Disclosed is the incorporation into a two-part metathesis-catalyst system of a halogen containing hydrocarbyl additive, where the hydrocarbyl additive contains at least one trihalogen substituted atom or at least one activated halogen atom. The use of this catalyst system to polymerize dicyclopentadiene results in a thermoset polymer containing low levels of residual monomer. Also disclosed is a preferred embodiment of dicyclopentadiene polymerization where the two parts of the metathesis-catalyst system, plus dicyclopentadiene are combined in the mixing head of a reaction injection molding machine and then injected into a mold where the dicyclopentadiene rapidly sets up into a thermoset polymer.