Process for preparing a (meth)acrylate copolymer containing tertiary amino groups by free-radical polymerization in solution

A process for preparing a (meth)acrylate copolymer, the process comprising:

• adding a polymerization initiator and a solvent or a solvent mixture to a monomer mixture, thereby obtaining a polymerization mixture,
• dissolving monomers of the monomer mixture and the polymerization initiator,
• polymerizing the polymerization mixture at the temperature from 30 to 120° C. over a period of from 2 to 24 hours, thereby obtaining a polymerization syrup with a conversion of the monomers to the copolymer of at least 99% by weight,
• subsequently degassing the polymerization syrup by distillation or by extrusion, thereby obtaining a degassed polymerization syrup, and comminuting the degassed polymerization syrup to a copolymer preparation as a granulate or powder,
• wherein the monomer mixture comprises:
• from 80 to 99% by weight of a C1-C4-alkyl ester of acrylic acid or a C1-C4-alkyl ester of methacrylic acid, from 1 to 20% by weight of the alkyl ester of acrylic acid or the alkyl ester of methacrylic acid with a quaternary ammonium group in an alkyl radical, and from 0 to 10% of a further-copolymerizable vinyl monomer,
• wherein the copolymer comprises the quaternary ammonium group,
• wherein the polymerization mixture comprises:
• at least 98% by weight of a total amount of following components comprising, from 50 to 80% by weight of the monomer mixture, from 0.01 to 5% by weight of the polymerization initiator, and from 20 to 50% by weight of the solvent or the solvent mixture, and
• wherein the copolymer preparation has:
• a weight average molecular weight of from 25,000 to 50,000, a polydispersity index of from 2.0 to 3.2, and a residual solvent concentration of less than 10,000 ppm by weight.