Handbook of Polymer Synthesis Second Edition Episode 12 pptx

This review concerns the synthesis routes, polymerization techniques, doping,orientation, and development of well-defined, highly conducting polymeric materials.Electrically conducting ma

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 tolearn that the benefits of these polymers lie less in providing substitutes for conventionalmetals than in opening up new fields of application

The Royal Swedish Academy of Sciences decided to award the Nobel Prize inChemistry 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 conductivepolymers’ [2]

Electrically conducting polymers are materials with an extended system of C¼Cconjugated bonds They are obtained by reduction or oxidation reactions (calleddoping), giving materials with electrical conductivities up to 105S/cm These materialsdiffer from polymers filled with carbon black or metals because the latter are onlyconductive if the individual conductive particles are mutually in contact and form acoherent 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 potentialapplications 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 decadesinterest grew in organic polymers as insulators, but not as electrical conductors

In the late 1950s organic semiconductors became the focus of investigations.Preliminary studies in this field up until the mid-1960s are reviewed in [4] Thesemiconducting polymers were termed ‘covalent organic polymers’, ‘charge-transfercomplexes’, and ‘mixed polymers’ Highest conductivity values reached about 103S/cm.Systematic work on this field began in the 1960s Oxidative coupling wassystematically extended and became established as the general structural method for

synthesizing poly(aromatic)s and poly(heterocycle)s [5] with conductivities of 10 S/cm.

At that time, the results aroused great astonishment, because it seemed a paradox thatinsulators well known as organic compounds should suddenly become conductive.Not only was this the highest value yet obtained for a polymer, but these were thefirst polymers capable of conducting electricity

The polymers also displayed photovoltaic and thermoelectric properties After thegreat surprise and no less incredulity as to how polymeric organic materials can suddenlyconduct electricity had subsided, the serious business of elucidating the structure, type ofcharge, mechanism, etc was pursued relentlessly and with some success

As early as 1969, it was pointed out that complex formation between electronacceptors 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(vinylmethylketone) to poly(cyclo-hexenone) or heterobridged or substituted poly(arylenes), e.g., by condensation of p,p0-dialkinylbenzene with reactive intermediates (pyrones, coumarins, cyclopentadienones)

In the search of easy-to-manufacture, highly-stable compounds with a knownnumber of double bonds, perylene derivatives of the imide-type and imidazole-type werestudied for their electrical photo- and dark conductivities

Interesting differences in the conductivity were found to be a function of thesubstituent and the crystallinity of the samples The formation of charge-transfercomplexes with tetracyanoquinone dimethane (TCNQ), tetracyanoethylene (TCNE) andiodine (I2) increased the conductivity by a factor of 1000, thereby allowing a conductivitysimilar to that of graphite (101S/cm) to be attained in some cases

Translating the system to polymeric charge-transfer complexes of the type polymerwith donor þ acceptor monomer, polymer with donor þ polymer with acceptor, orpolymer with acceptor þ donor monomer led to a new class of compounds [6] that haveelectrical conductivities of up to 102S/cm

The idea of inserting electron acceptors and donor groups alternatively inone 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., thermostablepolymers, coatings, organic electrical contacts, photoelectric devices, photocells as well aspigments with outstanding light-fastness and thermal stability

Other potential applications are resistance thermometers, thermistors, ductors, photodiodes, photoelements, solar batteries, electrical reproduction of informa-tion, electroluminescence, electrostatic storage batteries, image storage, and catalysis inchemical and biochemical systems [5]

photocon-In 1964 Little theoretically evaluated the possibility of superconductivity in polymersand 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 importantmilestone

Interest heightened and became acute from 1975 when IBM scientists showedthat crystalline poly(sulfurnitride), (SN)n, was superconductive [10] and MacDiarmid’s

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 theShirakawa 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 onthe vessel walks synthesizing cyclooctatetraene)

The important point is that science must blaze the trail for technology, and theefforts made and the successes scored in this direction are evident from scientific seminarsand publications This applies to the chemists, in the synthesis of polymers with goodmechanical properties and defined structures; to the physicists, in clarifying the relation-ships between charge carriers, mobility, and polymer structure; and to the engineer, inopening up virgin territory in finding applications for the new materials

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 azero 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 onexcitation of electrons from the valence band to the conduction band (e.g., by heating) Ifthe band gap is larger (3 eV), electron excitation is difficult; electrons are unable to crossthe gap and the material is an insulator

