Heterolytic C-H bond activation of ethylene on Cr-III-O has been proposed as the initial step in olefin polymerization on ( SiO)(3)Cr-III active sites of molecularly-defined analogues of the Phillips catalyst. Here, by using realistic amorphous periodic models that account for structural complexity, strain, and active site heterogeneity of well-defined silica-supported am-based polymerization catalysts, we show that this activation step is significantly favored due to the strain present on highly dehydroxylated silica. Furthermore, we find that initiation by insertion of ethylene into the Cr-O bond is even more favorable, especially for more strained sites, while both mechanisms can compete for less strained and thereby less active sites. Our results suggest a competing dual pathway for ethylene polymerization on Cr-III sites and are consistent with a distribution of active sites, the experimentally observed broad distribution of polymer molecular weight, and the increased polymerization activity upon high-temperature calcination in Phillips catalysts. (C) 2016 Elsevier Inc. All rights reserved.