Industrial applications of the upcoming class of high-performance biobased polybenzoxazines are currently impeded by the requirement of high polymerization temperatures. Valorization of elemental sulfur (S8), petroleum waste, has provided attractive technological opportunities. However, achieving sulfur copolymers at a low temperature remains a challenging task. Here, a unique facile synthetic strategy was adopted to form a fully sustainable solution-processable benzoxazine-sulfur copolymer via a ring-opening polymerization reaction of isoeugenol-furfurylamine (IE-fa, sourced from naturally abundant biobased feedstocks) and S8at room temperature. The IE-fa benzoxazine monomer was synthesized under neat conditions and reacted with varying percentages of S8as sodium polysulfide (Na2Sx) at unprecedented low temperatures (25 and 50 °C) under catalyst-free conditions. The covalent fixation of sulfur (90 wt %), via inverse vulcanization, within ring-opened benzoxazine resulted in an amorphous copolymer as supported by various techniques. The formation of S-rich and Bz-rich domains in the copolymers is governed by the feed-in ratio of monomers and reaction temperature. Copolymers exhibited high glass transition temperatures (10-86 °C) due to the synergistic effects induced by H-bonded ring-opened benzoxazine structures. The present work demonstrates the first example of benzoxazine as a comonomer to form low-temperature solution-processable benzoxazine sulfur benign random copolymers with mid-wave infrared transparency and may advance benzoxazine chemistry toward transparent optics. © 2022 American Chemical Society. All rights reserved.