Handbook of Polymer Synthesis Second Edition Episode 12 pptx

12 Conducting Polymers Herbert Naarmann (emerit) BASF AG Ludwigshafen I. INTRODUCTION Since the fascinating field of electrically conducting polymers was discovered more than 30 years ago [1], it has been the object of intense research. However, it took a long time to learn that the be nefits of these polymers lie less in providing substitutes for conventional metals than in opening up new fields of application. The Royal Swedish Academy of Sciences decided to award the Nobel Prize in Chemistry for 2000 to three scientists:  Professor Alan J. Heeger at the University of California at Santa Barbara, USA,  Professor Alan G. MacDiarmid at the University of Pennsylvania, USA, and  Professor Hideki Shirakawa at the University of Tsukuba, Japan, They are rewarded ‘for the discovery and development of electrically conductive polymers’ [2]. Electrically conducting polymers are materials with an extended system of C¼C conjugated bonds. They are obtained by reduction or oxidation reactions (called doping), giving materials with electrical conductivities up to 10 5 S/cm. These materials differ from polyme rs filled with carbon black or metals because the latter are only conductive if the individual conductive particles are mutually in contact and form a coherent phase. This review concerns the synthesis routes, polymerization techniques, doping, orientation, and development of well-defined, highly conducting polymeric materials. Electrically conducting materials are complied, their specific properties and potential applications are described. Numerous attempts have been made to synthesize ‘conductive organic materials’. The first was the synthesis of poly(aniline) by F. Goppelsroeder in 1891 [3]. After decades interest grew in organic polymers as insulators, but not as electrical conductors. In the late 1950s organic semiconductors became the focus of invest igations. Preliminary studies in this field up until the mid-1960s are reviewed in [4]. The semiconducting polymers were termed ‘coval ent organic polymers’, ‘charge-transfer complexes’, and ‘mixed polymers’. Highest conductivity values reached about 10 À3 S/cm. Systematic work on this field began in the 1960s. Oxidative coupling was systematically extended and became established as the general structural method for Copyright 2005 by Marcel Dekker. All Rights Reserved. synthesizing poly(aromatic)s and poly(heterocycle)s [5] with conductivities of 10 À1 S/cm. At that time, the results aroused great astonishment, because it seemed a paradox that insulators well known as organic compounds should suddenly become conductive. Not only was this the highest value yet obtained for a polymer, but these were the first polymers capable of conducting electricity. The polymers also displayed photovoltaic and thermoelectric properties. After the great surprise and no less incredulity as to how polymeric organic materials can suddenly conduct electricity had subsided, the serious business of elucidating the structure, type of charge, mechanism, etc. was pursued relentlessly and with some success. As early as 1969, it was pointed out that complex formation between electron acceptors and electron donors increase the conductivity by several orders of magnitude [6]. Analogous effects can be achieved by:  Increasing the degree of polymerization  Increasing the pressure  Raising the temperature  Irradiation. A crucial task was the search for defined structures with conjugated p systems, starting from characterized prepolymers, e.g., poly(vinylmethyl ketone) to poly(cyclo- hexenone) or heterobr idged or substituted poly(arylenes), e.g., by condensation of p,p 0 - dialkinylbenzene with reactive intermedi ates (pyrones, coumarins, cyclopentadienones). In the search of easy-to-manufacture, highly-stable compounds with a known number of double bonds, perylene derivatives of the imide-type and imidazole-type were studied for their electrical photo- and dark conductivities. Interesting differences in the conductivity were found to be a function of the substituent and the crystallinity of the samples. The formation of charge-transfer complexes with tetracyanoquinone dimethane (TCNQ), tetracyanoethylene (TCNE) and iodine (I 2 ) increased the conductivity by a factor of 1000, thereby allowing a conductivity similar to that