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Synthetic Metals 159 (2009) 103–112

Contents lists available at ScienceDirect

Synthetic Metals

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ynthesis and characterization of oxazinone and oxazoline substituted pyrroles:owards electrically conducting bi-functional copolymers

an Hegewald, Lothar Jakisch, Jürgen Pionteck ∗

eibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany

r t i c l e i n f o

rticle history:eceived 13 March 2008eceived in revised form 29 July 2008ccepted 5 August 2008vailable online 25 September 2008

a b s t r a c t

The synthesis of N-(3-aminopropyl)pyrrole and N-(2-carboxyethyl)pyrrole was reviewed and repeated.Both compounds were utilized as pre-cursors for the synthesis of oxazinone and oxazoline modifiedpyrroles by different synthetic pathways. The N-substituted pyrroles were characterized by 1H NMR,ATR-FTIR, and MALDI-TOF MS. The chemical oxidative copolymerization of pyrrole and its derivativeswas carried out in various solvent systems using FeCl3 and (NH4)2S2O8 as oxidants. Furthermore, sodium

eywords:xazolinexazinoneunctionalized polypyrrolelectrical conductivityluorescenceore-shell-like particles

poly(styrene sulfonate) was used as dopant leading to core-shell-like structures, and, after the copoly-merization of pyrrole with oxazinone or oxazoline functionalized monomers, to exceptionally increasedconductivities. The produced (co-)polymers were characterized in terms of their chemical composition,morphology, and electrical conductivity. Oxazinone and oxazoline moieties in the copolymers were con-firmed by ATR-FTIR. Furthermore, the retained reactivity of oxazinone and oxazoline functions afterchemical oxidative copolymerization was evidenced by model reactions with 1-pyrenemethylamine and1-pyreneacetic acid, respectively, and subsequent fluorescence microscopy and spectroscopy.

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. Introduction

Polypyrroles (PPys) are highly attractive materials and haveeen widely investigated for applications in fuel cells [1–3], elec-rochromic displays [2,4], actuator components [5–9], protective orioactive coatings [4,10–12], (bio-)sensors [13–16], bioadsorbents17], microsurgical tools [1], nerve repair conduits [1], and for tis-ue regeneration [1,18]. PPy’s versatility can be attributed to its goodlectrical conductivity concomitant with environmental stability toir and water [19,20]. Furthermore, the preparation of PPy is facile.Py can be synthesized electrochemically or chemical oxidative inhe forms of films or powders, respectively [21]. The properties,.g. morphology and electrical conductivity, can be influenced anddjusted by synthetic parameters, such as solvent, temperature,xidant or oxidative current/potential, and dopant [19].

To improve the solubility and processibility of PPy and to expandhe potential applications of PPy efforts of synthetic chemists haveeen directed to the synthesis of pyrrole derivatives. Substitution

f pyrrole has a negative impact on electroactivity and electri-al conductivity of the oxidized polymer due to a lack of ringlanarity [22,23]. This effect is stretched out with N-substitution24,25]. However, the synthetic effort for N-substituted monomers

∗ Corresponding author. Tel.: +49 351 4658393; fax: +49 351 4658565.E-mail address: pionteck@ipfdd.de (J. Pionteck).

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379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2008.08.003

© 2008 Elsevier B.V. All rights reserved.

s far less compared to 3-substitution. For biomedical applicationsodification with biological moieties is desired to enhance the

pecific covalent attachment of proteins, peptides, and enzymes.-(3-Aminopropyl)pyrrole [26,27] and N-(2-carboxyethyl)pyrrole

1,28,29] have been synthesized and utilized for cell adhesionxperiments [1] and as bioadsorbents [29]. Furthermore, they werenticipated for the use in immunodiagnostic assays [28,29]. Forhese applications the electroactivity of the conducting polymerss irrelevant: the value-added properties are its intense intrinsichromogenicity and the possibility of specific covalent linkage tohe biomolecules.

Besides carboxy and amino functions benzoxazinones and oxa-olines have been shown to possess biological activity. Oxazolinesre local anesthetics [30,31] and benzoxazinones are potent deac-ivators of chymotrypsin as well as inhibitors of human leukocytelastase and HSV-1 protease [32]. Furthermore, bisoxazinones andisoxazolines have been used as linking units, chain extenders,nd reactive compatibilizers in polymer blends [33]. Jakisch et al.escribed the synthesis of oxazinone and oxazoline bi-functionaloupling agents for use in reactive blending [34]. Model reac-ions showed that carboxylic acid groups reacted selectively with

xazoline groups, whereas the oxazinone groups were exclusivelyttacked by amino groups.

Herein various synthetic pathways (Scheme 1) towards oxazi-one and oxazoline functionalized pyrroles are reported. Both,xazoline and oxazinone modified pyrroles were copolymerized

104 J. Hegewald et al. / Synthetic Metals 159 (2009) 103–112

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Scheme 1. Synthetic routes towards N-ox

ith pyrrole using FeCl3 or (NH4)2S2O8 as oxidants to give elec-rical conducting polymers. Furthermore, sodium poly(styreneulfonate) was used, which acts as dopant improving the electri-al conductivity of PPy and as template leading to core-shell-liketructures, which enhance the applicability as described elsewhere3,29,35].

