Carothers decided that he wanted to work on polymers. At that time most chemists thought that polymers were actually groups of molecules clumped together and known as colloids; hence the name collodion, for the nitrocellulose derivative used in photography and in Chardonnet silk. Another opinion on the structure of polymers, championed by the German chemist Hermann Staudinger, was that these materials were extremely large molecules. The largest molecule synthesized up to that time—by Emil Fischer, the great sugar chemist—had a molecular weight of 4,200. In comparison, a simple water molecule has a molecular weight of 18, and a glucose molecule’s molecular weight is 180. Within a year of starting work in the Du Pont laboratory, Carothers had made a polyester molecule with a molecular weight of over 5,000. He was then able to increase this value to 12,000, adding more evidence to the giant molecule theory of polymers, for which Staudinger was to receive the 1953 Nobel Prize in chemistry.
Carothers’s first polymer initially looked as if it had some commercial potential, as its long threads glistened like silk and did not become stiff or brittle on drying. Unfortunately, it melted in hot water, dissolved in common cleaning solvents, and disintegrated after a few weeks. For four years Carothers and his coworkers prepared different types of polymers and studied their properties, before they finally produced nylon, the man-made fiber that comes the closest to having the properties of silk and that deserves to be described as “artificial silk.”
Nylon is a polyamide, meaning that, as with silk, its polymer units are held together through amide linkages. But while silk has both an acid end and an amine end on each of its individual amino acid units, Carothers’s nylon was made from two different monomer units—one with two acid groups and one with two amine groups—alternating in the chain. Adipic acid has acid groups COOH at both ends:
Structure of adipic acid, showing the two acid groups at each end of the molecule. The acid group -COOH is written backward as HOOC- when it is shown on the left-hand side.
or written as a condensed structure (below):
The condensed structure of the adipic acid molecule
The other molecular unit, 1,6-diaminohexane, has a very similar structure to that of adipic acid except there are amino groups (NH2) attached in place of the COOH acid groups. The structure and its condensed version are shown below:
The amide link in nylon, like the amide link in silk, is formed by eliminating a molecule of water between the ends of the two molecules, from the H atom from NH2 and the OH from COOH. The resulting amide bond, shown as -CO-NH- (or in reverse order as -NH-CO-) joins the two different molecules. It is in this respect—having the same amide link—that nylon and silk are chemically similar. In the making of nylon both the amino ends of 1,6-diaminohexane react with the acid ends of different molecules. This continues with alternating molecules adding to each end of a growing nylon chain. Carothers’s version of nylon became known as “nylon 66” because each monomer unit has six carbon atoms.
Structure of nylon, showing alternating molecules of adipic acid and 1,6-diaminohexane
The first commercial use of nylon, in 1938, was for toothbrush bristles. Then in 1939 nylon stockings were marketed for the first time. Nylon proved to be the ideal polymer for stockings. It had many of the desirable properties of silk; it did not sag and wrinkle like cotton or rayon; and most important, it was far less expensive than silk. Nylon hosiery was an enormous commercial success. In the year after they were introduced, some 64 million pairs of “nylons” were manufactured and sold. So overwhelming was the response to this product that the word nylons is now synonymous with women’s hosiery. With its exceptional strength, durability, and lightness, nylon quickly found a use in many other products: fishing lines and nets, strings for tennis and badminton rackets, surgical sutures, and coatings for electrical wires.
During World War II Du Pont’s main production of nylon shifted from the fine filaments used in hosiery to the coarser yarns needed for military products. Tire cords, mosquito netting, weather balloons, ropes, and other military items dominated the use of nylon. In aviation nylon proved to be an excellent substitute for silk parachute shrouds. After the end of the war production in nylon plants was quickly converted back to civilian products. By the 1950s nylon’s versatility was apparent in its use in clothing, skiwear, carpets, furnishings, sails, and many other products. It was also found to be an excellent molding compound and became the first “engineering plastic,” a plastic that is strong enough to be used as a replacement for metal. Ten million pounds of nylon were produced in 1953 for this use alone.
