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Polymer 52 (2011) 5795e5802

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Polymer

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Tailoring of chiroptical properties of substituted polyanilines by controllingsteric hindrance

Muhammad Naveed Anjum a, Lihua Zhu a,*, Zhihong Luo a, Jingchun Yan a, Heqing Tang b,*

aCollege of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR ChinabKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education, College of Chemistry and Material Science,South-Central University for Nationalities, Wuhan 430074, PR China

a r t i c l e i n f o

Article history:Received 25 June 2011Received in revised form14 October 2011Accepted 20 October 2011Available online 25 October 2011

Keywords:ChiralityPolyaniline nanofibersSteric hindrance

* Corresponding authors. Tel.: þ86 27 87543432; faE-mail addresses: lhzhu63@yahoo.com.cn (L. Zh

(H. Tang).

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.10.038

a b s t r a c t

Optically active polyorthoanisidine, polyorthotoluidine, polyorthoethylaniline and polyorthochloroani-line were synthesized with chemical polymerization of corresponding monomers in aqueous medium byusing D- or L-camphorsulfonic acid (D- or L-CSA) as chiral dopant, ammonium persulfate as oxidant, anddiaminodiphenylamine as initiator. By circular dichroism spectroscopic measurements, it was found thatPANI exhibited generally a reversed chirality in comparison with the used chiral dopant, but thesubstituted PANIs had the same one as the chiral dopant. This revealed that the substituent at orthoposition caused helical inversion of conformation in comparison with the parent PANI. Such effect wasfurther confirmed by the influence of the copolymerization of aniline and its derivatives on the chiralityof the copolymers. The effect of the substituent on the chirality of the copolymers was increased with theincrease of the steric hindrance of the ortho substituent. A mechanism was proposed to explain theeffects of steric hindrance on the chirality of PANIs. The clarified relationship between the sterichindrance and the chirality of the polymer can enable us to tailor the chiroptical properties of functionalpolymer materials for future application.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Chiral conducting polymers have attracted much attentionduring last decade due to their potential applications in variousfields such as chiral catalysis [1], asymmetric synthesis [2,3], chiralsensors [4e6] and separation of enantiomers employed as chiralstationary phase [7] and molecular imprinting [8e10]. Chirality inconducting polymers is often induced by either incorporating chiraldopant anions or introducing chiral substituent into the main chainof the polymer. The former route of inducing chirality by chiraldopant anions is more effective in synthesis of chiral polyanilines(PANIs), while the latter one is successful for synthesis of chiralpolypyrroles and polythiophenes.

Synthesis of chiral PANI was pioneered by Majidi et al. [11] andHavinga et al. [12], who carried out the electrochemical polymeri-zation of aniline and the doping of emeraldine base with D- or L-CSAas chiral dopant, respectively. Since then other chirality-inducingspecies have been reported, including amino acids, chondroitin

x: þ86 27 87543632.u), hqtang62@yahoo.com.cn

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sulfate, and b-cyclodextrin sulfate [13e16]. Chiral PANI nanofibersseem to be more interesting due to their one-dimensional nano-structure. The presence of aniline oligomer as initiator in thepolymerization solution proved successful in promoting the growthof PANI [17], framing its nanoscaled morphology [18] and inducingits chirality [19]. Recently, we clarified that the chirality of PANI isintrinsically related to nanofibrillar morphology by proposing an“electrical double layer model”, and developed an efficient way toprepare highly chiral PANI by synthesizing PANI nanofibers.According to this model, nascent PANI fibers formed at the earlystages of polymerization become positively charged due to theprotonation of N atom of amine and partial oxidation of PANI chainbeing surrounded by anions, resulting in the formation of a struc-ture of electric double layers at the interface between the nano-fibers seed and solution. The positive charges on the chains of PANIrepel the reactive species, which prevents the lateral growth andfavors the elongation growth of PANI chains via the polymerizationat head-sides. This growth pattern of PANI chain in one directionenables it to elongate, giving better fibrillar morphology withconcomitant higher optical activity [20].

In recent years, substituted PANIs attracted also considerableattention owing to their better resistance against microbialdegradation and higher dispersibility in different solvents than

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Fig. 1. Synthesis scheme of chiral PANIs by using monomers (1), initiator DDPA (2),oxidant APS (3) and chiral dopant D- or L-CSA (4a or 4b).

