PPPs precursor routes

Poly(p-phenylene) (PPP) is a widely explored conjugated polymer in the field of PLED materials, Fig. 14 [67]. The presence of large band gap in the polymer reveals its blue emission characteristics. PPPs are insoluble and intractable in nature and therefore researchers have explored routes to synthesize soluble PPP films and a variety of PPP precursor routes have been discussed in literature [68]. PPP precursor routes involve the thermal elimination such as the elimination of two equivalents of acetic acid (per monomer unit) from poly-l,4-(5,6-diaceto-2,3-cyclohexene). This... 

The strategy of Kaeriyama represents a so-called precursor route and was developed to overcome the characteristic shortcomings (insolubility, lack of process-ability) of previous PPP syntheses. The condensation reaction is carried out with solubilized monomers, leading to a soluble polymeric intermediate. In the final reaction step this intermediate is then converted, preferentially in the solid state allowing the formation of homogeneous PPP films or layers, into PPP (or other poly(arylene)s). 

Later on, Ballard et al. [42, 43] developed an improved precursor route starting from 5,6-diacetoxycyclohexa-1,3-diene (18), the so-called 1C1 route. The soluble precursor polymer 19 is finally aromatized thermally into PPP 1 via elimination of two molecules of acetic acid per structural unit. Unfortunately, the polymerization of the monomer does not proceed as a uniform 1,4-polymerization in addition to the regular 1,4-linkages ca. 10% of 1,2-linkages are also formed as result of a 1,2-polymerization of the monomer. 

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

FIGURE 8.13. Absorption, PL, and EL of PPP obtained from the precursor route. (Reproduced from Ref. 54.)... 

Due to the pronounced tolerance of the Suzuki reaction towards additional functional groups in the monomers, precursor strategies as well as so called direct routes can be applied for polyelectrolyte synthesis. However, the latter possibility, where the ionic functionalities are already present in the monomers, was rejected. The reason is too difficult determination of molecular information by means of ionic polymers. Therefore the decision was to apply precursor strategies (Scheme 1). Here, the Pd-catalyzed polycondensation process of monomers A leads to a non-ionic PPP precursor B which can be readily characterized. Then, using sufficiently efficient and selective macro-molecular substitution reactions, precursor B can be transformed into well-defined PPP polyelectrolytes D, if appropriate via an activated intermediate C. 

Phenylene-based polymers are one of the most important classes of conjugated polymers, and have been the subject of extensive research, in particular as the active materials in light-emitting diodes (LEDs) [1,2] and polymer lasers [3]. These materials have been of particular interest as potential blue emitters in such devices [4], The discovery of stable blue-light emitting materials is a major goal of research into luminescent polymers [5]. Poly(para-phenylene) (PPP, Scheme 1, 1) is a blue emitter [6], but it is insoluble and so films of PPP have to be prepared via precursor routes [7]. Substitution with long alkyl... 

Poly(para-phenylene) (PPP, 64) is insoluble and infusible and so films of PPP must be made by precursor routes (Scheme 6.19) [105]. The first of these, which was developed at ICI [106, 107], utilizes a radical polymerization of the diacetate 82 to produce a precursor polymer 83 that is thermally converted to PPP. This method produces PPP containing about 15% 1,2-linkages. A totally para-polymer can be made by the method of Grubbs involving the stereoregular nickel-catalyzed living polymerization of the bis(silyl ether) 84 to 85, and then its conversion to 83 [108-110],... 

To obtain soluble PPP homopolymers, two main strategies have been used. The first is the so-called precursor route. It consists in starting from soluble materials that are chemically or thermally converted into fully aromatic polymers by elimination of leaving groups. An example is the precursor route utilized by Ballard et al. [89] (Fig. 9.8). Synthesis is based on 5,6-cz s-dihydroxycyclohexa- 1,3-diene as starting material and originating from the bacterial oxidation of benzene by Pseudomonas... 

One of these was aimed at the elaboration of a precursor route. Kaeriyama et al. [12] reported on the Yamamoto coupling of l,4-dibromo-2-methoxycarbonylben-zene to poly(2-methoxycarbonyl-l,4-phenylene) (2) as a processable PPP precursor. The aromatic polyester precursor 2 is then saponified and thermally decarboxylated to 1 with CuO catalysts. However, the reaction conditions of the final step are quite drastic and cannot be carried out in the solid state (film). 

Films of PPP have to be prepared via precursor routes (previously reviewed by Gin and Conticello [36]). The route most often used to prepare films of PPP (1) is one developed at ICI (Scheme 3) [37,38]. This starts with a microbial oxidation of benzene to cyclohexadienediol 6. Radical-initiated polymerisation of the diacetate 7 gives the precmsor polymer 8, which is then thermally converted to 1. However, the material is not stereoregular as it contains about 10-15% of 1,2-linkages. This material has been used by Leising et al. to prepare blue-emitting LEDs (A,max = 459 nm) with efficiencies of up to 0.05% [39-41]. 

Scheme 23 Zirconocene-precursor route to substituted PPPs...

Among the main chemical procedures for preparing PPP samples, those of Kovacic and the Yamamoto syntheses have incontestably been the most popular techniques. The soluble polymer precursor route, first developed by Ballard et al., appeared very promising but it does not lead to conjugate lengths higher than with other classical methods. 

Several step owth routes to PPP have already been described. Despite some dever s)mthetic ideas, the end product is still a rigid, insoluble polymer, and all the step-growth methods discussed result in a low degree of polymerization. Thus, precursor routes to PPP have taken on much importance. 

Figure 31. A low temperature precursor route to PPP. 1,2-Linkages exist in this pol)mer but are not shown here.

In a classical multi-step route the critical point is to conduct (he ring closure quantitatively and regioseleclively. In the synthesis of I.PPP, the precursor polymer 13 is initially prepared in an aryl-aryl coupling from an aromatic diboronic acid and an aromatic dibromoketone. 

This idea was realized impressively in 1991 with the first synthesis of a soluble, conjugated ladder polymer of the PPP-type [41]. This PPP ladder polymer, LPPP 26, was prepared according to a so-called classical route, in which an open-chain, single-stranded precursor polymer was closed to give a double-stranded ladder polymer. The synthetic potential of the so-called classical multi-step sequence has been in doubt for a long time in the 1980s synchronous routes were strongly favoured as preparative method for ladder polymers. 

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