The invention is directed to catalyst systems for the oligomerization of ethylene. In some embodiments, the catalyst system includes ethylene, a solvent, and a catalyst activated with an aluminoxane co-catalyst. The system may further include an additive. Various modifications to the different reaction parameters may be made to fine-tune the reaction properties, such as productivity, activity and selectivity for a particular oligomer. For example, the solvent may be modified to include a mixture of at least two solvents. Alternatively, the co-catalyst may be modified by aging or partially fluorinating the co-catalyst. In another alternative, an additive may be included in the reaction system. Each modification to the reaction system effects a different reaction outcome, thereby enabling fine-tuning of the reaction properties by varying the modifications to the reaction system.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 The U.S. Government has certain rights in this invention pursuant to Grant No. DE-FG02-85ER13431 awarded by the Department of Energy.