Thermosetting materials (thermosets) are widely used organic materials derived from 3D-network forming monomers. Achieving high glass transition temperature (T-g) thermosets is often a challenging task due to the complexity of designing efficiently and cheaply monomers which are rigid enough to prevent molecular motions within the thermoset. We report here a very simple route to prepare epoxy thermosets with T-g as high as 350 degrees C, based on insertion polynorbornenes. The epoxy monomer (PNBE(epoxy)) is prepared by the epoxidation of poly(5-vinylnorbornene) obtained by catalytic insertion polymerization of S-vinylnorbornene. PNBE(epoxy) can be cross-linked with simple biosourced compounds. Alternatively, polar insertion polynorbornene can also be used as cross-linker in the formulation of an epoxy resin, once again resulting in epoxy resins with T-g higher than 300 degrees C and devoid of degradation at this temperature. Thus, this study clearly demonstrates the viability of catalytic polymerization to access epoxy thermosets with ultrahigh T-g.