Polyphenylenes with Side Chains

Polyphenylenes with Side Chains or Side Rings... 

Polyphenylenes with side chains or side rings have usually been prepared via organometallic polycondensation. Examples are shown in Schemes 4.10 and 4.11. 

S. Setayesh, A.C. Grimsdale, T. Weil, V. Enkelmann, K. Mullen, F. Meghdadi, E.J.W. List, and G. Leising, Polyfluorenes with polyphenylene dendron side chains toward non-aggregating, light-emitting polymers, J. Am. Chem. Soc., 123 946-953, 2001. 

Setayesh, S., et al. 2001. Polyfluorenes with polyphenylene dendron side chains Toward nonaggregating, light-emitting polymers. J Am Chem Soc 123 946. 

Another approach to CPL is the synthesis of conjugated polymers with intrinsic chiro-optical properties. A variety of polymers with CPPL have been synthesized so far. Most of them are based upon well-known conjugated polymers such as poly(thiophene)s [4,111], polyphenylene vinylene)s [123], poly(thienylene vinylene)s [124], ladder polymers [125], PPPs [126], polyphenylene ethynylene)s, [127] and poly(fluorenes) [128]. All of them have been modified with chiral side-chains, which induce the chiro-optical properties. 

Similar to the other rigid aromatic polymers, the insolubility of polyphenylenes constitutes the major problem associated with their synthesis and characterization [5] indeed, this is the very reason why most polyphenylenes synthesized to date have been decorated with solubilizing side chains. Such substituents not only cause the dihedral angles to be detrimental for conjugation when connected at ortho positions (see Section 22.5.1), but they also dilute the main-chain properties, particularly in bulk. Consequently, the aim has been to reduce the side chain content as much as possible for certain applications. Bearing this in mind, Schliiter and coworkers have synthesized a variety of alkyl chain-substituted PPPs with systematically decreased densities of the side chains (Figure 22.22) [34]. 

Recently, liquid-crystalline polyphenylene derivatives have been synthesized through substitution of a fluorine-containing chiral liquid-crystalline group into side chains, with an aim to develop ferroelectric liquid-crystalline conjugated polymers (Fig. 5 Suda and Akagi 2008). These are attracting interest because they can afford anisotropies in electrical and optical properties when they are macroscopically aligned. This study also elucidated that PPP can be used to prepare new types of polymer materials. 

Mullen and coworkers have prepared a polymer 129 with a first-generation polyphenylene dendron on the side chain (Scheme 60) [228]. Unlike Fr chet dendrimers, such dendrimers are rigid and shape-persistent. Interestingly, the bulky side chains produced no blue shift in the absorption or emission of the polymer compared with PDAFs such as 89, indicating that there is no increased steric repulsion between adjacent fluorene units. LEDs using this polymer display device characteristics (turn-on voltage, emission efficiency. 

Scheme 60 Synthesis of a polyfluorene with a polyphenylene dendron on the side chain...

Polyphenylenes with -SO3M or -NO2 Side Chains Prepared via Ullmann Coupling... 

Well developed fibrillar structures were formed when the polymerization was carried out with M0CI5 [27], whereas PPP aggregates of indefinite shape were observed with FeCE as catalyst. The formation of the fibrils was interpreted as resulting from simultaneous polymerization and crystallization, and was attributed to the fact that benzene reacts with polyphenylene chain ends aligned on the crystalline surface of already precipitated PPP [27]. In the case of FeCl3, the crystallinity of the PPP is less pronounced, and it was suggested that benzene could form polyphenyl side-chains or polynuclear structures, which would prevent the formation of the fibrillar structures [24]. 

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