Electrically conducting organic materials such as poly(phenylene), poly(acetylene)

or poly(pyrrole) are, however, peculiar in that the band theory cannot explain why thecharge-carrying species (electrons or holes) are spinless Conduction by polarons andbipolarons is now thought to be the dominant mechanism of charge transport in organicmaterials This concept also explains the drastic deepening of color changes produced bydoping A polaron (a term used in solid-state physics) is a radical cation that is partially

Figure 1 Model of band structure

delocalized over several monomer units (e.g., in a polymer segment) The bipolaron is adiradical dication Low doping levels gives rise to polarons, whereas higher doping levelsproduce bipolarons Both polarons and bipolarons are mobile and can move along thepolymer chain [13–16].

, Kþ, I

3, I

5, AsF6, FeCl4) The polymer can be switched between the doped,conductive state and the undoped, insulating state by applying an electric potential thatmakes the counterions move in and out This switching corresponds to charging anddischarging 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 beentreated in detail also by Pekker and Janossy [16]

Of the plethora of systems containing conjugated double bonds, poly(acetylene)s,poly(heterocycle)s, and poly(aminoaromatic) compounds are undoubtedly the mostpopular both in regard to their electrical conductivity and their stability and ease ofpreparation Poly(acetylene), poly(pyrrole), and poly(aniline) are the most intensivelystudied polymers

Figure 2 Comparison of the electrical conductivity (300 K) of organic and inorganic materials andthe effect of doping [2,13–15]

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 stabletrans-poly(acetylene) via the metastable trans-cisoid form

A historical overview was given in the first edition of this book [20] But two publicationsshould be mentioned:

In 1948 Reppe [21] prepared Cuprene film with a metallic luster In 1961 Hatanoreported the polymerization of acetylene with a AlEt3/Ti(OBu)4catalyst to give polymerswith 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 polymerization of acetylene [12] onthe surface of a high concentrated solution of Ziegler–Natta catalyst, receiving alsopoly(acetylene) films with a metallic lustre These small film pieces—inspite of theirimpurities (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 incase of other known poly(acetylene)s, e.g., the cuprene film with a lustre like copper ornickel and was produced in large quantities and sizes?

This question was the starting point for extensive studies [23] These showed thatpoly(acetylene) with lowest degree of crosslinking have the greatest crystallinity and

electrical conductivity, and that such highly crystalline polymers have the lowest capacityfor absorbing oxygen Furthermore, oxygen absorption considerably reduces thecrystallinity These results motivated researchers to make better polymers.

The search for easy-to-manufacture, highly stable compounds with a known number ofdouble 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 dopant systems [25])

self-(a) An interesting method is the polymerization of butenyne:

(b) The Feast method [26] for producing ‘Durham PAC’ proceeds according tothe following scheme: 7,8-Bis(trifluoromethyl)tricyclo(4,2,2,0)-deca-3,7,9-trienepolymerizes by undergoing ring opening and yields poly (acetylene) throughelimination of 1,2-bis(trifluoromethyl)benzene:

(c) In the Grubbs method [27] poly(benzvalene) is isomerized in the presence ofHgCl2to PAC:

Both of these methods start off with certain monomers that are converted tosoluble prepolymers that then yield insoluble perconjugated polymers afterthermal treatment

(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 adductthus formed can be opened by alkalis The reaction scheme for cyclo-addition ofchlorosulfonyl isocyanate and ring opening to substituted hydrophilic poly(acetylene) is asfollows:

With 3-chloroperbenzoic acid, the dominant reaction is the formation of oxiranestructures, which can react further Metal carbonyls, e.g., Fe3(CO)12, react only with cisoidunits Otherwise the metal atoms combine with two different units of the poly(acetylene)

or isomerization occurs, resulting in cis configurations All these types of reactions havebeen confirmed by IR spectroscopy CO insertion can also be observed with molybdenumcarbonyls 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 worthmentioning 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 thevarious types of poly(acetylene) [23] and revealed some astonishing correlations:conductivity was directly proportional to crystallinity and inversely proportional to thenumber of sp3 orbitals This discovery was the key to the production of newpoly(acetylene) types with fewer defects and greater stability Another important advancewas the modification of the polymerization conditions, e.g., using silicone oil or otherviscous media For instance, (CH)xcan 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

