Homo- and copolymerization of ethylene by cationic hafnocene catalysts based on tetrakis(pentafluorophenyl)borate

Ethylene homopolymerization and ethylene/1-hexene copolymerization were conducted at different temperature and ethylene pressure using several hafnocenes activated with dimethylanilinium tetrakis(pentafluorophenyl)borate (Me2PhNH·B(C6F5)4). rac-Ethylene(bisindenyl)hafnium dichloride (rac-Et(Ind)2-HfCl2) activated with Me2PhNH·B(C6F5)4 shows ten times higher activity than methylaluminoxane(MAO)-activated catalyst for ethylene polymerization at 40°C, and produces high molecular weight polyethylene with high activity even at 150°C independent of ethylene pressure. The molecular weight of ethylene/1-hexene copolymers synthesized at high temperature is influenced by the ligand structure, and a rac-(dimethylsilyl)bis(2,4-dimethylcyclopentadienyl)hafnium dichloride (rac-Me2Si(2,4-Me2Cp)2HfCl2)-based catalyst exceptionally produces only low molecular weight copolymers. These copolymers contain high amount of vinylidene end groups, indicating that β-H transfer from propagating chains containing primary inserted 1-hexene as terminal unit occurs frequently. At high ethylene pressure, the rac-Et(Ind)2HfCl2-based catalyst produces high molecular weight ethylene/1-hexene copolymers with high activity, but this activity is slightly lower than that of zirconium analogs.