An aqueous polymer dispersion is prepared by reacting at least one olefin in the presence of at least one polymerization catalyst and one emulsifier in an aqueous medium by a process wherein the polymerization catalyst is prepared in an in situ reaction by reacting the ligand compound 2,6-dichloro-para-benzoquinone (Ia) and/or 2,3,6-trichloro-para-benzoquinone (Ib) with a phosphine compound or diphosphine compound and with a metal compound and the polymerization reaction is effected in an aqueous medium.