||A series of high-k, ultrathin copolymer gate dielectrics was synthesized from 2-cyanoethyl acrylate (CEA) and di(ethylene glycol) divinyl ether (DEGDVE) monomers by a free radical polymerization via an one-step, vapor-phase method, initiated chemical vapor deposition (iCVD). The chemical composition of the copolymers was systematically optimized by tuning the input ratio of the vaporized CEA and DEGDVE monomers to achieve a high dielectric constant (k) as well as excellent dielectric strength. The dielectric film with the optimized composition showed the dielectric constant greater than 6 and extremely low leakage current whose thickness was only 20 nm. With the high-k dielectric layer, organic thin-film transistors (OTFTs) and oxide TFTs were fabricated, which commonly showed hysteresis-free transfer characteristics with the operating voltage less than 3 V. Furthermore, the flexible OTFTs retained their low gate leakage current and ideal TFT characteristics even in 2% of the applied tensile strain. We believe that the ultrathin, high-k organic dielectric films will play a crucial role in future soft electronics.