of graphite (10 1 S/cm) to be attained in some cases. Translating the system to polymeric charge-transfer complexes of the type polymer with donor þ acceptor monomer, polymer with donor þ polymer with acceptor, or polymer with acceptor þ donor monomer led to a new class of compounds [6] that have electrical conductivities of up to 10 2 S/cm. The idea of inserting electron acceptors and donor groups alternatively in one molecule was realized in the synthesis of substituted ladder-like poly(quinones) with –S–, –NH groups [7]. A large number of potential applications suggested themselves, i.e., thermostable polymers, coatings, organic electrical contacts, photoelectric devices, photocells as well as pigments with outstanding light-fastness and thermal stability. Other potential applications are resistance thermometers, thermistors, photocon- ductors, photodiodes, photoelements, solar batteries, electrical reproduction of informa- tion, electroluminescence, electrostatic storage batteries, image storage, and catalysis in chemical and biochemical systems [5]. In 1964 Little theoretically evaluated the possibility of superconductivity in polymers and suggested a model, consisting of a polyene chain with cyanine, dyelike substituents [8]. The work on CT complex radical cations by Heeger et al. [9] was another important milestone. Interest heightened and became acute from 1975 when IBM scientists showed that crystalline poly(sulfurnitride), (SN) n , was superconductive [10] and MacDiarmid’s Copyright 2005 by Marcel Dekker. All Rights Reserved. group [11] reported the doping of poly(acetylene) films prepared by Shirakawa [12] reaching conductivity values of 0.4 S/cm (bromine doped) and 38 S/cm (iodine doped), later Heeger reported a conductivity of 3000 S/cm (also iodine doped) [2]. Some kind of breakthrough was reached and led to new consideration because the Shirakawa poly(acetylene) was dopable, but not the polyene called cuprene or niprene. This material was in large quantities available as a film with metallic lustre (deposited on the vessel walks synthesizing cyclooctatetraene). The important point is that science must blaze the trail for technology, and the efforts made and the successes scored in this direction are evident from scientific seminars and publications. This applies to the chemists, in the synthesis of polymers with good mechanical properties and defined structures; to the physicists, in clarifying the relation- ships between charge carriers, mobility, an d polymer structure; and to the engineer, in opening up virgin territory in finding applications for the new materials. II. PRINCIPLES OF ELECTRICAL CONDUCTION The electrical properties of materials are determined by their electronic structure (Figure 1). The band theory accounts for the different behaviours of metals, semi- conductors, and insulators. The band gap is the energy spacing between the highest occupied energy level (valence band) and the lowest unoccupied energy level (conduction band). Metals have a zero band gap which means that they have a high electron mobility, i.e., conductivity. Semiconductors have a narrow band gap (ca. 2.5–1.5 eV), conductivity only occurs on excitation of electrons from the valence band to the conduction band (e.g., by heating). If the band gap is larger (3 eV), electron excitation is difficult; electrons are unable to cross the gap and the material is an insulator. Electrically conducting organic materials such as poly(phenylene), poly(acetylene) or poly(py rrole) are, however, peculiar in that the band theory cannot explain why the charge-carrying species (electrons or holes) are spinless. Conduction by polarons and bipolarons is now thought to be the dominant mechanism of charge transport in organic materials. This concept also explains the drastic deepening of color changes produced by doping. A polaron (a term used in solid-state physics) is a radical cation that is partially Figure 1 Model of band structure. Copyright 2005 by Marcel Dekker. All Rights Reserved. delocalized over severa l monomer units (e.g., in a polymer segment). The bipolaron is a diradical dication. Low doping levels gives rise to polarons, whereas higher doping levels produce bipolarons. Both polarons and bipolarons are mobile and can move along the polymer chain [13–16]. III. DOPING The process that transforms insulating