Aromatic ladder polymers, properties

Both theoretical and experimental studies show that not only are aromatic ladder polymers more thermally stable but they are also more highly conducting than analogously structured nonladder systems.In this communication, we report the synthesis and electronic properties of a ladder aromatic polymer, poly(8-methyl, 2.3-6,7-quinolino) (PMQ). The experimental procedures for preparation and characterization of PMQ are described in refs. 5 and 6. 

Table 14.4 Absorption and Emission Properties of the Angularly Annelated Polyacene Structures Derived from Monomers A and B as Fully Aromatic Ladder Polymers...

This approach leads to LPPPs with very unique optical and electronic properties, for more details see [67,68]. Following the general procedure outlined in Scheme 5.5, other aromatic building blocks (e.g., naphthalene, anthracene, thiophene, and carbazole) also have been incorporated into the main chain of such planarized arylene-type ladder polymers (Scheme 5.6). 

As already briefly mentioned, easily the most obvious change regarding optical properties appears in the transition from the open-chain precursor polymers to the planarized PPP ladder polymers (LPPP). The primary single-stranded coupling product (a benzoyl-substituted polyphenylene) from a Suzuki-type polycondensation between an alkyl- or alkoxy-substituted 2,5-dibromo-1,4-dibenzoylbenzene derivative and 2,5-dihexyl-l,4-phenylenediboronic acid is colorless with an absorption maximum Amax of 264 nm [n-TT transition c = 39,000 L/(moI cm)l (19]. This absorption behavior supports the strongly twisted structure of the main PPP chain. The mutual distortion of the aromatic subunits reduces the conjugative interaction to a minimum, so that the typical electronic properties of a conjugated polymer are almost completely lost. 

The incorporation of substructures into ladder polymers provides molecular properties with well-defined configuration and conformation. The difficulties, however, in synthesizing ladder-type polymers have been reported [50,51] For the synthesis of soluble ladder-type PPP, first, PPP precursor 51 with functional ketone groups was prepared by Pd-catalyzed coupling reaction of dihexyl-substituted aromatic boronic acid 29 with didecyl-substituted aryl bromide 50 in 60-80% yields. The values determined by GPC were 6100-9200 [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-... 

In general, the types of polymers which have the best thermal properties are aromatic in character (often with recurring heterocyclic units), have low hydrogen content, and often have stepladder or ladder structures. Although there are numerous articles in the literature which deal with the effects of structure on stability within a given class of heat-resistant polymers, only a limited number of publications are to be found which compare the stabilities of different classes of heat-resistant polymers under controlled conditions. From Ehler s TGA studies on different classes of heat-resistant polymers, as well as from other sources, a classification can be made of the effects of structure on heat stability for several classes of compounds. For... 

---