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

Ring-opening metathesis polymerization (ROMP) mediatedby Grubbs'first-generation catalyst [G1, (PCy3)(2)(Cl)(2)RuCHPh] and Grubbs' third-generation catalyst [G3,(H(2)IMes)-(Cl)(2)(pyr)(2)RuCHPh] can exhibitliving characteristics for some monomer classes, most commonly substitutednorbornenes. Here, we studied how various anchor groups, the seriesof atoms connecting the polymerizable norbornene unit to a functionalgroup, affect livingness in ROMP in a series of small-molecule exo-norbornene monomers. We first designed and calculatedthe HOMO energy of 61 monomers using density functional theory methods,finding that these energies spanned a range of 25 kcal/mol. We thenperformed kinetics experiments using H-1 NMR spectroscopyto measure the propagation rate constant (k (p,obs)) under identical conditions for eight selected monomers with differentanchor groups across the range of HOMO energies. We observed a positivecorrelation between the HOMO energy or the HOMO/LUMO energy gap andmeasured k (p,obs) values for both catalysts,revealing a 30-fold and a 10-fold variation in k (p,obs) values across the series for G1 and G3, respectively.Interestingly, we observed a plateau for the three monomers with thehighest HOMO energies for G3 catalyst, suggesting that above a certainlevel, the HOMO energy no longer influenced the rate-determining stepunder the conditions studied here. Chelation studies revealed thatonly one of the eight monomers showed measurable binding of electron-richgroups on the monomer to the catalyst, but with no apparent effecton k (p). Finally, we utilized H-1 NMR spectroscopy to measure the rate of catalyst decompositionin the presence of each monomer, a key termination pathway in ROMP.Ultimately, we determined that the anchor group did not substantiallyaffect catalyst decomposition, a proxy for the termination rate constant(k (t)). In sum, these combined computationaland experimental studies collectively demonstrate that livingnessin ROMP of exo-norbornene monomers using G1 and G3catalysts, as measured by relative k (p)/k (t) ratios, is primarily controlled by the k (p) of the anchor group, which is correlated withthe HOMO energy.

Automated summary

This study investigates the influence of different anchor groups on the activity of ROMP reaction through experiments and calculations. It is found that the HOMO energy of the anchor group is correlated with the propagation rate constant of the catalyst, indicating the significance of anchor group selection in ROMP reaction activity.