Polymerization behavior of α-olefins with rac- and meso-type ansa-metallocene catalysts: Effects of cocatalyst and metallocene ligand

Polymerizations of propene, 1-butene and 1-hexene were conducted with a mixture of rac- and meso-[dimethylsilylenebis(2-methylindenyl)]zirconium dichloride (1) combined with methylaluminoxane (MAO), triethylaluminium (AlEt₃)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (2) or triisobutylaluminium (AliBu₃)/2 as a cocatalyst. The polymerization profiles of propene with rac-1 and meso-1 were determined from the rate of overall propene consumption and the fractions of isotactic and atactic polymers which were sampled during polymerization. An induction time to reach the maximum R_p (rate of polymerization) followed by gradual decay was observed in the case of using the systems rac-, meso-1-MAO and rac-1-AliBu₃/2. Besides, a rapid drop of R_p from the initial value was found when using AlEt₃/2. Molecular weights of the isotactic and atactic polymers sampled do not change during polymerization, and it is suggested that the change of [C*] (number of active centers) is reflected in the profiles of R_p. The rate ratio of rac-1 to meso-1 (R_p(rac)/R_p(meso)) in propene and 1-butene polymerizations decreases in the following order: AlEt₃/2 > AIiBu₃/2 > MAO. In the case of 1-hexene polymerization, the highest R_p(rac)/R_p(meso) value was obtained. This result indicates that the coordination of 1-hexene to the sterically hindered site of meso-1 is difficult compared with propene and 1-butene.