Head polythiophenes

Xue C, Cai F, Liu H (2008) Ultrasensitive fluorescent responses of water-soluble, zwitter-ionic, boronic acid-bearing, regioregular head-to-tail polythiophene to biological species. ChemEur J 14 1648-1653... 

Pure polythiophene is neither soluble nor fusible and is therefore not useful for further processing. Side chains introduced on the polythiophene skeleton help to obtain organic materials that can be used in further processes <1999JMG1933>. When 3-substituted thiophenes are used in the polymerization process, three possible arrangements can be obtained head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT). 

Summaries on the synthesis, properties, and uses of polythiophenes are included in two general reviews on poly thiophenes [259,260]. A synopsis of important aspects of polythiophenes are also included in several reviews on various aspects of conducting polymers [221-226], Cation radicals are the propagating species in both electrochemical and chemical oxidative polymerizations of thiophene and its derivatives. The polymer obtained by this method is linked primarily by a,a-linkages. However, other types of linkages (a,f3 and /3,/3) are present in varying amounts (Fig. 59). Substituted thiophene derivatives can couple in a head-to-tail or head-to-head manner. 

Similar structures will occur in conjugated polymers such as polypyrroles and polythiophenes prepared from monomers with one or more different substituents at the positions on the aromatic ring not involved in the polymerisation, see Fig. 1.5. The reactivity of one particular site in the monomer usually predominates, so that the effect is relatively small. A 1 % head-to-head content may, however, seriously upset crystallisation. 

PAn also differs from PPy s and polythiophenes in that the N heteroatom participates directly in the polymerization process (PAn is a ladder polymer that polymerizes head to tail) and participates in the conjugation of the conducting form of the polymer to a greater extent than the N and S heteroatoms in PPy and poly thiophene. 

Despite the predominant 2,5 -coupling with both the 3-alkyl and 3-alkoxy thiophene monomer substrates, the presence of the ring substituents introduces a further structural complication. Coupling of the substituted thiophene units can now lead to four possible triad regioisomers for the resultant polymers, depending on whether the coupling occurs in a 2,5 -head-to-tail (HT), 2,2 -head-to-head (HH), or 5,5 -tail-to-tail (TT) manner. These structures 11-14 for the polythiophene products are depicted in Figure 6.4 (X = S). 

Fig. 6.10 Polythiophenes with varying distances between head-to-head dyads.

This group of polythiophenes is of considerable interest, both because the materials are intermediate between the plain PT and the PATs, and because they can be used to study the effect of head-to-tail versus head-to-head/tail-to-tail-configuTat ons. The materials may be considered as co-polymers of thiophene, T, and alkylthiophenes, AT and TA, with general compositions [(AT),(T),(TA),L ( exo ) or [(TA),(T),(AT),] Cendo ) (or also [(AT)r(T) (AT),] ), where AT and TA differ by the point at which the alkyl side chain is attached to the... 

Scheme 29.8 (a) Head-to-head, (b) head-to-tail, and (c) tail-to-tail coupling patterns in substituted polythiophenes. 

Polythiophenes and regioregular, head-tail poly(3-alkylthiophenes) synthesized by different methods, have been characterized by MALDl-TOF. More than one type of end- roup, depending on fhe synthetic method employed, were characterized. These polymers appear as molecular ions (M ) in the MALDl spectra recorded using dithranol matrix, and cationized maciomolecular chains were not detected. ... 

McCullough, R.D., S.P. Williams, M. Jayaraman, J. Reddinger, L. Miller, and S. Tristram-Nagle. 1994. Synthesis and physical properties of self-orienting head-to-tail polythiophenes. Mater Res Soc SympProc 328 (Electrical, Optical, and Magnetic Properties of Organic Solid State Materials) 215-220. 

McCullough, R.D., and S.P. Williams. 1993. Toward tuning electrical and optical properties in conjugated polymers using side-chains Highly conductive head-to-tail, heteroatom functionalized polythiophenes. / Am Chem Soc 115 (24) 11608-11609. 

Pilston, R.L., and R.D. McCullough. 2000. Toward highly fluorescent polythiophenes Head-to-tail coupled copolymers of 3-(methoxyethoxyethoxymethyl)thiophene and 3-(perfluoroalkyl)thio-phene. Thesis. 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

Figure 4.35 Synthesis of regioregular, head-to-tail polythiophenes. (Reprinted with permission from Journal of the American Chemical Society, 119, 633. Copyright (1997) American Chemical Society.)...

A. Iraqi, D. Clark, R. Jones and A. Krier. Synthesis, characterization and study of photoluminescent properties of head to tail poly(3-pentoxythiophene), poly(3-cyclohexylthiophene) and mixed alkoxy/cyclohexyl 3-substituted polythiophenes. Synth. Met. 102(1-3), 1220-1221 (1999). 

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