Rigid-rod heterocyclic ladder polymers

Among the wide variety of ladder or ladder-like polymers described in the literature, which are aimed at enhancing the thermal stability rather than at superior fiber-mechanical properties, the benzazole polymers of the so-called PBZ family have attained a semicommercial status as high-performance fibers processed via a liquid crystalline solution. A series of papers describing various aspects of these polymers appeared some time ago, and an excellent and comprehensive review appeared recently. 

The benzoxazole and the benzothiazole polymers which received most attention are the cis and trans isomers, respectively. PBT, which has been the object of most of the work, is prepared by first synthesizing 2,5-dimercapto-l,4-phenylenediamine PPD is converted into the bisthiourea with ammonium thiocyanate. This product is cyclized with bromine and hydrolyzed to the oxidatively exceedingly fragile 

The bis-HCl salt is easily oxidized also, but can be handled. The tetrafunctional monomer for PBO is prepared from resorcinol diacetate by nitration and hydrogenation. Care must be taken in view of explosive side products. A detailed description appeared recently. A probably more economical synthesis of 4,6-diaminoresorcinol has become available by nitration of 1,2,3-trichlorobenzene followed by alkaline hydrolysis and hydrogenation. The status of the work aimed at a commercial process for cis-PBO was reviewed recently.Diaminohydroquinone, the precursor for trans-PBO, can be prepared from chloranil by ammonolysis followed by hydrogenation  

Polymerization is brought about by dissolving the tetrafunctional monomer in polyphosphoric acid (PPA), by slowly heating to about 

An alternative route to polybenzazoles was devised, by polymerizing dinitro difunctional nucleophilic monomers with terephthaloyl chloride, followed by reduction of the nitro groups and cyclization. This approach has the advantage of oxidative stability of the dinitro monomers complete conversion in reduction and cyclization are unlikely to be easily achieved. 

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