The invention relates to a process for preparing a (meth)acrylate copolymer containing quaternary ammonium groups from a monomer mixture selected from: (a) 80 to 99% by weight of a C1-C4-alkyl ester of acrylic acid or methacrylic acid; and (b) 20 to 1% by weight of an alkyl ester of acrylic acid or methacrylic acid with a quaternary ammonium group in the alkyl radical; and (c) 0 to 10% of further copolymerizable vinyl monomers whereby one or more polymerization initiators, optionally one or more molecular weight regulators and one or more solvents or of a solvent mixture are added to the monomer mixture to give a polymerization mixture, which is polymerized at temperatures from 30 to 120 C over a period of 2 to 24 hours, where the polymerization mixture is polymerized to a polymerization syrup with a conversion of the monomers to the copolymer of at least 99% by weight, where the polymerization syrup is subsequently degassed by distillation or by extrusion and the degassed polymerization syrup is further comminuted to a copolymer preparation in the form of a granulate or powder and where the copolymer preparation is characterized by a molecular weight (Mw) of 25.000 to 50.000, a polydispersity index of 2.0 to 3.0 and a residual solvent concentration of less than 10.000 ppm by weight.