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

We report an experimental investigation on long chain branching (LCB) in et hylene slurry polymerization with bis(cyclopentadienyl) zirconium dichlorid e (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 o f branches per 10 000 carbons), polymer molecular weight, and shear thinnin g property (I-10/I-2) were systematically examined. The slurry polymerizati on process, with its associated polymer-rich phase and the partitioning of active sites, favors thr: LCB formation via an in situ copolymerization of ethylene macromonomers generated by beta-hydride elimination and chain tran sfer to monomer. Increasing the temperature from 60 to 80 degrees C reduced the LCBD from 0.33 to 0.10, while increasing the pressure from 2 to 20 psi g reduced the LCBD from 0.73 to 0.30. The LCB polyethylenes showed enhanced shear thinning properties, with melt flow index ratios (I-10/I-2) in the r ange 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. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.