Poly(ether nitrile) and poly(ether nitrile sulfone) copolymers with pendant methyl groups were prepared by the nucleophilic substitution reaction of 2,6'-dichlorobenzonitrile with methyl hydroquinone and with varying mole proportions of methyl hydroquinone and 4,4'-dihydroxydiphenylsulfone using N-methyl pyrrolidone as a solvent in the presence of anhydrous K2CO3. The polymers were characterized by different physicochemical techniques. Copolymer composition was determined using the FTIR technique. Thermogravimetric data revealed that all polymers were stable up to 420 degrees C with a char yield above 40% at 900 degrees C in a nitrogen atmosphere. The glass-transition temperature increased and the activation energy and inherent viscosities decreased with an increase in the concentration of the 4,4'-dihydroxydiphenylsulfone units in the polymer. Trimerization reactions were favorable with an increase in the concentration of methyl hydroquinone units in the polymer. The crystallinity of the polymer was also studied using wide-angle X-ray diffraction. (C) 2005 Wiley Periodicals, Inc.