The propylene polymerizations with different half-sandwich metallocene complexes of the general formula, Me2Si[Me4CpNR]TiCl2, result in the formation of polymers with increased isotacticity as compared to atactic polypropylene. This was observed if a sterically demanding aromatic system at the amido-ligand coordinates with the polymerization active metal center. In this case, the metal center is partly shielded and the steric demand of the ligand is similar to an ansa-metallocene. The monomer partly coordinates stereospecifically at the metal center. Additionally, the solvent polarity has a great influence on the microstructure of the produced polymers. Using the complex Me2Si[Me4CpNCHCH3-naphthyl]TiCl2 in toluene, ortho-dichlorobenzene and hexane polypropylenes of similar microstructures were formed (mmmm approximate to 45%) whereas in methylene chloride as solvent, the isotacticity of the polypropylene was decreased by 50%.