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

An intermolecular cross-linked metal complex between bipyridyl-branche d polyoxazoline and ferrous ion was prepared, in which the thermally r eversible nature was observed. The red 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 red so lution was concentrated slowly to dryness under atmospheric pressure t o give a red gel again in quantitative yield. This reversible intercon version between the gel and the soluble polymer was caused by the liga nd exchange reaction. The initial intermolecular complex gel was conve rted 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. F rom the results of the swelling behavior and the kinetics of the therm al cleavage reaction of the coordination gel at 40-degrees-C, it was f ound that the swelling equilibrium in water was the rate determining s tep at the first stage, then the rapid cleavage reaction took place, a nd simultaneously, the water content of the gel increased rapidly. A s eries 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. I n 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 b e regarded as a novel thermally reversible hydrogel system by means of kinetic control of the ligand exchange reaction.