Preparation and characterization of trifluoromethylaniline oligomers

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Synthetic Metals 94 (1998) 193-197

Preparation and characterization of trifluoromethylaniline oligomers Walter E. Rudzinski *, Lance Thrower, Ronald Sutcliffe, Mahnaz Bahrami Department of Chemisto,, Southwest Texas State University, San Marcos, TX 78666, USA Received 30 September 1997; accepted 15 December 1997

Abstract The oxidation of m-trifluoromethylaniline in the presence of trifluoroacetic acid leads to the preparation of poly(m-trifluoromethylaniline). The dimer and a mixture of higher oligomers have been prepared using both cyclic voltammetry for the preparation of thin films, and controlled potential electrooxidation for the preparation of powders. The CF3 moiety attached to the backbone shifts the oxidation potential of the thin film toward more positive potentials as might be expected because of the electron-withdrawing effects of the CF3 group. The conductivity of the film has been estimated to be on the order of 10 -8 S/cm when electrooxidized under the appropriate conditions of pH and potential. Extensive analysis of the products of electrooxidation indicates that the solution consists of oxidized monomer, while the dimer and higher molecular weight oligomers precipitate. The dimer can be separated from the other oligomers by vacuum sublimation. Gel permeation chromatography of the remaining precipitate indicates that the precipitate consists of 20% trimer and tetramer, 40% pentamer and hexamer, and about 40% higher molecular weight oligomers. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline and derivatives; Trifluoromethylaniline oligomers

1. Introduction In an attempt to prepare a soluble polyaniline, we have oxidized m-trifluoromethylaniline ( T F M A ) to yield oligomers with the following proposed structure:

/

/

Using an analogous line of reasoning as used in polyaniline chemistry [ 1 ] the chemical structure may vary from a fully reduced form (x = 1) to a fully oxidized form (x = 0) and may be protonated or unprotonated at the amino or imino groups. The oxidation state of the products as given by the value of x has not been determined definitively.

2. Experimental

for NMR: d3-acetonitrile, d-trifluoroacetic acid, D 2 0 and dchloroform were also obtained from Aldrich. All other chemicals and solvents were ACS reagent grade and used as received.

2.2. Electrochemisto' Electrochemical experiments were performed either using a BAS model CV 27 voltammetric analyzer attached to a BAS X - Y recorder or a Princeton Applied Research model 173 potentiostat-galvanostat, equipped with a model 179 digital coulometer and attached to a model 175 universal programmer and Houston Instruments model 2000 X-Yrecorder. For all electrochemical experiments, a conventional threeelectrode cell was used. The working electrode was either a glassy carbon electrode (GCE) which had been polished with 0.1 ~m alumina, In203/SnO2 conducting glass (PPG), Pt foil or Pt mesh. A P t wire was used as a counter electrode and Ag/AgC1 as a reference electrode. All potentials are reported with respect to Ag/AgCI.

2.3. Polymerization

2.1. Materials Trifluoromethylaniline ( T F M A ) , sodium trifluoroacetate, and trifluoroacetic acid were obtained from Aldrich. Solvents 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 7 9 - 6 7 7 9 ( 9 8 ) 0 0 0 0 2 - 2

Poly(m-trifluoromethylaniline), designated as 'poly( T F M A ) ' , was prepared using cyclic voltammetry (CV), and controlled potential electrooxidation methods.

194

W,E. Rudzinski et aL ~Synthetic Metals 94 (1998) 193-197

CV was performed at a sweep rate of 150 m V / s within the range - 0.4 to + 1.4 V. The GCE, Pt or In203/SnO2 working electrodes were immersed in either 0.05 M TFMA in (50: 50) acetonitrile:0.5-3.0 M trifluoroacetic acid, or 0.05 M TFMA in 1.5 M sodium trifluoroacetate (pH adjusted to between 0.18 and 1.29 with trifluoroacetic acid). Controlled potential electrooxidation was performed at a Pt foil or mesh poised at a potential of + 1.3 V for up to 22 h. The anode was immersed in 0.05 M TFMA in a (50:50) acetonitrile:0.5 M trifluoroacetic acid (pH = 1.1 ) solution. In order to perform an NMR analysis of the solution after electrooxidation, the electrooxidation was also performed in 0.05 M TFMA in (50:50) d3-acetonitrile:0.5 M d-trifluoroacetic acid ( p H = l . l ) in D20. All solutions were purged and maintained under a positive pressure of argon. After electrooxidation, a dark purple solution and a brown precipitate formed. The solution was filtered and the brown precipitate was washed with pentane or acetic acid, followed by dilute 6 M NH3 and then copious amounts of water. The black precipitate so obtained was dried in the oven at 110°C; after cooling, the precipitate turned into a brittle glass. The product was then sublimed under vacuum at 165-168°C. 2.4. Characterization methods"

