Understanding the Stereochemical Effect on the Properties of Emerging Thermosets: Sustainable Polybenzoxazines

Exploration of the properties of monomers by modulating their stereochemistry is not well reported in polybenzoxazines (PBzs). Isosorbide contains a rigid bicyclic ( endo-exo-configuration) core that has shown promise in high-performance thermoplastic polymers. In this report, new chiral PBzs were prepared from industrially available feedstocks, carbohydrate-derived (isosorbide), and lignin- or cashew nut waste-sourced renewable phenols (cardanol and guaiacol). A fully biobased benzoxazine monomer based on a one-step condensation reaction of isosorbide diamine (isa) with phenols was carried out for 15 min under microwave and solvent- and catalyst-free conditions. Structural elucidation of monomers was achieved by two-dimensional (2D) NMR spectroscopy [heteronuclear single quantum correlation (HSQC) and COSY (correlated spectroscopy)]. The "isa"-based monomer revealed a comparable ring-opening polymerization temperature and a good processing window for the widely available bisphenol-A/aniline-based monomer. The resultant polymers showed a higher T-g, better thermomechanical properties, and adhesive strength compared to other classical petrobased PBzs. The current work extends the importance of the major component of the sugar-derived isohexide family member, isosorbide, in polybenzoxazine chemistry and suggests that stereocontrol in the monomer is another approach to affect the thermomechanical properties of this upcoming class of high-performance phenolic polymer.