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Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

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Page 1: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

Water-Soluble Poly(ethylene oxide)-block-Poly(p-phenylene vinylene) Copolymer: Synthesis andCharacterization

CATHERINE BIANCHI, BRUNO GRASSL, BERNARD FRANCOIS, CHRISTINE DAGRON-LARTIGAU

Laboratoire de Physico-Chimie des Polymeres (Unite Mixte de Recherche 5067 Centre National de RechercheScientifique – Universite de Pau et des Pays de I’Adour), Helioparc 2 Avenue President Angot, 64053,PAU Cedex 9, France

Received 28 October 2004; accepted 23 April 2005DOI: 10.1002/pola.20903Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Water-soluble and photoluminescent block copolymers [poly(ethyleneoxide)-block-poly(p-phenylene vinylene) (PEO-b-PPV)] were synthesized, in two steps,by the addition of a-halo-a0-alkylsulfinyl-p-xylene from activated poly(ethylene oxide)(PEO) chains in tetrahydrofuran at 25 8C. This copolymerization, which was derivedfrom the Vanderzande poly(p-phenylene vinylene) (PPV) synthesis, led to partly con-verted PEO-b-PPV block copolymers mixed with unreacted PEO chains. The yield,length, and composition of these added sequences depended on the experimental con-ditions, namely, the order of reagent addition, the nature of the monomers, and theaddition of an extra base. The addition of lithium tert-butoxide increased the lengthof the PPV precursor sequence and reduced spontaneous conversion. The conversioninto PPV could be achieved in a second step by a thermal treatment. A spectral anal-ysis of the reactive medium and the composition of the resulting polymers revealednew evidence for an anionic mechanism of the copolymerization process under ourexperimental conditions. Moreover, the photoluminescence yields were stronglydependant on the conjugation length and on the solvent, with a maximum (70%) intetrahydrofuran and a minimum (<1%) in water. VVC 2005 Wiley Periodicals, Inc. J Polym

Sci Part A: Polym Chem 43: 4337–4350, 2005

Keywords: block copolymers; conjugated polymers; copolymerization photolumines-cence; poly(ethyleneoxide)-block-poly(p-phenylenevinylene);poly(p-phenylenevinylene)

INTRODUCTION

Since the pioneering work of the Cambridgegroup,1 various photoluminescent (PL) and elec-troluminescent (EL) polymers used in light ELdiodes, including the family of poly(p-phenylenevinylene) (PPV), have been developed. Itincludes insoluble films of unsubstituted PPVprepared by a precursor way,2,3 substituted

poly(p-phenylene vinylene) (s-PPV) soluble incommon organic solvents,4,5 block or graftcopolymers with PPV or s-PPV sequencesbonded to nonconjugated sequences, and PPV ors-PPV derivatives with modified backbones.4,6

Furthermore, blends of nonconjugated polymerswith PPV derivatives in certain proportions canresult in significant improvements in optoelec-tronic properties because of exciton confine-ment.7 Diblock copolymers of poly(p-phenylene)(PPP) or polythiophene (PT) with polystyrene(PS) or poly(methyl methacrylate) have beenshown to exhibit enhanced PL and EL proper-ties with respect to pure PPP or PT.8 In pre-

Correspondence to: C. Dagron-Lartigau (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 4337–4350

(2005)VVC 2005 Wiley Periodicals, Inc.

4337

Page 2: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

vious work, our preliminary results showed thatanionic chain ends of poly(ethylene oxide) (PEO)could react with two kinds of PPV monomers:a,a0-dichloro-p-xylene and a-chloro-a0-butylsul-finyl-p-xylene. Besides the functionalizationwith a PPV monomer, we observed that PPVsequences could be formed during the reaction.9

Such systems could be applied to EL electro-chemical cells by the improvement of the inter-mixture of conjugated and ionic conducting poly-mers. This was confirmed by systems based onPPV derivatives with short PEO chains eithergrafted on the conjugated backbone10 or alter-nating with it11 and with poly(ethylene oxide)-block-oligo(p-phenylene vinylene) (PEO–OPV) di-block copolymers.12

In this work, we developed an original use ofthe Vanderzande method to prepare water-solu-ble diblock copolymer precursors of PEO–PPV,in which the normally insoluble nonsubstitutedPPV sequence is bonded to potentially ionic con-ducting PEO. This method is based on the previ-ously described polymerization of a-halo-a0-Rsul-finyl-p-xylene (I; R ¼ alkyl, Rsulfinyl ¼ polar-izing group),3,13 which led to a precursorpolymer of PPV. We present a modification ofthis method to achieve the copolymerization ofPEO chains with four different monomers. In asecond step, the complete conversion into PEO–PPV can be achieve thermally. The mechanismfor the polymerization of the sulfinated mono-mer in basic media has up to now not been fullyunderstood. Some publications have claimed ananionic process, whereas some more recentarticles have favored a radical mechanism (seeref. 14 and references therein). Nevertheless,the route detailed here and the spectral studiescarried out during the copolymerization processhave permitted a further understanding of thepolymerization mechanism.

