Group 4 metallocene catalyst systems have been well established in olefin polymerization. In particular, chiral ansa metallocene systems have been intensively investigated to achieve stereochemical control of propylene polymerization by varying the ligand structure. On the other hand, in most cases, unbridged metallocenes give atactic microstructure for polypropylene due to free ligand rotation. When we consider the relative synthetic easiness of unbridged metallocenes compared to ansa-metallocenes, it is still of great interest to develop unbridged metallocene catalytic systems capable of inducing stereospecific polymerization of propylene. In this regard, we prepared unbridged metallocene catalytic systems, [1-(E-C₆H₄)-3,4-Me₂C₅H₂]₂ZrCl₂ (E= p-NMe2, p-OMe, p-SMe), that can produce highly isotactic polypropylene under MAO activation at atmospheric monomer pressure and liquid propylene condition. In spite of their aspecific nature, the generation of rigid rac-like cationic active species in situ by the interaction between basic sites of catalysts and acidic sites of [Me-MAO]^- counter anion is considered the origin of the observed isospecificity. Details of synthesis, structures and the polymerization properties of the foregoing unbridged zirconocenes will be presented.