INTRODUCTION:

The electrically conducting polymer is inherently interdisciplinary research subject. Interest in such materials is due to a wide variety of technological applications such as polymer batteries, light emitting diodes, photovoltaic devices, conducting fibers and so on. (1, 2) The discovery of conducting polymer led to the award of the Nobel prize for Chemistry in 2000.(3,4) The electrochemical polymerization is an elegant way to prepare conducting polymers because the polymerization reaction can be controlled easily. The success of electrochemical synthesis of polymers of high conductivity depends heavily on experimental conditions such as electrode materials, electrode potentials, impressed current density, solvents, temperature and supporting electrolytes. Five member heterocyclic compounds such as pyrrole and thiophene are generally polymerized electrochemically. Previously we reported the synthesis of conducting polythiophene in a solution of tetrabutylammonium hexachloroantimonate salt in DCE. The black to red colored film with metallic luster of polythiophene was deposited on Pt. and Au electrodes under potentiodynamic conditions. (5)

Here we describe the electrochemical formation of conducting polythiophene in DCE and acetonitrile containing boron fluoride etherate BF₃O (C₂H₅)₂as supporting electrolyte.

The use ofBF₃O(C₂H₅)₂ as supporting electrolyte was reported.(6, 7). Boron fluoride etherate may exist in diethyl ether as a polar molecule, (C₂H₅)₃O⁺, BF₄^- which furnishes the conducting medium(6, 8). The polythiophene film prepared electrochemically is quite stable in oxygen and moisture in doped and undoped conditions (9).

MATERIALS AND METHODS:

Thiophene, DCE, and acetonitrile were purified by fractional distillation and middle fractions were collected. Boron fluoride etherate was used without purification. Polymerization was conducted in a simple one compartment glass cell which could accommodate two Pt. electrodes of area 3.3cm². The cell was charged with the solution of known amount of monomer and supporting electrolyte in the solvent and was thermo stated at 30⁰C. The electrolysis was conducted under constant current conditions. The electrolysis was terminated at known time and the anode coated with a brown colored polymer mass was taken out. When the polymer film became dry, it was pressed in to a pellet which was made in Perkin – Elmer pellet die at a high pressure, obtained by Carver’s Hydraulic Press. The resistance of the pressed pellet of known diameter and thickness was measured with a high vacuum tube voltmeter and thus the conductivity was determined.

The polymerization was also carried out under potentiodynamic conditions using CV-27BAS(Bio Analytical System, USA)Cyclic voltammetry system equipped with BAS X-Y recorder. All electrical measurements were performed in a single compartment three electrodes cells under a N₂ atmosphere. The working electrode was a Pt. microelectrode, the counter electrode was a platinum wire and the reference electrode was Saturated Calomel Electrode (SCE) which was isolated from the cell by a salt bridge.

RESULT AND DISCUSSION:

Polymerization of Thiophene with (C₂H₅)₂OBF₃

The electrochemical polymerization of thiophene was investigated in DCE and acetonitrile containing BF₃O (C₂H₅)₂ as supporting electrolyte. As soon as the current passed through the solution , a brownish stream taking greenish tint emerged from the anode and finally the whole solution became deep brown. As usual the polymer formation took place at the anode surface. The yield and conductivity of the polymer formed in DCE and acetonitrile are summarized in Table-1 below

Table-1-Polymer Yield and Conductivity of Polythiophene formed from Thiophene (6.9 mole/l) in Different Solvents containing BF₃ (C₂H₅)₂ (2.20 mole/l) at 40mA; Electrolysis time-2 h; vol. of the reaction mixture- 18 ml at 30⁰C.

Solvent	Polymer Yield(mg)	Conductivity x10³(Scm^-1)
DCE	160	0.93
Acetonitrile	125	1.30

Table-1 shows that electrical conductivities are relatively higher in acetonitrile compare to that in DCE. Polymer films formed in DCE were black , floppy and brittle but those obtained in acetonitrile were smooth and flexible.

The Cyclic Voltammetry of Thiophene with (C₂H₅)₂OBF₃

Figure-1 shows the cyclic voltammogram of thiophene(0.63 mole/l) in DCE containing BF₃O(C₂H₅)₂(0.4 mole/l) was recorded in the range from -1.0 to 2.5V vs. Saturated Calomel Electrode (SCE) using microelectrode as working electrode and Pt. wire as auxiliary electrode in a single compartment cell.

