In the process according to the invention for the preparation of a polypropylene, a catalyst system is used which is based on the product of the reaction of a magnesium halide with an electron donor and a tetravalent titanium compound, a halogen-free organoaluminum compound and two different organosilicon compounds as stereo-regulator. Propylene homopolymers and copolymers having a very narrow molecular weight distribution and high isotacticity are obtained in high yields. The polymers are suitable, in particular, for injection molding.
