In spite of great importance of Phillips CrOx/SiO2 catalyst for commercial polyethylene production and long term research efforts

polymerization mechanism

especially initiation mechanism

still remains unclear.The effect of surface fluorination of silica support on initiation of ethylene polymerization during induction period over Phillips catalyst adsorbed with different amounts of formaldehyde molecules was investigated with the density functional theory.It was demonstrated that no reaction could be initiated over cluster models adsorbed with two formaldehyde molecules on account of steric hindrance.For cluster models adsorbed with one formaldehyde molecule

ethylene dimerization to 1-butene and metathesis to propylene took place via chromacyclopentane intermediate

and fluorination of silica support showed minor influence on both reactions.After a complete desorption of formaldehyde molecules

further ring expansion to chromacycloheptane occurred and surface fluorination of silica support showed improvement on this process.Fluorination of silica support was unfavorable to ring-opening of chromacycloheptane to give 1-hexene.It was also demonstrated that fluorination showed positive effect on chain propagation over models of CrⅢ-alkyl active sites during ethylene polymerization.