Cyclopentadiene (CPD) was selected as a target comonomer for reaction with sulfur because it is low-cost and readily prepared from dicyclopentadiene (DCPD) via a retro-Diels-Alder reaction (Fig. ). Furthermore, CPD is a simple diene that can provide a crosslinked polymer on reaction with sulfur (Fig ). The resulting sulfurized cyclopentane repeating unit was envisioned to impart shape persistence and provide limited modes of vibration for increased LWIR transparency.