Investigation of Ethylene Polymerization Using Soluble Cp2ZrCl2/MAO Catalytic System via Response Surface Methodology

Response surface methodology was used to investigate the kinetics of slurryhomopolymerization of ethylene which was catalyzed by soluble metallocenecatalytic system of Cp2ZrCl2/MAO. Polymerization temperature, monomer pressure, and molar ratio of cocatalyst to catalyst were considered as the main parameters which affect catalyst activity and viscosity average molecular weight of the final product. A Box-Behnken design was used to produce models for objective responses based on parameters that have significance probabilities (P-value<0.1). It was developed by using the three parameters at three levels including 50, 60, and 70°C for temperature; 2, 4, and 6 bar for pressure; and 3219, 4828.5, and 6438 for molar ratios of cocatalyst to catalyst. Analysis of variance showed that monomer pressure is the most affecting parameter on catalyst activity (P-value<0.0001). It was found that at high levels of cocatalyst concentration, the further increase of its concentration to higher levels decreases the catalyst activity which can be attributed to excessive complexation of MAO with active centres. Polymerization temperature was found to show the significant effect on viscosity average molecular weights of polymers (P-value = 0.0002). It is believed that higher activation energies of chain transfer and deactivation reactions relative to propagation reaction decrease the molecular weight at elevated temperatures. At higher molar ratio of cocatalyst to catalyst, molecular weight also decreases because of more chain transfer to cocatalyst.