living ROMP due to their fast initiation rates. Moreover, exo-norbornenes (NB) are often used as monomers due to their fast propagation rates, especially in comparison to their endo counterparts. Exo-NBs are, however, often more difficult to synthesize and more expensive than their endo isomers. Recent detailed studies of the mechanism and the effects of the exonorbornene monomer structure on the rate of propagation and degree of "livingness" of these polymerizations have led to new insights that may guide the design of next-generation initiator and monomer classes. For example, Matson and coworkers have suggested that the HOMO energy levels of exo-NBs correlate with their G3-initiated ROMP propagation rate. Inspired by this work, and seeking synthetically accessible, functional monomers that do not compromise polymerization kinetics and controlled polymerization behavior, we identified tricyclononenes (TCN) and tricyclononadienes (TCND) as ROMP monomers ripe for further study (Figure ). TCNs and TCNDs are tricyclic, strained olefins with fused cyclobutane and cyclobutene rings, respectively, connected to the two exo positions of the NB skeleton. Compared to other exosubstituted NB monomers, which often require high temperatures and/or precious metal catalysts for their synthesis, exo-TCNs and TCNDs can be conveniently assembled by thermal cycloadditions of activated olefins and quadricyclane with perfect exo selectivity. While TCN and TCND-based polymers have been synthesized using Mo (Schrock-type initiators) and Ru (Grubbs 1 st and 2 nd generation initiators) initiated ROMP and studied for applications in gas separations, no studies of their performance under "living" ROMP conditions initiated by G3 or post-polymerization functionalization have been reported. We hypothesized that they may be competitive with commonly used exo-NBs in terms of ROMP propagation rates, but that they could offer advantages of facile synthesis and, in some cases, post-polymerization modification. Moreover, by accessing a training set of monomers, we sought to investigate if the HOMO level was primarily responsible for propagation rate as proposed by Matson and coworkers, or whether other molecular features (e.g., steric parameters) of the monomers may also impact performance.