A process for the preparation of a catalyst system for olefin polymerization, including: A) providing said procatalyst obtainable via a process comprising: i) contacting a compound R4zMgX42-z with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product, ii) optionally contacting the solid product obtained in step i) with at least one activating compound selected from an activating electron donor or metal alkoxide compound; iii) contacting the first or second intermediate reaction product, with a halogen-containing Ti-compound and optionally an internal electron donor to obtain the procatalyst; and B) contacting the procatalyst with a co-catalyst and at least diethylaminotriethoxysilane as the external donor. Further disclosed is a catalyst system obtained by the process; a process for preparing a polyolefin by contacting at least one olefin with the catalyst system; a polyolefin obtained thereby; a composition comprising a propylene-ethylene copolymer; a shaped article thereof; and use of the polyolefin.