Diazonium grafting

The NH2 groups can be diazotized and reduced in the presence of thiosulphates and different metal ions. The effect of some metal ions, namely Fe ", Sn, Cu +, and Co on the graft yield of cotton modified with aryl diazonium groups via its reaction with 2,4-dichloro-6-(p-nitroaniline)-5-triazine in the presence of alkali and followed by reduction of nitro group was studied [4]. 

Richards, G. N. Diazonium derivatives of cellulose as initiators of graft copolymerization. J. Appl. Polymer Sci. 5, 553 (1961). 

If the diazonium groups result from the diazotation of poly-/>-amino-styrene, the macroradicals will initiate grafting. Contrarily, if >-(N-acetyl) phenylenediamine is diazotized and used as initiator of a first monomer, a polymer is obtained with an acetamino. phenyl end group (-CGH4-NH-Ac). After hydrolysis of this last and diazotation of the free amine group, the polymeric terminal diazonium salt can be used with ferrous ions for the synthesis of block copolymers. 

Fig. 15. The figure on the left shows the time dependence of the current density (A), pholuminescence intensity (B) and the change in photovoltage (C) during the grafting of various diazonium compounds at a potential of —1.2 V. The figure on the right plots the observed photovoltage changes as a function of the calculated effective dipole moment perpendicular to the surface. Reprinted from [90].

The versatility of the method is founded on the possibility of grafting a variety of functionalised aryl groups. This allows the attachment of a wide spectrum of substances[60]. In 1992, a study by Delamar and co-workers [61] demonstrated that reduction of diazonium salts at carbon surfaces resulted in a strongly attached surface layer. They attributed this to covalent bond formation between the aryl radical and the carbon surface [61,62], One electron reduction of aryl diazonium salts at carbon electrodes leads to grafting of aryl groups to the surface. Acetonitrile is often used as the modification medium. Reduction of the diazonium salt can be achieved by cyclic voltammetry or controlled potential electrolysis. The coupling reaction is favored both by the adsorption of the diazonium prior to its reduction and by the relatively positive potential of the diazonium prior to its reduction[62],... 

Recently, this difficulty was overcome when the starting diazonium was the aniline dimer (4-aminodiphenylamine or ADPA) [157]. In that case the grafted layer is electro-active and aniline oxidation remains possible in aqueous sulfuric acid and leads to a strongly adherent PANI film on the modified surface. The first experiments were performed on carbon and extension to an oxidizable metal has not yet been reported. However, as it has been proven that the covalent grafting of diazonium salts was also successful with iron and other oxidizable metals [178-182], we think that the same procedure might be applied in the case of iron. A strong increase of the adhesion should be expected, and therefore an improvement of the protection properties. 

V. MeveUec, S. Roussel, L. Tessier, J. Chancolon, M. Mayne-L Hennite, G. Deniau, P. Viel, and S. Palacin, Grafting polymers on surfaces a new powerful and versatile diazonium salt-based one-step process in aqueous media. Chem. Mat., 19,6323 330 (2007). 

Examples of postprocess systems under evaluation include surface oxidation using ozone, hydrogen peroxide, or nitric acid. Such approaches are used in the production of conductive blacks. Reaction with diazonium salts, plasma treatment, and polymer grafting are also under investigation. 

WEI 12] WEISSMANN M., CROSNIER O., BROUSSE T., et al, Electrochemical study of anthraquinone groups, grafted by the diazonium chemistry, in difierent aqueous media-relevance for the development of aqueous hybrid electrochemical capacitoP , Electrochimica Acta, vol. 82, pp. 250-256, 2012. 

Preparation and subsequent decomposition of polymers with diazonium salts can also be used to form graft cowlymers. An example is a nitrated polystyrene, reduced to the amine derivative and then diazotized. The decomposition of the diazonium salt results in the formation of radicals ... 

The material can be converted to diazonium salts and then decon x)sed with ferrous ions in the presence of some vinyl monomers to form graft copolymers. Aciylonitrile forms graft copolymer readily without formation of any homopolymers. Styrene and vinyl acetate, however, do not A modification of this technique is to conduct the diazotization reaction in the presence of emulsifiers. The amounts of graft copolymers that form with acrylic and methacrylic monomers and A vinylpyrrolidone depend upon the nature and pH of the emulsifiers, the reaction time, and the temperature. 

In order to construct an ordered monolayer on a carbon surface, a premodified smooth and functionalized film is needed. Electrochemical reduction of diazonium salts or oxidation of amine-containing compounds and arylacetate produces free radicals, resulting in covalent derivatization of the carbon surface in nonaqueous solutions (Figure 3). The premodification by free radical grafting methods is quite mild, simple, and rapid experimentally. The monolayers prepared as a precursor film for subsequent assembly are stable to harsh environments and can endure elevated temperatures, and especially are organized in order, compact, and charge-rich. 

The electroreduction of diazonium salts, described in Figure 9.2 and extensively used to graft hydrogenases onto electrodes, also proved to be an efficient alternative method to form covalent linkages between a model compound and a carbon electrode [27]. This was applied by Darensbourg and coworkers... 

Figure 10.3 Various structures of moiecuiar junctions commoniy used in moiecuiar eiec-tronics (a) Cross junction formed by perpen-dicuiariy oriented bottom and top contacts with a moiecuiar iayer sandwiched between the conductors. Typicai junction sizes range from severai square microns to a square mil-iimeter. (b) An aii-carbon molecular junction formed using pyrolyzed photoresist film (PPF) (on SiOj support) as a bottom contact, an evaporated carbon (eC) top contact, and a molecular layer consisting of a multilayer of biphenyl grafted using diazonium chemistry.

As to grafting with marked covalent character, the most widely investigated systems are based on diazonium salts that can be anchored at a number of quite different electrode surfaces, e.g., C or Pd (Fig. 5.3) [23-26]. The grafting occurs through chemical or electrochemical reduction of the diazonium moiety, leading to the release of nitrogen from the precursor molecules and concurrent formation of C-C or C-metal bonds. It is worth noticing that the reaction is not self-limited, so that formation of multilayers is also possible. 

Fig. 5.3 Reduction of a diazonium salt leading to covalent grafting on a surface (Reproduced from Ref [1] with the permission of Springer)...

Scheme 9.7 (a) Grafting from cobalt/carbon [82], (b) Water-soluble third-generation nanoparticles, attachment of monomer by ruthenium catalysts 49a-c, PEG oligomers diazonium precursor, copolymerization with attached in ortho, meta, and para positions, Grubbs second-generation ruthenium cat- phosphorylcholine in para position [84]. alyst, and reaction with palladium acetate... 

---