The low solubility of PPV with a degree ofpolymerization higher than 3–4 would induceaggregation in aqueous media, influencing theirPL properties. A detailed study of this aggrega-tion has been realized as a function of the sol-vent and concentration, and its drastic effect onthe PL yield is demonstrated.

EXPERIMENTAL

Solvents and Reagents

Schlenk techniques were used throughout thestudy unless otherwise stated; the vessels were

flame-dried and flushed with dry nitrogen. Tet-rahydrofuran (THF) was purified by double dis-tillation over a disodium benzophenone complexand stored under argon in a Schlenk vessel.Dimethylformamide was distilled in vacuo overCaH2 and stored over 4-A molecular sieves. Allother reagent-grade solvents were used withoutfurther purification.

Physical Measurements

Molecular weight determinations by size exclu-sion chromatography (SEC) with respect to PSstandards were carried out with a bank offour columns (HR 0.5, 2, 4, and 6) of Styragel(300 mm � 5 lm) at 40 8C, with THF as aneluent at a flow rate of 1.0 mL min�1, con-trolled by a Waters 2690 pump equipped withan ERC, Inc., 7515A refractive-index (RI)detector and a Waters 996 multiple-wavelengthultraviolet–visible (UV–vis) photodiode arraydetector. 1H NMR (400 MHz) and 13C NMR(100 MHz) spectra were recorded on a BrukerAvance 400 spectrometer. Fluorescence spectro-scopy was performed on a PerkinElmer LS50B.Thermogravimetric measurements were per-formed on a TA Instruments 2950 thermogra-vimetric analyzer under air at a heating rateof 10 8C min�1.

Synthesis of a-Halo-a0-alkylsulfinyl-p-xylene(3a, 3b, 4a, and 4b)

Four monomers (3a, 3b, 4a, and 4b, describedin Scheme 1) were synthesized with knownmethods.3,13 Dichloro-para-xylene in a water/toluene mixture in the presence of a phase-transfer agent (methyltrioctyl ammonium chlor-ide, i.e., Aliquat 336) was reacted with analkylthiol with a slow addition. The obtainedsulfide was then oxidized by hydrogen peroxidein the presence of tellurium dioxide, as detailedin Scheme 1.3,15

The brominated monomers (4a and 4b) wereobtained by the exchange of chlorine for bromineatoms from the chlorinated monomers.13,16 Afterrecrystallization from a mixture of hexane anddichloromethane, the yields were as follows: 3a,26%; 3b, 37%; 4a, 21%; and 4b, 27%. The purity,determined by 1H NMR against an internalstandard (hexamethyl cyclotrisiloxane, singlet at0.18 ppm), was greater than 98%.

4338 BIANCHI ET AL.

Page 3: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

Synthesis of Block Copolymers with PEO

Here is detailed the synthesis of poly(ethyleneoxide)-block-poly(a-halo-a0-alkylsulfinyl-p-xylene)(PEO-b-PASX) derived from mono- or difunction-alized PEO, which is denoted PEOFx, where F isthe functionalization by one or more PPV or PPVprecursor units and x is the experimental number.

The reaction was considered to operate asshown in Scheme 2.

Most of the experiments were carried outwith a monofunctional PEO [number-averagemolecular weight (Mn) ¼ 5000 g mol�1]. Theterminal hydroxyl groups of PEO in THF wereactivated by the slow addition of diphenylmethylpotassium (DPMK) in THF (0.8 mol L�1) undera nitrogen atmosphere, prepared according tothe literature.17 The red DPMK reacted immedi-ately, and the addition was stopped when aslightly red color became stable. The PEOalways contained a small amount of water,despite careful drying. Some potassium hydrox-ide was also produced during this step. Withbifunctional PEO, a strong aggregation of alkox-ide ends was observed and led to an inhomoge-neous medium.

The monomer was then added, at room tem-perature, to the PEO solution up to an equimo-lar proportion of the monomer and PEO chains(e.g., 1.07 equiv for PEOF2; see Table 1 for otherexperiments). Indeed, the polymerization of eachmonomer needed 1 mol of base, and the onlyavailable base was the terminal alkoxide PEO.Aliquots were withdrawn during the additionfor SEC studies. After a few hours, a clear, yel-low solution (absorbing at 390–400 nm) waspoured into an excess of cold diethyl ether. Ayellow, fluorescent solid precipitated (e.g., 91%overall yield of polymerization for PEOF2). Thedifferent experimental conditions for the copoly-merizations with PEO activated by DPMK arepresented in Table 1.

