Structure–Activity Correlation for Relative Chain Initiation to Propagation Rates in Single-Site Olefin Polymerization Catalysis

We have determined what makes the first monomer insertion
(initiation)
facile or slow for many homogeneous olefin polymerization catalysts.
Specifically, we have developed the first comprehensive and mechanistically
detailed quantitative structure–activity relationship (QSAR)
that successfully predicts relative chain initiation to propagation
rates for a large series of group 4 single-site olefin polymerization
catalysts. This QSAR correctly predicts (a) whether initiation is
facile or slow and (b) the ki/kp ratio for a catalyst family with slow initiation.
Monomer concentration versus time profiles were measured for batch
polymerization of 1-hexene catalyzed by 27 Cp′Ti­(OAr)­Me2 and Cp*Zr­(OC6H-2,3,5,6-Ph4)­J2 (J = Me, CH2Ph) complexes activated with B­(C6F5)3. Comparison of DFT calculations to experimental
data revealed that the underlying cause of slow versus facile initiation
is the difference in docking site opening sizes between the initiation
kinetically dominant ion pair (i-KDIP) and the propagation kinetically
dominant ion pair (p-KDIP). Specifically, initiation is facile if
the i-KDIP and p-KDIP have similar docking site opening sizes or the
i-KDIP docking site opening is not small but slow if the i-KDIP has
a small docking site opening and the p-KDIP has a much larger docking
site opening. The ion pairing dynamics was strongly influenced by
(a) the choice of solvent, (b) whether or not the catalyst exhibits
opportunistic ligand coordination, and (c) the type of initiating
group. DFT-computed transition states for selected systems confirmed
the underlying chemical mechanism that gives rise to this QSAR.