From "Greasy Chemistry" to "Macromolecule ... - ACS Publications

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From “Greasy Chemistry” to “Macromolecule”: Thoughts on the Historical Development of the Concept of a Macromolecule Pedro J. Bernal Department of Chemistry, Rollins College, Winter Park, FL 32789; *[email protected]

This paper is about the historical development of our understanding of the concept of a macromolecule and it argues that historical case studies can help students, particularly non-science majors, understand the nature of science. The following narrative, while not the case study itself, sketches out how the history of science might be used as a pedagogical tool.1 I begin by talking about the importance of the history of science to science education and then define and briefly illustrate the competing views about the structure of macromolecules using the example of Hevea rubber. Given that this controversy is recent, I try to give a flavor of the tone and nature of the debate as the issues were being settled by showing Hermann Staudinger in conversation with colleagues. I conclude with some thoughts on the tendency of some scientists to judge past scientific contributions without regard for historical context, an attitude that I term “presentism”. This is a story that has received a great deal of attention. Paul Flory (1), Herbert Morawetz (2), and, more recently, Y. Furukawa (3, 4) have presented the history of the development of polymer chemistry in great detail. Robert Olby (5 – 7) has written extensively and perceptively about the development of macromolecular chemistry and its connection to molecular biology. Hermann Staudinger told his version of the story in his autobiography (8). H. Zandvoort (9) has used the history of polymer chemistry as, in his words, a testing ground for the philosophy of science; and I. M. Pritykin (10) has written on the role of structural concepts in the development of the physical chemistry of polymers. In addition, a number of online educational resources are now available on this topic (11). A recent article attests to the continuing interest of this episode in the history of science (12). I have borrowed from all these sources to compose my narrative. The Importance of the History of Science to Science Education The claim that historical case studies can help students understand the nature of science is not new. James B. Conant (13, 14) championed a historical approach to the teaching of science more than fifty years ago. Students tend to think of science as a collection of facts acquired using an algorithm known as the scientific method. The history of science shows them that science, like other areas of intellectual activity, is an organized social effort to make sense of the world and that interpretation, imagination, and creativity are an integral part of the process. Observation and experience, no doubt, place boundaries on the range of valid interpretations, yet within those boundaries there is a great deal of room for creativity and imagination, particularly in the process of coming up with ideas that bring coherence to experimental observations. 870

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Studying the history of science, then, clearly illustrates the need for interpretation in science. Experiments, after all, are always interpreted in the context of prevailing assumptions and do not, in any sense, speak for themselves. The history of science is also useful for giving students a sense of place in the great enterprise that is science. It shows that scientists in the past were trying to make sense of the world with the experimental and conceptual tools at their disposal and that present scientists are, essentially, doing the same. True, our experimental tools are better and our theories may be more comprehensive, yet the nature of the engagement is the same. At times, we are tempted by the notion that our present scientific understanding is final and, in a sense, outside of history. We think that we have now arrived at the right answers and, that scientists in the past could have, in some cases, come to the same conclusions if they had carefully considered the available evidence. That is the problem that I have termed “presentism”. We do, as scientist and educators, claim that all scientific views are subject to revision although our attitude toward the past suggests that, on the whole, we don’t always hold that view. The history of science can also show how societal factors, such as personal prestige and national pride, can affect the scientific enterprise. What is the Story? A Description of the Aggregate and Chemical Theories of Colloidal Behavior Between the years 1860 and 1935, two theories of the nature of the chemical unit responsible for colloidal behavior were competing for acceptance. We can call them the chemical or molecular view, and the aggregate or colloidal hypothesis. From 1860 to about the turn of the century, the chemical view appeared to be well on its way to widespread acceptance as a result of the work of Kekulé and the development of molecular structural theory. Beginning around 1900, however, the entire chemical community, with notable exceptions, started to adopt the aggregate hypothesis to account for colloidal behavior. By 1915, the colloidal theory had prevailed. Approximately five years later, the chemical view started once again to gain prominence and, by the 1940s, it was fully established. What I want to explore, in a way that is pedagogically useful, is how and why this happened. The colloidal behavior (slow diffusion, high viscosity, and an inability to crystallize) shown by certain substances had to be explained. It was assumed that the behavior was due to the presence of large particles. What was the nature of these large particles? That was the question. The aggregate theory, based on the ideas of Thomas Graham, who coined the term colloid in 1861 (15), and the work of the botanist Nageli (16), who called them “micelles”, maintained that the particles were aggregates of smaller molecules

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held together by intermolecular forces of unknown origin. The chemical approach, on the other hand, claimed that the particles were molecules held together by normal Kekulé valences to form chains that could, in principle, aggregate, but it was argued—this is crucial—that colloidal behavior was not a property of the aggregate, rather, it was the property of the unit. This story is also about a rivalry between two research traditions. The proponents of the aggregate theory, the physicalist approach, maintained that aggregates were responsible for colloidal behavior and that the study of aggregates required the methods of physical and colloid chemistry. The proponents of the chemical view, the structuralist approach, argued that the unit responsible for colloidal behavior was the molecule and that the elucidation of colloidal properties required the deployment of the methods of classical organic chemistry such as isolation, crystallization, and analysis. Structural Theory: The Basis of the Chemical View (~1860–1900) Jacob Berzelius made the first reference to the “polymeric state” in 1832 (17). Berzelius used the word “polymer” to mean, in modern terminology, compounds with the same empirical formula, but different molecular formulas. For example, in Berzelius’s sense of the term, glucose would be considered a “polymer” of formaldehyde. CH2O formaldehyde

→

C6H12O6 glucose

This is, clearly, not what we mean by a polymer today. Berzelius made no reference to the concept of molecular structure. The development of the concept of molecular structure has its own history. For our purposes, we can somewhat arbitrarily suggest the publication in 1858 of Kekulé’s claim that carbon atoms are tetravalent (they form four bonds) as the event that begins the structuralist tradition to which the proponents of the chemical view belonged. Here is how Kekulé put it in a lecture he delivered at the University of Bonn (18): The separate atoms of a molecule are not connected all with all, or all with one, but, on the contrary, each one is connected only with one or a few neighboring atoms, just as in a chain link is connected with link.