Poly(a-Chloro-a0-butylsulfinyl-p-xylene) (Poly3a)

Vanderzande Method

For comparison, the homopolymerization of 3awas carried out with the Vanderzande method,and the obtained results were in good agree-ment with those described in the literature.3

Scheme 1. Schematic synthesis of a-halo-a0-alkylsulfinyl-p-xylene monomers.

Scheme 2. Schematic process of copolymerization.

WATER-SOLUBLE PEO-b-PPV COPOLYMER 4339

Page 4: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

Table

1.

Experim

entalCon

ditionsof

Syntheses

andAnalysisof

PEO-b-PASX

Cop

olymers

Sample

Addition

Process

nmono(equiv)a

Tem

perature

(8C)

Rea

ction

Tim

e(h)

Cop

olymer

Chains(%

)b

Mea

nPrecu

rsor

Units/mol

ofCop

olymer

c

Mea

nPPV

Units/mol

ofCop

olymer

d

Mea

nPPV

Seq

uen

ceLen

gth

eg c

opo

(%)f

FPEOF

(%)g

Overall

Polymer

Yield

(%)h

PEOF2i

PEO

on3a

1.07

25

90

12

3.0

1.5

>6

46

51

91

PEOF3i

PEO

on3a

1.06

40

65

114.8

1.5

>6

——

—PEOF4i

PEO

on3a

1.08

25

72

16

3.0

2.7

>6

69

78

88

PEOF5i

3aon

PEO

1.25

25

20

20

3.8

0.4

2–3

47

49

96

PEOF6i

PEO

on3b

2.00

25

20

19

5.8

0.03

170

90

78

PEOF7i

3bon

PEO

1.99

25

20

29

20.3

2–3

50

65

77

PEOF8i

4aon

PEO

1.41

25

20

35

1.9

0.3

2–3

46

48

95

PEOF9i

PEO

on4b

2.03

25

20

15

5.5

0.02

159

87

68

PEOF10j

3aon

PEO

2.43

25

20

22

3.4

0.6

3–4

33

54

90

PEOF11

k3aon

PEO

2.13

25

20

47

1.0

0.2

2–3

33

48

98

PEOF12l

PEO

on3a

1.08

25

20

0—

—1

——

95

anmono¼

molenumber

ofmon

omer.

bPercentageof

copolymerized

PEO

chains(determined

by

1H

NMR).

cMea

nnumber

ofnon

convertedprecu

rsor

unitsper

copolymer

chain

(determined

bySEC

andelem

entalanalysis).

dMea

nnumber

ofconvertedPPVunits(determined

bySEC

from

UV–vis

absorp

tion

ofthecopolymer

pea

k).

eMea

nlength

ofconvertedPPV

sequen

cesin

copolymer

chains(from

themaxim

um

absorp

tion

wavelen

gth).

fWeightpercentageof

mon

omer

incorp

oratedinto

copolymer

chains(determined

bySEC

andelem

entalanalysis).

gWeightpercentageof

mon

omer

incorp

oratedinto

copolymer

chainscomparedwiththeamou

ntof

mon

omer

polymerized

incopolymer

andoligom

erch

ains(determined

byelem

entalanalysis).

hPercentageof

polymerized

mon

omer

incopolymer

andoligom

erch

ains(determined

bySEC).

iPEO–OH,M

5000gmol

�1.

jHO–PEO–OH,M

20,000gmol

�1.

kHO–PEO–OH,M

1000gmol

�1.

lPEO–OH,M

5000gmol

�1,activatedbysec-butyllithium.

4340 BIANCHI ET AL.

Page 5: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

Homopolymerization with Our CopolymerizationConditions

The homopolymerization of 3a was also carriedout under the previously described copolymer-ization conditions. A clear, yellow, fluorescentsolution was obtained after 1 h at 20 8C. SECof the recovered yellow, fluorescent polymershowed a bimodal distribution: a peak corre-sponding to small oligomers weakly convertedand another peak at peak molecular weight (Mp)¼ 43,000 gmol�1 corresponding to partly convert-ed poly(a-halo-a0-alkylsulfinyl-p-xylene) (PASX;UV band at 320 nm).

RESULTS AND DISCUSSION

Chemical Characterization of the CopolymerizationProducts

To characterize the copolymerization process, sev-eral parameters were considered, such as thetime, temperature, and order of reagent addition.For each PEOF obtained, the precipitation proc-ess did not allow the separation of the unreactedPEO from the copolymer. Nevertheless, a partialseparation was realized with the lower criticalseparation temperature properties of these poly-mers. Indeed, it is well known that when waterPEO solutions are heated, the polymer precipi-tates.18 Moreover, it was observed that a PEOsubstituted with nonpolar groups precipitated ata lower temperature. The obtained copolymersand unreacted PEO mixture were enriched in thecopolymer by this method.

