Highlights

Abstract

Graphical abstract

Keywords

۱٫ Introduction

۲٫ Experimental

۳٫ Results and discussion

۴٫ Conclusions

Declaration of Competing Interest

Acknowledgement

Declaration of Competing Interest

Data Availability

Appendix A. Supplementary material

References

Abstract

Ring-opening polymerization of benzoxazine monomers is a complex process and various chemical structures including N,O-acetal structures, phenolic Mannich structures and arylamine Mannich structures are formed in polybenzoxazines. To understand the polymerization mechanism, the effects of temperature, time and solvent polarity on the polymerization routes, chemical structures and thermal properties were studied. It was discovered that the N,O-acetal structures and the phenoxy structures can be obtained in a low-polarity solvent like dichloromethane at low temperature (−۲۰ °C) with the aid of PCl5, while the arylamine Mannich structures can be readily generated in polar solvent like N,N-Dimethylformamide at high temperature (>80 °C) in the presence of PCl5. However, the phenolic Mannich structures can be directly formed at high temperature (>180 °C) without any catalysts. Upon prolonging the reaction time or elevating the temperature, the phenoxy structures easily rearranged into the N,O-acetal structures or the arylamine Mannich structures. Further increasing the temperature will cause the arylamine Mannich structures to rearrange into the phenolic Mannich structures and even the phenolic methylene structures. Therefore, both phenoxy structures and N,O-acetal structures showed poor thermal stability; while the arylamine Mannich structures possessed lower initial decomposition temperature but higher char yield compared with the phenolic Mannich structures because of the formation of thermally unstable iminium ions and the anchoring of dangling aniline moieties.

۱٫ Introduction

Polybenzoxazines (PBzs) are formed by the ring-opening polymerization (ROP) of nitrogen and oxygen containing six-membered heterocyclic compounds, 3,4-dihydro-2H-1,3-benzoxazines. They are labeled as a kind of new high-performance thermosetting resins that possess good properties similar to or even higher than phenolic resins and satisfactory processability resembling epoxy resins [1–۶]. Polybenzoxazines have aroused high research enthusiasm in academic communities and drawn much attention from industries because of their well-known merits: high strength, high modulus, good thermal stability, flame retardance, good dielectric properties, good dimensional stability, low water uptake, outstanding processability and tremendous design flexibility [7–۱۲]. More recently, polybenzoxaiznes have found many functional applications such as shape memory, self-healing, catalyst support, pollution treatment, super capacitor, CO2 absorption and corrosion prevention [13–۱۹]. Additionally, polybenzoxazines are of great potential in the future application as the fossil resources are decreasing rapidly but the amines and phenols that can be used to synthesize benzoxazine monomers are abundant and reproducible in nature [20–۲۴].