Process for the preparation of a polypropylene, wherein propylene is polymerized optionally with a comonomer selected from the group consisting of ethylene, a C4-C20 α-olefin and mixtures thereof, in the presence of a catalyst system comprising solid catalyst particles, wherein the solid catalyst particles (a) comprise a transition metal compound of formula (I) LmRnMXq (I) wherein “M” is a transition metal of anyone of the groups 3 to 10 of the periodic table (IUPAC), each “X” is independently a monovalent anionic σ-ligand, each “L” is independently an organic ligand which coordinates to the transition metal (M), each “R” is a bridging group linking two organic ligands (L), “m” is 2 or 3, preferably 2, “n” is 0, 1 or 2, preferably 1, “q” is 1, 2 or 3, preferably 2, m+q is equal to the valency of the transition metal (M), (c) comprise a cocatalyst (Co) comprising an element (E) of group 13 of the periodic table (IUPAC), preferably a cocatalyst (Co) comprising a compound of A1, wherein further the loss of activity of the solid catalyst particles during polymerization is at most 20%.