Aromatic Polyamides

In this section are discussed not only wholly aromatic polyamides, but also some mixed polyamides prepared from aromatic diacids and aliphatic diamines or vice versa. One such material was already described in Sect. 7.3.2. Another one, called Nylon 6T, is formed by interfacial polymerization of terephthaloyl chloride and hexamethylene diamine:

The polymer has good heat stability and the strength is unaffected by heating up to 185C for 5 h. The polyamide melts at 370C. When hexamethylene diamine is replaced with tetramethylene diamine, the melting point rises to 430C. The condensation product from isophthalic acid and tetramethylene diamine melts at 250C. Fully aromatic polyamides form from reactions of aromatic diacid chlorides and aromatic diamines [79, 80]. An example is formation of poly (m-phenylene diamine isophthalamide):

The polymer can be prepared in dimethylacetamide from isophthaloyl chloride and m-phenylene diamine in the presence of an acid scavenger at room temperature:

Because the polymer is soluble in dimethylacetamide containing 5% LiCl, fibers can be spun directly from solution. This polyamide melts at 371C with degradation. It is fire-resistant. The fibers are sold under a trade name of Nomex. Such aromatic polyamides with regular structure were reported to possess better flexibility and higher temperature resistance [74, 75]. Preparations of ordered copolyamides were described as follows: N,N-m-phenylene-bis(m-aminobenzamide) is formed first and then reacted with isophthaloyl chloride by interfacial condensation techniques to yield a product that melts at about 410C:

The above condensation, carried out with terephthaloyl chloride, yields a polymer that melts at 450C. Preparations of many other wholly aromatic polyamides from aromatic diacid chlorides and aromatic diamines were reported in the literature [79, 80]. In addition, several polymers are manufactured from both fully and partially substituted (para) structures. They carry the trade name of HT₄. As described in the previous section, direct polycondensation of various nylon salts is possible under mild conditions in the presence of polyphosphates and organic bases [84]. This reaction, useful in forming aromatic polyamides, takes place also in the presence of thionyl chloride [84]:

Metal salts, like lithium chloride, significantly enhance reactions of carboxylic acids with amines promoted by triphenyl phosphite [82]. This allows direct polycondensation of dicarboxylic acids with diamines and self-condensation of p-aminobenzoic acid [82]. The presence of a solvent markedly enhances the reaction with the best results being obtained in N-methyl pyrrolidone. High molecular weight polyamides form. Mixed solvents, like pyridine and N-methyl pyrrolidone, can be used to form polyisophthalamides [82]. This combination of solvents, however, yields only low molecular weight polyterephthalamides. On the other hand, when the reaction is carried out in the presence of polymeric matrices of poly(ethylene oxide) or poly(4-vinylpyridine), high molecular weight polyterephthalamides form [82]:

The Heck reaction was extended to carbonylation of aromatic dibromides with aromatic diamines in the presence of carbon monoxide [112]. High molecular weight aromatic polyamides form with the help of palladium catalysis:

The polymerization reaction takes place in a homogeneous dimethylacetamide solution, with catalytic amounts of PdCl2(PPh3)2 and an HBr scavenger. The carbonylation polycondensation proceeds rapidly at 115C and is almost complete in 1.5 h [112]. This reaction was also used to prepare many aromatic-aliphatic polyamides from corresponding aliphatic diamines with aromatic dibromides. Palladium is a relatively high-priced catalyst and it would be preferable if a lower-priced nickel catalyst could be used instead. All attempts, however, to form polymers by nickel-catalyzed carbonylation polycondensations of aromatic diamines with aromatic dibromides failed to yield high molecular weight materials [112]. Trimethylsilyl-substituted amines undergo a variety of reactions with electrophiles [113]. This reaction was extended recently to preparations of high molecular weight aromatic polyamides by low temperature solution polycondensation. N-trimethylsilylated aromatic diamines were condensed with aromatic diacid chlorides [114]at 10C in an amide solvent:

Preparations of poly-p-phenyleneterephthalamide by polycondensations with N-silylated diamine proceed more rapidly than with the parent diamine [113]. In addition, the products have higher molecular weights than similar commercial materials made from the parent diamine and sold under the trade name of Kevlar. Silylated diamines can also condense with diphenyl esters of aromatic dicarboxylic acids [114]:

The product, above shown aromatic polyamide, was reported to be of sufficiently high molecular weight and to be a useful material [113]. An example of a specialty aromatic polyamides is a fluorinated polyamide. It was prepared in an attempt to form a polymer with superior heat stability and resistance to hydrolytic attacks [76]:

Mitsuru et al. [113] reported a direct synthesis of Nomex, an aromatic polyamide, mentioned above, by a solid state polycondensation:

The reactions were carried out at the melting state of the reactants, at first at 360C and after oligomerization at 260C for 1 h.

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مواضيع ذات صلة

Special Reactions for Formation of Polyamides

Fatty Polyamides

Nylons

Polyesters from Lactones

Polycarbonates

Network Polyesters for Surface Coatings

Linear Unsaturated Polyesters

Copolyesters

Synthetic Methods

Linear Saturated Polyesters

Techniques of Polymer Preparation

Ring Formation in Step-Growth Polymerizations