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April 2001
Vol. 10, No. 04,
pp 93, 95, 97.
Chemistry Chronicles
Du Pont Strikes Pay Dirt at Purity Hall

Groundbreaking studies by Wallace Carothers paved the way for the development of nylon and neoprene.

1930s actress Marie Wilson waves from a chairlift beside a 35-foot Du Pont nylon display in Los Angeles.
1930s actress Marie Wilson waves from a chairlift beside a 35-foot Du Pont nylon display in Los Angeles.
Shortly before Christmas in 1926, Charles Stine proposed to the Du Pont executive committee what he termed “a radical departure from previous policy” regarding research. Stine, who was director of Du Pont’s Chemical Department, urged the committee to establish a modest program of fundamental research, which would have no clear-cut prospects for commercial development.

At first, the executive committee was skeptical. Only a handful of industrial firms did such research at that time, notably General Electric, American Telephone & Telegraph’s Bell Research Laboratories, and a couple of the big German chemical manufacturers. For nearly all companies, however, research was restricted simply to problem solving and process improvement. Stine argued that a program of fundamental research would enhance Du Pont’s prestige, make recruiting of Ph.D. scientists easier, and cement bonds to academic chemical departments. Finally, he added, just possibly something of practical value might come out of such an effort. This hope was more than justified by the creative chemical genius of Wallace H. Carothers, whose work launched the development of neoprene, the first U.S.-made synthetic rubber, and nylon, the first truly synthetic fiber.

The committee finally was persuaded and agreed in 1927 to fund the program at a rate of $300,000 a year. Stine planned to focus the company’s fundamental research on five then-hot areas: colloid chemistry, catalysis, the generation of chemical and physical data, organic synthesis, and polymerization. Finding a chemist to fill the organic and polymer post proved daunting, however. Several prominent candidates turned down his offers.

Then, he approached Carothers, a highly regarded young instructor of organic chemistry at Harvard University. Carothers initially was reluctant to leave Harvard. But he agreed to move to Du Pont when Stine convinced him that he would have free rein to work on whatever problems interested him and a generous budget for staff and supplies. In addition, Du Pont offered him an annual salary of $6000, nearly double what Harvard was paying him. He arrived at Du Pont’s Experimental Station, near Wilmington, DE, in early 1928.

Carothers was born in Burlington, IA, in 1896. He earned a master’s degree in chemistry at the University of Illinois in 1921, then became a chemistry instructor at the University of South Dakota. He returned to the University of Illinois—then the preeminent school in the United States for training organic chemists—to obtain a Ph.D. under Roger Adams in 1924. Adams would later call Carothers “the best organic chemist in the country”. Carothers stayed at the University of Illinois for two years before accepting a teaching post at Harvard.

Before he left Harvard, Carothers had been mulling over the chemistry of polymers. At that time, polymers were still poorly understood. Chemists were aware of many natural polymeric substances, especially cellulose, fibers, protein, and rubber. Other polymers had been made from such simple molecules as styrene, vinyl chloride, or acrylic acid. Leo H. Baekeland had achieved great commercial success after he had produced Bakelite, a hard resin prepared from phenol and formaldehyde, in 1907.

Polymers were known to be high-molecular-weight substances (up to 40,000 or more) that were aggregates of huge numbers of smaller chemical units. How these units were arranged and held together was a matter of controversy. Many chemists viewed them as something akin to colloids, consisting of small molecules bound by some unknown intramolecular force.

In the early 1920s, the great German organic chemist Hermann Staudinger, who would later be awarded a Nobel Prize in 1953, concluded that polymers were macromolecules consisting of small units linked to one another by the same covalent bonds found in smaller organic molecules. Carothers’s ideas about the structure of macromolecules generally were in line with those of Staudinger. Although Staudinger studied the polymer itself, trying to break it apart into its structural units, Carothers conceived of studying polymers by building them up from small organic molecules using well-understood reactions.

This was the research he decided to focus on when he reached Du Pont’s new fundamental research lab; the chemists who worked there had taken to calling it “Purity Hall”. He and the small group of Ph.D. chemists who worked under him undertook to form large molecules by stringing together small compounds with reactive groups on each end—specifically, by esterfying dibasic acids with dihydric alcohols to form polyesters. The scheme was roughly similar to linking together a long chain of paper clips.

Carothers’s group made rapid progress, and the polymers that they synthesized were theoretically interesting. But their molecular weights were limited to 4000 or fewer. Carothers decided that the water formed during esterfication restricted chain length. He discovered in 1930 that if the reactions were run in a molecular still, a recently invented device, the water could be frozen out. Soon polyesters with molecular weights of up to 25,000 were being prepared.

That same year, meanwhile, Carothers was asked by Elmer Bolton to take a look at polymers based on acetylene. Bolton became head of Du Pont’s Chemical Department after Stine had been promoted to corporate vice president and member of the company’s executive committee. Bolton, who previously had been in charge of research on dyestuffs, had a long-standing interest in synthetic rubber. He became aware, while attending an American Chemical Society meeting, that Father Julius A. Nieuwland, who taught chemistry at Notre Dame University, had polymerized acetylene by using a cuprous chloride catalyst. Bolton noted that the compounds prepared by Nieuwland were similar to isoprene, the basic structural unit of natural rubber.

