Thiophene-based monomers

STRUCTURAL CHARACTERIZATION OF THIOPHENE-BASED MONOMERS, POLYMERS, AND OLIGOMERS AS POWDERS, THICK, AND THIN HLMS... 

Table 1. C-C ring bond distance differences (8r), from crystal structure determinations of thiophene-based monomers, and corresponding solution oxidation potentials (OP, Volts versus SCE) ... 

Another very common thiophene-based monomer used to design low bandgap ECPs is EDOT, whose corresponding polymer PEDOT has a bandgap of 1.2 eV [214]. Zotti et al. have shown that bis-(3,4-ethylenedioxythienyl)-methane could give rise to thick polymer films with an electrochemical bandgap (based upon standard redox potentials) of 1.0 eV [215]. 

STM reveals the early stages of growth in epitaxial electrochemical polymerization of thiophene-based monomers... 

All C-H activation procedures for polymers reported thus far have been carried out under dry inert atmosphere using sealed vessels (Schlenk glassware). Similar to small-molecule procedures, polar (DMF, DMAc) and nonpolar (toluene, THE) aprotic solvents have been used and are degassed prior to use. Most thiophene-based monomers are not commercially available and must be synthesized according to literature procedures. It is very important that these monomers be extremely pure and free of all aryl impurities since other aryl bonds may undergo C-H activation and be incorporated into the polymer. All examples have employed palladium (II) acetate or the Herrmann-Beller catalyst. The latter can be prepared from Pd(OAc)2 and tris-(t)-tolyl)phosphine. All phosphine ligands, anhydrous bases and pivalic acid are commercially available and are stored under inert atmosphere. 

A new thiophene-based monomer bearing a condensed pyridine group (84) was synthesized to obtain a low-gap polymer but the maximum absorption of the polymer was found at a high energy (560 nm, 2.2 eV) [336]. A real lowering of the gap was obtained from a condensed thiadiazole unit (85) with maximum polymer absorption at 934 nm (1.3 eV) [337]. Recently by further insertion of a pyrazine ring (86) the maximum absorption has been increased to 990 nm (1.25 eV) and the threshold of absorption lowered to 0.3 eV [338] so... 

Tkachov R, Senkovskyy V, Komber H, Kiriy A (2011) Influence of alkyl substitution pattern on reactivity of thiophene-based monomers in Kumada catalyst-transfer polycondensation. Macromolecules 44 2006-2015... 

Electron-deficient thiophene-based monomers including thieno[3,4-c] pyrrole-4,6-dione (TPD), furo[3,4-c]pyrrole-4,6-dione (FPD) monomers, diketopyrrolopyrrole and isoindigo can be copolymerized with bromi-nated thiophene-based electron-rich monomers such as benzodithiophene, dithienosilole, and dithienogermole by direct arylation polymerization. Additionally, electron deficient 4,4 -dinonyl-2,2 -bithiazole and 1,2,4,5-tetra-fluorobenzene are highly reactive toward direct arylation polymerization. ... 

Polyrotaxanes have been prepared by threading multiple a-cyclodextrin units onto a bulky benzimidazole-based linear chain polymer with an aliphatic spacer where the cyclodextrin resides. The rotaxane forms as the cyclodextrin-bound precursor amine is polymerised. Compared to the polymer in the absence of the cyclodextrins, the glass transition temperature is raised by some 20 °C even though only ca. 16 % of the aliphatic spacers in the polymer are rotaxanated.28 A novel approach to polyrotaxanes involved the use of a metal ion such as Zn2+ to thread a phenanthroline-based macrocycle onto a thiophene-based complementary monomer. The resulting pseudorotaxane can then be electropolymerised and the Zn2+ ions removed to give a polyrotaxane, Scheme 14.5. The redox and conductivity properties of the polymer are very much dependent on whether a metal ion is bound or not.29... 

There are several ways to obtain multidimensional ECPs one of them consists of the formation of ladder polymers, i.e., polymers formed by coupling conjugated monomers by two bonds rather than only one. The main example of such ladder polymer is the one derived from benzimidazobenzophenanthroline (BBL, Scheme 18.8) and we will focus on the electrochemical properties of this polymer, although other examples of ladder ECPs exist, among which are those based on fused thiophene ring monomers [286]. 

Dispersion polymerization has been also carried out to prepare PPy nanoparticle by several research groups. The synthesis of sterically stabi-hzed PPy colloid was carried out using a tailor-made reactive polymeric stabUizer [210]. In this study, a copolymer stabihzer was formed by free radical copolymerization of thiophene-based vinyhc monomer with various hydrophihc vinyl monomers such as 2-(dimethylamino)ethyl methacrylate, 2-vinylpyridine, and N-vinylpyrrohdone. These copolymer stabihzers were grafted onto the surface of PPy nanoparticle during dispersion polymerization and contributed to effective steric stabihzation of the nanoparticle. The resulting nanopartide had the size distribution in the range of 50-100 nm. 

Wudl et al. synthesized a thienopyrazine-based monomer 3.21f (Chart 1.47) by attaching octyl groups at the 3-position of the thiophene moiety [334]. Monomer 3.21f was chemically polymerized with FeC to generate polymer P3.21f. The polymer covered a very broad absorption range from 300 to 980 nm. 

This section will describe some of the synthetic methods of selected PTs and PSTs. Examples of the different synthetic routes that are used to aehieve thiophene-based polymers with variable properties will be noted. The latter examples will also include monomer synthesis as this leads into the synthetic path chosen in many of the cases. The experimental procedures and characterization details will be included. 

Though both Suzuki and Stille reactions have been widely used to prepare conjugated polymers (including D-A copolymers), there are some subtle issues to consider when it comes to choose which reaction to use. For example, it is worth noting that the electron richness of stannyl aromatics decides whether these monomers are suitable for Stille-based polymerization or not. Mechanistically, relatively electron-rich thiophenes undergo the transmetalation step more readily than stannylbenzenes. Thus, stannylbenzenes experience low reactivity under Stille reaction conditions. Correspondingly, most thiophene-based aromatics are polymerized via Stille reactions, whereas a Suzuki reaction is a better option for benzene-based compounds. For example, fluorene and carbazole based polymers are usually prepared by Suzuki reaction, whereas polymers with cyclo-penta[2,l- ) 3,4-6 ]dithiophene, silolo[3,2- 4,5- ) ]dithiophene or benzo[l,2- 4,5-i Jdithiophene are often polymerized via Stille reaction. Due to its broader utilization over the Suzuki reaction in preparing D-A copolymers, Stille reaction-based polymerization will be the focus of this chapter, with a brief discussion on the Suzuki-based polymerization also included (Section 15.2.3). 

Prior to the Stille polymerization, all these thiophene-based donor molecules need to be converted into stannylated monomers, typically via BuLi in anhydrous THF followed by treating the lithiated anion with trialkyltin chloride. Usually MesSnCl is preferred albeit its stronger toxicity than the butyl analog i.e., BusSnCl) because the latter can render it much more difficult to purify monomers through recrystallization. 

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