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Synthetic Metals 160 (2010) 1920–1924

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ynthesis and characterization of poly[(R)-(−) andS)-(+)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate]ss chiral oligomers of pyrrole

oão Carlos Ramosa, Jéssica M. Monteiro Diasa, Rosa M. Souto-Maiora, Adriana S. Ribeirob,osealdo Tonholob, Valessa Barbierc, Jacques Penellec, Marcelo Navarroa,∗

Departamento de Química Fundamental, Universidade Federal de Pernambuco, UFPE, Cidade Universitária, 50670-901 Recife - PE, BrazilInstituto de Química e Biotecnologia, Universidade Federal de Alagoas, Campus A. C. Simões, 57072-970 Maceió-AL, BrazilInstitut de Chimie et des Matériaux Paris Est (ICMPE) - CNRS and Université Paris-Est, 2-8 rue H. Dunant, 94320 Thiais, France

r t i c l e i n f o

rticle history:eceived 20 April 2010eceived in revised form 7 July 2010

a b s t r a c t

Chiral polypyrrole oligomers were prepared from (R)-(−) (1) and (S)-(+)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate (2) by oxidative polymerization in the presence of FeCl3 as an oxidant.The new polymers were characterized by 1H NMR, FTIR and UV–vis (�máx = 251 nm). TGA analyses of the

ccepted 12 July 2010vailable online 19 August 2010

eywords:yrrole monomerhenylglycine

corresponding polymers, poly1 and poly2, demonstrate an excellent thermal stability up to 204 ◦C, fol-lowed by two consecutive weight losses at higher temperatures. Relative number-average molecularweights of about 1.4 × 103 and 2.3 × 103, for poly1 and poly2, respectively, were measured by SEC, corre-sponding to oligomeric chains. Specific optical rotation measurements (c 0.1, THF) for poly1 [�]D

20 = −44and poly2 ([�]D

20 = +44) suggest that chirality is retained during the polymerization. Transversal sectiondicat

olypyrrole

hiral oligomers

analysis of SEM images in

. Introduction

Shirakawa’s and Ikeda’s seminal contributions in the seventiesn self-supported films of polyacetylene obtained by the directolymerization of acetylene [1,2] provide one of the first examplesf an all-organic polymer semiconductor. The same authors [3] latereported that conductivities could be increased by up to 13 ordersf magnitude by treatment of the obtained polyacetylenes withewis acids or bases. These observations have led to a considerableevelopment for this class of materials. In some cases, conductingolymers exhibit indeed optical, magnetic, electrical and electronicroperties comparable to what is commonly observed for met-ls, while maintaining the ease of processing commonly associatedith conventional polymers [4]. As a result, numerous applicationsave been developed, from the manufacture of batteries [5], elec-rochromic devices [6] or light-emitting diodes [7], to the design ofrotection coatings against corrosion [8].

Polymers or oligomers of pyrrole and its derivatives are amonghe most widely studied conductive materials due to their excellenthermal stability, good electrical conductivity, as well as their rela-ive ease of synthesis [9,10]. These properties have induced a large

∗ Corresponding author. Tel.: +55 81 21267460; fax: +55 81 21268442.E-mail address: navarro@ufpe.br (M. Navarro).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.07.010

es the materials isolated right after the synthesis is highly porous.© 2010 Elsevier B.V. All rights reserved.

number of applications in high-tech areas such as for the immobi-lization of enzymes [11,12] or in the preparation of bio- [13,14] andpH sensors [15]. The synthesis of these materials generally occursby electrochemical or chemical oxidative polymerization of pyr-role monomers, in various organic solvents or water [4]. Severalinorganic salts may be used as oxidant: FeCl3 [16], Fe(ClO4)3 [17],AgNO3 [18], etc.

The use of polypyrroles in sensor design requires the synthe-sis of pyrrole derivatives with differentiated functional groups[19,20], as well as the possibility to access optically activepolymers [21]. A first approach to these chiral polypyrroleswas proposed by Delabouglise and Garnier who reporteda synthesis via the placement of a chiral amino acid at the3-position of the pyrrole [22]. In this work, we describe an alter-nate approach based on the preparation and characterizationof chiral oligomers poly[(R)-(−)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate] (poly1) andpoly[(S)-(+)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenyl glycinate] (poly2). The monomers (R)-(−) (1)and (S)-(+)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-

′

phenylglycinate (2) were obtained from 1-(3 -iodopropyl)pyrrole(3) via the nucleophilic substitution of chiral selectors (R)-(−) (4)and (S)-(+)-N-(3,5-dinitrobenzoyl)-�-phenylglycine (5), whichhave been successfully used by Pirkle as HPLC stationary phasesfor enantioselective separation [23]. The reported characterization

J.C. Ramos et al. / Synthetic Metals 160 (2010) 1920–1924 1921

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f these polymers and evaluation of their optical and thermalroperties constitute a key step in the development of theseaterials as chiral selectors for polymer-based chiral stationary

hases supported on silica, whose design will be described in aeparate account.

