Postsynthesis modification of solid polymethylaluminoxane (sMAO) with tris(pentafluorophenyl)borane or pentafluorophenol produces highly active metallocene supports “sMMAOs” for use in slurry-phase ethylene polymerization. Characterization of the sMMAOs using elemental analysis, BET isotherm, SEM-EDX, diffuse FT-IR, and solid-state NMR spectroscopy reveals that the surface methyl groups are exchanged for C6F5 and C6F5O moieties respectively, giving a material with reduced aluminum content and a lower specific surface area than sMAO. rac-Ethylenebis(1-indenyl) zirconium dichloride, {(EBI)ZrCl2} immobilized on B(C6F5)3- and C6F5OH-modified sMAO displayed activity increases of 66% and 71% respectively for ethylene polymerization compared to the same zirconocene catalyst precursor on unmodified sMAO. In the case of B(C6F5)3-modified sMAO, this enhanced polymerization activity is accompanied by excellent control of polymer particle size and morphology, and a small decrease in polymer molecular weight and polyd...