The present invention relates to an olefin polymerization process, wherein propylene and 1-butene and optionally ethylene are reacted in the presence of a Ziegler-Natta catalyst system so as to obtain a polypropylene, wherein the polypropylene comprises 1-butene-derived comonomer units in an amount of from 5 to 20 wt % and optionally ethylene-derived comonomer units in an amount of up to 3 wt %, and the Ziegler-Natta catalyst system comprises an external donor of the following formula (I): (R3)z(R2O)ySi(R1)x.