In propylene polymerization with MgCl2-supported Ziegler-Natta catalysts, it is known that the reduction of TiCl4 with alkylaluminum generates Ti3+ active species, and at the same time, leads to the growth of TiClx aggregates. In this study, the aggregation states of the Ti species were controlled by altering the Ti content in a TiCl3/MgCl2 model catalyst prepared from a TiCl3 center dot 3C(5)H(5)N complex. It is discovered that all the Ti species become isolated mononuclear with a highly aspecific feature below 0.1 wt.-% of the Ti content, and that the isolated aspecific Ti species are more efficiently converted into highly isospecific ones by the addition of donors than active sites in aggregated Ti species.