Formation, deactivation and transformation of stereospecific active sites con TiCl4/dibutylphthalate (DBP)/Mg(OEt)(2) Ziegler-Natta catalyst induced by short time reaction with triethylatuminum (TEA) cocatalyst (with TEA pretreatment time from 0 to 600 s) were investigated by stopped-flow propene polymerization combined with temperature rising elution fractionation (TREF) and GPC methods. It was demonstrated that both formation and deactivation of active sites with broad multiplicity in isospecificity on the catalyst are slow reactions with an induction period of ca. 0.2 s. It was most important to find that the formation of active sites with the highest isospecificity strongly depends on the interaction between the catalyst and cocatalyst (up to 60 s of pretreatment) even in the presence of internal donor. This newly observed phenomenon (according to our knowledge) suggested that the transformation of moncometallic active sites (aspecific or less isospecific) into bimetallic active sites (highly isospecific) through reversible complexing with TEA cocatalyst (or its reaction product diethylaluminum chloride (DEAC)) in Ziegler-Natta catalysts cannot be overlooked even in the presence of internal electron donor. The existence of -OC2H5 ligand in the catalyst most probably gave birth to a new group of active titanium species. The stability of active sites increases with increasing isospecificity in the early stage of pretreatment (up to 60 s of pretreatment). While all the active sites became relatively stable in the later stage of pretreatment (from 60 to 600 s of pretreatment). The extraction of internal donor DBP by TEA from the catalyst within the pretreatment procedure is found to initiate from 60 s of pretreatment resulting in slight transformation of isospecific active sites into aspecific sites. (C) 2002 Elsevier Science B.V. All rights reserved.