Since Shirakawa et al. discovered that polyacetylene possesses high electrical conductivity, the field of conducting polymers has attracted extensive interest among scientists. After nearly 20‑year development, conducting polymers have evolved into a relatively mature interdisciplinary research area. Such materials feature light weight, good processability, corrosion resistance and electrical conductivity. Among numerous conducting polymers, poly(3,4‑ethylenedioxythiophene) (abbreviated as PEDT), whose main‑chain structure is shown in Figure 1:

Figure 1 Structure of PEDOT

PEDOT has drawn wide attention owing to its high conductivity (600 S/cm), excellent stability and high visible‑light transmittance. Nevertheless, PEDOT itself is an insoluble polymer, which limits its practical applications. This processability problem can be solved by doping with water‑soluble polyelectrolyte polystyrene sulfonic acid (abbreviated as PSS). PEDOT/PSS is a dark‑blue water‑soluble polymer with good processability. PEDOT/PSS films deliver high conductivity (10 S/cm), considerable mechanical strength, high visible‑light transmittance (nearly transparent within visible‑light range), superior electrochemical performance and thermal stability. It can withstand over 1000 h at 100 °C with almost no decline in conductivity. Researchers at Bayer AG have applied it in various industrial sectors, including solid electrolytic capacitors, antistatic coatings, and electroplating for through‑hole circuit boards. Thereafter, new materials, processes and components based on PEDOT have achieved substantial development. However, relevant domestic research remains relatively backward, especially studies on the synthesis of monomer EDOT, for which few domestic reports have been published.

2. Conductive Mechanism of Conducting Polymers

For polymer molecules to conduct electricity, an essential prerequisite is a large conjugated system in the molecular chain (conjugated double bonds or conjugation coupled with heteroatoms such as N and S bearing non‑bonding p‑orbitals). Similar to metals that require free electrons and orbitals for electron migration, polymer conduction demands charge carriers and molecular orbitals for charge‑carrier transport. Since most polymers do not intrinsically contain charge carriers, the required charge carriers for conducting polymers are generated via the doping process. The well‑accepted mechanism for doped conducting polymers is briefly illustrated in Figure 2.

For polymers with conjugated or large‑π backbones under ideal conditions, electrons are delocalized over the whole backbone or conjugated segments. Orbital interactions among monomers form the valence band from the highest occupied molecular orbitals and the conduction band from the lowest unoccupied molecular orbitals. Excluding thermal motion and optical transitions, the valence band is fully filled with electrons while the conduction band remains empty. An energy gap exists between valence and conduction bands, resulting in generally low conductivity. Doping oxidizes high‑energy electrons from the valence band and generates holes (cation radicals) with energy levels lying between valence and conduction bands. Cation radicals stabilize themselves by polarizing surrounding media and are therefore termed polarons. Heavy doping of conjugated chains may further produce bipolarons or bipolaron bands on the basis of polarons. Polarons and bipolarons can migrate along conjugated backbones through double‑bond shifting and render the polymer conductive. The above‑mentioned mechanism is built upon the valence‑band theory of inorganic semiconductors. Although it satisfactorily explains experimental phenomena of conducting polymers, its full validity still requires further investigation.

3. Synthesis of Monomer 3,4‑Ethylenedioxythiophene (EDOT)

In 1998, J. D. Stenger‑Smith et al. reported an EDOT synthetic route starting from thiodiglycolic acid (HOOC‑CH₂‑S‑CH₂‑COOH). A series of reactions afforded 3,4‑ethylenedioxythiophene‑2,5‑dicarboxylic acid, which underwent catalytic decarboxylation to produce 3,4‑ethylenedioxythiophene. The synthetic route is presented in Figure 3.

This method suffers from low yield and high cost. Improving or developing new synthetic routes to raise EDOT yield and cut production cost remains a key task for researchers. Modifications were introduced in this work, such as phase‑transfer catalysts and zeolite molecular sieves, to enhance EDOT yield.

The above‑mentioned route represents the traditional five‑step synthesis and was the only reported method for many years. From an industrial perspective, it has several drawbacks: strong‑base requirements, high‑temperature conditions, and carcinogenic 1,2‑dibromoethane. In April 2004, Swedish scientists Fredrik von Kieseritzky et al. proposed an efficient novel route for EDOT preparation, as shown in Figure 4.

In this process, 2,3‑dimethoxy‑1,3‑butadiene reacts with SCl₂ in n‑hexane at 5 °C to yield 3,4‑dimethoxythiophene, which further reacts with ethylene glycol using p‑toluenesulfonic acid as catalyst to obtain EDOT. This route features cheap readily‑available raw materials, simple operations, mild conditions and high yield (60 %), and is highly suitable for industrial scale‑up. This critical technical breakthrough will greatly promote the development of thiophene‑based conducting polymers.

4. Synthetic Methods for Poly(3,4‑ethylenedioxythiophene) (PEDT)

Two major approaches are available for polymerization of EDT monomer and its derivatives: (1) chemical oxidative polymerization; (2) electrochemical polymerization.

(1) Chemical Oxidative Polymerization

The most classic approach employs oxidants FeCl₃ and iron(III) p‑toluenesulfonate [Fe(OTs)₃] for polymerization. Resulting polymers are difficult to characterize, yet analysis shows film conductivity can reach 550 S/cm. The reaction is shown in the corresponding figure.

Another chemical‑oxidative route is the Baytron P process developed by Bayer AG for PEDOT/PSS film preparation. EDOT is dissolved in polyelectrolyte solution such as PSS, with Na₂S₂O₈ as oxidant, yielding a dark‑blue aqueous PEDOT/PSS dispersion. The reaction proceeds at room temperature. A PEDOT/PSS polymer film is obtained after solvent evaporation. Such films exhibit high conductivity, good mechanical strength, transparency and solvent insolubility. The film structure is illustrated in Figure 6.

(2) Electrochemical Polymerization

Compared with chemical polymerization, electrochemical polymerization enables direct monomer oxidation, simple operation, and convenient monitoring and control over polymerization to acquire polymers with target performance. When preparing PEDT electrochemically, monomer concentration, supporting‑electrolyte type and polymerization potential exert significant influences on the conductivity of obtained polymers. Wemetnet has conducted extensive research in this field.

Excerpted from Chemical Intermediates