Long chain branching in ethylene polymerization using constrained geometry metallocene catalyst

Abstract

We report an experimental investigation on long chain branching (LCB) in ethylene polymerization with the Dow Chemical's constrained geometry catalyst system, CGC-Ti/TPFPB/MMAO, using a continuous stirred-tank reactor (CSTR) at 140°C, 3.45 × 103 kPa, and a mean residence time (τ) of 4 min. The effects of the catalyst (CGC-Ti) and co-catalyst (TPFPB and MMAO) concentrations on the catalyst activity, polymer molecular weight, and shear thinning were systematically examined. The boron cocatalyst had a great influence on the CGC activity. Increasing the ratio TPFPB/CGC-Ti from 0.66 to 5 gave ethylene propagation rates from 1.65 × 103 to 1.36 × 104 L · mol−1 · s−1. The addition of MMAO appeared to be essential, most likely acting as an impurity scavenger. The LCB polyethylenes showed enhanced shear thinning properties. The melt flow index ratios I10/I2 were in the range of 6.96 to 23.4, with the I2 of 0.172 to 0.681 g/10 min. The weight-average molecular weight Mw was correlated to I2 using a power equation within narrow I10/I2 ranges. The exponential factors were in the range of 4.24 to 6.31. The experimental and calculated Mw's were in a good agreement.