. Experimental

.1. Materials and methods

All reagents were purchased from Aldrich and Acros, and useds received. The solvents, of analytical grade, were dried by con-entional procedures and distilled prior to use [24].

NMR spectra were recorded using a Varian Unity Plus equipmentith frequency of 300 MHz for protons, in pyridine-d5 solutions.V–vis spectra were obtained with a Perkin-Elmer Lambda 6 spec-

rophotometer, in THF solutions. FTIR spectra were obtained with aruker IFS66 spectrophotometer using KBr pellets. Thermal analy-es were performed with a Schimadzu TGA50 thermobalance underir at a heating rate of 10 ◦C min−1. Optical rotations were mea-ured on a Perkin-Elmer 241 polarimeter at 20 or 26 ◦C using a cellf 10 cm path length; concentration (c) in g/100 mL and solvent areiven in parentheses. Scanning electron microscopy was performedith a JEOL 6360 equipment. Elemental analysis determinationsere performed with a Carlo Erba equipment. A Q-TOF mass spec-

rometer (Micromass, Manchester, UK) was used for fingerprintingnd ESI-MS analysis.

Number- and weight-average molecular weights (Mn, Mw)ere measured by size exclusion chromatography (SEC) againstolystyrene (PS) standards, using two Polymer Laboratories PLGelmm Mixed-C columns, a Shodex RI-71 RI detector and a ShimadzuC-10 AD pump, in THF, at a 1.0 mL min−1 flow rate.

.2. Synthesis

(R)-(−) (4) [74927-72-3] and (S)-(+)-N-(3,5-dinitrobenzoyl)-�-henylglycine (5) [90761-62-9] were synthesized according toirkle’s method [25] as slightly modified by Navarro and coworkers26], in 88 and 91% yields, respectively.

1-(3′-Bromopropyl)pyrrole [100779-91-7] was synthesized asescribed in the literature [26] in 82% yield (lit. 44%). 1H NMRCDCl3): 6.6 (t, J = 2.0 Hz, 2H, pyrrole ring); 6.0 (t, J = 2.0 Hz, 2H, pyr-ole ring); 4.0 (t, J = 6.0 Hz, 2H, CH2), 3.2 (t, J = 6.0 Hz, 2H, CH2); and.1 (m, J = 6.0 Hz, 2H, CH2). FTIR (KBr): 3098, 2964–2937, 1437, 1243nd 728 cm−1.

1-(3′-Iodopropyl)pyrrole (3) [167867-84-7] was synthesized as

escribed in the literature [26] in 86% yield (lit. 96%). 1H NMRCDCl3): 6.6 (t, J = 2.0 Hz, 2H, pyrrole ring); 6.0 (t, J = 2.0 Hz, 2H, pyr-ole ring); 3.9 (t, J = 6.0 Hz, 2H, CH2), 3.0 (t, J = 6.0 Hz, 2H, CH2); and.1 (m, J = 6.0 Hz, 2H, CH2). FTIR (KBr): 3098, 2932, 1440, 1215 and25 cm−1.

1.

(R)-(−) (1) [258344-87-5] and (S)-(+)-3-(1-pyrrolyl)propyl-N-(3,5-dinitrobenzoyl)-�-phenylglycinate (2) [258344-89-7] wassynthesized as described in the literature [25] in 65% yield (lit.70%) for the (R)-(−) monomer 1, m/z 453.19 [M+H]+ and 83% yield(lit. 73%) for the (S)-(+) monomer 2, m/z 453.19 [M+H]+. 1H NMR(CDCl3): 9.1 (t, J = 2.4 Hz, 1H, ArH), 8.9 (d, J = 2.4 Hz, 2H, ArH), 7.4–7.3(m, 6H, NH, Ar), 6.4 (t, J = 2.4 Hz, 2H, CH), 6.0 (t, J = 2.4 Hz, 2H, CH), 5.7(d, J = 6.3 Hz, 1H), 4.0–4.2 (m, 2H, CH2), 3.7 (t, J = 6.3 Hz, 2H, CH2),2.0–1.9 (m, 2H, CH2). FTIR (KBr): 3348, 3120, 2986–2870, 1743,1654, 1539, 1348 and 760–688 cm−1.

Optical rotations: [�]D20 = −80 +80, for 1 and 2, respectively (c

0.1, THF).