The reactivity of the oxazinone and oxazoline bi-functionalizedopolymers was proved by fluorescence spectroscopy and fluores-ence microscopy after conversion with 1-pyrenemethylamine and-pyreneacetic acid, respectively. The synthesized pyrrole deriva-

ives and the oxidized copolymers can be modified with otherunctionalities, thus giving a nice toolbox for a variety of reactionsnd applications.

ig. 1. Schematic diagram of the test stand for measuring the electrical bulk con-uctivity of powders, granulates, and disks.

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. Experimental

.1. Materials

Pyrrole (Aldrich) was freshly distilled and stored under nitro-en in a refrigerator at 4 ◦C. Tetrahydrofuran (THF) and diethylther were distilled prior use from anhydrous calcium chlo-ide (Fluka). N-(2-Cyanoethyl)pyrrole, lithium aluminum hydrideLiAlH4, 1.0 M solution in diethyl ether), ammonium persulfateNH4)2S2O8, iron(III)chloride (FeCl3), 2-(4-amino phenyl) oxazo-ine, 1,1′-carbonyldiimidazole, sodium poly(styrene sulfonate) (PSS,

w ≈ 70,000 g/mol), 1-pyrenemethylamine hydrochloride (PMACl), and 1-pyreneacetic acid (PAA) were purchased from Aldrichnd used as received.

.2. Instruments

1H NMR spectra (500.13 MHz) were recorded on a DRX 500MR spectrometer (Bruker). Dimethyl sulfoxide-d6 (DMSO-d6; ı

1H) = 2.50 ppm) was used as solvent, lock, and internal standard.he atom numbering corresponds to those given in Scheme 1.

ATR-FTIR spectroscopy was conducted with a Bruker Tensor 27T-IR spectrometer (Bruker Optics Inc.) with a Golden Gate MKIITR accessory (Specac). Each spectrum comprises 16 scans mea-ured at a spectral resolution of 4 cm−1. Absorbance spectra werecquired with OPUS MIR Tensor 27 software version 4.0 (Bruker

ptics Inc.).

The matrix-assisted laser desorption–ionization time-of-flightass spectrometry (MALDI-TOF MS) experiments were performed

n a biflex IV system (Bruker Daltonics) with delayed extrac-ion option. Desorption/ionization was performed by a pulsed N2

tic Me

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aser. The mass spectra were obtained from 19 kV accelerationoltage in the reflection mode. An external calibration was usedPEG 2000 standard, Polymer Laboratories). The matrix used with-ut further purification was 1,3,9-trihydroxyanthracene (Dithranol,igma–Aldrich). Solutions of analyte and matrix were preparedith a concentration of 10 g L−1 in chloroform or acetone and mixed

n a molar ratio of 1:10. No salt was used. The measurements werearried out with positive polarity.

Scanning electron microscopy (SEM) images of the dry, gold-oated powders were obtained from a SEM LEO 435 VP.

Fluorescence spectra (excitation wavelength �ex = 341 nm) wereeasured with a LS 50 luminescence spectrometer (PerkinElmer,K). The fluorimeter is corrected for the wavelength dependent

hroughput of the excitation part. Optical micrographs in the brighteld and with fluorescence filter (FITC) were obtained from aeica fluorescence microscope Improvision equipped with a CCDamera and a 20× immersion oil objective. The image acquisi-ion is made with a confocal photomultiplier during a time inhe order of 10 s. For both fluorescent spectra and micrographshe samples (0.002 g) were converted with the pyrene compo-ent (PMA HCl or PAA, 0.002 g) in N-methylpyrrolidone (0.5 mL) at00 ◦C for 10 min, filtered, and washed with methanol, acetone, andichloromethane. For the conversion with 1-pyrenemethylamineydrochloride equimolar amounts of triethylamine as auxiliaryase were added. The dry powders were applied to an object carrier,ispersed in glycerol, and then measured.

A bulk conductivity test stand was designed to measure thelectrical conductivity of dry (co-)polymer powders (Fig. 1). Con-entionally the conductivity of powders is measured with theour-point probe technique. The four probes penetrate the pre-ressed powder and determine the resistance only at the surfacer surface near area of the pellet. In addition the pellet often tendso crack when the needles penetrate [23,35]. Many authors do not

ention the pressure applied for pressing the powders, althoughhe measured electrical conductivity depends on it. The new PC-ontrolled bulk conductivity test stand applies a current and theoltage between the two gold-coated electrodes (both 5 mm iniameter) is measured. Whereas the lower electrode is fixed thepper electrode moves down controlled by distance or pressurend compresses the powder in situ to a pellet. Thus the bulkonductivity of the sample between the two electrodes is mea-ured in dependence on the pressure and surface effects can beeglected. Furthermore, knowing the mass of the powder insertednd analyzing permanently the distance between the electrodeshe bulk density of the compressed pellet in dependence on pres-ure can be determined, too. In general, the test stand can besed for powders, granules, and disks up to 5 mm in diameter.he measurement range is between 103 and 10−9 S/cm; a pressurep to 40 MPa can be applied. For comparison, few samples haveeen measured additionally with conventional four-point probeechnique.

