A defoaming agent obtainable by a process that includes: polymerizing (A) at least one polymerizable monomer in a composition, the composition comprising (C) a solvent and (B) at least one non-polymerizable polysiloxane dissolved in the (C) solvent, wherein the component (A) optionally comprises at least one Si-atom-containing monomer in an amount of less than 10 mass % on the basis of the total mass of the component (A).