Synthesis of aramids

An alternative route to aromatic pwlyamides is the p olyurethane-type hydrogen transfer reaction between an aromatic diisocyanate and a diamine [11]. The low-temperature reaction between them produces an intermediate pol5rmer that loses carbon dioxide in a subsequent heating to form the aramid. This reaction is not widely used because of the 

Commercial forms of aramids and their physical properties 

fibrids, papers and pulp are the usual aramids commercial forms. 

Aramid polymers have high melting points and melt with decomposition that makes impractical their processing to yams (fibers) by melt spinning. Among the commercial aramids, only the flexible chain homo- and copolymers of MPDI type can be dissolved in NMP and DMAc [15] to form isotropic solutions transformed into yarns by a dry spinning 

Aramid films have been in development since the late 1990s by several Japanese companies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded through flat dies to form films. The conventional wet process can be employed to produce unidirectional and bi-oriented films from isotropic 77j-aramid solutions. It is more complicated to obtain isotropic p-aramid films. It requires extrusion of PPTA/H2SO4 solutions, which are exposed to subsequent steam treatment, bi-axial stretching, drying and specific annealing [19]. p-Aramid films have many applications in electrical industry. 

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Commercially, 1,3-DNB has been used extensively as an organic intermediate for m -phenylenediamine, a chemical used in the synthesis of aramid fibers and spandex (HSDB 1994). 

Figure 13.5 Synthesis of Aramids from Diamines and Dichlorides diamine and...

As mentioned in Section 3, typical aramid-6-polyether elastomers are synthesized by the polycondensation reaction of polyether diol with the aramid compound I in the presence of transesterification catalysts. Under these conditions, the synthesis of aramid-6-polyester elastomers gave only low molecular weight elastomers with a broad segment length distribution due to transesterification reactions of the polyester segments. This result inidicated that in the obtaining of aramid-6-polyester elastomers, transesterification catalysts should be avoided. Later, a method for the obtaing of this type of elastomers was developed, which consisted in the copolymerization of an activated acyl lactam-terminated aramid compound II with polyester diols in the molten state, in the absence of transesterification catalysts [40,42]. Compound II was obtained by the reaction of N-(p-aminobenzoyl) caprolactam with terephthaloyl chloride, as shown in Scheme 8 [61]. 

The most widely employed synthetic route to aramids is based on the polycondensation of dicarboxylic acids with diamines in the presence of condensing agents. Good reviews on the synthesis of aramids have recently appeared (1-3). Recently, promising alternative synthetic routes to aramids have been reported and are described herein. These include the polycondensation of N-silylated diamines with diacid chlorides, the addition-elimination reaction of dicarboxylic acids with diisocyanates, and the palladium-catalyzed carbonylation polymerization of aromatic dibromides, aromatic diamines and carbon monoxide. 

The synthesis of aramids from N-silylated amines has been employed because N-silylated aromatic amines show higher reactivity relative to the parent diamines and the resulting trialkylsilyl halide does not lower the reactivity of unreacted amine functionality as is the case with amine protonation... 

Recently, Kraft and Osterod [157] reported the synthesis of poly(aramide) dendrimers possessing either electron-deficient 1,3,4-oxadiazole (70) or aromatic systems (71) linked by amide units to a central triphenylmethane unit (Fig. 31). 

Chidambareswarapattar C, Mohite DP, Lari-more ZJ, Lu H, Sotiriou-Leventis C, Leventis N. One pot synthesis of multifunctional aramid aerogels. MRS Proc 2012 1403 1403. 

Kovalev MK, Kalinina E, Androsov DA, Cho C. Synthesis of transparent and thermally stable polyimide-aramid nanocomposites -prospective materials for high-temperature electronic manufacture applications. Polymer 2013 54(l) 127-33. 

The same catalyst system was applied for the synthesis of poly(l,3-imino) phenylene ketones, which are isomers of the paro-aramide polymer, known as Kevlar (Scheme 13.63) [112]. 

The superior properties of aramid materials were the reason why significant research effort has been devoted to their synthesis. In 1989 there were at least 100 different chemical compositions of aromatic and aromatic-aliphatic polyamides [5] and there are indications that the number has doubled until today. This fact led the U.S. Federal TVade Commission to adopt a definition for aramid designating fibers of the aromatic polyamide type in which at least 85% of the amide linkages are attached directly to two aromatic rings. The compounds corresponding to this definition that have reached commercial stage are only four (Scheme 8.1) poly(r vphenylene isophthalamide) (MPDI), poly(p-phenylene terephthalamide) (PPTA), copoly(p-phenylene-3,4-diphenyl ether terephthalamide) (ODA-PPTA), and poly[5-amino-2(p-aminophenyl) benzimidazole terephthalamide] (SVM).The process of commercialization of every compound has always been the result of a constant trade-off between properties, processability and price. 

Armda E, Cao K, Siepermann C, Thouless M, Anderson R, Kotov N, Yang M, Waas. Synthesis and use of aramid nanofibers 2013, US patent Application US 2013/0288050. 

Yokozawa, T, Ogawa, M., Sekino, A. etai (2002) Chain-growth polycondensation for well-defined aramide. Synthesis of unprecedented block copolymer containing aramide with low polydispersity. Journal of the American Chemical Society, 124,15158-15159. 

This section overviews the synthesis of reactive aramid compounds and various synthetic routes leading to type I aramid elastomers with PTMO soft segments. 

Several methods for the synthesis of type I aramid elastomers have been proposed (i) transesterification reaction between the aramid compound I and PTMG (Route A) [33-35], (ii) polycondensation between PTMG end-capped... 

The polymerization Route A was successfully applied to the synthesis of type II aramid elastomers titanate-catalyzed transesterification between the aramid compound III and PTMG (Scheme 7). Using Route A, a high molecular weight elastomer with inherent viscosity of 2.8 dL/g was obtained [43]. 

Imai Y, Kajiyama M, Ogata S and Kakimoto M (1985) Improved synthesis of poly-ether-aramid multi-block copolymers by direct polycondensation. Polymer J 17 1173-1178. 

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