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 and that are necessary for the further discussion absolute configuration may be rendered, if of the basic units of cellulose. Emil Fischer sufficient intensity data are collected. synthesised glucose i.e. the basic monomeric For high molecular weight compounds, unit of cellulose. The anomeric α- and β- the analysis of configuration is more glucosidic series had been investigated by complex because of the distribution of chemical reaction and optical rotation as well molecular weights and imperfect as the D- and L-structures without crystallisation. Their structural analysis establishing absolute configurative relations. requires knowledge on the basic units The configuration of cellobiose belonged to (monomers) of the molecules, which can be the β-series according to Fischer and obtained by a mild degradation and evaluated Zemplén.2 The proposed pentagonal by subsequent examination of the products geometry of a glucose molecule originated by chemical or spectroscopic methods. from Böeseken3 as shown in Figure 2. That was the accepted configuration until 1925 Chemical constitution of cellulose (before when Haworth4,5 introduced a hexose in 1920) pyranose form (c.f. Fig. 1), i.e. a six- Constitution of the monomer unit membered ring. The structural chemistry of cellulose Cellulose [(C6H10O5)n by elemental experienced little noticeable progress until analysis] degrades mostly if not completely

353 PETER ZUGENMAIER to glucose (Ost and Wilkening6), which was charide by Skraup and König.13 Haworth and confirmed by several other researchers with Hirst14 showed that this compound consisted improved experimental techniques. The of two glucose units, which should be linked crystalline parts of cellulosic materials were by a 1-5 or 1-4 glycosidic bridge, and identified by orientation birefringence proposed a similar linkage between the (Ambronn7) and X-ray investigations monomer units of cellulose. In 1927, the 1-5 (Nishikawa and Ono,8 Scherrer,9 Herzog and linkage was excluded and a 1-4 linkage was Jancke10) and the resemblance of crystalline established for cellobiose (Haworth et al.,15 cellulosic materials in various plants and Zemplén;16 Fig. 3). wood pulp was recognised by Herzog and Jancke.11 Surprisingly, in 1919 biosan Valence chain (cellobiose) and triosan (cellotriose) were The conceptions of Tollens17 on cellulose, described as polysaccharides. represented in Figure 4, propose a high The treatment of cotton with sulphuric number of linked glucose residues (he acid and acetic anhydride degrades cellulose discusses 4 as well as 20), Nastukoff18 and and leads to remarkable amounts of Böeseken proposed at least 40 – the ends cellobiose octaacetate, first isolated by were joined to form large rings. Franchimont12 and recognized as a disac-

Figure 2: Geometric pentagonal configuration of α- Figure 3: Cellobiose, adapted from the representation glucose, adapted from Böeseken,3 here exemplified of Haworth et al.15 for α-methylglucoside

Figure 4: Constitution of cellulose, adapted from Tollens17

Cellulose forms derivatives with nitric Constitution and shape of cellulose (after and acetic acid, i.e. a trinitrate and triacetate, 1920) as well as a trimethylether. Primary valence chain Octaacetylcellobiose is obtained if cellulose The first attempts made at clarifying the is treated with a mixture of sulphuric acid constitution of cellulose were based on and acetic acid anhydrate (so-called limited methods of investigations. Progress acetolysis, Franchimont12). These in the methods of organic chemistry and investigations provided no information on especially of the physical investigations, in whether other sugars are present, on the solid state as well as in solution, led to partly steric or constitutional aspects of the type of new controversial ideas about the structure of linkage or the degree of branching. cellulose: is cellulose composed of long

