The irradiation of polyvinylimidazole (PVI) in the presence of a reactive monomer, such as vinylpyrrolidone (NVP), results in a copolymerization of the monomer with the double bonds of the pendant imidazole rings. This generates a three-dimensional network in which the PVI chains are linked together by sequences of PNVP. The structure of the correspoding hydrogel is very flexible, which provides excellent mechanical properties to the gel, even in a highly swollen state. (C) 1997 Elsevier Science Ltd.