Ethylene polymerization using β-diketimine complexes of zirconium

Ethylene polymerizations were conducted using beta-diketimine complexes of Zr [LZrX3 (1) and L2ZrX2 (2) with L = MeC(NAr)CHC(NAr)Me and X = Cl, R (R = Me, Bn)] as well as Cp(L)ZrX2 (3) in the presence of MAO. Complexes 1 possess low polymerization activities (ca. 10(5) g PE/(mol Zr h)) and provide PEs with a trimodal, molecular weight distribution. Both 2 and 3 behave as single-site catalysts under these conditions, with the activity of the latter being considerably higher than the former (ca. 10(6)-10(7) vs 10(5) g PE/(mol Zr h)). Induction periods of 10-20 min at 70 degrees C were observed in polymerizations involving 3 (Ar = Ph, X = Cl, Me) when activated by MAO. The use of [Ph3C][B(C6F5)(4)] and 3b (Ar = Ph, X = Me) in the presence of small amounts of MAO (ca. 100:1 Al:Zr) led to rapid monomer uptake and an increase in catalytic activity by about a factor of 4. Lower activities were observed in the presence of Me3Al or (Bu3Al)-Bu-i. Little or no polymerization activity was observed when excess 3b was used as a scrubbing agent (in the presence of [Ph3C][B(C6F5)(4)]), and rapid loss of activity was observed when excess 3b was added to a catalyst system already producing PE. Complex 3b reacts with [Ph3C][B(C6F5)(4)] in chlorobenzene-d(5) solution at 25 degrees C to provide [Cp(L)ZrMe][B(C6F5)(4)] (4) Complex 4 is transformed on heating in chlorobenzene to a dicationic (mu-Cl)(2) complex [(Cp(L)ZrCl)(2)][B(C6F5)(4)](2) (5), which was characterized by X-ray crystallography. Complex 4 forms dinuclear complexes with 3b or Me3Al in solution at room temperature. These complexes, [Cp(L)Zr(mu-Me)(2)AlMe2][B(C6F5)(4)] (3) or [(Cp(L)ZrMe)(2)-(mu-Me)][B(C6F5)(4)] (7), are fluxional in solution; at lower temperature, the solution NMR spectra are consistent with the structures shown. Complex 3b is a potent inhibitor of ethylene polymerization by 4, forming 7, which is resistant to dissociation. The presence of AlMe3 (or MAO) appears to reversibly displace 3b from 7, allowing the reaction of 3b with [Ph3C][B(C6F5)(4)] to proceed to completion.