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Polymer–fullerene nanocomposites consisting of linear polyurethane (PU) chains crosslinked via increasing loadings of polyhydroxylated fullerenes (C60 and Sc3N@C80, a metallic nitride fullerene) were prepared and characterized for their mechanical and dielectric properties using dynamic mechanical analysis (DMA) and broadband dielectric spectroscopy (BDS). Fullerene–polymer networks [C60‐PU and Sc3N@C80‐PU] having high gel fractions, good mechanical properties and thermal stabilities were produced. Polyhydroxylated fullerenes C60(OH)29 and Sc3N@C80(OH)18 were synthesized in high yield through a high‐speed vibration milling method and characterized using FTIR, matrix‐assisted laser desorption/ionization mass spectroscopy, and thermal gravimetric analysis. DMA of fullerene–PU networks indicates Tg ∼ −50°C, with a sub‐Tg relaxation due to local chain motions. BDS analyses of the fullerenes, before and after hydroxylation and before incorporation into the networks, revealed one relaxation and large real permittivity (ε′) values for C60(OH)29 relative to C60. Analogous samples for Sc3N@C80 exhibit two relaxations, where the extra relaxation is attributed to motions of the cage‐encapsulated Sc3N clusters. ε′ values for Sc3N@C80‐PU at a given frequency are higher than corresponding values for C60‐PU, likely because of the rotationally mobile Sc3N encapsulates. Surface and bulk resistivities of fullerene–PU networks were found to have a modest dependence on relative humidity. Capacitance versus voltage characteristics of the fullerene–PUs were also studied in the range of the applied dc bias voltage of −30 to +30 v. It is generally concluded, based on all the evidence that this class of materials can be rendered quite polarizable and could be used as high dielectric permittivity materials in capacitance applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40577.