2.3. Polymerization of monomers 1 and 2

Poly[(R)-(−)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate] (poly1) and poly[(S)-(+)-3-(1′-pyrrolyl)propyl-N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate](poly2) were synthesized by adding dropwise under N2 1 or 2(500 mg; 0.1 mmol) dissolved in dry CHCl3 (100 mL) to an FeCl3suspension (1.2 g; 7.2 mmol) in 100 mL of dry CHCl3. The mixturewas stirred for 48 h at room temperature. The polymer was pre-cipitated by addition of CH3OH, filtered and purified by Soxhletextraction with CH3OH. The purified polymer was dried undervacuum for 6 h. A reddish black solid was obtained: 288 mg (58%)for poly1 and 281 mg (56%) for poly2. FTIR (KBr): 3395, 3100,2970–2890, 1744, 1660, 1550, 1340 and 743–687. Anal. found forpoly1: C, 58.33; H, 3.96; N, 12.25. Anal. found for poly2: C, 58.99;H, 4.23; N, 12.00. Theoretical value for (C22H18N4O7)n: C, 58.67; H,4.03; N, 12.44.

Optical rotation: [�]D20 = −44 for poly1 and [�]D

20 = +44 forpoly2 (c 0.1, THF).

3. Results and discussion

Monomers 1 and 2 were synthesized in good yields of 65 and83%, respectively, following a procedure described in the literature[26] (Scheme 1). They were polymerized by oxidative polymeriza-tion with FeCl3 in CHCl3 (Scheme 2). The products, poly1 (58%) andpoly2 (56%), were obtained in good yields.

Polymer molecular weights as determined by SEC analysis usingpolystyrene calibration suggest that, as expected for these complexstep-growth polymerizations, mostly oligomers were obtained:Mn = 1.4 × 103 with a polydispersity index (Mw/Mn) of 3.1 for poly1and Mn = 2.3 × 103 with a polydispersity index of 2.0 for poly2.These numbers correspond to average degrees of polymerization

of 7 and 10, respectively, although these values have to be consid-ered as rough estimates, as the calibration with polystyrene (or anyother standard given the rigid-rod and grafted nature of the poly-mer) cannot be expected to provide an exact value. In the presentcase, a slight underestimation of the degrees of polymerization is

1922 J.C. Ramos et al. / Synthetic Metals 160 (2010) 1920–1924

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robable, given the rigid nature of the backbone and the large sizef the lateral substituents.

The FTIR spectra of monomers 1 and 2 (Fig. 1B) show characteris-ic bands at 3348 (N–H, amide stretching), 3120 (C–H, pyrrole ringtretching and phenyl group), 2986–2870 (CH2, asymmetric andymmetric stretching), 1743 (C O, ester), 1654 (C O, amide), 1539nd 1348 (NO2, symmetric and asymmetric axial deformations),

ig. 1. Infrared spectra of poly1 (A) and monomer 1 (B). Poly2 and monomer 2pectra are, as expected, strictly identical to A and B spectra, respectively.

.

and 722 cm−1 (pyrrole and benzene aromatic C–H bending). Asexpected, the corresponding polymers present the exact same typ-ical bands in their spectra (Fig. 1A). From the FTIR spectrum, poly1seems not doped possibly because of the distorted structure of theconjugated system as a consequence of steric hindrance of the func-tionalization on the N-atom. The most significant feature reflectinga doping process would be the appearance of a broad absorptionband above 2000 cm−1 attributed to an intra-chain (free-carrier)excitation [27,28], which is not observed in Fig. 1A. Elemental anal-yses of the synthesized polymers (see Section 2) point out theabsence of the expected dopant ion (Cl−), corroborating the resultsobserved in the IR spectrum.

Polymers poly1 and poly2 display identical UV–vis spectra(Fig. 2A), with an absorption band at �máx = 251 nm correspondingto �–�* transitions of the aromatic groups present in the oligomerand monomer structures; in addition, the expected shoulder isobserved for the oligomers between 300 and 400 nm, due to theextended conjugation of the �-system.

1H NMR spectra were obtained for poly2 and 2 in deuteratedpyridine as a solvent (Fig. 3), which provided a better solubil-ity for the polymer and an increase in resolution of the protonpeaks, compared with others tested solvents (acetone-d6, DMSO-d6 and THF-d8). All the peaks corresponding to the chiral selector

(i.e., the N-(3,5-dinitrobenzoyl)-�-phenylglycinate moiety) can beobserved on both the polymer and monomer spectra. Proton peakscorresponding to the hydrogens on the pyrrole ring cannot be iden-tified, which is expected for positions 2 and 5 of the ring given the2–5 links expected to form during the polymerization, but is more

Fig. 2. UV–vis spectra of poly1 and monomer 1.

J.C. Ramos et al. / Synthetic Metals 160 (2010) 1920–1924 1923

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Table 2Results of optical rotation measurements (c 0.1, THF).