The idea of long chains of repeating units held together by Kekulé’s valency forces, the chemical view, is a rather natural extension of Kekulé’s structural theory, which provided the impetus for the golden age of classical organic chemistry between 1860 and 1900. This was a period in which organic chemistry dominated German academic life and in which most of the participants in this story, such as Emil Fischer, Kurt Meyer, Rudolf Pummerer, Johannes Thiele, Heinrich Wieland, Richard Willstätter, and Hermann Staudinger, were trained at the Chemical Institute at the University of Munich. The Institute was founded in 1878 and was directed by Adolf von Baeyer who had been a student of Kekulé. By the turn of the century, however, organic chemists were becoming aware of the existence of a number of organic substances, such as proteins, rubber, starch, and cellulose, which exhibited colloidal properties and could not be conveniently studied using the conventional methods of organic chemistry. A sense www.JCE.DivCHED.org

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began to develop that perhaps the golden days of organic chemistry were over and that new approaches were needed. Adolf von Baeyer expressed the sentiment, “the field of organic chemistry is exhausted.... and then all that remains is the chemistry of grease [‘Schmiere’ ] ” (3a). During this period, in addition, physical and colloid chemistry started to challenge the dominance of organic chemistry. Colloid chemists were very much interested in “Schmiere”. Certain disenchantment with the ability of organic chemistry to tackle new problems had set in, but Hermann Staudinger thought that the best days of organic chemistry were ahead. His outstanding contribution was to maintain that the methods of classical organic chemistry were adequate for the elucidation of colloidal behavior. Around the turn of the century, however, when chemists tried to explain colloidal behavior the theory of choice was almost always the aggregate view. To that we now turn. The Predominance of Colloidal Theory, 1900–1920 Thomas Graham, in 1861, described materials that show slow diffusion, high viscosity, and do not crystallize as “colloidal” or “glue-like” (15). Substances that diffuse quickly and crystallize easily he termed “crystalloids”. Graham made no mention of the reasons for colloidal behavior. In 1891, the work of Alfred Werner on coordination compounds introduced the idea of two different kinds of bonding forces in chemical compounds: “primary” and “secondary” valences (19). According to Werner, atoms united by “primary valences” retained a “residual affinity” that allowed them to form aggregates. By the early 1900s, “colloid science”, the study of aggregates held together by “residual affinities”, was a fullfledged field of science regarded as the likely candidate to explain the behavior of “living matter” and of substances, such as rubber and cellulose, for which the methods of organic chemistry appeared inadequate. Wolfgang Ostwald, the man responsible for the establishment of colloid science as an independent branch of physical chemistry, put it as follows (3b): All those sticky, mucilaginous, resinous, tarry masses which refuse to crystallize and which are the abomination of the normal organic chemist; those substances which he carefully sets toward the back of his cupboard and marks “not fit for further use,” just these are the substances that are the delight of the colloid chemist.

The interest in colloid chemistry was also helped by the widely held assumption that understanding living systems would require a different conceptual framework from that of classical organic chemistry. This notion, called vitalism, was very influential, in part, because it could explain a great deal of the available empirical evidence. In 1861 Graham wrote (2a), “The colloid possesses energia. It may be looked upon as the probably primary source of the force appearing in the phenomenon of vitality.” Pflüger, a leading physiologist of his time, said almost 50 years later in 1910 (5a), “[I]n spite of the great exploits of Emil Fischer the synthesis of protein will take up another century and the synthesis of living protein is hardly likely ever to succeed.” The possibility of explaining the behavior of colloidal substances and living systems, it seems, was an important fac-

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tor in the predominance of the aggregate theory in the first twenty years of the 20th century. The disenchantment with the possibilities of organic chemistry coupled with the great hopes placed on colloid science provided the aggregate theory with a great impetus. What was at issue, however, was the nature of the unit responsible for colloidal behavior. In the 1880s, François-Marie Raoult (20, 21) and Jacobus Henricus van’t Hoff (22) had come up with a quantitative way to determine molecular weights in solution using physical methods. By 1900, a number of very large molecular weights had been reported for colloidal substances such as rubber (6500+), starch (38,000) and proteins (12,000 – 15,000), but these reports did not lead to the acceptance of the concept of large molecules (3c). Instead, between 1900 and 1926, aggregate structures were proposed for rubber by Carl Harries (23) and Rudolf Pummerer (24); for cellulose by Kurt Hess (25) and Paul Karrer (26); for starch by Max Bergmann (27), Hans Pringsheim (28), and Karrer (26); and for proteins by Bergmann (27) and Emile Abderhalden (29). Of these organic chemists influenced by the colloidalist view, Harries, Hess, Prinsheim, Bergmann, and Abderhalden were students of Emil Fischer who was highly suspicious of the existence of large molecules and contemptuous of the contribution of physical methods. (Fischer made popular the claim that substances with molecular weights higher than 5000 probably did not exist.) When told by Wilhelm Ostwald, one of the founders of classical physical chemistry, that organic chemists should thank physical chemists for developing new ways to determine molecular weights, Fischer replied (3d): “That was entirely unnecessary; I see directly the molecular weight of every substance, and do not need your methods.” Unfortunately the enmity was mutual, Wilhelm Ostwald despised organic chemistry and liked to claim that organic chemists did not consider him a chemist because he had never synthesized a compound. This methodological difference and disciplinary rivalry plays an important role in this narrative. Years later Morawetz attempted to explain why Staudinger, in Morawetz’s view, ignored evidence for the flexibility of macromolecules (30): Staudinger seems to have been haunted by the conviction that physical chemists look down on organic chemistry and that they expected, in particular, physical chemistry to provide all significant progress in the understanding of polymers. Since he believed that the study of polymers was opening a vast new area of activity for organic chemistry, he reacted to the perceived slight with passionate emotion.

