Poly-Paraphenylene—A Ring Polymer

We can go further and consider the effects of replacing the other C-H groups with something else. If we continue the perturbational treatment using the original polyacetylene functions, it is easy to see that now it will be the other band that will be affected tk — nja (Fig. 15-20b). 

There is a lot going on that we are ignoring. The C-N length is different from the C-C length, the COs change when we go to higher levels of perturbation theory, and Huckel methods are very approximate and become less reliable in systems with polar bonds. It should nevertheless be clear that, if one were looking for ways to predict the effects of chemical substitution on band gap, this sort of approach would provide the conceptual framework. 

We turn now to the tt COs of poly-paraphenylene (PPP). This system serves as an excellent illustrator of principles introduced in earlier sections.  

Chapter 15 Molecular Orbital Theory of Periodic Systems 

As its name indicates, PPP results when benzene molecules are attached in a planar chain, with each benzene being linked to two neighbors at para (opposite) positions (Fig. 15-2la). There is an infinity of ways we could select the repeating segment, or unit cell, for this polymer. Two of the most obvious are indicated in Fig. 15-21a, but any portion of length a (or na) would serve. Since selection of the unit cell detennines the basis set of MOs we will use to construct the COs, we choose the unit cell on the left and start with the familiar tt MOs of benzene, sketched in Fig. 15-2 lb. 

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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]. 

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