Iron Tris(bipyridine) PEG Hydrogels with Covalent and Metal Coordinate Cross-Links

Spontaneous gel formation of iron(II) tris(bipyridine)-centered poly(ethylene glycol) methacrylate ([Fe{bpy(PEG-MA)(2)}(3)](2+)) was observed without the addition of a cross-linking agent. BpyPEG(2) macroligands were first modified with methacrylate groups using methacrylic anhydride and then combined with FeSO4 to produce [Fe{bpy(PEG-MA)(2)}(3)]SO4. End group analysis by H-1 NMR spectroscopy verified quantitative methacrylation of the PEG hydroxyl chain ends. A series of experiments and control reactions were performed to investigate the conditions required for gel formation. Hydrogels of [Fe{bpy(PEG-MA)(2)}(3)]SO4 were produced both in the presence and in the absence of a photoinitiator. Controls using MA-PEG-MA also formed hydrogels in the presence of [Fe(bpy)(3)](2+); however, the addition of a radical scavenger, TEMPO, prevented formation of a polymer network, suggesting radical involvement. Treatment of preformed hydrogels of bpy(PEG-MA)(2) with aqueous solutions of FeSO4, CuBr2, and COCl2 also produced materials with color changes indicative of complexation.