Polar　groups　and　the　charge-transport　capability　play　significant　roles　in　the　dielectric　properties　of　organic　polymers,　and　thus　influence　the　electric　energy　density　upon　application　as　a　capacitor　material.　Here,　the　dielectric　properties　and　electric　conductivity　of　a　series　of　polymers　containing　2,2,6,6-tetramethylpiperidine-1-oxyl　(TEMPO)　radicals　are　investigated.　The　neat　radical　polymer　poly(TEMPO　methacrylate)　(PTMA)　has　a　high　dielectric　constant　but　poor　breakdown　strength.　Poly(methyl　methacrylate)　(PMMA)　is　introduced　as　an　insulating　polymer　with　high　resistivity　on　breakdown,　along　with　molecular　design　of　PTMA.　Copolymers　of　TEMPO　methacrylate　and　methyl　methacrylate,　P(TMA-r-MMA),　exhibit　high　breakdown　strengths　but　low　dielectric　constants.　PMMA　blended　with　TEMPO　exhibits　the　highest　electric　energy　density　of　7.4　J　cm−3　(that　of　PTMA　is　0.48　J　cm−3　as　a　control),　with　both　a　high　dielectric　constant　(≈6.8)　and　a　high　breakdown　strength　(≈500　MV　m−1).　It　benefits　from　long-range　but　not　bulk　charge　transport　in　the　blends,　which　is　different　from　the　bulk　charge　transport　in　PTMA　and　the　short-range　charge　transport　in　P(TMA-r-MMA).　These　results　indicate　that　the　TEMPO　moiety　located　in　the　high　breakdown　matrix　leads　to　a　high　energy-storage　density　in　the　capacitor.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim