Influence of the Norbornene Anchor Group in Ru-Mediated Ring-Opening Metathesis Polymerization: Synthesis of Linear Polymers

Abstract

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs’ first-generation catalyst [G1, (PCy3)2(Cl)2RuCHPh] and Grubbs’ third-generation catalyst [G3, (H2IMes)(Cl)2(pyr)2RuCHPh] can exhibit living characteristics for some monomer classes, most commonly substituted norbornenes. Here, we studied how various anchor groups, the series of atoms connecting the polymerizable norbornene unit to a functional group, affect livingness in ROMP in a series of small-molecule exo-norbornene monomers. We first designed and calculated the HOMO energy of 61 monomers using density functional theory methods, finding that these energies spanned a range of 25 kcal/mol. We then performed kinetics experiments using 1H NMR spectroscopy to measure the propagation rate constant (kp,obs) under identical conditions for eight selected monomers with different anchor groups across the range of HOMO energies. We observed a positive correlation between the HOMO energy or the HOMO/LUMO energy gap and measured kp,obs values for both catalysts, revealing a 30-fold and a 10-fold variation in kp,obs values across the series for G1 and G3, respectively. Interestingly, we observed a plateau for the three monomers with the highest HOMO energies for G3 catalyst, suggesting that above a certain level, the HOMO energy no longer influenced the rate-determining step under the conditions studied here. Chelation studies revealed that only one of the eight monomers showed measurable binding of electron-rich groups on the monomer to the catalyst, but with no apparent effect on kp. Finally, we utilized 1H NMR spectroscopy to measure the rate of catalyst decomposition in the presence of each monomer, a key termination pathway in ROMP. Ultimately, we determined that the anchor group did not substantially affect catalyst decomposition, a proxy for the termination rate constant (kt). In sum, these combined computational and experimental studies collectively demonstrate that livingness in ROMP of exo-norbornene monomers using G1 and G3 catalysts, as measured by relative kp/kt ratios, is primarily controlled by the kp of the anchor group, which is correlated with the HOMO energy.