Nature of active sites of supported chromic oxide polymerization catalysts

Abstract The active sites of chromic oxide polymerization catalysts form by interaction of the catalyst surface with monomer. Part of the Cr 6+ atoms are reduced to the trivalent state and alkylated. The active sites are alkylated trivalent chromium atoms in combination with a Lewis acid. By lowering the electron density on the Cr 3+ atom, the Lewis acid strengthens the Cr 3+ C bond in the polymer chain and increases the ability of the Cr 3+ atoms to coordinate with the monomer molecules. On the other hand, coordination weakens the Cr 3+ C bond, which grows stronger again after the monomer molecule enters the chain. Polymerization occurs as a result of alternative strengthening and weakening of this bond. An effective carrier increases the oxidative power of the chromic anhydride and enhances the donor-acceptor properties of the chromium atoms. There is a direct dependence between the oxidative power of chromic oxide catalysts and the polyethylene yield on them, which persists up to a certain limit. The subsequent drop in activity is due to decreasing stability of the catalyst.