Polymer Chemistry—the formative years


Polymers are substances made of giant molecules formed by uniting simple molecules or monomers by covalent bonds. The word comes from Greek and it means many parts. Polymers have high molecular weights, which gives them useful physical characteristics such as high viscosity, elasticity, and strength. Polymers are found everywhere. They are part of man himself: proteins and nucleic acid are polymers. Natural fibers such as wool and cotton are polymers. And of course many synthetics, such as plastics, nylon, and man-made rubber, are polymers.
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Today, the existence of macromolecules is readily accepted in the scientific world, and polymer science is a vital branch of chemistry. But that acceptance is fairly recent. As late as the early years of the 20th century, many of the most prominent chemists resisted the concept of macromolecules with molecular weights of thousands and even millions. For example, the Nobel Prize winning chemist Emil Fischer demonstrated the existence of polypeptide chains in proteins, but he remained convinced such chains could not exceed a molecular weight of 4000. Fischer’s prestige in the first decades of the 20th century was such that it “made it more difficult to see the macromolecular concept.”
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Jons Jacob Berzelius introduced the term polymer into the scientific lexicon in 1833. Berzelius recognized that two compounds could have the same composition but different molecular weights, but he never worked with substances of high molecular weight. For the next century, scientists continued to identify polymers, and in 1907 Leo Baekeland introduced Bakelite, the first synthetic polymer, plastic in this case, produced on a large commercial scale.

The development of polymer theory was derailed for a time by the popularity of the association theory, which grew out of the doubts of many organic chemists of the existence of macromolecules with high molecular weights. These researchers believed that the properties of what are now recognized as polymers could be explained by their colloidal nature. The association theory built on the work of Thomas Graham and it held that a substance could exist in a colloidal state just as it could occur under different conditions as a gas, a liquid, or a solid.

But some chemists continued to note the existence of substances with high molecular weights. Scientists like Michael Polanyi began employing the new techniques of X-ray diffraction to reveal that natural textile fibers, such as silk, cotton, and wool, had high molecular weights. Then in 1920,
Hermann Staudinger, a professor at the Eidengenössiche Technische Hochschule in Zurich, theorized the existence of very long chains with molecular weights reaching hundreds of thousands. Staudinger also claimed these chains were held together by normal covalent bonds. In the ensuing years, Staudinger demonstrated that polymerization led frequently to long chains of covalently bonded monomers. Staudinger, who won the Nobel Prize in Chemistry in 1953, coined the term macromolecules to refer to this phenomenon.

In 1926 Staudinger left Zurich to take a post at the University of Freiburg in Germany. In his farewell lecture, Staudinger discussed his concept of long chain molecules. When most of the scientists present resisted his ideas, Staudinger apparently ended by echoing Martin Luther’s famous challenge to papal authorities: “Here I stand; I can do no other.”

This encounter dramatized the differences between advocates of the association theory and the polymer faction and set the stage for a symposium on the topic in Dusseldorf, Germany, in September 1926. The first speakers attempted to refute Staudinger’s macromolecular theory by referring to what they called pseudo-high molecular weight substances. Staudinger responded by basing his macromolecular concept on the high viscosity of polymer solutions. Herman Mark, an expert in X-ray crystallography, presented another view. Mark argued that there are instances when a molecule can be larger than an elementary cell. He pointed to cellulose, which seems to be composed of small units that appear as a large molecule. Mark’s presentation, based on his experience in X-ray analyses, did not prove the macromolecular theory, but it also did not disprove it.

Mark would become a leading advocate of macromolecules and, many years later, a guiding light of polymer education in the United States. But he clashed with Staudinger because Mark doubted Staudinger’s view that macromolecules were long, thin rigid rods. Instead, Mark argued that long chain molecules rotated around covalent bonds.

Although Mark and Staudinger disagreed on the nature of macromolecules, they did agree on their existence. Their work helped the polymer concept gain acceptance in the scientific community. Another important influence came from the work of Wallace Carothers, whose investigations at DuPont demonstrated that polymers consisting of hundreds of monomers could be synthesized. This work led to the introduction of the first synthetic fiber, nylon, in the 1930s.

Another scientific conference, this one in Cambridge, England, in September 1935, demonstrated how radically attitudes toward macromolecules had shifted. The high point of the meeting, sponsored by the Faraday Society, came when Carothers reported on his work on polymerization. By 1935, less that ten years after the Dusselforf conference, the debate was not over the existence of macromolecules; instead, it was over their structure and properties, in essence, the differences between Staudinger and Mark. The debate was no longer over theory, but rather details.


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This discussion of polymers has benefited from the following: Herbert Morawetz, Polymers: The Origins and Growth of a Science (New York: Dover Publications, 1985); a number of the essays in G. Allan Stahl, ed., Polymer Science Overview: A Tribute to Herman F. Mark (Washington, D.C.: American Chemical Society, 1981); Yasu Furukawa, Inventing Polymer Science: Staudinger, Carothers, and the Emergence of Marcromolecular Chemistry (Philadelphia: University of Pennsylvania Press, 1998); Peter J.T. Morris, Polymer Pioneers: A Popular History of the Science and Technology of Large Molecules, (Philadelphia: Center for the History of Chemistry, 1986).

2 Herman Mark, From Small Organic Molecules to Large: A Century of Progress, in Profiles, Pathways, and Dreams: Autobiographies of Eminent Chemists, ed. Jeffrey Seeman (Washington, D.C.: American Chemical Society, 1993), p. 41.


 

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