Optimization of a silica supported bis(butylcyclopentadienyl)zirconium dichloride catalyst for ethylene polymerization

A series of heterogeneous alpha-olefin polymerization catalysts were prepar ed by the immobilization of bis(butylcyclopentadienyl)zirconium dichloride, (nBuCp)(2)ZrCl2, on a commercial silica support (Grace 948) using differen t procedures. The preparation parameters, namely, silica activation tempera ture, grafting temperature, grafting time, and solvent, were evaluated in t erms of metal content on silica and ethylene homopolymerization activity. M etal contents were determined by Rutherford back-scattering spectrometry (R BS). In the temperature activation range between 373 and 723 K, silica surf ace saturation in Zr was found to be around 0.34 wt.-% Zr/SiO2. However, po lymer polydispersity is shown to decrease with increasing support activatio n temperature. A better control in the generation of the active surface spe cies was achieved with thermal pretreatment temperatures close to 723 K. Th e grafting reaction was seen to be immediate. Longer grafting times or high er temperatures bore deactivated species. Practically all the systems were active in ethylene polymerization in the presence of MAO, but the highest y ield was obtained after grafting at 353 K for 1 h in toluene solution, empl oying silica pretreated at 723 K.