Unfortunately, Wallace Carothers did not live to see the success of his discovery. A victim of depression that became worse as he got older, he ended his life in 1937 by swallowing a vial of cyanide, unaware that the polymer molecule he had synthesized would play such a dominant role in the world of the future.
Silk and nylon share a similar legacy. It is more than just a comparable chemical structure and an eminent suitability for use in hosiery and parachutes. Both these polymers contributed—in their own way—to enormous changes in the economic prosperity of their times. Not only did the demand for silk open worldwide trade routes and new trade agreements; it also led to the growth of cities that depended on silk production or the silk trade and helped establish other industries, such as dyeing, spinning, and weaving, that developed alongside sericulture. Silk brought great wealth and great change to many parts of the globe.
Just as silk and silk production stimulated fashions—in clothing, furnishings, and art—in Europe and Asia for centuries, the introduction of nylon and a wealth of other modern textiles and materials has had a vast influence on our world. Where once plants and animals furnished the starting materials for our clothing, the raw products for many fabrics now come from by-products of oil refining. As a commodity, oil has taken over a position that once belonged to silk. As was once the case with silk, the demand for oil has forged new trade agreements, opened new trade routes, encouraged the growth of some cities and the establishment of others, created new industries and new jobs, and brought great wealth and great change to many parts of the globe.
Women rushed to buy—and wear—nylons after World War II when the polymer became available for hosiery again. (Photo courtesy of Du Pont)
7. PHENOL
THE VERY FIRST totally man-made polymer was produced about twenty-five years before Du Pont’s nylon. It was a somewhat random cross-linked material made from a compound whose chemical structure was similar to some of the spice molecules to which we attributed the Age of Discovery. This compound, phenol, started another age, the Age of Plastics. Linked to such diverse topics as surgical practices, endangered elephants, photography, and orchids, phenols have played a pivotal role in a number of advances that have changed the world.
STERILE SURGERY
In 1860 you would not have wanted to be a patient in a hospital—especially not to undergo an operation. Hospitals were dark, grimy, and airless. Patients were commonly given beds where the bedclothes were not changed after the previous occupant left—or more probably died. Surgical wards exuded an appalling stench from gangrene and sepsis. Equally appalling was the death rate from such bacterial infections; at least 40 percent of amputees died from so-called hospital disease. In army hospitals that number approached 70 percent.
Despite the fact that anesthetics had been introduced at the end of 1864, most patients agreed to surgery only as a last resort. Surgical wounds always became infected; accordingly, a surgeon would ensure that the stitches closing an operation site were left long, hanging down toward the ground, so that pus could drain away from the wound. When this happened, it was considered a positive sign, as chances were good that the infection would stay localized and not invade the rest of the body.
Of course, we now know why “hospital disease” was so prevalent and so lethal. It was actually a group of diseases caused by a variety of bacteria that easily passed from patient to patient or even from doctor to a series of
patients under unsanitary conditions. When hospital disease became too rife, a physician usually closed down his surgical ward, sent the remaining patients elsewhere, and had the premises fumigated with sulfur candles, the walls whitewashed, and the floors scrubbed. For a while after these precautions infections would be under control—until another outbreak required further attention.
Some surgeons insisted on maintaining constant strict cleanliness, a regime involving lots of cooled boiled water. Others supported the miasma theory, the belief that a poisonous gas generated by drains and sewers was carried in the air and that once a patient was infected, this miasma was transferred through the air to other patients. This miasma theory probably seemed very reasonable at the time. The stench from drains and sewers would have been as bad as the smell of putrefying gangrenous flesh in surgery wards, which would further explain how patients treated at home rather than in a hospital often escaped infection altogether. Various treatments were prescribed to counteract miasma gases, including thymol, salicylic acid, carbon dioxide gas, bitters, raw carrot poultices, zinc sulfate, and boracic acid. The occasional success of any of these remedies was fortuitous and could not be replicated at will.
