m surface-modified supported chromium catalyst; wherein the cocatalyst comprises a compound of formula MR3, where M is a Group 13 metal, and each R is independently a linear or branched C1 to C12 alkyl group, preferably a C4 to C8 alkyl group, more preferably wherein each R is an ethyl group; wherein M is boron, aluminum, or a mixture thereof; wherein the cocatalyst comprises tri-isobutylaluminum