Spectrochimica Acta Part A 62 (2005) 420–430

UV–vis spectroscopy for following the kinetics of homogeneous polymerization of diphenylamine in p-toluene sulphonic acid S. Nagarajan a , P. Santhosh a , M. Sankarasubramanian a , T. Vasudevan a , A. Gopalan a, ∗ , Kwnag-Pill Lee b a

Department of Industrial Chemistry, Alagappa University, Karaikudi, 630003 Tamil Nadu, India b Department of Chemistry Education, Kyungpook National University, Daegu, Korea Received 1 December 2004; accepted 21 January 2005

Abstract Kinetics of chemical oxidative polymerization of diphenylamine (DPA) was followed in aqueous 1 M para-toluene sulphonic acid (pTSA) using potassium peroxomonosulphate (PMS) or peroxodisulphate (PDS), independently as an oxidant. The medium was found to be homogeneous and became dark green in colour during the course of polymerisation. The course of polymerization was followed by UV–vis spectroscopy. Rate of polymerization (Rp ) was determined for various conditions by following the absorbance values corresponding to poly(diphenylamine) (PDPA) for different concentrations of DPA and PMS or PDS at various time intervals of polymerization. The observed dependences of DPA, PDS or PMS on Rp were used to deduce rate equations for PDS or PMS initiated polymerization of DPA. The rate constant for the formation of poly(diphenylamine), was estimated. In situ spectroelectrochemical studies on the polymerization of DPA were also carried out on an ITO electrode in 1 M p-TSA. The results are in accordance with the intermediates suggested in chemical oxidative polymerization. © 2005 Elsevier B.V. All rights reserved. Keywords: UV–vis spectroscopy; Kinetics; Oxidative polymerization; Diphenylamine; Rate equation; Mechanisms

1. Introduction Polyaniline (PANI) has attracted a great deal of attention due to its ease of synthesis by chemical or electrochemical methods [1,2], well behaved electrochemistry [3], moderately high conductivity upon doping with simple Bronsted acids [1–5], good environmental stability [1–3], good electronic properties and electrochromic effects [6]. Despite these characteristics, the principal problems associated with practical application of PANI are its insolubility, infusibility and hence non-processability [7]. Several methods have been developed to improve the processability of PANI [8–10]. The incorporation of long and flexible substituents in the polymer backbone has been tried for the preparation of soluble polymers with improved properties [11]. Many reports have been made on the solubility of oxidized conducting form of ∗

Corresponding author. Tel.: +91 4565 228836; fax: +91 4565 225202. E-mail address: algopal [email protected] (A. Gopalan).

1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.01.010

alkyl [12,13] and alkoxy [14] ring substituted and alkyl [15] and N-aryl [16] substituted polyanilines in organic solvents. It has been reported that mechanism of polymerization of diphenylamine (DPA) is different from the other N-substituted anilines. While polymerisation, N-alkyl substituted aniline has been reported to proceed through N–C coupling, polymerization of DPA is known to proceed through the 4,4 -phenyl–phenyl coupling mechanism [17]. FT-IR spectra of poly(diphenylamine) (PDPA) revealed the presence of benzidine band around 1607–1610 cm−1 [18] 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 [17], are found to be different from PANI and are not comparable with any other N-substituted aniline derivatives. Recently [19,20], attempts have been made to incorporate newer properties into PDPA by grafting non-conducting

