Polymerization behavior of alpha-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 mix ture of rac- and meso-[dimethylsilylenebis(2-methylindenyl)]zirconium dichl oride (1) combined with methylaluminoxane (MAO), triethylaluminium (AlEt3)/ triphenylcarbenium tetrakis(pentafluorophenyl)borate (2) or triisobutylalum inium (AliBu(3))/2 as a cocatalyst. The polymerization profiles of propene with rac-1 and meso-1 were determined from the rate of overall propene cons umption and the fractions of isotactic and atactic polymers which were samp led during polymerization. An induction time to reach the maximum R-p (rate of polymerization) followed by gradual decay was observed in the case of u sing the systems rac-, meso-1-MAO and rac-1-AliBu(3)/2. Besides, a rapid dr op of R-p from the initial value was found when using AlEt3/2. Molecular we ights of the isotactic and atactic polymers sampled do not change during po lymerization, 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 m eso-1 (R-p(rac)/R-p(meso)) in propene and 1-butene polymerizations decrease s in the following order: AlEt3/2 > AliBu(3)/2 > MAO. In the case of 1-hexe ne polymerization, the highest R-p(rac)/R-p(meso) value was obtained. This result indicates that the coordination of 1-hexene to the sterically hinder ed site of meso-1 is difficult compared with propene and 1-butene.