Recent Advances in the Design of Ionenes: Toward Convergence with High‐Performance Polymers

Abstract

Ionenes, condensation polymers wherein the charge (typically cationic) lies directly within the polymer backbone, have been known for over 85 years. Historically, ionenes have been synthesized from 3o diamines and α,ω‐dihaloalkanes, forming chains of ammonium cations tethered by flexible hydrocarbon linkages with “free” halide anions. However, the requisite building blocks of ionenes are by no means limited only to such molecules. In recent years, ionenes with more sophisticated backbone chemistries have been produced, with a trend toward the use of functionalities associated with classical “high‐performance” condensation polymers such as polyimides and polyarylamides. The expansion of ionenes is also catalyzed by the rapid growth of research in imidazolium‐based ionic liquids (ILs), wherein the combination of ionenes with ILs can yield unexpected behaviors. Furthermore, when considering the largely unexplored experimental space in anionic ionenes, the opportunities for new materials are virtually endless. This review primarily focuses on developments in ionenes published in the scientific literature since 2010, but also includes some older examples that may not have received sufficient attention at the time of their original publication yet can provide some key lessons for the future of ionene design.

Understanding the effects of backbone chemistry and anion type on the structure and thermal behaviors of imidazolium polyimide‐ionenes

“…Many ionenes feature structural elements common to ionic liquids (ILs) and poly(ILs), yet also offer greater variety in terms of the functionalities that can be included within their structures, provided that the requisite precursors are thoughtfully designed. The large majority of ionene polymers known in the literature have been synthesized via Menshutkin reactions between 3° diamines and dihaloalkane or similar species . Imidazole, a ubiquitous N ‐heterocycle with reactivity similar to 3° amines, is also highly suitable for ionene synthesis when starting from bis(imidazole) species such as 1,1′‐(1,6‐hexanediyl)bis(imidazole) …”

“…Research on imidazolium‐containing macromolecules is growing, due to their versatile nature, tunable structures and the coulombic interactions between polymer chains which are enhanced when additional ‘free’ IL is added to the polymer . Recently, we have developed new, broadly applicable approaches that can yield more thermally robust imidazolium ionenes, through the synthesis of bis(imidazole) monomers containing aromatic imide or amide linkages .…”

“…Research on imidazolium‐containing macromolecules is growing, due to their versatile nature, tunable structures and the coulombic interactions between polymer chains which are enhanced when additional ‘free’ IL is added to the polymer . Recently, we have developed new, broadly applicable approaches that can yield more thermally robust imidazolium ionenes, through the synthesis of bis(imidazole) monomers containing aromatic imide or amide linkages . Developing a rational understanding of the properties and behaviors of polyimide‐ionenes is of particular interest to our research in gas separation membranes as conventional polyimides built from species such as 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) have emerged as among the most promising polymers for separations involving CO 2 from N 2 or CH 4 .…”

“…The large majority of ionene polymers known in the literature have been synthesized via Menshutkin reactions between 3 ∘ diamines and dihaloalkane or similar species. 11,12 Imidazole, a ubiquitous N-heterocycle with reactivity similar to 3 ∘ amines, is also highly suitable for ionene synthesis when starting from bis(imidazole) species such as 1,1 ′ -(1,6-hexanediyl)bis(imidazole). [7][8][9]12 Research on imidazolium-containing macromolecules is growing, due to their versatile nature, tunable structures and the coulombic interactions between polymer chains which are enhanced when additional 'free' IL is added to the polymer.…”

“…1,11 Recently, we have developed new, broadly applicable approaches that can yield more thermally robust imidazolium ionenes, through the synthesis of bis(imidazole) monomers containing aromatic imide or amide linkages. 10,11,13 Developing a rational understanding of the properties and behaviors of polyimide-ionenes is of particular interest to our research in gas separation membranes as conventional polyimides built from species such as 4,4 ′ -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) have emerged as among the most promising polymers for separations involving CO 2 from N 2 or CH 4 . [14][15][16][17][18] Furthermore, utilizing the largely aromatic structures of these imides should yield enhanced thermal stability compared to analogous ionenes linked by n-alkyl groups and lacking aromatic imide or amide functionalities.…”