The preparation of PBI(IV) can be achieved by condensation reaction of diphenyl isophthalate (I) and 3,3’,4,4’-tetraaminodiphenyl (II) (Figure 1). The spontaneous cyclization of the intermediately formed animo-amide (III) to PBI (IV) provided a much more stable amide linkage.
This synthetic method was first used in the lab and later developed into a two step process. In a typical condition, starting materials were heated at 270 degree for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 360 degree for another 1 h to form the final commercial-grade product. The reason for the second step is due to the formation of the by-product phenol and water in the first step creating voluminous foam,[10] which leads to the volume expansion of several times of the original. This is the issue that must be considered by the industrial manufacturers. This foam can be reduced by conducting the polycondensation at a high temperature around 200 °C and under the pressure of 2.1-4.2 MPa.[11] The foam can also be controlled by adding high boiling point liquids such as diphenylether or cetane to the polycondesation. The boiling point can make the liquid stay in the first stage of polycondesation but evaporate in the second stage of solid condensation. The disadvantage of this method is that there are still some liquids which remain in PBI and it is hard to remove them completely.[11]