354 Cellulose linear chains of glucose residues connected continuous chain structure of Freudenberg by covalent bonds, or does it consist of a observing that starch, the composition of cluster of anhydroglucosidic rings held which is comparable with cellulose, can be together by strong secondary valences? completely degraded by enzymes to maltose Freudenberg19 (1921) improved the (the 1,4-linked dimer of α−glucose). The reaction by acetolysis and the yield of acetolysis of cellulose, which resulted in octaacetylcellobiose of Franchimont,12 Ost20 50% cellobiose in his experiments, should and Madson,21 who actually achieved the not be over-emphasised. Later it was highest yield. Freudenberg came to the proposed that the degradation of starch by following experimentally supported enzymes started from the end of the conclusions: depending on the reaction time, molecule and, therefore, did not fulfil the about 40% of the theoretical value of statistical degradation that had been assumed octaacetylcellobiose was isolated; during the by Freudenberg. reaction, about 20% of the already formed Karrer26,27 investigated amylose and octaacetate was degraded, which meant that natural starch and reached the conclusion about 60% of the glucose units, but not much that after derivatisation and comparison with more, were initially obtained in the state of identical derivatives of maltose, these two octaacetylcellobiose. This observation led materials (amylose and starch) consisted of Freudenberg to the following proposal maltose anhydride in colloidal form. The relying on the experimental result that molecular weight determination of a soluble cellulose consists of 100% glucose units: if but dissociated product, obtained by alkaline all glucose units in a chain are connected by methylation of starch, the reaction of which alike bonds and are split with equal proceeded without degradation in his probability, about 67% of biose molecules opinion, led to an upper limit of molecular should result according to the rules of particle weight – from 900 to 1200. Karrer probability. Freudenberg found 40% which, concluded that these particles were identical considering the loss, led to 60% – close to with the starch molecules and consisted of 4 the expected 67% – that is, considerably to 6 glucose units, some still in aggregated below 100%. Freudenberg concluded from form. He also proposed maltose anhydride these results that these findings cannot be aggregates for amylose due to the high value taken as a proof for a continuous cellulose of the heat of combustion. Exactly the same chain, although nothing speaks against. In his value for the heat of combustion was cellulose model, each glucose unit is linked observed for cellulose and, therefore, Karrer to the next one by a linkage that is excluded a long chain for the cellulose structurally and configurationally identical to molecule and proposed, in analogy to starch, the linkages in cellobiose. On the other hand, cellobiose anhydride as the basic unit of Polanyi22 determined the content of the X- cellulose. The content and symmetry of the ray unit cell to four glucose units. Therefore, X-ray unit cell, established by Herzog and cellulose could consist of either continuous Jancke28 (2 cellobiose anhydride units), linked cellobiose anhydrides to form chains – supported his idea that cellulose fibrils two chains running through the unit cell - or consisted of aggregated cellobiose of two rings of cellobiose anhydride. The anhydride. rings of cellobiose anhydride should lead to Similar ideas were developed by 100% cellobiose, in contradiction to the Herzog,29 as well as by Bergmann30 and observed value of 67%, and a chain structure Hess,31 who even proposed glucose can be concluded (Freudenberg23). No anhydride as the structural unit. On the other experimental evidence about the length of hand, Bergmann refused to apply the term the cellulose chains was accessible. “molecule” for insoluble or only colloidal dispersed organic materials. In solution, Association of low molecular weight these proposed small compounds form compounds aggregates or so-called micelles (c.f. Karrer,25 who shared the 1937 Nobel Appendix) arranged similarly to micelles in a Prize with Haworth, opposed the projected soap solution. This proposal of a micellar

355 PETER ZUGENMAIER concept of cellulose dominated the scientific determined by the Scherrer X-ray method in literature for almost a decade. Especially crystalline fibres as the size of micelles biologists were attracted to it, since the idea (crystallites). They proposed a micelle model followed the thoughts of C. v. Nägeli,24 (Fig. for cellulose fibres – shown in Figure 6 – 5), who showed by microscopy that cotton consisting of bundles of parallel primary crystallises in domains, and the so-called valence chains of cellulose (c.f. the micelles formed would also necessitate a discussion on the cellulose structure). The colloidal character in solution. This idea was micelles are held together by special micellar confirmed by diffusion experiments carried forces and are still present in the solution. out on cellulose nitrate by Herzog.29 He This concept of K. H. Meyer34 was well- found the same size for the micelles accepted among chemists. By inter-micellar (crystallites) in the solid state and those in swelling, these micelles would be loosened colloidal solution, the interpretation being and finally dissolved as colloidal particles in that the micelles of the crystalline structure solution. Osmotic measurements provide transfer without changes into solutions. Hess micellar weights and not molecular weights. et al.32 concluded from these data that the This interpretation appeared quite plausible, properties of cellulose are not based on the since the solutions of many macromolecular size of the molecules, but rather on a skin materials behave like colloidal soap that surrounds the cellulose micelle. solutions. Therefore, McBain,35 who worked Therefore, cellulose degradation depends to a primarily in the field of soaps, stated that large extent on the slow destruction of these cellulose and similar materials show a skin systems. micellar form. Meyer and Mark33 expressed similar thoughts, interpreting the domains

