Chain “stationary” insertion mechanism and production of isotactic polypropylene with C₁ symmetric catalyst systems

In this contribution, the stereochemistry of propylene insertion/propagation reactions with a variety of C-1 symmetric metallocene catalysts, containing bridged cyclopentadienyl-fluorenyl ligand for the preparation of highly stereoregular polypropylene is presented. The impact of the distal substituent's size and composition and changes that the catalytic sites undergo upon such substitution is elaborated. A comprehensive mechanism is proposed to explain the resulting catalytic changes that bring about the irreversible C-s/C-1 site transformation and tactic behavior inversion. Furthermore the cyclopentadienyl's combined distal/proximal and fluorenyl's frontal substituent effects on molecular weight, regio-, and stereoregularity of the final polymers are discussed. Finally, stereoselectivities of C-2 and C-1 symmetric catalyst systems are compared. It is shown that current high performance C-1 symmetric catalyst systems with central site chirality can be isotactic selective as well or even better in certain aspects than the C-2 symmetric bridged bisindenyl-based metallocene catalysts.