Elucidating the Role of Conjugated Alkenyl Functionalities at the Oxazine Ring in Governing the Polymerization Mechanism of 4th Generation-Biobased Benzoxazine Thermosets

The fourth-generation oxazine ring-substituted polybenzoxazines have recently gained attention as promising high-performing thermosets. This work successfully investigates the role of the conjugated alkenyl moiety at the oxazine ring in influencing the course of polymerization with dual benefits: lowering the ring-opening polymerization (ROP) temperature and regulating the mass-loss phenomena. By employing biosourced precursors, viz., cinnamaldehyde and trans-4-stilbene carboxaldehyde, a facile methodology for monomer synthesis is demonstrated. The structural characterization of these benzoxazines is achieved using high-resolution mass spectrometry (HRMS), nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy, which indicate the successful inheritance of the reactive conjugated alkenyl functionalities into the oxazine ring-substituted benzoxazine monomers. The thermal behavior of the benzoxazine monomers is examined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to realize lowered ROP temperature (∼195 °C) and mass-loss (∼7%). Moreover, thermal polymerization and degradation kinetics, as well as relevant spectroscopic analyses, are performed to study the effect of the (conjugation vs extended conjugation) alkenyl functionality in determining the polymerization mechanisms of herein reported monomers. Prior to onset, ROP is observed to proceed via fragmentation, i.e., bond cleavage of the zwitterion intermediates and subsequent cycloaddition-adduct formation from in situ generated species. However, at a later stage, complete polymerization occurs through a more complex route, including the ROP of the oxazine ring and the participation of other adducts in the cross-linking process. The current strategy offers an intriguing avenue for modifying oxazine ring carbon centers with reactive functional organic skeletons, which may play an instrumental role in exploring potential high-temperature applications.