Contribution to the Historical Development of Macromolecular Chemistry – Exemplified on Cellulose

CELLULOSE CHEMISTRY AND TECHNOLOGY CONTRIBUTION TO THE HISTORICAL DEVELOPMENT OF MACROMOLECULAR CHEMISTRY – EXEMPLIFIED ON CELLULOSE PETER ZUGENMAIER Institute of Physical Chemistry, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany Dedicated to Professor Elfriede Husemann, on the occasion of her 100th birthday in December 2008. She was an admirable and internationally highly recognized scientist and the first director of the Institute of Macromolecular Chemistry (Hermann-Staudinger-Haus) of the Albert-Ludwigs-Universität Freiburg; the foundation of the institute owing to the eminent scientific success and recognition of the work of Hermann Staudinger, leading to the Nobel Prize in the field of macromolecular chemistry. Received October 20, 2009 The development of the structure determination for cellulose and its derivatives as macromolecules is described from the beginning of the 20th century to the 1940s. The first correct presentation of the constitution of cellulose as a linear chain macromolecule of 1-4 linked β-D-anhydroglucopyranose, with the help of organic chemistry, dates from 1928. The size and shape of cellulose molecules still remained a controversial topic for some time. On the one hand, there were proposals of micelles i.e. aggregates of cyclic mono- or oligoanhydroglucose or micelles of small macromolecules of 30-50 glucose units. On the other hand, cellulose was seen as large macromolecules with more than 3000 glucose units for structures considered in solution as well as in fibres. The final clarification of the cellulose structure as a semi-flexible macromolecule of high molecular weight was extremely hindered by the inadequate interpretation of experimental results. Later, additional experimental and theoretical methods led to a consistent picture of the cellulose structure with high precision. Keywords: cellulose constitution, structure, molecular weight; primary valence bonds of cellulose and polyoxymethylene as model INTRODUCTION For thousands of years, cellulose served als were discovered and applied to cellulose humans’ needs, in various forms, such as with the goal of improving its physical fibrous materials (cotton, ramie, hemp, etc.) properties and satisfying scientific curiosity. or composite materials, e.g. wood, even As a biopolymer and the main constituent of though it was much later that humans organic renewable materials, cellulose acquired knowledge of its chemical played a major role in the development of constitution, configuration or molecular macromolecular chemistry. conformation. This was also true of the Cellulose is defined as a linear cellulose derivatives, when they became macromolecule, a non-branched chain of th available in the 19 century. It was only at variable length of 1-4 linked β-D- the beginning of the 20th century that new anhydroglucopyranose (Fig. 1). Cotton and improved methods for evaluating materi- comes very close to this ideal of cellulose, Cellulose Chem. Technol., 43 (9-10), 351-378 (2009) PETER ZUGENMAIER since it contains, beside cellulose, only ca. impurities, because of its better oriented 5% impurities. Ramie is often chosen as a crystalline fibrils. model cellulosic fibre, despite its ca. 25% Figure 1: Representation of a cellulose molecule fragment as 1-4 linked β-D-anhydroglucopyranose and labelling of the C-atoms. The atoms linked to C, i.e. O and H, are assigned the same numbers. A dimer is observed at repeat distance (fibre repeat) in the crystalline state of native cellulose Today’s knowledge of the cellulose others rejected this law in an endorsement structure results from a great number of address supporting Einstein’s membership to investigations carried out by many scientists. the Prussian Academy of Sciences (c.f. The clarification and development of the Appendix). Einstein’s theory of relativity cellulose structure has been for a long time was regarded by many scientists as more marked with controversial opinions and important and Einstein himself chose the ideas. The influence and progress of topic “Grundgedanken und Probleme der ingenuous ideas and their scientific Relativitätstheorie (Fundamental Ideas and recognition can be studied here in various Problems of the Theory of Relativity)” for ways. his Nobel Lecture. It is difficult to judge and assess The structure of the hydrogen atom, as it knowledge within a certain field in a certain was proposed by Bohr, certainly represents period of time, since we have to rely on the an ingenuous idea for his times and explains information available at that time, but we the optical line spectrum of the hydrogen actually judge it according to our today’s atom. However, the model cannot be knowledge. We all recognize and appreciate transferred to helium