LMSA29858

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LMSA_A41_011

Techset Composition Ltd, Salisbury, U.K.

8/24/2004

JOURNAL OF MACROMOLECULAR SCIENCEw Part A—Pure and Applied Chemistry Vol. A41, No. 11, pp. 1285–1301, 2004

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Copolymer Formation Between Two N-Substituted Anilines—Electrochemical and Spectroelectrochemical Studies

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M. Sankarasubramanian, P. Santhosh, A. Gopalan,* and T. Vasudevan

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Department of Industrial Chemistry, Alagappa University, Karaikudi, India

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ABSTRACT

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Electrochemical polymerization of diphenylamine, DPA with N-methyl aniline, NMA was performed using cyclic voltammetry in a 4 M sulfuric acid medium. The electrochemical parameters representing the polymer deposition showed a strong dependence on the molar concentration ratios of DPA or NMA in the feed. In situ spectroelectrochemical studies were performed during the electropolymerization with different molar concentration feed ratios of DPA. The results reveal the formation of intermediates together with DPA and NMA units. Derivative cyclic voltabsorptograms (DCVAs) were deduced at the wavelength of absorbance corresponding to the intermediates and explained with redox characteristics in cyclic voltammogram. Results from cyclic voltammetry and spectroelectrochemical studies favor copolymer formation between DPA and NMA. Copolymers were prepared for different molar concentrations feed ratios of DPA and the composition of the monomer units in the copolymers were determined. Reactivity ratios of DPA and NMA were deduced using Fineman–Ross and Kelen–Tudos methods and correlated with the results from cyclic voltammetry and spectroelectrochemical studies.

35 36 37 38

Key Words: Electrocopolymerization; Cyclic voltammetry; In situ spectroelectrochemical studies; UV-VIS spectroscopy; Copolymerization; Copolymer composition; Reactivity ratios.

39 40 41 42 43

*Correspondence: A. Gopalan, Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, India; E-mail: [email protected].

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DOI: 10.1081/MA-200029858 Copyright # 2004 by Marcel Dekker, Inc.

1060-1325 (Print); 1520-5738 (Online) www.dekker.com

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INTRODUCTION

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94

Polyaniline (PANI) is one of the interesting conducting polymers due to its environmental stability, processablity, electrical, optical, and electrochemical properties,[1] electrochromism,[2] ease of doping,[3] and preparation. In order to make PANI a more technologically viable conducting polymer, attention is focused on improving its processability. Polymerization of ring[4,5] or N-substituted[6] aniline derivatives and copolymerization of aniline with aniline derivatives have been proved effective in this respect. Polymers of alkyl or aryl substituted anilines are soluble in common organic solvents.[7] It has been reported that the mechanism of polymerization of diphenylamine (DPA) is different from the other N-substituted anilines. While polymerization N-alkyl substituted aniline has been reported to proceed through N –C coupling, polymerization of DPA is known to proceed through the 4,40 -phenyl – phenyl coupling mechanism.[8] FT-IR spectra of poly(diphenylamine) (PDPA) revealed the presence of benzidine band around 1607 –1610 cm21[9] and supported C –C coupling in the structure of PDPA. Due to the benzidine type of reaction during polymerization, an extraordinarily high rate has been reported for the polymerization of DPA. Many properties of PDPA, which include electrochemistry, conductivity, and electrochromism,[8] are found to be different from PANI and are not comparable with any of the other N-substituted aniline derivatives. PDPA exhibit properties of both poly( p-phenylene) and PANI.[10] Interestingly, on polymerization of DPA, the formed PDPA is expected to posses both poly(phenylene) and poly(aniline) type structures in the backbone. The formation of such a structure is expected to arise from the intermediates of different origin than reported in polymerization of simple aniline/aniline derivatives. Poly(N-methyl aniline) (PNMA) an alkyl N-substituted PANI derivative, is one of the PANI derivatives which has received attention.[11 – 13] UV-VIS spectroelectrochemistry has been used for following early stages of electro oxidation of N-methyl aniline (NMA)[14] at transparent indium tin oxide (ITO) coated glass electrodes. It is frequently noted that reactive intermediates are formed in the initial stages of the electro-oxidation of aniline or aniline derivatives. Subsequently, these reactive species react with solution species, mostly neutral monomers and generate oligomers/polymers. Conventional techniques are difficult to detect these intermediate species as the intermediates are short lived and reactive in nature. In situ UV-VIS spectroelectrochemistry has been proved to be a powerful tool for the study of the early stages of the electropolymerization process for the identifying such intermediates.[15] In the present study, electrochemical copolymerization studies were performed involving two N-substituted anilines, DPA and NMA. While DPA is expected to generate diphenyl benzidine (DPB) type intermediates,[16] NMA is known to undergo polymerization through anilinium type cation radical.[17] A systematic in situ UV-VIS spectroelectrochemical study would therefore provide information on the changes in spectral characteristics corresponding to probable formation of new intermediates while performing polymerization with DPA and NMA. Additionally, the characterization of copolymers prepared with different molar feed compositions of DPA or NMA, would confirm such a possible copolymerization mechanism.