The copolymerization medium was first char-acterized with an SEC instrument, which alloweda good separation of oligomers and was equippedwith two detectors: a refractometer (RI) and adiode-array UV–vis spectrometer. A completeUV–vis spectrum could be extracted for each elu-tion time, and chromatograms could be extractedfor different absorption wavelengths. The elutiontimes were corrected to consider the distancebetween the RI and UV–vis detectors.

The solutions were filtered through a 0.2-lmTeflon filter before analysis. Practically nothingwas retained on the filter.

A typical example of such chromatograms isgiven in Figure 1.

The raw reaction medium was injected intothe SEC instrument at the end of the copoly-merization with monomer 3a. After the copoly-merization reaction, the initial PEO RI peak

was shifted to a lower elution volume (highermolecular weights), and this proved the copoly-merization. This shift was significant but small;this meant that only small sequences had beenadded onto PEO chains. As pure PEO has noUV–vis response, the chromatograms recordedwith UV–vis at 254 or 390 nm showed the clearaddition of a PASX block partly converted into aPPV sequence onto the PEO chain. The UV–visabsorption corresponded to the left part of theRI peak, indicating that only some of the PEOchains has been copolymerized. This point isdeveloped in the following. A UV–vis spectrumrecorded at the maximum of the copolymer peakis shown in Figure 2 for different reaction times.

At an elution volume higher than 23 mL, bothdetectors exhibited the presence of some partlyconverted oligomers (elution volume ¼ 23–27.5 mL). Their absorption at 390 nm was veryweak in comparison with the absorption of thecopolymer. A peak of the residual monomer wasnoticeable at 28 mL. Even with the very sensi-tive light scattering detector, no peak corre-sponding to soluble aggregates was detected atan elution volume lower than 20.5 mL. In addi-tion, we noticed that after its precipitation andpurification, the copolymer was much less soluble.

We concluded from these qualitative analysesthat only a fraction of the PEO chains werecopolymerized; the attached sequences of PASXwere short and were partly converted into PPV.Moreover, a small amount of partly converted

Figure 1. SEC analysis of the copolymerizationmedium with refractometer and UV–vis spectroscopydetectors (end of the synthesis of PEOF2).

WATER-SOLUBLE PEO-b-PPV COPOLYMER 4341

Page 6: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

oligomers was formed, and a residue of themonomer was observed.

To complete these observations, differentparameters have been quantified for samplesafter copolymerization (PEOFs) as follows (seeTable 1 for the results).

The fraction of PEO (percentage of copolymerchains) that added at least one monomer unitwas determined from the 1H NMR analysis ofthe samples dissolved in dimethyl sulfoxide-d6.This was calculated from the area of the 1HNMR peak H1 (4.53 ppm, triplet) of the left ter-minal OH of unreacted PEO and from peak H2

(4.49 ppm, singlet) of the protons of ��OCH2Ph

typical of the attachment of a monomer unitonto a PEO chain [Fig. 3(a)].

The position of this last peak was confirmedby the synthesis of a model molecule by thereaction of activated PEO with an excess ofdichloroparaxylene [with t-BuONa as the basein THF for 40 h at 40 8C; Fig. 3(b)].

The percentage of copolymerized PEO chainsis given by

gf ¼ H2=2

H2=2þH1� 100

The mean PASX content of the copolymerizedsequences (mean precursor units/mol of copoly-

Figure 3. 1H NMR spectra of mixtures of OH-terminated PEO and PEO reactedwith (a) monomer 3a or (b) dichloro-p-xylene. The ��CH2�� band at 4.49 ppm allowsa quantitative determination of the mixture composition.

Figure 2. UV–vis spectra of the copolymer peak (at 21.5 cm�3) extracted from theSEC analysis of the copolymerization medium at various reaction times.

4342 BIANCHI ET AL.

Page 7: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

mer) was determined from the elemental analysisof the S content in the samples after copolymer-ization and by the consideration of the previouslydetermined percentage of copolymerized PEO.

The amount and mean lengths of the convertedPPV sequences (mean PPV units/mol of copoly-mer) were evaluated from the maximum absorp-tion of the copolymer UV–vis spectra recordedwith the SEC UV–vis detector. These calculationswere based on the published relationship betweenthe maximum wavelength of absorption and thedegree of polymerization of oligo-PPV.19 It wasthought that a saturation of the wavelength shiftoccurred for a degree of polymerization equal to6. The extinction coefficient for a PPV unit wastaken to be 20.000 L mol�1 cm�1.20

The overall yield of polymerization (overallpolymer yield) was determined from the varia-tion of the monomer peak in the SEC analysis ofthe reaction medium. The fraction of polymer-ized monomer incorporated into a block copoly-mer or in the oligomers was deduced from SECanalysis with a UV–vis detector at 254 nm.

