Back-Skip of the Growing Chain at Model Complexes for the Metallocene Polymerization Catalysis

Abstract

A molecular mechanics analysis on model metallocene complexes, as possible intermediates for the propene polymerization, in which the two coordination positions available for the monomer and the growing chain are diastereotopic, is presented. The energy difference between the corresponding diastereoisomeric preinsertion intermediates appears to be relevant for the model complexes based on the meso-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) ligand (1), the isopropylidene[(3-methyl(η5-cyclopentadienyl))(η5-9-fluorenyl)] ligand (2), and the rac-ethylene[1-(η5-9-fluorenyl)-1-phenyl-2-(η5-1-indenyl)] ligand (3), while it appears to be small for those based on the rac-ethylene[1-(η5-cyclopentadienyl)-1-phenyl-2-(η5-1-indenyl)] ligand (4). It is suggested that these energy differences can be related to an increased probability of a back-skip of the growing chain toward the outward coordination position after the monomer insertion and prior to the coordination of a new olefin molecule. The kinetic competition between the back-skip of the chain outwards and the monomer coordination is able to rationalize the kinds of defects in prevailingly hemi-isotactic chains obtained with systems based on 2, as well as the increased stereoregularity at decreasing monomer concentration for systems based on 3 (not shown by systems based on 4). The molecular mechanics analysis shows that the nonbonded interactions are also able to account for the influence of the ethylene bridge conformation on the stereospecificity of the propene polymerization in ansa-zirconocene complexes.

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]−

“…25,61,62 In contrast, an "alternating mechanism" 63 explained by Miller and Bercaw for ansa-[Me 2 C(3-tert-butyl-Cp)(Flu)ZrCl 2 ] operates through a less stereoselective site in the transition state due to the lack of polymer chain flipping for the formation of isotactic products. 64 Both mechanisms are shown in the scheme in the later section.…”

Ethene/Norbornene Copolymerization using the Catalyst System Pri[(3‐Pri‐Cp)Flu]ZrCl2: Determination of the Copolymerization Parameters and Mechanistic Considerations

“…Theoretical considerations of this kind of mechanism have been described in a publication on propene homopolymerization. [22] The formation of ethene/norbornene copolymers containing an alternating structure without any norbornene microblocks can be explained by the steric interaction between the norbornene backbone and the ligand framework of the metallocene, both of which have a high steric demand. In the case of a chain migration mechanism the [7,10,11] In the case of the retention mechanism, ethene and norbornene are inserted from the same coordination position during the entire copolymerization process.…”

NMR evidence for B(C6F5)3 attack on the inward position of a highly hindered meso ansa-zirconocene

“…However, these isomers allow efficient synthesis of ethylene-based polymers with high molecular weights and narrow molecular weight distributions [6]. Therefore, also these isomers can have important technological uses [7][8][9], and the mechanisms responsible for the growth of the polymer chain on the metal site have been investigated [10,11].…”