Indian Journal of Chemistry Vol. 43A. March 2004, pp. 487-493

Poly (o-toluidine-co-o-chloroaniline) copolymers: Effect of polymerization conditions on the properties of the copolymers

P Savitha & D N Sathyanarayana*

Department of Inorgan ic and Physical Chemistry, Indian Institute of Science. Bangalore 560012. India

Email: dns@ ipc.iisc .ernet.in

Received 30 December 2003

Fully substituted polyaniline derivatives are sy nthesized by chemical copolymerization of a-toluidine with o-ch loroaniline by three different methods to note the effect of synthesis conditions. The structures of the copolymers have been determined by IR, Raman, NMR and their conductivity data. The copolymer compositions have been determined by proton NMR spectroscopy. The copolymers are completely so luble in DMSO and their conductivity is generally hi gher than that of the hornopolymers. poly(o-toluidine) and poly(o-ch loroaniline) .

In the past few years, aromatic conducting polymers

such as polyaniline, polypyrrole, polythiophene, etc have attracted much attention. Among these polymers ,

polyaniline (PANI) has been described as the

'Conducting Polymer for Technology' 1

• It is easily

synthesized and exhibits good environmental and

thermal stability. As a result, polyaniline and its

derivatives can be used as active e lectrode materials

in microelectronics and electrochromic display devices~. However, app lications of polyaniline are limited by poor solubility in common organic solvents.

To increase the processibility of the polymer, the use

of substituted polyanilines is attractive.

Many properties of the ring and N-substituted

polyanilines differ from those of the parent polyaniline' . For example, substituted polyanilines are

considerably more soluble than polyaniline.

Substituted PANI can be synthesized by modifying the polymer chain in the following ways: (i) the post-substitution of the parent PANJ, (ii) the chemical and

electrochemical homopolymerization of ani line derivatives, and, (iii) copolymerization of aniline with

ring and N-substituted derivatives.

Studies on the synthesis and characterization of

the homopolymer, poly(toluidine) have been reported from this laborator/ 5 . Chemical poly.merization of

aniline with electronegative substituents has been little reported. Chemical synthesis of poly(a-chloroan iline}

has been achieved using only a stronger oxidizing agent

namely chromic acid in aqueous HCI solution and conductivity in the region of l0.5 -10-7 S/cm has been reported6• The aim of the present work is to synthesize

polyaniline derivatives with two different substituents

present simultaneously in the polymer chain by

chemical copolymerization of a-toluidine with a-chloroaniline. Three different methods have been tried

to note the effect of the synthesis conditions. The copolymers have been synthesized using solution ,

emulsion and inverse emulsion methods. The effect

of temperature, the so lvent used to carry out the

polymerization and the oxidant employed have been

investigated using spectroscopic, thermal and electrical

methods.

Materials and Methods a -Toluidine (Merck) was distilled twice before use.

a -Ch loroaniline (A ldri ch) and all other chemicals were

used as procured.

Preparation of the copolymers

Emulsion method

Chloroform (or toluene) (100 mL ) was taken in a

500 mL beaker and 2.25g of sodium Iaury! sulphate (emulsifier) in 50 mL of water (0.1 M) was added to it with constant stin·ing to obtain a milky white emulsion.

a-Toluidine (0.05 M, l.3 mL) and a-chloroaniline (0.05

488 INDIAN J CHEM. SEC A. MARCH 2004

M, 1.3 mL) were then added to it followed by the

dropwise addition of 50 mL of the dopant HCI (I M)

and 50 mL of an aqueous solution of the oxidant,

ammonium persulphate (5.7g. 0.1 M). The reaction

was allowed to proceed for 24 h with stirring. The

organic phase was separated and washed repeated ly

with water. It was then added to a beaker containing 600 mL of a non-solvent, namely. acetone when the

copolymer precipitated out. After 10 h, the precipitate was filtered, washed with some more acetone and dried

under vacuum for 72 h.