The copolymerization yield corresponded tothe weight percentage of the monomer incorpo-rated into copolymer chains in comparison withthe initial weight of the monomer. It was deter-mined from SEC and elemental analysis results.

The weight percentage of monomer unitsincorporated into copolymer chains, in compari-son with the monomer that polymerized eitherin copolymer or oligomer chains, was deter-mined from elemental analysis to quantify theproportion of the copolymerization process.

It is clear from the results presented in Table 1that a partial conversion of the precursor sequen-ces to PPV took place simultaneously with thecopolymerization process. Because no conversionwas observed for the previously reported homopo-lymerization,3 we carried out the homopolymeriza-tion of 3a with our copolymerization conditions. Asa clear, yellow, fluorescent solution was obtainedafter 1 h at 20 8C; this experiment confirmed thata partly converted PASX homopolymer could beformed. The SEC analysis of the recovered yellow,fluorescent polymer showed a bimodal distribu-tion: a peak corresponding to small oligomersweakly converted and another peak correspondingto partly converted PASX (UV band at 320 nm).

Furthermore, we can make two remarksabout the temperature and time conditions:

� A comparison of the results obtained forPEOF3 with PEOF2 and PEOF4 showed

that heating at 40 8C slightly favored thelength of PASX, whereas it minimized thepercentage of copolymer chains. However,for all the other samples, the experimentswere performed at 25 8C.

� No significant improvements were observedfor times longer than 20 h.

The percentages of copolymerized PEO chainsdetermined by 1H NMR were confirmed by thermo-gravimetric analysis (TGA). It was indeed ob-served that the unreacted PEO bearing aterminal OH was much less thermally stablethan a functionalized PEO. This allowed thequantitative determination of the functionalizedPEO content. Figure 4 shows the TGA curvesobtained under air of the initial pure PEO, amodel PEO terminated by an anthracene,21 andthe sample PEOF2. The heating rates were opti-mized to get the best separation between thethermal decomposition of pure PEO and of thecopolymer: from 100 to 200 8C at 20 8C min�1,50 min at 200 8C, and a ramp from 200 to 6008C at 100 8C min�1. Although the OH-termi-nated PEO decomposed during the first ramp,the anthracene-terminated PEO was stable at200 8C. The PEOF2 showed two steps corre-sponding to the decomposition of nonfunctional-ized PEO and to the decomposition of the co-polymer. The percentage of copolymer chainsmeasured by this method (13%) was in goodagreement with the value deduced from NMR(12%).

The comparison of PEOF5 with PEOF8 andPEOF6 with PEOF9 showed no significant effect

Figure 4. TGA curves of pure PEO terminated byOH or anthracene and of a mixture of PEO–OH andPEO block copolymer.

WATER-SOLUBLE PEO-b-PPV COPOLYMER 4343

Page 8: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

of the leaving group nature (Cl or Br), althoughadding the step of Cl/Br exchange was not foundto be essential. On the other hand, the order ofreactant addition played a role in the composi-tion of the samples. Indeed, adding PEO ontothe monomer tended to increase the PASX blocklength, whereas the reverse process favored anincrease in the PEO chain amount involved incopolymerization. However, a specific feature of

the desired copolymer could be obtained bychanges in the addition process.

The nature of the R substituent of the mono-mer influenced the length of PASX as well asthe proportion of converted PPV. Indeed, copoly-mers from monomers with an octyl group(PEOF6 and PEOF9 with 3b and 4b, respec-tively) exhibited 1.5–3 times longer PASX blocksthan those with a butyl group (PEOF5 and

Scheme 3. Copolymerization mechanism from PEO chains in basic media.

4344 BIANCHI ET AL.

Page 9: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

PEOF8 with 3a and 4a, respectively), whereasthe mean number of converted PPV units was10 times lower with an octyl group. A strongincrease in the solubility in THF was noticed forthe copolymers with octyl groups, unlike thosewith butyl groups.

No significant differences were observed withdifunctional PEO (PEOF10 and PEOF11), exceptfor the formation of a gel in THF preventing theaddition of activated PEO onto the monomer.Furthermore, using a PEO with a lower molecu-lar weight favored the functionalization to thedetriment of the PASX length.

The replacement of DPMK by another basewas not favorable. As seen in experimentPEOF12, the activation of PEO by sec-butyl-lithium led essentially to nonconverted homopoly-mers with a negligible amount of the copolymer.

Eventually, from these data, one can see thatunder these experimental conditions (activationof PEO with an organo K compound and mono-mer/active PEO chain end ratio � 1), the propor-tion of copolymerized PEO chains was in therange of 11–47%. The mean total length of thecopolymerized sequences was rather short, inthe of range 1.2 (PEOF11) to 6.3 (PEOF3).

