Pi-Conjugated Polymers: The Importance of Polymer Synthesis

JOHN R. REYNOLDS

School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, GA 30332-0400, USA

Email: reynolds@chemistry.gatech.edu

1.1 Historical Perspective

When one considers the early days of conjugated and conducting polymer synthesis, the early work by Letheby on the oxidation of aniline, presumably forming polyaniline (1), and Dall'Olio et al. on polypyrrole (2) are often referred to as landmark developments in the field. To gain an important historical view, the reader is directed to the work of Rasmussen who provides a perspective on the field where the work of Weiss on polypyrrole, as well as that of Buvet and Jozefowicz on polyaniline, are highlighted. These materials, while completely insoluble and infusible as formed from oxidative polymerization from the parent monomers, served as the basis for inducing electroactivity into polymer systems. Over the years, numerous review articles, book chapters and reviews have issued around the field, most often directed to a class of polymers or type of property they impart, with the 1986, 1998, and 2007 editions of the "Handbook of Conducting Polymers" providing a deep and complete scientific overview (Chart 1.1).

The 1950s saw the Nobel Prize winning work of Karl Ziegler and Giulio Natta on coordination polymerization of unsaturated molecules, which provided a route to structurally well-defined polymers. Most known for the development of commercially important polyolefins (e.g. polyethylene and polypropylene), the work of Natta demonstrated that acetylene polymerization could yield the conjugated polymer polyacetylene (CH)X(3) as an infusible grey powder. While the 1960s saw the development of many aromatic units containing polymers, it also became understood that pi-conjugation led to semi-conducting material properties. In one fascinating study, Berets and Smith examined the vapor phase treatment of (CH)X powders with various Lewis acids and bases. In fact, when using BF3 as the reacting dopant, they measured conductivity enhancements by a factor of 1000 on pressed pellets. In this work, they also treated (CH)X with Cl2, yet only saw small conductivity increases by a factor of 5. Interestingly for the discussion that will follow, this work did not use iodine or bromine as an oxidizing system.

Serendipity and careful observation go hand-in-hand in science, and this has been especially important for many discoveries in the conjugated and conducting polymer field. An especially important discovery was the formation of free-standing films of (CH)X by Ito et al. in 1974 caused by rapid polymer- ization of acetylene at a quiescent Ziegler–Natta polymerization initiator system solution. While the Shirakawa research group was mainly dedicated to understanding structural properties (e.g. cis–trans ratios), the fact that these films were mechanically flexible, with a metallic silver luster, suggested important underlying electronic properties.

In a landmark series of experiments, Heeger, MacDiarmid, and Shirakawa combined efforts to study the electronic properties and gas-phase doping of polyacetylene films. Their discovery that treatment of these films with controlled amounts of Cl2, Br2, I2 and AsF5 could yield conductivity enhancements greater than 107, and ultimately yield electronic conductivities in excess of 500 S/cm, demonstrated unprecedented conductivity in an organic polymer. While charge transfer salts, such as those based on tetrathiafulvalene and tetracyanoquinodimethane were known to exhibit high conductivity and metallic properties, the fact that such properties were accessible in the more disordered and flexible polymer films was especially stunning. Researchers around the world quickly picked up on this, and it was demonstrated that the high level of conjugation in the polymer, along with pi-stacking and interchain interactions, all played an important role in the electronic properties. Chemists quickly realized that many polymer structures could be prepared that were fully conjugated; thus, the stage was set for a major synthetic effort. This work has now spanned 30 years and has led from insoluble, infusible, materials that were highly unstable conductors to well-characterized, solution processable polymers with fine structural control that are finding utility across a broad number of applications.

With this background in mind, this book seeks to teach the details of synthetic preparative polymer chemistry in all of the major classes of pi-conjugated polymers that have been developed to researchers in the field. The authors of each chapter have carefully overviewed the various polymer types employed in the field with a special focus on experimental details that yield reproducible and high-quality materials. Prior to moving to those specific chapters, let's take some time to review the general concepts in polymerization that are important for the development of such materials.

1.2 Considerations in Polymerizations

Fundamental polymer chemistry teaches us two main relevant mechanisms for polymerization; specifically step-growth and chain-growth methods. These methods provide polymers with distinctly different structures in terms of repeat unit functionality, molecular weight, and dispersity. As these molecular structures relate to higher-level macromolecular considerations, such as chain–chain interactions and the development of material morphology, it is important that the mechanism be understood for any system under study. Step-growth polymerization sees the step-wise buildup of molecular weight as a function of the extent of conversion of reactive monomer functional groups. As taught by Carother's equation, high molecular weight polymers are obtained at high extents of conversion requiring especially high degrees of monomer purity. The excess of any one monomer type (more formally the excess of any one functional group in polymerization) limits the molecular weight considerably where oligomers can provide non-optimal properties.

