Abstract: A catalyst system obtainable with a process comprising the following steps: i) contacting a group 4 metal compound of formula (I) MX4??(I) wherein M is a metal of group 4 of the periodic table of the element, and X is a halogen atom or an organic radical; with a compound of formula (II) ii) adding to the reaction mixture of step i) one or more boron compounds having Lewis acidity wherein the molar ratio between the boron compound and the compound of formula (I) ranges from 0.9 to 100; iii) adding the reaction mixture obtained in step ii) to a silica support. with the proviso that the catalyst system is not treated with alumoxanes.