Electronically active polymers

Another convenient way to disperse platinum-based electrocatalysts is to use electron-conducting polymers, such as polyaniline (PAni) or polypyrrole (PPy), which play the role of a three-dimensional electrode.In such a way very dispersed electrocatalysts are obtained, with particle sizes on the order of a few nanometers, leading to a very high activity for the oxidation of methanol (Fig. 10). 

In this chapter we focus on two electrochemically relevant, active polymer types ionically conductive polymers and electronically conductive polymers, and discuss new developments emerging in their context. 

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

All important electronically conducting polymers, except perhaps for polyacetylene, can be prepared electrochemically by anodic oxidation of the monomers. The reaction is initiated by splitting off two hydrogen atoms from the monomer molecule (H—M—H), which subsequently polymerizes by interconnecting thus activated sites ... 

Metallophosphazenes are a new type of macromolecule designed to bridge the gap between polymers and metals. Although still at an exploratory stage of laboratory development, they may provide access to electronically-conducting polymers, magnetically-active polymers, macromolecular catalysts, electrode mediator systems, or polymers crosslinked by metal atoms. 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

Buchwald has shown that, in combination with palladium(II) acetate or Pd2(dba)3 [tris(dibenzylideneacetone)dipalladium], the Merrifield resin-bound electron-rich dialkylphosphinobiphenyl ligand (45) (Scheme 4.29) forms the active polymer-supported catalysts for amination and Suzuki reactions [121]. Inactivated aryl iodides, bromides, or even chlorides can be employed as substrates in these reactions. The catalyst derived from ligand (45) and a palladium source can be recycled for both amination and Suzuki reactions without addition of palladium. 

Three processes can control the rate of homogeneous charge transport through a redox-active polymer film, i.e. electron self-exchange between redox-active centers,... 

Activity in the battery area using electronically conducting polymers has been intense (see Fig. 11.19). The possibilities (MacDiarmid, 1997) point to the provision of cheap (because of the relative cheapness of organic compounds) batteries, e.g., for massive use as a power source for bicycles. The counter-point is the limited stability and hence lifetime of such structures, and the availability of, e.g., rechargeable MnOj-Li cells, in which the (bismuth doped) Mn02 is also a relatively cheap material. 

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