Electronic spectral data were obtained on a Beckman DU 7500 UV-Vis spectrophotometer. IR spectra were obtained on a Perkin-Elmer model 1600 FT-IR by thin film specular reflectance off a Pt electrode, or transmittance through a KBr pellet or Nujol mull. NMR spectra were obtained on an IBM NR 80 with TMS as an internal standard. C, H, N and F elemental analyses were performed by Desert Analytics (Tucson, AZ). X-ray reflectivity scans were performed on a Bede 200 system which incorporated a Bede EDR detector, channel cut collimator, with X-ray generator settings of 35 kV and 35 mA. Molecular weights were determined by gel permeation chromatography (GPC). The system consisted of a Tracor 951 HPLC pump, Rheodyne injector, and Tracor 970 A U V Vis variable wavelength detector attached to an HP 3390 A integrator or a BAS X - Y recorder. The GPC column was either a Shodex K-80M, 7.8 × 300 mm (Waters) or a ProgelTSKP H, G 1000HXL, 7.8 × 300 mm (Supelco, Bellefontane, PA). The eluent was tetrahydrofuran (THF) adjusted to a flow rate of either 1.0 or 2.0 ml/min, while the detector wavelength was set at 254 nm. Polystyrene standards and ethyl benzene were used to construct the molecular weight calibration curve for the Shodex K80M column, while polystyrene (M,~= 1320), N,N'-diphenyl-l,4-phenylenediamine (Mw = 2 6 0 ) and trifluoromethyl aniline (Mw = 161 ) were used to construct the molecular weight calibration curve for the Progel G 1000HXL column (exclusion limit= 1 × 103). The R z values for the calibration curves were better than 0.993. Thermogravimetric analysis ( T G A ) was performed on a 1000 S T A + thermal analyzer. The thermal stability was

determined using a heating rate of 20°C/min in nitrogen within the temperature range of 25 to 300°C. The solution viscosities were obtained with a Canon-Fenske viscometer using 0.02-0.41 g/dl of solute in THF. The intrinsic viscosities were then determined at infinite dilution.

3. Results and discussion 3.1. Polymerization by cyclic v o h a m m e t ~

For the electropolymerization of 0.05 M TFMA monomer at a GCE in 2.25 M trifluoroacetic acid (pH = 0 . 6 0 ) , it was observed that as the potential is swept to values greater than + 1.05 V, the monomer is oxidized. Upon scan reversal, two peaks appear at + 0.38 and - 0 . 0 4 V. On the second sweep toward oxidizing potentials, one prominent peak at + 0.67 V (E~.~) and two small peaks at +0.33 ( E ~ ) , +0.43 (E~z) begin to appear. There are no further changes after the second sweep and passivation occurs just as observed by LaCroix et al. [2] for the oxidation of 0.1 M TFMA in 2 M H2SO4 . The specular reflectance 1R spectrum for a film prepared at a Pt electrode under the same conditions then poised at - 0 . 2 V reveals a broad absorption envelope between 3600 and 2600 c m - ~. The absorption envelope is characteristic for protonated amino or imino groups [ 3,4]. An intense band at 1681 cm ~ ascribed to C = O as well as three bands at 850, 795 and 728 cm-~ associated with -CF3 are due to trifluoroacetate. As a basis of comparison, the IR spectrum of sodium trifluoroacetate has bands which appear at 1685, 847, 800 and 725 cm ~. The IR data clearly indicate that the film is protonated and that trifluoroacetate serves as a chargecompensating anion. The following structure is proposed: CF3CO0-

CF3CO0-

F

If the same film is then poised at a potential of + 0.8 V, the broad absorption envelope disappears and peaks appear at 3458, 2912, 2845 and 2634 cm J. These peaks may be attributed to N - H and C - H stretches [5]. The bands diagnostic for trifluoroacetate remain and the counterion is not lost upon oxidation of the trifluoromethylaniline. Even though the TFMA deprotonates, the trifluoroacetate remains to provide charge compensation for the oxidized film. The following structure is proposed: CF3CO0-

CF3GO0-

-

~--%

\

\.

