Preparation of catalyst system

A process for the preparation of a supported catalyst system, wherein the catalyst system comprises:

• (i) a metallocene complex;
• (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst;
• and
• (iii) a porous inorganic support;
• whereby the process comprises the steps of:
• a) reacting the porous inorganic support with a first portion of the aluminoxane cocatalyst in a first hydrocarbon solvent to obtain an aluminoxane cocatalyst treated support;
• b) reacting the metallocene complex with a second portion of the aluminoxane cocatalyst in a second hydrocarbon solvent,
• wherein the first portion of the aluminoxane cocatalyst added in step a) is from 75.0 to 97.0 wt % of the total amount of the aluminoxane cocatalyst and the second portion of the aluminoxane cocatalyst added in step b) is from 3.0 to 25.0 wt % of the total amount of aluminoxane cocatalyst;
• c) adding the boron containing cocatalyst to the solution obtained in step b) to obtain a solution of the metallocene complex, boron containing cocatalyst, and aluminoxane cocatalyst, wherein the metallocene complex includes a metal ion M and whereby the boron containing cocatalyst is added in an amount such that the molar ratio of boron/M is in the range of from 0.1:1 to 10:1;
• d) adding the solution obtained in step c) to the aluminoxane cocatalyst treated support obtained in step a) to form the supported catalyst system;
• and optionally
• e) drying the so obtained supported catalyst system.