This was the world in which the physician Joseph Lister was practicing surgery. Born in 1827 to a Quaker family from Yorkshire, Lister completed his medical degree at University College in London, and by 1861 was a surgeon at the Royal Infirmary in Glasgow and a professor of surgery at the University of Glasgow. Though a new modern surgical block had been opened at the Royal Infirmary during Lister’s tenure, hospital disease was just as much a problem there as it was elsewhere.
Lister believed its cause might not be a poisonous gas but something else in the air, something that could not be seen with the human eye, something microscopic. On reading a paper that described “The Germ Theory of Diseases,” he immediately recognized its applicability to his own ideas. The paper had been written by Louis Pasteur, a professor of chemistry in Lille, northeastern France, and mentor to Chardonnet of Chardonnet silk fame. Pasteur’s experiments on the souring of wine and milk had been presented to a gathering of scientists at the Sorbonne in Paris in 1864. Germs—microorganisms that could not be detected by the human eye—were considered by Pasteur to be everywhere. His experiments showed that such microorganisms could be eliminated by boiling, leading of course to our present-day pasteurization of milk and other foodstuffs.
As boiling patients and surgeons was not practical, Lister had to find some other way to safely eliminate germs on all surfaces. He settled on carbolic acid, a product made from coal tar that had been used successfully to treat stinking city drains and that had already been tried as a dressing on surgical wounds, without very positive results. Lister persevered and met with success in the case of an eleven-year-old boy who came to the Royal Infirmary with a compound fracture of the leg. At the time compound fractures were a dreaded injury. A simple fracture could be set without invasive surgery, but a compound fracture, where the sharp ends of broken bones had pierced the skin, was almost certain to become infected, despite a surgeon’s skill at setting the bone. Amputation was a common outcome, and death from an uncontrollable persistent infection was likely.
Lister carefully cleaned the area in and around the boy’s broken bone with lint soaked in carbolic acid. Then he prepared a surgical dressing consisting of layers of linen soaked in carbolic solution and covered with a thin metal sheet bent over the leg to reduce possible evaporation of the carbolic acid. This dressing was carefully taped in place. A scab soon formed, the wound healed rapidly, and infection never occurred.
Other patients had survived their infections from hospital disease, but this was a case in which infection had been prevented, not just outlasted. Lister treated subsequent compound fracture cases the same way, producing the same positive outcome, convincing him of the effectiveness of carbolic solutions. By August 1867 he was using carbolic acid as an antiseptic agent during all his surgical procedures, not just as a postoperative dressing. He improved his antiseptic techniques over the next decade, gradually convincing other surgeons, many of whom still refused to believe in the germ theory as “if you can’t see them, they are not there.”
Coal tar, from which Lister obtained his carbolic acid solution, was readily available as a waste product from the gaslight illumination of city streets and houses during the nineteenth century. The National Light and Heat Company had installed the first gas street lighting in Westminster, London, in 1814, and widespread use of gas lighting followed in other cities. Coal gas was produced by heating coal at high temperatures; it was a flammable mixture—about 50 percent hydrogen, 35 percent methane, and smaller amounts of carbon monoxide, ethylene, acetylene, and other organic compounds. It was piped to homes, factories, and streetlamps from local gasworks. As demand for coal gas grew, so did the problem of what to do with coal tar, the seemingly unimportant residue of the coal-gasification process.
Coal tar was a viscous, black, acrid-smelling liquid that would eventually prove an amazingly prolific source for a number of important aromatic molecules. Not until huge reservoirs of natural gas, consisting mainly of methane, were discovered in the early part of the twentieth century did the coal-gasification process and its accompanying production of coal tar decline. Crude carbolic acid, as first used by Lister, was a mixture distilled from coal tar at temperatures between 170°C and 230°C. It was a dark and very strong-smelling oily material that burned the skin. Lister was eventually able to obtain the main constitutent of carbolic acid, phenol, in its pure form as white crystals.
