Synthesis of Polyamides

Nylon is a generic term for a family of synthetic polymers known generi-cally as aliphatic polyamides, first produced in 1935 at DuPont s research facility at the DuPont Experimental Station led by Carothers. As one of the most popular thermoplastics. Nylon has different chemical structures and trade names such as nylon-6,6, nylon-6, nylon-6,9, and nylon-11. 

Two molecules of water are given off and the nylon is formed. Its properties are determined by the R and R groups in the monomers. In nylon-6,6, R = 4C and R = 6C alkanes, but one also has to include the two carboxyl carbons in the diacid to get the number it donates to the chain. In Kevlar, both R and R are benzene rings. 

In early discovery of polymers, of course there were a lot of arguments on the basics of polymer science and unclear concepts and terms. Carothers criticized Berzelius [10,13] about his misleading concept on polymers as the term polymers indicated the presence of the same atoms in the same proportions. Carothers described a condensation that requires as starting materials compounds in which at least two functional groups are present in the same molecule (e.g., hydroxy acids, HO-R-COOH, might lead to polyesters, HO[RCOO] RCO- and amino acids to polyamides, NH2[RCONH]j,RCO-.) [10]. In addition, he 

It was found that the monomer of ethyl oxalate crystals was also not stable in ambient condition. 

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Other Preparative Reactions. Polyamidation has been an active area of research for many years, and numerous methods have been developed for polyamide formation. The synthesis of polyamides has been extensively reviewed (54). In addition, many of the methods used to prepare simple amides are appHcable to polyamides (55,56). Polyamides of aromatic diamines and aUphatic diacids can also be made by the reaction of the corresponding aromatic diisocyanate and diacids (57). 

Figure 5.22 Synthesis of polyamide-imide by catalytic carbonylation of bis-4-chlorophthalimide in presence of aromatic diamine.

The way in which this is done has a major effect on the formation of a particular nanocomposite and this is discussed further below. A wide range of w-amino acids have been intercalated between the layers of MMT [14]. Amino acids have been successfully used in the synthesis of polyamide... 

Fig. 3.8 Variations to solid phase synthesis of polyamides. Use of Fmoc monomers on jS-Ala-Wang resin (left) provides polyamides containing a jS-alanine residue near the C-ter-mini. Polyamides synthesized on the Kaiser...

Baird, E.E. and P.B. Dervan. Solid phase synthesis of polyamides containing imidazole and pyrrole amino acids./. 

WvRTZ, N.R., J.M. Turner, E.E. Baird, and P. B. Dervan. Fmoc solid phase synthesis of polyamides containing pyrrole and imidazole amino acids. Org. Lett. 2001, 3, 1201-1203. 

Hexamethylenediamine (HMDA), a monomer for the synthesis of polyamide-6,6, is produced by catalytic hydrogenation of adiponitrile. Three processes, each based on a different reactant, produce the latter coimnercially. The original Du Pont process, still used in a few plants, starts with adipic acid made from cyclohexane adipic acid then reacts with ammonia to yield the dinitrile. This process has been replaced in many plants by the catalytic hydrocyanation of butadiene. A third route to adiponitrile is the electrolytic dimerization of acrylonitrile, the latter produced by the ammoxidation of propene. 

The azolide method has also been used for the synthesis of polyamides and polyimides. These can be obtained by several routes First by condensation of two dihomofunctional components (dicarboxylic acid diimidazolides and diamines), secondly by condensation of a heterodifunctional compound (amino carboxylic acid and CDI), or through reaction on a polymer (for example, polymeric carboxylic acid imidazolides and amines). 

The relationship given in Equation 4.21 is called the Carothers equation because it was first found by Carothers while working with the synthesis of polyamides (nylons). For an essentially quantitative synthesis of polyamides where p is 0.9999, the DP is approximately 10,000, the value calculated using Equation 4.21 ... 

Proposed mechanisms for polycondensations are essentially the same as those proposed in the organic chemistry of smaller molecules. Here, we will briefly consider several examples to illustrate this similarity between reaction mechanisms for small molecules and those forming polymers. For instance, the synthesis of polyamides (nylons) is envisioned as a simple Sn2 type Lewis acid-base reaction, with the Lewis base nucleophilic amine attacking the electron-poor, electrophilic carbonyl site followed by loss of a proton. 