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polymer onto the backbone. Interestingly, PDPA has both poly(phenylene) as well as poly(aniline) type structures in the backbone [21]. The formation of such structure is expected to arise from intermediates of different origin than reported in polymerization of simple aniline/aniline derivatives. It is frequently assumed that reactive intermediates are formed in the initial stages of the electro oxidation of aniline or similar monomers. These highly reactive species subsequently react with solution species, yielding oligomers and polymers. Since the intermediate species are very reactive and leading to fast further reactions, it becomes difficult to detect them with usual techniques. Bacon and Adoms [22] also suggested that the initial stages of aniline oxidation involve the formation of a radical cation. The cation radical intermediates formed during the oxidation of aniline have been examined by experiments like rotating ring–disc electrode [23] and electrochemical thermo spray mass spectrometry [24]. Mu et al. [25] noticed the formation of radical cation intermediate in the initial stages of electrochemical polymerization of aniline. Genies and Lapkowski [26] identified the presence of short-lived intermediates during electro-oxidation of aniline by in situ spectroelectrochemistry. Wu et al. [27] reported the formation of short-lived intermediates showing peaks in the UV–vis regions for their electronic states 310 and 500 nm during electro-oxidation of diphenylamine in 4 M H2 SO4 medium by in situ spectroelectrochemistry. In the present study, we have used in situ UV–vis spectroscopic method of following the course of polymerization of diphenylamine initiated by chemical oxidative polymerization with potassium peroxomonosulphate (PMS) or potassium peroxodisulphate (PDS) as oxidant in 1 M p-toluene sulphonic acid (p-TSA) medium. The kinetics was followed systematically as a function of monomer/oxidant concentrations. For obtaining the amount of PDPA formed at any of the polymerization time, a calibration approach using the UV–vis absorbance data of PDPA was used.

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containing 1.69 g in 250 ml of 1 M p-TSA was then added drop wise to the DPA solution with stirring over a period of 1 h. A dark green precipitate was formed. The precipitate was filtered through a sintered glass crucible and washed with 1 M p-TSA till the filtrate became colourless. The p-TSA-doped PDPA was dried under vacuum for 48 h at room temperature. 2.3. UV–vis Spectroscopy UV–vis spectra of PDPA were recorded in p-TSA using Shimadzu UV–vis Spectrophotometer-2401PC model. Polymer sample, PDPA was diluted suitably with p-TSA and the UV–vis spectra were recorded. The absorbance value of PDPA with a definite concentration was used to calculate the molar extinction coefficient and further used to determine the amount of PDPA formed at any polymerization time during the course of oxidative polymerization of DPA. 2.4. Course of polymerization by UV–vis spectroscopy Chemical oxidative polymerization of DPA was carried out in 1 M p-TSA medium. PMS or PDS was used as an oxidizing agent. The course of polymerization was followed by using Shimadzu UV-2401 PC UV–vis Spectrophotometer. Kinetic studies were performed by monitoring the absorption spectra of the reaction medium at different polymerization times. All polymerization reactions were carried out in a temperature controlled quartz cuvette at 30 ◦ C. Polymerization experiments were made at different monomer and initiator concentrations. Rate of polymerization, Rp was determined as follows. The amount of PDPA formed at any specified time of polymerization was estimated from the calibration data obtained from absorbance values of known concentration of PDPA. Rp was determined as the amount of polymer formed per unit time of polymerization. 2.5. In situ spectroelectrochemical study

2. Experimental details 2.1. Chemicals Diphenylamine (E-merck), para-toluene sulphonic acid (CDH), potassium peroxomonosulphate (E-merck) potassium peroxodisulphate (E-merck) were used as received. All the reagents were prepared in p-TSA medium. An aqueous solution of p-TSA was prepared from doubly distilled water. 2.2. Preparation of PDPA PDPA was prepared by chemical oxidative polymerization using PDS or PMS as the oxidant. A typical procedure is as follows: a solution of PDPA (20 mM) prepared by dissolving 0.846 g of DPA in 250 ml of 1 M p-TSA was cooled below 273 K. A pre-cooled solution of PMS/PDS (25 mM)

In situ spectroelectrochemical studies on DPA were also made by controlled potential electrolysis of a defined concentration of DPA and recording the UV–vis spectra simulataneously. The electrolysis were carried out in the quartz cuvette of 1 cm path length by assembling as a cell with an optically transparent indium tin oxide (ITO) coated glass as working electrode (with a specific surface conductivity of 10
/
) installed perpendicular to the light path, platinum wire as counter electrode and Ag/AgCl as reference electrode. A potential of 1.0 V (versus Ag/AgCl) was applied to a DPA solution in p-TSA by using BAS 100 BW Potentiostat/Galvanostat electrochemical analyzer. Before the spectroelectrochemical experiment, the ITO electrode was degreased with acetone and rinsed with a solution of p-TSA. For each experiment, a new ITO electrode was used.

