Nylon 6,6, synthesis from adipic acid

Until recently, the feedstock for all polymer synthesis has been petroleum that is, the monomers for virtually all polymer syntheses are made from crude oil, a nonrenewable raw material. As an example, nylon 6,6 is prepared industrially from adipic acid [HOOC(CH2)4COOHl and 1,6-diaminohexane [H2N(CH2)6NH2J, both of which originate from benzene, a product of petroleum refining (Figure 30.8). 

Of far-reaching and more lasting importance for the industrial use of benzene was the commencement of the production of styrene by IG Farbenindustrie in 1929, and the hydrogenation of benzene to cyclohexane as a feedstock for nylon production, after the discovery of nylon synthesis from hexamethylenediamine and adipic acid by Wallace H.Carothers of Du Pont in 1935. 

In the synthesis of nylon 66, hexanedioic acid (adipic acid) and 1,6-hexanediamine (hexamethylenediamine) are dissolved in aqueous ethanol, where they react to form a one-to-one salt called nylon salt. Nylon salt is then heated in an autoclave to 250°C, where the internal pressure rises to about 15 atm. Under these conditions, —COO groups from adipic acid and — NHj+ groups from hexamethylenediamine react with loss of H O to form amide groups. 

Adipic Acid. Nylon 66, produced from adipic acid and hexamethylenediamine (HMDA), currently is the largest-volume domestic nylon. About 90 percent of all adipic acid goes to make nylon 66 fibers and resins. Although HMDA can be made from adipic acid, a major source is from adiponitrile. The commercial synthesis of adipic acid is a two-step reaction starting with either cyclohexane or phenol. In both cases, a cyclohexanone/cyclohexanol mixture is formed as an intermediate. This mixture then is catalytically oxidized with nitric acid to the adipic product. It also can be manufactured as a by-product of the caprolactam process. 

Adipic acid (1,4-butanedicarboxylic acid/hexanedioic acid) is tiie most essential aliphatic dicarboxylic acid and is white crystalline powder in nature. Adipic acid is largely used for the production of nylon 6,6. The synthesis of adipic acid is as follows biosynthesis of cisfiis-muconic acid by the fermentation of glucose, followed by catalytic hydrogenation to adipic acid. In addition to tiie optimization of culture conditions for its production, the separation of adipic acid from an aqueous medium at high purity levels is necessary for polymer-grade products. The catalytic conversion of muconic acid to adipic acid requires to be furtiier explored. 

Wacker oxidation of styrene has also been performed in [bmim][BF4] and [bmim][PF6], at 60 °C with H2O2 and PdCF as a catalyst [19]. This system gave yields of acetophenone as high as 92 % after 3 h. Hydrogen peroxide may also be used under phase transfer conditions for alkene bond cleavage, to produce adipic acid (an intermediate in the synthesis of nylon-6) from cyclohexene (Scheme 9.9). 

A synthesis of great industrial interest is the electrochemical anodic reductive dimerisation of two molecules of acrylonitrile to give adiponitrile, from which adipic acid and 1,6-hexanediamine are prepared by hydrolysis and reduction, respectively, of the two nitrile groups. Polycondensation of the resulting products leads to Nylon 66 (Scheme 5.27). 

There are nine chemicals in the top 50 that are manufactured from benzene. These are listed in Table 11.1. Two of these, ethylbenzene and styrene, have already been discussed in Chapter 9, Sections 5 and 6, since they are also derivatives of ethylene. Three others—cumene, acetone, and bisphenol A— were covered in Chapter 10, Sections 3-5, when propylene derivatives were studied. Although the three carbons of acetone do not formally come from benzene, its primary manufacturing method is from cumene, which is made by reaction of benzene and propylene. These compounds need not be discussed further at this point. That leaves phenol, cyclohexane, adipic acid, and nitrobenzene. Figure 11.1 summarizes the synthesis of important chemicals made from benzene. Caprolactam is the monomer for nylon 6 and is included because of it importance. 

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 repeating units in nylon 66 and in most step reaction polymers consist of residues from both reactants. The following equation shows the synthesis of nylon 66 in this case the reactants are a dicarboxylic acid (adipic acid) and a diamine (1,6-hexanediamine). 

Adipic acid is a most important petrochemical product which is mostly used for the synthesis of nylon 6.6 from its condensation with hexamethylenediamine. Cyclohexane is transformed to adipic acid in two steps (a) oxidation of cyclohexane to a cyclohexanol-cyclohexanone mixture (ol-one) via the formation of cyclohexyl hydroperoxide followed by (b) oxidation of the ol-one mixture to adipic acid by nitric acid (equation 239). 

Another example of bulk (or melt) polymerization is the synthesis of polyamides through the direct interaction between a dicarboxylic acid and a diamine. Nylon 66, for example, can be produced from the reaction between hexamethylenediamine and adipic acid. In practice, it is preferable to ensure the existence of a 1 1 ratio of the two reactants by prior isolation of a 1 1 salt of the two. The overall procedure is summarized by the reaction scheme ... 

Problem 5.21 Consider the synthesis of nylon-6,6 from hexamethylene diamine and adipic acid, the mole ratio of the diacid to diamine being 0.99. Estimate the degree of polymerization of species which has the maximum yield by weight at the completion of the condensation. 

The first industrial success in step polymerisation was the synthesis and development by Carothers and co-workers at DuPont of the famous polyamide Nylon 6-6, starting from hexamethylene diamine and adipic acid dichloride. As shown in Equation 3.1, this reaction is favoured by elimination of hydrochloric acid. 

The name oxalic acid is derived from one of its sources in the biological world, namely, plants of the genus Oxalis, one of which is rhubarb. Oxalic acid also occurs in human and animal urine, and calcium oxalate (the calcium salt of oxalic acid) is a major component of kidney stones. Adipic acid is one of the two monomers required for the synthesis of the polymer nylon 66. The U.S. chemical industry produces approximately 1.8 billion pounds of adipic acid annually, solely for the synthesis of nylon 66 (Section 16.4A). 

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