.3. Monomer synthesis

.3.1. Synthesis of N-(3-aminopropyl)pyrrole (M477)N-(3-Aminopropyl)pyrrole (M477) was synthesized by the

eduction of N-(2-cyanoethyl)pyrrole (M470) according to the pro-edure described in the literature [26,27]. N-(2-Cyanoethyl)pyrrole21.6 g, 0.18 mol) was dissolved in 100 mL anhydrous diethyl ethernd added dropwise to 100 mL of 1 M LiAlH4 in anhydrous diethyl

ther. The mixture was then refluxed for 2 h. Excess of LiAlH4 wasestroyed with water and N-(3-aminopropyl)pyrrole was collecteds an orange oil in a yield of 82% after filtration, drying over Na2SO4nd rotary evaporation of the ether phase. The structure was veri-ed by 1H NMR, ATR-FTIR, and TGA.

y0wRw

tals 159 (2009) 103–112 105

.3.2. Synthesis of N-(2-carboxyethyl)pyrrole (M479)N-(2-Cyanoethyl)pyrrole (M470) was hydrolyzed to N-(2-

arboxyethyl)pyrrole (M479) [23,28,29]. For that, N-(2-yanoethyl)pyrrole (24 g, 0.2 mol) was added to 60 mL of 6.7 MOH. The mixture was heated to reflux under an inert atmospheref nitrogen until the solution became orange and a litmus testhowed that no more NH3 evolved. After 2 h the solution wascidified to pH 5 using 8 M HCl, extracted with dichloromethane,nd dried over Na2SO4. The crude product was obtained afterotary evaporation. Recrystallization from n-heptane resulted inhite needle-like crystals (yield 51%). The chemical structure was

onfirmed by melting point 60–66 ◦C, 1H NMR, and ATR-FTIR.

.3.3. Synthesis of M488N-(3-Aminopropyl)pyrrole (M477) (8.5 g, 0.07 mol) and com-

ound CAA [34] (10.05 g, 0.034 mol) were dissolved in toluene andeated to reflux under an inert atmosphere of nitrogen. The conver-ion was observed by taking samples out of the reaction mediumnd analyzing them by ATR-FTIR. The intensity of the oxazinoneand at 1766 cm−1 decreases whereas the amide band at 1671 cm−1

ncreases with reaction time. After 2 h the oxazinone band disap-eared in the ATR-FTIR spectra and the toluene was removed byotary evaporation. The resulting oil was dissolved in THF (25 mL)nd precipitated in an excess of n-hexane (200 mL). The brownisholored product was filtered off, washed with n-hexane and driednder vacuum. Yield 63%, mp 95 ◦C.

1H NMR (500 MHz, DMSO-d6) ı: 12.45 (s, 1H, NH-Ar), 9.16 (s,H, H8), 8.97 (t, 1H, NH-Alk), 7.96 (d, 2H, H11), 7.91 (d, 1H, H6), 7.67d, 1H, H7), 7.65 (t, 1H, H13), 7.60 (t, 2H, H12), 6.77 (t, 2H, H2), 5.97 (t,H, H1), 4.46 (t, 2H, H9), 4.01 (t, 2H, H10), 3.96 (t, 2H, H3), 3.29 (q, 2H,5), 1.98 (m, 2H, H4). ATR-FTIR: 1671 (C O, NH–C O), 1643 cm−1

C O, N C–O).

.3.4. Synthesis of M489Compound CAB [33] (10 g, 0.033 mol) was dissolved in

00 mL anhydrous THF. N-(3-Aminopropyl)pyrrole (M477) (4.35 g,.035 mol) and a molar excess of triethylamine (7.08 g, 0.07 mol)ere dissolved in 60 mL anhydrous THF and added dropwise to the

tirred solution. The reaction mixture was cooled with an ice bathuring the addition of the reactants and left to proceed for 1 h atT under an inert atmosphere of nitrogen. Then the mixture waseated to reflux for 10 min. The excess of compound CAB and the

ormed triethylamine hydrochloride were filtered off and the sol-ent was removed by rotary evaporation. The crude product wasissolved in THF (25 mL) and precipitated in an excess of n-hexane200 mL). The resulting product was dissolved in ethanol, heated toeflux, filtered, and dried under vacuum. An ocher product with aield of 75% and a melting point of 287–296 ◦C was obtained.

1H NMR (500 MHz, DMSO-d6) ı: 8.84 (t, 1H, NH), 8.24 (d, 1H, H8),.23 (d, 2H, H9), 8.16 (s, 1H, H6), 8.02 (d, 1H, H7), 7.69 (t, 1H, H11),.62 (t, 2H, H10), 6.80 (t, 2H, H2), 5.99 (t, 2H, H1), 3.97 (q, 2H, H3),.29 (t, 2H, H5), 1.99 (m, 2H, H4). ATR-FTIR: 1757 (C O), 1651 cm−1

C O, NH–C O).

.3.5. Synthesis of M520N-(2-Carboxyethyl)pyrrole (7 g, 0.05 mol) and 1,1′-carbonyl-

iimidazole (8.16 g, 0.05 mol) were dissolved in 60 mL anhydrousHF and stirred for 1 h at RT under nitrogen atmosphere. Carbonioxide evolves during the formation of imidazol-1-yl-3-pyrrol-1-

l-propan-1-one (IPP). Then 2-(4-amino phenyl)oxazoline (8.05 g,.05 mol) was added dropwise to the stirred solution. The reactionas stirred under nitrogen for 1 h at RT, 40 min at 50 ◦C, 40 min atT, and left overnight, successively. The product was filtered off,ashed with n-hexane and dried under vacuum. A white solid was

106 J. Hegewald et al. / Synthetic Metals 159 (2009) 103–112

Table 1Effect of synthesis condition on the electrical bulk conductivity of polypyrrole and N-oxazinone and/or N-oxazoline modified polypyrrole powders

Sample Pyrrol/comonomer molar ratio Oxidant/dopant Solventd Powder bulk conductivity (S/cm)

