A DENSITY-FUNCTIONAL STUDY OF CHAIN GROWING AND CHAIN TERMINATING STEPS IN OLEFIN POLYMERIZATION BY METALLOCENE AND CONSTRAINED GEOMETRY CATALYSTS

Nonlocal density functional (DF) calculations have been carried out on the insertion of ethylene into the metal-CH3 bond of the Kaminsky type metallocenes: CP2ZrCH3+ (2a), (SiH2CP2)ZrCH3+ (3a), CP2ScCH3 (4a), and the constrained geometry catalyst (CGC) (SiH2(Cp)NH)ZrCH3+ (5a). The objective has been to study the energy profile for the insertion involving 2a and how it is modified by introducing the silane bridge in 3a, replacing one Cp with NH in 5 or switching to the neutral system 4a. The DF calculations reveal that the insertion into the bis-Cp systems proceeds with a modest barrier of 3, 4, and 14 kJ/mol for 2-4, respectively. This barrier is marginally influenced by going from the charged species CP2ZrCH3+ (2a) to the neutral system CP2ScCH3 (4a) and is unchanged in going from the unbridged system CP2ZrCH3+ (2a) to the silane bridged system (3a). The systems 2a, 3a, and 4a all form a pi-complex with a very shallow minimum at the beginning of the insertion process. The insertion process for the zirconium CGC (5a) exhibits a clear activation barrier and a pronounced minimum for the pi-complex. The deeper potential well of the pi-complex in 5a compared to 2a, 3a, and 4a can be related to a reduction in the steric interaction between ethylene and the ligands on the metal center as one Cp ring is replaced by a NH ligand. The higher barrier in the case of 5a is a consequence of the stable pi-complex which has to be abandoned in order to proceed to the insertion product. Calculations have also been carried out on two chain terminating steps. The first step involved beta-hydride elimination. It is concluded that this process is unfavorable for early transition metal centers with an endothermicity of 176 kJ/mol. The second step is concerned with the activation of an ethylene C-H bond by the metallocene to form an alkane and a vinylzirconocene. It is concluded that this process is viable as a chain terminating step.