Organic Radical Polymers

In the modern era, organic compounds (i.e., carbon-based small molecules and polymers) have earned a status of great import across various fields of modern material sciences. Beyond the realms of mere curiosity and fundamental research, the commercialization of organic electronics [e.g., organic light-emitting diodes (OLEDs)] has boosted the impetus to elucidate the fundamentals regarding the vast chemistry and related physical properties and device opportunities of organic materials (Fig. 1.1) [1]. This is because organic materials are often preferred over their inorganic counterparts in electronic devices where their relatively lower costs of production, earth-abundant materials compositions, ease of fine-tuning and processing, mechanical robustness and flexibility, and relatively benign environmental hazards are of primary concern [2].

What forms, follows functions; however, even the best of intentions can only be realized if and only if a successful formation (i.e., synthesis) of the targeted material can be achieved. Thus, a discussion of the viable and facile synthetic routes for the formation of the desired radical polymers is itself an important field of research. This is because one needs to understand and contain the reactivity of the pendant radical units [1]. Ambient stable radicals can be, and often are, reactive towards various chemical species including other radicals [2]. For instance, radical polymerizations of radical-containing stable monomers may not be a viable approach for the synthesis of radical polymers due to their reactivity. Thus, the choice of polymerization and the associated choice of monomer units are crucial in design and formation of any given radical polymer or polyradical (Fig. 2.1).

The modern revolution of organic material sciences cannot be justly described if one does not account for the contributions of polymeric materials [1]. Even at this fast pace of development, no other class of materials can match the versatility of macromolecules with regard to their fine-tuneable physical or chemical properties and ease of processing. It is not an exaggeration to claim that any specific functional property of any given polymer would eventually find (if it has not already found) its own importance in the upcoming avenues of science and engineering. In most cases, demand drives discovery of such materials. However, in many cases, mere curiosity drives discovery, broadening the scopes of the applications of various materials. In the case of radical polymers, the story is quite unique, as are the materials [2]. As noted in an earlier chapter, even though the successful synthesis of PTMA was known since 1972 [3], it required three decades for the community to appreciate the vast opportunity of such materials in any viable application. This spark encouraged an entire generation of researchers towards the broader opportunities of radical polymers, and the most significant impact of radical polymers has been in the development of modern approaches towards fully organic energy storage devices [4]. This opportunity is feasible due to the inherent redox-active electronic properties of this class of compounds.

The properties of organic materials in bulk solids and thin films are key to their applications in modern optoelectronic systems [1]. That is, unlike most commercial batteries, other devices (e.g., field-effect transistors, light-emitting devices, and organic photovoltaic cells) often demand the application of functional materials in the solid state. As most such applications of radical polymers have only been examined in less than the last 5 years, this area of research is practically the youngest frontier of radical polymers. Nevertheless, even in this relatively short timeframe, there have been several significant discoveries of the properties of radical polymers, which are further boosting the research efforts of these classes of compounds. In fact, the journey of the community started with the idea to utilize the spin systems in radical-containing macromolecules. In the earliest of these examples, which occurred at the dawn of twenty-first century, radical-containing macromolecules (e.g., polyradicals) formed the early examples of organic polymer magnets [2].

After being shelved for a rather long time after their initial discovery in the 1970s, radical polymers are currently finding their unique identity in the modern frontiers of organic electronics. The intriguing chemistry associated with the radical sites opens a number of opportunities in a variety of unique and exhilarating directions. Assuredly, open-shell-bearing stable radical polymer structures have their own emerging opportunities in the field of organic electronics. The redox-active radical sites in the polymers allow for reversible redox reactions, opening opportunities in charge storage, which in turn diverges towards the broader opportunities of organic batteries, supercapacitors, and memory devices. The role of organic radical polymers in revolutionizing the field of organic energy storage systems cannot be emphasized enough. The initial reports on the opportunities of organic polymers in such systems have had a literal domino effect, crushing many of conventional prejudices about the versatility of organic materials. That is, they are not only restricted to the proof-of-concept, and radical polymers are proving to be pioneering in the development of large-scale yet cost-effective redox flow batteries. Furthermore, the emergence of flexible device systems is redefining the borders of flexible organic electronics.