polymers (e.g., poly(acetylene), conductivity 0.1 S/cm) to excellent conductors (Figure 2) is the formation of charge-transfer complexes by electron donors such as sodium or potassium (n doping, reduction) or by elect ron acceptors such as I 2 , AsF 5 , or FeCl 3 (p doping, oxidation). The doped polyme r backbone becomes negatively or positively charged with the dopa nt forming oppositely charged ions (Na þ ,K þ ,I À 3 ,I À 5 , AsF À 6 , FeCl À 4 ). The polymer can be switched between the doped, conductive state and the undoped, insulating state by applying an electric potential that makes the counterions move in and out. This switching corresponds to charging and discharging when these materials are used as electrodes in rechargeable batteries [2,13–15]. The chemistry of doping and the distribution of doping in poly(acetylene) has been treated in detail also by Pekker and Janossy [16]. IV. MEASUREMENT Electrical conductivity is a measure of the flow of current through a material for a given applied voltage. The electrical conductivity s is reciprocal ohms or siemens per centimetre ( À1 cm À1 or S/cm) [16]. V. TYPES OF ELECTRICALLY CONDUCTING ORGANIC MATERIALS Of the plethora of systems containing conjugated double bonds, poly(acetylene)s, poly(heterocycle)s, and poly(aminoaromatic) compounds are undoubtedly the most popular both in regard to their electrical condu ctivity and their stability and ease of preparation. Poly(acetylene), poly(pyrrole), and poly(aniline) are the most intensively studied polymers. Figure 2 Comparison of the electrical conductivity (300 K) of organic and inorganic materials and the effect of doping [2,13–15]. Copyright 2005 by Marcel Dekker. All Rights Reserved. VI. POLY(ACETYLENE) Poly(acetylene) (PAC) exists in various isomeric forms: The cis-cisoid PAC has not yet been prepared in pure form. Model reactions, however, have shown that cyclic and helical structures are flexible [17–19]. Cis-poly(acetylene) is relatively unstable and reverts to the thermodynamically stable trans-poly(acetylene) via the metastable trans-cisoid form. VII. VARIOUS TYPES AND SYNTHESIS OF POLY(ACETYLENE) A historical overview was given in the first edition of this book [20]. But two publications should be mentioned: In 1948 Reppe [21] prepared Cuprene film with a metallic luster. In 1961 Hatano reported the polymerization of acetylene with a AlEt 3 /Ti(OBu) 4 catalyst to give polymers with conductivities up to 0.001 S/cm [22]. Since then intensive work has been carried out on the various polymer types, reviews are given in [13–15,20]. Later in 1974 Japanese scientists published the polymerizat ion of a cetylene [12] on the surface of a high concentrated solution of Ziegler–Natta catalyst, receiving also poly(acetylene) films with a metallic lustre. These small film pieces—inspite of their impurities (O $ 1.0%, Ti þ Al $ 0.5%)—had one remarkable property they were ‘dopable’ reaching values of up to 2500 S/cm. What was the reason for that unusual behaviour in case of other known poly(acetylene)s, e.g., the cuprene film with a lustre like copper or nickel and was produced in large quantities and sizes? This question was the starting point for extensive studies [23]. These showed that poly(acetylene) with lowest degree of crosslinking have the greatest crystallinity and Copyright 2005 by Marcel Dekker. All Rights Reserved. electrical conductivity, and that such high ly crystalline polymers have the lowest capacity for absorbing oxygen. Furthermore, oxygen absorption considerably reduces the crystallinity. These results motivated researchers to make better polymers. VIII. CONSEQUENCES: NEW TYPES AND METHODS The search for easy-to-manufacture, highly stable compounds with a known number of double bonds also focused on perylene derivatives. Further investigation led to the concept of ribbon-like polymers (e.g., by repetitive Diels–Alder addition [24] and ladder-like self- dopant systems [25]). (a) An interesting method is the polymerization of butenyne: (b) The Feast method [26] for producing ‘Durham PAC’ proceeds according to the following scheme: 7,8-Bis(trifluoromethyl)tricyclo(4,2,2,0)-deca-3,7,9-triene polymerizes by undergoing ring opening and yields poly (acetylene) through elimination of 1,2-bis(trifluoromethyl)benzene: (c) In the Grubbs method [27] poly(benzvalene) is isomerized in