Radical cation stair-step mechanism

Figure 71 Radical cation stair step mechanism of polyphenylene synthesis. (From Refs. 307 and 311.)...

Three reaction mechanisms of C—C coupling in oxidative polymerization of benzenes have been reported (Fig. 9). The first step of each mechanism is one electron oxidation of a monomer to give a cation radical. The next step may be (i) conpling of two cation radicals (233,234), (ii) coupling of one cation radical with one neutral molecule (226,228), or (iii) coupling of one cation radical with many neutral molecules (stair-step mechanism) (235). 

However, a slightly different mechanism was postulated by Kovacic et al. [3,15-16] for the polymerization of benzene by catalyst-oxidant systems. According to Kovacic et al, benzene can be oxidized to a radical-cation by a variety of acid catalysts with or without oxidants, but will react on the substrate so that polymer chain growth occurs according to a stair-step mechanism, in which the radical-cation is delocalized over the whole chain (11) before forming a new chain of non-conjugated cyclohexadiene moieties (12) which after oxidation yields the PPP (Scheme 6.4)... 

This stair-step mechanism was supported by ab initio calculations on the stability of the radical-cation dimer [16], considered as the first stage of the propagation reaction. The authors concluded that the stability of the stair-step dimer was the same as that of... 

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