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3. Results and discussions PMS or PDS was separately used as an oxidant for following the kinetics of oxidative polymerization of diphenylamine in p-TSA. UV–vis spectra of the reaction mixture were recorded for various time intervals to follow the course of the polymerization. The following general observations were noted. The reaction medium was homogenous during the entire course of polymerization. The medium of the reaction changed to green colour during the course of polymerization. It is important to note that there was no absorption in the visible region before the addition of the oxidant. On adding the oxidant, new peaks could be seen in the visible region. After proper identification of peaks corresponding to the intermediates and polymer, absorbance values were followed for different conditions of polymerization at the peak corresponding to PDPA. The amount of PDPA formed at different time intervals during the course of polymerization was estimated by calibrating the absorbance value at the peak corresponding to PDPA. Rate of polymerization (Rp ) was determined under various conditions of polymerization. A kinetic equation was established relating the Rp with [DPA] and [initiator]. The deduced rate equation for the polymerization was used to determine the rate constant for the formation of p-TSA doped PDPA. Detailed discussions are presented below.

Fig. 1. UV–vis absorption spectra for the course of polymerization of diphenylamine in p-toluene sulphonic acid medium; [DPA] = 1.6 × 10−4 M; [PMS] = 5.0 × 10−3 M (spectra were recorded for every minute).

an increasing trend with increasing [DPA]. Otherwise, for a fixed [PMS], the absorbance of the band around 490 and 520 nm was found to depend on [DPA]. This gives a clue that the peaks around 490 and 520 nm may correspond to the intermediates formed during chemical oxidation of DPA by PMS, which subsequently reacts with DPA monomer in a bimolecular reaction to produce oligomers/polymers. Pre-

3.1. PMS initiated polymerization of DPA in p-TSA Polymerization was carried out in room temperature by keeping [PMS] = 5 × 10−3 mol l−1 and changing the [DPA] in the range from 1.6 × 10−4 to 2.4 × 10−4 mol l−1 . Fig. 1 represent the UV–vis absorption spectra collected at various time intervals during the course of polymerization of DPA for a selected condition. In a similar way, by varying the [PMS] for a fixed [DPA], polymerization experiments were carried out. A typical course of polymerization of DPA for a selected condition is given in Fig. 2. Close analysis of the UV–vis spectra reveals the peak that corresponds to the intermediates generated during the polymerization and formed PDPA. Four bands in visible region could be identified during the course of polymerization. At the initial stage of polymerization, a peak at 490 nm and a broad band around 520 nm were noticed (Fig. 1). During the later stages of polymerization, the band at 490 nm shifted to more-longer wavelength and merged with the band around 520 nm. Two other bands were prominently noted around 370 and 650 nm, at the later stages of polymerization. The bands at 490 and 520 nm showed progressive increase in absorbance with time. However, the growth of absorbance for the band around 650 nm proceeds relatively slow at the initial stages and also showed

Fig. 2. UV–vis absorption spectra for the course of polymerization of diphenylamine in p-toluene sulphonic acid medium; [DPA] = 2.8 × 10−4 M; [PMS] = 4.25 × 10−3 M (spectra were recorded for every 5 min).

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Table 1 Determination of molar extinction coefficient at 650 nm for poly(diphenylamine) Concentration of PDPA ×104 mol l−1

Absorbance (λmax = 650 nm)

Molar extinction coefficient (ε) (M−1 cm−1 )

1.6 1.8 2.0 2.2 2.4

0.064 0.086 0.092 0.105 0.117

400 477 460 480 490

vious report [27] on the constant potential electrochemical polymerization of DPA identifies the bands around 400 and 500 nm for DPA correspond to the intermediates generated during the polymerization. Recording the UV–vis spectrum of PDPA separated from the reaction medium also identified the electronic bands corresponding to PDPA. Polymerization was allowed to proceed for 24 h and the formed polymer was isolated as described in the experimental section. The dark green coloured product, poly(diphenylamine) was again dissolved in the medium and UV–vis spectrum was recorded. The bands around 370 and 650 nm, which appeared during course of polymerization were also seen in the spectrum of PDPA. Hence, the absorbance around 650 nm was used as a measure of PDPA formed. The following quantitative approach was made to estimate the amount of PDPA formed at any time of polymerization. UV–vis spectra of different known concentrations of PDPA in p-TSA were recorded and the absorbance values at 650 nm were used to calculate the molar extinction coefficient (ε650 ) at 650 nm (Table 1). The consistency of the ε650 is evident from Table 1. The average value of ε650 was used further to estimate the amount of polymer formed at any time intervals, by noting the absorbance at 650 nm.

