Binary blends of biphenyl phthalonitrile and the n = 4 oligomeric phthalonitrile were prepared and characterized by differential scanning calorimetry and rheology studies. The blended phthalonitriles also were polymerized from the melt in the presence of bis[4-(4-aminophenoxy) phenyl]sulfone and the dynamic mechanical and thermal properties of the resulting copolymers were investigated. The properties of the phthalonitrile blends and copolymers were compared with those of the neat polymers. The phthalonitrile blends were found to have larger processing windows relative to that observed for the biphenyl phthalonitrile. The size of the processing window depended on the n = 4 phthalonitrile content in the blend. Dynamic mechanical measurements and thermogravimetric analysis showed that temperatures up to 425 degrees C were necessary to completely cure the phthalonitrile copolymers. The dynamic mechanical measurements also revealed that the fully-cured phthalonitrile copolymers did not soften or exhibit a glass transition temperature upon heating to 450 degrees C. Thermogravimetric analysis showed that the phthalonitrile copolymers exhibited excellent thermal stability along with long-term thermo-oxidative stability. Published by Elsevier Ltd.