Molecules Featuring the Azaheterocycle Moiety toward the Application of Flame-Retardant Polymers

Flame retardancy is a desirable property of polymer materials, and the molecular design of novel flame-retardant structures considers three main aspects: (i) the chemical structure should have suitable thermal degradation temperature and a high tendency to form char; (ii) the flame-retardant molecule should not compromise the mechanical properties of the polymer matrix; and (iii) the molecule should have low inherent toxicity and generate minimal or no harmful substances to humans and the environment during degradation. Azaheterocycles and their derivatives are an important class of nitrogen-containing flame retardants, which achieve good flame retardancy by diluting the concentration of oxygen and fuel gas on the polymer surface in the gaseous phase and promoting the amount of char residue in the condensed phase. At the same time, azaheterocycle-containing flame retardants possess excellent designability and appropriate thermal degradation temperature of most of their structures, producing gases with low toxicity when subjected to thermal decomposition. Moreover, azaheterocycles and their derivatives have promising potential in the synthesis of novel flame retardants and flame-retardant polymers. This review mainly summarizes the methods of improving the flame retardancy of polymeric materials using azaheterocycle-containing compounds and the applications of azaheterocycle-containing flame retardant molecules. Meanwhile, this review also identifies the main challenges for the future development of azaheterocycle-containing flame retardants.