M.N. Anjum et al. / Polymer 52 (2011) 5795e58025796

parent PANI [21e23]. Su and Kuramoto synthesized optically activePANIs in organic media by using 2,3-dichloro-5,6-dicyanobenzoquinone as oxidizing agent and chiral CSA as chiralinductor, and found that the PANI derivatives with various ringsubstituents exhibited similar chiroptical property [24]. Yuan andco-workers reported electrochemical synthesis of helical poly (2-methoxyaniline) by using b-cyclodextrin sulfate as chiral dopantand studied the effect of pH and salt concentration on chiropticalproperties [25,26]. However, there were few reports on thesynthesis of chiral PANI derivatives in aqueousmedium by chemicalmethods due to the much limited polymerization caused by sterichindrance of substituents [16].

Because the steric hindrance of substituents increases much thedifficulty in preparation of PANIs, we attempted to investigate theeffects of steric hindrance on the chemical synthesis of PANIderivatives and their chirality in the present work. As a result, wefor the first time synthesized highly chiral ortho-substituted PANIssuch as polyorthoanisidine (POA), polyorthotoluidine (POT), poly-orthoethylaniline (POE) and polyorthochloroaniline (POC) inaqueous medium by chemically polymerizing o-toluidine, o-eth-ylaniline, o-anisidine and o-chloroaniline and copolymerizingthese monomers and aniline with D- or L-CSA as chiral dopant andammonium persulfate as oxidant in presence of dia-minodiphenylamine (DDPA). Moreover, we first time report thatthese ortho-substituted PANI derivatives intrinsically possessopposite chiroptical properties in comparisonwith non-substitutedparent PANI. The variation of the chirality of the ortho-substitutedPANI derivatives was clearly explained by considering the effects ofsteric hindrance on the polymerization and the related CSA doping.

2. Experimental

2.1. Chemical reagents

Aniline, o-toluidine, o-ethylaniline and o-anisidine werepurchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai,China). Diaminodiphenylamine (DDPA) was purchased from TokyoChemical Industries (TCI). Chiral dopants D- and L-camphorsulfonicacids were purchased from Aladdin Chemical Reagent Corporation(Shanghai, China). All chemical were of analytical grade and used asreceived. Double distilled water was used for synthesis, washingand making dispersion of polymers.

2.2. Polymerization

In a typical procedure, of monomer (0.24 mmol) was dissolvedin 2.5 mL of D- or L-CSA (3 M), with addition of DDPA (0.01 mmol).This mixture solution, being referred to as solution A, was warmedto 60 �C for 1 min and cooled down to room temperature (25 �C). Asolution of the oxidant (solution B) was prepared by dissolving APS(0.24 mmol) into 2.5 mL of 3 M D- or L-CSA solution. Solution B wasrapidly added into solution A and then subjected to vigorous stir-ring forw30 s. After the polymerization reactionwas conducted for2 h, the crude product was washed with water and then separatedby centrifugation. The purified product was dispersed in distilledwater. The dedoping and redoping were conducted by using 1 MHCl and 1 M ammonia water, respectively. For synthesis of copol-ymers, total concentration of the used monomers was controlled at0.48 mmol, and the initial additions of APS and DDPAwere 0.48 and0.02 mmol, respectively.

2.3. Characterization and analysis

UVevisible absorption and CD spectra of the polymers wererecorded on a UVevisible spectrometer (Varian Cary 50) and

a spectropolarimeter (Jasco 710), respectively. FT-IR spectra wererecorded by using an IR spectrophotometer (Bruker Vertex 70)between 400 and 4000 cm�1 and the samples were prepared in thepellet form by using spectroscopic grade KBr powder. Since thechiral intensity is in direct proportion to absorption intensity of theproducts [20], the relative ratio of ellipticity and absorptionintensity has been expressed as the optical activity of the chiralproduct obtained. The morphological images of the polymers wereobtained using SEM (Hitachi S-4800, Japan).

3. Results and discussion

3.1. Polymerization and characterization

Chiral PANI and its derivatives (POA, POT, POE and POC) wereprepared (Fig.1) in aqueousmedium by using DDPA as initiator, APSas oxidant and D- or L-camphorsulfonic acid as chiral dopant. Themorphology of these polymers was observed on SEM, parts of theSEM photos were shown in Fig. 2, which demonstrates that allthese polymers have the one-directional fibrillar structure.

