Several new in situ tungsten catalyst systems for ring-opening metathesis polymerizations (ROMP) by reaction injection molding (RIM) have been developed by adding BF 3 promoter to binary catalyst systems, by using metal hydride cocatalysts, and by altering the ligands on the procatalyst metal center. BF 3 etherates improved catalyst efficiency and reduced induction times for formation of active catalysts from reaction of aryloxytungsten complexes [e.g., (ArO) y WX x )] with organotin hydrides. Coordinatively unsaturated cationic intermediates, such as [(ArO) y WX x01 ] / BF 3 X 0 , are proposed to facilitate formation of the active catalysts. Tougher poly(dicyclopentadiene) (polyDCPD) composites were produced using Γ΅ 5 wt % of styrene-butadiene block copolymers due to formation of small ''shell-core'' rubber morphologies when BF 3 promoter was added to the catalyst system. Nonalkylating metal hydrides besides R 3 SnH, including (PPh 3 ) 2 CuBH 4 , (PPh 3 CuH) 6 , and Cp 2 ZrClH, were shown to be cocatalysts. The optimum 2 : 1 stoichiometric ratio of organotin hydride cocatalyst to tungsten, revealed by BF 3 -promoted catalyst systems, and W V EPR resonances (g Γ 1.7) observed in the reaction of aryloxytungsten with organotin hydride are consistent with an overall reduction and reoxidation mechanism for formation of the active metathesis catalysts. Some tungsten complexes derived from 9-hydroxyfluorene, 2,2 -(and 4,4 )-biphenols, and 1,4-hydroquinones were found to be very reactive procatalysts, even in the absence of cocatalyst in some cases. These procatalysts also were paramagnetic, characterized by unusual EPR spectra consistent with W V (g Γ
1.6-1.9) and ''ligand-centered'' (g Γ
2.003) resonances. Valence tautomeric species, analogous to catecholate-semiquinonate complexes, are proposed.