The polymerization of ethylene and ethylene/1-octene in a semibatch solution reactor using rac-dimethylsilylbis(indenyl)hafnium dimethyl and tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)(4)](-)[Me2NHPh](+)) as the catalytic system and trioctylaluminum (TOA) as the scavenger was investigated. Ethylene and 1-octene concentrations, polymerization temperature, borate/catalyst ratio, and TOA concentration were changed to study the ethylene polymerization kinetics with this system. The mode of addition for catalyst, borate, and TOA was also studied. When TOA and borate were added sequentially to the reactor followed by the catalyst, the polymerization activity was low and the molecular weight distribution (MIND) bimodal. Contrarily, when TOA was added first to the reactor and a mixture of catalyst and borate were added to the reactor, the polymerization rate was much higher and the MIND unimodal. An augmented 2 X 2 central composite design was used to investigate this phenomenon. Borate helped stabilize the active sites and reduce potential deactivation reactions with excess TOA.