Thiophene-Rich Benzoxazines with an Amide Moiety: Integration of Structural and Hydrogen Bonding Influence on the Polymerization Mechanism by Experimental and Computational Studies

The future development of benzoxazine thermosetting resins as advanced polymeric materials depends on designing more attractive chemical structures and reaching a deeper understanding of the structure-property relationship at the molecular level. Herein, we synthesized a new series of benzoxazines containing the thiophenecarboxamide or thiopheneacetamide functionality at the ortho- and para-positions with respect to oxygen in the oxazine ring. Successful syntheses of thiophene-rich benzoxazine monomers and their corresponding molecular characterizations were confirmed by Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR), and high-resolution mass spectroscopy. Ring-opening polymerization processes of benzoxazines and thermal properties of their resulting polybenzoxazines were studied by differential scanning calorimetry, in situ FT-IR spectroscopy, and thermogravimetric analysis. In addition, a combined experimental and computational study using both 1D and 2D NMR as well as density functional theory calculations provided kinetic insights into the effects of structural and hydrogen bonding on the polymerization mechanism. With the designed varieties of hydrogen bonds in these novel benzoxazines, it has been found that oxygen in the oxazine ring involved in stronger hydrogen bonding can generate a relatively weaker O-CH2 bond, leading to the reduced ring-opening polymerization temperature. The fundamental investigations presented in the current work provide us a deeper understanding of the hydrogen bonding interaction in benzoxazine as well as its role in polymerization behavior. Moreover, these newly obtained thiophene-rich benzoxazines also give us ample opportunities to develop high-performance polybenzoxazine thermosets with promising applications.