Transition metal imido catalysts for ethylene polymerisation

The imido complexes CpV(N-2-MeC6H4)Cl-2 (1), Cr(N-'Bu)(2)Cl-2 (2), CpNb(N-2 -'BuC6H4)Cl-2 (3) and Mo(N-'Bu)(2)Cl-2 (4) have been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium ch loride or methylaluminoxane (MAO) co-catalysts. The vanadium precursors giv e the highest activities but are short-lived while the chromium system give s a long-lived catalyst of moderate activity. The niobium and molybdenum de rivatives gave relatively low activities under all test conditions. The pol yethylene generated by the vanadium and chromium catalysts is of high molec ular weight with little branching. The dialkyl complexes Cr(N-'Bu)(2)(CH2Ph )(2) (5), Cr(N-2,6-Pr-2'C6H3)(2)(CH2Ph)(2) (6), Cr(N-2,6-Pr-2'C6H3)(2)Me-2 (7), Mo(N'Bu)(2)(CH2Ph)(2) (8), Mo(N-2,6-Pr'2C6H3)(2)(CH2Ph)(2) (9), (C5Me5 )Nb(N-2,6-Pr'2C6H3)Me-2 (10) and (C5Me5)Ta(N-'Bu)(CH2Ph)(2) (11) have been prepared by treatment of the dihalide precursors with appropriate alkylatin g reagents and investigated as precursors to well-defined cationic alkyl ca talysts. Treatment of (5) with [Ph3C][B(C6F5)(4)] affords the cationic eta( 2)-benzyl species [Cr(N-'Bu)(2)(eta(2)-CH2Ph)][B(C6F5)(4)] (12) while its r eaction with [PhNMe2H][B(C6F5)(4)] liberates toluene to give a mixture of t he mono and bis(dimethylaniline) adducts [Cr(N-'Bu)(2)(eta(2)-CH2Ph)(NMe2Ph )][B(C6F5)(4)] (14) and [Cr(N-'Bu)(2)(eta(2)-CH2Ph)(NMe2Ph)(2)][B(C6F5)(4)] (15). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N-'Bu)(2)(eta(2)-CH2Ph)(PMe3)][B(C6F5)(4)] (13). Solutions cont aining cationic species (12, 14, 15) are active for ethylene polymerisation in the absence of co-catalyst, affording high-molecular-weight polyethylen e with relatively broad molecular-weight distributions. (C) 1999 Elsevier S cience S.A. All rights reserved.