Catalyst compositions for olefin polymerizing

A composition, which can be used as catalyst for producing an olefin polymer having a multimodal molecular weight distribution, is provided that comprises a transition metal-containing catalyst, a metallocene, and a boron-containing organoaluminoxane, wherein each component of the composition is present in an effective amount so that an olefin polymer having a multimodal molecular weight distribution can be produced using the composition as catalyst, preferably in the presence of a cocatalyst. Also provided is a process for preparing the composition which comprises: (1) contacting a transition metal-containing catalyst with an organoaluminoxane dispersed in a solvent to form a slurry; (2) combining the slurry with a boroxine to prepare a combination of a transition metal-containing catalyst and boron-containing organoaluminoxane; and (3) contacting the combination with a metallocene. Further provided is a process for polymerizing an olefin to prepare an olefin polymer using the composition.