Infrared spectroscopy was used next to assess transparency in the MWIR and LWIR regions (Fig ). Free standing polymer windows were prepared at a thickness of 1 mm for all polymers by melt casting the polymer into a silicone mold before curing. Important for precision optics, the sample surfaces were smooth, with an average surface roughness of 2.68 ± 0.47 nm for 50-poly(S-r-CPD) and 1.96 ± 0.79 nm for 67-poly(S-r-CPD) (as determined by AFM). These windows were tested for transmission between 2 and 20 µm and compared to simulated infrared spectra for model polymers (Fig. ). The simulated spectra, generated using DFT methods, show good agreement in both the MWIR region and LWIR regions and corroborate the proposed structure. These simulations-the first for this class of sulfur co-polymers-also reveal the contribution of each potential chemical and structural feature of the polymer to its IR spectrum (see Supplementary Materials). As expected, there was an absorbance due to C-H stretching in all polymers at a wavelength of around 3.3 µm, but the rest of the MWIR region (3 µm to 5 µm) showed very little absorption. The 67-poly(S-r-CPD) polymer, for instance, transmitted >50% of MWIR radiation. This transparency was calculated by integrating over the transmission spectrum region and dividing by the wavelength range. The transmittance of 67-poly(S-r-CPD) in the MWIR region is among the highest reported for synthetic polymers. For comparison, the previously reported poly(S-r-DIB) polymer system prepared at either the same mass or molar feed ratios exhibited ~25% transmittance (Fig S18 ). Importantly, the LWIR transparency for the 67-poly(S-r-CPD) was also excellent for both the reflux and gas phase synthesis methods. For example, 67-poly(S-r-CPD) made using the gas phase synthesis was found to have an average transmission of 9.0% in the LWIR range (7 µm to 14 µm) for 1 mm thick windows. When comparing directly to previously described synthetic polymers , this is the highest 5 LWIR transparency reported to date for a window of this thickness (Fig. ). The MWIR (3 µm -5 µm) and LWIR (7 µm -14 µm) regions are highlighted. (D) Simulated IR spectrum for model polymer and measured spectrum for 67-poly(S-r-CPD). The "saturated" model polymer is one in which all alkenes have reacted with sulfur. The "unsaturated" model polymer is one that contains unreacted alkenes. There was good agreement between the simulated and experimental spectra in the LWIR and MWIR regions, however a peak at ~1700 cm -1 in the synthesized 67-poly(S-r-CPD) could not be assigned. (E) Average LWIR (7 µm -14 µm) transmittance of 67-poly(S-r-CPD) is higher than all other reported sulfur-based synthetic polymers (organic monomer shown) (F) The refractive index in the MWIR and LWIR region for 67-poly(S-r-CPD) was n = 1.85-1.90. (G-H) LWIR images of a human subject taken through a 1 mm thick window of 67-poly(S-r-CPD) using a FLIR E6 thermal camera (7.5 µm -13 µm).