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 MgCl 2 support (TiCl x /MgCl 2 ) and without MgCl 2 (TiCl y ), were fabricated on an Au substrate in ultrahigh vacuum. Once activated with AlEt 3 vapor, both catalysts were active for propylene polymerization in the absence of excess AlEt 3 during polymerization. The TiCl x /MgCl 2 catalyst produced both atactic and isotactic polypropylene, while the TiCl y catalyst without the MgCl 2 support produced exclusively isotactic polypropylene. For the same catalysts, the mesitylene TPD revealed that TiCl x /MgCl 2 had two adsorption site structures, a basal plane structure and a nonbasal plane structure, while TiCl y 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.