Site selective ligand modification and tactic variation in polypropylene chains produced with metallocene catalysts

This review covers the stereochemistry of propylene insertion/propagation reactions with a variety of Cs and C1 symmetric, bridged cyclopentadienyl-fluorenyl ligands containing metallocene catalysts for the preparation of highly stereo-regular polypropylene. The article discusses the syndioselectivity of metallocenes with local Cs symmetry and two enantio-topic coordination positions, and proof that from all the different possible substitutional positions, only those performed in the frontal positions of the fluorenyl ligand are effective in improving the syndio-selectivity of the final catalysts. It further establishes that the bilateral symmetry selection rule is not a sufficient condition and will be “ignored” by the system, whenever symmetric “excess” in steric bulk is present. An unsymmetrical “excess” in steric forces, induced by a bulky distal substituent on the other hand, will provoke a full tactic inversion as a consequence of an irreversible Cs/C1 site transformation. The review also covers the combined distal, proximal, and frontal substituent effect and their impact on molecular weight and regio- and stereoregularity of the final polymers. Finally, isoselective C2 and C1 symmetric systems are compared with respect to their potential for commercial application.