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

Redox behavior and thermally reversible nature were observed in an int ermolecular cross-linked metal complex between bipyridyl-branched poly oxazoline and the cobalt(III) ion. The complex gels were much swollen in water and stable enough at ambient temperature for a few days, whil e they turned soluble in hot water within 30 min. The resulting brown orange solution was concentrated slowly to dryness under atmospheric p ressure to regenerate the cross-linked product in a quantitative yield . This reversible interconversion between the gel and the soluble poly mer was caused by the ligand exchange reaction. The initial intermolec ular complex gel was converted to the entropically favorable intramole cular one (soluble polymer) by swelling in water. The thermal accelera tion of the ligand exchange reaction changed these complex gels to the thermally reversible hydrogel. The results of the swelling behavior a nd 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 reac tion took place, and simultaneously the water content of the gel incre ased rapidly. This gel was also completely dissolved in water by the r apid reduction of the central cobalt ion from the trivalent state to t he divalent one. This interconversion from a hydrogel to a soluble pol ymer by reduction can be regarded as a novel redox reversible hydrogel system by means of kinetic control of the ligand exchange reaction co upled with the redox of the metal ions. Thus, this system can be recog nized as the first example of a multisensitive hydrogel, i.e., the red ox and thermally reversible hydrogel system.