The production and properties of a new family of materials synthesised via ring opening metathesis polymerisation (ROMP) is described. The monomers offer potential alternatives to dicyclopentadiene (DCPD) for either reaction injection moulding (RIM) or resin transfer moulding (RTM) use and provide new materials with an extended range of properties vis-a-vis poly(DCPD). improvements include lower monomer odour together with control of both the reaction rate and the crosslinking reaction. A series of monofunctional imidonorbornene monomers, with different N-alkyl side chains, were polymerised to give a range of linear polymers, and, by copolymerisation with difunctional. monomers with different N,N ' -alkylene spacer lengths, a range of crosslinked materials. Processing conditions were established which,gave over 95% conversion of the monomer for the linear materials, and over 95% gel fraction for the crosslinked materials. For the crosslinked materials, a value of the monomer: initiator ratio was established which gave homogeneously cured materials. The glass transition temperature (T-g) of the linear polymers was found to depend on the length of the alkyl side chain, while a sub T-g beta relaxation was observed, the position of which was independent of the side chain length. For the crosslinked polymers, the molecular weight between crosslinks was found to be lower than expected at low percentage loading of the crosslinking unit, and this was attributed to the contribution of physical crosslinks. At higher percentage loading of the crosslinking unit, the molecular weight between crosslinks was higher than expected, suggesting that under these conditions not all the crosslinking units were fully bound into the network, i.e. bound at both ends. Mechanical properties of the various materials were measured using the three-point bend geometry. Of particular interest were the high values of yield strength and toughness of the crosslinked polymers, which are comparable to known 'high toughness' materials, such as polycarbonate.