An examination of the colloidal view of the structure of Hevea rubber proposed by Harries and others gives us some detail of the approach used by the aggregate theory. After some comments about the contribution of X-ray crystallography to the predominance of the aggregate theory, we turn to Staudinger’s vigorous defense of the chemical view.

and 1920 was the notion that classical organic chemistry was not going to provide the means to understand the behavior of living systems and colloidal substances. Consider for a minute that this is not in any sense part of the “evidence” for the adoption of a theory. It is, rather, an instance of the importance of background assumptions that provide the context in which theories are tested. The chemical theory was very much in line with classical organic chemistry and what was required, it was claimed, was a different theoretical framework. Colloid chemistry was expected to provide that new theoretical framework that, presumably, was going to be able to explain colloidal behavior in a way that organic chemistry could not. Related to this argument is the importance of research traditions. The aggregate theory was the result of a tradition that emphasized a physical approach to solve problems. The chemical view maintained that the traditional methods of organic chemistry were adequate for studying the nature of the unit responsible for colloidal behavior. This is, no doubt, a subtle point for students to grasp: the way that scientists define and attack problems depends, in part, on the tradition in which they have been trained. Not-so-faint echoes of the rivalry between physical and organic chemistry can still be heard in the halls of chemistry departments today. Hevea Rubber and Colloidal Behavior Rubber played a crucial role in the development of macromolecular chemistry. For that reason, it serves as a good example to illustrate the issues outlined above. As early as 1828, Faraday came up with the formula C5H8 for rubber (31). As a result of work done in the early 1900s, Carl D. Harries proposed that the rubber molecule must be composed of units of the following formula (23, 32–34):

?

At this point in the narrative we have the first opportunity to comment on an important aspect of science that has pedagogical value. The suggestion here is that an important factor in deciding which theory was adopted between 1900 872


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H C

C

C ? H2

H3C

But what is the arrangement of these isoprene units that ends up producing the observed colloidal behavior of rubber? In 1899, Johannes Thiele (35) had suggested that in compounds that contain carbon–carbon double bonds, “the strength of the affinity is not fully used and on each atom there is still an affinity residue or a partial valence” (2b). Harries believed that the units were cyclic dimers held together by “residual affinities”.

H3C

H3C ..... .....

Pedagogical Comments I: On Theory Adoption

H2 C

C HC

C H2 C H2

.....

C CH H2 ..... C C H2 CH3

C HC

C H2 C H2

C CH .... H2 ..... C C H2 CH3

Harries’ interpretation of the structure was challenged by Samuel S. Pickels (36), who argued that if interactions between the double bonds of small dimers were responsible

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for aggregation, then breaking the double bonds through bromination will lead to the disappearance of the colloidal behavior. The colloidal behavior persisted after the reaction. Based on his results, Pickels argued that the formation of rubber from isoprene was a “purely chemical process” and proposed the following formula:

H3C C .........H2C

CH3

CH H2C

C HC

“[A]ccording to the most recent investigations the principle of the long chain formula must be abandoned” (1a). Four years later, in 1926, E. Ott, a renowned X-ray crystallographer said, “the hypothesis of an extremely high degree of polymerization in the chemistry of rubber cannot be maintained” (38). At this point, the idea of long chain molecules could not be reconciled with the results of X-ray crystallography and solubility studies. Vitalism and the end-group problem also appeared to be on the side of the aggregate theory. Pedagogical Comments II: On Experimentation

CH2

......

The units responsible for colloidal behavior, Pickels maintained, were large rings of eight monomers. Like Harries, he was proposing a ring structure, although with a different ring size. But why rings? The answer is that unless a ring is formed, there was no way to account for the way in which the growth of the chain stops. The problem can be called the “end-group” problem. The ring structure had the virtue of solving it. The Role of X-Ray Crystallography in the Debate X-ray crystallography played a decisive role in the birth of macromolecular chemistry. At the time, the assumption among crystallographers was that the atoms or molecules that form a crystal could not be larger than the unit cell. The unit cell, to put it simply, is the smallest repeating unit that forms the crystal. The empirical evidence showed that for crystalline rubber and cellulose the unit cell was similar in size to those of simple substances. Therefore, it was argued, high molecular compounds (defined operationally as compounds that could not be vaporized) could not contain large molecules. Where did the assumption that the molecule could not be larger than the unit cell came from? That is a complex story well told by H. Morawetz (2) and R. Olby (5). For our purposes, let us point out that X-ray crystallography had developed mostly through the study of crystal lattices in which the unit cell is always composed of several atoms. Here is how Sir William Bragg, who received a Nobel prize for his work in X-ray crystallography, put it in 1922 (37): X-ray analysis shows that the unit cell nearly always contains the substance of more than one molecule; generally of two, three, or four. The crystal unit must contain the substance of an integral number of molecules; this is a simple consequence of the fact that the atoms of the different elements are present in the same proportion in both liquid and solid.