\ F

If TFMA is electropolymerized at a pH of 1.3, dramatic changes are observed (see Fig. 1 and [6] ). On the second

W.E. Rudzinski et al. / Synthetic Metals 94 (1998) 193-197

/F F--C--F

195

/F F--- C - - F

l 50rtA

I -0.40

I -0.05

l +0.30

I +0.65

I +1.00

Volts

Fig. 1, CV depicting the electropolymerizationof 0.05 M TFMAat a GCE in a solution containing (50:50) acetonitrile:l.5 M sodiumtrifluoroacetate (pH adjusted to 1.29 with trifluoroaceticacid). and subsequent scans, two peaks appear at + 0.51 V (Ea2) and +0.73 V (E~,3). Two cathodic peaks also appear at + 0.29 and at - 0 . 1 9 V. On each successive sweep, the CV shows a monotonic increase in current. Both the anodic and cathodic peaks also shift toward increasingly higher positive and negative potentials. This increase in the difference between the anodic and cathodic peak potentials is indicative of an increase in kinetic or charge transfer limitations. The optimal pH for electropolymerization appears to be about unity, so long as the linear potential sweeps encompass the range - 0.4 to + 1.4 V. The CV bears a superficial resemblance to the CV of polyaniline which also exhibits two oxidation peaks and two reduction peaks [7]. The first oxidation peak for poly(TFMA) has a higher positive potential (by about +0.30 V) when compared to the first oxidation peak of polyaniline. This shift to more positive potentials may be ascribed to the -CF3 group attached to the polyaniline backbone. The -CF3 moiety is an electron-withdrawing substituent which makes the polymer more difficult to oxidize. The following structure is proposed as the conducting form of poly(TFMA) :

\/

3.2. Polymerization by controlled potential electrooxidation Elemental analysis of the purified poly(TFMA) yielded:

/ F--C--F

The results also indicate that the pK, value of the trifluoromethylanilinium salt lies between 0.6 and 1.3 since the poly(TFMA) deprotonates within this pH range. A low-angle X-ray reflectivity scan of an electropolymerized film of poly(TFMA) on In203/SnO 2 conducting glass indicates that a film with a thickness of 30 nm may be prepared after 20 scans. If the film thickness increases linearly upon each successive scan, then the rate of film thickening may be estimated as 1.5 nm/scan. The reflectivity data conclusively indicate that a film forms of appreciable thickness and that polymerization may be initiated on a conducting glass substrate and thereafter sustained on a m-trifluoromethylaniline underlayer. If an aqueous TFA solution is used as the medium for electropolymerization instead of (50:50) acetonitrile:TFMA buffer solutions, then additional anodic peaks begin to appear after about 12 scans [ 8]. The CV shows a monotonic increase in current on each successive sweep and the amber-colored film never passivates the electrode. Three oxidation peaks appear at +0.55, +0.8 and +0.9 V, while two reduction peaks appear at + 0.15 and less than - 0.3 V. The appearance of the third oxidation peak is attributed to some sort of polymer degradation at high potentials which is observable only in aqueous solutions. The resistance of the film after 32 scans and after the passage of 166 txC is 2.6 kl). If the film thickness is presumed to be 50 nm (vide supra), and the area of the electrode is 0.06 cm 2, then the specific conductivity may be estimated as 3.3 × 10 ~ S/cm. This specific conductivity is lower than that observed for polyaniline films (on the order of 1 S) [1,9].

CF3CO0_

F_3J

The specular reflectance IR spectrum for a film prepared at a Pt electrode under the same conditions, then poised at a potential of + 0.8 V, is quite distinct from that prepared under more acidic conditions. There is no appreciable peak intensity around 1680 c m - ~, and any peaks attributable to the -CF3 moiety of trifluoroacetate are missing. The IR results indicate that poly (TFMA) is not protonated in the oxidized form ( see the proposed structure below):

Anal. Found (Calc. for pernigraniline form of poly (TFMA) base): C, 52.55 (52.83); H, 2.32 (2.52):. N, 7.71 (8.80); F, 34.51 (35.85)%. If the product is sublimed under vacuum at 165-168°C, a purple residue (60-70%) remains on the bottom of the flask, but about 8-15% of the total forms an orange precipitate on the cold finger. GPC analysis of the orange precipitate yields a molecular weight of 3.3 × 102 daltons; see the peak eluting at 4.79 min in Fig. 2(b). The molecular weight corresponds to that of a dimer while the elemental analysis confirms the empirical formula, CF3C6H4NH2. Anal. Found (Calc.): C, 52.53 (52.83); H, 2.49 (2.52); N, 8.75 (8.80); F, 35.79 (35.85)%. GPC analysis of the purple residue (see Fig. 2(c)) indicates a molecular weight distribution of 7% trimer (tR = 4.41

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W.E. Rudzinski et al. / Synthetic Metals 94 (1998) 193-197

to the N - H and C-N stretches, respectively, of a primary amine [ 5 ]. In the dimer and oligomer, these bands are absent. The proton NMR spectrum of the dimer (see Fig. 3(a)) consists of two pairs of multiplets: one pair further downfield at 8.1 and 8.2 ppm (integrated area of three), and a second pair at 7.6 and 7.7 ppm (integrated area of four). The imine proton absorbs at 1.5 ppm. The oligomer (see Fig. 3 ( b ) ) shows a broad absorption envelope which extends from 6.5 to 8.3 ppm with an imine proton absorption at 1.3 ppm. The following structure is proposed for the dimer:

(a) (c) (b) ,~

[Abs]

/

g

i

~r Retention

F

F--C~F

Time(min)

Fig. 2. GPC of a prepared mixture in THF of (a) 0.56 mg/ml TFMA monomer, (b) 0.36 mg/ml TFMA dimer, and (c) 0.18 mg/ml TFMA oligomer.