Chain-growth polymerizations to form addition polymers are most often accomplished using monomers with multiple bonds and loss of unsaturation. In this mechanism, a reactive intermediate is first created in an initiation step and subsequently propagates via repeated monomer addition to provide a macro- molecule. When the reactive intermediate is ionic, impurity termination or quenching processes can kill the reactive intermediate, while in the case of radical polymerization, coupling termination can lead to an overall doubling of the average molecular weight. Many early attempts at forming conjugated polyarylenes and polyheterocycles attempted to use step-growth polymerization under non-optimized conditions, thus yielding low molecular weight polymers. Significant efforts detailed in this book demonstrate how careful control of the reagents and polymerization conditions now lead to quality polymers as high molecular weight, well-defined chemical systems. In fact, in some instances where it was believed that step-growth couplings were occurring, detailed studies show that indeed chain-growth (and in some instances living) polymerizations were in fact underway. While one of the benefits of a chain-growth polymerization can be the formation of high molecular weight polymers at a low degree of monomer conversion, the fact that unsaturation is lost tends to limit simple chain-growth polymerizations to directly form conjugated polymers to alkyne derivatives.

1.3 Side Chains, Processability and Molecular Weight

One of the most important physical limitations that have been addressed by synthetic chemists over the years is the inherent insolubility of pi-conjugated polymer chains. With a tendency towards rigidity and strong interchain pistacking interactions, the inherent systems tend to be completely insoluble and infusible, as illustrated by the structures of unsubstituted (R1/4H) polythiophenes (4), poly(p-phenylenes) (5), and poly(p-phenylene vinylene) (6). A major success of synthetic efforts over the years has been to create highly soluble pi-conjugated polymers that can be processed into thin-film and fiber forms for potential applications. Overcoming these solubility issues was one of the most important early contributions the synthetic community made to the field. The introduction of pendant flexible side chains (R 1/4 alkyl and alkoxy in 4, 5, and 6) on conjugated polymers provides conformational entropy that induces solubility into the polymer product. As a generality, alkyl groups on the order of 6–8 carbons in length (hexyl to octyl), provide sufficient conformational disorder to induce solubility in the high molecular weight polymers with simple single aryl ring repeat units. This method is illustrated throughout this text, as it has become the main approach for preparing usefully processable conjugated polymers (Chart 1.2).

Considering the high molecular weights possible with chain-growth polymerizations, the synthesis of soluble and processable polymer precursors to fully conjugated materials has proven to be an excellent route for preparing useful materials. An early example of this is the synthesis of poly(p-phenylene vinylene) (PPV) via the polymerization of bis-sulfonium salts of bis-dichloro-methylbenzene. Basic treatment of the bis-sulfonium salt leads to in situ formation of a quinoidal structured intermediate (not isolated), which subsequently polymerizes to form a nonconjugated polyelectrolyte that is soluble in alcoholic media. This soluble precursor polymer solution can be stored for quite some time, and subsequently processed into thin films by any number of solution processing methods. Thermal treatment of the solid material leads to elimination of HCl and dialkylsulfide or tetrahydrothiophene yielding the final conjugated PPV derivative. This general concept of soluble precursor polymer synthesis has found use in the preparation of various polyacetylenes, poly(p-phenylenes), and poly(thienylene vinylenes) along with numerous other poly(arylene vinylene) systems in general. A major benefit of this methodology is that high molecular weight polymers can be obtained, even at low monomer conversion, with the precursor polymers tending to solution process well. A major drawback of the polymer precursor route is the chemical purity of the final conjugated polymer. As with any reaction on a macromolecule, complete conversion is not possible. In addition, many of the conversion reactions are thermally driven eliminations where side reactions are induced.

