The mechanochemical synthesis of environmentally benign fully biobased 4th generation benzoxazines and their polymers: mechanistic insights into the catalytic activity of latent catalysts

Abstract

Sustainable methodologies, such as mechanochemical mixers, have revolutionized the way compounds are synthesized in minimal time and with better yields. In this work, a series of latest 4^th generation benzoxazine monomers were synthesized via a facile, highly scalable, efficient mechanochemical ball-milling process, making them more viable than those synthesized via traditional synthesis route. The 4^th generation benzoxazine (BZ) monomers contained 100% biosynthons (vanillin, ortho-vanillin and furfuryl amine) and were purified without strenuous separation techniques. We found that the variation in the nature of aryl substitution (with and without phenolic–OH) at the reactive oxazine C₂ centre governs the polymerization temperature, volatilization of monomers, kinetics of polymerization, thermal and mechanical properties. As expected, the control monomer without inherent phenolic–OH showed a high polymerization temperature and the advantages offered by vanillin regioisomers facilitated the polymerization with minimal volatile release. The latent catalytic effect in the monomer was confirmed by both temperature-dependent NMR and SCXRD studies. Interestingly, distal phenolic–OH was found to be more labile and polymerized easily at a lower temperature than when it was present at the ortho-position. Additionally, the former monomer structure led to a well-defined polymer network with an appreciable T_g (116 °C) and thermal stability (T_max of 348 °C and char yield of 36%). Furthermore, it also revealed an excellent storage modulus and good adhesion properties compared to many classical petroleum-based polybenzoxazines. Overall, we showcased the viability of employing the benefits of copolymerizing two generations of benzoxazines at low temperature and explored this new class of latest benzoxazine monomers as greener adhesives with improved stability to enable future designing and open up the possibility of using them in several practical and innovative applications.