The polymerization of the vinyl derivatized water-soluble polymers can be carried out with an initiator system composed of a peroxydisulfate and N,N,N9N9-tetramethylene-diamine (TEMED). This is a very efficient system, which gives rapid formation of the gel even under mild conditions (room temperature, pH 7). However, unreacted peroxydisul-fate and TEMED as well as their degradation products have to be extracted from the gel before in vivo application. Moreover, this initiator system can also damage proteins once they are present during preparation of the gels.