Polybenzoxazines obtained especially from green synthons are facing challenges of the requirement of high ring-opening polymerization (ROP) temperature of the monomer, thus affecting their exploration at the industrial front. This demands effective structural changes in the monomer itself, to mediate catalyst-free polymerization at a low energy via one-step synthesis protocol. In this regard, monomers based on disulfide-linked bisbenzoxazine were successfully synthesized using cystamine (biobased) and cardanol (agro-waste)/ phenol. Reduction of the disulfide bridge in the monomer using dithiothreitol under mild conditions in situ transformed the oxazine ring in the monomer, via neighboring group participation of the −SH group in a transient intermediate monomer, into a thiazolidine structure, which is otherwise difficult to synthesize. Structural transformation of ring-opening followed by the ring-closing intramolecular reaction led to an interconversion of O−CH 2 −N containing a six-membered oxazine ring to S−CH 2 −N containing a five-membered thiazolidine ring and a phenolic-OH. The structure of the monomer with the oxazine ring and its congener with the thiazolidine ring was characterized by spectroscopic methods and X-ray analysis. Kinetics of structural transformation at a molecular level is studied in detail, and it was found that the reaction proceeded via a transient 2-aminoethanethiol-linked benzoxazine intermediate, as supported by nuclear magnetic resonance spectroscopy and density functional theory studies. The thiazolidine-ring-containing monomer promotes ROP at a substantially low temperature than the reported mono-/bisoxazine monomers due to the dual mode of facilitation of the ROP reaction, both by phenolic-OH and by ring strain. Surprisingly, both the monomer structures led to the formation of a similar polymer structure, as supported by thermogravimetric analysis and Fourier transform infrared study. The current work highlights the benefits of inherent functionalities in naturally sourced feedstocks as biosynthons for the new latest generation of benzoxazine monomers.

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 S8 at room temperature. The IE-fa benzoxazine monomer was synthesized under neat conditions and reacted with varying percentages of S8 as sodium polysulfide (Na2S x ) 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.

No abstract

No abstract