Inverse emulsion method

A solution of 0.2 M of the oxidant, benzoyl peroxide (12.1 g), was taken in 100 mL of the solvent

and 0.1 M of sodium Iaury I sulphate (emul sifier) in 50 mL of water was added to it with vigorous stirring to

obtain a milky white em ul sion . Then, 0.05 M of a-chloroaniline was added followed by 0 .05 M a-

toluidine. Then, 100 mL of I M HCI was added to it

dropwise and the polymerization was allowed to

proceed for 24 h. The product was precipitated

using acetone. It was co ll ec ted and purified as

mentioned above.

Solution method

In a typical experiment, a mixture of 1.3 mL of a-

toluidine (0.05 M) and 1. 3 mL of o-ch loroaniline (0.05

M) was dissolved in 100 mL of 1 M HCJ. An aqueous sol ution of O.l M ammonium persulfate (5.7 g) was added to it dropwise over a period of 20 minutes with

continuous stirring. The total volume of the reaction

mixture was kept at 250 mL. The react ion was allowed

to proceed for 24 h with stirring. The resulting dark

green precipitate was filtered, washed repeatedly with distilled water until the filtrate became colourless. It

was then washed with methanol followed by acetone.

The product was dried at room temperature for 72 h

under vacuum. The reaction also carried out at oo 0 C. The UV-visible absorption spectra were recorded

using a Hitachi U-3400 spectrophotometer for

solutions prepared in DMSO. Fr-IR spectra for solid

sa mples were obtained with a Bruker-IFS 55

instrument using the KBr pellet technique. FT-Raman

spectra were recorded with a Bruker RFS-100/S spectrometer for the powdered polymers using a Nd3+-

YAG laser having a laser power of 100 mW at the

sample. 1H NMR spectra in DMSO- d6 were obtained

using a Bruker AMX 400 MHz spectrometer with

tetramethyl silane as an internal reference.

Conductivity measurements were made using the four-

probe tech niq ue on pr :::; sed pellets obtained by

s ubj ec tin g the powder to a pressure of 50 kN. Thermogravimetric (TGA) measmements were made

using a Meltler To ledo Star System at a heating rate of l0°C per minute under tiir atmosphere. Solubility

of the copolymers in the ir salt form in DMSO was evaluated as the amount of solvent required to di ssolve

10 mg of the copolymer.

Results and Discussion Copolymers of a-toluidine and a-chloroaniline at

va rying ratios were chemically synthesized to form

poly(a-toluidine-ca-o-chloroaniline). Figure 1 shows

the general structure of the copolymers formed.

The names of the copolymers have been

abbreviated on the basis of the tem perature , the oxidant, the solvent, and the method used for their

synthesis. For example, the copolymer synthesized

employing 1: I feed ratio of the comonomers a-

toluidine (OT) and a-chloroaniline (OC) by solution

method (S) at 0°C (L) is denoted as OTOCllSL while

the copolymer synthesized at room temperature (R) is

denoted as OTOCllSR. The copolymer synthesized

by theemulsion technique employing ammonium persulphate (A) as the oxidant and chloroform as the

R Cl jj_ R Cl >li ----0-~l\0-N~~

R= -CH:J /-CI

Fig. I- Rcpe

SA VITI-lA el a/.: POLY (o-TOLUIDINE-CO-o-CHLOROANILINE) COPOLYMERS 489

Table l- Yield, conductivity. UV -visible spectra and solubility data of the copolymers

Copolymer Oxidant Solvent Yield Conductivity A. nmx (nnl) Solubility (%) (S/crn·') (g/L)

Sol/Ilion me/hod

OTOCIISL APS Water, 35 2.79 X JO·l 315 435 618 799 partiall y (OOC) soluble

OTOCIISR APS Water. 39 7.21 X 10-4 312 610 818 2.5 (270C)

Emulsion 111elhod

OTOCIIAC APS CHCI _. 36 5.10 X IO·-' 312 439 609 3.3 OTOCIIAT APS toiucne 32 1.15 x IO l 316 611 777 5.0

