HIGH MOLECULAR WEIGHT CONJUGATED POLYMER NANOPARTICLES FROM STEP-GROWTH COUPLING
Moritz Baier, Johannes Huber, and Stefan Mecking*
Chair of Chemical Materials Science Department of Chemistry University of Konstanz, Germany Introduction
Conjugated polymers are studied intensely due to their unique properties, such as electro- and photoluminescence. These are the basis of their utilization in, for example, organic light emitting devices (polymer- OLEDs).1 The intrinsically rather low solubility of conjugated polymers, and the high viscosities of polymer solutions, complicate the processing of conjugated polymers to such devices. Also, the preparation of multilayer devices, without undesired redissolution of already applied layers, is challenging. These issues could be resolved by processing from polymer nanoparticle dispersions.2-4
Full color displays require organic layers with different emission colors.
The emission color of conjugated polymers can be influenced by fluorescent dyes. A possible approach is to blend the polymer with a dye. Energy transfer from the polymer to the dye can be very effective such that emission occurs exclusively from the dye. Employing appropriate dyes the emission color can be adjusted.5,6 Microphase separation of the dye and the polymer can detoriate device performance, however. To prevent this separation the dye can be covalently bound to the polymer.7
Aqueous polymer dispersions can be obtained from emulsion polymerizations. In industry free-radical emulsion polymerizations are widely used. The resulting polymer dispersions are employed in environmentally friendly coatings. Since the beginning of this decade catalytic polymerizations of olefins in aqueous emulsion have been studied increasingly also.8-10 Up to now, these reactions have been restricted to chain growth polymerizations. In step growth polymerizations the active metal center is released from the polymer chain after each reaction step. This can enhance catalyst deactivation, if the intermediates are water sensitive. Such a deactivation would also be detrimental, as in step-growth polymerization high conversions are necessary to achieve reasonable molecular weights. Very recently, the synthesis of aqueous dispersions of poly(p-phenylene vinylene) with molecular weights around Mn 103 g mol-1 by acylic diene metathesis polymerization (ADMET) was demonstrated.11
Most often conjugated polymers for OLEDs are produced by step growth reactions. Therefore a high yielding reaction is needed for the polymerization to achieve high molecular weights. Catalytic carbon carbon coupling reactions such as Suzuki, Heck, Sonogashira or Yamamoto coupling reactions are widely used for the preparation of light emitting polymers.12
The Glaser reaction, that is the oxidative coupling of monosubstituted alkynes to the diethynyls, is a high yielding reaction which occurs under very mild conditions. Air can serve as the oxidant, and the reaction can be carried out in aqueous solution.13,14 In organic solvents, the preparation of conjugated polymers by Glaser coupling has been demonstrated.15-18 In terms of classical polycondensation, the reaction is akin to polymerization of an AB monomer (with the A and B functional group identical). Unlike A-A + B-B polycondensation, exact adjustment of stoichometry, which could be particularly challenging in multiphase systems like aqueous emulsions, is not an issue. These features prompted us to study nanoparticle synthesis by Glaser coupling.
Experimental
General Methods and Materials. 4,4’-Dinonyl-2,2’-bipyridine (dNbpy; Aldrich) and tetradecyltrimethylammonium bromide (TTAB; Fluka) were used as received. Molecular weights were determined by gel permeation chromatography (GPC) on a Polymer Laboratories PL-GPC 50 instrument with two PLgel 5 µm MIXED-C columns in THF at 40°C against polystyrene standards. TEM images were obtained on a Zeiss Libra 120 instrument (acceleration voltage 120 keV). Dispersions were dialyzed for TEM analysis to remove any free surfactant, and applied to a copper grid. Samples were not contrasted. Fluorescence spectra were recorded on a custom-made setup consisting of a xenon flash lamp, a monochromator (Oriel 77250 1/8M), a spectrograph and a nitrogen-cooled CCD camera enabling photoluminescence
detection from 310 nm to 940 nm.19 For measurements in solution spectroscopic grade chloroform (Uvasol, Merck) was used. 1,4-Diethynyl-2,5- di(2-ethylhexyloxy)benzene (1) was prepared according to literature procedures.20-23 The perylene dye 2 was synthesized according to modified literature procedures.7,24,25 Compounds were fully characterized by 1H and 13C NMR.
General Procedure for Emulsion Polymerizations. CuCl and dNbpy were mixed with 0.5 ml of toluene and stirred until the solids had dissolved completely. The green catalyst solution obtained was mixed with a solution of the monomers in 0.5 ml of toluene and immediately added to 30 ml of an aqueous 1 % TTAB solution with a syringe. The mixture was ultrasonicated for 2 minutes (Bandelin HD 2200 with a KE76 tip operated at 120 W). The emulsion was stirred at room temperature in a Schlenk tube open to air. An aliquot was precipitated in an excess of methanol to determine the polymer yield, and for molecular weight determination on the isolated polymer. The polymer dispersions were dialyzed to remove residual surfactant and catalyst, followed by filtering through a paper filter.