Given that experimental X-ray diffraction studies had shown that crystalline rubber and cellulose contained small unit cell volumes; it was natural to assume that the molecules were also small. Aggregates of these small molecules were responsible for colloidal behavior, the aggregate theory maintained. At the time it was also widely assumed that the high solubility of substances such as polystyrene was incompatible with a long chain structure. “Everyone knew” that solubility decreased with molecular size. In 1922, E. Heuser, summarizing years of research on cellulose and starch, stated www.JCE.DivCHED.org

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Notice that the assumption that the molecule cannot be larger than the unit cell is not the result of an experiment rather, it is part of the context in which experiments are interpreted. These contextual assumptions cannot be tested by a single experiment. They arise from the fact that making them renders experiments intelligible. An understanding of the nature of the materials to which X-ray methods had been applied during its development made it an entirely reasonable assumption. The question arises, then, if these statements cannot be tested, how do we ever find out that they are wrong? The answer is that, over time, experimental evidence accumulates that becomes hard to interpret if one continues to hold that the initial assumption is correct. Scientists start to notice that giving up an assumption begins to make better sense of the entire picture. As a result, assumptions are given up and new ones are adopted, although not because a single experiment shows that they are wrong. That is the notion that is captured by the famous phrase of Pierre Duhem, that “hypotheses are tested in bundles”. Notice that the empirical evidence is always interpreted in the light of some assumptions. That is why some historical accounts of scientific processes tend to suggest that empirical evidence available at the time was ignored. The way a piece of experimental evidence is interpreted changes with time. Here, for example, is a statement that illustrates the point (1b): The fallacy in the assumption that the molecule could not be larger than the unit cell had been pointed out earlier, but it remained for Sponsler and Dore to show in 1926 that the results of X-ray diffraction by cellulose fibers are consistent with a chain formula composed of an indefinitely large number of units.

This is 1926, the year in which E. Ott (38) was saying, as noted above, that the hypothesis of a high degree of polymerization had to be given up. Fallacy is a strong word to describe an assumption that was almost universally held. That bit about “bundles” is extremely important. Staudinger Argues the Chemical View Hermann Staudinger (1881–1965) almost single-handedly changed scientific opinion in favor of the existence of macromolecules. Around 1920, at the age of thirty-nine and widely regarded as one of the leading organic chemists in the world, Staudinger decided to turn his attention to “higher molecular compounds”. Earlier in his career, around 1911, Staudinger studied the structure of rubber and in a publication on isoprene made reference to Harries’s interpretation of colloidal behavior using the aggregate theory (39). Six years

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later, in a lecture in October of 1917, he explicitly rejected the aggregate theory and endorsed the long-chain structure proposed by Pickels, which asserted that polymerization was strictly a chemical process. Pickels had proposed a ring structure because of the “end-group” problem. In this same lecture, Staudinger introduced a long-chain formula for rubber (40). In 1920, he stated his doctrine that high molecular compounds are covalently bonded long chain molecules (41). He claimed, that is, that polymerization could be explained using normal Kekulé valences and that it was a chemical reaction that leads to products of the same composition but higher molecular weight. In addition, he provided long-chain formulas for polyoxymethylene (repeat unit CH2O) and polystyrene (–CH–(C6H5)–CH2–) and attributed the colloidal properties to the size of the molecules, which he speculated was about 100 units. He proposed unsaturated free atoms at the end of the chain because, he argued, in large molecules the reactivity of the end groups is radically diminished. Staudinger’s 1920 publication did not include any experimental support for his claims. Harries had a few years earlier suggested that “it would be important to reduce rubber since the product could probably be distilled in a high vacuum without decomposition allowing an unambiguous determination of its constitution” (2c). In 1922, Staudinger and his student Jakob Fritschi performed the experiment and obtained what he called “the first evidence for the existence of macromolecules” (3e). The aggregate theory predicted that hydrogenation of rubber would lead to a normal molecular weight substance and therefore to the disappearance of colloidal behavior. Saturation of the double bonds would force the cancellation of the “residual affinities” forming the aggregate. Staudinger and Fritschi hydrogenated rubber at high temperature and pressure and obtained a product with properties that were very similar to natural rubber, which is to say that the colloidal behavior persisted. They concluded, therefore, that colloidal particles of rubber were not aggregates, they were long-chain molecules. In 1922, Staudinger introduced for the first time the word “macromolecule” (42) to designate these large molecules and defined it two years later as follows (43), For those colloidal particles in which the molecule is identical to the primary particle, where the individual atoms of the colloidal molecule are those bound by the normal valency activity, we suggest the term Macromolecule.

His colleagues did not like his decision to work with “high molecular compounds”. H. Wieland, whom he later succeeded in Freiburg, wrote to him at the end of the 1920s (8a): Dear colleague, let me advise you to dismiss the idea of large molecules, there are no organic molecules with a molecular mass over 5000. Purify your products, like for instance rubber, and they will crystallize and reveal themselves as low-molecular weight substances.

Staudinger recalled in his memoirs (8b), My colleagues were very skeptical about this change, and those that knew my publications in the field of lowmolecular chemistry asked me why I was neglecting this interesting field and instead was working on very unpleasant and poorly defined compounds, like rubber

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and synthetic polymers. At that time the chemistry of these compounds often was designated, in view of their properties, Schmierenchemie [greasy chemistry].

His 1925 farewell lecture to the Zürich Chemical Society has been described as follows by Frey-Wyssling (5b): I remember Staudinger’s lecture to the Zürich Chemical Society in 1925 on his high polymer thread molecules with a long series of Kekulé valency bonds. It was impossible to accommodate his view in the unit cell as established by X-ray analysis. All the great men present: the organic chemist, Karrer, the mineralogist, Niggli, the colloidal chemist, Wiegner, the physicist, Scherrer, and the X-ray crystallographer (subsequently cellulose chemist), Ott, tried in vain to convince Staudinger of the impossibility of his idea because it conflicted with exact scientific data. The stormy meeting ended with Staudinger shouting ‘Here I stand, I can do no other’ [Hier stehe ich, ich kann nicht anders] in defiance of his critics.

Rudolf Signer, a graduate student of Staudinger’s at the time and later professor of organic chemistry at Bern, recalled his graduate student years this way (5c): [I] … was very impressed by Staudinger. He was completely sure that his idea of the existence of macromolecules was right and he had practically all his colleagues against him and his opinions. And so it was a very interesting situation to see this man already having a great experience in this field, having a special conviction, and having against him all his colleagues. The crystallographer Niggli said that each substance in the pure state should crystallize and if these materials of polystyrenes and other polymers which Staudinger had synthesized were pure they should form crystals. After the meeting Wieland told Staudinger that in his opinion molecules with more than forty carbon atoms should not exist.

