High-performance dielectric materials have gained widespread attention, due to their great potential for various applications, such as flexible electronics and capacitive sensors. In this study, sulfamide (SA) and sulfanilamide (SAN), containing sulfonyl groups, were introduced into polyurethane (PU) as chain extenders. The polyurethane with the best performance was end-capped with 3-methylsulfonyl-1-propanol (3-MSP) to investigate the effects of the SAN/SA molar ratio and the capping agent on the microstructure, dielectric properties, and mechanical performance of the PU. The experimental results indicated that, at a frequency of 10