The high pressure polymerization of aromatic dicyanates was carried out by using a piston cylinder type hot-pressing apparatus, which led to the formation of crosslinked polycyanurate resins. In a model compound study, aromatic cyanates were found to cyclotrimerize under high pressure to give aromatic cyanurates, with the application of high pressure accelerating the cyclotrimerization process. Aromatic dicyanates polymerized at 150-280-degrees-C under pressures of 200-450 MPa, affording polycyanurates through a cyclotrimerization reaction. Polymerization under higher pressure required both higher temperatures and longer reaction times. After further curing of these crosslinked polymers, very hard resins were produced, with Vickers hardness values of 330-530 MPa and with a high modulus (>2 GPa), when compared with the same polymers synthesized under normal atmospheric pressure.