The synthesis of a new A(2)X-type difluoride monomer, N-2-pyridyl-4',4 "-bis(4-fluorobenzenesulfonyl)-o-terphenyl-3,6-dimethyl-4,5-dicarboxylic imide (3), is described. The monomer 3 was incorporated into a series of copoly(aryl ether sulfone)s by polymerization of 4,4'-isopropylidenediphenol and 4,4'-difluorophenylsulfone. The incorporation of monomer 3 had an observable effect on both the glass-transition temperature of poly(aryl ether sulfone)s and the tendency for macrocyclic oligomers to form during polymerization. Replacement of the pyridyl imide group via a transimidization reaction with propargyl amine proceeded quantitatively and without polymer degradation. The acetylene containing copoly(aryl ether sulfone) could be crosslinked by simple thermal treatment,resulting in an increase in the glass-transition temperature and solvent resistance.