Both anodic and cathodic peak currents increased gradually in first several cycles after which they become constant with continued cycling. Increasing waves are due to the growth of polymer films. The electrode becomes passivated owing to gradual formation of the strongly associated polymer anion salt. The polymer oxidation and reduction cycles are associated with inclusion or exclusion of anion BF₄^- of supporting electrolyte. Figure-1 shows that the redox peaks are not symmetrical about the potential value. This is due to the difference in the background current and some kinetic limitations. The peaks are also broad which is due to the presence of mixed valence species such as cation and dication of polymer anion salts. The colour of the oxidized polymer on the working electrode was black and that of reduced form was red.

Figure-2 shows that redox peak current increased with increasing scan rate. This suggests that the thickness of conducting films increases with the increase of scan rate. The polythiophene coated microelectrode was washed properly to make it thiophene free. Then it was dipped into DCE containing only BF₃O(C₂H₅)₂ and multisweep cyclic voltammograms were recorded as shown in Figure-3. The redox peak current increases initially for a few cycles and then became constant. On continued cycles the redox peak current remained unchanged suggesting that polymer film did not suffer noticeable degradation and the film is able to stand several cycles from positive to negative potentials.

In the redox process the polymer film is insulating in reduced form and conductive in the oxidized form. The insulator - to – conductor reaction is expressed as

P_y^- + X_s^- ----à P_y⁺ - X_p^- + e^-

Where P_y^- and P_y⁺ symbolize reduced and oxidized monomer units respectively and X^–is a charge compensating anion. X^– is initially present in a contacting solution phase but upon oxidation of the polymer, it is incorporated into the polymer phase. Thus the switching reaction involves a charge transport process in which oxidized monomer sites and charge compensating counter ions move through the polymer film. This switching process plays an important role in the application of electronic conductive polymers.

Mechanism:

The polymerization mechanism appears to be the same as discussed in detail by G. Tourillon(10). We also mentioned in our previous paper the mechanism as below (5).

Radical cation is initially formed by the anodic oxidation of the monomer which dimerises with the expulsion of 2H⁺ to yield a neutral dimer. The process is repeated with 2e^- and 2H⁺ involved in each addition step to finally yield oxidized conducting polymer.

Electrical conductivity of polythiophene, therefore, is dependant both on the oxidation and deprotonation.

A similar mechanism is also reported in the literature (11).

CONCLUSION:

The electrochemical polymerization of thiophene was carried out in organic solvents containing boron fluoride etherate as a supporting electrolyte. Polymer films formed in acetonitrile are flexible but the films formed in DCE is brittle. Cyclic voltammetry results show that the films are quite stable.

REFERENCES:

1. Nalwa H. S. Editor, Handbook of Organic Conductive Material and Polymers. Wiley, New York. 1997.

2. Skotheim T. A. et al, Editors, Handbook of Conducting Polymers.2^nd Ed., Marcel Dekker Inc., New York.1998.

3. Mac Diarmid A. G. “Synthetic Metals”:A Novel role for organic polymers(Nobel Lecture). Angewandte Chemie. Int. Edition 40; 2001: 2581-2590.

4. Heegar A. J. Semiconducting and metallic polymers: The fourth generation of polymeric materials. American Chemical Society. 105(36); 2001: 8475-8491.

5. Bhadani SN, Gupta MK, and Sen Gupta SK. Conductivity and cyclic voltammetric investigations of electrically synthesized polythiophene. Journal of Polymer Material. 9; 1992: 147-152.

6. Bhadani SN, Sen Gupta SK. Gupta MK, and Prasad J. Electrosynthesis OF conducting poly (p-phenylene). Journal of Applied Polymer Science. 47; 1993: 1215-1218.

7. Santoso HT, Singh V, Kalaitzidou K, and Cola BA. Enhanced molecular order in polythiophene films electropolymerized in a mixed electrolytes of anionic surfactants and boron trifluoro diethyl etherate. Applied Materials Interfaces. 4; 2012: 1697-1703.

8. Eley D.D, Chemistry of Cationic Polymerization, Plesch P. H., Ed.,Macmillan, New York. 1963: 393.

9. Udum YA, Pekzez K, and Yildiz A. Electropolymerization of self doped polythiophene in acetonitrile containing FSO₃H. Synthetic Metals. 142; 2004: 7-12.

10. Tourillon G.`` Polythiophene and its Derivatives`` in Handbook of Conducting Polymers edited by Skothiem TA et al. Marcel Dekker Inc, New York,1986: p. 293.

11. Das I, Agrawal NR, Ansari SA, and Gupta SK. Pattern formation and oscillatory electropolymerization of thiophene. Indian Journal of Chemistry. 47A December; 2008: 1798-1803.

Received on 17.02.2013 Modified on 01.03.2013

Asian J. Research Chem. 6(3): March 2013; Page 241-243