In the past decades, 4-phenylethynyl phthalic anhydride (4-PEPA) has been the most important endcapper used for thermoset polyimide. As the isomer of 4-PEPA, 3-phenylethynyl phthalic anhydride (3-PEPA) has attracted our interest. In this article, 3-PEPA was synthesized and a comparative study with 4-PEPA on curing temperature, curing rate, thermal and mechanical properties of oligomers and cured polymers was presented. The new phenylethynyl endcapped model compound, N-phenyl-3-phenylethynyl phthalimide, was synthesized and characterized. The molecular structure of model compound was determined via single-crystal X-ray diffraction and the thermal curing process was investigated by Fourier transform infrared. Differential scanning calorimetry clearly showed that the model compound from 3-PEPA had about 20 degrees C higher curing onset and peak temperature than the 4-PEPA analog. This result was further proved by the dynamic rheological analysis, that the temperature of minimum viscosity for oligomers end-capped with 3-PEPA was above 20 degrees C higher than that of the corresponding 4-PEPA endcapped oligomers with the same calculated number average molecular weight. The cured polymer from 3-PEPA displayed slightly higher thermal oxidative stability than those from 4-PEPA by thermogravimetric analysis. The thermal curing kinetics of 3-PEPA endcapped oligomer (OI-5) and 4-PEPA endcapped oligomer (OI-6) fitted a first-order rate law quite well and revealed a similar rate acceleration trend. (C) 2008 Wiley Periodicals, Inc.