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LMSA29858

LMSA_A41_011

Techset Composition Ltd, Salisbury, U.K.

Copolymer Formation Between Two N-Substituted Anilines

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EXPERIMENTAL

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Chemicals

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DPA, NMA, and H2SO4 were purchased from Merck, Darmstadt, Germany and used as received. Solutions of DPA and NMA were prepared in aqueous 4 M H2SO4.

101 102

Electrochemical Copolymerization

103 104 105 106 107 108 109 110 111 112

For the electrochemical copolymerization studies, a mixture of DPA and NMA was used in 4 M H2SO4. Experiments were done for different molar feed ratios of DPA (ratio of molar concentration of DPA to total molar concentration of both DPA and NMA). Electropolymerization was achieved by sweeping the potential in the range 0.0 –0.80 V for 50 cycles while keeping the sweep rate as 100 mV/sec. The cyclic voltammograms (CVs) were recorded simultaneously with synthesis. Polymerization experiments involving DPA or NMA alone were also carried out in the same potential range.

113 114

Spectroelectrochemistry

115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132

Spectroelectrochemical experiments were carried out by making use of an optically transparent ITO coated glass plate (specific surface resistance 10V) as a working electrode, Ag/AgCl and platinum wire as reference and counter electrodes, respectively. The course of electropolymerization of DPA with NMA was followed for different polymerization conditions. Polymerization was performed in a quartz cuvette of 1 cm path length assembled as an electrochemical cell by keeping the working electrode perpendicular to the light path. Before the spectroelectrochemical experiment, the ITO electrode was degreased with acetone and rinsed with distilled water. For each experiment, a new ITO coated glass plate was used. By employing BAS 100 BW, constant potential (0.8 V) was applied. A UV-VIS spectrophotometer (Shimadzu UVPC-2401) was used for collecting the spectra continuously at different polymerization time intervals. Copolymerization studies were performed in an aqueous 4 M H2SO4 solution. Electrochemical copolymerization was also carried out by cycling the potential in the range of 0.0 – 0.8 V for a solution of monomers (DPA and NMA) in different feed ratios. UV-VIS spectra were collected while sweeping the potential. Polymerization of DPA or NMA alone was also carried out. UV-VIS spectra were collected simultaneously while performing cyclic voltammetery or constant electrolysis.

