Transition Metal Imido Catalysts for Ethylene Polymerisation

The imido complexes CpV(N-2-MeC6H4)Cl2 (1), Cr(N-tBu)2Cl2 (2), CpNb(N-2-tBuC6H4)Cl2 (3) and Mo(N-tBu)2Cl2 (4) have been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium chloride or methylaluminoxane (MAO) co-catalysts. The vanadium precursors give the highest activities but are short-lived, while the chromium system gives a long-lived catalyst of moderate activity. The niobium and molybdenum derivatives gave relatively low activities under all test conditions. The polyethylene generated by the vanadium and chromium catalysts is of high molecular weight with little branching. The dialkyl complexes Cr(N-tBu)2(CH2Ph)2 (5), Cr(N-2,6-Pri2C6H3)2(CH2Ph)2 (6), Cr(N-2,6-Pri2C6H3)2Me2 (7), Mo(N-tBu)2(CH2Ph)2 (8), Mo(N-2,6-Pri2C6H3)2(CH2Ph)2 (9), (C5Me5)Nb(N-2,6-Pri2C6H3)Me2 (10) and (C5Me5)Ta(N-tBu)(CH2Ph)2 (11) have been prepared by treatment of the dihalide precursors with appropriate alkylating reagents and investigated as precursors to well-defined cationic alkyl catalysts. Treatment of (5) with [Ph3C][B(C6F5)4] affords the cationic ¿2-benzyl species [Cr(N-tBu)2(¿2-CH2Ph)][B(C6F5)4] (12) while its reaction with [PhNMe2H][B(C6F5)4] liberates toluene to give a mixture of the mono and bis(dimethylaniline) adducts [Cr(N-tBu)2(¿2-CH2Ph)(NMe2Ph)][B(C6F5)4] (14) and [Cr(N-tBu)2(¿2-CH2Ph)(NMe2Ph)2][B(C6F5)4] (15). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N-tBu)2(¿2-CH2Ph)(PMe3)][B(C6F5)4] (13). Solutions containing cationic species (12, 14, 15) are active for ethylene polymerisation in the absence of co-catalyst, affording high-molecular-weight polyethylene with relatively broad molecular-weight distributions