Effect of Al‐alkyl cocatalyst on ethylene polymerization behavior using silyl chromate (SC)/SiO2 catalyst

Cr/SiO2 catalysts are important industrial ethylene polymerization catalysts, mainly including Phillips catalyst (also called oxo chromium/SiO2) and silyl chromate/SiO2 systems (named as SC/SiO2, also called UCC S-2 catalyst). Compared with Phillips catalyst, SC/SiO2 catalyst is not fully investigated. In this work, SC/SiO2 catalyst was prepared, and triethylaluminum or triisobutylaluminium was used as cocatalyst for ethylene homopolymerization. The effects of cocatalysts on polymerization kinetics and polymer microstructure were investigated. It is found that the kinetic curves using SC/SiO2 catalyst in the presence of triethylaluminum or triisobutylaluminium seem to be a hybrid type composed of two types of basic polymerization kinetics: one is a fast built-up and fast decay type, and the other is a slow built-up and slow decay type, which indicates the existence of two types of polymerization active sites. Plausible mechanisms of the formation and transformation of active sites were proposed to rationalize the unique polymerization behavior. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.