A process for the preparation of a polypropylene in a sequential polymerization process including a pre-polymerization reactor and at least two polymerization reactors connected in series. The polymerization in the at least two polymerization reactors takes place in the presence of a Ziegler-Natta catalyst. The Ziegler-Natta catalyst includes (a) a pro-catalyst that has a compound of a transition metal, a compound of a metal which metal is selected from one of the groups 1 to 3 of the periodic table (IUPAC), and an internal electron donor, (b) a co-catalyst, and (c) an external donor. The Ziegler-Natta catalyst is present in the pre-polymerization reactor. Ethylene and propylene are fed to the pre-polymerization reactor in a feed rate ratio of 0.5 to 10.0 g/kg.