Overview and Historical Background of Polybenzoxazine Research

This chapter points out that naming of polybenzoxazine is very complex due to the rich variety of structures from similar raw materials available. The general overview of benzoxazine chemistry and polybenzoxazine properties is provided here. The benzoxazine chemistry offers the richest molecular design flexibility among all classes of polymers, thus leading to advances in many fields with tailored properties. Benzoxazines exhibit various unusual properties, such as near-zero shrinkage upon polymerization, fast development of properties at low conversions, very high char yield, and hydrophobicity and low dielectric constants, despite having many hydrophilic groups. The cross-linked polybenzoxazine derived from bisphenol-A and aniline-based monomeric benzoxazine (BA-a) is expressed as poly (BA-a). It states that Benzoxazine is a molecule where an oxazine ring is attached to a benzene ring. There are several benzoxazine structures depending on the position of the heteroatoms. It explains that if benzoxazine monomers crystallize is pure then it tends to retain a small amount of solvents and impurities rather tenaciously and thus purification can sometimes be difficult. Upon polymerization, polybenzoxazine exhibits amorphous structure, as they are cross-linked. The synthesized compound is subjected to thermally accelerated, cationic ring-opening polymerization with or without an added initiator. If the initiation takes place with a labile proton initiator, such as phenol, it leads to a polymer with a phenolic structure. On the other hand, if a nonlabile proton initiator, such as Lewis acid, is used to polymerize at a relatively low temperature, it could lead to an arylether structure. However, the arylether structure is thermally unstable and can undergo structural transformation to the phenolic-type at an elevated temperature.