The structure of active centers in Ziegler–Natta catalysts is well established only for metallocene catalysts. An idealized and simplified metallocene complex CpZrCl represents a typical precatalyst.  It is unreactive toward alkenes.  The dihalide reacts with MAO and is transformed into a metallocenium ion CpCH, which is ion-paired to some derivative(s) of MAO. A polymer molecule grows by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:
Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center.  The Cossee–Arlman mechanism describes the growth of stereospecific polymers. This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center.