A catalyst system useful for polymerizing olefins is disclosed. The catalyst system comprises a cocatalyst and a supported transition metal prepared from a halogen-containing Group 4 6 transition metal compound and an organoaluminum-siloxane containing mixture. The latter mixture, which is prepared from an organoaluminum compound and an organomagnesium-siloxane reaction product, incorporates a chelating ligand. The invention includes a method for making the catalyst system and a process for polymerizing olefins using the catalyst system. The process is convenient and avoids expensive catalyst components used in known olefin polymerization processes.