Results and Discussion
Commonly, hydrophilic amines are utilized as ligands for the copper catalyst in Glaser coupling. A key to conducting such reactions in aqueous emulsion was found to be the utilization of the lipophilic bidentate ligand dinonylbipyridine (dNbpy) in the copper(I/II) catalyst employed. It solubilizes the catalyst in the organic monomer phase, rather than in the aqueous phase.
The polymerizations were performed in a Schlenk tube open to air by stirring at room temperature. Under these conditions the toluene slowly evaporated as an azeotrope during stirring. After a few days of stirring, intensely yellow colored polymer dispersions were obtained which were very stable for at least several months (Figure 4).
N O
O
N
O O O
O O
O x n
1 2
Figure 1. Polymer obtained from Glaser coupling polymerization.
Dispersions of poly(1,4-diethynyl-2,5-di(2-ethylhexyloxy)benzene) (poly-1) and a copolymer of 1 and a perylene monomer 2 (poly-1-co-2) were prepared (Table 1). A perylene derivative was chosen as perylene dyes are known to be photostable and to emit with high quantum yields.7 The incorporation of dye does not affect the progress of the polymerization reaction significantly versus the homopolymerization of 1.
Table 1. Polymerization Experiments in Aqueous Emulsion
entry 1 2 3 4
monomer/catalyst 6.9 6.8 8.8 7.0
perylene [mol-%] 0.0 0.0 1.0 2.1
t [h] 48 48 120 72
Mn [g/mol] 23 200 41 300 36 800 32 600 Mw [g/mol] 75 700 254 100 106 500 94 600
Mw / Mn 3.3 6.1 2.9 2.9
Mn,NMR [g/mol] [a] 18 200 57 000 24 000 18 200
polymer yield [%] >95 >95 90 >95 Amount of Catalyst: 6 mg (60 µmol) CuCl and 24 mg (60 µmol) dNbpy. Entry 2:
polymerization at 50°C under O2 atmosphere. a) from integrals of the endgroups (CDCl3 at 25°C).
By comparison to polymerization at room temperature, conducting the reaction at a slightly elevated temperature of 50 °C under an oxygen atmosphere resulted in substantially higher polymer molecular weights (entry 2). Apparent molecular weights vs. polystyrene standards of up to Mn = 4.1 x 104 g mol-1 were determined by GPC. These match well with absolute
Polymer Preprints 2008, 49(1),1124
First publ. in: Polymer Preprints 49 (2008), 1, pp. 1124-1125
Konstanzer Online-Publikations-System (KOPS) URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-73696
URL: http://kops.ub.uni-konstanz.de/volltexte/2009/7369/
molecular weights determined by end group analyses from 1H NMR spectroscopy. As expected for polymers with a rigid backbone, GPC somewhat overestimates the molecular weight. Average particle sizes around 30 nm are observed in TEM images (Figure 2).
Figure 2. TEM image of nanoparticles from poly-1 dispersion (entry 1).
The incorporation of small amounts of the perylene dye lead to an effective tuning of the emission color of the polymer dispersions due to energy transfer from the polymer backbone to the perylene dye. An intense emission band arises in the fluorescence spectra of the perylene-containing polymers which corresponds to the emission of the perylene dye. The observed emission of the polymer backbone decreases with higher amounts of incorporated perylene (Figure 3 and Figure 4). At a perylene content of 2 mol-%, emission virtually occurs only from the dye. In polymer solutions no such effective energy transfer can be observed.
450 500 550 600 650 700 750 800 850 900
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1
wavelength [nm]
nor maednnsyliz iteit
Figure 3. Normalized fluorescence spectra of aqueous dispersions of poly-1 (solid black, excitation wavelength 450 nm), poly-1-co-2 with 1.0 % perylene (solid gray, excitation wavelength 450 nm) and poly-1-co-2 with 2.1 % perylene (dashed gray, excitation wavelength 460 nm).
Figure 4. Photograph of polymer dispersions (left: poly-1; center and right:
poly-1-co-2 with 1 mol-% and 2 mol-%, respectively, of 2).
Conclusions
In summary, high-molecular-weight conjugated polymer nanoparticles have been prepared for the first time by a catalytic coupling reaction in
aqueous emulsion. An effective energy transfer to incorporated dye occurs in the poly(aryl diethynyl)s prepared.
Acknowledgements
We thank Andreas Zumbusch and his group for access to the optical spectrometer. TEM was carried out by Marina Krumova. GPC analysis were conducted by Lars Bolk. S.M. is indebted to the Fonds der Chemischen Industrie.
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