Inverse emulsion mellwd

OTOCIIBC BPO CHCI3

37 7.52 x 1 o-• 311 433 606 836 3.3 OTOCIIBT BPO toluene 70 1.11 x 1 o--' 315 600 785 rartially

APS- ammonium persulphate, BPO- benzoyl peroxide

solvent (C) is denoted as OTOCIIAC, whereas OTOCIIAT denotes a copolymer synthesized under similar conditions in toluene (T) as the solvent. Similarly. the copolymer OTOCIIBC was synthesized using benzoy l peroxide (B) as the oxidant in chloroform (C) med ium.

Solubi li ty of the copolymers synthesised by different methods was determined in DMSO. Most of the copolymers showed fairly good solubility in DMSO as shown in Table I except for the copo lymer OTOCllSL prepared by the solution method at 0 °C and that prepared in toluene medium by the inverse emulsion method , OTOCllBT. The fonnercopolymer has lower o-ch loroaniline content as seen from theIR and NMR studies discussed below. The insoluble nature of OTOC llBT cou ld not be understood. The copolymer OTOC llAT which has a rather high o-chloroani line content shows the highest solubility . Irrespective of the method of synthesis, the yield of the copolymers calcu lated on the basis of the monomers taken was between 32 and 37% except in one case. The yield was exceptiona ll y high fo r the copolymer synthesized in toluene medium by the inverse emulsion method.

Although homopolymerization of o-ch loroan iline requires a stronger oxidizing agent, i.e. , chromic acid in HCI medium, interestingly the copolymers of o-chloroani li ne with toluidine cou ld be prepared with

soluble

milder oxidizing agents. viz., ammonium persulphate and benzoyl peroxide.

Conductivity The conductivity of the copolymers is considerably

higher than those of the corresponding homopolymers, poly(toluidine) and po ly(chloroaniline). The conductivity of the copolymers varies over the range 2.8 X 10'2 - 8 X 10.4 s em·'. The copolymer synthesized at room temperature exhibits a lower conductivity than that synthesized at 0°C for the same comonomer feed ratio. This may be due to the larger number of defects which may occur in the copolymer synthesi zed at ambient temperature . On the other hand , the copolymers synthesized by the emulsion method show higher conductivity comparee: to those obtained by the inverse emulsion method . Interestingly benzoy l peroxide which has a lower oxidation potential supports the copolymerization of o-chloroaniline to a relatively greater degree. It results in lower conjugation and hence lower conductivity. The copolymers prepared in to luene medium exh ibit high er conductivity than those prepared in other solvents demonstrating the effect of the solvent medium on the structure of the polymer chain formed and on doping as verified from the presence of the polaron peak around 800 nm in the absorption spectra of the copolymers OTOCIIAT and OTOCIIBT di sc ussed

490 INDIAN J CHEM. SEC A. MARCH 2004

below. Some of the copolymers have conductivity

higher than that of the homopolyme r, POT7, which

exhibits a conductivity of 1.6 x 10'3 S cm·1• Poly(a-chloroaniline)6' 8 exhibits a lower conductivity of 10'5 - 10'7 s cm'1.

The electrical conductivity of the copolymers,

OTOCllSR and OTOCllBC, is lower than that of

the corresponding aniline copolymers9 or parent poly

(a-toluidine) due to diminution in 'IT-conjugation along

the chain caused by the steric effects of the -Cl and

-CH3 groups. Electron-withdrawing cr substituent in the copolymer fu1ther decreases the conductivity by

lowering the basicity of the imine units. However, the

electron donating effect of the -CHl groups increases

the imine basicity . As a consequence, a-toluidine

copolymers with moderate o-chloroaniline content

such as OTOC11AT and OTOCLlSL exhibit higher

conductivity. Sometimes they are higher than those of the copolymers of aniline with chloroaniline9.

