did not cause any response, signifying the potential of the proposed concept for sensing applications