Polymer-based CMUT microfabrication approaches allow low-cost fabrication of flexible transducers. However, they are characterized by processing limitations mainly related to the low glass transition temperature of polymers, reducing the possible choice of materials that can be used for electrodes and in-cavity passivation layers, the latter having a major impact on transducer performance and reliability. In this paper, we experimentally evaluate the electrical properties of these two materials in terms of dielectric dispersion and losses, and high electric field response with respect to state-of-the-art materials.