Figure 6: Arrangement of chains and micelles in Figure 5: Representation of the micelle structure, ramie fibres: a – chain with glucose residues, b – adapted from v. Nägeli and Schwendener24 micelle, c – inter-micellar gap; adapted from Meyer34

In a survey on the constitution of molecules that surpass the longest dimension carbohydrates, Haworth1 expressed the of the X-ray unit cell. For cellulose in opinion that Hermann Staudinger most solution, the concept of micelles was effectively opposed the association theory. contradicted by the results of Stamm,36 who By synthesis as well as by investigations of detected molecular disperse particles (single high molecular weight materials, Staudinger molecules) by studies with an had supported the classical principles of the ultracentrifuge. He complained that the formation of biopolymers as very large diffusion measurements by Herzog and chains of monomer units linked by primary Krüger (1926)37 showed particle weights that valences. were too high because they were carried out In an interdisciplinary research project, at insufficient low concentrations to actually chemists and physicists in Freiburg showed obtain free diffusion. Also, their evaluation that the basic units of polyoxymethylene are did not take into account the fact that the connected by chemical valences – advanced particles were not spherical. by Kekulé – to give long chains of single

356 Cellulose Molecular disperse solutions of The position of the hydroxyl groups was cellulosics were later detected by many detected by Denham and Woodhouse42 by authors. the methylation and hydrolysis of cotton. Bergmann and Machemer38 corrected The product obtained was identified as 2,3,6 their own misconception as well as that of trimethyl glucose. Irvine and Hirst43 showed Herzog and that of Hess and Friese,39 that complete methylation yielded only 2,3,6 according to which cellulose consisted of trimethyl glucose, and proposed a ring- associated cellobiose anhydride or associated shaped trisaccharide as a model for cellulose. glucose anhydride. They investigated the They favoured the structure shown in Figure degradation products of cellulose acetate by 7, i.e. trianhydroglucose, which may be titration of the end-groups with iodine and realised by various isomeric units, as the found a mixture of oligosaccharides of 7-9 simplest form for a model of cellulose with a and 9-11 hexose residues, respectively, 1-5 linkage of monomers. The maximum depending on preconditioning, and excluded theoretical yield of cellobiose amounts to cellobiose anhydride as the structure of 70%, with 37% of glucose. The researchers cellulose. Haworth and Machemer40 did not exclude larger odd-numbered rings of hydrolysed trimethyl cellulose and isolated anhydroglucose residues, but were unable to both the expected 2,3,6 trimethyl make assumptions on the size of the glucopyranose and 2,3,4,6 tetramethyl cellulose molecule. They refused to accept glucopyranose. They concluded that their the hypothesis that cellulose was based on a cellulosic material contained not less than 50 molecule containing an even number of and not many more than 100 cellobiose units. hexose units. Their proposed model, plotted This proof on trimethyl cellulose is of in Figure 7, is in disagreement with the X- importance since the methyl group cannot be ray results of Polanyi22 and with the dimeric transferred during the reaction to other models obtained by researchers later. Irvine positions in the molecules. Further evidence and Hirst stressed that the fragments of against cellobiose anhydride as representing cellulose consist of 1-5 linked cellulose in the fibres came from the anhydroglucose – this linkage is also present hydrolysis of cellulose performed by in cellobiose - and that all glucose units are Willstätter and Zechmeister,41 who found identical in the cellulose molecule. The cellotriose and -tetraose as degradation isolated 2,3,6 trimethyl glucose excludes a products. number of structural models for cellulose, e.g. the comb model of Hess, described in Side groups and ring structure the paper of Irvine and Hirst.43 Their model Cotton derivatisation led to the is certainly strongly influenced by the conclusion that three hydroxyl groups are discussion of the cellulose structure as present in the monomer unit, which means aggregations of small ring-like molecules. that trinitrate and triacetate can be formed.