Fig. 3 compares the UVevisible absorption spectra of thesepolymers in both the doped and dedoped states. Being similar tothe reported observations for PANI [19,20,27e30], the spectrum ofthe as-prepared PANI show the absorptions at 831 nm and 400 nmascribed to the p-p* transition of the polarons, indicating that PANIis in its doped state [11,29]. The excitonic transition of dedopedemeraldine oxidation state appears at 603 nm, which correspondsto an equal number of oxidized and reduced aniline units along thepolymer chain [18,28].

The UVevisible absorption spectrum of POA salt showed threecharacteristic absorption peaks at 310, 410, and 825 nm. Theabsorption peak at 310 nm was ascribed to p-p* transition of thebenzenoid ring, whereas the peaks at 410 and 825 nm wereattributed to polaron-p* transition and p-polaron transition,

Fig. 2. SEM images of PANI (a), POA (b) and the copolymer of aniline/o-anisidine ¼ 5/1 (c).

M.N. Anjum et al. / Polymer 52 (2011) 5795e5802 5797

respectively [31,32]. The band at 566 nm in the spectrum ofdedoped POA was due to n-p* transition from non-bondingnitrogen lone pair to the conduction band (p*) [33,34]. The spec-trum of PANI emeraldine salt showed only a shoulder at higherwavelength near 831 nm with free carrier tail in the near infraredregion, which suggests a partial “extended coil” conformation[35,36]. In comparison with PANI, the spectrum of POA exhibiteda well-defined intense peak at 825 nm with hypsochromic shift,which revealed an increase in band gap created by an increase intorsion angle between the CeNeC plane and the plane of benzenering. This ultimately decreased the conjugation length due to thepresence of methoxy group, which twisted the torsion angle andgave the polymer chain a “compact coil” conformation [37].

The UVevisible absorption spectrum of POT showed theabsorption peak at 801 nm, which was relatively blue shifted incomparison with PANI. This can be attributed to stronger sterichindrance of eCH3 than H in PANI. Thus, the shifting of absorptionmaxima towards shorter wavelength reveals that POT chainpossesses higher degree of “compact coil” conformation than PANIand POA because the steric effect of eCH3 is stronger than eOCH3,which is due to easy rotation of O-C bond in eOCH3 [36,37].Similarly, the spectrum of POE salt also showed blue shift to748 nm, confirming that the stronger steric hindrance from theethyl group results in increased “compactness” of the polymerchain. The spectra of dedoped POA, POT, POE and POC displayedlmax at 566, 584, 594 and 595 nm, respectively, which are blueshifted in comparison with PANI (603 nm). This suggests that

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introduction of substituent at ortho position changes ‘extendedcoil’ to ‘compact coil’ conformation.

The FT-IR spectra of the polymers were recorded ( Fig. S1), andthe positions and assignments of the absorption peaks were shownin Table 1. The strong bands in the vicinity of 1500 and 1600 cm�1

are assigned to the non-symmetric C6 ring modes. The higherfrequency vibration at 1600 cm�1 has amajor contribution from thequinoid rings, while the lower frequencymode at 1500 cm�1 showsthe presence of the benzenoid ring units. The presence of boththese bands clearly shows that the polymers are composed of theamine and the imine units [35e41]. In FT-IR spectra of the PANIderivatives, the bands around 940, 880, 790, 712 and 600 cm�1 arelinked to the ortho substituted aromatic ring [33]. In particular, theband at 879 cm�1 in the spectrum of POT is due to methyl groupattached to phenyl ring [39]. In the spectrum of POA, the weak peakat 1455.8 cm�1 corresponds to C-H bending of the eOCH3 group.The bands at 1261.1 and 1025.0 cm-1 correspond to the CeOeCstretching of the alkyl aryl ether linkage [42].

Fig. 4 showed the circular dichroism (CD) spectra of the PANIsprepared with chiral CSA dopant. When PANI was prepared with D-CSA, it presented the Cotton effect or CD peak around 430 nm,which is induced by the chirality of D-CSA. The negative peak at430 nm, which is assigned to p-p* transition of polarons, can beascribed to right-handed helical conformation of PANI because CDspectrum of D-CSA shows only positive peak at 290 nm ( Fig. S2).The CD spectrum of PANI prepared with L-CSA showed the mirrorimage with a positive peak, demonstrating its left handed helical

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Table 1FT-IR characteristics of PANI, POA, POT and POE in the doped state with D-CSA.

