Aromatic cyclized ladder

Stabilization process which is carried out in air (oxidative stabilization) constitutes the first and very important operation of the conversion of the PAN fiber precursor to carbon as well as activated carbon fiber. During stabilization, the precursor fiber is heated to a temperature in the range of 180-300 °C for over an hour. Because of the chemical reactions involved, cyclization, dehydrogenation, aromatization, and oxidation and cross linking occur and as a result of the conversion of CH N bonds to C=N bonds fully aromatic cyclized ladder type structure forms [36, 49]. 

Cyclization is a key reaction in the production of carbon fibers from polyacrylonitrile (PAN) (acrylic fiber see Sec. 3-14d-2). The acrylic fiber used for this purpose usually contains no more than 0.5-5% comonomer (usually methyl acrylate or methacrylate or methacrylic acid). Highly drawn (oriented) fibers are subjected to successive thermal treatments—initially 200-300°C in air followed by 1200-2000°C in nitrogen [Riggs, 1985]. PAN undergoes cyclization via polymerization through the nitrile groups to form a ladder structure (XXVII). Further reaction results in aromatization to the polyquinizarine structure (XXVIII)... 

Zipping-up reaction very seldom leads to polymers with full ladder structure. Very often the reaction proceeds with a break in the ladder, and isolated reactive groups are present in the product. Moreover, structure investigations are very difficult because ladder polymers are mostly insoluble. Decrease in transmission made IR spectra unintelligible as in the case of cyclization and aromatization of polybutadiene. NMR analysis by simple techniques is also impossible. 

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. 

Aromatic ladder polymers. Ladder polymers which withstand red heat are obtained by cyclization of polyacrylonitrile [293, 294] ... 

The polymers from the three compounds will have ladder-type structures. However, the polymers from compounds B and C will have at least one and two skeletal bond(s) per repeating unit while a double-cyclization polymer will result from compound A. The presence of one or two consecutive single bonds between aromatic units will enhance polymer susceptibility to thermal degradation. In principle, therefore, under identical conditions the thermal stability of the resulting polymers will be A more than B more than C. 

Conj ugated Ladder Polymers. Since the 1930s double-stranded, ladder-type polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(l,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-t5q)e polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-l,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels-Alder polyaddition of AB-type diene-dienophiles (200), AA/BB-type Diels-Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene imits (202),... 

Polyimidazopyrrolone n (ladder pyrrone, polypyrrolone) An aromatic, heterocyclic polymer that results from the reaction of an aromatic dianhydride with a tetramine. Due to the double-chain or ladder-like structure, these polymers have outstanding resistance to radiation, chemicals, and heat (no weight loss to 550° C). However, this structure also makes them difficult to process. To overcome this difficulty pyrrone prepolymers in the form of solutions and salt-like powders have been made available. The powders can be molded under conditions that complete the cyclization or conversion of the ladder-like molecular... 

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