Polycyanurates from 2,4-dichloro-6-methoxy-1,3,5-triazine with bisphenol A or 9,9-bis(4-hydroxyphenyl)fluorene were Subjected to thermal treatment under argon to induce the thermal rearrangement of the polycyanurates into the corresponding polyisocyanurates. Thermally rearranged polymers with a wide range of conversions were prepared by simply changing the thermal treatment time. It was found that the refractive index of the thermally rearranged polymer was tunable by adjusting the conversion of the thermal rearrangement. A significant increase in the refractive index, up to 0.0513, was achieved by the thermal rearrangement of polycyanurate containing a fluorene structure in the main chain.