The mechanochemical Friedel‐Crafts polymerization as a solvent‐free cross‐linking approach toward microporous polymers

Abstract

Herein we report the mechanochemical Friedel‐Crafts alkylation of 1,3,5‐triphenylbenzene (TPB) with two organochloride cross‐linking agents, dichloromethane (DCM) and chloroform (CHCl3), respectively. During a thorough milling parameter evaluation, the DCM‐linked polymers were found to be flexible and extremely sensitive toward parameter changes, which even enables the synthesis of a polymer with a SSABET of 1670 m2/g, on par with the solution‐based reference. Contrary, CHCl3‐linked polymers are exhibiting a rigid structure, with a high porosity that is widely unaffected by parameter changes. As a result, a polymer with a SSABET of 1280 m2/g could be generated in as little as 30 minutes, outperforming the reported literature analogue in terms of synthesis time and SSABET. To underline the environmental benefits of our fast and solvent‐free synthesis approach, the green metrics are discussed, revealing an enhancement of the mass intensity, mass productivity and the E‐factor, as well as of synthesis time and the work‐up in comparison to the classical synthesis. Therefore, the mechanochemical polymerization is presented as a versatile tool, enabling the generation of highly porous polymers within short reaction times, with a minimal use of chlorinated cross‐linker and with the possibility of a post polymerization modification.

The mechanochemical synthesis of polymers

“…Recently, the same group calculated the green metrics of a mechanochemical Friedel–Crafts polymerisation. 271 In this study, 1,3,5-triphenylbenzene was cross-linked with either DCM or CHCl 3 in the presence of the Lewis acid AlCl 3 , which served as the bulk material and the catalyst. The authors provided a detailed calculation of AE, MI, MP, and the E-factor for the solvent-based synthesis, for the mechanochemical synthesis, and for the mechanochemical synthesis without additional bulk material.…”

“…Although the mechanochemical synthesis drastically reduced the environmental impact as well as the synthesis/workup time and the toxicity of the liquids required for workup, the resulting porous polymer outperformed its solvent-based equivalent in terms of SSA BET and yield. 271 …”

“…While the neat-grinding Scholl reaction yielded a polymer with an SSA BET of 658 m 2 g −1 , 53 the introduction of a cross-linker allowed the synthesis of highly flexible polymers with SSA BET values of up to 1670 m 2 g −1 . 271 Nevertheless, the surface area of polymers generated by the mechanochemical Friedel–Crafts reaction was highly dependent on the monomer building blocks used, as investigated by the group of Horike ( Fig. 19 ).…”

“…Despite the similarities between both reactions, the mechanochemical Friedel-Crafts reaction yields porous polymers with higher surface areas than Scholl coupling under similar conditions. 53,267,271 While the mechanochemical Scholl reaction of various monomers leads to the formation of conjugated porous polymers, the additional crosslinker induced by the Friedel-Crafts polymerization breaks the conjugation and functions as a spacer between the monomers, allowing a higher flexibility during network formation, accompanied by an enhanced SSA BET of the resulting polymer. Recently, the Borchardt group has compared the Scholl and Friedel-Crafts polymerisations of 1,3,5-triphenylbenzene.…”

“…Although the mechanochemical synthesis drastically reduced the environmental impact as well as the synthesis/workup time and the toxicity of the liquids required for workup, the resulting porous polymer outperformed its solvent-based equivalent in terms of SSA BET and yield. 271 Exemplary for other mechanochemical polymerization reactions, the Suzuki and the Friedel-Crafts polymerization can therefore be considered as extremely advantageous in comparison to the solvent-based approach, since the avoidance of solvent waste drastically increases the overall sustainability and the time reduction further promotes energy efficiency as well as cost reduction. 90,271 Although mechanochemical polymerisations can generally be declared as more environmentally benign than their solutionbased counterparts, other synthetic methods may even exceed the sustainability of mechanochemical procedures.…”