Polyamides direct amidation

Amide interchange reactions of the type represented by reaction 3 in Table 5.4 are known to occur more slowly than direct amidation nevertheless, reactions between high and low molecular weight polyamides result in a polymer of intermediate molecular weight. The polymer is initially a block copolymer of the two starting materials, but randomization is eventually produced. 

Direct Amidation. The direct reaction of amino acids to form Type AB polyamides (eq. 1) and diacids and diamines to form type AABB polyamides (eq. 2) ate two of the most commonly used methods to produce polyamides. The... 

Direct amidation is generally carried out ia the melt, although it can be done ia an iaert solvent starting from the dry salt (46). Because most aUphatic polyamides melt ia the range of 200—300°C and aromatic-containing polyamides at even higher temperatures, the reactants and products must be thermally stable to be polymerized via this method. 

The synthesis of polyamides follows a different route from that of polyesters. Although several different polymerization reactions are possible, polyamides are usually produced either by direct amidation of a diacid with a diamine or the self-amidation of an amino acid. The polymerization of amino acids is not as useful because of a greater tendency toward cycliza-tion (Sec. 2-5b). Ring-opening polymerization of lactams is also employed to synthesize polyamides (Chap. 7). Poly(hexamethylene adipamde) [IUPAC poly(iminohexanedioylimi-nohexane-l,6-diyl) or poly(iminoadipoyliminohexane-l,6-diyl)], also referred to as nylon 6/6, is synthesized from hexamethylene diamine and adipic acid [Zimmerman, 1988 Zimmerman and Kohan, 2001]. A stoichiometric balance of amine and carboxyl groups is readily obtained by the preliminary formation of a 1 1 ammonium salt (XU ) in aqueous solution at a concentration of 50%. The salt is often referred to as a nylon salt. Stoichiometric... 

The most important method used in the preparation of polyamides is direct amidation, usually through the intermediate formation of a salt of the diamine and dicarboxylic acid, but without it in the case of aminoacids or for pairs of monomers that do not readily form a salt. Esters can react with diamines to form polyamides with liberation of alcohol or phenol. Diamines can be reacted with diamides yielding polyamides and freeing ammonia. Polyamides have been prepared by acidolysis of acyl derivatives of diamines (compare Section 5.4 for acidolysis in polyester preparation). Bis-anhydrides react with diamines to form polyamides and, if reacted further, polyimides. The low-temperature reaction of acid chlorides with diamines has been used, interfacially or as a solution technique, to prepare certain polyamides (compare Section 5.7 for related reactions in polyester synthesis). 

Attempts to prepare polyamides by direct amidation can fail for various reasons. Thus, lactams are readily formed from 7- or 5-amino acids. For example... 

Direct Amidation. Formation of polyamides by condensation of amine and carboxyl groups is illustrated by... 

Most polyamides are prepared by a direct amidation reaction between diacids and diamines, and numerous combinations of monomers are possible. This reaction is accompanied by the elimination of water, and the amount of water released can serve as a measure of the extent of the reaction. In addition to this, the intramolecular reaction of the amino acids can also produce a polyamide (Fig. 3.2). 

Generally speaking, polyamides can be synthesized by direct amidation where an amine reacts with a carboxylic acid with the removal of water. The reactive amine and acid groups may be on a single amino acid molecule ... 

For aliphatic polyamides, the method of direct amidation is used predominantly. The syntheses of the commercially important nylons may be represented by the general Equation 2.1 through Equation 2.3. Equation 2.1 refers to the formation of AABB-type nylons ... 

On this basis, it is not surprising that only a few kinds of reaction have so far been successfully applied in the synthesis of high-molecular linear polycondensates. High-molecular-weight polyesters (Section 26.4) or polyamides (Section 28.3) are obtained both by direct esterification or direct amidation of acids and by fr ns-esterification or Schotten-Baumann type reactions (see Section 17.4). The synthesis of polyurethane from diisocyanates and diols represents an addition of H to the N= double bonds (Section 28.5). Other addition reactions, such as HS—R—SH -h CH2=CH—R —CH=CH2 HS—R—S—CH2—CH2—R —CH=CH2, have not achieved any commercial significance. Poly(alkylene sulfides), on the other hand, are produced by nucleophilic substitutions (Section 27.1), while the polycondensation of benzyl chloride is an electrophilic substitution ... 

Four types of reactions have been applied to the preparation of polyamides in general, but only one of these, the reaction of an acid chloride with an amine, has been broadly used to prepare lyotropic polyamides. The direct amidation of acids and amines through a crystalline salt intermediate, as used for the preparation of aliphatic polyamides (termed a nylon salt ), is not applicable for aromatic amine-acid monomer pairs, but three other types of reactions have recently been developed for polyamides these are ... 

In recent years there has been considerable interest in aromatic polyamide fibres, better known as aramid fibres. These are defined by the US Federal Trade Commission as a manufactured fibre in which the fibre-forming substance is a long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. ... 

SOLUTION (a) Amide groups are present in the backbone, and so the polymer is a polyamide. Because the amide groups face in opposite directions, there are two different monomers, one with two acid groups and the other with two amine groups. We split each amide group apart and add a molecule of water to each amide link. 

Amidation is particularly well adapted to use as a polymer-forming condensation reaction. The reaction is rapid above 180° to 200°C, it is remarkably free from side reactions, no catalysts are required (indeed, none are known), and the process is of the second order so that the molecular weight increases directly as the time of reaction. Molecular weights of 20,000 to 30,000 are attainable with no great difficulty under favorable conditions. This is not true of particular polyamide reactants susceptible to side reactions, as, for example, in the reaction of a diamine with glutaric acid wherein the inherent instability of the glutaric amide unit leads to decomposition. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

Nylon-6,6 and nylon-6 have competed successfully in the marketplace since their respective commercial introductions in 1939 and 1941, and in the 1990s share, about equally, 90% of the total polyamide market. Their chemical and physical properties are almost identical, as the similarity of their chemical structure might suggest the amide functions are oriented in the same direction along the polymer chain for nylon-6, but are alternating in direction for nylon-6,6. 

The fact, that almost all basic functions in caprolactam or polyamide medium occur in the form of amidic anions [-CO-N-]- arose the idea that these particular anions are directly responsible for the polymerization reaction. In analogy to the transesterification mechanism an intermediate addition complex might be formed by combining the amidic group with the respective anion... 

Upon dissolution of physical mixtiues (blends) of PTFE micropowders (without carboxylic acid groups) and PA-6 in formic acid, pure PTFE separates very fast from the solution. If the new melt-modified PTFE polyamide materials (PTFE irradiated 2,000 kGy) are dissolved in formic acid, the separation is very difficult. After separation of the insoluble content by centrifugation and ehmination of the soluble pure polyamide, the presence of an amide bond R1-CF2-CO-NH-CH2-PA between the PTFE and e.g. PA-6 was proved by IR spectroscopic investigations. The 1,708-cm band in the IR difference spectrum (Fig. 8 blue line) deriving from the direct PTFE polyamide linkage was in agreement with the IR spectra of low molecular model substances. 

Since the reactions with phosphorus compounds give relatively high yields of carboxylic amides and esters and ureas, they have been extended to the direct polycondensations of dicarboxylic acids with diamines and bisphenols, of free a-amino acids or dipeptides, and of carbon dioxide and carbon disulfide with diamines under mild conditions to give polyamides, polyesters, polyureas, and polythioureas. 

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