STEREOSPECIFIC POLYMERIZATION BY METALLOCENE ALUMINOXANE CATALYSTS

In the past, only heterogeneous Ziegler-Natta catalysts were known to produce highly isotactic polyolefins. In recent years a large number of different chiral metallocenes have been found that, in combination with methylaluminoxane, also give isotactic polymers. The homogeneous systems allow the polymerization of cyclic olefins without any ring-opening reaction. The metallocene and the aluminoxane form a complex that is nearly colloidal. The complexation is followed by an alkyl exchange and a beta-hydrogen abstraction, forming methane. The methane production depends on the catalyst concentration and leads to polymerization-inactive compounds.