Pyrrole/thiophene copolymers

Because their oxidation potentials are similar, substituted pyrroles can be copolymerized with pyrrole, allowing the limiting conductivity of the fully-doped polymer to be varied 194,19S). The oxidation potentials of the monomers, and hence their reactivity ratios, are sensitive to the substituent196). Inganas et al.197) reported the synthesis of pyrrole-thiophene copolymers starting from terthiophene, whose oxidation potential is similar to that of pyrrole. Sundaresan et al.198) copolymerized pyrrole with 3-(pyrrol-l-yl)propanesulphonate to give a polymer in which the sulphonate counter-ion is a part of the polymer structure. 

Because of the problems associated with copolymerizations of monomers of very different reactivity, many authors have looked at an alternative approach which is to synthesise appropriate sections of the desired polymer chain, then couple them electrochemically to get the final polymer. Naitoh et al.199) synthesised the dimer, 2,2 -thienylpyrrole and used this as a monomer to prepare the alternating pyrrole-thiophene copolymer. They claimed that the copolymer film obtained with HSO as the counter-ion is more conductive than either of the corresponding homopolymers by a factor of 10 to 20. McLeod et al. 200) synthesised 2,5-dithienylpyrrole and polymerized it electrochemically with silver p-toluenesulphonate as the electrolyte. They obtained films of polymer whose conductivity could be varied in the range 10 8 to 0.1 Scm-1. Surprisingly, some low conductivity films were soluble in acetone or acetonitrile and evaporation of the solvent gave a powder of similar conductivity. Based on the shift in the absorption maximum in the visible spectrum on polymerization, it was concluded that the soluble films were polymers with molecular weights of 4000. 

A third route to a pyrrole/thiophene copolymer involves the electrochemical polymerization of 2,5-di-(2-thienyl)pyrrole (Fig. 10b) as studied by McLeod et al. [206]. The monomer was chemically synthesized [207] from a 0.2 M acetonitrile solution containing 0.07 M p-toluene sulfonate. Black films with a range of conductivities up to 0.1 S cm" were synthesized by applying -0.5 to -0.2 V (versus SCE). Films with conductivities around 10" S cm" were soluble in both acetonitrile and acetone. Once the solvent evaporated, the polymer could also be recovered as a black powder with conductivities similar to that of the original films. 

A low band-gap (Eg 1.6 eV) conjugated thiophene copolymer 468 with pyrrole and BT units was synthesized by Stille coupling [568]. They showed emission in the NIR region (AEL 800 nm) with turn-on voltage below 4 V but with very low efficiency (Chart 2.113). 

Fig. 10.24 Structures of polymers absorbing beyond 600 nm (a) poly(dithieno(3,4-b 2, 3 -d)thiophene), (b) tbiophene/isothianaphthalene copolymers, and (c) a thienyl-pyrrole/benzothiazole copolymer.

Random copolymers have been prepared by electropolymerization of a mixture of terthienyl derivates and pyrrole [348]. Otherwise, a thiophene-pyrrole block copolymer showing electrical and electrochemical properties intermediate between those of polypyrrole... 

Strictly, this is an alternating copolymer of bithiophene and pyrrole, rather than a random copolymer of thiophene and pyrrole. The copolymer, deposited on either transparent electrodes or platinum, was characterized by in-situ visible/near IR spectroscopy as a function of oxidation level and by FTIR, four-point probe conductivity, and CV... 

The conductivity of the (70/30) pyrrole/ben-zo[b]thiophene copolymer decreases slowly during the first 6 days and less so thereafter. 

Polypyrrole copolymers with perfectly alternating units of pyrrole with another single heterocycle are not known. Alternating copolymers of pyrrole with bithiophene are prepared by electrochemical polymerization of the three-membered oligomer, 2,5-di-2-(thienyl)-pyrrole [11,12]. Even the homopolymerization of 2-thienylpyrrole does not really lead to alternating units of pyrrole and thiophene because this monomer must dimerize after oxidation. What results is a copolymer of bipyrrole and bithiophene because of the different values for the oxidation potentials of pyrrole, thiophene, bithiophene, etc. 

Copolymerization of thiophenes and pyrroles offers the promise of combining the higher stability associated with thiophene-based polymers with the higher conductivity characteristic of polypyrroles. Three different methods have been used to prepare such copolymers. These are (1) direct copolymerization of pyrrole and thiophene monomer mixtures, (2) copolymerization of pyrrole with bi- or a-terthiophene, and (3) polymerization ofa-linked pyrrole-thiophene monomers. The last case includes both two- and three-ring systems. 

In another study, pyrrole was chemically polymerized with a 3-substituted thiophene [3-(methoxyethoxy-ethoxymethyl)thiophene], XLVIll [76]. This copolymer had a pyrrole/thiophene ratio of 7 1 and a conductivity of 1 S/cm. Pyrrole acted as a initiator of the polymerization and produced a material that would be less expensive while still maintaining the solubility properties of the substituted thiophene. 

Hanack et al. [145-148] have developed a series of polyarenemethylidenes that are copolymers containing mixtures of pyrrole, thiophene, thieno[c]thiophene, and isothianaphthene rings linked by a methine carbon (LXXXII). These polymers are characterized by having low Eaup values (—1.0 eV). 

One of the unique features of the alkene/CO copolymer is the existence of multiple carbonyl groups in the main chain. Thus, versatile chemical transformations of the carbonyl groups were examined to provide new polymers (Scheme 16). The 1,4-diketone structure ethylene/CO copolymers can be transformed into pyrroles, thiophenes, and furans upon treatment with primary amines,phosphorus pentasulfide, and phosphorus pentoxide, respectively. ... 

The synthesis, characterization, electrical conductivity, and field effect mobility of a series of novel soluble N-alkyl dithieno[3,2-b 2, 3 -d]pyrrole (DTP) and thiophene (TH)-based copolymers (DTP-co-THs) were reported (06MM1771 08JA13167). The incorporation of DTP units extends n conjugation, and the introduction of thiophene subunits imparts good solubility, high conductivity, and high charge carrier mobility. Therefore, the incorporation of DTP units and various substituted thiophenes into the polymer backbone affords the ability to enhance the solubility, lower the band gap, and achieve the enhanced electronic properties. 

There are several examples of graft copolymers obtained from macromonomer prepared by the metal-catalyzed polymerizations. Conventional radical copolymerization of TV-vinylpyrrolidione and vinyl acetate-capped polystyrene synthesized with copper catalysts gave graft copolymers G-2, which formed hydrogels in water.337 Electrochemical copolymerization of pyrrole and thiophene-capped poly(MMA) affords G-3.335... 

Yilmaz, F. Sel, O. Guner, Y. Toppare, L. Hepuzer, Y. Yagci, Y. Controlled synthesis of block copolymers containing side chain thiophene units and their use in electrocopolymerization with thiophene and pyrrole. J. Macromol. Sci., Pure Appl. Chem. 2004, A41, 401—418... 

Photoluminescence spectra of copolymer of pyrrole and bithiophene films whose thiophene content is higher than 50% consist of three peaks around 2.0, 1.8, 1.7eV corresponding to phonon side bands, at lOK. These peaks have been considered to be radiative relaxation from self-trapped exciton levels. The peak at the highest energy reflects band gap. 

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