Abstract
Polymeric materials, undergoing thermally stimulated reaction cascades, are promising materials and techniques for future custom tailoring of high-performance materials, such as in coating or film, heat resistance, and energy applications. The high atom economy and stimulation in the solid state of readily processable precursors, which lead to complex and usually difficult-to-process materials, make them highly attractive when it comes to the design of new materials. Polyimides and polybenzoxazoles are both known as thermally resistant high-performance materials. Polyimides have been used for the conversion to polybenzoxazoles (PBO) at high temperatures. In our study, we report a set of allylated ortho-hydroxy polyimides (HPIs) that are capable of undergoing a set of consecutive thermally stimulated reactions in a reaction cascade-like manner. The reactions have been investigated in detail by thermokinetic and spectroscopic experiments, supported by means of quantum chemical and molecular dynamic simulations. A significant change regarding the extent of consecutive Claisen rearrangement reactions was observed, depending on the type of allyl derivative and course of annealing. βM-PI thus showed a very high conversion to benzofuran rings, followed by an HPI-to-PBO rearrangement to full conversion at an annealing procedure using only 350 °C, which is the highest conversion at a sub-400 °C annealing protocol for a fluorinated HPI. This is even surpassed in our study by γE-PI, which shows the lowest ever reported onset temperature for a modified HPI and especially for a hexafluoroisopropylidene group-containing HPI backbone, which has been identified as a promising backbone material for membrane applications. These results point out that applying thermal reaction cascades, using modified allyl groups in polyimides, could be a universal strategy to improve the materials’ performance of polyimides for various applications.
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