Thermoset resins of dicyclopentadiene were viscosity-modified to achieve the rheology necessary for Direct Ink Writing, with the deposited ink solidified on-the-fly by frontal polymerization. This rapid modification strategy incorporated the ruthenium-based Grubbs' third-generation catalyst and a ligand exchange reaction to decrease the gelation time for resins to under 10 min, an order of magnitude less than existing strategies. The gelled resins are rheologically stable and retain their reactivity for over 500 days. Viscosity modification enables 3D-printing of these resins, producing polymers with thermomechanical properties that match or surpass those from resins gelled more slowly. Replacing the solvent eliminates reduction in glass transition temperature from plasticization while maintaining extended resin pot life.