A process for the preparation of a procatalyst suitable for preparing a catalyst composition for olefin polymerization, the procatalyst obtained or obtainable by the process; and a catalyst composition for olefin polymerization comprising the procatalyst. In particular an activator according to Formula (I) can be used in the preparation of a supported Ziegler-Natta type procatalyst useful for a process for the preparation of polyolefins.