Polypyrrole–Palladium Nanocomposite Coating of Micrometer-Sized Polymer Particles Toward a Recyclable Catalyst

A range of near-monodisperse, <i>multimicrometer-sized</i> polymer particles has been coated with ultrathin overlayers of polypyrrole–palladium (PPy–Pd) nanocomposite by chemical oxidative polymerization of pyrrole using PdCl₂ as an oxidant in aqueous media. Good control over the targeted PPy–Pd nanocomposite loading is achieved for 5.2 μm diameter polystyrene (PS) particles, and PS particles of up to 84 μm diameter can also be efficiently coated with the PPy–Pd nanocomposite. The seed polymer particles and resulting composite particles were extensively characterized with respect to particle size and size distribution, morphology, surface/bulk chemical compositions, and conductivity. Laser diffraction studies of dilute aqueous suspensions indicate that the polymer particles disperse stably before and after nanocoating with the PPy–Pd nanocomposite. The Fourier transform infrared (FT-IR) spectrum of the PS particles coated with the PPy–Pd nanocomposite overlayer is dominated by the underlying particle, since this is the major component (>96% by mass). Thermogravimetric and elemental analysis indicated that PPy–Pd nanocomposite loadings were below 6 wt %. The conductivity of pressed pellets prepared with the nanocomposite-coated particles increased with a decrease of particle diameter because of higher PPy–Pd nanocomposite loading. “Flattened ball” morphologies were observed by scanning/transmission electron microscopy after extraction of the PS component from the composite particles, which confirmed a PS core and a PPy–Pd nanocomposite shell morphology. X-ray diffraction confirmed the production of elemental Pd and X-ray photoelectron spectroscopy studies indicated the existence of elemental Pd on the surface of the composite particles. Transmission electron microscopy confirmed that nanometer-sized Pd particles were distributed in the shell. Near-monodisperse poly­(methyl methacrylate) particles with diameters ranging between 10 and 19 μm have been also successfully coated with PPy–Pd nanocomposite, and stable aqueous dispersions were obtained. The nanocomposite particles functioned as an efficient catalyst for the aerobic oxidative homocoupling reaction of 4-carboxyphenylboronic acid in aqueous media for the formation of carbon–carbon bonds. The composite particles sediment in a short time