Long-chain branching in slurry polymerization of ethylene with zirconocene dichloride/modified methylaluminoxane

We report an experimental investigation on long chain branching (LCB) in ethylene slurry polymerization with bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2)/modified methylaluminoxane (MMAO) using a semi-batch reactor. The effects of the reaction temperature, pressure, MMAO concentration, and catalyst feeding method on the long chain branching density (LCBD, number of branches per 10 000 carbons), polymer molecular weight, and shear thinning property (I10/I2) were systematically examined. The slurry polymerization process, with its associated polymer-rich phase and the partitioning of active sites, favors the LCB formation via an in situ copolymerization of ethylene macromonomers generated by β-hydride elimination and chain transfer to monomer. Increasing the temperature from 60 to 80°C reduced the LCBD from 0.33 to 0.10, while increasing the pressure from 2 to 20psig reduced the LCBD from 0.73 to 0.30. The LCB polyethylenes showed enhanced shear thinning properties, with melt flow index ratios (I10/I2) in the range of 8.8–21.5. The feeding sequence of reactants also had a significant effect on the LCB formation. It was observed that feeding ethylene monomer before zirconocene catalyst produced polyethylenes having much higher LCBD than feeding catalyst before monomer.