Poly -phenylene s

This article has been focussing on poly(phenylene)s with 1,4-(pnra-)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 (1D-) structures, but has more recently developed into 2D- and 3D-structures as well, the latter serving as functional shape-persistent nanoparticlcs. 

On the other hand, it has also become clear that a materials-oriented synthesis of conjugated poly(phenylene)s cannot narrow its attention to properties of molecules only in solution, but has to include aspects of processing and supramolecu-lar ordering as well. The rigid-rod character of PPPs therefore suggests the use of chain stiffness as a structure-forming principle in the design of supramolecular motifs. 

Ni(0)-mediated homocouplings of 2-subslituted l,4-phcnylenebis(triflate)s have been reported by Percec et al. [15] to provide substituted poly(/ -phenylene)s 7 containing alkyl, aryl or ester substituents in the 2- and 3-positions of the 1,4-phenylcnc skeleton. This method of preparation appears to be broad in scope, especially due to the ease of preparation of the bis(lriflatc) monomers starting from the corresponding hydroquinone derivatives. 

Kim and Webster [57] were the first to show that trifunctional benzene-based monomers can also be used to synthesize poly(phenylene)s, in this case hyperbranched structures 31 based on 1,3,5-trisubstituled benzene cores. They self-condensed l,3-dibromophenyl-5-boronic acid leading to the formation of soluble, hyperbranched PPP-type macromolecule 31. 

Dendritic Poly(phenylene)s and Giant Polyaromatic Hydrocarbons (PAHs)... 

Rod-shaped systems such as 54, extended versions of the widely studied poly(phenylene)s, are available by coupling the para-isomer 45 with a poro-dihaloaromatic unit in the presence of a palladium catalyst [33] (Scheme 13) ... 

Poly(phenylene)s expected as a new class of n-type conjugate polymers were synthesized in high yields by the Barbier procedure 27.52... 

An electrophilic substitution reaction has been used for the key ladderforming step in the synthesis of soluble ladder-type poly(phenylene)s [51-53]. These aromatic polymers have a ribbon-like rigid, planar structure. They are of interest because of their optical and electronic properties [51,54,55]. The preparation of these polymers was accomplished by two basic steps. The first step was the construction of a substituted poly(p-phenylene) backbone. The ladder structure was obtained by a subsequent intramolecular electrophilic ring closure reaction. For example, the syn-... 

Figure 17 Synthesis of soluble ladder-type poly(phenylene)s by a two-step process Construction of the Poly(p-phenylene) backbone and intramolecular ring closure. (From Ref. 51.)...

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

The target structures in the final example are fully aromatic polymeric hydrocarbons, consisting of all-carbon six-membered rings - so-called angularly annulated polyacenes 91 [55]. The structural difference between those and the methylene-bridged poly(phenylene)s is the replacement of the benzylic methylene bridges by vinylene moieties. 

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