The activation parameters for the ring-opening metathesis polymerization (ROMP) of endo(1) and exo-dicyclopentadiene (2), endo-1,2-dihydrodicylopentadiene (3), and norbornene (4) in the presence of Grubbs' catalyst were determined using in situ NMR. The exo isomer of DCP was found to be more than an order of magnitude more reactive than the endo, isomer. endo-DCP was found to have reactivity similar to its partially saturated counterpart 3, suggesting that the cause of the rate difference between the two isomers of DCP is primarily steric in nature. This interaction is shown to be predominantly entropic and is suspected to originate from an interaction of the penultimate repeat unit and the incoming monomer. Additionally, the alkylidene generated during the polymerization of endo-DCP was found to form an intramolecular complex, but this complex only affects the rate slightly.