Figure 7: Preferred structure of cellulose, adapted from Irvine and Hirst43

The model given in Figure 7 was not mixture (c.f. also Willstätter and confirmed by later experiments of Zechmeister41). However, the length of the Zechmeister and Tóth,44 nor by their joint original chain of cellulose remained publication with Mark,45 who isolated undetermined and linkages, other than the cellotriose, tetraose and -hexaose with β−glycosidic ones, were not excluded with glycosidic linkages from the hydrolysis certainty. In contrast, Freudenberg and co-

357 PETER ZUGENMAIER workers46 rejected the presence of non β- which was the energetically favoured glycosidic bonds by iodine-controlled structure confirmed by experimental hydrolytic scission and confirmed a chain observations (Fig. 8). This proposal was later structure of cellulose. confirmed by the X-ray single crystal Until 1925, chemists had accepted a five- analysis of β-D-glucose (Ferrer,49,50 Chu and membered ring for glucose, cellobiose and Jeffrey51) and of the 1-4 linked β-cellobiose cellulose, as consistent with the experimental (Jacobson et al.,52 refined by Brown,53 and facts. The formulation of glucose as a six- further by Chu and Jeffrey51) or of methyl membered ring by Haworth4,5 changed the cellotrioside (Raymond et al.54) and of situation completely, above all clarifying the cellotetraose (Gessler et al.,55 Raymond et representation of cellobiose as a 1-4 linked al.56) as a fragment of cellulose. With β-D-glucose (Haworth15). The twisting of the sufficient precise data from X-ray analysis, second ring by ca. 180o is necessary to obtain the conformation and absolute configuration a reasonable glycosidic bond angle (Fig. 1) can be determined. Adequate data are not and a linked sequence of cellobiose available for crystalline polymer fibres, molecules leading to an extended cellulose additional data being necessary, besides the chain, which would meet the observed 10.3 X-ray intensities, for a spatial evaluation of Å fiber repeat distance. The spatial shape of their molecular shapes. the glucose molecule was proposed by 47 4 Reeves as a C1 trans-Sachse chair conformation in today’s nomenclature,

4 1 Figure 8: Representation of the C1 and C4 chair conformations of glucose, adapted from Lehmann48

