The crosslinked poly(2,6-dimethylphenol (95 mol %)-co-2,6-diphenylphenol (5 mol %)) (poly(2,6-DMP95-co-2,6-DPP5)) was successfully developed as an insulating material separating conducting elements with a low dielectric constant and dissipation factor. The crosslinked poly(2,6-DMP95-co-2,6-DPP5) was prepared by the oxidative coupling polymerization of 2,6-DMP with 2,6-DPP, followed by the reaction with 4,4-methylenebis[2,6-bis(methoxymethyl)]phenol (MBMP) as a crosslinking agent. The crosslinked poly(2,6-DMP95-co-2,6-DPP5) exhibited a good thermal stability and glass transition temperature. The dielectric constant and dissipation factor of the crosslinked poly(2,6-DMP95-co-2,6-DPP5) were 2.6 and 0.004 at 10 GHz, respectively. Moreover, a flexible double layer copper clad laminate based on the crosslinked poly(2,6-DMP95-co-2,6-DPP5) composite was successfully prepared, indicating a useful material for high-speed and high-frequency electrical applications. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3218-3223