Linear para-phenylenes

Remarkably enough, dialkoxyterephthalic acids, both in 2D and 3D, form linear hydrogen bonded strands which are reminiscent of the rigid rods formed from alkyl substituted poly-para-phenylenes. 

Similar processes for producing conducting polymeric films of benzene and its derivatives had been studied earlier [2-4], Necessary conditions for the successful realization of these processes are the use of a platinum electrode and a polar solvent in the presence of catalysts (Lewis acids) and thermostatting of the reactor at -75°C. A poly(para)phenylene polymerizate of the linear structure H-(-C6FLr)n-H with the degree of polymerization n, which varies between 3 and 16, is formed. Forced convection of monomeric molecules facilitates the polymerization reaction in the diffusion layer near the electrode and the formation of a dense film on the electrode surface and prevents the formation of poly(para)phenylene in the bulk. 

This article has been focussing on poly(phenylene)s with 1,4-(para-)phenylene units since these polymers play a key role in the synthesis-driven search for electronic materials. From this article it has become clear that poly(phenylene) chemistry has not restricted its attention to linear (ID-) structures, but has more recently developed into 2D- and 3D-structures as well, the latter serving as functional shape-persistent nanoparticles. 

FIGURE 8.9 Linear unsubstituted conjugated polymers poly(p-phenylene), poly(para-phenylene vinylene), polythiophenes, and polyfluorenes. 

At first sight the excited state lattice distortions of poly(para-phenylene) represented in Fig. 11.15 do not resemble those of a linear polyene. We therefore might enquire whether the geometrical defects (for example, solitons, polarons, etc.) and their associated mid-gap electronic states also exist in an analogous maimer in poly(para-phenylene). To show that bond defects do exist in an analogous manner to linear polyenes we again consider the summed bond distortions, defined by eqns (11.18) - (11.20). 

Figure 26 Frechet-type monodendrons (generations 1, 2, and 3) possessing various reactive groups at the focal point. Rod-shaped, dendritic-linear architectural copolymers obtained by coupling respective dendrons to linear poly(para-phenylene) (PPP) backbone. (Courtesy J. Am. Chem. Soc. 119 3297, 1997. Copyright 1997 American Chemical Society.)...

As an alternative approach, flexible coil parts in rod-coil molecules can be structurally modihed into dendritic shapes. In comparison to linear-type coils, dendritic coils would cause a larger steric repulsion at the rod/coil interface, which influences rod-packing structures. Lee et al. devised a dumbbellshaped molecule (12) based on an elongated dodeca-para-phenylene rod block and aliphatic polyether dendritic coils with chiral carbon centers, and investigated the self-assembly behavior in aqueous solution [54]. In remarkable contrast to ordinary one-dimensional fibers, the aggregate structures... 

More recently, (26) salts of para-bromophenols have been reacted with a variety of oxidizing agents as initiators under anhydrous conditions in the presence of aprotic, dipolar solvents. Under these conditions it is remarkable that the product from 2,6-dichloro-4-bromophenol appears to be the high molecular weight, linear 1.4-phenylene ether (VI). 

Using the same methodology, from ortho, meta, and para dicyanobenzenes compounds 82, 83, and 84 were synthesized in yields of 75%, 85%, and 92% <1991CC369>. As expected, para derivative 84 gave the most stable crystal lattice, presumably due to its linear nature. Further work in the same laboratory involved the synthesis of the 5,5, 5"-(l,3,5-phenylene)tris(l,3,2,4-dithiadiazolium)tri(hexafluoroarsenate) salt 85 in 80% yield. 

Fig. 14.4. (a) 2D solid-state NMR spectra of a bundle of oriented, industrial PET fibers. The diagonal peaks are labeled according to Fig. 14.5. The 180° phenylene motion about the para-axis is reflected in the sharp exchange peak between positions 2 and 3. The high intensity along the diagonal is due to carbon atoms which have not changed their position and, therefore, their frequency during the mixing time r , 1 s. (b) Contour plot of the spectrum shown in Fig. 14.4(a). The 20 linearly spaced lines between 1.5 and 17% of the maximum height of the spectrum indicate clearly the 180° flip motion. In addition, a cut through the spectrum at peak 2 is shown.

---