A novel unsymmetrical phthalimide-containing phthalonitrile (PIPN) was successfully synthesized. The chemical structure of the PIPN monomer was confirmed by various spectroscopic techniques. Rheology and differential scanning calorimetry (DSC) revealed that the self-promoted curing reaction of the PIPN was an extremely sluggish process. The fully cured PIPN polymers showed excellent thermal properties revealed by thermogravimetric analysis (TGA) and DSC. IR and solid-state UV-Vis diffusion reflectance spectra confirmed the formation of the triazine and phthalocyanine ring during the curing reaction. Rheometric studies suggested that the curing reaction of phthalonitrile with a phthalimide group was faster compared to the reaction with a benzimidazole ring, and a nucleophilic addition reaction mechanism was successfully introduced to explain this phenomenon.