According to Staudinger’s own recollection, the crystallographer Niggli simply said with respect to the macromolecule, “such a thing does not exist” (8c). In the same year, 1926, at a meeting of the Society of German Naturalists and Physicians in Düsseldorf, a discussion of the nature of colloids was held. Max Bergmann, from the Kaiser Wilhelm Institute for fiber chemistry, claimed that classical structural theory, based on the ideas of Kekulé and the molecular concept of Avogadro, could not account for colloidal behavior. Here is how Bergmann’s presentation ended (44): [W]hat is at the present time especially necessary for the chemistry of pseudo-high molecular substances is the development of a structural and spatial chemistry the object of which lies outside the molecule, outside the individual group—a structural chemistry, a spatial chemistry of aggregating forces and of aggregates.

Herman Mark, a pioneer in the application of X-ray techniques to the study of macromolecules (who later, in 1942, founded the Polymer Research Institute at the Polytechnic Institute of New York), called for further X-ray studies on good crystals. Hermann Staudinger, to conclude the sym-

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posium, presented an impressive amount of experimental evidence in support of the chemical hypothesis. He presented: (a) His experiments on the bromination and hydrogenation of rubber (b) The results on the viscosity, melting points, and solubility of fractionated polymers obtained by his students in the years 1923–1926 (c) The demonstration that the hydrogenation of polystyrene was a normal chemical reaction that obeyed the laws of stoichiometry

It was clear, he said, based on the evidence, that: “…the monomers are united by main valences” (3f ). In Düsseldorf, Pringsheim and Bergmann also presented experimental arguments on the structure of proteins, inulin, and other complex carbohydrates that, according to their interpretation, supported the aggregate theory. In addition, at this conference Staudinger noted, for the first time, that the molecular weight of polymers could only be average values and separated polymers into what he called “hemicolloids” (molecular weights up to 10,000) and “eucolloids” (molecular weights up to 100,000). In spite of all the experimental evidence he presented, only Richard Willstätter, who chaired the meeting, sided with Staudinger. The supporters of the aggregate theory claimed that no convincing argument against the problem posed by X-ray crystallography had been offered. Here is how the X-ray crystallographer Rudolph Katz put it (3g): “Staudinger’s conception did not seem to many of us really convincing, nor was the decisive value which X-ray spectrography could have for the subject yet understood at this meeting”. Most of the audience seems to have shared the attitude expressed by one of the participants (3h), “We were shocked like a zoologist would be if told that somewhere in Africa an elephant was found who was 1500 feet long and 300 feet high”. Pedagogical Comments III: On Background Assumptions We can begin with Staudinger’s reference to the fact that the chemistry of high molecular compounds was designated as “greasy chemistry”. This, of course, is a reference to their properties, yet it is also a derogatory term that carries the implication that distinguished chemists, like Staudinger, should not waste their time working on these compounds. The question is why? Why was it that these compounds were not deemed worthy of study? One of the assumptions of classical organic chemistry was that meaningful work could only be done in pure substances in which, by definition, all the molecules are identical. High molecular compounds, everyone agreed, are mixtures of molecules of different sizes and therefore not suitable to do meaningful work. Notice that here which theory, the chemical or the aggregate, turns out to be right makes an enormous difference. If high-molecular compounds are aggregates, they can be separated by physical methods into identical molecules and one can proceed. If the chemical theory is right, high-molecular compounds are inevitably mixtures and studying them does not hold that much promise. This was one of the factors that favored the aggregate theory. In arguing the chemical view, as Zandvoort (9)

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points out, Staudinger had to redefine the notion of a pure substance. He argued that mixtures of molecules of different molecular weights could be studied profitably and for that reason, they were, in a sense, “pure” even though all the molecules were not identical. That is why Staudinger’s admission that the molecular weight of polymers could only be average molecular weights was so important. He was in effect saying, yes they are mixtures, so what? Notice, again, that all these important factors have nothing to do with the scientific evidence. They are background notions that provide the context in which scientific work is done. Another issue that comes to mind as one reads the last section is the role of “conviction”. Is conviction a good thing in science? Staudinger’s use of the words of Martin Luther (“Hier stehe ich, ich kann nicht anders”) brings this to mind. As he made the case for macromolecules, we see him arguing brilliantly for a view that is not widely shared. There is a caricature of science that depicts scientists as disinterested observers of the scientific evidence. Clearly that is not what is going on here. We now know that on the issue of the structure of macromolecules Staudinger was right and for that we recognize him as the great chemist that he was. But what if he had turned out to be wrong as he did on the issue of the flexibility of polymers? Would we say that he was just a bad scientist? Notice, incidentally, that Staudinger’s claim about the “endgroup” problem and about polymers always having the same chemical composition as the monomers turned out to be mistaken. Staudinger’s attitude gives us a chance to examine the neglected issue of the role of conviction in science. The Beginning of the End of the Aggregate Theory The aggregate theory prevailed at Düsseldorf, but the 1926 symposium marks the beginning of the end of the colloidal theory. In 1927, one of Staudinger’s students, Rudolph Singer, was able to split a long strand of polyoxymethylene (repeating unit CH2O) into groups with different chain lengths (oligomers). Gustav Mie and J. H. Hengstenberg studied the oligomers using X-rays and showed that the interference pattern varied with the number of repeating units in the chain (45). The unit cell contained only four repeating units. They concluded, “the molecular size of high molecular compounds cannot be determined by X-ray measurements” (3i). Katz, who had not been impressed a year earlier in Düsseldorf, was now convinced. These results, he said, “provide the first direct experimental evidence of Staudinger’s hypothesis and a decisive one” (46). After 1927, the size of the unit cell was no longer an issue. In 1928, Kurt Meyer and Herman Mark proposed what Staudinger called the “new micelle theory”, which claimed that the units responsible for colloidal behavior were “micelles” formed by long-chain molecules held together by “special micellar forces”. Note that the “new micelle theory” combines both the chemical and the aggregate view. For example, Mark and Meyer proposed that a cellulose micelle was formed by 40–60 long-chains each composed of 30–50 glucose molecules (47). It was in this environment that a meeting of the “Kolloid Gesellschaft” was held in Frankfurt in 1930. At this meeting of colloid chemists, Staudinger introduced his viscosity law, which claimed that there was a relationship between viscos-