133 134 135

Synthesis and Characteristics of Copolymer

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Synthesis of Copolymers

138 139 140 141

Copolymers were prepared by keeping different molar feed compositions of DPA or NMA and performing bulk electropolymerization (0.8 V vs. Ag/AgCl) or chemical oxidative polymerization.[18]

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Copolymer Composition by UV-VIS Spectroscopy

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UV-VIS spectra of the copolymers synthesized with different molar feed compositions were recorded in dimethyl formamide (DMF) for specific concentrations. UV-VIS spectra of homopolymers, PDPA and PNMA, were also recorded in DMF for different concentrations and the molar extinction coefficients at specified wavelengths were calculated and used to obtain copolymer composition.[19,20]

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RESULTS AND DISCUSSION

153 154

Electrochemical Copolymerization/Homopolymerization

155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188

Figure 1(a – d) presents the CVs recorded for 50 cycles during polymerization of DPA with NMA using different molar feed ratios of DPA (0.375, 0.50, 0.625, and 0.875), with an anodic potential limit as 0.80 V, which is sufficient to oxidize DPA and NMA simultaneously to produce their respective cation radicals.[21,22] A close analysis of CVs recorded at any specified cycle of potential during the polymerization with different molar feed ratios of DPA reveals that redox pattern of the formed species on the electrode is strongly dependent on the feed ratio of DPA. In the first cycle, the current of anodic peak corresponding to oxidation of combined monomers (DPA and NMA) is comparatively much higher than the oxidation current of the respective individual monomers (DPA or NMA) for the specific concentration maintained in the mixture. This infers that both NMA and DPA are simultaneously oxidized at the potential window to produce the respective cation radicals, diphenylamine cation radical (DPACR), and N-methyl amine cation radical (NMACR). There can be two possibilities for further reactions. In the first, DPACR and NMACR can undergo selfpolymerization which results in the respective polymer, PDPA or PNMA. In such a case, the CVs of subsequent cycles, during polymerization of mixture of DPA and NMA should resemble the CVs of PDPA and/or PNMA. Alternatively, the reactivities of DPACR and NMACR favor cross-reaction between them, which would result a new intermediate consisting of DPA and NMA units. In such a case, the CVs of subsequent cycles during polymerization of mixture of DPA and NMA should show difference than the CVs observed for PDPA and PNMA formation. CVs recorded during electrochemical copolymerization with a mixture containing DPA and NMA showed strikingly different electrochemical characteristics in comparison with individual homopolymerization. It is interesting to note that the peak potential during the first anodic scan strongly depends on the changes in the molar feed compositions of DPA or NMA (Fig. 2). On increasing the feed concentration of DPA in the feed, the peak around 0.45 V (for the molar composition of DPA as 0.38) was positively shifted and observed at 0.75 V (for the molar composition of DPA as 0.88). In the reverse scan, two peaks were noticed at around 0.39–0.40 V and 0.56–0.61 V, respectively. These peaks are assigned for the reduction of dimeric or oligomeric products produced from the chemical reactions of the oxidized monomer species, DPACR and NMACR. The CVs of the subsequent cycles of potential during copolymerization reveal few interesting characteristics. The increasing peak current values at the redox processes

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LMSA29858

LMSA_A41_011

Techset Composition Ltd, Salisbury, U.K.

Copolymer Formation Between Two N-Substituted Anilines

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189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220

Figure 1. CVs recorded during the electrochemical copolymerization of DPA and NMA with molar feed ratio of DPA: (a) 0.375; (b) 0.50; (c) 0.625; and (d) 0.875. Total concentration of DPA and NMA is 40 mM (a– d); CVs of every 10 subsequent potential cycles are given.

221 222 223 224 225 226 227 228 229 230 231 232 233 234 235

with number of cycles infer a continuous deposition of copolymer of DPA and NMA on the surface of working electrode. The CV pattern showed striking variations upon changing the molar feed ratios of two monomers (DPA and NMA) in the copolymerization, which also favor deposition of such a copolymer. We envisage that this type of changing trend in the CV pattern would arise as a consequence of the changes in the composition of the monomer units in the deposited copolymer during polymerization. Since both DPACR and NMACR can have differences in reactivities towards DPACR, the electroactitvity of the deposited copolymer is expected to show variations with reminiscence of the proportional extent of the monomer units in the copolymer. This supposition is critically analyzed through a careful comparison of peak potentials and peak current of the redox processes representing the copolymer. Furthermore, this was supported with the determination of molar composition of DPA and NMA in the copolymer, reactivity ratios of DPA and NMA and in situ spectroelectrochemical studies.

