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.