1a Py (100) (NH4)2S2O8 H2O 0.0302b Py (100) FeCl3 H2O 1.713b Py/488 (83/17) FeCl3 H2O/acetone (80/20, vol.%) 3.1 × 10−9

4b Py/489 (83/17) FeCl3 H2O/acetone (80/20, vol.%) 2.1 × 10−8

5a Py/488 (83/17) (NH4)2S2O8 H2O/acetone (80/20, vol.%) <1 × 10−9

6a Py/489 (83/17) (NH4)2S2O8 H2O/acetone (80/20, vol.%) <1 × 10−9

7b Py/488 (83/17) FeCl3 DCM/H2O (80/20, vol.%) 9.9 × 10−6

8b Py/489 (83/17) FeCl3 DCM/H2O (80/20, vol.%) 1.8 × 10−3

9b Py/488/489 (70/15/15) FeCl3 DCM/H2O (80/20, vol.%) 3.3 × 10−4

10c Py (NH4)2S2O8/PSS H2O 0.1411c Py/488 (83/17) (NH4)2S2O8/PSS H2O 0.01812c Py/489 (83/17) (NH4)2S2O8/PSS H2O 0.029

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btained with a yield of 80%, mp 238–241 ◦C. The solubility in DMSOs poor.

1H NMR (500 MHz, DMSO-d6) ı: 10.18 (s, 1H, NH), 7.81 (d, 2H,5), 7.67 (d, 2H, H6), 6.74 (t, 2H, H2), 5.96 (t, 2H, H1), 4.36 (t, 2H,7), 4.20 (t, 2H, H8), 3.92 (t, 2H, H3), 2.80 (t, 2H, H4). ATR-FTIR: 1684

C O, NH–CO), 1640 cm−1 (C O, N C–O).

.3.6. Synthesis of M523N-(2-Carboxyethyl)pyrrole (3 g, 0.02 mol) and 1,1′-

arbonyldiimidazole (3.5 g, 0.02 mol) were dissolved in 50 mLnhydrous THF and stirred for 1 h at RT under nitrogen atmo-phere. Anthranilic acid (2.96 g, 0.02 mol) was dissolved withriethylamine (2.18 g, 0.02 mol) in anhydrous THF and added to theolution, stirred 1 h at RT, and 1 h at 50 ◦C under an inert atmo-phere of nitrogen. The solvent was removed by rotary evaporationnd the resulting oil was treated with aqueous HCl. The white pre-ipitate was filtered off, thoroughly washed with distilled water,nd freeze-dried (yield 63%). 50 mL acetic anhydride and a catalyticmount of sodium acetate were added to the intermediate. Theixture was heated to reflux for 2 h under nitrogen atmosphere.fter cooling, 50 mL of acetic acid were added and the solutionas slowly poured into 500 mL distilled water. The yellowishrecipitate was filtered off, washed with water, and freeze-driedoverall yield 42%, mp 75–77 ◦C).

1H NMR (500 MHz, DMSO-d6) ı: 8.11 (d, 1H, H5), 7.92 (t, 1H, H7),.61 (t, 1H, H6), 7.60 (d, 1H, H8), 6.81 (t, 2H, H2), 5.96 (t, 2H, H1),.34 (t, 2H, H3), 3.14 (t, 2H, H4). ATR-FTIR: 1753 cm−1 (C O).

.4. Chemical (co-)polymerization of pyrrole and pyrroleerivatives

The PPys were prepared by chemical oxidative (co-)olymerization in acetone/water or dichloromethane/waterolutions using FeCl3 or (NH4)2S2O8 as oxidant (Table 1). A typicalomopolypyrrole synthesis was carried out as follows: Pyrrole0.838 g, 12.5 mmol) was diluted in 100 mL dichloromethane andooled with an ice bath. A solution of FeCl3 (4.67 g, 28.8 mmol)n 25 mL deionized water was dropped slowly into the stirredolution and the polymerization was allowed to proceed for 48 h.he final product was centrifuged at 20,000 rpm for 20 min andedispersed in deionized water using an ultrasonic bath. This

entrifugation–redispersion cycle was repeated three times inrder to purify the black sediment. The aqueous dispersionsere then freeze-dried using a CHRIST Alpha 1-2/LDplus freezeryer. Essentially the same procedure was utilized for functionalopolymers, except that pyrrole monomer was partially replaced

tm

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y the corresponding functionalized monomers (compositions seeable 1).

.5. Chemical (co-)polymerization of pyrrole and pyrroleerivatives in the presence of sodium poly(styrene sulfonate)

PPy/PSS composites were prepared by chemical oxidationf 0.14 M (0.938 g) pyrrole in a 0.042 M aqueous solution ofodium poly(styrene sulfonate) (0.865 g). Equimolar amounts ofNH4)2S2O8 (3.195 g) to monomer were dissolved in water anddded dropwise to the ice bath cooled solution. After 48 h the blackrecipitate was separated from the reaction mixture by centrifug-

ng, washed with deionized water, and freeze-dried.

. Results and discussion

.1. Monomer synthesis

Scheme 1 displays synthetic routes to a series of N-substitutedyrrole derivatives with pendant oxazoline and oxazinone moi-ties. The synthesis started from N-(2-cyanoethyl)pyrrole (M470).he reduction towards N-(3-aminopropyl)pyrrole [26,27] and thelkaline hydrolysis to N-(2-carboxyethyl)pyrrole [1,28,29] were car-ied out as described elsewhere. The nucleophilic ring openingeaction between Monomer M477 and the coupling agent CAA [33]rovided the oxazoline functionalized pyrrole M488. The oxazi-one functionalized pyrrole M489 was obtained in good yield afterhe conversion of M477 with the acid chloride CAB [34] in theresence of an auxiliary base. Alternatively, M520 and M523 wereynthesized utilizing M479 as pre-cursor.

