Blends of phthalonitrile (CN) and alicyclic imide compounds synthesized from tetrahydrophthalic anhydride (CC), methyl tetrahydrophthalic anhydride (MCC) and hexahydrophthalic anhydride (HCC), respectively, were prepared and characterized. Results from rheological studies demonstrated the scope of a novel Thermal Synergistic Polymerization (TSP) effect between phthalonitrile and the alicyclic imide compounds. FTIR data showed a unique phenomenon, which is, the -C equivalent to N of the phthalonitrile compound could be completely and rapidly consumed without generating any triazine ring. Further works on cured products of CC/CN system showed comparable or even higher thermal oxidation stability, low boiling water absorbance and moderate dielectric constant (e' in the range of 4.8 similar to 4.5) with almost no voids compared with that of the phthalonitrile resins promoted by traditional curing system of active hydrogen. Model system was designed to study the mechanism of TSP effect of alicyclic imide moiety/phthalonitrile. The systematic characterizations by FTIR, UV-Vis, MALDI-TOF MS, free radical scavenger (DPPH), EPR, etc. showed that the TSP reaction undergoes both copolymerization and homopolymerization with no triazine formation, involving a free radical process. Structural analysis of the oligomers suggested that phthalocyanine rings were a minor component of the thermoset networks, while polyisoindolines were the primary structural motif. (C) 2016 Elsevier Ltd. All rights reserved.