Abstract:
Since the discovery of TiCl4/AlEt3 catalyst for olefin polymerization in the early 1950s, many generations of such Ziegler-Natta catalysts have been developed. The most active generation is TiCl4 supported on MgCl2. At the same time, other homogeneous-type catalysts have also been synthesized and used in ethylene polymerization. It was not until the mid 1980s that the first highly active uniform site catalyst for polyethylene was developed. It consisted of a Group IV transition metal complex, called metallocene, used together with excess methylaluminoxane (MAO). This catalyst system affords a wide range of polyolefins with controlled molecular weight and stereostructures. Because high amount of MAO over catalyst is required for acceptable activity, other kinds of cocatalyst have been developed to reduce the amount of MAO. In this work, bis(cyclopentadienyl) zirconium dichloride (Cp2zrCl2) was used with trimethylaluminum (TMA) and tris(pentafluorophenyl)borane (B(C6Fs)3) to compare with TiCl4/MgCl2 Ziegler catalyst. The morphology of the polyethylene produced using these two catalyst systems was studied. MAO-free metallocene systems show less activity than the highly active Ziegler-Natta catalyst but still retain the advantages of a uniform site catalyst.
