The mechanism by which Phillips catalysts function involves the activation of ethylene through the formation of a chromium-carbon bond. The active site in these catalysts is often a Cr(VI) species, which is reduced under the polymerization conditions to form the actual catalytically active sites, generally believed to be Cr(II) or Cr(III). The resulting active sites facilitate the insertion of ethylene into the growing polymer chain, thus enabling the polymerization process.