THE TRIGGER MECHANISM FOR POLYMERIZATION OF ALPHA-OLEFINS WITH ZIEGLER-NATTA CATALYSTS - A NEW MODEL BASED ON INTERACTION OF 2 MONOMERS AT THE TRANSITION-STATE AND MONOMER ACTIVATION OF THE CATALYTIC CENTERS

The trigger mechanism involves a two-monomer transition state, where the entering of a new monomer unit triggers the insertion of the already complexed monomer. The catalytic center is activated through the insertion of the first monomer, and a reaction scheme (the dynamic steady-state approach) for a repeated activation/deactivation of the active center is formulated. The model explains kinetic phenomena as a reaction rate order vs monomer concentration between 1.0 and 2.0, the complex kinetics of ethene/α-olefin copolymerization, and phenomena observed when reaction parameters are changed during the polymerization. If the catalytic center is an octahedral titanium complex with three facial ligands bridging a support surface, an isospecific insertion is predicted as a result of the interaction between the two monomers and between the monomers and the surface. The model predicts a very high and self-correcting microisotacticity, self-correcting stereoselectivity, and a lower regularity for the first insertion. A reaction rate higher for the isospecific centers compared to that of nonspecific centers is rationalized. Syndiospecific polymerization is predicted for nonchiral homogeneous catalysts. © 1991.