(790f) Effects of Pendant Ligand Binding Affinity On Chain Transfer for 1-Hexene Polymerization Catalyzed By Group IV Amine Bis-Phenolate Single-Site Catalysts
The kinetics of 1-hexene polymerization using a family of five zirconium amine bis-phenolate catalysts, Zr[tBu-ONXO]Bn2 (where, X = THF (1), pyridine (2), NMe2 (3), furan (4), SMe (5) has been investigated to uncover the mechanistic effect of varying the pendant ligand X. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction specific rate constants involved in a given polymerization process. The mechanism of polymerization for 1-5 was similar and the necessary elementary reaction steps included initiation, normal propagation, misinsertion, recovery from misinsertion, and chain transfer. The latter reaction, chain transfer, featured monomer independent β-H elimination in 1-3 and monomer dependent β-H transfer in 4 and 5. Of all the rate constants, those for chain transfer showed the most variation, spanning two orders of magnitude (ca. 0.1-10 x 10-3 s-1 for vinylidene and 0.5-87 x 10-4 s-1 for vinylene). A quantitative structure-activity relationship was uncovered between the logarithm of the chain transfer rate constants and the Zr-X bond distance for catalysts 1-3. However, this trend is broken once the Zr-X bond distance elongates further, as is the case for catalysts 4 and 5, which operate primarily through a different mechanistic pathway. A similar linear relationship appears to be holding for the monomer independent chain transfer rates for the hafnium analogs of systems 1- 3, which exhibit a much weaker dependence on the bond length as the slope of this correlation is much more gentle. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multi-response data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure-activity relationships.