Carothers assigned Arnold Collins to make a very pure sample of divinylacetylene, one of Nieuwland’s polymers. In the process, Collins recovered a small fraction of an unknown liquid that he set aside. He found a few days later that the unknown material had congealed. When he dropped the homogenous mass, it bounced. Analysis showed that it was a polymer of chloroprene that was formed with chlorine in the catalyst. Inadvertently, Collins had made a synthetic rubber.

Du Pont began producing the rubber commercially in 1932 under the name Duprene—later changed to neoprene. Neoprene never really rivaled natural rubber; it is difficult and expensive to produce. But its resistance to weather, oil, chemicals, and heat won it several relatively small but profitable applications.

In 1930, Carothers closest research associate, Julian Hill, after synthesizing a high-molecular-weight polyester, happened to touch a glass rod to the molten mass. He found he could pull out, like taffy, what he called a “festoon of fibers”. Once the fibers cooled, moreover, further pulling made them strong and elastic as the previous helter-skelter arrangement of the polymer molecules became oriented along a single axis. Hill had formed a truly synthetic fiber.

Hill prepared a wide variety of polyester fibers using aliphatic carboxylic acids. They were scientific marvels. But they all melted at a relatively low temperature—about 1000 °C—and they softened in water and dissolved in many organic solvents. Consequently, they were of no use for making textiles.

Carothers and Hill turned their attention to polymerizing dibasic acids with diamines—molecules that had an amino group at each end—instead of dihydric alcohols. Fibers could be drawn from these polyamides, but they seemed to be of no more practical use than the polyesters. Their melting points were very high, and they were relatively insoluble. Discouraged, Carothers stopped working on polyamides in 1931. Bolton, nonetheless, was sure that a useful fiber was possible. He urged that research be resumed. Unlike Stine, moreover, he strongly believed that Du Pont’s research should be targeted at specific commercial applications. Under him, the days of freewheeling fundamental research at Purity Hall were fading.

With Bolton’s prodding, the polyamide fiber project got back in gear. Several polymers were synthesized, at least five of which looked promising as fibers. One of these was made using adipic acid and hexamethylenediamine on February 28, 1935. Because each of its components had six carbon atoms, it was named fiber 66. After cold drawing, the fibers were tough, elastic, and unaffected by water and most solvents. Du Pont picked it for commercial development in July 1935.

By then, Carothers was largely out of the picture. Even while a student at Illinois, he had shown occasional signs of depression. In a letter to Du Pont before he accepted his position there, he had warned, “I suffer from neurotic spells of diminished capacity.” His periods of depression deepened with time. In 1934, he spent several weeks at a Baltimore clinic for psychotics.

In the spring of 1936, against his will, he was admitted for several months to a mental institution in Philadelphia, which he described to a friend as an “elaborate semi-bug house”. While Du Pont was hard at work putting in place a commercial process for its fiber 66, Carothers’s career had essentially ended. Not only was he increasingly depressed and erratic, but he also was drinking heavily.

On April 29, 1937, he checked into a hotel in Philadelphia; later that day, his body was found on the floor of his room. Beside it was an empty vial showing traces of cyanide and a squeezed lemon. Carothers had long carried a capsule containing cyanide on his watch chain.

Back in Delaware, Du Pont was learning how to turn out fiber 66 on a large scale. Du Pont engineers had to design and build a plant to manufacture a product that had never been made before. About 230 chemists and chemical engineers worked on the project at one time or another before Du Pont started construction on its initial plant at Seaford, DE, in 1938. The plant had a startup capacity of 3 million pounds of yarn annually—soon to be expanded to 8 million pounds.

Several names were proposed for fiber 66, including Delawear, a play on the firm’s home state and Duparooh, derived from “Du Pont pulls a rabbit out of a hat.” Finally, the company settled on nylon.

Nylon was announced to the public by Stine at a forum of women’s clubs in New York City on October 27, 1938. Because of the new fiber’s silklike qualities, Du Pont targeted it toward women’s hosiery. (Actually, small quantities were used to make bristles for Dr. West’s toothbrushes earlier in 1938.) Company office employees had a chance to buy sample stockings in March 1939; a limited quantity was sold to the public in Wilmington later that year. But nationwide, sales were not slated until May 15, 1940. Demand exceeded expectations; nearly 800,000 pairs of stockings were snapped up the very first day. By 1941, nylon had captured 30% of the U.S. hosiery market. Once the United States entered World War II in December 1941, all output of nylon was shifted to military needs for parachutes, airplane tire cord, glider towrope, and tents. After the war, consumer demand rebounded as nylon moved into other outlets than hosiery, such as lingerie and, in the 1950s, tire cord and carpets. Nylon was to make more money for Du Pont than any other product.

Last November, the site near Wilmington where Carothers conducted his pioneering studies of polymers was named an International Historic Chemical Landmark by the American Chemical Society. In 1995, Du Pont’s first nylon plant at Seaford was likewise designated.

David M. Kiefer, former assistant managing editor of Chemical & Engineering News until his retirement in 1991, is a consulting editor for Today’s Chemist at Work. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th st N.W., Washington, DC 20036.

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