We synthesized rubbers including cyanoethyl groups and evaluated the effect of side-chain structures on dielectric properties, electrical properties at room temperature, and glass-transition temperatures (T-g). New monomers including cyanoethyl were synthesized via the thiol-ene reaction and esterification; these included 3-((2,4-dicyanobutyl)thio)propyl acrylate (DCEA3M) and 6-((2,4-dicyanobutyl)thio)hexyl acrylate (DCEA6M). Copolymers consisting of 2-cyanoethyl acrylate (CEA) and DCEA3M or DCEA6M were synthesized to obtain a rubber with a high dielectric constant. Reference copolymer samples with varying CN wt % consisted of CEA and acrylate monomers were also synthesized. Undesirable gelation occurred during polymerization of monomers including cyanoethyl; however, gelation could be resolved by changing the monomer concentration and reaction time. The copolymers exhibited a T-g of 5 degrees C or below, and the relative dielectric constant at 100 Hz was above 10. pDCEA3M exhibited the highest dielectric constant of 20.3 corresponding to a high CN wt % of 20.6%, and pDCEA6M displayed the lowest T-g of -33.2 degrees C due to the internal plasticization corresponding to the long side chain; however, the dielectric losses of pDCEA3M and pDCEA6M were as high as 3.2 and 7.7, respectively. On the other hand, p(CEA-DCEA3M) and p(CEA-DCEA6M) copolymerized with CEA could reduce the dielectric loss (similar to 2.4) while maintaining high dielectric constant (16.3-18.9) and low T-g (-22.9--3.36 degrees C). The copolymers also exhibited a high dielectric constant at a comparable volume resistivity (10(10)-10(12) Omega.cm) compared to commercial rubbers. Such elastomers with high dielectric constant and low dielectric loss are expected to improve the performance of soft actuators.