Benzene poly-paraphenylene

The first method reported for the synthesis of poly(paraphenylene) is the coupling of benzene by Lewis acid catalysis in the presence of an oxidant. In 1963, poly(paraphenylene) was synthesized by stirring benzene, aluminum chloride, and copper(II) chloride yielding an insoluble light brown powder [98]. The most favored mechanism is shown in Scheme 29.13. Benzene is oxidized to its radical cation that then propagates cationically. A second oxidation step produces the bis-cation, which looses two protons rearomatizing the terminal rings. Further oxidation of the dihydro structures finally affords the polymer [99]. 

Other Lewis acid/oxidant systems have been employed including ASFj/AsFj [100] and liquid SO2 or sulfuric acid and aluminum chloride [101]. Poly(paraphenylene) has been synthesized by the electrochemical oxidation of benzene in solvents such as liquid SO2 [102] and concentrated sulfuric acid [103], or with the addition of Lewis acids including aluminum chloride [104], CuCl2/LiAsFg [105], and BF30Et2 [106], affording polymeric films. To improve the solubility, poly(paraphenylene) has been sulfonated [107] and alkylated [108] with propyl halides to give materials with enhanced solubility. 

The extent to which the COs conform to the rule of increasing number of nodes with energy (or k) will be seen in the example of a planar conformation of poly-paraphenylene. On the left-hand side of Fig. 9.17 we have the valence rr-orbitals of benzene ... 

Perhaps the most elegant route to poly(paraphenylene) has been developed by workers at ICI, and involves a microbiological synthesis of a monomer which is used to prepare a processable precursor polymer. The starting material for this route is benzene, which is microbiologically converted to c/s-5,6-dihydroxycyclohexa-l,3-diene this diol is converted to the dimethyl carbonate shown as the monomer in Figure 4.6. Radical initiated polymerization gives a soluble and processable polymer which can be converted to poly(paraphenylene) as required (7). 

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