A visible light photoinitiator system to produce acrylamide based smart hydrogels: Ru(bpy)3+2 as photopolymerization initiator and molecular probe of hydrogel microenvironments

A novel visible light bimolecular photoinitiator system (tris(2,2′-bipyridine)ruthenium(II)/N,N-dimethylaniline) is shown to be able to polymerize N-isopropylacrylamide (NIPAM) and 2-Acrylamido-2-Methylpropanesulfonic Acid (AMPS), in aqueous solution, to render high molecular weight polymers and crosslinked gels. The photoinitiator is especially useful to synthesize thermosensitive polymers and gels, because it could be used at temperatures below the phase transition, allowing the polymer chain to grow in its uncoiled state. The polymerization and conversion rates are affected by the structure of the monomer, decreasing in the order NIPAM > AAm > AMPS. The properties of the gels agree with literature data, suggesting that the method is able to produce conventional and smart hydrogels. The microenvironments present near linear polymers and inside crosslinked gels were investigated by measuring fluorescence lifetimes and steady state anisotropy of the metallic complex (Ru(bpy)3+2, present in the solution. Clear effects of the polymer presence on the photophysical properties of the complex are observed. Therefore, the same metallic complex could be used as photoinitiator of vinyl polymerization and as molecular probe to sense the hydrogel microenvironments.