Advances in modern electronics require the development of polymer-based dielectric materials with high dielectric constant, low dielectric loss, and high thermal stability. Fundamental dielectric theory suggests that strongly dipolar polymers have the potential to realize a high dielectric constant. In order to achieve high thermal stability, these polymers should also possess a high glass transition temperature Tg. However, it has been observed in many dielectric polymers that the dielectric constant decreases markedly at temperatures below Tg due to dipole freezing. This study shows that a high energy density polymer with low loss and high operating temperature can be realized in a family of weakly-coupled strongly-dipolar polymers, such as aromatic urea and thiourea. Our experimental results uncover that disorder (or blending) in these polymers leads to a significantly larger free volume at temperatures far below Tg, thereby enabling easier reorientation of dipoles in response to an electric field. The net result is a substantial enhancement in the dielectric constant while preserving low dielectric loss and very high breakdown field. The results here pave the way for creating high energy density polymers with low loss and high operating temperature.