The aromatic diphenol monomers, 4-(1-adamantyl)-1,3-benzene diol (AdRES) and 4-(6-(4-hydroxyphenyl)undecan-6-yl)phenol (BISPR5R5) were successfully synthesized and used to synthesize a new series of poly (aryl ether ketone) copolymers (AdRES-BISPR5R5-PEEKs) with different percentages of two diphenol monomers by nucleophilic aromatic substitution polycondensation. The structure of copolymers was confirmed using FTIR and H-1 NMR spectroscopy and their thermal and mechanical properties and solubilities were determined. It was found that the flexibility and toughness properties of the copolymers could be controlled by varying the molar ratio of diphenol monomers containing the pendent rigid adamantyl group and the flexible long side chains. They also showed low dielectric constants ( = 2.50-2.88 at 1 MHz) and low water absorptions. It revealed that the symmetrical long aliphatic side chains were more conducive for reducing the polarity of the polymer chains, loosening the polymer packing, and subsequently reducing polymers' density and dielectric constants than the pendent rigid adamantyl group. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013