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ity and molecular weight. He presented experimental data on polystyrene, polyoxymethylene, cellulose, and rubber and showed that he could calculate the molecular weights using viscometry. Staudinger concluded his presentation by contrasting his results with those predicted by the aggregate theory and the “new micelle theory”. For rubber he proposed an average chain length of 1000 monomers compared to 100 for Mark and 8 for the aggregate theory championed by Pummerer (48). After the presentation, Herzog, Hess, and Pummerer defended the aggregate theory of cellulose and rubber, but they could sense that the climate of opinion was changing rapidly. Even Wolfgang Ostwald, the “high priest of the colloid concept” started to change his mind. Staudinger’s wife wrote to V. E. Yarsley (49), saying that: My husband encountered opposition in all his lectures. Only in the autumn of 1929 when in a lecture to the South West German chemical dozenten in Frankfurt he put forward his viscosity formula, for the first time there was no opposition. This both astonished and pleased us.

E. Ott, the X-ray physicist, later said, “Staudinger succeeded where others failed because he knew and believed in organic chemistry” (3j). The story, at this point, was essentially over. A word should be said about the role of Wallace Carothers in the controversy about the nature of macromolecules. The work of Carothers clearly demonstrated the macromolecular nature of polymers, although he was not an active participant in the controversy. He always assumed that the chemical hypothesis was the correct one. His contribution was to take a synthetic approach that differed from the predominantly analytical one used by Staudinger. Here is how Carothers put it in a letter in 1927 (3k), For some time I have been hoping that it might be possible to tackle this problem from the synthetic side. The idea would be to build up some very large molecules by simple and definite reactions in such a way that there could be no doubt as to their structure.

When he came to work at Dupont in 1928 Carothers tackled the problem with extraordinary success. The foundations of polymer chemistry as we know it were essentially set by the publication of Carothers’s paper “Polymerization” published in Chemical Reviews in 1931 (50). Carothers defined polymers, in what is surely one of the classical definitions in the history of science, as substances whose “structure may be represented as –R–R–R– where “–R–” are bivalent radicals which, in general, are not capable of independent existence” (51). One Final Argument Recall the difference between hemicolloids and eucolloids. By this time, the consensus was that “hemicolloids” (molecular weights < 10,000) were long chain compounds held together by covalent bonds, but what about “eucolloids” (molecular weights > 10,000)? Staudinger always maintained that colloidal properties were the result of the structure of the long-chain molecule. Herman Mark and Kurt Meyer, however, claimed that the X-ray diffraction evidence was contrary 876


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to such an extrapolation and that the colloidal properties depended on the micellar structure of the colloidal particles. They argued that eucolloids were hemicolloids aligned into a crystalloid pattern by intermolecular forces. Here is how Meyer and Mark put it in 1929 (8d): We do not agree with Staudinger’s assumptions on the structure of particles in solution which are responsible for the osmotic pressure. While Staudinger assumes that these particles are identical in every case with the long-chain molecule (that is, with the primary valence chain, ‘eucolloids’), we are of the opinion that these particles consist of groups or bundles of such chains. …[O]smotic pressure measurements do not necessarily determine the primary valence chain, but a more or less complicated mixture of aggregated chains. With this assumption one of Staudinger’s main proofs on the structure of his synthetic high molecular compounds becomes questionable; the molecular weights he determines on polymer products may be several times too high.

According to Meyer and Mark, the aggregates persisted in very dilute polymer solutions, an assertion that Staudinger vigorously denied. Staudinger maintained that this “new micelle theory” was a return to the physicalist approach that he loathed. Mark and Meyer, as late as 1940, wrote in the first volume of Hochpolymere Chemie (2d): [A] separation into independently moving molecules takes place only at extreme dilution such as cannot be obtained either in solutions used in practice nor in those employed for scientific study.

The controversy about the existence of macromolecular aggregates in dilute solutions continued for a number of years and eventually ended as a result of the accumulation of experimental evidence. Later results showed that aggregation does happen but is not as common as assumed by Meyer and Mark. The controversy, however, created a great deal of animosity between Meyer and Staudinger. When the second volume of Hochpolymere Chemie appeared (Mark was no longer listed as co-author), Staudinger had a sheet glued to the cover of the volume in the Freiburg University library which included the following passages (2e): This book is not a scientific work but propaganda.… Meyer takes essentially the results of the Freiburg laboratory without citing them. This scientific expropriation is disguised to the uninitiated by the distortion and the arrogant criticism of the Freiburg researchers. …[A]ccording to Meyer, only two of the thirteen recent publications worth a mention are German. This could be taken by the outside world as an indication of the decline of German science.

