Aqueous Polyalkyne Dispersions

INTRODUCTION The discovery of the conductive properties of conjugated organic polymers 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. 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. 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. 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. 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 > 10 4 g mol) can be achieved in some cases. Catalyst activities are moderate, with several hundred turnovers total. Polymer dispersions were not reported. Catalytic polymerization in aqueous emulsions has evolved as a general route to polymer nanoparticle dispersions. We have reported the synthesis of aqueous polyacetylene dispersions. While neat polyacetylene was considered unprocessable until recently, from the dispersions it can be ink-jet printed to functioning circuit paths.