Microwave-Assisted Solvent-Free Synthesis of Novel Benzoxazines: A Faster and Environmentally Friendly Route to the Development of Bio-Based Thermosetting Resins

Microwave-assisted organic synthesis (MAOS) is a well-established technique that has been used in the enhancement of chemical reactions. Here, the versatility of MAOS is explored describing an environmentally friendly one-pot route to novel bio-based benzoxazines under solvent-free conditions. The lignin derivative, guaiacol, along with paraformaldehyde and different conjugated and nonconjugated amines are successfully fused into guaiacol-derived 3,4-dihydro-2H-1,3-benzoxazines. The reactions conducted under microwave irradiation are completed much faster than those under traditional heating, reducing the reaction time from hours to only 6 min, with good yields. The chemical structures of novel benzoxazines are confirmed by H-1 and C-13 NMR spectroscopy, FTIR, and HR-MS. The thermal behavior of the resins are evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), showing that these polymers have good thermal stability and wide processing-window, with onset temperature of polymerization above 230 degrees C. These results indicate dramatic improvement over the traditional methodologies for the production of this class of resins, which are usually obtained by time-consuming procedures and in the presence of toxic solvents. Therefore, MAOS can be considered a green and efficient strategy for the synthesis of eco-friendly benzoxazines. (C) 2017 Wiley Periodicals, Inc.