Comparison of Selected Zirconium and Hafnium Amine Bis(phenolate) Catalysts for 1‑Hexene Polymerization

The kinetics of 1-hexene polymerization using a family of three zirconium and hafnium amine bis-phenolate catalysts, M­[<i>t</i>-Bu-ON^XO]­Bn₂ (where M = Zr (<b>a</b>) or Hf (<b>b</b>), and X = THF (<b>1</b>), pyridine (<b>2</b>), or NMe₂ (<b>3</b>)), have been investigated to uncover the mechanistic effect of varying the metal center M. 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. This study builds upon the mechanism of polymerization for <b>1a</b>–<b>3a</b>, which has been previously reported by applying the same methodology to the hafnium containing analogues, <b>1b</b>–<b>3b</b>. It has been observed that each elementary step-specific rate constant that involves the insertion of a monomer is reduced by an order of magnitude. As previously reported for catalysts <b>1a</b>–<b>3a</b>, a quantitative structure–activity relationship was uncovered between the logarithm of the monomer-independent chain transfer rate constants and the Hf–X bond distance for catalysts <b>1b</b>–<b>3b</b>. However, this dependence on the pendant ligand is 2.7 times weaker for the Hf-containing analogues versus those containing Zr. 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