Aqueous Polyalkyne Dispersions
Johannes Huber and Stefan Mecking*
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Germany
INTRODUCTION
The discovery of the conductive properties of conjugated organic polymers1 in the late 1970s initiated intense investigations of their synthesis, chemistry and physical properties. Today, they are widely studied for various applications, e.g. optoelectronics in displays or organic solar cells.2,3 Beyond general academic curiosity towards the generation and properties of unusual nanostructures, nanoparticles of conjugated polymers can be of interest for the preparation of thin films, of highly disperse multiphase films, and of multilayer devices. These examples illustrate that nanoparticles may contribute to resolve the notorious issue of processing of conjugated polymers. The rigid conjugated polymer backbone generally results in very low solubilities in organic solvents and in thermal properties that prohibit thermoplastic processing. This can be overcome by introducing substituents as side chains, which however also alters the electronic properties (which may be desirable or undesirable) and often requires additional synthetic effort.
Aqueous polymer dispersions are attractive in this context as they are compat ble with printing techniques, also water is a non-solvent for the majority of organic compounds and polymers. The non-flammability and environmentally benign nature of water can be advantageous in some cases. Accordingly, the preparation of secondary dispersions, that is, post-polymerization emulsification of conjugated polymers has been studied.4,5 However, the necessity of polymer solubility in an organic solvent or of fusability, and the high viscosity of polymer melts and also of polymer solutions in organic solvents (which also have to be removed from dispersions subsequently) limit the scope of secondary dispersions.
Polyanilines and polythiophenes are common conjugated polymers with heteroatoms in the main chain. They are accessible by oxidative polymerization, typically in water as a reaction medium. Aqueous dispersions of these polymers, that is colloidally stabilized particles of water-insoluble polyanilines and polythiophenes, have been studied.6,7
Conjugated polymers with hydrocarbon main chains, namely polyacetylene and substituted polyalkynes, poly(p-phenylene)s, and poly(phenylene vinylene)s, are accessible by catalytic polymerization.3 The catalyst studied by Shirakawa initially for polymerization of acetylene was a homogeneous, Ziegler-type catalyst soluble in organic solvents derived from Ti(OBu)4-AlEt3.8 Such highly electrophilic early transition metal catalysts are very reactive and sensitive towards water.
Late transition metal complexes are less sensitive towards water, and indeed insertion polymerization of substituted alkynes, mainly phenylacetylene and p-(4-methylphenyl)acetylene in aqueous biphasic or homogeneous systems has been studied. Rhodium and in some cases also iridium complexes were studied. These studies demonstrate that high molecular weights (Mw > 104 g mol-1) can be achieved in some cases. Catalyst activities are moderate, with several hundred turnovers total. Polymer dispersions were not reported.9-14
Catalytic polymerization in aqueous emulsions has evolved as a general route to polymer nanoparticle dispersions.15 We have reported the synthesis of aqueous polyacetylene dispersions.16 While neat polyacetylene was considered unprocessable until recently, from the dispersions it can be ink-jet printed to functioning circuit paths.16
EXPERIMENTAL
General methods and materials: All manipulations involving phosphines were carried out under an inert atmosphere in a dry box or by standard Schlenk techniques. Dynamic light scattering (DLS) on diluted dispersion samples was performed on a Malvern Nano-ZS ZEN 3600 particle sizer (173° back scattering). The autocorrelation function was analyzed using the Malvern dispersion technology software 3.30 algorithm to obtain volume and number weighted particle size
distributions. Conductivities were determined on compressed pellets by the four-point-method.17 1,3-Bis-(di-tert-butylphosphino)propane was prepared according to [18]. Polyacetylene dispersions were prepared as reported previously.16
Preparation of catalyst solution (example): A solution of 33.7 mg (150 µmol) Pd(OAc)2 in 5 mL acetonitrile, and a solution of 149.5 mg (450 µmol) 1,3-Bis-(di-tert-butylphosphino)propane in 5 mL ethanol were mixed and the solvent was evaporated in vacuo. The residue was dissolved in a mixture of 0.2 mL of ethanol and 4.8 mL of hexane, to afford a catalyst solution with a concentration of 30 µmol Pd/mL.
Synthesis of polyphenylacetylene dispersion:
In a 100 mL roundbottom Schlenk flask closed with a septum an aqueous solution of 0.5 g SDS in 39 g water was topped by a layer of 10 ml phenylacetylene. To this layer 1.0 mL catalyst solution (30 µmol) was added. The mixture was ultrasonicated for 2 min (Bandelin HD 2200 with a KE76 tip, operated at 120W) followed by addition of one drop methanesulfonic acid while stirring. The initially pale-yellow emulsion turned to an intense yellow while the temperature increased significantly. After one hour of polymerization an intensely yellow dispersion was obtained.