Rp versus [DPA]2 (Fig. 5b) was drawn. This was found to be linear and passing through the origin confirming the second order dependence of Rp on [DPA]. On varying the [PMS] by keeping [DPA] as constant (Fig. 2), Rp was found to have first power dependence on [PMS]. The plots of Rp versus [PMS] (Fig. 6; plots a and b) were straight lines with negligible intercepts. These dependences of PMS and DPA on Rp were used to deduce a rate expression for the polymerization of DPA in the present study. Tzou and Gregory [28] followed the kinetics of chemical polymerization of aniline in a heterogeneous medium where polyaniline precipitated during the polymerization. A kinetic scheme involving the effects of surface and auto acceleration by polymer was proposed. For such heterogeneous conditions, the kinetics of electropolymerization of aniline was explained by including the surface effects of the formed polymer [29]. A kinetic equation has been deduced for the electrochemical deposition of PANI as: Rp = k[M] + k [M][P]

3.2. Kinetics of DPA polymerization initiated by PMS To analyze the kinetics of polymerization of DPA and obtain an expression for Rp , a systematic approach was made to establish the dependences of Rp on [PMS] and [DPA]. By changing [DPA] and keeping [PMS] as constant and vice versa, the course of DPA polymerization was followed. UV-spectra collected at a specified time of polymerization for the polymerization conditions with a fixed value of [DPA] and different values of [PMS] are presented in Fig. 3. It can be clearly seen that increasing the concentration of [PMS] increases the absorbance at 650 nm. Obviously, the amount of PDPA formed depends on [PMS]. Fig. 4(i) and (ii) represents the Rp versus time plot for the polymerization of DPA performed for the various conditions [PMS] and [DPA]. Rp showed an increasing trend with time in all the cases. Rp values were used to obtain the dependence of DPA. The plot of log Rp versus log [DPA] was found to be linear with a slope value of two (Fig. 5a). This informs the second order dependence of Rp on [DPA]. For verifying this, the plot of

Fig. 3. UV–vis spectra recorded during the polymerization of DPA in 1 M p-toluene sulphonic acid medium (45th min); [DPA] = 2.8 × 10−4 M (a–d); [PMS] = 1.875 × 10−3 M (a), 2.5 × 10−3 M (b), 3.75 × 10−3 M (c) and 4.25 × 10−3 M (d).

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Fig. 4. Effect of time on Rp for the polymerization of DPA in 1 M p-toluene sulphonic acid (i) for the concentration variations of DPA; [PMS] = 5 × 10−3 M (a–e); [DPA] = 1.6 × 10−4 M (a), 1.8 × 10−4 M (b), 2.0 × 10−4 M (c), 2.2 × 10−4 M (d) and 2.4 × 10−4 M (e); (ii) for the concentration variations of PMS [DPA] = 2.8 × 10−4 M (a–e); [PMS] = 1.25 × 10−3 M (a), 1.875 × 10−3 M (b), 2.5 × 10−3 M (c), 3.75 × 10−3 M (d) and 4.25 × 10−3 M (e).

Fig. 6. Effect of [PMS] on Rp for the polymerization of DPA in 1 M p-toluene sulphonic acid medium.

where k is the rate constant of formation of PANI on bare platinum electrode and k is the rate constant on PANI coated platinum electrode. Subsequently, Shim et al. [30] proposed a kinetic equation for polymerization of aniline including the auto acceleration effect as: Rp = k [ANI][Oxi] + k [ANI][TAS] where k and k are rate constants for PANI formation on platinum bare and PANI coated platinum electrodes, respectively. TAS is the total area of surface. The above two equations were used to explain the kinetics of PANI formation when auto acceleration by precipitated polymer and surface effect on polymerization were dominant. Our research groups have reported kinetic equations for the polymerization of ANI and o-toluidine [31–36] in the presence of added substrates, while the polymer was precipitating during course of polymerization. 3.3. Deducing the rate expression for the homogeneous polymerization of DPA

Fig. 5. Effect of [DPA] on Rp for the polymerization of DPA in 1 M p-toluene sulphonic acid medium.

