Correlation between Catalyst Surface Structure and Polypropylene Tacticity in Ziegler−Natta Polymerization System

The correlation between the surface structure of the Ziegler−Natta catalysts and the stereospecificity in propylene polymerization was shown experimentally using model catalysts and temperature-programmed desorption (TPD) of mesitylene as a nondestructive surface probe. Two types of titanium chloride model catalysts, with MgCl2 support (TiClx/MgCl2) and without MgCl2 (TiCly), were fabricated on an Au substrate in ultrahigh vacuum. Once activated with AlEt3 vapor, both catalysts were active for propylene polymerization in the absence of excess AlEt3 during polymerization. The TiClx/MgCl2 catalyst produced both atactic and isotactic polypropylene, while the TiCly catalyst without the MgCl2 support produced exclusively isotactic polypropylene. For the same catalysts, the mesitylene TPD revealed that TiClx/MgCl2 had two adsorption site structures, a basal plane structure and a nonbasal plane structure, while TiCly had only the latter structure. To our knowledge, this was the first direct experimental evidence revealing the structural difference of the catalyst surfaces that exhibit different stereospecificity in propylene polymerization.