Copolymerizations of ethylene and 1-hexene were carried out in hexane at 60 degreesC by using ethylenebis(indenyl)zirconium dichloride as catalyst and methylaluminoxane (MAO) as cocatalyst in presence of triisobutylaluminum (TIBA). The influence of the alkylaluminum concentration and Al-TOTAL/Zr molar ratio on the catalyst activity and comonomer incorporation are evaluated. Increasing the TIBA concentration, the MAO solubility augments in the polymerization milieu, as well as the catalyst activity and the comonomer incorporation. Increasing the Al-TOTAL/Zr molar ratio, catalyst activity becomes higher, while comonomer incorporation remained constant at around 8 wt% for molar ratios between 0 and 5000. For Al-TOTAL/Zr molar ratios higher than 5,000 comonomer incorporation increased gradually. Under the same experimental conditions dimethylsilylbis(indenyl)zirconium dichloride showed higher thermal stability and led to higher comonomer incorporation.