Mechanism of the Reactions

These different results can be interpreted if oneconsiders that the PEO chain ends are used inseveral simultaneous reactions (Scheme 3).From the hypothesis of these reactions, we candraw some observations. Most of the reactionsare initiated with a base; in our case, activatedPEO chains are the only available bases.

The relatively low fraction of copolymerizedchains and the short sequence lengths areexplained by the assumption of the simultaneityof the five reactions. Depending on their positionin the copolymer sequence and on their conver-sion state, each monomer unit consumed one tothree PEO–OK chain ends. In the case of oneconverted monomer unit attached to a PEOchain, three PEO chains were necessary: one foractivation, one to fix the monomer unit, andanother one to convert it. In our experiments,PEO chains were introduced in an equimolaramount with respect to the monomer, and thisexplains the partial consumption of the latter.

The possibility of PEO–OK converting precur-sor units into PPV was directly checked by agradual addition of PEO–OK in THF to a solu-tion of the homopolymer of 3a prepared withthe Vanderzande method. The UV–vis spectraobtained during this process are shown inFigure 5. We observed a roughly proportionalincrease of the absorption around 400 nm versusthe PEO–OK addition. When the PEO–OK/monomer unit ratio reached unity, the partlyconverted polymer slowly precipitated, and thisconfirmed that our experimental conditions ofcopolymerization could convert PASX into PPV.This was confirmed by the determination of ahigher extinction coefficient than that describedin the literature for PPV.20 We assume that thecomplexation of K cation by the PEO chainmade this alcoholate specially reactive.

A radical mechanism, though not excludedduring the polymerization step, could not fully

Figure 5. Conversion of poly3a to PPV by the grad-ual addition of PEO–OK in a THF solution at 25 8C(optical path ¼ 0.2 cm; OD ¼ optical density).

Figure 6. Spectral evolution of the copolymerizationmedium as a function of the added amount of mono-mer (O.D. ¼ optical density).

WATER-SOLUBLE PEO-b-PPV COPOLYMER 4345

Page 10: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

explain these copolymerization reactions. Pre-vious results from the literature showed ananionic mechanism during Wessling polymeriza-tion, with the attachment of the base onto thereactive monomer.22 More recently, Vander-zande’s group showed the coexistence of bothradical and anionic mechanisms during the poly-merization process in aprotic solvents.23 Webelieve that our inert experimental conditionsand the activation by a PEO–OK chain in theaprotic solvent THF probably favored the an-ionic processes.

This assumption of anionic processes was alsosupported by the color evolution during thecopolymerization. Indeed, several color changeswere observed during the reaction; within a fewminutes, the color changed from violet dark tobrown and finally to bright, fluorescent yellow.

To confirm these observations, UV–vis spectraof the reaction medium were recorded duringthe gradual addition of the monomer (3a) to aTHF solution of PEO–OK (preparation ofPEOF5), and they are reported in Figure 6. Atthe beginning, two bands were observed at 350and 510 nm. The first one could correspond tothe activated monomer with a p-quinodimethanestructure or to the terminal carbanion formedby the attachment of a monomer onto activatedPEO. At the end of the reaction, this band washidden by the absorption of the partly convertedpolymer.

The more noticeable point was the presenceof an absorption band around 510 nm. Such apeak was not observed in the reported homopo-lymerization of 3a. It could correspond to alargely delocalized carbanion connected to a con-verted PPV sequence. It reached a maximum for0.28 monomer equivalents and then decreased,in agreement with the assumed mechanismspresented previously, that between one to threePEO chains could be consumed per monomer. Atthis stage, the carbanionic species that formedprobably reacted as a base when all the PEO–OK chains were consumed.

Improvement of These Syntheses by the Addition ofExtra Bases

As the activated PEO was used in all thedescribed reactions (initiation of the copolymer-ization, propagation, and conversion to PPV), weneeded to add an extra base to improve the yieldof one or several of these reactions. This was donein two different experiments by the addition of T

able

2.

Experim

entalCon

ditionsandCharacteristics

ofProductsObtained

bytheRea

ctionbetwee

nActivatedPEO

5000andMon

omer

3ain

the

Presence

ofanAdditionalBase

Sample

aAddition

Process

Extra

Base

nmono

(equiv)

nbase

(equiv)

Cop

olymer

Chains(%

)b

Mea

nPrecu

rsor

Units/mol

ofCop

olymer

c

Mea

nPPV

Units/mol

ofCop

olymer

d

Mea

nPPV

Seq

uen

ceLen

gth

eg c

opo

(%)f

FPEOF

(%)g

Overall

Polymer

Yield

(%)h

PEOF5

3aon

PEO

—1.25

—20

3.8

0.4

2–3

47

49

96

PEOF13L

3aon

PEO

tBuOK

19.90

9.30

46

2.4

0.3

2–3

—22

—PEOF2

PEO

on3a

—1.07

—12

3.0

1.5

>6

46

51

91

PEOF14L

PEO

on3a

tBuOLi

4.92

3.92

13

14.9

0.05

1—

85

aPEO–OH,M

5000gmol

�1.