F ~ C

\ F

min), 13% tetramer (tR = 4.13 min), about a 43% mixture of pentamer and hexamer, and 38% higher oligomer. The lowest molecular weight observable in the residue corresponds to that of a trimer. The UV-Vis spectra of TFMA monomer, dimer and oligomer in the base form show that the monomer is transparent, the dimer has an absorbance maximum at 440 nm (absorptivity, k=2.1 × l03 cm2/g), while the oligomer has an absorbance which begins in the UV and extends well into the visible. Comparisons of the IR and NMR spectra of the monomer with the spectra of both the dimer and oligomer clearly show polymerization. The monomer has two peaks at 3403 and 3491 c m - J and a peak at 1255 c m - ~which can be attributed

TGA of the oligomer in nitrogen shows no change up to 200°C, after which there is a gradual weight loss. The results indicate that there are no waters of hydration, entrained dimer or any species which may volatilize below 200°C. In order to characterize the purple solution remaining after polymerization, the electrooxidation of TFMA was repeated using a solution of 0.5 M TFMA in (50:50) d3-acetonitrile, d-trifluoroacetic acid in D20. After 22 h at + 1.3 V, GPC analysis of the solution yielded 97% monomer.

4. Conclusions The electrochemical oxidation of m-trifluoromethylaniline (TFMA) in the presence of trifluoroacetic acid leads to the

I

(b)

I

'l./

F

(a)

t ] Z I. I, I 5.0 4.0 3.0 2.0 1.0 0.0 PPM Fig. 3. H NMR spectra in d-CHC13 of TFMA (a) dimer and (b) oligomer. Asterisks indicate d-chloroform. t

I

g

L

9.0

8.0

7.0

6.0

W.E. Rudzinski et al./Synthetic Metals 94 (1998) 193-197

preparation of poly (m-trifluoromethylaniline). The polymer has been prepared using cyclic voltammetry, and the dimer and oligomers using controlled potential electrooxidation techniques. The oxidation of TFMA using cyclic voltammetry yielded a thin film with a conductivity estimated at 10 -~ S/cm and with oxidation potentials shifted towards more positive potentials when compared with polyaniline. Controlled potential electrooxidation of the monomer in an acidic, hydroorganic solvent mixture yielded oxidized monomer in solution, with dimer, trimer and higher molecular weight oligomers precipitating out. The dimer may be separated from oligomer by vacuum sublimation, while the oligomers remain stable up to 200°C. The oligomer consists primarily of a mixture of about 40% pentamer and hexamer, and about 40% higher molecular weight n-met. The oligomer though soluble in a variety of organic solvents is not as conducting as polyaniline or polyaniline derivatives with electron-donating moieties. Both the increase in solubility and the decrease in conductivity may be attributed to the electronwithdrawing effects of the -CF3 group.

197

Acknowledgements The authors would like to thank the Office of Research and Sponsored Programs of Southwest Texas State University for support of this project.

References [1] A.G. MacDiarmid, A.J. Epstein, Faraday Discuss. Chem. Soc. 88 (1989) and Refs. therein. [2] J.-C. Lacroix, P. Garcia, J.-P. Audiere, R. Clement, O. Kahn, New J. Chem. 14 (1990) 87. [3] S.-A. Chen, H.-T. Lee, Synth. Met. 47 (1992) 233. [4] Y. Wang, M.F. Rubner, Synth. Met. 47 (1992) 255. [5] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, Wiley, New York, 5th edn., 1991. [6] W.E. Rudzinski, L. Thrower, L. Pin, Polym. Prepr. (Am. Chem. Soc.. Div. Polym. Chem.) 35 (1994) 313-314. [7] W.E. Rudzinski, L. Lozano, M. Walker, J. Electrochem. Soc. 137 (1990) 3132-3136. [ 8 ] W.E. Rudzinski, R. Sutcliffe, L. Thrower, Polym. Prepr. ( Am. Chem. Soc., Div. Polym, Chem.) 34 (1993) 449-450. [9 ] W.E. Rudzinski, M. Walker, C.P. Horwitz, N.Y. Suhu, J. Electroanal. Chem. 335 (1992) 265-279.