Process for production of post-crosslinkable thermoplastic resins by bulk-polymerizing a polymerizable composition (A) comprising (I) a monomer fluid containing a cyclic olefin (α) having two or more metathetical ring-opening reaction sites in the molecule in an amount 10 wt % or above based on the total amount of the monomers or a monomer fluid containing a norbornene monomer and a crosslinking agent, (II) a metathetical polymerization catalyst, and (III) a chain transfer agent; thermoplastic resins obtained by this process. These thermoplastic resins are free from odor due to residual monomers and are excellent in storage stability. Process for producing crosslinked resins and composite materials which comprises laminating the thermoplastic resin to a substrate and then crosslinking the thermoplastic resin. These crosslinked resins and composite materials are invention are excellent in electrical insulation properties, mechanical strength, heat resistance, and dielectric characteristics, and are useful as electrical materials.