LIVING POLYMERIZATION OF ETHYLENE CATALYZED BY DIENE COMPLEXES OF NIOBIUM AND TANTALUM, M(ETA(5)-C(5)ME(5))(ETA(4)-DIENE)X(2) AND M(ETA(5)-C(5)ME(5))(ETA(4)-DIENE)(2)(M= NB AND TA), IN THE PRESENCE OF METHYLALUMINOXANE

Catalyst systems of MX(2)(eta(5)-C(5)R(5))(eta(4)-diene) (M = Nb and Ta; R = H and CH3; X = Cl and CH3) in a combination of methylaluminoxane (MAO) are the precursors of the living polymerization of ethylene. Mono-diene complexes MCl(2)(eta(5)-C(5)R(5))(eta(4)-diene) [M = Nb, R = CH3 (1); M = Ta, R = CHS (2); M = Ta, R = H (3); M = Nb, R = H (4); diene = buta-1,3-diene (a), isoprene (b), 2,3-dimethylbuta-1,3-diene (c)] were prepared by the reaction of MCl(4)(eta(5)-C(5)R(5)) with 2 equiv of the methylated allyl Grignard reagents in THF. Reaction of 2 and 3 with MeMgI afforded dimethyl complexes, Ta(eta(5)-C(5)R(5))(eta(4)-1,3-butadiene)Me(2) [R = CH3 (5); R = H (6)], respectively. The polymerization of ethylene catalyzed by 1/MAO, 2/MAO, and 5/MAO at low temperature (-20 degrees C) gave polyethylene with very narrow polydispersities (M(w)/M(n) as low as 1.05). The niobium complexes are superior to the tantalum complexes in terms of the catalyst activity and the polydispersity. When the ligand was Cp instead of Cp*, the catalyst activity of 3 for the polymerization of ethylene at -20 degrees C increased but the polydispersity of the obtained polyethylene broadened (M(w)/M(n) = 1.40). Protolytic reaction of 5b with 1 equiv of TfOH produces TaCp*(eta 4-isoprene)(CH3)(OSO2CF3) (7), whose structure is determined by single-crystal X-ray diffractometry. Compound 7 crystallizes in the orthorhombic space group P2(1)2(1)2(1) (No. 19) with a = 13.388(6) Angstrom, b = 16.196(4) Angstrom, c = 9.214(4) Angstrom, Z = 4, V = 1998(1) Angstrom(3), D-calcd = 1.823 g/mL, and R = 0.041 based on 2191 reflections. The molecular structure shows that 7 is not a cationic complex. The tantalum atom is in a chiral center surrounded by four different ligands: eta(1)-triflate, eta(5)-Cp*, eta(4)-isoprene, and methyl. A cationic species derived from 5b and B(C6F5)(3) was detected by H-1 NMR spectroscopy and found to be active in ethylene polymerization. A system of one of the bis-diene complexes of the type M(eta 5-C(5)R(5))(eta(4)-2,3-dimethyl-1,3-butadiene)(2) [M = Nb, R = H (8); M = Nb, R = CH3 (9); M = Ta, R = CH3 (10)] in the presence of a large excess of MAO was also found to be an active catalyst for the polymerization of ethylene.