page_title: Polythiophene
section_title: Routes employing chemical oxidants
hierarchical_section_title: Polythiophene / Synthesis / Routes employing chemical oxidants
context_page_description: Polythiophenes are polymerized thiophenes, a sulfur heterocycle. They are white solids with the formula ₙ for the parent PT. The rings are linked through the 2- and 5-positions. Polys have substituents at the 3- or 4-position. They are also white solids, but tend to be soluble in organic solvents.
  PTs become conductive when oxidized. The electrical conductivity results from the delocalization of electrons along the polymer backbone. Conductivity however is not the only interesting property resulting from electron delocalization. The optical properties of these materials respond to environmental stimuli, with dramatic color shifts in response to changes in solvent, temperature, applied potential, and binding to other molecules. Changes in both color and conductivity are induced by the same mechanism, twisting of the polymer backbone and disrupting conjugation, making conjugated polymers attractive as sensors that can provide a range of optical and electronic responses.
  The development of polythiophenes and related conductive organic polymers was recognized by the awarding of the 2000 Nobel Prize in Chemistry to Alan J.
context_section_description: In contrast to methods that require brominated monomers, the oxidative polymerization of thiophenes using ferric chloride proceeds at room temperature.  The approach was reported by Sugimoto et al. in 1986. The stoichiometry is analogous to that of electropolymerization.
  This method has proven to be extremely popular; antistatic coatings are prepared on a commercial scale using ferric chloride.  In addition to ferric chloride, other oxidizing agents. Slow addition of ferric chloride to the monomer solution produced poly(3-(4-octylphenyl)thiophene)s with approximately 94% H–T content. Precipitation of ferric chloride in situ (in order to maximize the surface area of the catalyst) produced significantly higher yields and monomer conversions than adding monomer directly to crystalline catalyst. Higher molecular weights were reported when dry air was bubbled through the reaction mixture during polymerization. Exhaustive Soxhlet extraction after polymerization with polar solvents was found to effectively fractionate the polymer and remove residual catalyst before NMR spectroscopy. Using a lower ratio of catalyst to monomer (2:1, rather than 4:1) may increase the regioregularity of poly(3-dodecylthiophene)s. Andreani et al. reported higher yields of soluble poly(dialkylterthiophene)s in carbon tetrachloride rather than chloroform, which they attributed to the stability of the radical species in carbon tetrachloride. Higher-quality catalyst, added at a slower rate and at reduced temperature, was shown to produce high molecular weight PATs with no insoluble polymer residue. Factorial experiments indicate that the catalyst/monomer ratio correlated with increased yield of poly(3-octylthiophene). Longer polymerization time also increased the yield.
  In terms of mechanism, the oxidative polymerization using ferric chloride, a radical pathway has been proposed.
   Polymerization was only observed in solvents where the catalyst was either partially or completely insoluble (chloroform, toluene, carbon tetrachloride, pentane, and hexane, and not diethyl ether, xylene, acetone, or formic acid).  The polymerization may occur at the surface of solid ferric chloride. Quantum mechanical calculations also point to a radical mechanism.  The mechanism can also be inferred from the regiochemistry of the dimerization of 3-methylthiophene since C2 in [3-methylthiophene]⁺ has the highest spin density.
  A carbocation mechanism is inferred from the structure of 3-(4-octylphenyl)thiophene prepared from ferric chloride.
  Polymerization of thiophene can be effected by a solution of ferric chloride in acetonitrile.  The kinetics of thiophene polymerization also seemed to contradict the predictions of the radical polymerization mechanism. Barbarella et al. studied the oligomerization of 3-(alkylsulfanyl)thiophenes, and concluded from their quantum mechanical calculations, and considerations of the enhanced stability of the radical cation when delocalized over a planar conjugated oligomer, that a radical cation mechanism analogous to that generally accepted for electrochemical polymerization was more likely. Given the difficulties of studying a system with a heterogeneous, strongly oxidizing catalyst that produces difficult to characterize rigid-rod polymers, the mechanism of oxidative polymerization is by no means decided. The radical cation mechanism is generally accepted.
caption: Proposed mechanisms for ferric chloride oxidative polymerizations of thiophenes.
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