Aromatic rigid-rod polymers

C.C. Wu, P.Y. Tsay, H.Y. Cheng, and S.J. Bai, Polarized luminescence and absorption of highly oriented, fully conjugated, heterocyclic aromatic rigid-rod polymer poly-p-phenylenebenzoxazole, J. Appl. Phys., 95 417 123, 2004. 

Aromatic, rigid-rod polymers play an important role in a number of diverse technologies, including high-performance engineering materials, conducting polymers, and nonlinear optical materials. 

Aromatic rigid-rod polymers play an important role in a number of diverse technologies including high-performance engineering materials, conducting polymers, and nonhnear optical materials. The cross-coupling reaction of aryldiboronic acids and dihaloarenes for the synthesis of poly(p-phenylenes) was first reported by Rehahn et al. The method has been extensively applied to water-soluble poly(p-phenylene), planar poly(p-phenylenes) fixed with the ketoimine bonds, poly(phenylenes) fused with polycyclic aromatics,and nonlinear optical materials (Scheme 14). 

Poly(p-pheny lene)s, PPPs, constitute the prototype of rigid-rod polymers and are currently being intensively investigated [1]. The key role of PPPs follows from their conceptually simple and appealing molecular structure, from their chemical stability, and from their superior physical properties [2], In turn, this is the result of important advances made in aromatic chemistry over the last few years. The following section gives an overview of the most common methods to generate poly(p-phenylene)s via different synthetic approaches. 

It appears that the main reasons for the high thermal stability ofPNT-N and PNT-F arise from their inherent rigid-rod polymer structure and the high aromatic content of the repeating unit. However, the fact that the thermal stability of PNT-F is higher than PNT-N might be the result of strong intermolecular interaction between chains because of dipole-dipole interactions. 

The last two decades of research on high performance rigid-rod polymers and molecular composites have produced a wide variety of material systems for evaluation. The purpose here is not to present a comprehensive review but rather to describe these relatively recent research developments with regard to the synthetic methods and approaches that have been utilized. In doing this some properties of the polymers are discussed as well as some of the problems encountered in conjunction with their synthesis and evaluation. In an effort to limit the scope of the review, we will address only aromatic heterocyclic rigid-rod polymers, extended chain polymers and derived molecular composites. 

Horie and his co-workers [131-135] have made extensive use of fluorescence spectroscopy to probe the microstructures of MCLC polymers. The first such polymer to be studied [131,135] was the all aromatic, rigid rod polyester, 65, with long, flexible side-chains attached to a pyromellitic ester mesogenic group. 

Aromatic heterocyclic rigid-rod polymers that have exceptional thermal oxidative stability have been prepared using 1,5-naphthylene dicarboxylic acid and 2,5-diamino-1, 4-benzenedithiol. These heterocyclic rigid-rod polymers materials are useful as protective garments in ballistic vests and abrasion- and flame-resistant fabrics. 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

An all carbon conjugated ladder polymer (graphite ribbon) was synthesized by a novel electrophile-induced cyclization reaction to provide fused benzenoid aromatic hydrocarbon in quantitative yield [161]. Suzuki cross-coupling of dieneyne 105 with 1,4-didode-cylbenzene-2,5-diboronic acid (106) gave rigid-rod polymers 107, which was further treated with TFA to produce 108 graphite ribbon as a yellow/orange solid. 

In addition to those formed by surfactant amphiphiles, two other types of lyotropic mesophases are generally recognized, neither of which exhibits a cmc. The first of these are lyotropic phases of rigid-rod polymers that can form mesophases in both aqueous and non-aqueous solvents " these mesophases are of the nematic or hexagonal type. Examples include polymeric metal acetylide complexes and DNA." The other type is usually formed from flat and largely aromatic molecules which stack to give lyotropic columnar phases, also referred to as chromonic phases." " This latter class is formed from systems with ionic or strongly hydrophilic peripheral functions, and forms mesophases... 

Rigid linear aromatic polyester Rigid-rod polymers... 

Aromatic polyamides with terminal acetylenic groups were formed from 2,2 -diiodo-diphenyl-4,4 -dicarbonyl chloride reacted with aromatic diamines. The phenylethynyl groups were introduced by reacting the iodine moieties with copper phenyl acetylide. Thermal treatment converted the prepolymers to 9-phenyl dibenzanthracene based rigid-rod polymers that fail to melt below 500... 

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