Active sites formation and their transformations during ethylene polymerization by the Phillips CrOx/SiO2 catalyst

•Many potential initiation and propagation mechanisms are computationally studied.•Cr(II), Cr(III), Cr(III)OH and Cr(V) oxide species are considered as the precursors.•Cr(III)OH and Cr(III) vinyl sites can be effective for ethylene polymerization.•Silica defect sites can play a role in transformation of Cr(II) to Cr(III) species.