Ethylene polymerizations were conducted using β-diketimine complexes of Zr [LZrX 3 (1) and L 2 ZrX 2 (2) with L ) MeC(NAr)CHC(NAr)Me and X ) Cl, R (R ) Me, Bn)] as well as Cp(L)ZrX 2 (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 °C were observed in polymerizations involving 3 (Ar ) Ph, X ) Cl, Me) when activated by MAO. The use of [Ph 3 C][B(C 6 F 5 ) 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 Me 3 Al or i Bu 3 Al. Little or no polymerization activity was observed when excess 3b was used as a scrubbing agent (in the presence of [Ph 3 C][B(C 6 F 5 ) 4 ]), and rapid loss of activity was observed when excess 3b was added to a catalyst system already producing PE. Complex 3b reacts withwhich was characterized by X-ray crystallography. Complex 4 forms dinuclear complexes with 3b or Me 3 Al in solution at room temperature. These complexes, [Cp(L)Zr(µ-Me) 2 AlMe 2 ][B(C 6 F 5 ) 4 ] (6) or [(Cp(L)ZrMe) 2 -(µ-Me)][B(C 6 F 5 ) 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 AlMe 3 (or MAO) appears to reversibly displace 3b from 7, allowing the reaction of 3b with [Ph 3 C]-[B(C 6 F 5 ) 4 ] to proceed to completion.