(753c) Structure-Activity Correlation for Relative Chain Initiation to Propagation Rates In Single-Site Olefin Polymerization Catalysis
Using experiments and density functional theory (DFT) calculations, we systematically studied factors controlling relative chain initiation to propagation rates in homogeneous olefin polymerization catalysis. These catalysts typically contain Ti or Zr bound to several organic ligands. The active part of the catalyst is a cation coordinated to a weakly bound anion (aka counterion). This ion pair can exist in various forms: (a) in the inner sphere ion pair, the counterion is in direct contact with the counterion, (b) in the outer sphere ion pair, the counterion is in the metal's outer coordination sphere but there are no solvent molecules between the cation and counterion, and (c) in the solvent separated ion pair, the cation and counterion are separated by two or more solvent layers. Since partial sepation of the counterion from the cation is required for the monomer to enter the docking site, catalysts with lower ion pair separation energies have higher reactivity. Twenty-seven such catalysts containing mixed cyclopentadienyl/aryloxide ligation were synthesized and used to polymerize 1-hexene in toluene, bromobenzene, or 1,2-dichlorobenzene solvents. For some of the catalysts, the rate of first monomer insertion (i.e. chain initiation) was comparable to the rate of subsequent monomer insertions (i.e. chain propagation). For other catalysts, the chain initiation rate was three orders of magnitude slower than the chain propagation rate. A comprehensive set of DFT calculations was performed to discover the underlying cause of slow versus facile chain initiation. First, DFT computations were used to identify the lowest energy ion pair form for each catalyst in various solvents. Then, appropriate electronic and steric descriptors were identified and correlated to chain initiation and propagation rate constants. Comparison of DFT calculations to experimental data revealed the underlying cause of slow versus facile initiation is the difference in steric congestion between the initiation kinetically dominant ion pair and the propagation kinetically dominant ion pair. These descriptors were used to construct a quantitative structure-activity relationship (QSAR) that accurately predicts whether chain initiation is facile or slow for each catalyst. Transition state analysis performed for representative catalysts confirmed the underlying chemical mechanism controlling facile versus slow initiation. This underlying chemical mechanism was consistent with the developed QSAR.