M]0 = 5 M. A concentration of 5 M is fairly typical of a step polymerization that is often carried out with only the reactant(s) present (without solvent). The lowering of [H20] to achieve a particular Xn is less the more favorable the equilibrium (that is, the larger the K value). Thus the synthesis of polyamides (with typical K values > 102) is clearly easier from the equilibrium viewpoint than polyester synthesis (K 0.1-1). Polyesterification requires a greater lowering of [H20] than does polyamidation. It should be understood that simply to lower [H20] as much as possible is not the desired approach. One needs to control the [H20] so as to obtain the desired degree of polymerization. 

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... 

Scheme 1 Major preparative methods for the synthesis of polyamides.

The second common method of polymer synthesis involves the stepwise coupling of small molecules which are difunctional by virtue of reactive functional groups. A typical example of step-reaction polymerization would be the synthesis of polyamides by reaction of a diamine with a diacid. In these systems the chain is built up slowly by reaction of any pair of functional groups in the system and it is common for the coupling to involve elimination of a small molecule. Conventionally these polymerizations allow more control over the chain structure but difficulties in reaching very high conversions and problems of reagent purity usually lead to much shorter... 

The third major method for achieving difunctionality involves the ring-opening polymerization of a cyclic monomer, typified for example by the synthesis of polyamides from cyclic lactams. Reactions of this type proceed by chain-reaction mechanisms but yield polymers more typical of step-reactions, in that they contain functional groups within the chain. 

In the present context, solid-phase synthesis has been used primarily for the preparation of peptide and glycopeptide dendrimers [23]. For example, a second-generation dendrimer could be prepared by successive addition of branched polyproline building blocks to a solid phase [24]. The divergent synthesis of polyamide dendrimers on polystyrenes was accomplished [25] by Frechet el al. 1991 (cf. Fig. 2.8) [26]. PAMAM dendrons could be grown up to the fourth-... 

Imai Y (1996) A new facile and rapid synthesis of polyamides and polyimides by microwave-assisted polycondensation. In Hedrick JL, Labadie JW (eds) Step-growth polymers for high-performance materials. Am Chem Soc Washington, p421... 

Coordination carbonylation polycondensation has been extended from the synthesis of polyamides [scheme (15)] and polyarylates [scheme (16)] to reactions using other nucleophilic monomers that, with dihaloarenes and carbon monoxide, yield poly(imide-amide)s, poly(acylhydrazide)s, and poly(benzoxa-zole)s [165,170,171],... 

Condensation polymerization (e.g., synthesis of polyamides, synthesis of polymers such as polyethylene terephthalate) ... 

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 ... 

The recent application of this synthetic methodology to the carbamation of diamines [25b] is of great interest as di-carbamates are the source of di-isocyanates used in the synthesis of polyamides. Interestingly, the catalyst can be used for the selective synthesis of both mono- and di-carbamates. 

Will, D. W., Breipohl, G., Langner, D., et al. (1995) The synthesis of polyamide nucleic acids using a novel monomethoxytrityl protecting-group strategy. Tetrahedron, 51, 12069-12082. 

The synthesis of polyamides illustrates both groups of polymerization reactions ... 

Just as the comparatively weak metal-metal bonds pose problems for the synthesis of the difunctional dimers, they cause similar problems in the synthesis of the polymers. The relative weakness of the metal-metal bonds makes them more reactive than the bonds found in standard organic polymers thus, under many standard polymerization reaction conditions, metal-metal bond cleavage would result. For example, metal-metal bonds react with acyl halides to form metal-hahde complexes. Therefore, the synthesis of polyamides using metal-metal bonded diamines and diacyl chlorides would simply lead to metal-metal bond cleavage rather than polymerization. Likewise, metal-metal bonded complexes are incompatible with many Lewis... 

Table 1. Direkt synthesis of polyamides by using phosphites and phosphonites in NMR-Py solution3...

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