The present invention relates to an olefin polymerization process for producing propylene copolymer composition (P), wherein propylene, C4 to C10 α-olefin and optionally ethylene are reacted in the presence of a Ziegler-Natta catalyst in a multistage polymerization process comprising at least two polymerization reactors, wherein the copolymer composition is bimodal with respect to the content of C4 to C10 α-olefin and, if present, to ethylene. Further, the invention is directed to the propylene copolymer composition being bimodal with respect to the content of C4 to C10 α-olefin and optionally to ethylene, and use of said propylene copolymer compositions for producing articles.