Compositions and methods for catalyzing and controlling the rate of olefin metathesis reactions including Ring Opening Metathesis Polymerization (ROMP) reactions. The molding of polymer articles using ROMP polymers. The composition includes a Ruthenium or Osmium carbene complex catalyst and a gel modification additive. The Ruthenium or Osmium carbene complex catalyst having the formula ##STR1## where M may be Os or Ru; R and R.sup.1 may be the same or different and may be hydrogen or a substituent group including C.sub.2 -C.sub.20 alkenyl, C.sub.2 -C.sub.20 alkynyl, C.sub.1 -C.sub.20 alkyl, aryl, C.sub.1 -C.sub.20 carboxylate, C.sub.1 -C.sub.20 alkoxy, C.sub.2 -C.sub.20 alkenyloxy, C.sub.2 -C.sub.20 alkynyloxy, aryloxy, C.sub.2 -C.sub.20 alkoxycarbonyl, C.sub.1 -C.sub.20 alkylthio, C.sub.1 -C.sub.20 alkylsulfonyl and C.sub.1 -C.sub.20 alkylsulfinyl; X and X.sup.1 may be the same or different and may be any anionic ligand; and L and L.sup.1 may be the same or different and may be neutral electron donor. The gel modification additive may be a neutral electron donor or a neutral Lewis base including phosphine, sulfonated phosphine, phosphite, phosphinite, phosphonite, arsine, stibine, ether, amine, amide, sulfoxide, carboxyl, nitrosyl, pyridine, or thioether. The gel modification additive may be a trialkylphosphine or triarylphosphine. Examples of gel modification additives include P(cyclohexyl).sub.3, P(cyclopentyl).sub.3, P(isopropyl).sub.3, P(phenyl).sub.3, and pyridine.