bPercentageof

copolymerized

PEO

chains(determined

by

1H

NMR).

cMea

nnumber

ofnon

convertedprecu

rsor

unitsper

copolymer

chain

(determined

bySEC

andelem

entalanalysis).

dMea

nnumber

ofconvertedPPVunits(determined

bySEC

from

UV–vis

absorp

tion

ofthecopolymer

pea

k).

eMea

nlength

ofconvertedPPV

sequen

cesin

copolymer

chains(from

themaxim

um

absorp

tion

wavelen

gth).

fWeightpercentageof

mon

omer

incorp

oratedinto

copolymer

chains(determined

bySEC

andelem

entalanalysis).

gWeightpercentageof

mon

omer

incorp

oratedinto

copolymer

chainscomparedwiththeamou

ntof

mon

omer

polymerized

incopolymer

andoligom

erch

ains(determined

byelem

entalanalysis).

hPercentageof

polymerized

mon

omer

incopolymer

andoligom

erch

ains(determined

bySEC).

4346 BIANCHI ET AL.

Page 11: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

t-BuOK or t-BuOLi to the reaction medium. Theexperimental conditions and results are pre-sented in Table 2.

In the first experiment, the monomer (3a)was added to a heterogeneous mixture of PEO–OK and t-BuOK in a twofold excess versus thetotal base (experiment PEOF13). After the reac-tion, the medium was filtered, and only the yel-low, fluorescent solution (PEOF13L), containing64% of the total solid, was studied. In compari-son with PEOF5, the copolymer initiation yield

had been improved (46% of the PEO chainsadded at least one monomer), but the meanlength of the sequences was still very short (ca.three units).

In the second experiment (PEOF14), t-BuOLiwas solubilized in a reaction medium containinga 20-fold molar excess of the monomer (3a) ver-sus the PEO molecules. The reaction mediumwas filtered at the end of the reaction. Only thesolution containing 89% of the total solid wasanalyzed (PEOF14L). These results showedthat, in comparison with PEOF2, the initiationyield had not been improved (12%), but themean sequence length was strongly increased(15 units), and these sequences were practicallynot converted. This confirmed the results ofexperiment PEOF12 with PEO–OLi, which onlyled to nonconverted oligomers.

Thermal Conversion of the PEO–PASX BlockCopolymer to PEO–PPV in Solution

As previously seen, the addition of t-BuOLi toPEO–OK allowed the synthesis of the PEO–PASXblock copolymer with a reasonable PASX length(15 units). The problem was then to convert thisprecursor to a soluble PEO–PPV copolymer. Theconversion to PEO–PPV in THF orN-methylform-amide (MMF) at room temperature by the addi-tion of a base such as t-BuOK failed. A thermaltreatment inMMF solutions at 140 8C allowed thisconversion, as shown in Figure 7. In a first step,

Figure 7. UV–vis spectra of the thermal conversionof the block copolymer PEO–PASX (PEOF14L) intoPEO–PPV in an MMF solution at 140 8C (O.D. ¼optical density).

Figure 8. Kinetics of the formation of the band at 365 nm during the thermal con-version of the PEO–PASX copolymer (PEOF14L).

WATER-SOLUBLE PEO-b-PPV COPOLYMER 4347

Page 12: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

relatively short sequences absorbing around330 nm were formed. This peak leveled off after30 min. Simultaneously, the absorption of longersequences at 365 nm increased with time up to amaximum value after about 150min.

The variation with time of the absorption at365 nm, which corresponds to the final maxi-mum absorption, fits very well eq 1, whichassumes an apparent first-order reaction for theconversion process and a complete conversion ofthe available precursor units (Fig. 8):

ðA� yÞ=ðA� CÞ ¼ expð�B� tÞ ð1Þ

where A and C are the optical absorption at 365nm at the end and at the beginning of the reac-tion, respectively, and t is time expressed inminutes. The measured rate constant of the ther-mal conversion was B ¼ 0.033 min�1 at 140 8C.

Nevertheless, this maximum wavelength waslower than the one obtained with copolymersdirectly converted during the copolymerizationprocess (392 nm, PEOF3).

Moreover, the apparent extinction coefficient,calculated from the maximum absorption of thethermally converted copolymer and from its con-centration and composition in the medium, wasfound to be 9980 L mol�1 cm�1, a value smallerthan expected for a complete conversion. Thismeans that a part was not converted or was con-verted into very short sequences (two to threeunits). A treatment at higher temperatureswould probably be more efficient.

