Poly(vinyl alcohol)-based hydrogels formed by "click chemistry"

Novel poly(vinyl alcohols) (PVA) functionalized with pendant acetylene and azide groups were prepared by carbonyldiimidazole (CDI)-mediated couplings of the amines terminated with functional groups, 1-azido-2-aminoethane, propargylamine, or N-methylpropargylamine, to PVA. Low degrees (1-5%) of PVA modification were required in order to retain solubility in water. Azide-modified PVA and alkyne-modified PVA components were cross-linked by mixing of their solutions together with Cu(I) catalyst, a type of Huisgen's 1,3-dipolar azide-alkyne cycloaddition, recently defined as a powerful "click" chemistry. Reaction of the two different polymers results in a chemoselective coupling between alkynyl and azido functional groups with the multiple formation of triazole cross-links to give hydrogel formation. In another version the PVA-based hydrogels were obtained by cross-linking of alkyne-modified PVA with the telechelic bifunctional poly(ethylene glycol)diazide cross-linker. The hydrogels prepared by these two methods were characterized by their equilibrium swelling in water, by their viscoelastic properties in the swollen state, and by their soluble fraction. It was demonstrated that network properties are affected by the type of cross-linker with polyfunctional PVA cross-linkers having higher gelation capacity than the bifunctional PEG cross-linker. The approach we describe here presents a promising alternative to a common chemical hydrogel preparation technique, which utilizes bifunctional low-molecular-weight cross-linkers.