An olefin polymerization process is described. The process comprises preparing a catalyst system by premixing a first organoaluminum with a supported boraaryl catalyst and then polymerizing an olefin in the presence of the premixed supported catalyst, an activator and a second organoaluminum. The process produces polyolefins having broad and/or bimodal molecular weight distributions compared to the process in which either the first or the second organoaluminum is not used.