Polymerization of propene in the presence of MgCl2‐supported Ziegler‐Natta catalysts, 1. The role of ethyl benzoate as “internal” and “external” base

The effect of ethyl benzoate (EB) as a component of the solid catalyst (“internal” base) and/or of the co‐catalyst (“external” base) on propene polymerization in the presence of different MgCl2‐supported Ziegler‐Natta systems was investigated systematically. From the experimental results, it is suggested that internal EB mainly prevents TiCl4 from coordination on the MgCI2 crystal faces where mostly non‐stereospecific active sites would be formed, whereas external EB inhibits selectively the non‐stereospecific centres and decreases the reaction of the alkylaluminium with the internal base. A possible model of active sites for isotactic 1‐alkenes polymerization in the presence of MgCl2‐supported catalysts is presented, which is in good agreement with a large number of known experimental facts.