Whereas the monomers M523 and M520 are characterized bythylene spacers between the pyrrole ring and the pendant func-ional units M488 and M489 are distinguished by trimethylenepacers. Recently, Li et al. have shown that tetramethylene unitsre long enough to fully decouple the electronic effect of the bulkyunctional group on the pyrrole ring [36]. Reducing the distanceetween pyrrole ring and functional unit will result in a shortfficient conjugation length and hence in poor conductivities. Byontrast Allcock et al. observed lower conductivities for polymersith longer spacers linking the functional unit to the backbone and

xplained these findings by means that longer spacers increase

he distance between the polypyrrole chains within the polymer

atrix, thus the interchain hopping distance [23].However, it turned out that the oxazinone function (linked to

he aliphatic spacer) in monomer M523 is vulnerable to hydrolysis.urthermore, the solubility of M520 is poor. Therefore the focus in

J. Hegewald et al. / Synthetic Me

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ig. 2. MALDI-TOF mass spectrum of N-substituted pyrrol derivatives (structuresee Scheme 1).

he following is drawn on the pyrrole derivatives with trimethy-ene spacers, M489 and M488, carrying an oxazinone or oxazolineunction, respectively.

500 MHz-NMR, MALDI-TOF MS (Fig. 2), and ATR-FTIR (Fig. 3)ere carried out to confirm the structure of the new pyrroleonomers. Although 1H NMR spectra clearly evidenced the struc-

ure and purity of the monomers MALDI-TOF MS spectra wereecorded to determine the absolute molar mass of the monomers.ig. 2 shows the MALDI-TOF MS spectra of the matrix (only foromparison) and the aforementioned monomers. The solid arrowsndicate the peaks corresponding to the absolute molecular weightf the monomers M523, M520, M488, and M489. Besides theonomer peaks dimer species of M488 and M489 were recorded

dotted arrows). The reason for the dimer formation is not clearet, but we assume that the dimers were formed during theynthesis. However, the minor quantities of formed dimers arexpected to have no negative effect on the (co-)polymerization ofhe monomers. Since the NMR spectra are in agreement with pure

onomers the other peaks in the MALDI-TOF MS spectra of M488re most likely caused by fragmentation and coalescence (recom-ination of fragments with fragments or intact molecules in the ionource) [37] during the MALDI-TOF process. The matrix assistanceith M523 is obviously very poor, hence only a weak signal corre-

ig. 3. ATR-FTIR spectra of PPy and PPy copolymers polymerized inichloromethane/water (80/20, vol.%) with FeCl3 (solid lines) as well as ofhe corresponding oxazoline and oxazinone modified monomers used foropolymerization (dotted lines, numeration see Table 1).

ott

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tals 159 (2009) 103–112 107

ponding to M523 is detected compared to the more pronouncedatrix peaks in the spectra.

.2. Chemical (co-)polymerization of pyrrole and pyrroleerivatives

Several PPys and copolymers from pyrrole with oxazinoneM489), oxazoline (M488), and also amino (M477) and carboxyM479) functionalized monomers were synthesized. However,n the following only the PPy homopolymers in comparison toopolymers of pyrrole with M488 and/or M489 are discussed.he polymers were synthesized as described in the experimen-al section and characterized to compare physical and chemicalroperties.

Scheme 2 depicts the copolymerization of pyrrole with M489nd M488 to give a bi-functionalized copolymer. Comonomer feedatios of 83:17 for mono-functionalized polymers and 70:15:15 fori-functionalized polymers were chosen in order to obtain a certainegree of functionalization concomitant with reasonable electricalonductivities. It is known that high degrees of functionalizationause a decrease in the electrically conductivity of the bulk material22]. More importantly, just a few chemical groups might be reac-ive toward other functional groups, e.g. of immobilized proteinsr other biomolecules [29]. Table 1 gives a representative overviewf the synthesis conditions and the electrical conductivities of theesultant powders measured with the bulk conductivity test standt 30 MPa. It can be depicted that the comonomer feed, the sol-ent, and the oxidant have an enormous effect on the electricalonductivity of the conducting (co-)polymers. The conductivitiesange from 1.7 S/cm for PPy polymerized with FeCl3 in aqueousolution down to values below 1E−09 S/cm for copolymers oxidizedith ammonium persulfate in a mixture of acetone and water. It

hould be noted that the electrical conductivities determined withhe bulk conductivity test stand are in general one to two ordersf magnitude lower than the values determined by the conven-ional four-point van der Pauw method (same pressure applied tohe powder, 30 MPa), e.g. a value of 0.03 S/cm for 1 determined byhe bulk conductivity test stand corresponds to a value of 0.46 S/cmetermined by the four-point probe technique. However, as afore-entioned the van der Pauw four-point method exhibits some

istinct disadvantages: The four probes determine the resistancenly at the surface or surface near area of the pellet. In additionhe mechanical stability of the pre-pressed pellets is crucial for thisechnique [23,35].

