Preparation of catalyst system

A process for the preparation of a polypropylene homopolymer or polypropylene copolymer with one or more C2 to C8 alpha olefin comonomers, said process comprising polymerising propylene and optionally one or more C2 to C8 alpha olefin comonomers in the presence of a 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 silica support;
• wherein the metallocene complex is of formula (VI):
• wherein
• each X independently is a sigma-donor ligand,
• M is a group 4 metal;
• L is a carbon, silicon, or germanium based bridge in which one to four backbone atoms link the ligands;
• and
• each Ind is a substituted or unsubstituted indenyl, or a substituted or unsubstituted fused indenyl;
• wherein the boron containing cocatalyst is either of formula (B) or a borate containing an anion of formula (C):
• wherein
• each Y independently is a hydrogen atom, an alkyl group of from 1 to about 20 carbon atoms, an aryl group of from 6 to about 15 carbon atoms, alkylaryl, arylalkyl, haloalkyl or haloaryl each having from 1 to 10 carbon atoms in the alkyl radical and from 6-20 carbon atoms in the aryl radical or fluorine, chlorine, bromine or iodine;
• or
• Z is an optionally substituted phenyl derivative, said substituent being a halo-C1-6-alkyl or halo group, and as counterions protonated amine or aniline derivatives;
• and
• whereby the catalyst system is prepared by the steps of:
• a) treating the porous inorganic support with the aluminoxane cocatalyst in a hydrocarbon solvent, optionally followed by thermal treatment of the aluminoxane treated support;
• b) combining the metallocene complex with the boron containing cocatalyst and optionally with the aluminoxane cocatalyst in the hydrocarbon solvent;
• c) loading the solution of step b) onto the treated support of step a);
• wherein the amount of aluminoxane cocatalyst added in step a) is 75.0 to 100.0 wt % of the total amount of aluminoxane cocatalyst and the amount of aluminoxane cocatalyst added in step b) is 0.0 to 25.0 wt % of the total amount of aluminoxane cocatalyst.