nor explain chemical certain conceptions, which in the end may bonds. John von Neumann, who has converge to an accepted representation of influenced a great deal the basics of physics disputed ideas, structures or models, and computers, expressed his concept of however, it is impossible to predict such a models as follows: „The Sciences do not try development in a field at the time when those to explain, they hardly even try to interpret, conceptions were introduced. they mainly make models. By a model is The importance of some scientific meant a mathematical construct which, with discoveries, such as the discovery of X-rays the addition of some verbal interpretation, or neutrons, is indisputable. However, there describes observed phenomena. The have been discoveries that led to Nobel Prize justification of such a construct is solely and awarding and were still disputed when the precisely that it is expected to work.” prize was granted. Michelson’s work The development of models of cellulose established the constant velocity of light in in solid state or in solution shows that vacuum with and against the rotation of the scientific knowledge and scientific progress earth, leading to the exclusion of an ether. rely on dispute. After overcoming the This was the basic experiment for the special controversial conceptions and difficulties theory of relativity, but its author was that were mainly related to the lack of exact honoured “for his optical precision methods at the beginning of the development instruments and the spectroscopic and of macromolecular chemistry, the 1953 metrological investigations carried out with Nobel Prize in chemistry was awarded to their aid”. Einstein won the Nobel Prize “for Hermann Staudinger, in recognition of “his his services to theoretical physics, and discoveries in the field of macromolecular especially for his discovery of the law of the chemistry”. Herein, we will try to follow the photoelectric effect”. Planck, Nernst and path of the development of macromolecular 352 Cellulose chemistry in elucidating the study of 1920 because, except for chemical analysis, cellulose. the methods needed for evaluating the structures were not developed yet or did not Constitution possess the necessary precision. In 1919, The chemical constitution of a low or when Emil Fischer, the senior champion of high molecular weight organic compound organic chemistry, as he was called by can be determined in the following steps. Haworth,1 passed away, the bases for the Elemental analysis leads to a quantitative structural knowledge on simple crystalline composition formula for the substance. The and soluble sugars were developed. Cellulose materials must be pure and uniform. Purity is and its derivatives however were mostly easy to confirm for low molecular weight considered insoluble, representing fibrous compounds, since the materials will possess materials with no sharp melting point. the same definite physical properties after Attempts to determine the molecular weight fractional distillation or crystallisation. As a led to negative or indefinite results. The further step, the mass or respectively, the required criteria of homogeneity and purity molecular weight of the kinetic active of cellulose were not fulfilled, but particles in solution or vapour should be investigations were carried out and the determined. The size of a particle (molecule) results obtained led to contradictory provides information about how many atoms interpretations due to inadequate evidence. are linked by primary valences. Care has to The composition of mixtures could vary, be taken, because primary valence depending on their individual preparation, (covalently linked) molecules could be which led to confusion, also caused by associated by secondary interactions, such as divergent formulation of the data obtained van der Waals forces or hydrogen bonding. with essentially the same methods. Thus, Such association should be ruled out by confusion spread instead of the expected changing the solvent, the temperature or by clarification of the structural chemistry of the influence of chemical reactions. The cellulose. However, one basic idea was clear emerging materials may be characterized by to many scientists – the physical properties analysis, e.g. by the melting point. Today, of cellulose and its derivatives in the solid materials are easily analysed by state or in solution required a molecular spectroscopic methods – among others, weight that was considerably larger than the NMR. Notably, crystalline materials can be monomeric unit. precisely described by single crystal X-ray This review will also present some results analysis, their composition may be detected on sugars that were known before 1920 and and, in addition, both their conformation

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