Ethylene carbon monoxide copolymer (ECO) has physical and thermal properties essentially identical to those of low density polyethylene, including a melt temperature near 100C and a low water vapor transmission rate. Unlike polyethylene, ECO absorbs and can be heated and melted with microwave and radio frequency energy. A conventional method of sealing flexible polymer containers is to apply the external heat to the walls with heated dies. The time of sealing is directly related to the thickness of the wall structure, its composition, and the temperature of the heated dies. The die temperature is often limited by the softening or melt temperature of the external polymer layer and/or degradation of surface inks or applied graphics. High frequency energy applied to a properly designed polymer structure deposits the energy into the internal absorbing layers, which transform that energy into heat. No heated dies are required. External layers of high frequency non-absorbing polymer act as thermal insulators, keeping the generated heat in at the seal location. Sealing rates can be several times faster than by conventional means.