Phenol is a simple aromatic molecule consisting of a benzene ring, to which is attached an oxygen-hydrogen or OH group.
Phenol
It is somewhat water soluble and is very soluble in oil. Lister made use of these characteristics by developing what became known as the “carbolic putty poultice,” a mixture of phenol with linseed oil and whitening (powdered chalk). The resultant paste (spread on a sheet of tinfoil) was placed poultice side down on the wound and acted like a scab, providing a barrier to bacteria. A less concentrated solution of phenol in water, usually about one part of phenol to between twenty and forty parts of water, was used to wash the skin around a wound, the surgical instruments, and the surgeon’s hands, and it was also sprayed onto an incision during an operation.
Despite the effectiveness of his carbolic acid treatment, as shown by the recovery rates of his patients, Lister was not satisfied that he had achieved totally antiseptic conditions during surgical operations. He thought that every dust particle in the air bore germs, and in an effort to prevent these airborne germs from contaminating operations, he developed a machine that continually sprayed a fine mist of carbolic acid solution into the air, effectively drenching the whole area. Airborne germs are actually far less a problem than Lister had assumed. The real issue was microorganisms that came from the clothes, hair, skin, mouths, and noses of the surgeons, the other doctors, and the medical students who routinely assisted with or watched operations without taking any antiseptic precautions. Modern operating-room protocol of sterile masks, scrub suits, hair coverings, drapes, and latex gloves now solves this problem.
Lister’s carbolic spray machine did help prevent contamination from microorganisms, but it had negative effects on the surgeons and others in the operating room. Phenol is toxic, and even in dilute solutions, it causes bleaching, cracking, and numbing of the skin. Inhaling phenolic spray can lead to illness; some surgeons refused to continue working when a phenol spray was in use. Despite these drawbacks Lister’s techniques of antiseptic surgery were so effective and the positive results so obvious that by 1878 they were in use throughout the world. Phenol is rarely used today as an antiseptic; its harsh effect on the skin and its toxicity make it less useful than newer antiseptics that have been developed.
MANY-FACETED PHENOLS
The name phenol does not apply only to Lister’s antiseptic molecule; it is applied to a very large group of related compou
nds that all have an OH group attached directly to a benzene ring. This may seem a bit confusing, as there are thousands or even hundreds of thousands of phenols, but only one “phenol.” There are man-made phenols, like trichlorophenol and hexylresorcinols, with antibacterial properties that are used today as antiseptics.
Picric acid, originally used as a dye—especially for silk—and later in armaments by the British in the Boer War and in the initial stages of World War I, is a triple-nitrated phenol and is highly explosive.
Trinitrophenol (picric acid)
Many different phenols occur in nature. The hot molecules—capsaicin from peppers and zingerone from ginger—can both be classified as phenols, and some of the highly fragrant molecules in spices—eugenol from cloves and isoeugenol from nutmeg—are members of the phenol family.
Capsaicin (left) and zingerone (right). The phenol part of each structure is circled.
Vanillin, the active ingredient in one of our most widely used flavoring compounds, vanilla, is also a phenol, with a very similar structure to that of eugenol and isoeugenol.
Vanillin is present in the dried fermented seedpods from the vanilla orchid (Vanilla planifolia), native to the West Indies and Central America but now grown around the world. These long, thin, fragrant seedpods are sold as vanilla beans, and up to 2 percent of their weight can be vanillin. When wine is stored in oak casks, vanillin molecules are leached from the wood, contributing to the changes that constitute the aging process. Chocolate is a mixture containing cacao and vanillin; custards, ice cream, sauces, syrups, cakes, and many other foods depend partly on vanilla for their flavor. Perfumes also incorporate vanillin for its heady and distinctive aroma.
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