Head-to-tail 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... 

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. 

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.)...

Fig. 9.7 Regioregular, head-to-tail polythiophene can be planar. Irregular poly(3-alkylthiophene) is not planar.

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. 

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). 

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. 

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. 

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). 

Figure 1. Steric interactions in a head-to head, head-to-tail triad of an alkyl substituted polythiophene. Steric clashes between alkyl side chains and between side chains and the sulfur lone pairs are indicated.

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