the presence of HgCl 2 to PAC: Both of these methods start off with certain monomers that are converted to soluble prepolymers that then yield insoluble perconjugated polymers after thermal treatment. Copyright 2005 by Marcel Dekker. All Rights Reserved. (d) Elimination reactions [28,29]: (e) Cyclooctatetraene is also polymerized to give soluble polyenes [30]: A. Modification of Poly(acetylene) 1. Cycloaddition A variety of chemical modifications result from radical addition or cyclo-additions to the (CH) x backbone, e.g., with chlorosulfonyl isocyanate. The ring of the adduct thus formed can be opened by alkalis. The reaction scheme for cyclo-addition of chlorosulfonyl isocyanate and ring opening to substituted hydrophilic poly(acetylene) is as follows: With 3-chloroperbenzoic acid, the dominant reaction is the formation of oxirane structures, which can react further. Metal carbonyls, e.g., Fe 3 (CO) 12 , react only with cisoid units. Otherwise the metal atoms combine with two different units of the poly(acetylene) Copyright 2005 by Marcel Dekker. All Rights Reserved. or isomerization occurs, resulting in cis configu rations. All these types of reactions have been confirmed by IR spectroscopy. CO insertion can also be observed with molybdenum carbonyls. Cyclo-addition of maleic anhydride (MA) and 3,4-dichloromaleic anhydride (DCMA) leads to adducts like that shown below. The adduct formed by DCMA is worth mentioning because it gives rise to fusible poly(acetylene) (165–80  C) [31]. 2. Modification of Polymerization Conditions An important progress (concerning (CH) x properties) occurred by a comparison of the various types of poly(acetyl ene) [23] and revealed some astonishing correlations: conductivity was directly proportional to crystallinity and inversely proportional to the number of sp 3 orbitals. This discovery was the key to the production of new poly(acetylene) types with fewe r defects and greater stability. Another important advance was the modificat ion of the polymerizat ion conditions, e.g., using silicone oil or other viscous media. For instance, (CH) x can be polymerized at room temperature to yield a new (CH) x poly(acetylene) of at least the same quality as the standard (CH) x obtained at À 78  C by Shirakawa and co-workers [22]. Ageing of the standard catalyst brings about another surprising impro vement in the (CH) x properties. The resulting reduction in the number of sp 3 orbitals, i.e., the production of a defect-free system, is of great benefit—you can stretch this (CH) x [17]. Special techniques were applied to orient the (CH) x in order to attain high conductivities (i.e., values up to 100 000 S cm À1 [32] and parallel fibrils. Similarly, it is possible to make transparent (CH) x films with a conductivity of over 5000 S cm À1 . The poly(acetylene) is produced on a plastic film and stretched together with the supporting material. Later it is complexed, e.g., with iodine, under standard conditions. The standard Shirakawa type is crosslinked and contains an sp 3 fraction of approximately 2%. The new BASF technique involves polymerization at room temperature (instead of À 78  C) and the use of a tempered catalyst. The stretched poly(acetylene) product has parallel fibrils. It is linear (no sp 3 fractions), is highly orientable (can be stretched by up to 660%), and has a conductivity exceeding 10 5 S/cm À1 . A convincing demonstration of the high anisotropy (1 : 100) in the stretched polymer are laid across each other, polarized light (sunlight) is extinguished in the region of overlap [33] in a manner similar to the effect of crossed Nicol prisms. Figure 3 shows the equipment for the new BASF technique and process. As seen polymerization doesn’t occur in a shaken or stirred vessel but on an even polymerization desk. This process was also developed as a continuous one. Copyright 2005 by Marcel Dekker. All Rights Reserved. Details are given under [17], also the preparation procedure of various poly- (acetylene) types. IX. CATALYST A crucial point mainly in acetylene polymerization is the catalyst influence of impurities, preparation of the catalyst system, changes in the catalyst according to the preparation temperature, examination by IR or NMR annealing of the catalyst and modifications, including preparation of the catalyst, details under [17]. X. ORIENTATION PROCESSES