The effects of support friability ((D) and ethylene/comonomer ratios were investigated over supported metallocene/methylaluminoxane catalysts prepared with nine different porous polymeric supports and various comonomer concentrations with a 2-L reactor operated in the sernibatch gas-phase mode at 80 degrees C and 1.4 MPa. Phi of the supports was measured with a newly devised method. The performance of the supported catalysts depended on support Phi as follows. The average homopolymerization activities varied from less than 6 t of polyethylene (PE) (mol of Zr)(-1) h(-1) for low-Phi catalysts to 10-20 t of PE (mol of Zr), h(-1) for moderate-Phi catalysts and up to 100 t of PE (mol of Zr)(-1)h(-1) for the high-Phi catalysts. The presence of 1-hexene and propylene comonomers increased the activity of the low-Phi catalysts by up to 20-fold and 50-fold, respectively; that is, there were very marked comonomer effects. Activity enhancement by 1-hexene was less than 3-fold for the moderate-Phi catalysts, whereas the high-Phi catalysts showed little activity enhancement. Sometimes, 1-hexene even resulted in activity reductions. Very different particle morphologies were obtained with the catalysts of different Phi's. (c) 2007 Wiley Periodicals, Inc.