In general, the electrical conductivity of the copolymers isower compared to PPy homopolymers. It is known that the addi-ion of substituents to the pyrrole ring is linked with a failuren ring planarity during polymerization, resulting in disruptionf the delocalized �-electron backbone, which in turn limits theonductivity of the resulting polymer [1,24]. The dark color of con-ucting polymers is also associated to the degree and perfectnessf conjugated sequences in the polymer chains [23]. Indeed, theomopolymerization of PPy (1, 2) and the copolymerization inichloromethane/water (7–9) yielded dark black insoluble pow-ers, whereas the copolymers prepared in acetone/water (3–6)re brownish. Best conditions for the synthesis of whether oxa-oline or oxazinone modified copolymers or for oxazoline andxazinone bi-functionalized PPys, neglecting the later discussedse of PSS as dopant, were found utilizing FeCl3 as oxidant inichloromethane/water (80/20, vol.%).

The presence of oxazinone and oxazoline functionalities innsoluble conducting polymer powders was confirmed using ATR-TIR spectroscopy. Fig. 3 shows the ATR-FTIR spectra of theon-modified PPy (2), the copolymer of pyrrole with M489poly(py-co-489), 8), the copolymer of pyrrole with M488 (poly(py-

108 J. Hegewald et al. / Synthetic Metals 159 (2009) 103–112

s cont

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Scheme 2. Copolymerization of pyrrole with N-substituted pyrrole derivative

o-488), 7), the copolymer of pyrrole with M489 and M488poly(py-co-488-co-489), 9) as solid lines and of the monomers

489 and M488 as dotted lines.The spectrum of 9 shows in comparison to the spectrum of non-

odified PPy (2) a new band at 1780 cm−1 and an adumbratedhoulder (partially superimposed from PPy) at 1685 cm−1 corre-ponding to the oxazinone and oxazoline moieties, respectively. Inddition, the strong C–H and N–H stretches at 2970 and 3290 cm−1

f the monomers are evident also in the spectra of the oxazoline and

xazinone modified copolymers. The characteristic oxazinone bandt 1780 cm−1 appears also in 8 and the oxazoline band is expresseds shoulder in 7 at 1696 cm−1. All the characteristic bands for bothxazinone and oxazoline functions in the polymer spectra of 7–9 are

pd

n

Fig. 4. SEM images of pure and modified PPy powders (nume

aining an oxazoline (M488) and/or an oxazinone group (M489), respectively.

hifted to slightly higher wavenumbers compared to the monomerpectra, which can be caused by interactions (hydrogen bonding) ofhe pendant moieties with the polypyrrole backbone [38]. Thus it isuggested that the side chains are lying out of plane with the pyrroleings. This effect is particularly pronounced in 7, due to the bulkyendant unit of M488. The spectra were not baseline corrected.he broad absorption band above 1800 cm−1 is characteristic tohe highly conjugated structure of the polypyrrole and, in contextf visual diagnostic assays, makes these intrinsically chromogenic

olypyrroles an interesting alternative to conventional extrinsicallyyed polystyrene latices [29].

Fig. 4 presents SEM micrographs of dry powders of PPy, oxazi-one modified PPy, oxazoline modified PPy, and bi-functionalized

ration see Table 1). The white bar corresponds to 2 �m.

J. Hegewald et al. / Synthetic Metals 159 (2009) 103–112 109

F 1) oxit

Psc(tlpiawiamBldcmlotmiPiyu

3i

nabfisirpFatoLit

iacbtveabttWbpPdsBs2roanbnwb

3cc

otpoebb

ig. 5. Representative SEM (a) and TEM (b) images of PPy/PSS composites (10, Tablehe cauliflower-like surface.

Py. Generally the PPy particles tend to agglomerate and pos-ess a cauliflower-like structure. These features are visible inase of PPy and poly(py-co-489). Whereas the non-modified PPy2) has a rather compact and homogeneous globular surfacehe oxazinone modified PPy (8) is distinguished by smaller andess densely packed subdomains. This can be attributed to theendant side chains, which typically hinder a compact packag-

ng. However, the morphology is also influenced by the solventnd the polymerization temperature [39]. PPy homopolymers (2)ere polymerized in water. The copolymers 7–9 were oxidized

n dichloromethane/water (80/20, vol.%). On the contrary to theforementioned oxazinone modified PPy (8), the other two copoly-ers (7 and 9) exhibit significant differences in their morphology.

oth reveal a rather heterogeneous structure with globular andeaf-like features. This might be explained with the rigid, bulky pen-ant unit of M488, which promotes interactions between the sidehain and the polypyrrole backbone, thereby hampering the poly-erization process, which results in reduced efficient conjugation

engths. This assumption is supported by the low conductivitiesbtained for the oxazoline modified PPys. Furthermore, we suggesthat during the oxidative chemical polymerization not only copoly-

er species were formed. Thus we ascribe the globular structuresn the SEM micrograph of 7 partially to the collateral formation ofPy homopolymers. The effect is diminished during the copolymer-zation of pyrrole with both, oxazoline and oxazinone monomersielding bi-functional polypyrrole. Computational simulations tonderlay these findings are subject of our present work.

