Method for preparing a catalyst composition for oligomerization of ethylene and respective catalyst composition pre-formation unit

A method for preparing a catalyst composition for the oligomerization of ethylene comprising the steps:

• a) preparing in a first solvent, a first solution by mixing of a co-catalyst and a modifier, wherein the co-catalyst is selected from trialkyl aluminum, alkyl aluminum sesquichloride, dialkyl aluminum chloride, alkyl aluminum dichloride, wherein alkyl is preferably methyl, ethyl, isopropyl or isobutyl, methylaluminoxane (MAO) or mixtures thereof, and wherein the modifier is selected from ammonium or phosphonium salts of the type [H4E]X, [H3ER]X, [H2ER2]X, [HER3]X or [ER4]X or HX or RX with E=N or P, X═Cl, Br or I and R=alkyl, cycloalkyl, acyl, aryl, alkenyl, alkynyl or the corresponding bridging di-, tri- or multiunits, or ammonium or phosphonium salts based on cyclic amines;
• and
• b) adding a chromium compound and a ligand to the first solution obtained in step a) to obtain a second solution.