78
C by Shirakawa and co-workers [22] Ageing of the standard catalyst brings aboutanother surprising improvement in the (CH)x properties The resulting reduction in thenumber of sp3orbitals, i.e., the production of a defect-free system, is of great benefit—youcan stretch this (CH)x [17]

Special techniques were applied to orient the (CH)x in order to attain highconductivities (i.e., values up to 100 000 S cm1 [32] and parallel fibrils Similarly, it ispossible to make transparent (CH)x films with a conductivity of over 5000 S cm1 Thepoly(acetylene) is produced on a plastic film and stretched together with the supportingmaterial Later it is complexed, e.g., with iodine, under standard conditions

The standard Shirakawa type is crosslinked and contains an sp3 fraction ofapproximately 2%

The new BASF technique involves polymerization at room temperature (instead of

78
C) and the use of a tempered catalyst The stretched poly(acetylene) product hasparallel fibrils It is linear (no sp3fractions), is highly orientable (can be stretched by up to660%), and has a conductivity exceeding 105S/cm1 A convincing demonstration of thehigh 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 ofcrossed Nicol prisms

Figure 3shows the equipment for the new BASF technique and process As seenpolymerization doesn’t occur in a shaken or stirred vessel but on an even polymerizationdesk This process was also developed as a continuous one

Details are given under [17], also the preparation procedure of various (acetylene) types.

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 preparationtemperature, examination by IR or NMR annealing of the catalyst and modifications,including preparation of the catalyst, details under [17]

Orientation processes are powerful methods that are used to improve conductivity andother material properties (e.g., transparency, anisotropy) Orientation can be achieved inseveral ways, including stretching

Mechanical stretching can be performed after polymerization, e.g., in noncross-linkedpolymers 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 105S/cm

Figure 3 Glove-box—pilot plant

Continuous electrochemical polymerization (e.g., of poly(pyrrole)) on the surface of

a rotating drum permits simultaneous peeling off, mechanical stretching, and orientation

s  200 S/cm Greater stretching rates and therefore greater conductivities are reported inpoly(pyrrole perchlorate) films (s up to 103S/cm) [35] Biaxially stretched films yieldedconductivities of 800 S/cm parallel to the stretching direction and 290 S/cm in the crossdirection

Stretched poly(phenyl vinylenes) and poly(thienyl vinylenes) yielded conductivities

of ca 103S/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] Variantsare 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 liquidcrystal side chains that contain sulfonate groups [39] The sulfonate groups act as ‘selfdopants’ and the liquid crystal side chains are responsible for orientation

Polymerization of extremely thin poly(acetylene) films (<1 mm) on crystal surfaces byepitaxial growth (e.g., on frozen benzene) [40] also induced orientation in the depositedpolymer layer Substituted poly(pyrrole) films with a high anisotropy can be produced bythe Langmuir–Blodgett technique [41–43]

The importance of stability was recognized early and in particular, oxygen absorption andstorage 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 ofthe 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 extremelyshort for disposable articles, such as those used for personal hygiene or in medicine, but itmay 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 ofthe new materials as well as of organic polymers in general The problem of oxidativedamage has therefore been the object of intensive research Poly(acetylene) (CH)x,manufactured with Ziegler–Natta, Luttinger or other catalysts were used as modelcompounds

All organic polymers degrade on exposure to oxygen, particularly in the presence ofsunlight, 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)xsynthesized with different catalysts increases in the order:Luttinger type L(CH)x< Shirakawa type S(CH)x< new type N(CH)x

The BASF type –N–(CH)x, is the optimum material Due to its special method ofsynthesis, it has a minimal sp3fraction, a high cis content (80% cis isomer synthesized atRT), a high density, very thin fibrils, and a high conductivity after doping with iodine BothN–(CH)xand highly stretched (CH)xhas greater stability than the usual systems (such asthose of Shirakawa and Luttinger), probably due to the higher density and very low defectrate of the former [17] and less impurities (only ppm amounts of O, Ti, Al and Si)

The mechanism of polymer degradation usually entails the absorption of energy(thermal or UV), leading to the formation of active free radicals that partake in chainscission and crosslinking Pristine S–(CH)x appears to undergo some spontaneous orthermal degradation at all practical temperatures Even at  78
C, cis–trans isomerizationoccurs.