Peak position (cm�1) Band characteristics

PANI POA POT POE POC

587.8 584.4 584.9 586.1 585.5 C-H out of plane bending794.0 792.8 793.6 790.6 792.2 Para substituted aromatic rings

(polymer formation)1044.9 1044.1 1042.9 1043.7 1043.7 SO3 group of dopant acid1139.8 1173.6 1171.6 1171.4 1176.0 C-H in plan bending vibrations1375.3 1371.8 1375.8 1375.6 1375.27 C-N stretching (secondary

aromatic amine)1484.6 1494.9 1487.7 1486.0 1506.4 C-N stretching of benzenoid rings1581.4 1602.5 1607.3 1605.6 1595.9 C-N stretching of Quinoid rings1740.4 1740.3 1740.1 1741.3 1740.7 C-O stretching of dopant acid3229.1 3240.0 3233.2 3225.7 e Aromatic C-H stretching

M.N. Anjum et al. / Polymer 52 (2011) 5795e58025798

conformation. According to earlier work [19,20,27], the PANIprepared by using D-CSA as chiral dopant results in right-handedhelix as more stable conformation for D-CSA doped PANI, and thedoping-induced right-handed helix conformation can be kept afterthe doped PANI is dedoped with ammonia water.

Unlike PANI, all the prepared PANI derivatives (POA, POT, POEand POC) exhibited opposite chiroptical properties. When D-CSAwas used, these PANI derivatives gave a positive peak, showing theleft handed helical conformation; when L-CSA was used, they gavea negative peak, showing the right-handed helical conformation.The opposite chiroptical properties of these polymers in compar-ison with PANI should be attributed to the effect of ortho substi-tution of methoxy, methyl, ethyl and chloro group on the benzenering of aniline. It was also noted that the CD peak position of thepolymers is shifted in the order POA (395 nm), POC (387 nm), POE(380 nm) and POT (370 nm).This shift towards shorter wavelengthis the same as observed in UVevisible spectra suggesting thatsubstitution at ortho position causes change of conformation from‘extended coil’ to ‘compact coil’.

3.2. Effects of reaction conditions on the chirality of PANIderivatives

Effects of reaction conditions including temperature andconcentrations of monomer, oxidant, dopant and initiator and weresystematically investigated on the chirality of the PANI derivativesby using POA as the representative.

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Fig. 4. CD spectra of POA, POE and POT prepared with D- or L-CSA as the chiral dopant.Inset shows the corresponding CD spectra of PANI.

As shown in Fig. 5a, when other reaction conditions were notchanged, increasing the monomer concentration resulted indecreasing of the chirality of the polymer. The increase inmonomerconcentration means a decrease in CSA/monomer ratio anda decrease of formation of CSA-monomer complex ( Fig. 7), ulti-mately resulting in decreasing of the chirality. The same trend wasobserved when CSA concentration was increased for a fixedmonomer concentration. The CD spectra (Fig. 5b) show that thechirality is increased with increasing D-CSA concentration.However, when D-CSA concentration is over-high, the chirality ofthe resultant polymer is decreased. This is ascribed to high viscosityand low diffusion, which inhibit the one-directional growth ofpolymer chain and result in the decrease of optical activity. Fig. 5c,shows the influence of APS concentration on the chirality of POA.Lower chirality was observed at lower APS concentration, becausethe relatively fast consumption of APS leads to the unavailability ofthe oxidizing agent for the elongation of polymer chains. Over highAPS concentration also decreases the chirality, because thecompetition between the chiral dopant and the generated dopantsulfate anions from the decomposition of APS will affect theamount of CSA-monomer complex and the chiral doping of thepolymer product [19].

Effect of DDPA concentration was investigated (Fig. 5d). As anamine-amine-capped initiator, DDPA has lower redox potentialthan that of monomer molecules and is oxidized more easily thanthemonomer, but it cannot form polymer by itself [44]. It may havetwo functions: it accelerates the rate of polymerization, and acts asseed to initiate chain growth by adding monomer units to bothends (Fig. 1).

To study the effect of temperature on chirality, chiral POA waspolymerized at different temperatures. Fig. 5e shows that thechirality of POA increases with rise of temperature from 0 to 35 �Cand then drops at higher temperatures. This variation trend of theoptical activity is same to that of PANI as reported previously [20].The UVevisible absorption data of the polymers being prepared atdifferent temperatures showed a fluctuation of conformationbetween “extended and compact” coil conformation ( Fig. S3). Asdiscussed above, the chirality in PANI and its derivatives is theoutcome of the interaction between CSA and monomer molecules.Because the strength of electrostatic attraction and H bondbetween CSA and monomer molecules varies at different temper-atures, the chirality and the conformation of the polymer aredependent on the polymerization temperature. The appearance ofthe optimal temperature is related to the relaxation effects ofincreased temperature on POA chain.