Meyer, for his part, would tell colleagues that what Staudinger was doing was “not science at all” (3l). This was in wartime and by then both Meyer and Mark, who were of Jewish descent, had been forced to leave Germany. This is not to suggest that Staudinger was supportive of the Nazi regime. Staudinger, in fact, had been a pacifist during the First World War and was constantly harassed by the Nazi regime. His funds were cut, his publications blocked and his

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students could not find academic positions. Between 1928 and 1944, all but one of his students went to work in industry and that may have been what allowed him to retain his position during wartime. The industrialists recognized the importance of Staudinger’s work to the war effort. His research facilities were almost completely destroyed by allied bombing in November of 1944 but he was able to resume work after the war and his interest turned more and more to the biological aspects of macromolecules. Staudinger, while not sympathetic to National Socialism, however, wanted German science and technology to be self-sufficient and recognized as the best in the world. After the war, he wrote to Otto Hahn, the president of the Max Planck society, “ I cannot calmly see the results of my lifework move to the United States and other places” (3m). In 1951, at the age of seventy and after twenty-five years in the Freiburg faculty, Staudinger retired but continued to work until 1956 as the director of the State Research Institute of Macromolecular Chemistry. In 1953, at the age of seventy-two, he was awarded the Nobel Prize in Chemistry for his “discoveries in the field of macromolecular chemistry”. Staudinger had actually been the founder of macromolecular chemistry and was fortunate enough to live until he was officially recognized as such. He died in 1965, at the age of eighty-four. Pedagogical Comments IV: Social Influences The debate over the nature of macromolecules serves as a good example to illustrate the way in which personal, professional, and national conflicts can affect the conduct of science. In the macromolecular debate, there is not much evidence that ideology distorted the content of the debate. There was never an “Aryan Polymer Science” the way there was an “Aryan physics” or a “Russian genetics” under Lysenko. What we have, rather, is an enhancement of professional and personal hostility as a result of political allegiance and the influence of events on a scientific story deeply connected to industrial developments during wartime. The Problem of Presentism for Science and Education In this section I want to say a few words about the tendency of some scientists to write about the history of science without regard for historical context. I present some examples from two eminent polymer scientists that were only a few years removed from the events they were writing about. In his excellent book on the history of macromolecular chemistry, The Origins and Growth of a Science (2), Herbert Morawetz refers to the first-ever use of the word “polymeric”, by Jacob Berzelius in a 1833 article, as follows: “it is ironic that the first reference to the polymeric state should contain two obvious errors” (2f ). The two obvious errors alluded to are the use of the formula CH2 for ethylene (C2H4) and the use of “oil of wine” rather than butene as an example of an “ethylene polymer”. These “errors” are only obvious if one thinks of Berzelius’s understanding as being similar to ours. To regard them as obvious errors on the part of Berzelius (1779–1848) who remarked “the devil may write chemical textbooks because every few years the whole thing changes” (52) underestimates the difficulties of the conceptual environment in which www.JCE.DivCHED.org

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Berzelius was working. Morawetz’s book (2), in addition, commonly uses terms such as “surprising”, “truly astonishing”, and “incredible” to refer to the interpretation of experimental results made by scientist in the first half of the 20th century. Paul J. Flory (1910–1985), Nobel Prize recipient for his contributions to macromolecular chemistry, published his classic book Principles of Polymer Chemistry in 1953 (1). The book begins with a historical introduction including the following statements about the view that polymers are long chains of covalently bonded repeating units (1c, 1d, 1e): This elementary concept did not gain widespread acceptance before 1930, and vestiges of contrary views remained for more than a decade thereafter. It may appear incredible that significant investigations from this obvious point of view were not undertaking in so manifestly important a field before about 1930, and that noteworthy advances have occurred principally since 1940. The reasons for the delay in the evolution of a rational approach to the study of high polymers would be difficult to explain adequately in a few words. In 1929, the work of Carothers disspelled the attitude of mysticism then prevailing in the field.

It has been shown that rather than being irrational, the approach to the study of polymers up to about 1940 was an argument among the best scientists in the world over a period of more than sixty years. How difficult the interpretation of the experimental evidence actually was has just been described. It is helpful to remember that Flory was writing less than twenty years removed from what he called an “attitude of mysticism” that kept scientists from arriving at an “obvious point of view” and prevented the use of a “rational” approach. Unfortunately as I look at the sources that I’m working with for this article, the pattern keeps coming up. I. M. Pritykin criticizes Staudinger because it turned out that his ideas about the flexibility of macromolecules were not quite right. According to Pritykin, Staudinger “had at his disposal direct experimental evidence of the flexibility of the macromolecules” (10). Morawetz claims that Meyer, Mark, and Kuhn provided “incontrovertible evidence for the flexibility of chain molecules, but Staudinger and his students chose to disregard them” (30). Here one sees evidence being interpreted retrospectively. No wonder John Turner has claimed: “the relationship of the working scientist to the history of science is that, as far as he or she is concerned, there isn’t one” (7a). Rather than a dismissive attitude toward the (recent?) past, what is needed is a description of the conceptual difficulties and the nature of the presuppositions that scientists take for granted as they work on questions that are not yet fully resolved. Final Thoughts on the Pedagogical Importance of the History of Science At the beginning of this paper I did not attempt to justify the claim that a case study approach is a good way to teach science. The following questions are of great relevance to science educators: Can case studies communicate the con-

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tent and practice of science as effectively as a traditional approach with little historical context? Are case studies important enough to sacrifice content? To what extent should they be relied on in science courses for majors? My view is that the practice of science is communicated better by a historical approach, although the proper coverage of content using a case study method requires a lot more time. For that reason, for majors a historical approach should be integrated into the fabric of the course to give the students a sense of the practice of science, but I do not advocate that a case study method be used as a substitute for a standard majors course because, given the available time, it would require the sacrifice of too much content. I think majors would greatly benefit from studying the history of science, but the course need not be one of their major courses. A case study approach is ideally suited for non-majors if one thinks that understanding the nature of science is as important as knowing some science. The advantage of a case study approach is that, in order to follow the details of the case, concepts can be introduced on a “need to know” basis; in my view, which case study is used is not particularly important. I support the use of historical case study courses to satisfy the science requirement for non-majors provided that enough content is introduced to make the scientific details understandable. Conclusions Here are some of the points I have tried to convey and a few of the lessons of the case study approach that have pedagogical importance. 1. Historical case studies show scientists trying to do the best they can with the conceptual and experimental tools at their disposal. Nothing else, after all, is possible. 2. The conceptual context in which experiments are interpreted is affected by cultural and historical factors. Put differently, empirical evidence is always interpreted in the context of traditions of inquiry that result from a complicated interaction between theory, experimental results, and broader conceptions of the nature of things. Think, for instance, about the notion of “vitalism.” Where did that notion come from? How did it relate to the experimental data? Clearly “vitalism” was able to explain a great deal of the experimental data, but it was more than that. It was, and is, a metaphysical notion that transcends the experimental data. It may be consistent with it, but it is not being tested by it. It is, rather, an overarching view within which experiments are interpreted. 3. Science is logical but it is not logic. Scientific controversies are not settled by formal arguments; rather, they are decided by the accumulation over time of experimental data that renders particular interpretations difficult to support. This is why crucial experiments are hard to find. Scientific controversies, such as the one described here, cannot often be settled by a single experiment.