In the present study, the medium of polymerization was homogeneous during the experimental conditions in which the kinetic results were obtained. Hence, we envisage that the kinetic equation may be different from the previous reports on aniline polymerization. Taking into account the observations made in the present study, a kinetic equation for Rp is deduced in the present study. The medium was homogeneous through out the polymerization, which confirmed the fact that the formed polymer was soluble in p-TSA. The observed green

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Scheme 1. PMS initiated polymerization of DPA.

colouration during course of polymerization and the green colour of the isolated polymer justified the solubility of the formed polymer in the medium of polymerization. Hence, considering the fact that the plots of Rp versus [DPA]2 (Fig. 5b) and Rp versus [PMS] (Fig. 6b) are linear and passing through the origin and homogeneous conditions for polymerization, the following kinetic equation is proposed: Rp = k1 [DPA]2 [PMS]

(1)

This obviously means that for the polymerization of DPA, a mechanism as similar to PANI would be operative with negligible contribution from auto acceleration by the formed polymer. However, a second order dependence of [DPA] on Rp was found here in contrast to first power dependence on ANI polymerization. A different initiation path for the polymerization of DPA would therefore be probable. The direct interaction of PMS with DPA is therefore expected to be the initiation step to produce the intermediate cation radical (Scheme 1). The rate expression to describe the chemical oxidative polymerization is now described through conversion of

monomer into polymer: −d[DPA] = k1 [DPA]2 [PMS] dt

(2)

where [DPA] and [PMS] are concentrations of DPA and PMS at any time intervals. The variation of [DPA] with time is then defined by: [DPA] = [DPA]0 − n[P]

(3)

where n is the number of monomeric units in the polymer molecule, [DPA]0 the initial concentration and [P] is the concentration of polymer formed at any time of polymerization. Assuming that 2.5 electrons are necessary to oxidize one molecule of monomer [28] and 1.25 mol of PMS are consumed in the oxidation of 1 mol of DPA, makes to write at any time ‘t’ as: [PMS] = [PMS]0 −

[DPA]0 − [DPA] 1.25

where [PMS]0 is the initial concentration of PMS.

(4)

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Substituting equation (3) in (4) and rearranging: [PMS] = [PMS]0 − 0.8n[P]

(5)

Substituting equations (3) and (5) in equation (2), assuming d[DPA]/dt = 0 and rearranging d[P] = Rp = k1 [DPA]20 ([PMS]0 − 0.8np) dt + k1 n2 p2 ([PMS]0 − 0.8np) +2k1 np[DPA]0 (0.8np − [PMS]0 )

(6)

Since the kinetics was studied by following absorbance of the polymer, equation (5) can be conveniently converted using the experimental parameter, namely, absorbance (A) at 650 nm. A = ε650 nm b[P];

Since b = 1 cm,

[P] =

A ε650 nm (7) Fig. 7. UV–vis absorption spectra for the course of polymerization of diphenylamine in p-toluene sulphonic acid medium; [DPA] = 2.2 × 10−4 M; [PDS] = 5.00 × 10−3 M (spectra were recorded for every 5 min).

Substituting the terms in equation (7) into (6) makes d[A] = Rp = k1 [DPA]20 ([PMS]0 ε − 0.8nA) dt
 k 1 n2 A 2 A + [PMS]0 − 0.8n ε ε
 A +2k1 nA[DPA]0 0.8n − [PMS]0 ) ε

(8)

