Thermal conversion of hydroxy-containing polyimides to polybenzoxazoles. Does this reaction really occur?

Hydroxy-containing polyimides with a low level of structural defects have been produced by the reaction of aromatic tetra-carboxylic acids with hydroxy containing aromatic diamines in water, as this method avoids the normal ester impurities formed by the standard polyimide synthesis techniques using tetra-carboxylic dianhydrides in dry polar solvents. Detailed FTIR studies focused on the controlled thermolysis of these purified alpha-hydroxypolyimides and their model compounds has revealed that the hydroxy groups are lost at significantly lower temperatures and times than the imide groups. This observation does not support previous literature reports which suggested that benzoxazole formation occurs and has implications for the style of materials ultimately produced. We propose a new mechanism and provide evidence to indicate that the polymers initially produced by the thermolysis reaction of alpha-hydroxypolyimides are actually crosslinked, imide containing biphenylene derivatives.