MOLECULAR GIANTS
"We are shocked like zoologists would be if they were told somewhere in Africa an elephant was found who was 1500 feet long and 300 feet high."
-German chemist after a 1926 public debate on macromolecules
Inside a polymer.
An X-ray crystallograph of the DNA molecule. The dots and rings show where the X-rays were diffracted by atoms in the molecule. Photo courtesy of the Nobel Foundation, ©1963.
In the visible world, giants are both large in size and few in number. They immediately attract our attention, whether a seven-foot tall athlete, a fifteen-hundred-mile wall in China, or a hundred-foot long blue whale.
Early in the twentieth century the discovery of giants in the invisible world of molecules created excitement among scientists. These giant molecules are called polymers. They fascinate us not because they are scarce, but because they are everywhere about us. Polymers are the messengers of heredity, the components of important natural resources, and the key to many of the materials that people have used for clothing and shelter for centuries. To understand how natural polymers are formed and how they function was a challenge. Chemists and engineers have met that challenge. They have also found how to create synthetic polymers of unparalleled variety. These new materials are woven into every aspect of our age.
A century ago the idea that matter was composed of discrete physical aggregates of atoms, or molecules, was still contentious. Chemists had discovered that some substances containing similar proportions of carbon and hydrogen differed greatly in the properties they exhibited. It seemed incorrect to conclude that these materials shared the same basic structure. Certain compounds melted at higher temperatures than others. This characteristic led chemists to believe that these substances had more complex relationships among the atoms in their molecules. Hence the first speculations arose that these high-melting-point materials might be made up of building blocks of other simpler, constituent molecules.
Thomas Graham. Photo courtesy of the Edgar Fahs Smith Collection, Van Pelt-Dietrich Library, University of Pennsylvania.
In 1861 the British chemist Thomas Graham (1805-1869) noted another unusual property of some organic compounds such as starches and cellulose (wood fiber). When dissolved in solutions, they would not pass through fine filters. Nor could they be purified into a crystalline form. Graham believed that these substances represented a completely different organization of matter. He called them colloids, after kolla, the Greek word for glue, another material unable to penetrate fine membranes. Graham thought that cellulose and other colloids consisted not of molecules with an unusual number of atoms, but of large numbers of structurally simple molecules held together by "association." Association forces were not as strong as those forces binding the atoms of a molecule together. Yet they were sufficiently powerful to keep colloids from behaving like their "crystalloid" chemical relatives. After Graham, two generations of chemists would disregard the possibility that the sticky impurities fouling their distillations were actually giant molecules.
Manufacturers used a number of colloidal substances as bases for new materials in the nineteenth century. The most widely developed of these was cellulose. In 1870 the American inventor John Wesley Hyatt (1837-1920) marketed a pioneer plastic material, celluloid. Hyatt found that a chemically modified cellulose, cellulose nitrate, when combined with camphor (the same material used today in many lip balms), could be shaped and hardened into a permanent form by the application of heat and pressure.
An advertisement for Bakelite. Courtesy of the Edgar Fahs Smith Collection, Van Pelt-Dietrich Library, University of Pennsylvania.
Celluloid was transformed into keepsake photograph cases, countless combs and toiletry items, and collar stays in fashionable apparel. It was the first flexible photographic film, and early motion-picture audiences swooned to the stars of the celluloid cinema.
In 1887 Count Hilaire de Chardonnet (1839-1924) revealed his discovery of a method of spinning strands of cellulose nitrate into an artificial textile fiber. These first fibers were highly flammable because of their nitrogen content, a quality which prompted some to label the new material "mother-in-law silk." The Frenchman solved the problem by treating his fibers with acid sulfide solutions, and Chardonnet silk, the first synthetic fiber, entered commercial production in 1890.
The first plastics were modifications of natural materials. Industrial chemists later produced plastics solely from chemical reactions. In 1909 Belgian-born Leo Baekeland (1863-1944) began small-scale production of a resin formed by the combination of phenol with formaldehyde. Mixing this resin with suitable fillers and, like Hyatt, then applying heat and pressure, Baekeland created Bakelite, the first wholly synthetic plastic. Within two years Bakelite was being produced in both Europe and America.
For lack of lac for shellac.
Shellac was made from a special resin secreted by the East Asian lac bug. The resin was harvested by scraping the hardened deposits from trees inhabited by the insects, then processed. The search for a cheap substitute for this imported material led Leo Baekeland (left) to Bakelite. Photo courtesy of the Edgar Fahs Smith Collection, Van Pelt-Dietrich Library, University of Pennsylvania.