3.4. Evaluation of rate constant The rate constant, k1 value appearing in equation (8) was estimated through statistical analysis of dA/dt for various ‘t’ obtained at each set of [DPA] and [PMS]. The consistency of k1 values calculated for different set of conditions support the selection of rate expression for Rp . This was also verified by calculating the value of k1 from the slopes of the plots Rp versus [DPA]2 (Fig. 5b) and Rp versus [PMS] (Fig. 6b). The closeness of k1 values obtained for different conditions can also be seen here (Table 2) and confirms that equation (4) is the right choice for representing Rp for DPA polymerization. The average value for k1 for DPA polymerization is found to be 0.53 M−2 s−1 at the room temperature.

of polymerization was followed by changing the [DPA] in the range 1.60 × 10−4 to 2.40 × 10−4 mol l−1 while keeping [PDS] = 5.0 × 10−3 mol l−1 as constant. The spectra obtained during the course of polymerization are similar to PMS initiation with peaks corresponding to intermediates and polymer. Also, a consecutive type of kinetics therefore follows in which the intermediate is formed in the first step, is further consumed in the second step resulting the formation of PDPA. For a fixed time interval, increasing the [DPA] increases the optical density values at 650 nm as similar to DPA–PMS system. 3.6. Kinetics of PDS initiated DPA polymerization Rp was found to be higher in this case under identical conditions in comparison with PMS initiated polymerisation Table 2 Evaluation of rate constant from rate measurements for the polymerization of DPA initiated by PMS Variation of concentration

Condition

k1 (M−2 s−1 )

[DPA] ×104 mol l−1

1.6 1.8 2.0 2.2 2.4

[PMS] = 5 × 10−3 M l−1

0.53a

[PMS] ×10−3 M l−1

1.250 1.875 2.500 3.750 4.250

[DPA] = 2.8 × 104 mol l−1

0.59b

3.5. Polymerization of DPA initiated by PDS in p-TSA Figs. 7 and 8 represent the spectra for the typical course of polymerization by varying DPA and PDS concentrations for few selected conditions. With this oxidant also, the intermediates seemed to transform into product. This was evident from the peak at 650 nm with a similar response to time as noticed with PMS as initiator. To obtain the rate expression for DPA polymerization initiated by PDS, the dependences of Rp on [PDS] and [DPA] were determined. The course

a b

From the slope of the plot, Rp vs. [DPA]2 (Fig. 5). From the slope of the plot, Rp vs. [PMS] (Fig. 6).

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Scheme 2. PDS initiated polymerization of DPA.

(Fig. 9 (i) and (ii)). The plot of log Rp versus log [DPA] was found to be linear with a slope of value one (Fig. 10a). This indicates that a first power dependence of Rp on [DPA]. The plot of Rp versus [DPA] (Fig. 10b) was drawn. This was found

to be linear and passing through the origin confirming the first power dependence of Rp on [DPA]. Similarly, in another set of experiment for the variation of [PDS] with fixed value of [DPA], a first power dependence of [PDS] was noticed. The plot of Rp versus [PDS] (Fig. 11b) was found to be linear with negligible intercepts. The dependences of PDS and DPA for Rp are used now to obtain the rate expression for the DPA polymerization. The observed first power dependence of Rp on DPA suggest that initiation of polymerization proceeds in the case as similar to ANI polymerization [31–36] (Scheme 2). Rp = k2 [DPA][PDS]

(9)

With the usual assumption as mentioned for PDS system: [PDS] = [PDS]0 −

[DPA]0 − [DPA] 1.25

(10)

where [PDS]0 is the initial concentration of PDS. Substituting equation (3) in (10) and rearranging: [PDS] = [PDS]0 − 0.8n[P] Fig. 8. UV–vis absorption spectra for the course of polymerization of diphenylamine in p-toluene sulphonic acid medium; [DPA] = 2.8 × 10−4 M; [PDS] = 1.25 × 10−3 M (spectra were recorded for every 5 min).

(11)

Substituting equations (3) and (11) in equation (9), assuming d[DPA]/dt = 0 and rearranging.

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Fig. 9. Effect of time on Rp for the polymerization of DPA in 1 M p-toluene sulphonic acid (i) concentration variations of DPA; [PDS] = 5 × 10−3 M (a–e); [DPA] = 1.6 × 10−4 M (a), 1.8 × 10−4 M (b), 2.0 × 10−4 M (c), 2.2 × 10−4 M (d) and 2.4 × 10−4 M (e). (ii) Concentration variations of PDS [DPA] = 2.8 × 10−4 M (a–e); [PDS] = 1.25 × 10−3 M (a), 1.875 × 10−3 M (b), 2.5 × 10−3 M (c), 3.75 × 10−3 M (d) and 4.25 × 10−3 M (e).