3.3. Effects of the copolymerization of aniline and its ortho-substituted derivatives on the chirality of the copolymers

In order to study the extent of steric-hindrance posed by theortho substituent on polymer conformation and chirality, thecopolymers of aniline with o-anisidine, o-toluidine, o-ethylanilineand o-chloroaniline were synthesized in the presence of D-CSA aschiral dopant by changing the ratio between twomonomers, wherethe total concentration of the two monomers were kept at 0.1 M.SEM observations indicated that all the copolymers have the one-directional fibrillar structure, like PANI (Fig. 2).

As shown in Fig. 6a, parent PANI (corresponding to 1:0 ofaniline/o-anisidine) had a strong chirality with an ellipticity valueof about�400mdegree, which is reverse to the chirality of the usedchiral dopant (D-CSA). When the relative concentration of o-anisi-dine was increased, the chirality of the copolymer was decreased,that is, the absolute value of the ellipticity was decreased. Thechirality of the copolymer prepared with aniline/o-anisidine¼ 10:1became very weak. When the relative concentration of o-anisidine

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Fig. 5. Dependences of CD spectra of POA on (a) monomer concentration, (b) D-CSA concentration, (c) APS concentration, (d) DPPA concentration, and (e) reaction temperature.

M.N. Anjum et al. / Polymer 52 (2011) 5795e5802 5799

was increased to that corresponding to aniline/o-anisidine ¼ 5:1,the chirality of the copolymer was reversed, which had an ellip-ticity value of aboutþ25mdegree. When only o-anisidine was usedas the monomer, the obtained POA had an ellipticity value ofaboutþ80mdegree (Fig. 4). Similar effects of the monomer ratio onthe chirality of the copolymers were observed when the monomero-anisidine was substituted with o-ethylaniline, o-toluidine and o-chloroaniline (Fig. 6bed). These substituents were classified intotwo categories: one with larger size (C2H5- and CH3O-) and otherwith smaller size (CH3- and Cl-). The copolymers of the larger andsmaller ortho substituents showed the chirality inversion at the

monomer ratio of aniline/o-derivative ¼ 5:1 and 1:2, respectively.In case of former category, the inversion of conformation ofcopolymer can be rationalized in terms of the ‘sergeants andsoldiers’ principle in which small number of monomer units withlarger size o-substituent act as ‘sergeants’ and dictate the confor-mation to a large number of ‘cooperative soldiers’- monomerswithout substituent but in the latter case the ‘majority rules’principle holds. The inversion effect of the four ortho-substitutedaniline derivatives on the chirality of the copolymer seems tofollow the order of o-ethylaniline > o-anisidine > o-toluidine > o-chloroaniline, which is the same as the order of the size of the

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Fig. 6. CD spectra of the copolymers obtained by copolymerizing aniline and the ortho substituted derivatives of (a) o-Anisidine, (b) o-Ethyl aniline, (c) o-Toluidine and (d) o-Chloroaniline with different monomer molar ratios. The other reaction conditions were 0.1 M APS, 3 M D-CSA and 0.02 mmol DDPA.

Fig. 7. Scheme for the formation of complexes between D-CSA and aniline and ortho substituted aniline dimer and their polymerization to form right and left handed helixrespectively. R ¼ C2H5-, CH3O-, CH3- or Cl-.

M.N. Anjum et al. / Polymer 52 (2011) 5795e58025800

M.N. Anjum et al. / Polymer 52 (2011) 5795e5802 5801

groups: C2H5- > CH3O- > CH3-> Cl-. This further confirms that theinversion of chirality and conformation is quantitatively connectedwith extent of steric hindrance posed by the substituent on thebenzene ring of aniline.

