100th Anniversary of Macromolecular Science Viewpoint: Fundamentals for the Future of Macromolecular Nitroxide Radicals

Abstract

Macromolecular radicals, radical polymers, and polyradicals bear unique functionalities derived from their pendant radical groups. The increasing need for organic functional materials is driving the growth in research interest in macromolecular radicals for batteries, electronics, memory, and imaging. This Viewpoint summarizes the current state-of-knowledge regarding the macromolecular nitroxide radicals’ redox mechanism, conductivity, chain conformation, controlled polymerization, network structure, conjugated forms, and applications. The nitroxide radical group is the focus because it is the most widely studied. Although most literature focuses upon applications, an emerging body of work is highlighting the fundamental physicochemical properties of macromolecular radicals. To this end, this Viewpoint recommends areas of opportunity in fundamental studies and best practices in reporting.

Making Nonconjugated Small-Molecule Organic Radicals Conduct

“…This is consistent with previous computational efforts, which have highlighted that the change in exchange energy associated with molecular rearrangement is necessarily zero between neighboring radical active sites. 47,48 This is because, unlike in conjugated material systems, the charge-transfer site is heavily on the nitroxide group. Thus, any changes in structure by crystalline domain, added or disrupted conjugation, or alternation to the configuration (e.g., kinks along the chains) do not impact the ability for charge transport of radical polymers.…”

Design Strategies for Enhanced Conductivity in Metal–Organic Frameworks

“…Finally, the presence of solvent structuring in nanoconfined spaces likely restricts flexibility of the medium and impacts the overall ion mobility. Indeed, diffusion-limited electron transport is commonly observed in nanoconfined redox-active systems. − Most notably, ion conductivity is one of the critical parameters for the performance of rechargeable batteries because it directly affects charging and discharging through the processes of intercalation and deintercalation . Given the importance of ion transport in these processes, current efforts are focused on improving the mobility by careful electrode structuring or by incorporating secondary ion-transport channels. , …”

Design of an n-type low glass transition temperature radical polymer

“…[9][10][11][12][13][14][15][16][17][18][19][20][21][22] Whereas, only over the last handful of years have the solid-state electrical conductivity properties of radical polymers been evaluated in full. 8,[23][24][25][26][27][28] In many of the early solid-state electrical conductivity evaluation efforts, the conductivity of a model radical polymer, poly (2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), was quantified by multiple groups with the a highest value of ∼10 −4 S m −1 being reported. 23,26,29,30 This solid-state electrical conductivity was improved due to an improved radical polymer design in that poly(2,3-bis(2′,2′,6′,6′-tetramethylpiperidinyl-Noxyl-4′-oxycarbonyl)-5-norbornene) (PTNB) had a higher radical content than what was typically observed in the PTMAbased macromolecular design case, and a thin film of this material achieved a solid-state electronic conductivity of 7 × 10 −3 S m −1 .…”