The porosity of Phillips Cr/silica catalysts plays a major role in determining the molecular weight (MW) and molecular weight distribution obtained during ethylene polymerization. A survey of the many different families of silica was made to evaluate this connection. No one single physical catalyst property. such as surface area, pore volume, radius of curvature, or average pore diameter, could be universally correlated with the MW of polyethylene. Instead the data seem to suggest that the trends observed are the result of two independent influences. First, pore Volume determines the fragility of the catalyst, which governs the degree of fragmentation during polymerization, and the fragment size. Secondly, pore diameter controls how easily the polymer can escape from the interior of the fragment. These variables are explored, and possible explanations for the observed behavior are discussed. (C) 2008 Elsevier Inc. All rights reserved.