COBALT(III) BIPYRIDYL-BRANCHED POLYOXAZOLINE COMPLEX AS A THERMALLY AND REDOX REVERSIBLE HYDROGEL

Redox behavior and thermally reversible nature were observed in an intermolecular cross-linked metal complex between bipyridyl-branched polyoxazoline and the cobalt(III) ion. The complex gels were much swollen in water and stable enough at ambient temperature for a few days, while they turned soluble in hot water within 30 min. The resulting brown orange solution was concentrated slowly to dryness under atmospheric pressure to regenerate the cross-linked product in a quantitative yield. This reversible interconversion between the gel and the soluble polymer was caused by the ligand exchange reaction. The initial intermolecular complex gel was converted to the entropically favorable intramolecular one (soluble polymer) by swelling in water. The thermal acceleration of the ligand exchange reaction changed these complex gels to the thermally reversible hydrogel. The results of the swelling behavior and the kinetics of the thermal cleavage reaction of the complex gel at 30-degrees-C revealed that the swelling equilibrium in water was the rate determining step at the first stage, then the rapid cleavage reaction took place, and simultaneously the water content of the gel increased rapidly. This gel was also completely dissolved in water by the rapid reduction of the central cobalt ion from the trivalent state to the divalent one. This interconversion from a hydrogel to a soluble polymer by reduction can be regarded as a novel redox reversible hydrogel system by means of kinetic control of the ligand exchange reaction coupled with the redox of the metal ions. Thus, this system can be recognized as the first example of a multisensitive hydrogel, i.e., the redox and thermally reversible hydrogel system.