AbstractThe synthesis of 3‐phenylethynylphenol, and its applicability as a high temperature cross‐linking endcap for highTgpolyarylene ethers is described. It was synthesized in high yields and purity using the palladium catalyzed coupling reaction between the protected 3‐bromo or iodo phenol and phenylacetylene. The yield of the reaction was found to be highly dependent on the structure of the halide used, the reaction temperature, and the concentration of phenylacetylene. The use of the protected phenol in the palladium catalyzed reaction was also extended to the high yield synthesis of 3‐ethynylphenol and protected 4‐ethynylphenols. The complete synthesis of 3‐phenylethynylphenol, 3‐ethynylphenol, and protected 4‐ethynylphenol in high yields has been demonstrated and is discussed herein. Three new phenylethynyl functionalized arylene ether matrix resins have been synthesized in high yields and purity by reacting 3‐phenylethynylphenol with 4,4′‐dichlorodiphenyl sulfone, 4,4′‐difluorobenzophenone, and bis(4‐fluorophenyl)phenyl phosphine oxide, via nucleophilic poly(arylene ether) synthesis conditions. These low molecular weight materials undergo thermally induced chain extension/branching to yield an insoluble three‐dimensional network at reaction temperatures of around 380°C. The low molecular weight arylene ethers endcapped with the phenylethynyl group demonstrate excellent flow characteristics and a wide processing window of about 250°C. Crosslinking of the 4,4′‐bis(3‐phenylethynyl phenoxy)diphenyl sulfone system for 30 min at 350°C in air afforded aTgvalue of 265°C by differential scanning calorimetry measurements. Trace metal analysis for palladium and copper showed absence of these metals that would otherwise detract from the excellent thermal stability. The synthesis and characterization of these phenylethynyl endcapped arylene ether matrix resins is