Dielectric polymers are attractive for applications in electrical energy storage with reduced volume, weight and cost. This talk will describe our efforts to develop new polymer based dielectric materials with high energy densities. We have developed a modular approach toward poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers exhibiting high dielectric constants (K =50 @ 1 kHz). This approach allows the chemical compositions of the resulting polymers to be precisely controlled, leading to a library of ferroelectric polymers with tunable dielectric constants. The structural dependence of dielectric breakdown strength for the ferroelectric polymers has also been investigated. It is found by numerical calculations that an electromechanical breakdown agrees quantitatively with experimental characteristics of the breakdown field with polymer composition.