STEREOSELECTIVE POLYMERIZATION OF ALPHA-OLEFINS BY HETEROGENEOUS CHIRAL ZIEGLER-NATTA CATALYSTS

MgCl2-supported TiCl3 catalysts were synthesized to contain either chiral (racemic or optically active) or nonchiral Lewis basic esters. They were compared along with ordinary TiCl3 in the stereospecific polymerization of propylene and in the stereoselective and stereoelective polymerizations of chiral 3- and 4-methyl-substituted olefins. The number and type of active sites, kinetic parameters, stereoregularity and optical purity of the polyolefins produced, and optical purity of unpolymerized monomer have been determined. The results suggest that active sites differing in steric constraints are responsible for the polymerizations of various monomers. The catalyst modified with the optically active Lewis base possesses several times more active centers of both stereospecific as well as nonstereospecific kinds than the catalyst modified with the corresponding racemic Lewis base in both propylene and racemic 4-methyl-1-hexene polymerizations. The results indicate a mechanism of site stereoselection by the chiral Lewis base. Polymerization of racemic 4-methyl-1-hexene, 3-methyl-1-hexene, and 3,7-dimethyl-1-octene by an optically active catalyst preferentially consumes one antipode showing stereoelectivity for the process.