Most of these early synthetic materials generated little interest among academic chemists. But the creation of a synthetic rubber was a different matter. In 1860 the English researcher Greville Williams (1829-1910) distilled from natural rubber a simple compound, isoprene, in which, he suggested, eight hydrogen atoms were bound to five carbon atoms. Two decades later the French chemist Gustave Bouchardat (1842-1918) created a rubberlike substance in an experiment utilizing isoprene. The relation between the two substances, rubber and isoprene, seemed obvious. Rubber must be a polymer (from the Greek "polymeres," "having many parts") of isoprene, as it is made out of many parts, each like an isoprene molecule.
Yet how were these units of isoprene linked to form rubber? Chemists knew that the units had certain limits in their abilities to combine with each other. Carbon, for example, was tetravalent, meaning it could form up to four bonds with other atoms, including bonds with other carbon atoms. Beginning in 1904, Germany's Carl Harries (1866-1923) published several studies of the isoprene-rubber relationship. He concluded that within the modified isoprene units in natural rubber only one carbon-carbon double bond existed, leaving a unit with the ability to share a bond at each of its terminal carbon atoms. Harries considered the possibility that rubber might be a long, end-to-end chain of these modified units. But if rubber was a long-chain molecule, could one explain how such a molecular chain would begin and end with these same units? What would make an "end group" of exactly the same composition as its neighbor fail to combine with another unit at one of its available carbon bonds? Ingeniously Harries suggested that in fact there were no end groups, that these modified units combined to form a ring structure, two units to a ring. These ring structures remained closely packed due to strong "association forces."
The word plastic comes from the Greek "plastikos," meaning "moldable." Most polymer plastics are formed through the application of heat and pressure. Thermoplastic polymers can be reheated and reshaped after once being molded. Thermosetting polymers retain their shapes even at relatively high temperatures and cannot be reformed.
The tie that binds.
Most polymers contain atoms held in a molecular structure by covalent bonds. These bonds are formed through the sharing of electrons, electrically charged particles within an atom. Sharing electrons reduces the energy level of the individual atoms. These bonds are maintained until other chemical reactions encourage atom combinations which yield lower energies.
In 1920 a German organic chemist, Hermann Staudinger (1881-1965), began a spirited campaign to convince his fellow scientists of the existence of long-chain molecules. Staudinger had studied polyketenes, materials derived from the compound ketene and believed by many to have a ring structure similar to that proposed by Harries for rubber. Staudinger believed that the properties of polyketene should not be explained by invoking the presence of "association forces" that cause ketene molecules to aggregate. Polyketene was a different molecular species: a long-chain molecule in which a repeating sequence of atoms could be found, but in which all the bonds within and without a sequence were of exactly the same character. Staudinger suggested that the end groups on this long chain were no different from the other groups in the molecule. Polymerization--the creation of the long chain through the joining of smaller groups--ceased whenever the length of the molecule became so great that the chain would no longer seek out additional groups. The spirit of an end group electron might be willing to bond, but the body of the chain would no longer be able.
Staudinger proposed long-chain formulas for rubber and several other compounds, calling this group of materials "high polymers" (because of the large number of units making up each chain) or, as he preferred, macromolecules. Staudinger also derived a mathematical formula to explain the relationship between macromolecular viscosity--the resistance of polymer solutions to flow--and a dissolved polymer's molecular weight, a measure of the size of the molecule. This formula proved to be a powerful theoretical argument in the battle with supporters of a colloidal explanation of high polymers.
New instruments and new experimental results secured the triumph of the macromolecular hypothesis by the end of the 1920s. Chemists succeeded in crystallizing a number of high polymers, including biological enzymes, something that by definition could not be done to a true colloid. The Swedish chemist The Svedberg (1884-1971) developed the ultracentrifuge, which spun samples of colloidal solutions so rapidly that forces 100,000 times as strong as earth's normal gravity separated particles in solution. Very accurate determinations of molecular weight could be made with the new machine. Svedberg discovered that hemoglobin, the oxygen-carrying protein that gives blood its red color, had a molecular weight of over 66,000--four times the highest weight ever assigned to it before.
In Germany chemists Kurt Meyer (1883-1952) and Herman Mark (b. 1895) used X-rays to examine the internal structure of cellulose and other polymers. The work of Mark and Meyer convinced researchers that cellulose and other high polymers could have structures larger than a single unit. Finally the great utility of the macromolecular hypothesis was shown by the work of a brilliant young industrial chemist in America, Wallace Hume Carothers (1896-1937). With his research group at the Du Pont Experimental Station, Carothers synthesized and analyzed an extensive series of new polymeric substances, including polyesters, neoprenes, and nylons.
Polymers al dente?
Staudinger and Mark disagreed about the shape and properties which a macromolecule has. Staudinger proposed a rigid, crystalline form, like that of an uncooked strand of spaghetti. Mark, holding a model of Staudinger's molecule in this 1986 photograph, believed that the polymer would be much more flexible in its behavior, taking on an appearance more like that of a cooked strand of spaghetti. Mark's view has proven correct for most polymers. Mark photo courtesy of the Chemical Heritage Foundation.
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