A series of novel heterogeneous catalysts were developed by immobilizing imidazolium-based functionalized ionic liquids on an FDU-type mesoporous polymer. Various techniques such as X-ray diffraction, N2 adsorption–desorption, and high resolution transmission electron microscopy were used to characterize the physicochemical properties of the catalysts. Ionic liquid-functionalized mesoporous polymers serve as efficient catalysts in the solvent free cycloaddition reactions of CO2 with epoxides without the use of any co-catalyst. The catalysts can be easily recovered and reused without a significant loss of activity. In comparison with the polystyrene (PS) and mesosilica SBA-15 supports, the FDU mesopolymer possesses both the mesoporous organic framework and abundant phenolic hydroxyl groups, which are key factors in improving the catalytic performance of supported ionic liquids.