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236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263

Figure 2. CVs of the first potential cycle recorded during the electrochemical copolymerization of DPA and NMA with molar feed ratio of DPA: (a) 0.375; (b) 0.50; (c) 0.625; and (d) 0.875.

264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282

Two anodic peaks are easily observable at 0.39 –0.55 V with complementary cathodic peaks at 0.33 –0.49 V, respectively for the copolymerization with molar feed ratios of DPA as 0.375 and 0.50, [Fig. 1(a) and (b)]. The peak potentials observed here are totally different from those of homopolymerization of DPA/NMA and hence, would represent the oxidation of a new material, the copolymer, which could be deposited on the electrode surface. For the polymerization of DPA, the anodic processes have been reported to occur at 0.65– 0.74 V with cathodic counter parts at 0.57 – 0.69 V, respectively.[21] The two anodic peaks observed during polymerization of DPA are assigned for the generation of N, N0 diphenyl benzidine type radical (DPB.þ, polaronic form of PDPA structure) and N, N0 diphenyl benzidine dication (DPB2þ, bipolaronic form of PDPA), respectively. For the copolymerization, with mixture of DPA and NMA, the oxidation peak at the lower potential (0.39 V) would then correspond to cation radical originated from NMA units in the copolymer, whilst, the oxidation peak at higher potential (0.55 V) would correspond to the generation of N,N0 - diphenylbenzidine radical from DPA units. For the copolymerization with a molar feed ratio of DPA as 0.625 and 0.875, the two redox couples become identifiable at 0.59– 0.69 V during the initial scan in the CVs [Fig. 1 (c) and (d)]. On subsequent cycling, the two peaks merged into single peak and observed at

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LMSA_A41_011

Techset Composition Ltd, Salisbury, U.K.

Copolymer Formation Between Two N-Substituted Anilines 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297

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a higher potential, which indicates the incorporation of more number of DPA units into the copolymer at the later stages of polymerization. The peak current values for all of the redox processes were found to increase progressively with a number of cycles, while carrying out polymerization with increasing feed ratios of DPA. Similar observations have been noticed for the electropolymerization of aniline with substituted anilines[9,23] and attributed to the continuous build up of electroactive polymer film on the surface of the working electrode during electropolymerization. In fact, a green colored deposit could be seen on the surface of the working electrode. Hence, it becomes obvious that the copolymer, poly(DPA-co-NMA), was deposited continuously on the electrode surface. The presence of DPA and NMA units together in the deposited copolymer, poly(DPA-co-NMA), makes changes in the redox processes. As the molar feed composition of DPA or NMA changes, the copolymer would have different proportions of DPA or NMA units and display a changing CV pattern during copolymerization. The CVs of copolymer film reflect this changing trend.

298 299 300

Electrochemical Behavior of Copolymer Films

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Figure 3 represents the CVs of the copolymer films deposited with different molar feed ratios of DPA recorded in 4 M H2SO4. The CV’s have different redox characteristics, which can be attributed to the differences in the proportion of monomer units in the copolymer. Nearly, more than 95% of the charge utilized for deposition of the copolymer was retained in the copolymer film. This informs the absence of co-deposition of oligomers and stable nature of copolymer film on the surface of working electrode.

