Long chain branching in ethylene polymerization using constrained geometry metallocene catalyst

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 degrees C, 3.45 x 10(3) kPa, and a mean residence time (tau) 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 x 10(3) to 1.36 x 10(1) 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 I-10/I-2 were in the range of 6.96 to 23.4, with the I-2 of 0.172 to 0.681 g/10 min. The weight-average molecular weight (M) over bar(W), was correlated to I-2 using a power equation within narrow I-10/I-2 ranges. The exponential factors were in the range of 4.24 to 6.31. The experimental and calculated (M) over bar(W)'s were in a good agreement.