Alkoxy polythiophenes

Smeets et al. developed a similar approach to prepare a diblock copolymer carrying two different poly(thiophene) derivatives, namely 3-alkyl and 3-alkoxy polythiophenes, using a CuAAC reaction as the final coupling reaction between the two different polythiophene blocks [41]. 

Despite their low molecular weights, these unsubstituted polythiophenes are insoluble in THF and other common organic solvents, and are also infusible. Their poor processability has therefore led to extensive studies of alkyl- and alkoxy-sub-stituted polythiophenes in the hope of enhancing solubility in organic solvents and allowing melt processing. Synthetic approaches to these substituted polythiophenes are described in the following text. 

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). 

An alternative, less sophisticated approach to regioregular substituted polythiophenes is the use of asymmetric 3,4-disubstituted thiophene monomers as substrates, resulting in the selective activation of either the 2- or 5- positions on the thiophene ring. Using this method, poly(3-alkoxy-4-methylthiophenes) with > 95% HT couplings have been prepared via FeCl3 oxidation.48 59... 

The solubility of polythiophenes has also been increased by attaching alkoxy groups to bithiophere monomers.12 Other workers131415 have been concerned with producing pyrroles and thiophenes with alkyl-sulfonated chains attached to increase the water solubility of the polymer. 

The phenomenon is found with many, but not all substituted polythiophenes. Studies of 3,3 -di-alkyl-substituted polythiophenes revealed no temperature-induced colour change [17]. But many other structures will give thermochromism and solvatochromism. Polythiophenes substituted both with alkoxy and alkyl side chains may show it both purely amorphous and partially crystalline polythiophenes show it short and long-chain polymers may show it both regular and irregular polymers show it (see Figure 15.1),... 

M.Leclerc and G.Daoust, Structural effect in alkyl and alkoxy-substituted polythiophenes,... 

If the three positions of the aromatic rings in PPy or PTh are substituted with alkyl (or alkoxy) chains, polymers are obtained that are structurally similar to the parent polypyrrole or polythiophene. An example of this substitution is the series of poly(3-alkylthiophenes) (P3 AT) which exhibit the useful properties of solubility in common solvents and of enhanced environmental stability. Kawai et al who investigated the changes in the lattice vectors of poly(3-alkylthiophenes) with doping levels of BFJ, AsF and toluene sulphonyl anions, conclude that the dopant ions assume sites between the chains and are coplanar with them. In relation to polymers in adjacent planes the anions constitute intercalants, although their sites are not between neighboring pairs of thiophene rings. 

Sheina, E.E., S.M. Khersonsky, E.G. Jones, and R.D. McCullough. Highly conductive, regioregular alkoxy-fimctionalized polythiophenes A new class of stable, low band gap materials. 2005. Chem Mater 17 (13) 3317-3319. 

Zotti, G., R. Salmaso, M.C. Gallazzi, and R.A. Marin. 1997. In situ conductivity of a polythiophene from a branched alkoxy-substituted tetrathiophene. Enhancement of conductivity by conjugated cross-linking of polymers chains. Chem Mater 9 791-795. 

Daoust, G., and M. Leclerc. 1991. Structure-property relationships in alkoxy-substituted polythiophenes. Macromolecules 24 455-459. 

Soluble conducting polymers can be solvent cast to form coatings. The addition of appropriate substituents to the polymer backbone or to the dopant ion can impart the necessary solubility to the polymer. For example, alkyl or alkoxy groups appended to the polymer backbone yield polypyrroles [117,118], polythiophenes [118], polyanilines [119,120], and poly(p-phenylenevinylenes) [97] that are soluble in common organic solvents. Alternatively, the attachment of ionizable functionalities (such as alkyl sulfonates or carboxylates) to the polymer backbone can impart water solubility to the polymer, and this approach has been used to form water-soluble polypyrroles [121], polythiophenes [122], and polyanilines [123]. These latter polymers are often referred to as self-doped polymers as the anionic dopant is covalently attached to the polymer backbone [9]. For use as a corrosion control coating, these water-soluble polymers must be cross-linked [124] or otherwise rendered insoluble. 

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). 

Due to the length of the alkoxy and alkyl substituents, substituted polythiophenes, such as poly(4,4 -dimethoxy-3 -methyl-2,2 5, 2 -terthiophene) (PDMTT) and poly(4,4 -dipentoxy-3 -dodecyl-2,2 5, 2 -terthiophene) (PDDTT), differ in their spectroelectrochemical and photoelectrochemical properties [56]. It was demonstrated that the maximum absorption wavelength of poly 3-[12-(jt -methoxyphenoxy) alkyljthiophene] also depends on the alkyl chain length of the substituted monomer [15],... 

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