A standard question asked in any new polymer study is, what is the magnitude of the molecular weight that is required to provide the limiting properties for a particular application? This will be an important concept addressed throughout the many chapters in this book, as the polymerization chemistry used to attain conjugated systems can be quite varied. As just one example, consider the evolution of the optical absorption spectra of conjugated polymers as a function of chain length and the effect on the resultant color (important when considering electrochromic applications) transmitted or rejected by the final polymer films. In a combined size exclusion chromatography/UV-Vis spectroscopy experiment, it was found that a series of cyanovinylene-linked dioxythiophene polymers attained their limiting spectra at a GPC estimated number average molecular weight of approximately 10 kg/mol. Simultaneously, this molecular weight also provided materials with sufficient film-forming properties for stable and reproducible electrochemical switching and, as such, this molecular weight is adequate for this specific electrochromic polymer application. In general, many step-growth polymerization methodologies can provide conjugated polymers of sufficient molecular weight for the application at hand where the materials are used as thin, electrode-supported, films. Standard equilibrium controlled step-growth polymerizations have degrees of polymerizations controlled by Carother's equation. The necessity for a high degree of functional-group conversion for molecular weight creates the situation in which the synthetic chemist must be especially careful about monomer purity and functionality. At the same time, it has been demonstrated that higher molecular weights, beyond which there is no visible change in the spectroscopic signature of a conjugated polymer, can provide elevated power conversion efficiencies in solar cell and field effect transistor applications. These considerations of molecular weight are subtle from polymer to polymer, and application to application, and must be addressed separately for each system. These concepts are illustrated nicely throughout this book.

1.4 Structural Control via Repeat Unit and Functionality

When one considers how synthetic chemistry has impacted the development of conjugated polymers, there is no better example than the poly(3-alkylthiophenes) (P3ATs, 4). Early work focused on oxidative polymerization methods as a means of preparing soluble forms of this polymer. Subsequently, Grignard coupling reactions were able to prepare the polymer directly in the reduced state, such that there were no residual charge carriers in the materials (this would ultimately prove useful in the concepts of using P3ATs as semi-conducting and charge-transporting organic electronic materials). Disorder through the formation of head/head and tail/tail defects led researchers to develop controlled polymerizations that provided regioregular P3ATs with a high degree of order. Even finer control has been brought through the utilization of Grignard metathesis reactions and the examination of polymerization catalytic processes, such that the polymerization can be carried out under living conditions. It is just these considerations that are the major driving force that led us to edit this book. It is crucial that synthetic chemists obtain polymers with high repeat unit purity, backbones with no branching or crosslinking, high molecular weights with low dispersity, and overall high purity in the removal of residual chemical species formed during polymerization, such as entrapment of metallic catalyst impurities. Further, the controlled introduction of end groups on the conjugated polymer chains provides another degree of purity, and depth of structural understanding.

This book is designed to provide the reader with a comprehensive view of how the various classes of conjugated polymers are synthesized. Contained within these 20 chapters are overviews of the reactions, structures, and synthetic conditions required for effective polymer formation, along with experimental details. Throughout the text, the evolution of structural build up is a focus; moving from simple polymer repeat units, to highly functionalized polymers, to more complicated structures with specific property design in mind. Building on the fundamental conjugated polymer systems introduced above, a number of chapters are directed to various forms of polyarylenes such as the poly(phenylene ethynylenes) (7, Chapter 8), polyfluorenes (8, Chapter 5), and polycarbazoles (9 and 10, Chapter 6), to name just a few. Property modification becomes evident through the extent of conjugation provided in these types of polymers. For example, comparison of the 2,7- and 3,6-linked polycarbazoles allow examination of the effects of full compared to broken conjugation, where the latter structure leads to discrete chromophores. Ultimately, the ability to form these specific linkages in polymers plays an important role in determining ultimate properties as the 2,7-linked carbazole units are found to be useful in high-performance solar polymers, while the electron-rich 3,6-linked carbazoles find use in easily switchable, redox-active electrochromic polymers26 (Chart 1.3).

As noted earlier, polythiophene has served as an easily functionalized system where the nature of the side chains, and their regio-orientation, provides a broad range of controllable solution and solid-state order properties. Many functionalized conjugated polymers are illustrated throughout this collection of chapters where the side chains bring added functionality. Using the oligoether and naphthylene containing side-group-substituted polythiophenes 11 and 12, respectively, (Chapter 9) as examples, polar ion coordinating and liquid-crystalline behavior can be introduced into the resultant materials. Dioxythiophene chemistry, led by poly(3,4-ethylenedioxythiophene) (PEDOT) (Chapter 10), provides a class of polymers that are easily oxidized, thus providing highly stable conducting materials. The poly(3,4-propylenedioxythiophene) (PPro-DOT, 13) family of polymers can be prepared using oxidative, Grignard metathesis, and direct arylation conditions to yield a family of polymers that are especially vibrantly colored in their neutral states and transmissive in their oxidized forms, as desired for electrochromic applications (Chart 1.4).