The cis content (%) of N–(CH)xfilm versus storage time in months at 22
C undernitrogen decreases very slowly compared with that of S–(CH)x to reach 78% after 3months This higher stability of N–(CH)x at 22
C is also confirmed by thermalisomerization studies, which show a higher energy barrier (25 kcal mol1) for N–(CH)xcompared with 17 kcal mol1for typical S–(CH)x Also, we observed that N–(CH)x filmthat has been stored for 3 months under nitrogen gives, after doping with iodine,conductivities as high as 2000 S/cm, whereas 1 month-old S–(CH)xonly 150 S/cm1 Theinfrared spectrum of the 3 month-old N–(CH)xsample is similar to that of the initial one[17]

More recently, Wegner et al [44] studied the properties of oxidized poly(acetylene)

by EFTEM (energy-filtering transmission electron microscopy) They demonstrated thatoxidation proceeds homogeneously and the anion distribution is uniform without any sign

of nucleation

Since doped poly(acetylene)s were shown to possess metallic conductivity, this class oforganic polymers has been studied intensively Regardless of the catalyst system used, it is

an inherent disadvantage of the washing process which is required to remove the catalyst

or residues from the desired polymer preparation Therefore a method was developed thatavoids this pitfall and provides polyene films that form a solid layer on glass surfaces,ceramic plates, tubes, etc

Figure 4 Autooxidation of poly(acetylene) [20]

Allene (propadiene) is condensed under high vacuum into a 2-litre round bottomedflask and, after several freeze-and-pump cycles to remove residual oxygen and volatileimpurities, the reaction vessel is sealed (internal pressure at room temperature approx-imately 1000 mbar).

After 20 hours at 640
C (a high-temperature drying oven) the inner surface of theflask is covered completely by a deep-black, shiny film which can be removed in patchesand whose electrical conductivity is 108S/cm1 When this film is doped by treating it for

30 min with a saturated solution of iodine in carbon tetrachloride its appearance changes

to a shiny gold After solvent removal, the conductivity is found to have increased by afactor of 109 Several other alkynes were polymerized under above conditions and theresults of these experiments are summarized in Table 1

Poly(pyrrole) and poly(thiophene), both first described in 1963 as electrically conductingmaterials [5], experienced a renaissance when Diaz and Street gave a new attention to theelectrochemical oxidation of pyrrole and Garnier to the poly(thiophene) field transistor.Poly(phenylenevinylene), poly(aniline), poly(phenylenesulfide), poly(carbazole), poly(in-dole), poly(pyrene) and polyene fulvene are just a few of the large number of electricallyconducting polymers with specific properties and interest [20]

The synthesis of polymers from substituted acetylene monomers is directed toward thepreparation of substituted, conjugated chains which ameliorate the negative properties ofpoly(acetylene)s (e.g., sensitivity to air, insolubility, and infusibility) while maintaining thedesired electrical properties of acetylene’s conjugated backbone alkyl- and aryl substitutedpolymers result They are soluble (e.g., in toluene and cyclohexane) and proccessible, buthave low conductivities (<0.1 S/cm) compared with the unsubstituted poly(acetylene)

A new development are polymers from phenylacetylene substituted Schiff ’s basemonomers with conductivities of 102S/cm but high environmental stability up to 300
Cand above [43,46]

In contrast to poly(acetylene), poly(diacetylene)s have limited electrical conductivity(0.1 S/cm), but can be obtained as large, single crystals

Table 1 High-temperature polymerization of alkynes and diynes [17]

Monomer Electrical conductivity of polymerfilm [S/cm]

The polymerization of diacetylene is an example of a topochemical polymerization inwhich 1,4-addition of 1,3-diyne units takes place in the crystalline state The reaction doesnot require a catalysts and is performed by irradiation of the diacetylene crystals withvisible or UV light, X-rays, g-rays, or by annealing the crystals below their melting point.The unreacted monomer is then extracted with a suitable solvent (e.g., hexane, toluene),leaving a single, dark red crystal of poly(diacetylene) This unique polymerization processwas already observed in 1882 [47] and has now been intensively studied [48] A correctinterpretation of the phenomena was first provided in the early 1970s [13–15,49].More details concerning mono or multilayer applications and the synthesis ofdiacetylenes are summarized in [20].