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4. National pride and personal and professional rivalries, can, and sometimes do, affect the conduct of scientists. Science is a social process.

Let me conclude by saying that the controversies in the history and philosophy of science over the last forty years are not particularly helpful to the student trying to understand the nature of science (53, 54). They are simply too abstract, too far removed from the actual practice of science. What students need is a description of (a) the interaction between theory and experiments embedded within a broader historical narrative illustrated by examples, and (b) the process by which scientists go about proposing and testing hypotheses in particular historical situations. A recent paper in this Journal (55) considered the question of what we should teach non-major students. While this paper does not directly address the kinds of questions proposed in that contribution, I submit that historical case studies provide a convenient approach to the changes that non-major science courses need to become better general education courses. Note 1. Admittedly the narrative presented here does not contain enough experimental detail to be used in the classroom as a case study without using additional sources. In this particular topic there are excellent resources available for educators interested in this kind of approach. References 2, 3, and 11 certainly contain the additional information necessary.

Acknowledgments I would like to acknowledge Fabio Zuluaga of the Chemistry Department of the Universidad del Valle in Cali, Colombia, and Jennifer McCluan, a Rollins College graduate, for their help at different stages in the preparation of this manuscript. Literature Cited 1. Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953; (a) p 11; (b) p 22; (c) p 3; (d) p 4; (e) p 23. 2. Morawetz, H. Polymers: The Origins and Growth of a Science; John Wiley and Sons, Inc.: New York, 1985; (a) p 51; (b) p 34; (c) p 91; (d) p 97; (e) p 97; (f ) p 5. 3. Furukawa, Y. Inventing Polymer Science: Staudinger, Carothers, and the Emergence of Macromolecular Science; University of Pennsylvania Press: Philadelphia, PA, 1988; (a) p 17; (b) p 20; (c) listed on p 20; (d) p 21; (e) p 65; (f ) p 10; (g) p 74; (h) p 74; (i) p 75; (j) p 67; (k) p 125; (l) p 82; (m) p 203. 4. Furukawa, Y. Polymer Science: From Organic Chemistry to an Interdisciplinary Science. In Chemical Sciences in the 20th Century: Bridging Boundaries, Reinhardt, Carsten, Ed.; Wiley: New York, 2001; p 228. 5. Olby, R. The Path to the Double Helix; University of Washington Press: Seattle, WA, 1974; (a) p 5; (b) p 7; (c) p 8; (d) p 23. 6. Olby, R. J. Chem. Educ. 1970, 47, 168; (a) p 89. 7. Olby, R. C., Canor, G. N., Cristie, J. R. R., Hodge, M. J. S., Eds. Companion to the History of Modern Science; Routledge: New York, 1990.

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Chemistry for Everyone 8. Staudinger, H. From Organic Chemistry to Macromolecules; Wiley: New York, 1970; (a) p 79; (b) p 77; (c) p 85; (d) p 89. 9. Zandvoort, H. Stud. Hist. Phil. Sci. 1988, 19, 489. 10. Pritykin, L. M. ISIS 1981, 72, 446. 11. Chemical Heritage Foundation Classroom Resources. http:// www.chemheritage.org/classroom/class.html (accessed Apr 2006). 12. Mülhaupt, R. Angew. Chem. Int. Ed. 2004, 43, 1054. 13. Conant, J. B. Harvard Case Histories in Experimental Science: Harvard University Press: Cambridge, MA, 1957. 14. Conant, J. B. Understanding Science; Yale University Press: New Haven, CT, 1947. 15. Graham, T. Phil. Trans. Roy. Soc. 1861, 51, 183. 16. Katz, J. R. In Chemie der Zellulose, Hess, K., Ed.; Akad. Verlag: Dresden, Germany, 1928; p 605. 17. Berzelius, J. Jahresber. Fortsch. Phys. Chem. 1833, 12, 63. 18. Kekulé, A. Nature 1878, 18, 210. 19. Werner, A. Vierteljahrsschrift der Züricher Naturforscher Gesellschaft 1891, 322, 261. 20. Raoult, F. M. Compt. Rend. 1882, 95, 1030. 21. Raoult, F. M. Compt. Rend. 1887, 104, 1430. 22. van’t Hoff, J. H. Phil. Mag. Ser. 5, 1888, 26, 81. 23. Harries, C. Ber. 1905, 38, 1195. 24. Pummerer, R.; Burkard, P. A. Ber. 1922, 56, 3458. 25. Hess, K. Liebigs Ann. Chem. 1924, 435, 1. 26. Karrer, P. Helv. Chim. Acta 1920, 3, 620. 27. Bergmann, M. Ber. 1926, 59, 2973. 28. Pringsheim, H. Ber. 1926, 59, 3008. 29. Abderhalden, E. Naturwiss. 1924, 12, 716. 30. Morawetz, H. Angew. Chem. Int. Ed. Eng. 1987, 26, 93. 31. Faraday, M. Quart. J. Sci. Arts, Roy. Inst. Great Britain 1836, 21, 1819. 32. Harries, C. Ber. 1902, 35, 3256. 33. Harries, C. Ber. 1903, 36, 1937.

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