3.4. A mechanism for the generation of the chirality in PANI and itsderivatives

The formation of chiral PANI occurs through oxidative polymer-ization, inwhich aniline is oxidized to aniliniumcation,which reactswith other aniline or anilinium cation to form aniline dimer. Theformation of chiral anioneaniline complex before the polymeriza-tion is of vital importance, and the complexes are also formedbetween anions and dication or diradical cation in PANI chainsduring the polymerization, inwhich such complexes are attacked bymonomer in the form of monomereacid complex, leading to theadditionof an anilineunit into thepolymer chain [30]. Therefore, theinduction of chirality in PANI bychiral dopant acid can be ascribed tothe formation of complex between chiral acid and aniline, whichshould be suitable for the synthesis of chiral PANIs. This is supportedby the observation that higher concentrations of chiral CSA (�3 M)favors the increase of the chirality of the PANIs (Fig. 5b), becausehigher CSA concentration is favorable to the formation of thecomplex betweenmonomer and CSA. It is noted that the chirality ofthe polymer being prepared with 4 M chiral CSA was weaker thanthat with 3 M CSA. The possible reason for this is ascribed to highviscosity and low diffusion, which inhibit the one-directionalgrowth of polymer chain and result in decrease of optical activity.

Similar observation about the influence of the chiral dopantanion on the optical activity of polyaniline was reported by Ashrafand co-workers, who suggested that chiral conformation in PANI isdue to both the electrostatic bonding of CSA� anion to PANI HN�þ

and H bonding between carbonyl groups of CSA and HN site of PANI[29]. As CSA is a bidental chiral dopant, at initial stage of poly-merization the complexes are formed between CSA and anilinethrough the interactions between the oxygen atoms (carbonyloxygen and (SO3)-) in CSA and both NH sites of anilinium unit byhydrogen bonding and/or electrostatic attraction. Zhang et alproposed that polymerization of aniline takes place throughformation of aniline dimer, and Zaidi et al reported that the pres-ence of substituent at ortho position affects the internal geometryof aniline dimer (or polymer) in a systematic fashion by changingdihedral and inter planar angles [45,46]. Therefore, the geometry orshape of dimer-CSA complex is affected by the presence ofsubstituent at ortho position, and the CSA molecule interacts withdimer on the opposite site as shown in Fig. 7 due to presence ofsubstituent.

In the case of aniline dimer-D-CSA, the addition of monomertakes place on the right hand side in the axis of propagation allalong the polymer, giving right-handed helical conformation. Onthe other hand, o-substituted aniline dimer-D-CSA allows theaddition of monomer on the left side in the axis of propagation,making left-handed helical conformation [47]. Furthermore, thischirality inversion phenomenon due to the presence of substituentat ortho position has also been demonstrated by copolymerizinganiline with its ortho substituted derivatives. The steric hindrancewas introduced at ortho position by copolymerizing, which resul-ted in inversion of conformation. Yan et al introduced sterichindrance of methyl at ortho position by copolymerizing anilinewith o-toluidine and observed no effect on the chiroptical prop-erties and helical conformation [43]. Contrary to this report, wehave observed that the presence of substituent (such as methyl,ethyl, methoxy and chloro group) at ortho position is a decisivefactor in shaping the helical conformation of polymer chain andconsequently affect the chirality of the product.

4. Conclusion

The chiral ortho substituted PANIs, POA, POT, POE and POC andtheir copolymers with aniline were synthesized chemically inpresence of chiral CSAwith APS as oxidant in aqueous medium. Thechirality of ortho substituted PANIs was found to be totally oppositeto that of simple PANI. The characterization of chiral polymers byCD and UVevisible spectroscopy revealed that steric hindranceposed by the substituent at ortho position caused the helicalinversion from right hand to left hand with concomitant change ofconformation from extended to compact one. Moreover, thechirality of copolymers of aniline with its ortho substituted deriv-ative showed the relation between chirality and extent of sterichindrance of different substituent. It was found that the substituentposing higher steric hindrance can reverse chirality with lowerratio of aniline derivative to aniline. In other words chirality of thepolymers can be easily tuned by incorporation of substituent atortho position. Therefore, besides the induction of optical activityby chiral dopant, the change in substituent at ortho position canalso tailor the chirality, which may be helpful in manipulating thehelical conformation and composition of macromolecular materialto broaden the future horizons.

Acknowledgements

This work was supported by the National Science Foundation ofChina (Grant Nos. 20877031, 21077037 and 21177044), the Brain-stormProject of Science andTechnologyofWuhanCity, China (GrantNo. 201060623258), and the Fundamental Research Funds for theCentral Universities of China (Grant Nos. 2011TS121 and CZZ11008).The Analytical and Testing Center of HuazhongUniversity of Scienceand Technology is appreciated for its help in CD measurements.

Appendix. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.polymer.2011.10.038.

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