Polymer obtainable by a multistage polymerization process which involves conducting, in a first step, an aqueous-phase polymerization of at least one ethylenically monofunctional compound alone or together with at least one-ethylenically difunctional or poly-functional compound together in the presence of a polyesterpolyol, polyurethane and/or polyacrylate; followed by a reaction of the resulting product with a crosslinker; and its use.