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Early History of Conjugated Polymers: From Their Origins to the Handbook of Conducting Polymers
Seth C. Rasmussen
1.1 Introduction
1.2 Basic Synthesis and Doping Processes of Conjugated Polymers
1.3 Polyaniline
Early Reports of the Oxidation of Aniline • Determination of the Structure of Aniline Oxidation Products • Buvet, Jozefowicz, and Conducting Polyaniline
1.4 Polypyrrole
Angeli and Pyrrole Black • Ciusa and ‘Graphite’ from Pyrrole • Weiss and Conducting Polypyrrole • Pyrrole Black at the University of Parma • Diaz and Electropolymerized Polypyrrole Films
1.5 Polyacetylene
Natta and the Polymerization of Acetylene • Tokyo Institute of Technology and Continued Studies of Polyacetylene • Shirakawa and Polyacetylene Films • Smith, Berets, and Doped Polyacetylene • MacDiarmid, Heeger, and Poly(sulfur nitride) • Doped Polyacetylene Films
1.6 Polythiophene
Yamamoto and Polythiophene via Catalytic Cross-Coupling • Lin and Related Catalytic Cross-Coupling Methods • Polythiophene via Electropolymerization • Polythiophenes via Chemical Oxidation
1.7 The Rise of Synthetic Metals and a Developing Field of Conductive Polymers
Synthetic Metals • Dedicated Literature
References
1.1 Introduction
Modern society is largely a plastic-based culture in which plastics developed from organic polymers have become more ubiquitous than other common materials such as metals, glass, or ceramics. This has led some to postulate that there is sufficient justification to refer to the period beginning with the 20th century as the Age of Plastics [1]. Although common organic plastics comprised of polymers such as polyethylene or polystyrene are electrically insulating materials, it was discovered in the 1960s that certain types of organic polymers could be made to exhibit semiconducting properties, with even metallic conductivity demonstrated by the late 1970s.
The most common and successful examples of such conductive organic polymers are developed from conjugated organic polymers (Figure 1.1), a class of organic semiconducting materials that exhibit enhanced electronic conductivity in their oxidized or reduced states [2, 3]. These organic materials thus combine the electronic properties of classical inorganic materials with many of the desirable properties of organic plastics, including mechanical flexibility and low production costs. This combination of properties has led to substantial fundamental and technological interest, resulting in the current field of organic electronics and the development of a variety of modern technological applications. Such applications commonly include sensors, electrochromic devices, organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and field effect transistors (FETs) [2–7]. In addition, the flexible, plastic nature of the organic electronic materials used as the active layers in such devices has led to the realistic promise of flexible electronics in the near future [4–7].
FIGURE 1.1 Common parent conjugated polymers and the years of their first reports in the literature.
The ability to imbue organic polymers with electrical conductivity is typically viewed as a somewhat recent advancement, and discussions of the history of these materials generally begin in the mid- to late-1970s with the collaborative work of Hideki Shirakawa, Alan G. MacDiarmid, and Alan J. Heeger on conducting polyacetylene [8–12]. The common view that the field essentially began with this polyacetylene work was further reinforced with the awarding of the 2000 Nobel Prize in Chemistry to these investigators “for the discovery and development of electrically conductive polymers” [13]. Such a view, however, overlooks the fact that reports of electrically conductive conjugated polymers date back to the early 1960s [14–16], with the study of conjugated polymers in general dating nearly back to the very beginning of the 19th century [17]. In fact, it has been recently argued that polyaniline is the oldest known fully synthetic organic polymer [17], with a nearly continuous string of publications on this material dating back over the last 180+ years.
Recently, the current author has worked to educate the conjugated materials community with a series of publications detailing the early history of conjugated polymers and the discovery of their conductive nature when treated with appropriate oxidizing or reducing agents [17–24]. Along with these contributions, two additional historical accounts have been published during this time frame that have also tried to highlight some of the contributions from research that predate the polyacetylene work of the 1970s [25, 26]. In continuing these collective efforts, the present chapter will provide an overview of the known history of four of the primary parent conjugated polymers—polyaniline, polypyrrole, polyacetylene, and polythiophene—from the origins of polyaniline in the early 19th century up through the development of polythiophenes in the 1980s. While early work in the field may not have been as dramatic or as fully realized as the later polyacetylene studies, many aspects and relationships attributed to the work recognized by the 2000 Nobel Prize can be seen in these earlier contributions.
1.2 Basic Synthesis and Doping Processes of Conjugated Polymers
Much of the following history predates the ability to determine the molecular structure of the polymeric materials in question, and even predates the macromolecular model of polymers as introduced by Hermann Staudinger (1881–1965) in the 1920s [27–31]. As such, it is worthwhile to briefly review our modern understanding of the polymerization methods under discussion, as well as the basic redox processes involved in the doping of conjugated polymers, in order to provide context to what will be presented in the following sections. The majority of modern conjugated materials are produced via various transition metal-catalyzed cross-coupling methods, including Kumada, Stille, and Suzuki crosscoupling [32–35], as well as recent efforts in direct arylation polymerization [36–39]. In contrast, however, the early period of conjugated polymers was dominated by oxidative polymerization as the primary synthetic method for the generation of these materials.
Electron-rich monomers polymerize anodically via either chemical or electrochemical oxidation of the π-system to form the corresponding radical cation intermediate [40–46], for which multiple resonance forms exist. For the 5-membered heterocycles (thiophene, pyrrole, furan, etc.), spin density studies support the localization of the unpaired electron at the α-position (Figure 1.2) [40, 42, 43]. As such, coupling of the radical cations occurs predominately through the α-positions, followed by deprotonation to give the neutral α,α′-dimer [40–43]. Chain propagation then continues through a step-growth mechanism involving sequential oxidation, coupling, and deprotonation steps.
FIGURE 1.2 Oxidative polymerization mechanism of 5-membered heterocycles.
Aniline undergoes oxidative polymerization in a similar manner, but with some important differences [43–46]. As with the heterocycles above, oxidation results in the formation of the corresp...
