With increasing use of infrared imaging in medical diagnostics, military and civilian surveillance, and navigation of autonomous vehicles, there is a need for low-cost alternatives to traditional materials used in infrared optics such as germanium. Sulfur-rich copolymers hold promise, as they are made from low-cost feedstocks and have a high refractive index. In this report, cyclopentadiene was copolymerized with sulfur to provide a plastic with the highest long-wave infrared transparency reported to date for this class of materials. Diverse lens architectures were accessible through melt casting or reactive injection molding. The featured copolymer was black, which enabled its use as an infrared-transparent blind for protection of thermal imaging equipment and clandestine surveillance. These findings portend expanded use of sulfur copolymers in infrared optics.