Fig. 11. Effect of [PDS] on Rp for the polymerization of DPA in 1 M ptoluene sulphonic acid medium.

d[P] = Rp = 0.8k2 [P]2 − 0.8k2 [DPA]0 [P] − k2 [PDS]0 [P] dt k2 [PDS]0 [DPA]0 + (12) n Equation (12) can also be conveniently converted as d[A] 0.8k2 A2 = − 0.8k2 [DPA]0 A − k2 [PDS]0 A dt ε k2 ε[PDS]0 [DPA]0 + n

(13)

3.7. Evaluation of rate constant for PDS initiated polymerization of DPA Now, the rate constant, k2 value appearing in equation (13) was calculated through by employing the conditions of different [DPA] and [PDA] for various time intervals (Table 3). The calculated k2 values agree each other for different set of conditions and support the selection of rate expression (equation (9)) for Rp . The average value for k2 for polymerization of DPA is found to be 0.83 × 10−3 M−1 s−1 . 3.8. In situ spectroelectochemical studies on polymerization of DPA

Fig. 10. Effect of [DPA] on Rp for the polymerization of DPA in 1 M ptoluene sulphonic acid medium.

In situ spectroelectrochemical studies were made to follow the early stages of DPA polymerization in p-TSA. The aim is to obtain the information concerning the early stages of the electropolymerization of DPA. By performing electropolymerization on ITO coated glass plate, it was possible

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Table 3 Evaluation of rate constant from rate measurements for the polymerization of DPA initiated by PDS Variation of concentration

Condition

k1 M−1 s−1

[DPA] ×104 mol l−1

1.6 1.8 2.0 2.2 2.4

[PDS] = 5 × 10−3 M l−1

0.75a

[PDS] ×10−3 M l−1

1.250 1.875 2.500 3.750 4.250

[DPA] = 2.8 × 104 mol l−1

0.83b

a b

From the slope of the plot, Rp vs. [DPA] (Fig. 10). From the slope of the plot, Rp vs. [PDS] (Fig. 11).

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to identify the intermediates and compare with chemical oxidative polymerization of DPA. The potential was kept constant at 1.0 V versus Ag/AgCl for a solution of DPA at different concentrations, 2, 5 and 7 mM. Fig. 12a–c represents UV–vis spectra recorded during the electropolymerization of DPA in p-TSA (for [DPA] of 2, 5 and 7 mM, respectively). After switching the potential, three absorbance bands started appearing at 320, 460 and 620 nm. The peak at 320 nm was due to ␲–␲* transition of the phenyl rings. The band around 460 nm is assigned for the intermediate formed during the polymerization of DPA (Fig. 12a–c). The time versus absorption (not shown) curves shows similarity with the chemical oxidative polymerization and indicated a similar type of consecutive kinetics for the electrochemical polymerization.

Fig. 12. (a–c) UV–vis spectra recorded during the electropolymerization of DPA in p-toluenesulphonic acid medium [DPA] = 2 mM (a), 5 mM (b) and 7 mM (c), potential = 1.0 V vs. Ag/AgCl.

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4. Conclusions A smooth kinetics was noticed for the polymerization of diphenylamine as the medium, p-toluene sulphonic acid, provides solubility to poly(diphenyl amine). For the polymerization with PMS as initiator, rate of polymerization showed second power dependence on DPA and first power dependence on PMS. This is in contrast to the first order dependence of rate of polymerization on DPA with PDS as initiator. Mechanisms for the polymerization of DPA were proposed with different initiation paths. Kinetic expressions were deduced for the rate of polymerization and used to evaluate the rate constants by correlating the absorbance corresponding to the polymer and experimental conditions. Results from the in situ spectroelectrochemcial studies on polymerization with PDS or PMS are also in accordance with consecutive-type kinetics for the oxidative polymerization of DPA using peroxosalt as an initiator.

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