Reaction mechanism and products of the thermal conversion of hydroxy-containing polyimides

Thermal treatments of hydroxyl-containing polyimide materials at high temperatures are believed to lead to unique structural and chemical changes to the polymer backbone and side groups. These rearrangements when applied to specific polymeric membranes improve both their permeability and selectivity over conventional materials and this development has been attributed to the formation of nanoporous cavities in the treated products. However, the mechanisms of the possible chemical reactions that take place and the structures of the end-products have been poorly understood. With validated evidence from experiments and virtual screening of different precursors and possible products by molecular simulation, we have determined that hydroxy-containing polyimides can be thermally rearranged into poly(biphenylene bisimide) polymers under designed conditions. The proposed reaction mechanism is novel and calls into question the previously reported polybenzoxazole mechanism of thermal rearrangement. The new results indicate that the exact conditions of synthesis and thermal treatment of the starting polyimides will be critical to the properties of the membranes produced.