Orientation processes are powerful methods that are used to improve conductivity and other material properties (e.g., transparency, a nisotropy). Orientation can be achieved in several ways, including stretching. Mechanical stretching can be performed after polymerization, e.g., in noncross-linked polymers. In the case of poly(acetylene)s prepared with aged Ziegler–Natta catalysts [34] stretching increases conductivity from 2500 S/cm to values as high as 10 5 S/cm. Figure 3 Glove-box—pilot plant. Copyright 2005 by Marcel Dekker. All Rights Reserved. Continuous electrochemical polymerization (e.g., of poly(pyrrole)) on the surface of a rotating drum permits sim ultaneous peeling off, mechanical stretching, and orientation s 200 S/cm. Greater stretching rates and therefore greater conductivities are reported in poly(pyrrole perchlor ate) films (s up to 10 3 S/cm) [35]. Biaxially stretched films yielded conductivities of 800 S/cm parallel to the stretching direction and 290 S/cm in the cross direction. Stretched poly(phenyl vinylenes) and poly(thienyl vinylen es) yielded conductivities of ca.10 3 S/cm [36]. Orientation can also occur during polymerization or by performing polymerization in an oriented matrix consisting of liquid crystals and using magnetic fields [37]. Variants are the use of liquid crystal matrices during the electromechanical synthesis of poly- (heterocycle)s [38] and the synthesis of polymers (e.g., substituted thiophenes) with liquid crystal side chains that contain sulfonate groups [39]. The sulfonate groups act as ‘self dopants’ and the liquid crystal side chains are responsible for orientation. Polymerization of extremely thin poly(acetylene) films (<1 mm) on crystal surfaces by epitaxial growth (e.g., on frozen be nzene) [40] also induced orientation in the deposited polymer layer. Substituted poly(pyrrole) films with a high anisotropy can be produced by the Langmuir–Blodgett technique [41–43]. XI. WHAT ABOUT STABILITY? The importance of stability was recognized early and in particular, oxygen absorption and storage stability were investigated. Stability is a relative term, being generally understood to mean the constancy of material properties. In practice it means that the properties of the materials used to make a product should undergo no changes during normal use (including storage), at least for the duration of their life cycle. The life cycle is extremely short for disposable articles, such as those used for personal hygiene or in medicine, but it may be several years for products such as domestic appliances, tools, machines and cars, and even decades for construction materials for bridges, buildings, etc. The stability, particularly the susceptibility to autooxidation, is the Achilles’ heel of the new materials as well as of organic polymers in general. The problem of oxidative damage has therefore been the object of intensive research. Poly(acetylene) (CH) x , manufactured with Ziegler–Natta, Luttinger or other catalysts were used as model compounds. All organic polymers degrade on exp osure to oxygen, particularly in the presence of sunlight, but the extent of degradation varies markedly with the structure of the polymer. Normal (CH) x is particularly susceptible to reaction with oxygen (Figure 4). The stability of (CH) x synthesized with different catalysts increases in the order: Luttinger type LÀ(CH) x < Shirakawa type SÀ(CH) x < new type NÀ(CH) x The BASF type –N–(CH) x , is the optimum material. Due to its special method of synthesis, it has a minimal sp 3 fraction, a high cis content (80% cis isomer synthesized at RT), a high density, v ery thin fibrils, and a high conductivity after doping with iodine. Both N–(CH) x and highly stretched (CH) x has greater stability than the usual systems (such as those of Shirakawa and Luttinger), probably due to the higher density and very low defect rate of the former [17] and less impurities (only ppm amounts of O, Ti, Al and Si). Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... conductivities of ca 0.01 S/cm Poly(indole) has found applications as an organic polymer coating The performance of layered semiconductors has been shown to be improved by the electropolymerization of layers of poly(indole) on the defective sites of the surface Carbon fibers may be coated with poly(indole) by electropolymerization More recently, poly(indole) has been employed for the polymer coating... were found by IR and 1H-NMR analysis of the toluene soluble fractions Studies of the homopolymerization of thiophenol in the presence of sulfuric acid have been carried out by Zuk [96] and Viswanathan [97] The oxidative condensation of thiophenol with thionyl chloride in the presence of Lewis acid was studied by Wejchan-Judek [97] The high-temperature condensation of aromatic halides (diphenylsulfide,... reactions of benzene with sulfur) in the presence of aluminium chloride was examined by Sergeyev et al [98] The reaction of 1,4-dichlorobenzene or 1,4-dibromobenzene with sodium sulfide at 195  C in N-methylpyrrolidone (NMP) has been studied under normal atmospheric pressure, the kinetic of polymerization was found to be of second order The same reaction in NMP was revised by Russian authors [99] The polymerization... involves self-condensation of a halogenated thiophenol resulting in ‘head-to-tail’ polymerization [93]: The electrooxidative polymerization of thiopenol in nitromethane at room temperature was studied by Tsuchida et al [100] The elctropolymerization was carried out in the presence of 1.5 M trifluoracetic acid The presence of sulfonic acid or stannic chloride produced also PPS The white polymer precipitates... the synthesis and is soluble in NMP with m.p 180–190  C and Mw > 1000 The variation of solvent and acid on the polymer yield and molecular weight was examined According to the mechanism of polymerization proposed by the authors, the reaction proceeds at first with the formation of diphenyldisulfide as intermediate, which would be rearranged subsequently to PPS Recently a new process for the synthesis of. .. amount of dichloromethane and subjected to (air)oxidation polymerization at 20  C in the presence of vanadylacetylacetonate produced high yield of PPS The same author succeeded in the electrolytic polymerization of DPS to produce PPS by using tetra-n-butylammoniumtetrafluorborate in dichloromethane with addition to trifluoraceticacid and trifluoraceticanhydride XXVII DOPING OF PPS [90] Doping of PPS... can be accounted for in terms of electrons hopping between localized states under the assistance of proton transfer, for which the presence of water plays an essential role [107] (Figure 12) Poly(aniline) is primarily of interest because it can be used as electrode material It is the preferred choice of all conducting polymers Its discharge capacity is greater than that of poly(pyrrole)(þ)/perchlorate(À)... poly(pyrene) and polyene fulvene are just a few of the large number of electrically conducting polymers with specific properties and interest [20] XIV SUBSTITUTED POLY(ACETYLENE)S [20,45] The synthesis of polymers from substituted acetylene monomers is directed toward the preparation of substituted, conjugated chains which ameliorate the negative properties of poly(acetylene)s (e.g., sensitivity to air,... presence of the respective polymers (Lit cit under [20]) Poly(hexylthiophene) doped with electrons shows superconductivity [70] The synthetic methods used to polymerize the 3-alkyl thiophene do not differ substantially from these employed for thiophene The good choice of the solvent is important to ensure a complete dissolution of the monomer and the electrolyte in the electrosynthesis case Chemically polymerization... to handling of chemicals and safety considerations Several other synthetic routes have been attempted to prepare PPS XXVI SYNTHESIS OF PPS The Macallum synthesis of PPS was studied in detail by Lenz and co-workers [93] who developed an improved synthetic route based on the self-condensation of metal-p-halogenothiophenoxide This method although used less drastic conditions than Macallum’s synthesis required . monomer, polymer with donor þ polymer with acceptor, or polymer with acceptor þ donor monomer led to a new class of compounds [6] that have electrical conductivities of up to 10 2 S/cm. The idea of. fulvene are just a few of the large number of electrically conducting polymers with specific properties and interest [20]. XIV. SUBSTITUTED POLY(ACETYLENE)S [20,45] The synthesis of polymers from substituted. University of Pennsylvania, USA, and  Professor Hideki Shirakawa at the University of Tsukuba, Japan, They are rewarded ‘for the discovery and development of electrically conductive polymers’