Kinetic Modeling of the Effect of MAO/Zr Ratio and Chain Transfer to Aluminum in Zirconocene Catalyzed Propylene Polymerization

A kinetic model based on the coordination−insertion mechanism was developed to characterize metallocene-catalyzed propylene polymerization using two different catalyst systems:  rac-Et(Ind)2ZrCl2/MAO (I/MAO) and rac-Et(4,7-Me2-1-Ind)2ZrCl2/MAO (II/MAO). Slurry propylene polymerizations were performed in a semibatch reactor at 40 °C to investigate the effects of propylene partial pressure and MAO/Zr ratio. The kinetic model accounts for the formation of regioirregularities, the occurrence of chain transfer to trimethylaluminum (TMA), and -hydride chain transfer to both monomer and metal to predict the effects of propylene partial pressure and the MAO concentration on polymer molecular weight and the formation of isobutyl end groups. A systematic optimization strategy was applied to estimate the kinetic parameters from on-line measurements of the reaction rate and end-of-batch measurements of the molecular weights and percentages of end groups. The formation of 2,1-insertions was more frequent for catalyst II/MAO (ks 97.4 L mol-1s-1vs ks 49.4 L mol-1s-1for I/MAO). The Mwof polymer produced with I/MAO decreased at low pressures due to the high rate of monomolecular -hydride transfer to the metal (kH 26.5 s-1). Chain transfer to TMA was more significant with the catalyst II/MAO (kAl 5.46 × 103L mol-1s-1vs kAl 1.97 × 103L mol-1s-1for I/MAO).