SYNTHESIS AND REDOX GELATION OF DISULFIDE-MODIFIED POLYOXAZOLINE

A polyoxazoline hydrogel having a disulfide bridge was prepared by the reduction-oxidation of thiol-protected poly(N-acetylethylenimine) (PAEI) (anthracene-SS-modified PAEI) or by the cross-linking reaction of the partially hydrolyzed PAEI with bifunctional disulfide compounds. As a disulfide cross-linking agent, dithiodi-2,1'-ethanediyl diisocyanate/1,8-diazabicyclo[5.4.0] undec-7-ene or 3,3'-dithiopropanoic acid/dicyclohexylcarbodiimide gave good results. The reduction of thiol-protected PAEI produced the thiol-modified PAEI, which was susceptible to air oxidation to form the networks. These gels based on highly hydrophilic PAEI were stable and swollen in water as a hydrogel. A series of PAEIs having varying amounts of the functional groups were prepared and subjected to cross-linking reactions by reduction-oxidation. The degree of swelling in water increased with a decrease of the degree of substitution in the prepolymer. The disulfide-bridged PAEI gel became soluble to form the linear polymer by the treatment with reducing agents such as sodium hydrosulfite, sodium borohydride, or triphenylphosphine. This solubilization of gel was caused by the reductive cleavage of the disulfide bridge at the cross-linking points. This interconversion from a hydrogel to a soluble polymer can be regarded as a novel redox-reversible hydrogel system by means of reversible conversion between disulfide and thiol groups.