308 309 310

In Situ Spectroelectrochemical Studies

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In situ UV-VIS spectra were followed during the course of polymerization. Studies were specifically performed under different molar feed compositions of DPA to obtain evidence for copolymer formation. On close comparison of the spectroelectrochemical results of the polymerization of mixture of DPA and NMA with different feed ratios of DPA and polymerization of DPA (or) NMA, clear differences in spectral characteristics between them could be noticed. These differences can therefore be expected due to the formation of copolymer having various proportions of DPA and NMA units. It is inferred that UV-VIS spectra recorded during polymerization of DPA with NMA were strongly dependent on the molar composition of DPA or NMA. On switching the potential to 0.8 V of the ITO electrode for the mixture having 0.7 [Fig. 4(a)] as the molar feed composition of DPA, two peaks at 450 and 570 nm with a broad band around 700 nm were observed. On the contrary, for the mixture in which the molar feed ratio of DPA was 0.3 [Fig 4 (b)], two peaks at 490 and 575 nm and a broad band at 790 nm were observed. For electropolymerization having the molar feed ratio of DPA as 0.5 [Fig. 4 (c)], three absorption peaks around 450, 590, and 700 nm have been observed. These patterns of electronic bands are quite different from the ones noticed for the polymerization of DPA or NMA. During electropolymerization of NMA [Fig. 4(d)], only

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Figure 3. CVs of the films of poly(DPA-co-NMA) in 4 M H2SO4. Copolymer films were deposited for different molar feed ratios of DPA: (a) 0.125; (b) 0.375; (c) 0.50, (d) 0.625, and (e) 0.75 .

359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376

two absorption peaks at 450 and 720 nm were observed. Based on the assignments made for the polymerization of aniline derivatives, a peak observed for NMA polymerization at 450 and 720 nm can be assigned for the polaronic and bipolaronic transitions of aniline units.[22] On electropolymerization of DPA, three peaks around 430, 500, and at 600 nm have been noticed,[16] which represent the formation of aniline cation radical (or) oxidized benzidine dimer, generation of diphenyl benzidine type oligomer cation radical (DPB.þ), and N,N0 diphenyl benzidine type dication (DPB2þ) of the oligomer, respectively. Knowing spectral bands for the individual polymerization of DPA and NMA, the assignment of spectral bands for the copolymerization of DPA with NMA was made. The prominent peak observed around 570 nm during the copolymerization [Fig. 4(a–c)] was not witnessed during the polymerization of NMA or DPA alone. Hence, the peak around 570 nm can be attributed to the formation of N,N0 -diphenylbenzidine type cation radical (DPB.þ) of the oligomer or polymer formed during copolymerization. It is known from the polymerization of DPA[9,21] that polymerization occurs through para-diphenylene units as a result of a 4-40 -C–C-phenyl–phenyl coupling mechanism which is in contrast to the PANI formation from aniline.

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Techset Composition Ltd, Salisbury, U.K.

Copolymer Formation Between Two N-Substituted Anilines

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377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409

Figure 4. UV-VIS spectra recorded during constant potential electropolymerization of DPA with NMA for different molar feed ratios. Potential ¼ 0.80 V vs. Ag/AgCl; Molar feed ratio of DPA: (a) 0.7; (b) 0.3; and (c) 0.5; and (d) homopolymerization of NMA; [NMA] ¼ 20 mM.

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In the present case, as a result of 4-40 -C– C-phenyl – phenyl coupling during polymerization of DPA, DPB.þ type intermediate having both DPA and NMA units is expected to be generated with an absorbance band around 570 nm (Sch. 1). The presence of phenyl substitution at nitrogen atom can hinder the – C –N-coupling. The presence of a peak around 450 nm can be attributed to the formation of anilinum type cation radical in such an intermediate/polymer. This may be preferably formed from NMA units in oligomer/copolymer. NMA, an N-substituted aniline derivative favors the – C –N coupling[22] (Sch. 1). Furthermore, analysis was made by monitoring the absorbance changes at the peak positions of aniline type cation radical (450 nm) and diphenylamine benzidine type cation radical (DPB.þ) (570 nm). After a period of applying potential (0.8 V) for the poly merization of DPA with NMA, the applied potential 0.8 V was switched off and spectra

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424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470

Scheme 1. Mechanism of electropolymerization of DPA with NMA.