Absorption spectra

F igure 2 shows the UV -vi s ibl e spectra of the

copolymers OTOC 11AC and OTOC 11 SL recorded in

DMSO solution as typical of copolymers. The spectral

bands for all the copolymers investigated are listed in

Table 1. The spectra consist of four major absorption

bands. The first band in the region 3 Ll-316 nm is

assigned to the n-n* transition of the benzenoid ring based on the earlier studies on pulytoi uidines

4'6'

10• It is

related to the extent of conjugation between the phenyl

rings along the polymer chain. The second absorption

,..-... Vl -s::

" .0 ... "' ., (.)

s::

"' .0 ... 0

.0

<

300

__ .. -- --- --- -·--- -- -.

·-- --...

400 500 600 700

Wavelength (nm)

(a)

------(b)

800 900

Fig. 2- Absorption spectra of (a) OTOC II AC and (b) OTOC II SL

at 600-618 nm is assigned to the "exc iton" transition

from HOMO of benzenoid to LUMO of quinoid ring.

It is sensitive to the overall oxidation state of the

polymer. Both the 430 and 800 nm absorptions are

assigned to the polaron transitions. They are observed

only when the degree of doping and solubility of the

copolymer in the salt form are considerably high since

they are weak absorptions. Most of the copolymers

exhibit a band around 800 nm. The 450 nm absorption band is observed as a very weak band and small shifts

of the neighboring peaks lead to overlapping. Thus,

the weak band near 450 nm is often not observed since

it may merge with the high intensity 3 15 nm peak or

the 600 nm peak depenciing upon its position in the

spectrum.

A noticeab le feature of :he spectra of the

copolymers is a small bathochromic shift of the 600

nm peak when compared to that of pure POT (where

it occurs at 602 nm). This shift may be due to the

lowering of the band gap of the copolymer due to the

presence of o-toluidine (with a donor -CH3 group)

alternating to o-chloroaniline (with an acceptor -CI

group) 11 as shown in Fig.l. However, the bathochromic

shift is not observed for the copolymers synthesized

by the inverse emulsion method. Since benzoyl

peroxide is a milder oxidizing agent than ammonium

persulphate, probably a higher amount of a-

chloroaniline is incorporated into the copolymer as

noted earlier, significantly decreasing the electron

density on the ring and hence copolymers with

diminution in conjugation are obtained.

The temperature at which the solution

polymerization is carried out seems to influence the

absorption spectra. The copolymer synthesized at 0 °C displays increased bathochromic shift of the

exciton band (which occurs at 602 nm in POT) probably due to smaller amount of a-c hloroaniline segments in the copolymer. The solvent used to carry out the copolymerization by emulsion and inverse

emulsion methods does not seem to hav e any

significant effect on the absorption spectra .

FT-IR spectra

In the JR spectra ofOTOC11SL and OTOC11BC as typical of copolymers, a small weak band at -3200

cm·1

in poly(a-toluidine) spectra corresponding to the

hydrogen bonded N-H vibrations is found shifted to

SAYITHA era/. : POLY (o-TOLUIDINE-CO-o-CHLOROANILINE) COPOLYMERS 491

........ 0 -0 ~ u c:: 0

.:;:: E C/l c::

~

3500 2000 1000 wavenumber ( cm-1 )

Fig. 3- Ff-IR spectra of (a) OTOC II SL and (b) OTOC II BC

3215 cm·1 in the copolymers (Fig. 3). The characteristic bands of the emeraldine form of poly(a-toluidine) at 1587, 1491 , 1381 , 1315, 1213, 1153, 1109 and 808 cm·1 occur with small shifts in the spect ra of copolymers confirming the presence of o-toluidine units in the copolymer. For example, the spectrum of OTOCllAC shows two main bands at 1589 and 1483 cm·1 assigned to the ring stretching vibrations of the

quinoid and the benzenoid rings respectively . A medium intensity peak at 1300-1325 cm· 1 is due to C-

