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Effects of Pendant Ligand Binding Affinity on Chain Transfer for 1-Hexene Polymerization Catalyzed by Single-Site Zirconium Amine Bis-Phenolate Complexes

Steelman, D. Keith, Xiong, Silei, Pletcher, Paul D., Smith, Erin, Switzer, Jeffrey M., Medvedev, Grigori A., Delgass, W. Nicholas, Caruthers, James M., Abu-Omar, Mahdi M.
Journal of the American Chemical Society 2013 v.135 no.16 pp. 6280-6288
binding capacity, catalysts, ligands, polymerization, pyridines, quantitative structure-activity relationships, zirconium
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), and 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 2 orders of magnitude (ca. (0.1–10) × 10–³ s–¹ for vinylidene and (0.5–87) × 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. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure–activity relationships.