FORMATION MECHANISM OF STEREOBLOCKS IN POLYPROPYLENE PRODUCED BY SUPPORTED ZIEGLER-NATTA CATALYSTS

Stereoblock structures are found in the poly(propylene) fractions of l ow isotacticity produced by supported Ziegler-Natta catalysts. The inf luences of electron donors on the length of stereoblock and AlEt3 on t he isotactic index of polymer were investigated. The formation of ster eoblock structures is correlated to the interaction between AlEt3 and active sites. The nature of the active sites which produce different s tereostructures and the reason that they can interconvert are explaine d with the concept of equilibrium. When electron donors are present in the catalysts, electron donors may exist in the environment of aspeci fic active sites and can be extracted by AlEt3, and these reactions ar e reversible. In the case of catalyst containing no electron donor, Al Et3 can complex with active sites, and there is an equilibrium reactio n between monometallic and bimetallic active sites. These equilibrium reactions are responsible for the formation of stereoblock structures.