The SEC analysis of the copolymer after con-version showed clearly the strong increase inthe UV–vis band of the copolymer and did notshow any formation of oligomers, which couldarise from the cleavage of the PASX/PPVsequence during the conversion process.

Table 3. Maximum Absorption and Photoluminescence of Prepared PEO–PPVBlock Copolymer Solutions at Given Concentrations

Sample Solvent kUV (nm)a kFluo (nm)bgPL (Concentration)

[% (mg mL�1)]

PEOF2 THF 390 511–544 70 (0.06)Water 399 511–542 4 (0.04)DMSO 388 510–542 48 (0.05)Cyclopentanone 392 511–542 30 (0.05)Chlorobenzene 391 511–543 43 (0.04)

PEOF3 Cyclopentanone 392 513–544 42 (0.04)Chlorobenzene 396 512–544 43 (0.04)

PEOF4 Cyclopentanone 392 511–542 45 (0.04)Chlorobenzene 391 511–542 57 (0.04)

PEOF5 THF 367 455–504 58 (0.04)Water 320 509 2 (0.04)

PEOF6 THF 318 427–450 23 (0.1)MMF before conversion 310–320 428–453 6 (2.3)MMF after conversion 375 512–541 1.5 (2.3)

PEOF7 THF 368 457–509 70 (0.04)Water 321 510 34 (0.04)

PEOF8 THF 366 456–507 70 (0.04)Water 311 507 5 (0.04)

PEOF9 THF 320 426–451 16 (0.1)PEOF10 THF 366 430–453 59 (0.04)

Water 315 508–538 7 (0.04)PEOF11 THF 324 462 0.4 (2.32)PEOF13L THF 326 458 78 (0.04)

Water 329 488–508 30 (0.04)PEOF14L THF 305–320 428 10 (0.1)

MMF before conversion 304–318 425 4 (2.3)MMF after conversion 370 513–544 2.5 (2.3)

a kUV ¼ maximum UV–vis absorption wavelength.b kFluo ¼ maximum PL emission wavelength.

4348 BIANCHI ET AL.

Page 13: Water-soluble poly(ethylene oxide)-block-poly(p-phenylene vinylene) copolymer: Synthesis and characterization

PL Properties in Solution

Because of the synthesis process, we get a mix-ture of pure, unreacted PEO (Mn ¼ 5000 g mol�1)and block copolymer PEO–PPV with the samePEO sequence. They have not been separatedin these studies, and the given concentrationsrefer to the previously detailed mixture. Althoughthese copolymers were perfectly soluble in theirsynthesis medium, some of them became hardlysoluble after drying. Dimethyl sulfoxide (DMSO)was found to be the best solvent, and no losswas observed after filtration through a 0.2-lmTeflon filter. In THF and especially in water, bigaggregates were eliminated by this filtration. Inevery case, poly(vinylidene fluoride) (PVDF) fil-ters strongly retained the copolymers because ofstrong adsorption.

A PerkinElmer spectrofluorimeter was usedfor the PL studies. The quantum yields of fluo-rescence (gPL) were measured with solutions ofpreviously described copolymers with quininesulfate in 1 N sulfuric acid as a reference.24 Theused concentrations are given together with gPL,and the obtained results are reported in Table 3.

The PL yields were strongly dependent on thesolvent and on the copolymer composition.

The PL yield increased with kmax of absorp-tion and then with the conjugation length, witha maximum value around 70% in THF for sam-ples with a kmax value higher than 360 nm.Whatever kmax was, gPL was always very low inwater (<1%), probably because of a strongaggregation of PPV units.

CONCLUSIONS

Soluble block copolymers (PEO–PPV) were syn-thesized by the polymerization of a-halo-a0-alkyl-sulfinyl-p-xylene monomers at the end of acti-vated PEO. The mechanism of reaction seemedto be largely anionic, and an important conver-sion from the precursor to PPV was realizedduring this copolymerization process. Moreover,we showed that the length of the PPV precursorsequences could be increased by the addition ofan extra base. The lithium tert-butoxide basewas efficient, but in that case, the conversioninto PPV did not occur during the copolymeriza-tion process and had to be realized in a secondstep.

The nature of the solvent and the concentra-tion were found to play important roles in the

absorption and luminescence spectra of thesecopolymers.

The authors thank D. Vanderzande and his group fortheir kind help in the syntheses of some of the mono-mers. This work was supported by the Saint-GobainCo. and by the European Training Research Mobility(TRM) contract Synthetic Electroactive OrganicArchitectures (SELOA). The authors gratefully ack-nowledge Laurent Billon for his help with thermogra-vimetric analysis experiments and Abdel Khoukh fornuclear magnetic resonance measurements.

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4350 BIANCHI ET AL.