Due to their outstanding and versatile properties, polybenzoxazines have quickly occupied a great niche of applications. Developing the ability to polymerize benzoxazine resin at lower temperatures than the current capability is essential in taking advantage of these exceptional properties and remains to be most challenging subject in the field. The current review is classified into several parts to achieve this goal. In this review, fundamentals on the synthesis and evolution of structure, which led to classification of PBz in different generations, are discussed. Classifications of PBzs are defined depending on building block as well as how structure is evolved and property obtained. Progress on the utility of biobased feedstocks from various bio-/waste-mass is also discussed and compared, wherever possible. The second part of review discusses the probable polymerization mechanism proposed for the ring-opening reactions. This is complementary to the third section, where the effect of catalysts/initiators has on triggering polymerization at low temperature is discussed extensively. The role of additional functionalities in influencing the temperature of polymerization is also discussed. There has been a shift in paradigm beyond the lowering of ring-opening polymerization (ROP) temperature and other areas of interest, such as adaptation of molecular functionality with simultaneous improvement of properties.

This paper deals with the preparation of sustainable benzoxazines that exhibit enormous potential to compete with the existing petro-based advance performance thermosets. The phenolic component used for the synthesis of benzoxazine is derived from naturally occurring cardanol, which is obtained from cashew nut tree, Anacardium occidentale. Polyethylene terephthalate (PET) was chosen as a sustainable feedstock for the amine fraction used to prepare the benzoxazine monomer containing amide linkages. Microwave-assisted aminolysis of PET was performed to obtain bis­(amino-ethyl) terephthalamide (BAET) and α,ω-aminoligo­(ethylene terephthalamide) (AOET), which were employed as the difunctional amine for the preparation of bis-benzoxazines. In comparison to the traditional method, microwave-assisted aminolysis of PET was found to be significantly faster, and the reaction completion time could be brought down appreciably. Mannich-like condensation of cardanol with PET-derived terephthalamides and paraformaldehyde led to the formation of bis-benzoxazines with amide linkages, the structure of which was confirmed through FT-IR and 1H NMR spectroscopy. The curing behavior of the bis-benzoxazines was studied using nonisothermal differential scanning calorimetry. The presence of amide linkages in addition to the polar group formed during the ring opening of benzoxazines led to the improvement in adhesive strength, which was quantified in terms of lap shear strength.

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