The Phillips Cr(VI)/silica catalyst, used worldwide to manufacture polyethylene, must first be reduced and alkylated to display polymerization activity. This is accomplished commercially by exposure to ethylene, in a reaction that is still uncertain after 60 years of investigation. Here we report that other nonolefinic hydrocarbons can also be used in this self-alkylation mechanism, and we track, through isotopic labeling and other analyses, the fate of such hydrocarbons when used to initiate (reduce and alkylate) Cr (VI). A part of the reductant hydrocarbon is converted into oxygenated ligands, which remain attached as a permanent component of the active catalyst, thus influencing its behavior. Another part is used to alkylate the Cr, and it becomes incorporated into the first polymer chain. Each hydrocarbon reductant produces a unique catalyst, with unique ligands, displaying its own activity and sometimes producing distinctive polymer. The implication of this situation, in contrast to more traditional findings from the field, is one of opportunity to create novel varieties of new Phillips catalysts. (c) 2020 Elsevier Inc. All rights reserved.