Zirconium salicylaldiminates in the presence of AlBu3/[Ph3C][B(C6F5)(4)] are inactive for the homopolymerization of p-tert-butylstyrene (TBS), but the addition of hexene leads to a highly active system. Analysis of the polymers by NMR and MALDI indicates surprisingly that most of the product chains contain no hexene, while some contain a single hexene unit. Addition of dihydrogen also leads to the production of poly(TBS) without substantial reduction in molecular weight and with main-chain end-group unsaturation. These and other observations lead us to two related mechanisms for the hexene- and dihydrogen-promoted reactions. Synthetic and catalytic studies indicate that the active species is not a typical bis(salicylaldiminato) compound but an amido/alkoxido system. An authentic compound of this class is highly active for the TBS polymerization even in the absence of added aluminum alkyl.