Polymerization Behavior of <i>C</i>₁-Symmetric Metallocenes (M = Zr, Hf): From Ultrahigh Molecular Weight Elastic Polypropylene to Useful Macromonomers

Abstract

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)/[CPh3][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.