Adipic acid, reaction with hexamethylene

Concept Check 15.12 Nylon 6,6 may be formed by means of a condensation polymerization reaction in which hexamethylene diamine [NH2—(CH2)e—NH2] and adipic acid react with one another with the formation of water as a by-product. Write out this reaction in the manner of Equation 15.9. Note The structure for adipic acid is... 

The diamine-terminated nylon 6/66 copolyamide oligomers (CPA, Scheme 6), having the values shown in Table 7 were synthesized by the melt polycondensation reaction of e-caprolactam (CA), adipic acid (AA), and hexamethylene diamine (HA) [37,39]. An excess of HA over AA was used to obtain CPA with terminal amine groups and the molecular weight was controlled by a stoichiometric imbalance of reactants, ie., by varying the AA/HA feed molar ratio at a fixed CA/AA feed molar ratio (Table 7). The diamine-terminated nylon 6... 

The main use of acrolein is to produce acrylic acid and its esters. Acrolein is also an intermediate in the synthesis of pharmaceuticals and herhicides. It may also he used to produce glycerol hy reaction with isopropanol (discussed later in this chapter). 2-Hexanedial, which could he a precursor for adipic acid and hexamethylene-diamine, may he prepared from acrolein Tail to tail dimenization of acrolein using ruthenium catalyst produces trans-2-hexanedial. The trimer, trans-6-hydroxy-5-formyl-2,7-octadienal is coproduced. Acrolein, may also he a precursor for 1,3-propanediol. Hydrolysis of acrolein produces 3-hydroxypropionalde-hyde which could he hydrogenated to 1,3-propanediol. ... 

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

In 1930, DuPont launched the synthetic fiber industry with the discovery of nylon-6,6.2 In 1938, a pilot plant for nylon-6,6 production was put into operation, and in 1939, production was commenced at a large-scale plant in Seaford, Delaware. The classical method for the synthesis of nylon-6,6 involves a two-step process. In the first step, hexamethylene diamine (HMDA) is reacted with adipic acid (AA) to form a nylon salt. Polymerization of the aqueous salt solution is carried out at temperatures in the range of about 210-275°C at a steam pressure of about 1.7 MPa. When 275°C is reached, the pressure is reduced to atmospheric pressure and heating is continued to drive the reaction to completion. 

The condensation polymers are formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units. In these pol3nnerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take place. The examples are terylene (dacron), nylon 6, 6, nylon 6, etc. For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid. 

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... 

Nylon is the common name for polyamides. Polyamides are generally made from reactions of diacids with diamines. The most common polyamide is called nylon 6,6 because it is made by reaction of a six-carbon diacid (adipic acid) with a six-carbon diamine. The six-carbon diamine, systematically named hexane-1,6-diamine, is commonly called hexamethylene diamine. When adipic acid is mixed with hexamethylene diamine, a proton-transfer reaction gives a white solid called nylon salt. When nylon salt is heated to 250 °C, water is driven off as a gas, and molten nylon results. Molten nylon is cast into a solid shape or extruded through a spinneret to produce a fiber. 

Polyhexamethylenecarbodiimide, an insoluble condensation reagent for the synthesis of peptides, is obtained in the reaction of hexamethylene diisocyanate with a phospholene oxide catalyst in NMP. The isocyanate end groups are reacted with ethanol. Linear polycarbodiimides, upon reaction with adipic acid, form polyureides. For example, reaction of the 2,4-TDI derived carbodiimide with adipic acid in DMF produces the polymer, mp295°C.222... 

Quite often polymerisation proceeds by interaction of pairs of complementary monomers. Thus Nylon-6,6 is formed by reaction of adipic acid with hexamethylene diamine ... 

Condensation reactions can involve the splitting out of molecules other than water. For example, nylon 6,6 can be made from hexamethylene diamine and adipoyl chloride (instead of adipic acid). In this case a molecule of HCI is split or condensed out. There is a neat trick you can perform with this system that is commonly called The Nylon Rope Trick. The acid chloride will dissolve in an organic solvent, such as chloroform, while the diamine will dissolve in water. These two solutions do not want to mix and when carefully added to a beaker they form a phase-separated system. Polymerization can then occur at the interface between the phases (an iriterfacial polymerization), as illustrated in Figure 3-13. 

A/f-copolymers have a unique situation among macromolecular compounds. They have an ordered structure of the type -[A-B-]n, which can be viewed as the structure of a homopolymer. The fact that a/f-copolymers can be formed from two starting monomers is not their unique property, and many homopolymers formed in step reactions have an -[A-B-]n formula. For example. Nylon 66, being formed from adipic acid and 1,6-hexandiamine, can be considered an a/f-copolymer and named poly(hexamethylene-diamine-a/f-adipic acid), or it can have the name poly(hexamethylene adipamide) or poly(iminohexa-methylene iminoadipoyl) and be viewed as a homopolymer with the structure -[NH-(CH2)6-NHC(0)-(CH2)4-C(O)-]n. Many other examples of the same type can be listed. 

In the case of nylon-6,6, basic hydrolysis is the preferred treatment. Depolymerization occurs by reaction with sodium hydroxide, hexamethylene diamine and sodium adipate being the initially formed products. In a second step, the latter is converted into adipic acid via acidification with HC1. 

A polycondensation is a series of such reactions, which takes place between bifunctional molecules. For example, the polycondensation of adipic acid with hexamethylene-diamine gives nylon 6-6. In fact, the chemical method of production is not so simple, but we may (naively) write the reaction in the form... 

As a last point, let us consider how copolymerization relates to the polymer growth mechanism. Eirst, most step-growth polymerizations (e.g., the production of nylon 6/6 by the reaction of hexamethylene diamine with adipic acid) use two monomers to produce the final polymer. One can say that these are inherently copolymerizations. Considering... 

In contrast with ethylene monomer production, many stages are needed to produce the monomers adipic acid and hexamethylene diamine for nylon 6,6 (Fig. 2.11). None of these stages is 100% efficient, although new catalysts have increased the efficiency of some stages. Since energy is consumed in each reaction stage, and the capital cost of the many reactors is high, the cost per tonne of nylon 6,6 is four or five times that of polyethylene. Moreover the scale of production is smaller by a factor of 50 the implications of this are explored later. 

In the preparation of nylon-6,6 salt, DuPont discloses a process for making highly concentrated solutions of nylon salt at maximum solubility [192], In the first step of this process, a concentrated salt solution for nylon-6,6 is made with 73.5-77.5 wt% of adipic acid and 22.5-26.5 wt% of hexamethylene diamine at 55-60°C. The solution contains 60-69.5 wt% solute as compared to an ordinarily stoichiometric solution containing 56% diacid and 44% diamine with a maximum solute concentration of about 59 wt%i. The second step is to remove water from the solution by evaporation to a solution concentration of 93-96 wt%. When the concentrated solution is ready to be polymerized, addition hexamethylene diamine is added to complete the reaction. A process with similar reaction modifications is developed to prepare an essentially anhydrous mixture of adipic acid and hexamethylene diamine [193], The reaction mixture is heated to 120-135°C to allow evaporation of water while reacting. The resulting product has a ratio of adipic acid to hexamethylene diamine of 81 19. The molten acid-rich mixture is withdrawn in a continuous process. 

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