.2.1. Synthesis of oxazoline and oxazinone functionalizedntrinsically conducting polypyrrole core-shell-like particles

In order to make the previously discussed oxazoline and oxazi-one functionalized copolymers more accessible for potentialpplications, e.g. in (bio-)sensors or in visual diagnostic assays, asioadsorbent or for cell adhesion experiments, we have tried tond a way to produce these intrinsically conducting polymers withome degree of controlled dimension and shape by a method, whichs simple, cheap, fast, and can be easily scaled up. Qi and Pickupeported the chemical polymerization of size controlled sphericalolypyrrole particles using poly(styrene sulfonate) as template andeCl3 as oxidant [35]. Such polypyrrole/polyanion composites havettracted much interest in recent years, since they have high elec-

rical, ionic, and proton conductivity. Numerous publications existn the electrochemical preparation of such composites. However,i and Kumacheva found out that chemically prepared compos-tes are more porous than electrochemically prepared films andherefore have higher ionic conductivities [40]. Based on these find-

alrob

dized in aqueous solution of sodium poly(styrene sulfonate) with (NH4)2S2O8. Note

ngs we dispersed the monomers in an aqueous solution of PSSnd added the oxidant ((NH4)2S2O8) dropwise. The resulting parti-les were insoluble and easily separated from the reaction mediumy centrifugation. The electrical conductivities in dependence onhe used comonomers are presented in Table 1. The conductivityalues are increased compared to those without PSS. This positiveffect is especially pronounced for the copolymers with function-lized monomers. Conductivities above 0.01 S/cm are reached foroth, oxazinone and oxazoline modified copolymers. We ascribehis effect to �–� interactions between PSS, pyrrole, and excep-ionally M488 and M489 that carry rigid, bulky pendant side chains.

e believe that the monomers interact with the PSS via hydropho-ic forces. This enhances the local concentration and facilitates theolymerization resulting in compact PPy/PSS structures. Thus theSS acts as molecular template and is incorporated into the oxi-ized polymer as dopant. This assumption is supported by thecanning and transmission electron micrographs shown in Fig. 5.oth images represent spherical particles with a raspberry-likeurface morphology and an estimated average diameter of around50 nm. Dynamic light scattering of a dispersed aqueous solutionevealed an average hydrodynamic radius of 230–275 nm. More-ver, the transmission electron micrograph (see Fig. 5b) revealscore-shell-like structure of the PPy/PSS composite. It should beoted that PPy/PSS composites are due to the excess of immo-ile sulfonate group’s cation exchangers at reduction. Since cationsormally move faster than anions [41], the charge transport rateill be faster making these particles interesting for applications in

atteries, supercapacitors, and fuel cells [35,42–44].

.3. Verification of the oxazinone and oxazoline moieties in theonducting PPy copolymers by model reaction with amino- andarboxy-containing fluorophores

ATR-FTIR spectra (Fig. 3) have indicated that oxazoline andxazinone moieties were correctly incorporated into the syn-hesized copolymers and withstood the oxidative chemicalolymerization process. Fluorescence measurements were carriedut to verify the presence of the oxazinone and oxazoline moi-ties in the copolymer and to proof their reactivity, which shoulde influenced by the conjugated sequences of the polypyrroleackbone and its electrostatic interactions with the counterions

nd the surrounding media. Selective model reactions of oxazo-ine and oxazinone moieties with carboxy and amino functions,espectively, were reported by Jakisch et al. [33,34]. Although flu-rescence microscopy can only image the surface of the insoluble,lack powders we converted PPy (2) and functionalized copoly-

110 J. Hegewald et al. / Synthetic Metals 159 (2009) 103–112

F ylaminf yrenem were1

msmP(otflhraoPtoco

(

wsmtwaceaPb

cc

ig. 6. Phase contrast (top) and fluorescence (bottom) images of (A): 1-pyrenemethunctionalized PPy (poly(py-co-489) 8, Table 1) converted with PMA (right); (B) 1-P

odified PPy (poly(py-co-488), 7, Table 1) converted with PAA (right). The samples00 pixels (=62.5 �m).

ers (7–9) with amino and carboxy-containing fluorophores. Fig. 6hows both, fluorescence (bottom) and the corresponding opticalicroscopy images (top). Panel A demonstrates the conversion of

Py (2) and oxazinone modified PPy (8) with 1-pyrenemethylaminePMA). To act as reference the pure fluorophore (PMA) is depictedn the left. Both samples (2 and 8) were subjected to the samereatment; however, there is no noticeable fluorescence in theuorescence image of the non-reactive pure PPy 2, whereas aeterogeneous coverage of 8 with the PMA is detected. This indi-ectly validates the coupling reaction of oxazinone groups withmino functions of PMA. Panel B represents similar results for thexazoline-acid combination. PPy (2) and the oxazoline modifiedPy (7) were treated with 1-pyreneacetic acid (PAA). Again, whereashe non-modified PPy (2) exhibits no noticeable fluorescence spots

f the fluorophore after the treatment, the bright fluorescence islearly visible in the image of 7 indicating the chemical conversionf the oxazoline groups with the carboxy groups of the PAA.

These results were confirmed by fluorescence spectroscopyFig. 7). The bi-functionalized PPy (9) and non-modified PPy (2)

tmflul

e (PMA, left), pure PPy (2, Table 1) converted with PMA (middle), and N-oxazinoneacetic acid (PAA, left), pure PPy (2) converted with PAA (middle), and N-oxazoline

applied to an object carrier and dispersed in glycerol. The white bar corresponds to

ere treated with PMA and PAA, respectively. Comparing the emis-ion spectra after treatment with the amino and the carboxyodified fluorophore only the bi-functionalized PPy samples show

he characteristic emission bands for pyrene at 377 and 397 nm,hile from the non-reactive PPy the fluorophores PAA as well

s PMA are washed out completely during the preparation pro-ess (see Section 2.2). It is important to note that no fluorescencemission spectra were recorded from dry PPy powders. To obtainnalyzable emission spectra of the inherently black and insolublePy powders it was necessary to increase their specific surface areay dispersing them in glycerol.

