Mechanism of Isotactic Polypropylene Formation with C_1-Symmetric Metallocene Catalysts

The mechanism for isotactic polypropylene formation by the C_1-symmetric catalyst system [Me_2C(3-tert-butyl-C_5H_3)(C_(13)H_8)]ZrCl_2/MAO (MAO = methylaluminoxane, C13H8 = fluorenylidene) has been examined. Evidence supports an alternating mechanism, where both sites of the metallocene wedge are utilized for monomer insertion, rather than the previously proposed site epimerization (inversion at Zr) following each monomer insertion. As the polymerization temperature increases (0 to 60 °C) with lower concentrations of propylene, the site epimerization mechanism begins to compete, as evidenced by an increase in isotacticity. The alternating mechanism also accounts for polypropylene microstructures obtained with Me_2C(3-R-C_5H_3)(Oct)ZrCl_2/MAO, where Oct = octamethyloctahydrodibenzofluorenylidene and R = methyl, cyclohexyl, diphenylmethyl, and with Me_2C(3-tert-butyl-4-Me-C_5H_2)(Oct)ZrCl_2/MAO. For an Oct-containing catalyst system with R = 2-methyl-2-adamantyl, unprecedentedly high (for a fluorenyl-based metallocene catalyst) isotacticity ([mmmm] > 99%) is obtained; the polymer prepared at 0 °C has T_m = 167 °C and M_w = 370 000.