Disclosed is a method to synthesize hydrogels with increased viscosity from a solution that contains water, a copolymer and a dissolved salt of an alkaline metal, an earth alkaline metal and/or an organic amine by addition of a zirconium compound as a crosslinker which comprises i) providing a copolymer containing 0.005 to 20 weight % of structural units of formula I, 4.995 to 40 weight% of structural units of formula II and 5 to 95 weight % of structural units of formula III wherein R1, R4 and R6 are independently of one another hydrogen or C1-C6-alkyl,R2, R3 and R5 are independently of one another hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, R7 and R8 are independently of one another hydrogen or C1-C6-alkyhA is a covalent C-P bond or a two-valent organic bridge group, and B is a covalent C-S bond or a two-valent organic brigde group, and wherein the percentage of the structural units refers to the total mass of the copolymer, preparing an aqueous solution by adding the copolymer prepared in step i) to a solution comprising water and the salt of the alkaline metal, the earth alkaline metal and/or the organic amine, forming a network of a hydrogel by addition of at least one Zirconium- compound to the aqueous solution prepared in step ii), and selecting the amount of the salt of the alkaline metal, the earth alkaline metal and/or the organic amine in the hydrogel to range from 0.15 and 10 weight%, referring to the total mass of the hydrogel. The method can be used to prepare hydrogels used in fracturing operations.