N stretching vibration in alternate units of quinoid-benzenoid-quinoid rings, while the band at 1375-1381 cm· 1 corresponds to C=N+ stretching vibration. The 1170-1190 cm·1 band in the spectra of copolymers can be assigned to a vibrational mode of a B-NH+=Q structure, which is formed during the protonation process. It indicates the existence of positive charges in the chain and the distribution of the dihedral angle between the quinoid and benzenoid rings. It is said to increase in intensity with the increase in degree of doping in the polymer backbone12 • The bands become more intense due to enhancement of the oscillator strengths of the backbone related vibrations by coupling with the proton induced charge. A band near 1107 cm·1 is assigned to C-H in plane bending. This

n01mally infrared-inactive mode becomes active when

a protonation process induces confom1ational changes

in the polymer chain, i.e ., forming po larons or

bipolarons. The spectra of the copolymers exhibit a fairly intense absorption at 758-764 cm·

1 due to the

characteristic C-CI stretching which occurs as a strong absorption at 746 cm·1 in the monomer indicati ng the

presence of a-chloroaniline units in the copolymers. The strong C-H out of plane bending vibrations

of the 1,2,4-trisubstitued benzene rings appear around 880 and 810 cm·1 indicating that the monomers in the

copolymers are bonded head to tail in agreement wi th

the expected structures. The IR spectra of a-toluidine and o-chloroaniline copolymers show the quinoid and benzenoid absorptions at about 1580 and 1495 cm·

1

for the doped st ructure . The IR spectra of the copolymers show characteristic bands of the functional group7 -CH3 at ca. l005 cm·

1•

An intense C-CI absorption was found in the spectra of the copolymers OTOC II AC and

OTOC11BC which were synthesized in chloroform

medium using ammonium persulphate and benzoyl peroxide respectively as the oxidants. The spectra of the other copolymers and particularly the spectrum of OTOC11AT show a weak band for C-CI stretching. It clearly indicates that the amount of o-c hloroaniline copolymerized depends on the synthesis conditions.

FT-Raman spectra

The FT-Raman spectrum of the homopolymer POT exhibits intense peaks at 1610, 1498, 1361 and 1257 cm· 1• These bands are found in the o-toluidine -

o-chloroaniline copolymers with small shifts

supporting the presence of a-toluidine units in the copolymer. The band at 1610 cm·1 is attributed to the C=C stretching and the peak at 1498 cm·1 to the C=N stretching of the quinoid and benzeno:d rings respectively. The spectrum of a-chloroaniline shows strong bands at 1025, 838 , 680 anc! 563 cm-

1• The

spectra of the copolymers exhibit these characteristic peaks with small shifts indicting the presence of a-chloroaniline units in the copolymer. As noted earlier from the IR spectra, the intensity of the peaks arising from o-chloroaniline is highest when the synthesis was carried out in chloroform as the solvent medium. The spectra of both OTOCI iAC and OTOCllBC show more inte nse Raman bands arising from

492 INDIAN J CHEM. SEC A. MARCH 2004

o-chloroaniline units consistent with the FT-IR studies.

'H NMR spectra

The 1H NMR spectra consist of resonance peaks

in the region of 0 .8-1.0 ppm and 1.8-2.2 ppm corresponding to the methyl protons of the quinoid and benzenoid rings respectively of POT demonstrating the presence of o-toluidine segments in the copolymer. The resonance peaks of -NH protons along the polymer chain occur at 3.6 ppm. while the monomeric or chain end -NH2 protons are found at

5.2 ppm. The aromatic protons of o-chloroaniline occur

in the region of 6.8-7.2 ppm and those of o-toluidine occur at 7.4-8.0 ppm. For the copolymers prepared by the emulsion/inverse emulsion method, the quinoid protons are masked by the surfactant impurities. On the other hand, the copolymers prepared by the solution method show peaks du e to the monomeric -NH2 protons which are totally absent in other copolymers. This could be due to the presence of a larger olumber of low molecular weight chains which are not completely removed by washing.

