The nature of the active sites and the mechanism of ethylene polymerization over the Phillips CrOx/SiO2 catalyst are still under debate. In this work, a number of potential initiation, propagation and termination mechanisms for ethylene polymerization are investigated using density functional theory and cluster models. Cr(II), Cr(III), Cr(III)-OH and Cr(V) oxide species are considered as the active site precursors. It is predicted that the oxachromacycle ring expansion mechanism is more probable than the routes involving equivalent to Si(OH)Cr(II)-vinyl or equivalent to Si(OH)Cr(III)-vinyl sites. We also show that the hydroxylated Cr(III)-OH species, further transforming into the active Cr(III) vinyl sites, can be effective for ethylene polymerization. Additionally, we propose that defect sites in the amorphous silica framework can play a role in low-temperature transformation of the Cr(II) species into the active Cr(III) sites. A potential polymerization mechanism involving minor Cr(V) oxide species is also calculated. (C) 2017 Elsevier Inc. All rights reserved.