Poly(pyrrole) is a polysalt that can be produced in the form of powders, coatings, or films;

it is intrinsically conductive, and exceptionally stable and can be quite easily producedalso continuously, e.g., by electrochemical techniques (Figure 5)

The preparation of poly(pyrrole) (pyrrole red and pyrrole black) by oxidation ofpyrrole dates back to 1888 [50] and by electrochemical polymerization to 1957 [51] Afairly long period elapsed before this organic p-system attracted general interest and wasfound to be electrically conductive [5] in 1963

Conductive poly(pyrrole) films are obtained directly by anodic polymerization ofpyrrole in aqueous or organic electrolytes [52] They are black and under suitable reactionconditions, can be detached from the anode in the form of self-supporting films Theconducting salt use in the electrolyte solution is incorporated in the film as a counterion

In contrast to poly(acetylene), poly(pyrrole) has a high mechanical and chemicalstability and can be produced continuously as flexible film (thickness 80 mm; trade name:Lutamer, BASF) by electrochemical techniques [53]

Other electrochemical polymerizable heterocycles are thiophene, furan and theirsubstituted and oligomeric derivatives

The polymerization starts initial by an oxidation step, followed by a radical cationformation, coupling reaction, deprotonation, and a one-electron oxidation in order toregenerate the aromatic system [54] scheme [20]

Also important is the reaction introducing oxygen—forming a labile –CO–NH–group in the pyrrole ring system

Figure 5 Electrochemical poly(pyrrole) synthesis Left discontinuous process [52] center and rightBASF methods, center-continuous by rotating-drum electrode (US Patent 4468 291, June 27, 1993)and right-rotating band electrode

The quality of the polymers is greatly influenced by many factors, e.g., impurities,electrode material, pressure, concentrations, temperature and comonomers The mostdecisive, however, are the current density and the electrolyte, particularly the condutinganion Xbecause it is incorporated into the polymer as a counterion.

The properties of the counterion (e.g., its size, geometry, charge) influence theproperties of the polymer

In general, one anion is incorporated for every three pyrrole units Exceptions arepyrrole- or thiophenesulfonic acids where the counterion is coupled directly to themonomer (self doping) [55] Some typical conducting anions are fluoroborate, perchlorate,aromatic sulfonic acids, penicillin, n-dodecyl sulfate phthalocyanine sulfonic acid,poly(styrenesulfonicacid), styrene sulfonic acid, and heparin [13]

Interesting conductive salts that affect optical activity in poly(pyrrole) are (þ) or camphor sulfonic acid (used for racemate separations) [20]

Poly(pyrrole) functionalized with peptides are reported in Refs [57–59]

By changing reaction conditions, polymers with different surface morphologies (e.g.,

an open porous structure) can be obtained (Figure 6)

Figure 6 Poly(pyrrole) with defined holes and counterion [60]

Preparation procedures of films and powders are given under [20], e.g chemical production of poly(pyrrole)films—discontinuously or continuously:

electro- Preparation of Finely Divided Poly(pyrrole)

Optimization of Synthesis Conditions of Poly(pyrrole) from Aqueous Solution Coating Surfaces with Poly(pyrrole) (e.g Poly(ester), Poly(amide))

Chemical Modification of Surfaces (e.g ceramics, glass)

Properties and potential uses are shown in Table 2 and Figure 7

The conductivity of poly(pyrrole) film suggests applications such as flexibleconductive paths in printed circuits, heating films, and film keyboards

Poly(pyrrole) films show good electromagnetic shielding effects of about 40 dB over awide range of frequencies (0–1500 MHz) [20]

Although the new systems appear to be promising, stability problems may beencountered

Poly(pyrrole) is sensitive to moisture because this leads to leaching of the counterionand thus to a decrease in conductivity This can be avoided by use of appropriatehydrophobic or polymeric counterions (e.g., camphor sulfonic acid or poly(styrenesulfo-nicacid) or by incorporating hydrophilic compounds

Poly(pyrrole) in which the counterion is 4-hydroxyphenyl sulfonic acid do notundergo any change in conductivity if they are exposed to nitrogen for two month at

140
C No change in conductivity was observed when these polymers were stored in thelaboratory for three years at room temperature and 55% R.H

