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[t-Bu-ONXO]­Bn2 (where M = Zr (a)
or Hf (b), and X = THF (1), pyridine (2), or NMe2 (3)), 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 1a–3a, which has been previously reported by applying the same methodology
to the hafnium containing analogues, 1b–3b. 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 1a–3a, 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 1b–3b. 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.