5-(3-Cyclohexen-1-yl)-2-norbornene [CHNB] has been shown to form a very lightly crosslinked polymer (T-g = 127 degrees C) with good elongation via ring-opening metathesis polymerization (ROMP). Based on swelling behavior with added norbornadiene dimer, the low crosslink density is ascribed to much less than 0.5% participation by the cyclohexenyl ring. Compared to dicyclopentadiene (DCPD), CHNB polymerizations were less exothermic, required less catalyst, and exhibited greater molding latitude, which are advantageous for Reaction injection Molding (RIM). Styrene-isoprene and sty rene-ethylene/butylene block copolymers were effective impact modifiers for polyCHNB, forming large particle morphologies. Small rubber particles formed from styrene-butadiene block copolymers were not effective for impact enhancement of polyCHNB, in contrast to polyDCPD. Rubber-modified polyCHNB retained impact resistance four to six times longer than polyDCPD samples when aged in air at 50-70 degrees C. Related RIM-ROMP of liquid monomer mixtures prepared by cyclopentadiene cycloadditions with 4-vinyl-1-cyclohexene, cis-1,3-divinylcyclopentane, 3,5-divinylcyclopentene or cis-2,4-divinylbicyclo [3.3.0] oct-6-ene formed highly crosslinked, less ductile copolymers with T(g)s as high as 206 degrees C.