Successful FROMP requires favorable reaction kinetics with fast rates to ensure that sufficient quantities of heat are released rapidly enough to support front propagation. Given the close proximity of the bridgehead methyl substituents in 4a and 5a to the reactive olefin, we hypothesized that resulting steric effects could influence propagation kinetics. Considering the initiation of ROMP is sensitive to monomer structures and reaction conditions, we conducted solution-state competitive ROMP experiments to assess the reactivities of various monomers under the same conditions. Equimolar quantities of 3a, 4a, and 5a were mixed with the 2nd generation Grubbs catalyst ([(SIMes)Ru(=CHPh)(PCy3)Cl2]; G2) in CDCl3 (Figure and Supplementary Figure ). Reaction kinetics were quantified by H NMR spectroscopy with dichloroethane (DCE) as an internal standard (Figure and Supplementary Table , Figure ). The parent monomer (3a) completely polymerized within 25 min, whereas the monomethyl (4a) and dimethyl substituted (5a) monomers required ≈2.5 h and 2 days, respectively, for full conversion. The first-order decay kinetics indicated that 3a, 4a, and 5a exhibited half-lives of ≈500, 5000, and 60000 s, respectively. We hypothesize that methyl substituents hinder catalyst access to the ROMP-active olefin, thereby interfering with the polymerization propagation and decreasing the observed reaction rates. We next investigated the solution ROMP reactivity of the HEN-type monomers. When mixed with G2, monomers 3b and 3c rapidly reacted at room temperature preventing full catalyst dissolution, likely by encapsulating the catalyst surface. To decrease their reactivity, we added two equiv of tributylphosphite (TBP) relative to G2 as a catalytic inhibitor. In contrast, monomers 4a, 4b, 4c, and 5a did not require inhibition with TBP to generate persistent resins. Differential scanning calorimetry (DSC) quantified the exotherms of ROMP in resins freshly prepared from neat monomer and G2 (Figure ). Monomers without bridgehead substituents exhibited lower exotherms (14.8 kcal/mol for 3b, 12.9 kcal/mol for 3c) compared to those with monomethyl substitution (17.9 kcal/mol for 4a, 18.8 kcal/mol for 4b, and 19.7 kcal/mol for 4c). The disparity between these results and thermodynamic estimates from DFT calculations may stem from the additional influence of ROMP kinetics which could result in non-stoichiometric ring-opening during polymerization. For instance, background ROMP reactivity of 3b and 3c during resin preparation was indicated by their gelation within 30 min even in the presence of TBP. Conversely, monomer 5a exhibited minimal exothermic events even at 250 °C (0.65 kcal/mol), which suggested low monomer conversion.