The invention relates to aqueous polymer dispersions, comprising a water insoluble polymer P in the form of dispersed polymer particles, which is obtainable by free radical aqueous emulsion polymerization of ethylenically unsaturated monomers M, which comprise a) at least one monomer M1 which exhibits two ethylenically unsaturated moieties which are connected by a linker which comprises a disulfide moiety, and b) at least one monoethylenically unsaturated monomer M2, which has a low water solubility. The invention also relates to processes for preparing such an aqueous polymer dispersion, to solid polymer compositions in powder form, obtainable by drying such polymer dispersions, to the use of such polymer dispersions in a coating material, adhesive or sealant and to a methods of coating a surface of a substrate which comprises applying an aqueous coating composition, which contains such an polymer dispersion.