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were collected continuously for a further period of time. Figure 5 shows that the absorbance at 450 and 570 nm, (the absorbance at the peaks representing the aniline type cation radical and DPB.þ) decrease on switching off the potential [Fig. 5(a) and (b)]. This trend confirms that the peaks are corresponding to the intermediates. The formed intermediates can then undergo subsequent reaction with the neutral DPA or NMA

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Figure 5. (a) Changes in the absorbance at 450 nm corresponding to the anilinium cation radical during and after interruption of electrolysis; (b) changes in the absorbance at 570 nm corresponding to the DPB type cation radical during and after interruption of electrolysis: potential ¼ 0.8 V vs. Ag/ AgCl; molar Feed ratio of DPA: (i) 0.2; (ii) 0.5; (iii) 0.7; and (iv) 0.8.

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molecule to result in an oligomer or polymer. Also, this can be taken as evidence for the probable cross reaction between the intermediates cation radical generated from NMA and DPA with NMA or DPA, respectively. Moreover, the rate of decrease in absorbance with time (a rate of decay of intermediate cation radical) was found to depend on the molar feed ratio of DPA or NMA maintained in the polymerization. Hence, the extent of incorporation of DPA or NMA units in the copolymer is expected to depend on the molar feed composition of DPA or NMA. These results clearly predict the nature of the intermediates formed during the copolymerization and hence, the composition of the two monomers in the copolymer depends on the composition of the monomers used for the polymerization. In order to authenticate such a formation of intermediate during polymerization, spectra were collected while cycling the potential. UV-VIS spectra were recorded continuously during the potential cycling in the range of 0.0– 0.8 V at a scan rate of 1 mV/sec for a mixture of DPA and NMA. The dependence of absorbance on electrode potentials at specific wavelengths (570 and 450 nm), under which the intermediate cation radicals are expected to be generated was deduced by establishing the derivative cyclic voltabsorptogram (DCVA) (Fig. 6). A correlation could be seen between the peak noticed in CV and the peaks in DCVA. The DCVA deduced at 570 nm [Fig. 6(a)] has a peak around 580 mV that coincides with the oxidation peak observed in CV [Fig. 6(a)]. Otherwise, the species corresponding to the absorption band around 580 nm is generated around 550 mV, as noticed from CV. But, in the case of oxidative polymerization of DPA[16] or NMA [Fig. 4(d)], there is no such band around 570 nm. Hence, the intermediates generated with the mixture of DPA and NMA is totally different from the intermediate formed during DPA or NMA polymerization. The band around 570 nm is, therefore, assigned for the oligomeric cation radical/polymer generated from the cross reaction of aniline type of cation radical in NMA with DPA of DPB type cation radical (Sch. 1). Also, the oligomeric intermediate cation radical get reduced at around 380 mV [Fig. 4(a)]. The DCVA derived at the wavelength of 430 nm [Fig. 6(b)] shows an anodic peak around 700 mV, and this can be assigned for the generation of aniline type cation radical. The cyclic spectrovoltammery results, hence, predict copolymer formation during electropolymerization of DPA with NMA. Furthermore, the compositions of the two monomers in the copolymer have been determined by UV-VIS spectroscopy based on the method developed by Ramelow and Baysal.[19] UV-VIS spectra were collected for copolymer samples prepared with different molar feed ratios of DPA and NMA. By taking spectra for PDPA and PNMA for different concentrations, the specific extinction co-efficients, 11 and 12 were determined at 341 and 366 nm, respectively. From the spectra recorded for the copolymer samples synthesized with different molar concentrations of DPA or NMA, the molar extinction co-efficient, 112, was determined for the copolymer (Table 1). The composition of DPA or NMA units in the copolymer was determined[20] (Table 1). Clearly, the composition of DPA or NMA in the copolymer varied with the feed composition of two monomers (Fig. 7) employed in the polymerization. The composition of DPA or NMA in the copolymer determined for various feed compositions of DPA or NMA were used to evaluate the reactivity ratios of DPA and NMA. Reactivity ratios were determined by using the Kelen – Tudos[24] and Fineman – Ross[25] methods [Fig. 8(b) and (a)] as 0.315 and 0.441 and 0.363 and 0.423 for DPA and

