IRON(II) BIPYRIDYL-BRANCHED POLYOXAZOLINE COMPLEX AS A THERMALLY REVERSIBLE HYDROGEL

An intermolecular cross-linked metal complex between bipyridyl-branched polyoxazoline and ferrous ion was prepared, in which the thermally reversible nature was observed. The red 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 red solution was concentrated slowly to dryness under atmospheric pressure to give a red gel again in 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 reversibility of this hydrogel was caused by the thermal acceleration of the ligand exchange reaction. From the results of the swelling behavior and the kinetics of the thermal cleavage reaction of the coordination gel at 40-degrees-C, it was found 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. A series of poly(N-acetylethylenimine)s (PAEIs) having varying amounts of the functional groups were prepared and subjected to a cross-linking reaction. The degree of swelling and the stability of the gel in water depended on the content of the functional groups in the prepolymer. In the case of less substituted polymer, e.g., 3.3% bipyridyl-branched PAEI, the water uptake was up to 56 multiples of its own weight in the dry state. This gel was completely dissolved into water rapidly. This interconversion from a hydrogel to a soluble polymer by heating can be regarded as a novel thermally reversible hydrogel system by means of kinetic control of the ligand exchange reaction.