Polyesters and Polyamides Step-Growth Polymers

When an amine reacts with an acid chloride, an amide is formed. What would happen, though, if a diamine and a diacid chloride were allowed to react Each partner could form two amide bonds, linking more and more molecules together until a giant polyamide resulted. In the same way, reaction of a diol with a diacid would lead to a polyester. 

The alkene and diene polymers discussed in Sections 7.10 and 14.6 are called chain-growth polymers because they are produced by chain reactions. An initiator adds to a C = C bond to give a reactive intermediate, which adds to a second alkene molecule to produce a new intermediate, which adds to a third molecule, and so on. By contrast, polyamides and polyesters are called step-growth polymers because each bond in the polymer is formed independently of the others. A large number of different step-growth polymers have been made some of the more important ones are shown in Table 21.2. 

Wallace Hume Carothers (1896-1937) was born in Burlington, Iowa, and received his Ph.D. at the University of Illinois in 1924 with Roger Adams. He began his career with brief teaching positions at the University of South Dakota, the University of Illinois, and Harvard University, but moved to the DuPont Company in 1928 to head their new chemistry research program in polymers. A prolonged struggle with depression led to his suicide after only 9 years at DuPont. 

The best known step-growth polymers are the polyamides, or nylons, first prepared by Wallace Carothers at the DuPont Company by heating a diamine with a diacid. For example, nylon 66 is prepared by reaction of adipic acid (hexanedioic acid) with hexamethylenediamine (1,6-hexanediamine) at 280 °C. The designation 66 tells the number of carbon atoms in the diamine (the first 6) and the diacid (the second 6). 

Nylons are used both in engineering applications and In making fibers. A combination of high impact strength and abrasion resistance makes nylon an excellent metal substitute for bearings and gears. As fiber, nylon is used in a variety of applications, from clothing to tire cord to ropes. 

T he most generally useful polyester is that made by reaction between dimethyl terephthalate (dimethyl 1,4-benzcncdicarboxyIate) and ethylene glycol (1,2-ethanediol). The product is used under the trade name Dacron to make clothing fiber and tire cord and under the name Mylar to make recording tape. The tensile strength of poly(ethylene terephthalate) film is nearly equal to that of steel. 

As an example of how acetyl CoA is used in nature, N-aeetylglucos-amine, an important constituent of surface membranes in mammalian cells, is synthesized by an aminolysis reaction between glucosamine and acetyl CoA. We ll look at some reactions of acetyl CoA in more detail in Chapter 29. 

CHAPTER 21 fl Carboxylic Acid Derivatives and Nucleophilic Acyl Substitutions 

Another example of a nucleophilic acyl substitution reaction on a thioester—this one a substitution by hydride ion to effect partial reduction of a thioester to an aldehyde—occurs in the biosynthesis of mevaldehyde, an intermediate in terpenoid synthesis (Chapter 7 Lagniappe and Section 23.7). In this reaction, (3S)-3-hydroxy-3-methylglutaryl CoA is reduced by hydride donation from NADPH. 

CHAPTER l6 CARBOXYLIC ACID DERIVATIVES NUCLEOPHILIC ACYL SUBSTITUTION REACTIONS 

Some Common Step-Growth Polymers and Their Uses 

There are two main classes of synthetic polymers chain-growth polymers and step-growth polymers. Polyethylene and other alkene and diene polymers like those we saw in Sections 8.10 and 14.6 are chain-growth polymers because 

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Polyamides and Polyesters Step-Growth Polymers 818 21.10 Spectroscopy of Carboxylic Acid Derivatives 822... 

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