Oxidative Coupling Copolycondensation of 2,6-Dimethylphenol with 2,5-Dimethylphenol: Highly Thermostable Poly(phenylene ether)Copolymerization of 2,5- and 2,6-Xylenol

2,5-Dimethylphenol (25DMP) was successfully copolymerized with 2,6-dimethylphenol (26DMP) using the di-μ-hydroxo-bis[(N,N,N′,N′-tetramethylethylenediamine)copper (II)]chloride (CuCl-TMEDA) catalyst with 10 equiv (to copper ion) of tetramethylethylenediamine (TMEDA) in toluene at 20 °C for 7 h under oxygen. Conversion of the two monomers was monitored by gas chromatography, which revealed that 25DMP was slightly less reactive than 26DMP. Infrared spectra of a series of the copolymers indicated that the composition of the copolymer could be controlled by the monomer feed ratio. The resulting copolymer shows a higher thermal air stability of about 70 °C along with improved mechanical properties in comparison with the conventional poly(2,6-dimethyl-1,4-phenylene ether) (PPE). PPE with a methyl end group, prepared by the endcapping reaction of PPE with methyl iodide in the presence of potassium hydroxide in N,N-dimethylformamide, shows a higher thermal stability than the as-prepared PPE under air, and similar stability under nitrogen, indicating that thermal degradation mainly occurs from the polymer end group under air. Thus, the higher thermal stability of the copolymer is attributable to the 25DMP unit located at the end of the copolymer.