RESULTS AND DISCUSSION
For the polymerization of phenylacetylene in aqueous emulsion a catalyst prepared in situ from Pd(OAc)2 and 1,3-Bis-(di-tert-butyl- phosphino)propane,19 which has previously been shown to polymerize acetylene in aqueous emulsion,16 was employed. For the preparation of colloidally stable polymer dispersions, in the case of monomers which are liquid under reaction conditions such a lipophilic catalyst can be miniemulsified as a solution in the monomer, providing polymerization does not occur at this stage but can be triggered subsequently.20 A miniemulsion of monomer containing dissolved Pd(OAc)2 / phosphine was prepared, and activated by addition of water soluble acid. The function of acid is l kely displacement of acetate groups to form a weakly coordinated, electrophilic phosphine coordinated Pd(II) complex.21 In the system studied, this would occur at the aqueous/organic interface. A poss ble subsequent route to the polymerization active species could be direct reaction with phenylacetylene to form [(phosphine)Pd-C≡CPh]+.
By this procedure, colloidally stable dispersions of polyphenylacetylene were obtained with very high polymerization rates (Table 1). Dispersions with polymer solids contents of up to 36 wt.-%
were prepared (entry 3). Varying the monomer content of the reaction mixture, dispersions with average particle sizes ranging from 50 to 150 nm were obtained (entries 1 to 3). In these experiments 30 µmol of palladium(II) catalyst precursor were employed. Even upon reducing the amount of catalyst precursor to 0.4 µmol, complete conversion of monomer was still observed (entry 4). This corresponds to a catalyst productivity of 2.2×105 mol(phenyacetylene) mol(Pd)-1, in a one hour polymerization run. Employing a microemulsion procedure,22 extremely small nanoparticles of polyphenylacetylene were obtained. A monomer microemulsion was mixed with a catalyst microemulsion to produce a transparent polymer dispersion with a volume average particle size of 25 nm (polymer solids contents 6 wt.-%).
Table 1. Synthesis of Polyphenylacetylene Dispersions.
Entry 1 2 3 4
Catalysta) 30 µmol 30 µmol 30 µmol 0.4 µmol Particle sizeb) 57 nm 130 nm 142 nm 112 nm Polymer contentc) 6 % 18 % 36 % 17 % Color of dispersion yellow yellow orange yellow
Total amount of reaction mixture: 50 g; 1% SDS. Reaction time: 1 h. a) Pd(OAc)2 / tBu2P(CH2)3PtBu2 = 1:3. b) volume average particle size determined by dynamic light scattering. c) polymer solids content of dispersion.
TEM micrographs of the dispersions prepared from monomer miniemulsions show spherical particles (Figure 1).
First publ. in: Polymeric Materials: Science and Engineering 96 (2007), pp. 306-307
Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/6605/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-66050
Figure 1. Transmission electron microscopy (TEM) image of polyphenylacetylene particles (dispersion from entry 3 in Table 1).
Preliminary studies of the conductivity of the materials prepared were carried out with polyphenylacetylene prepared with the same catalyst as used in the emulsion polymerization in methanol. After doping with iodine, a conductivity of 3.3×10-5 S cm-1 was observed.
The polyphenylacetylene dispersions could be processed by inkjet printing. Dispersions were used as obtained, adding 5 vol.-% of glycerole to adjust the drying rate. Photo-quality printouts were obtained.
The yellow color of the printouts immediately turned black upon treatment with bromine vapor. This ‘doping’ reaction further illustrates the conjugated nature of the polymer.
The resolution and accuracy of printing was studied in more detail with polyacetylene dispersions. As reported, they can be printed to functioning circuit paths on paper.16
Figure 2. Pattern ink-jet printed with polyacetylene dispersion (top, width ca. 5 cm). Close-up (bottom left), and close up of pattern printed with original printing ink (bottom, right).
The patterns depicted in Figure 2 were printed on high gloss photo paper. With the HP Deskjet 920C printer used (which has a nozzle diameter of 40 µm according to the manufacturer), the quality of the printout with the polyacetylene dispersion was not inferior to printouts with original printing ink (Figure 2). Continuous lines were obtained (Figure 2, bottom left). After doping with iodine, all six lines were found to be conductive. The resistance of the lines was between 2 and 4 MΩ, depending on the quality of the printout. Moreover, no crossta k in the form of short circuits was observed. The resistance between the lines was >20 MΩ.
ACKNOWLEDGEMENTS
We thank Ralf Thomann (Freiburg) for TEM analyses.
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