Polymers, A Brief History
Herman Francis Mark in 1986 displaying Hermann Staudinger’s rigid-rod model of macromolecules, CHF Collections. Photograph by Jim Bohning.
The advances made in synthetic-rubber creation and production during World War II were coincident with advances made in the larger sphere of polymer science, of which rubber is one part. A polymer is a large molecule composed of many smaller units. The American Chemistry Council encourages students to think of a polymer as a chain that is made up of many links and that can be rigid or pliable. The earliest polymer science involved modifications of naturally occurring polymers. Vulcanization of rubber, invented by Charles Goodyear in the nineteenth century, is the best example of this type of modification. Natural rubber was an amazing resource but could prove excessively tacky or brittle in cold winters. Through the addition of sulfur the natural rubber-polymer chains form crosslinked bridges. In the early twentieth century, Leo Baekeland created the polymer Bakelite, a completely synthetic polymer with hard properties mimicking the naturally occurring resources of shellac and ivory. Other synthetic polymer advances included neoprene, the first synthetic rubber, in 1930 and nylon, the famed polymer fiber invented in 1935.
Although rubber and other polymers had been of interest to chemists since the nineteenth century, by the early twentieth century there was still only vague understanding of what polymers were. The large molecules, like natural rubber were often described as colloids—simple associations of larger molecules. Herman Mark discusses the controversy that was the subject of a 1926 symposium in Dusseldorf:
Then I prepared myself for the lecture by reading a little more about Staudinger's articles and by reading a little more about the articles of the opposition. The situation was appalling. There were three types of opposing views. One was a kind of a subjective view of bad feeling. The classical organic chemists like [Heinrich] Wieland or Willstätter had worked all their lives with molecules having molecular weights between three hundred and five hundred; and they had much experience with these little things. They just couldn't swallow it that somebody should come and say, "I'm working with molecules which have a molecular weights of five hundred thousand." So that was a more or less personal view. The next group were the colloid chemists, Freundlich, Herzog, and others who said, "Well, the phenomena are there; there's no question about that; high viscosities and gel formation. But we feel that they can be explained on the basis of known phenomena in colloid science and one doesn't need the extravagant hypothesis of a molecule with a molecular weight of five hundred thousand." In other words, they were nearer to reality. They didn't say, “We don't like it”; they said, “We don't need it.” There was a third group, the crystallographers. Their argument was this: it has been established, mainly in our laboratory by Dr. Rudolf Brill, Dr. [Johann R.] Katz and myself, that the elementary cells of rubber, cellulose, and silk are small; so small that only a molecule of a molecular weight of about five hundred could be accommodated inside it. Then this group said that crystallography shows that the molecules can't be larger than the elementary cell: therefore, the molecules must be small. Thus it can't be. The first was, “We don't like it”; the second was “It's not necessary”; and the third was “It's impossible. ” (13)
In 1929 Wallace H. Carothers, of DuPont, in the Journal of the American Chemical Society, attempted to define the term polymer in a modern sense based on new chemical understanding developed in his laboratory.
The overall situation surrounding polymers was difficult, with chemists from different disciplines attacking Staudinger’s view. Mark “had worked with both classes of these materials” (13). He explained:
My essential contribution was that the presently existing experimental evidence in the x-ray field cannot prove that macromolecules exist, but it also cannot prove that they cannot exist. It was a kind of a soft position, but it was true and it took away the edge of the third group which said, “It can't be,” by saying, “It can be.” (14)
While Mark remained agnostic on the topic of Staudinger’s macromolecules, by 1928, when he was working for I.G. Farben, he had become a believer in polymers after seeing his own x-ray crystallography work on the structure of cellulose.
Through the work of Staudinger and Mark an understanding of polymers, or macromolecules, as Staudinger called them, developed in which the smaller molecules were joined by strong covalent bonds into the longer polymer chains. Staudinger was awarded the 1953 Nobel Prize in Chemistry, and Mark helped found the Polymer Research Institute at Brooklyn Polytechnic.
Hear Albert Clifford: Our program had been directed primarily and was quite similar to that which Dr. Marvel mentioned in his interview, namely to see what practical materials might be obtained in large commercial quantities at a reasonable cost. We had already ascertained that butadiene was suitable for this purpose, and we were in quest of other materials that could be used as co-monomers with it. As this work went on, it became more and more apparent that we would probably have to settle for either styrene or acrylonitrile as the quick way of getting into synthetic rubber production. We developed pilot plant facilities in the mid 1930s, and larger scale equipment became available in the late 1930s, largely through the efforts of the organic section, who worked out the types of vessels that would be necessary for polymerization under pressure at temperatures of the order of 40 to 60° Centigrade or higher. The kind of enamel for the lining of these vessels and the kind of stirrers to be employed in them and so on were important. So, that by about the late 1930s, we had graduated, Herb [interviewer], from vessels of one-liter size, which itself was quite an accomplishment from the glass tube or bottle, to a five-gallon reactor, and then fifteen-gallon reactors, fifty-gallon reactors, and finally to one hundred-gallon reactors. As I remember, by about 1938, we had five hundred-gallon reactors and were in the process of engineering in the Goodyear pilot plant, twelve hundred-gallon experimental reactors for producing synthetic rubber legacies. (5-6)