NMR spectroscopy is used to directly monitor the behavior of an aluminum alkyl during the polymerization of 1-hexene by rac-(C₂H₄(1-indenyl)₂Zr(Me)(MeB(C₆F₅)₃).  The rates of polymerization are not inhibited by Al(iBu)₂(BHT), Al(Me)(BHT)₂, or Al(iBu)₃ (BHT = 2,6-di-tert-butyl-4-methylphenyl).  Detailed measurement of polymerization rate and catalyst speciation demonstrate that BHT modified aluminum alkyls protect active sites from decomposition in the presence of protic impurities such as methanol.