Abstract: The invention relates to a process for the preparation of a polymer comprising at least aliphatic or aromatic hydrocarbyl C2-C20 mono or multiolefins in the presence of a catalyst, an activator wherein the catalyst and the activator are combined in a catalyst system according to formula (I) wherein R, R1, R2, R3, R4, R5, R6, R7, R8 and R9 can independently be chosen from substituted or unsubstituted alkyl or aryl groups having 1 to 20 carbon atoms, M is a group 4, or 5 metal; with valency v, La is a coordinating Lewis base with a=0, 1, or 2, being the number of ligands L and, Xi is an anionic ligand, with i=v?3, being the number of anionic ligands, the catalyst does not comprise boron and fluor atoms.