Mechanochemically-Assisted Synthesis of Polyimides

Abstract

Polyimides were obtained in 99 % yield in under 1 h through the "beat and heat" approach, involving solvent-free vibrational ball milling and a thermal treatment step. The influence of a plethora of additives was explored, such as Lewis acids, Lewis bases, and dehydrating agents, and the mechanochemical reaction was identified to run via a polyamic acid intermediate. The protocol was adopted to a range of substrates inaccessible through solution-based processes, including perylene tetracarboxylic acid dianhydride and melamine. Furthermore, quantum chemical calculations were conducted to identify the water removal as the crucial step in the reaction mechanism. The presented method is substantially faster and more versatile than the solution-based process.

Keywords: beat and heat; green chemistry; mechanochemistry; polymers; solvent-free.

Figure 1

Formation of polymeric structure under mechanochemical influence. Our results show that a thermal step is necessary to reach a closed imide ring.

Figure 2

IR spectra of the model compound produced from pyromellitic dianhydride (PMDA, bottom) and 1,3,5‐tri(4‐aminophenyl)benzene (TAB, middle) after subsequent washing with water, ethanol, and acetone, and drying at 80 °C.

Figure 3

Yield of the polymer formation from PMDA and TAB with NaCl as bulking agent. Three regions are visible: chain growth (yellow), polymer network (green), and breakage (red).

Figure 4

TEM images of samples from the model compound after washing and drying at 80 °C (left) and subsequent heat treatment at 200 °C (right).

Figure 5

Enthalpies of imidization reaction calculated by means of PW6B95D3/6‐31G*//PW6B95D3/6‐31G*.