Poly(pyrrole)s obtained by synthesis in aqueous electrolytes maintain theirconductivity at a level of about 20 S/cm Poly(pyrrole)s with perchlorate as a counterionare unstable under atmospheric conditions but can be used as electrodes in rechargeablebatteries [13]

Poly(pyrrole) is a suitable electrode material for rechargeable electrochemical cells.Storage of poly(pyrrole) films (80 mm thickness) over 19 years (1982–2001) in aplastic wrapper showed a loss of conductivity <5% and no change in flexibility [14] Theinfluence of oxygen on the properties is described in [15]

The advantage of polymer electrodes is that they can be easily shaped, allowing novelbattery design Polymer cells with poly(pyrrole) and lithium electrodes have beendeveloped [13] In the flat cell, the poly(pyrrole) and lithium films are sandwiched together,

in the cylindrical cell, the two films are wound concentrically Their energy per unit mass

Table 2 Properties of conductive polymers

and their discharge characteristics are similar to those of the nickel-cadmium cells now

on the market More than 500 charging and recharging cycles have been achieved withlaboratory cells Applications include dictaphones and pocket radios [13]

Under oxidation conditions (electrochem or chem.), starting from substitutedbenzenes or derivatives, triphenylene structures (benzo[1,2 : 3,4 : 5,6]tris[arylenes]) are

Figure 7 Some typical application for conductive polymers (e.g poly(pyrrole)s)

1 The resulting poly(pyrrole) showed no C–H signals after pulsing (1H-NMR).

2 When tetradeutero (2,3,4,5)pyrrole was used as the starting monomer, theresulting poly(pyrrole) showed no detectable amounts of D

3 Poly(pyrrole) degradation (pyrolysis at 600
C or anodic over-oxidation) gavebenzene, indole, carbazole, etc, fragments

4 When a poly(pyrrole) film was treated with an aq CuSO4solution, Cu2þ wassequestered (verified by spectroscopy) [63,64]

All these phenomena are in accordance with the proposed macrocyclic structure andnot with a linear one

This condensation seems to be a general approach to the synthesis of new types ofdisc-like tridentate polymeric structures The pyrrole units can be imagined to conform

to a three-dimensional fullerene type structure in which one carbon from each of thepentagonal units is replaced by a nitrogen (Figures 8and9)

It is worth mentioning that indophenins, including both oligomers and polymers,represent a new type of electrically conducting materials

Since the first report of thiophene polymerization in 1883 [65] decades passed until 1963thiophene lead to an electrically conducting material [5] and was considered to be anattractive monomer for conducting polymers [66] Interest is still growing becausethiophene is easier to handle than pyrrole (less sensitive to oxygen) and allows thesimple preparation of substituted monomers leading to soluble polymers Three differentmethods have been used to produce poly(thiophene): chemical oxidation coupling withorganometallic agents or by the Grignard reaction and electrochemical oxidation Thepreparation techniques for films, powders, and coatings are similar to those of pyrroles

but thiophene needs stronger oxidants for its polymerization than pyrrole or otherheterocycles Standard oxidation potentials follow:

The oxidation potential correlates to the ease of polymerization [66]

Figure 10 illustrates the correlation between UV–VIS absorption and the tion potential of thiopene oligomers [67] The oxidation potential of the series(thiophene)n¼n ¼1–6 decreases with the increasing number of thiophene units

Almost parallel to the polymerization of thiophene, much work has also beendone with the electropolymerization of 3-methylthiophene The structure, morphology,electronic, and electrical properties of poly(3-methylthiophene) have been studied byseveral authors [20,68].

The 3-alkylsubstituted thiophenes with alkylchain size larger or equal than butylwere found to be soluble in common organic solvent and therefore an important amount

of work is being developed since their discovery [69]

Polysubstituted thiophenes with other groups different to n-alkyl chain are alsoknown For example, alkylsulfonate, alkoxy, amide, poly(ether), and acylgroup wereintroduced in 3-position in thiophene and their electroobtained polymers were studied

A water soluble poly(alkane)sulfonatederivative of thiophene has also been reported[55] which would be an intrinsically conducting polymer (self-doped) Composites ofpoly(thiophene)s with poly(methylmethacrylate) and poly(vinylchloride) were prepared as

Figure 9 Model of poly(pyrrole) macrocycle with counterion [17]

Figure 10 Correlation between oxidation potential (a) and maximum absorption wavelength (b) ofthiophene oligomers [67]; Oxidation potential measured in 0.5 mol/L Et4NBF4in acetonitrile versussaturated calomel electrode

transparent films by electropolymerization of thiophene in the presence of the respectivepolymers (Lit cit under [20]) Poly(hexylthiophene) doped with electrons showssuperconductivity [70].