Entry Compound Optical rotation [�]D T (◦C)

1 1 −80.0a 202 2 +80.0a 203 4 −76.2b 284 5 +76.0b 285 poly1 −44.0 206 poly2 +44.0 207 poly6 −29.0b 288 poly7 +28.4b 28

when compared to the [�] values of the monomers (entries 1and 2). The same behavior was observed for the correspondingthiophene-substituted monomers (i.e., (R)-(−)-2-(3′-thienyl)ethylN-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate (6) and (S)-(+)-2-(3′-

ig. 3. 1H NMR spectra of poly2 and monomer 2 in pyridine-d5 (the Py label on thegure indicates pyridine proton peaks).

urprising for positions 3 and 4. This difficulty in observing themight be either due to very long relaxation times or to a very large

roadening of the corresponding peaks [29].Thermogravimetric curves (under air at a heating rate of

0 ◦C min−1 as summarized in Table 1) present a first decompo-ition step to volatile products ranging from about 204 to 411 ◦C,ith a maximum weight loss at 283 ◦C and a combined loss of 32%.second decomposition step, from 412 up to 944 ◦C, generates

n additional weight loss of 31%, with a residue above that tem-erature of 36%. This behavior is similar to TGA results obtainedor polythiophenes containing the same chiral selector, as recently

escribed in the literature [30].

The above thermogravimetric results strictly apply to polymeramples that have been carefully dried prior to the analysis. Poly1nd poly2 samples are indeed very hygroscopic, most probably due

able 1GA results obtained for poly1 and poly2 under air and at a heating rate of 10 ◦Cin−1.

Polymer (poly1 or poly2)d Decomposition temperature (◦C)

Tia Tmáx

b Tfc

Step 1 204 283 411Step 2 411 694 944

a Initial temperature.b Maximum temperature.c Final temperature.d Identical results were obtained for both polymers.

a Ref. [26].b Ref. [28].

to the presence of a –(C O)–NH– group in each repeat unit, and,if wet, display some additional weight loss of variable intensityfrom 100 to 180 ◦C, corresponding to water loss. This hygroscopiccharacter can also be detected by elemental analysis, samples con-taining uu to 1 to 2 equivalents of water molecules per repeat unit(3–7 wt%) having been observed occasionally.

Optical rotation values [�]D20 (c 0.1, THF) were determined for

both polymers poly1 and poly2 and are summarized in Table 2.As can be observed, the optical activity is retained during thepolymerization (entries 5 and 6), although the values diminish

Fig. 4. SEM images of the polymers as isolated right after the polymerization. Top:poly1 1000× magnification (scale bar 10 �m); bottom: poly2 1000× magnification(scale bar 10 �m).

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hienyl)ethyl N-(3′′,5′′-dinitrobenzoyl)-�-phenylglycinate (7)) andhe corresponding polymers, as previously described in the litera-ure [29], for which the [�]D

28 value for the polymers had half thealue of the monomers.

In summary, the characterization data obtained on the synthe-ized polymers strongly suggest that the presence of the chiral sideroups do not affect significantly the polymerization of the pyrrolenit, despite the known tendency for oxidative routes to polypyr-ole derivatives to induce side-reactions as a result from both theeactivity of the highly reactive cationic species involved and theresence of strong oxidants.

Morphological characterization of as-synthesized samples ofoly1 and poly2 were obtained by scanning electron microscopy.ransversal section analysis of the powders shows highly porousaterials are obtained (Fig. 4), with pore diameters ranging from 1

o 10 �m.

. Conclusion

Chiral polypyrrole oligomers poly1 and poly2 have beenbtained in good yields from the corresponding enantiopureonomers, and characterized by several analytical techniques. SEC

nd spectroscopic analysis suggests oligomeric species have beenbtained, whose structure is compatible with the expected one.he solid materials isolated right after the synthesis, i.e. withoutny further treatment, displayed a high porosity and an excellenthermal stability (204 ◦C), with values similar to those previouslybserved for polythiophene equivalents [30]. The optical activ-ties of the polymers were lower than those measured for the

onomers, as also observed for polythiophene derivatives. Over-ll, the obtained results suggest that the chirality and the structuref the side groups are retained throughout the polymerization andhat oligomers can be obtained whose properties are compatibleith the requirements needed for the development of chiral sta-

ionary phases as well as other applications involving the use of

he chiral groups for enantioselective molecular recognition. Thisncludes the modification of metallic and non-metallic electrodesn such a way a chiral layer is added either for recognition pur-oses (sensor design) or for catalysis (enantioselective synthesis)31].

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ls 160 (2010) 1920–1924

Acknowledgements

The authors would like to acknowledge granting authoritiesCNPq and CAPES for financial support.

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