Electro synthesis of Conducting Poly (methyl phenylene)

 

Reena Bhadani¹* and S.N. Bhadani²

¹Department of Chemistry, Ranchi Women’s College, Ranchi.(Jharkhand)

²P. G. Department of Chemistry, Ranchi University, Ranchi.

*Corresponding Author E-mail: reenabhadani@yahoo.com

The polymerization of toluene was carried out in 1, 2-dichloroethane and nitrobenzene using BF₃O(C₂H₅)₂ as supporting electrolyte at platinum electrodes. The polymers were formed as a black mass only at the anode surface. The yield and conductivity of Poly(toluene)or poly(methyl phenylene) depend on the nature of solvents. Cyclic voltammetry measurements suggest that the polymer film is fairly stable. 

 

 

 

INTRODUCTION:

The electrically conducting polymers are an active research field^(1,2). Such materials have a unique combination of properties not available in any other known materials.^(3,4)The electrochemical formation of conducting polymers of aromatic compounds was well studied. The oxidation of the corresponding monomers yields conducting films^{(1, 2)}. Poly(paraphenylene) is more attractive but less investigated because of the high oxidation potential of benzene that restricts the choice of electrolytic solution^(5,6). Previously we reported the polymerization of benzene with boron fluoride etherate, BF₃O(C₂H₅)₂ under constant current electrolysis conditions as well as under potentiodynamic controls ⁽⁷⁾. In this article, we describe the electrochemical polymerization of toluene using BF₃O(C₂ H₅)₂as supporting electrolyte.

 

EXPERIMENTAL:

Materials

Toluene, nitrobenzene and dichloroethane were purified by fractional distillation and middle fractions were collected. BF₃O(C₂H₅)₂ was used without further purification.

 

Polymerization

Under constant current electrolysis, polymerization was carried out in a simple one-compartment glass cell that could accommodate two platinum electrodes of the area 2.5×1.7cm².

 

 

The cell was charged with known amounts of monomer, solvent, and supporting electrolyte and the resulting solution was saturated with N₂ gas to purge oxygen. Then, the cell was thermo stated at 30^o C and subjected to electrolysis at 40 mA. The electrolysis was terminated at a known time and the anode that was heavily covered with a black polymer mass was taken out. The deposited polymers were removed with a sharp razor which were dried and weighed. The polymer mass was pressed into pellet in Perkin-Elmer Pellet Die at a high pressure obtained by Carver’s Hydraulic Press. The surface resistance of press pellet of a known diameter was measured at ambient temperature and the conductivity was calculated.

 

Cyclic Voltammetry

Cyclic voltammograms were recorded with CV-27 BAS (Bio Analytical System, U.S.A.). Cyclic Voltammetry System is equipped with BAS X-Y recorder. All electrical measurements were performed in a single compartment three electrode cell under a N₂ atmosphere. The working   electrode was a Pt. Microelectrode, the counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE).

 

RESULTS AND DISCUSSION:

Polymerization of Toluene

The polymerization of toluene (spectroscopic grade thiophene free) was investigated in nitrobenzene and in DCE, using boron fluoride etherate as supporting electrolyte. When BF₃O(C₂H₅)₂is added to the solution of toluene in nitrobenzene the solution turned little yellowish, but the polymerization did not take place if the above solution was not subjected to electrolysis. As soon as electrolysis was started, a black polymer mass formed on the anode surface and finally the whole anode turned dark black. The polymer formation occurred only on the anode surface into electrochemical cell and not in the body of the solution.  The polymers so formed are highly brittle and can be removed easily from the electrode.  The density of the polymer was found to be 1.28gm/cc.  The yield and the electrical conductivity of the resulting polymers which were synthesized at different current levels in nitrobenzene are summarised in table 1.

 

Table-1 : Yields and electrical conductivities of polytoluene prepared from toluene in Nitrobenzene containing BF₃O(C₂H₅)₂ (1.10 mole/L) at different current levels. Concentration of toluene in nitrobenzene-0.53mole/L, Volume of reaction mixture-17.5mL and Time of electrolysis- 2h

Current (mA)	Polymer yield (mg)	Conductivity x 106 (S/cm)
10	12	106.00
15	16	57.00
20	22	29.00
30	32	01.90

 

It is evident that the polymer yield increases with current level, whereas the electrical conductivity decreases.  On the other hand, in DCE, both the polymer yield and the electrical conductivity increase with current levels as shown in Table 2.

 

The polymer yield is found to be higher in nitrobenzene than in DCE whereas the electrical conductivity is higher in DCE. The polymers formed in nitrobenzene were rough, highly brittle and threadlike whereas the polymers formed in DCE were smooth, relatively less brittle and uniform(plate like). These results demonstrate that yield and conductivity of polytoluene prominently depend on the nature of solvents, which also seem to have an influence on the morphology of this film.

 

Table-2: Yields and electrical conductivities of polytoluene prepared from toluene in DCE containing BF₃O(C₂H₅)₂ (1.10 mole/L) at different current levels.  Volume of reaction mixture-17.5mL, Time of electrolysis- 2h and Temperature-30⁰C

Current (mA)	Polymer yield (mg)	Conductivity x 102 (S/cm)
10	8	44.40
20	12	48.50
30	18	54.70
40	25	85.30

 

Cyclic Voltammogams

Multi sweep cyclic voltammograms of polytoluene on Pt microelectrode in the solution of toluene (1.0mole/L) in the nitrobenzene containing BF₃O(C₂H₅)₂(0.3 mole/L) at 50 mv/s were recorded and shown in figure 1. The redox peaks are not symmetrical about the potential value and also broad. This might be due to the difference in the background current.

 

Figure 1 shows redox peak current increases initially for a few cycles and then becomes constant. On continued cycles the redox peak current remained unchanged indicating that polymer film did not suffer degradation and the film is able to stand several cycles from positive to negative potentials

 

 

Mechanism

The electron donating effect of –CH₃ group increases the overall electron densities of the ring and results in a lower oxidation potential for toluene. Polymer linkages may go to ortho or meta positions. The meta linkages between conducting segments interrupt conjugation along the polymer chain consequently produce with lower conductivity ⁽⁸⁾. The polymerization mechanism is believed to be the same as discussed for five membered heterocyclic monomers ⁽⁹⁾ and p-methoxi toluene ⁽¹⁰⁾. 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 reaction is followed by a further electro oxidation, dimerization and deprotonation leading to a progressive increase in the chain length through the same sequence until finally yields conducting polymer ^{(11, 12)}.

 

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Received on 07.12.2014         Modified on 18.12.2014

Accepted on 05.01.2015         © AJRC All right reserved