The effect of synthesis conditions on the composition of the copolymers could be observed from

I the NMR spectra. H NMR spectra of the copolymers OTOCllSR, OTOCllAC and OTOCIIBC showed fairly intense aromatic signals arising from o-chloroaniline , whereas the other copolymers exhibited signal s chiefly of o-toluidine show ing that the synthesi s carried out at 0 °C as well as in to luene medium provide copolymers with low o-chloroaniline content.

T he compositions of the copolymers were semiquantitatively verified from 1H NMR spectra by comparing the intensities of the aromatic protons of o-toluidine and o-chloroaniline. The highest intensity of o-chloroaniline peaks is observed for the copolymer OTOCllB C which has an o-tol uidin e to o-chloroaniline copolymer composition ratio of 2.3: I. The copolymer OTOC II R had a -toluidine and o-chloroaniline in the ratio of 4.2: 1, whereas the composit ion rati o o f OTOC 11 E was 7:1. For OTOCllAT, OTOC!lSL and OTOCI tBT, the o-chloroaniline content was very low (less than 5%).

Thus, the spectroscop ic studi es re veal the formation of true copolymers and not a mixture of the hom0polyrners.

Thermogravimetric analysis

The TGA curves for the copolymers show a three-step weight loss; the results are summarized in Table 2. The weight loss up to -l30°C is due to the loss of moisture. The second weight loss step occurring

up to -300°C is attributed to the loss of substituent/ dopant Cr/HCI. The final step which starts around 400°C leads to the complete degradation of the copolymer chain

14• The copolymers thus have better

thermal stabil ities when compared to the homopolymer POT for which the degradation temperature5 is 335°C.

The copolymer synthesized by the solution method at 0°C was found to have a higher thermal stability. This could be due to the formation of highly ordered and strongly conjugated structures of high molecular weight at low temperatures. Among the copolymers synthesized at room temperature, OTOC I LAC has a higher stability, while the copolymer OTOCllBC is

the least stable. The lower stabi lity could be the result of the diminution in conjugation due to of the e lectron withdrawing effect of the chlorine substituent. At the same time, the presence of moderate level of o-

chloroaniline units (as in the copolymer OTOC L 1 AC) could lead to a higher degree of hydrogen bonding as well as inter chain linking due to the presence of -CI as the ring substituent and hence increase in the thermal stability.

Conclusion Electronic and ste ric effects of the aniline

substituents markedly affect the copolymerization of 2-substituted aniline derivatives namely o-toluidine and o-chloroaniline. Although homopolymerization of o-chloroaniline requires a stronger oxidising agent (chromi c acid in HCI) , the copo lymers of

Table 2-Thermogravimetric data or the copolymers

Sample Temp. range. °C (wt. loss. %)

I" step 2"" step 3'" step

OTOCIISL 50--133 ( I 0) 158-308 ( 16) 444

OTOC IISR 50--117 (3) 127-227 (9) 381

OTOCIIAC 50--138 (8) 187-300 ( 15) 413

OTOCII/\T 50--127 (7) 188-28 1 (7) 404

OTOCJ IBT 50--144 (I 0) 223-358 (2 1) 400

OTOCI IBC 50-- 129 (5) 185- 300 (18) 362

SAVITHA eta/.: POLY (o-TOLUIDINE-CO-o-CHLOROANILINE) COPOLYMERS 493

o-chloroaniline with toluidine could be readily

synthesized using milder oxidants, viz ., benzoyl

peroxide and ammonium persulphate . The

conductivity of the copolymers range from 2.8 x 10·2

- 7.2 x 10-~ S cm-1, which is higher than that of the homopolymers, poly(o-toluidine) and poly(o-chloroaniline) . The copolymer, OTOCllBC,

synthesized by the inverse emulsion method using

chloroform as the solvent has a higher o-chloroaniline

content, but its thermal stability and conductivity are

low. On the other hand, the copolymer OTOCllAC

synthesized by emulsion method also possesses higher

o-chloroani line content but exhibits higher thermal

stability and conductivity.

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