Polymerization behavior of C ₁-symmetric metallocenes (M = Zr, Hf): From ultrahigh molecular weight elastic polypropylene to useful macromonomers

The C₁-symmetric metallocenes rac-[1-(9-η⁵- fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η⁵-indenyl)ethane] zirconium dichloride (1) and rac-[1-(9-η⁵-fluorenyl)-2-(5,6- cyclopenta-2-methyl-1-η⁵-indenyl)ethane]hafnium dichloride (2) are known to produce elastic polypropylene. They were investigated concerning their temperature stability during the polymerization of propene. After activation of these complexes with triisobutylaluminum (TIBA)/[CPh ₃][B(C₆F₅)₄], first polymerization experiments (80-100 C) afforded moderate to high activities. However, at these polymerization temperatures, the molecular weights of the produced polymers are significantly decreased, resulting in a waxy appearance and, therefore, a loss of the elastic behavior. The main reason for this behavior, especially for the more significant decrease of the molecular weight in the case of hafnocene 2 compared with zirconocene 1, was revealed to occur due to a fast β-methyl transfer reaction. Hence, hafnocene 2 can form polymer chains with a high selectivity toward allylic chain ends. These macromonomers can be used in the catalytic insertion polymerization for the formation of new grafted copolymers. Initial copolymerization experiments with ethene were conducted.