There remains a need for inexpensive and processible polymers that have a high refractive index (n >1.7) and high transparency of LWIR radiation. These properties are particularly important for imaging humans because the maximum intensity of infrared light emitted at normal body temperature is located in the LWIR region at a wavelength of approximately 10 µm. In this study, new polymerization techniques and materials are reported to meet this need. Specifically, gaseous cyclopentadiene and molten sulfur were copolymerized to provide a high refractive index material with the highest LWIR transparency reported for a synthetic polymer. The material is processible by melt casting or injection molding, which allowed access to complex lens architectures. And while the LWIR transparency of this polymer is high, it absorbs visible lighta property that was leveraged for new applications in clandestine surveillance in low-light environments.