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565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611

Figure 6. DCVA at different wavelengths derived from cyclic spectroelectrochemical results on copolymerization of DPA with NMA (first cycle) [DPA] ¼ 8 mM; [NMA] ¼ 2 mM; wavelength: (a) 570 nm and (b) 450 nm.

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Table 1. Molar composition of DPA or NMA units in poly(DPA-co-NMA) by UV-VIS spectroscopy.

614 615

Molar composition of the copolymer

Molar concentration

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DPA(mM)

NMA (mM)

Concentration of copolymer 1022 (g/L)

Average 1
1022 L/g

DPA

NMA

35 25 20 15 5

6, 3 6, 3 6, 3 6, 3 6, 3

0.32 0.29 0.18 0.15 0.10

0.11 0.24 0.46 0.74 0.90

0.89 0.76 0.54 0.26 0.09

619 620 621 622 623 624 625 626

5 15 20 25 35

Note: 1PDPA ¼ 0.316
102 L/g; lmax(PDPA) ¼ 341 nm; (PNMA) ¼ 366 nm; lmax Poly(DPA-co-NMA) ¼ 359 nm.

1PNMA ¼ 0.077
102L/g;

lmax

627 628 629 630 631 632 633 634 635 636 637 638 639

NMA, respectively. The approximate values determined in these two approaches were close to each other. From the reactivity ratio values, it is expected that the change in the feed composition of the monomers could necessarily affect the compositions of the monomer in the copolymer. The spectroelectrochemical results reported in the present study are in agreement with this. In the earlier studies,[16,26] the electrochemical and spectral behavior of the copolymers formed from aniline derivatives showed a strong resemblance to the polymer, which was expected to be produced from a highly reactive monomer in the mixture. In the previous reports on copolymer formation between DPA with 2-amino diphenylamine[26] and DPA with anthranilic acid,[21] the CVs of copolymer were found to display redox characteristics close to PDPA, and this is attributed to the much higher reactivity of DPA in

640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658

Figure 7. Plot of composition of NMA units in the copolymer vs. molar feed composition of NMA.

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659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687

Figure 8.

(a) Fineman – Ross and (b) Kelen – Tudos plots for poly(DPA-co-NMA).

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comparison to the other monomer. In the present study, the values of reactivity ratios of DPA and NMA, suggest that the copolymer would have proportions of DPA and NMA depending on the feed composition. The observed results from cyclic voltammetry and spectroelectrochemical studies are the consequence of reactivity of DPA or NMA towards their cation radical (DPACR and NMACR) during electrochemical polymerization.

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CONCLUSION

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The results from electrochemical and UV-VIS spectroelectrochemical studies on the copolymerization of DPA with NMA show that the electrochemical and spectral characteristics are dependent on the molar feed compositions of DPA or NMA. The presence of a band around 570 nm during electropolymerization of a mixture of DPA and NMA indicates the formation of an intermediate generated as a result of cross-reaction between radicals from DPA and NMA with neutral DPA or NMA. DCVA deduced at

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the wavelength at 450 and 570 nm corresponding to intermediates correlate with the information from cyclic voltammetry. The differences in reactivity ratios computed by Kelen – Tudos and Fineman – Ross methods (DPA: 0.315, NMA: 0.441) inform that the variations in the UV-VIS spectral and electrochemical characteristics during electropolymerization could arise from the differences in reactivities of DPA and NMA towards their radicals.

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ACKNOWLEDGMENT

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The authors gratefully acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi, India (SP/S1-H12/97).

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Techset Composition Ltd, Salisbury, U.K.

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Received February 2004 Accepted May 2004