Inferential, the reaction of the reactive pyrroles with fluorophorean also be utilized to induce fluorescence to the electricallyonducting material. This is particularly interesting in the con-

ext of sensor applications. One can also imagine to use these

aterials in polymers or polymer blends. The incorporation ofuorophores into a polymer matrix will protect them from itsndesirable interaction with oxygen and enhances the fluorescence

ifetime [45]. Thus electrical conductivity and fluorescence can

J. Hegewald et al. / Synthetic Me

Fig. 7. Fluorescence emission spectra of pure PPy (2, Table 1) and of the oxazolineagI4

banara

4

oTOMtvdcmMstocasmed

tttitffmwrhei

gt

cnaampcoptWaMao

aptFlcma

A

fUtfosOt

R

[

[[

[[

[[

[

nd oxazinone containing poly(py-co-488-co-489) (9, Table 1), both dispersed inlycerol and treated with the reactive pyrenes PMA or PAA, respectively; I1 = 377 nm,

3 = 397 nm, �ex = 341 nm. For comparison, the spectrum of non-treated poly(py-co-88-co-489) is shown.

e brought to the polymeric material simultaneously. Besides theforementioned applications these novel oxazoline and/or oxazi-one functionalized PPys can be employed in diagnostic assaysnd as bioadsorbents. Thereby, morphological features, like theevealed porous surface adjustable by the comonomer feed are ofdvantage.

. Conclusions

With the synthesis of pyrrole derivatives bearing oxazoline andxazinone moieties, four new pyrrole monomers were produced.he basic compound for the synthesis was N-(2-cyanoethyl)pyrrole.ne route leads beyond N-(3-aminopropyl)pyrrole to monomers488 and M489 characterized by a trimethylene spacer between

he pyrrole ring and the pendant functional units. A second pathwayia N-(2-carboxyethyl)pyrrole yields in monomers M520 and M523istinguished by ethylene spacers. Although both methods provideompounds in high yield and high purity, the latter mentionedonomers have distinct disadvantages. The oxazinone group in523 is vulnerable to hydrolysis and M520 turned out to be poorly

oluble. Therefore, only M488 and M489 were then used for oxida-ive copolymerization with pyrrole to yield novel oxazoline andxazinone mono- and bi-functionalized intrinsically conductingopolymers. A new bulk conductivity test stand was developed tollow the measurement of the electrical bulk conductivity of theynthesized powders in dependence on the pressure. Best poly-erization conditions targeting functional copolymers with high

lectrical conductivities were found utilizing FeCl3 as oxidant inichloromethane/water (80/20, vol.%).

The synthesized (co-)polymers were further characterized inerms of their chemical and physical properties by ATR-FTIR spec-roscopy, SEM, and TEM. The ATR-FTIR measurements confirmedhat the oxazinone and oxazoline functionalities were properlyncorporated into the synthesized copolymers and both func-ional groups withstood the polymerization conditions. However,or further applications it was important to show, whether theunctionalities retain their reactivity in the copolymeric environ-

ent. For that purpose non-modified PPys and functionalized PPys

ere converted with carboxy and amino modified pyrene fluo-

ophores. Whereas no noticeable fluorescence was observed for PPyomopolymers both, fluorescence microscopy and fluorescencemission spectra of the functionalized copolymers indirectly val-dated the coupling reaction between the oxazinone and oxazoline

[

[[

tals 159 (2009) 103–112 111

roups of the copolymers with the amino and carboxy groups ofhe fluorophores, respectively.

The SEM image of PPy homopolymers shows the well-knownompact cauliflower morphology. In contrast, the surface of oxazi-one modified PPys is due to the pendant groups less compactnd reveals smaller subdomains. The SEM images of oxazolinend bi-functionalized copolymers exhibit a more heterogeneousorphology. We ascribe these findings to the extremely bulky

endant side chain of M488 hampering the polymerization pro-ess. This becomes also evident from the poor conductivity ofxazoline-modified copolymers. However, this drawback is com-ensated employing PSS as dopant. Electrical bulk conductivities inhe range of >0.01 S/cm are obtained for functionalized copolymers.

e believe that hydrophobic interactions between the PSS chainsnd the monomers are beneficial for the copolymerization process.oreover, this method provides a facile route to control dimension

nd shape of PPy particles, without using steric stabilizers [46,47]r auxiliary templates [40,48,49].

The utilization of the novel oxazoline and oxazinone function-lized copolymers under discussion will be the subject of furtherublications. We are currently working on the optimization ofhe oxidative (co-)polymerization with organic oxidant systems.urthermore, studies are underway to apply these novel oxazo-ine and/or oxazinone functionalized copolymers e.g. as electricalonductive interfacial modifier and compatibilizer in reactive poly-er blends in analogy to silane-containing bi-functional coupling

gents [50].

cknowledgements

The authors thank D. Kuckling (Technische Universität Dresden)or the help in MALDI-TOF MS, H. Komber for 1H NMR spectroscopy,. Oertel and B. Pilch for fluorescence spectroscopy, K. Alberti for

he help in fluorescence microscopy, and K. Riess and H. Kunathor their support in the laboratory. P. Fedorko (Slovak Universityf Technology, Bratislava) is acknowledged for conductivity mea-urements using the van der Pauw method. We are grateful for M.mastová for the helpful discussions. The work was supported by

he Deutsche Forschungsgemeinschaft (DFG) within the SFB 287.

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