The synthetic methods used to polymerize the 3-alkyl thiophene do not differsubstantially from these employed for thiophene The good choice of the solvent isimportant to ensure a complete dissolution of the monomer and the electrolyte in theelectrosynthesis case Chemically polymerization is based on the Grignard couplingmethod used by Yamamoto et al [71] and later revised by Kobayashi et al [72] Somepolymerizations carried out with chemical oxidants are also known with poly(3-alkyl-thiophene) [73]

The electropolymerization of alkylthiophene is carried out mostly in a compartment cell, using platinum as working electrodes and nitrobenzene as solvent.Lower temperatures, preferentially between 5–15
C and current densities ranging 1–2 mA/

one-cm2 are applied in galvanostatic conditions [74] Dihydroisothiophene and aromaticbridged thiophene see [75]

A new type of a substituted thiophene represents bis(thienyl) coronene, leading to analternating polymer with bis-thiophene and coronene units [76] Synthesis and properties

of processable poly(thiophene) are described in [77]

The oligothiophenes have reached more prominence in recent years The progress of thesematerials is summarized by Ba¨uerle [78] Dithienylpolyenes, thienylene polyenyleneoligomers and polymers are described by Spangler and He [77]

A number of different characterization methods have been performed on poly(thiophene)and poly(alkylderivative)s NMR of electropolymerized poly(thiophene) films has beenstudied by Hotta et al and Osterholm et al An infrared study about vibrational key band

on poly(thiophene) films and FT-IR spectra were also published Resonance Ramanscattering on poly(methylthiophene) and x-ray scattering on poly(thiophene) wereperformed; x-ray photoelectron spectroscopy has been reported on FeCl3-dopedpoly(hexylthiophene) and electrochemically obtained poly(thiophene) [20] Time resolvedfluorescence studies on thiophene oligomers are given by Chosrovian et al [79] Forstudies on oligothiophene films see [80,81]

Xanes (x-ray absorption near edge structure) investigations showed that polymerchain variations decrease with increasing size of alkyl-side chains on the thiophenebackbone [82]

Poly(p-phenylenevinylene) [83] was first synthesized by the Wittig condensation ofterephthaldehyde and p-xylenebis(triphenylphosphonium)chloride The method which is

now mainly employed starts from a soluble poly(sulfonium salt) [28] for example:

The poly(sulfonium salt) is obtained by the reaction if a,a-dichloro-p-xylene withexcess dimethyl sulfide (50
C, 20 h) and polymerization with sodium hydroxide (0
C, 1 h).Instead of the dimethylsulfonium salt often tetrahydrothiophene or tetrahydro-pyrane salts are used

The starting precursor poly(sulfonium salt)s are prepared by polymerizing themonomer sulfonium salt with an equimolar amount of NaOH in aqueous solution at 0
Cfor 1 h Further details and similar preparation techniques are cited in [20]

A complete different route to PPVs [84] starts from tetrahalogenated derivatives and by an dehalogenation reaction yielding aryl substituted PPVs

xylylen-But comparing both PPVs synthesized by the sulfonium route with those prepared

by the dehalogenation route, you’ll find remarkable differences; PPVs prepared by thesulfoniumroute:

contain impurities (S, O, Cl up to 1% and more)

show electrically conductivity (104S/cm)

show electroluminescence

PPVs prepared by the dehalogenation route:

are very pure (impurities <10 ppm)

are not electrically conducting (<1012S/cm)

show no electroluminescence [85]

The first doped poly(phenylenevinylene) was reported by Karasz using AsF5 asdopant; its conductivity was 3 S/cm Stretching the precursor film at high temperature anddoping with AsF5or SO3yielded conductivities up to 2800 S/cm

Substituted derivatives, copolymers, and blends show better thermal stability Highlyconductive graphite films have been prepared by pyrolysis of poly(phenylenevinylene)(>3000
C), stretched samples doped with SO3 had conductivities of 105S/cm [20].Poly(phenylenevinylene) can be used as tunable polymer diodes or luminescentelectrodes [83]