Model compound polyoxymethylene chemical units in the chain. Isoprene was (POM) also discussed, representing a linear rubber C. Priesner57 praised the review on chain, according to Pickels,59 who did not polymerisation published by Hermann specify a chain length. Cellulose was not Staudinger58: “Of fundamental importance mentioned in this review. In the following for the investigation of polymers of natural years, Staudinger concentrated his and synthetic origin has been an article investigations mainly on three domains: 1) “Über Polymerisation” published in Berichte rubber and synthetic isoprene polymers der Deutschen Chemischen Gesellschaft (Staudinger and Fritschi,60 Staudinger61); 2) issued on June 12, 1920. It can be said synthetic polymers, especially polymeric without exaggeration that this paper has formaldehyde (polyoxymethylene, pointed the direction for modern Staudinger and Lüthy,62,63 Staudinger64); 3) macromolecular chemistry. The within cellulose. Staudinger regarded the included thesis has been wholly confirmed engagement with synthetic polymers as by researches in the following years, extremely important, since the monomeric or appearing surprisingly modern in its basic units are well-known, and products formulation.” with graduated polymerisation are Formaldehyde was an example, accessible. The results obtained for polymerising to a short linear chain of polyoxymethylene were then transferred to paraformaldehyde or to a longer chain called cellulose by analogy. From 1929 on, polyoxymethylene with the same basic Staudinger worked experimentally with 358 Cellulose cellulose (Priesner), concentrating his derived from morphological considerations numerous investigations on the size and (Fig. 9). With these data, the length of a - macromolecular character of cellulose. CH2O- monomer unit amounts to c/2 = 1.77 His studies on the macromolecular Å, somewhat shorter than the expected and character and structure of polyoxymethylene calculated distance of 2.37 Å between the are of exceptional importance, since they two carbon atoms of C-O-C (bond length C- combine data of organic chemistry with X- O and O-C 1.410 Å, angle C-O-C 114o). This ray investigations to give sound results. shortening has been explained by a tilt of the These results concern primary valence monomer unit toward the c axis.66 The small- chains, the length of a linear chain molecule angle X-ray reflection shows for compound and the formation of fibres. By the scission (CH3CO)2*O*(CH2O)n a spacing of 25.3 Å of high molecular weight polyoxymethylene for n = 9 and leads to an increment of 1.8 Å with acetic anhydride, short chains of for each additional monomer CH2O-unit. polyoxymethylene diacetates were produced. This value corresponds to c/2 (1.77 Å) for After fractional distillation and the high molecular weight POM and crystallisation, they were separated into distinctly shows that the same elongation of single individual compounds (up to a degree the chain is obtained by adding a monomer of polymerisation DP = 22). End group unit to the chain of polyoxymethylene determination and osmosis resulted in the diacetates. This increment corresponds to the same molecular weights. The analysis of the same distance of the continuous monomer undissolved residue showed an average DP units of the chain of high molecular weight of ca. 50 (Staudinger et al.65,66). The POM in the c-direction. Therefore, it can be subsequent X-ray investigations on concluded that a continuous primary valence crystalline materials by the Debye-Scherrer chain with covalent chemical bonds is method led to the results listed below, with running through the unit cell. significant consequences. Actually, the POM chain forms a helical All high molecular weight POMs with structure with a 21 screw axis parallel to the c various end groups led to the same X-ray axis,67 (Fig. 9) and the POM macromolecule diagrams, from which the same unit cell size does not exhibit the maximal possible length. was concluded. This unit cell contains the Because of regular gauche conformations same number of molecular units that have along the skeleton of the chain, a shortening the same basic molecular structure. The of the chain occurs in the c-direction of the shorter polyoxymethylene diacetate with 9 to unit cell. The projection of the distance of 19 formaldehyde groups exhibit the same adjacent C…C atoms of 2.37 Å of a rings in the Debye-Scherrer (powder) monomeric unit on the c-axis amounts to diagrams as high molecular weight polymers 1.77 Å as observed. The ball and stick do, but they are less sharp and are sometimes models with known standard bond lengths split into several fine rings, under the and angles that Sponsler and Dore (1926)68 influence of end groups. Further reflections successfully used for the determination of the appear at very small diffraction angles in the crystal structure of cellulose could have also innermost region of the diagram. The been effectively applied to POM. That spacings of those reflections are influenced modelling would have demonstrated that by the length of the crystallites of the various Staudinger’s idea that regarded macromo- fractions. The diagram of POM can be lecules as rigid rods with the most extended indexed with an orthorhombic (quasi- chain length was not adequate either for the hexagonal) unit cell with a = 4.56 Å, b = solid state, or for solution. POM also 7.89 Å, c = 3.54 Å. According to density provided evidence that the shape of the chain considerations, a unit cell contains four is flexible even in the solid state. A second monomeric units placed in two chains – to be crystalline modification of POM was consistent with the size of the unit cell, obtained and investigated by Hengstenberg69 which means that one dimer unit is placed and Sauter70 with samples from Staudinger’s along each chain within the unit cell. The laboratory, requiring further changes in the chains are located parallel to the c axis conformation (shape) of the chain skeleton.

359 PETER ZUGENMAIER Figure 9 was drawn with the coordinates high molecular weight materials are of a later performed single crystal X-ray unreliable at best. analysis (Gramlich67) with a bond length C- The sublimation of POM in vacuum led O of 1.410 Å, angle C-O-C of 114.1o and O- to fibres, whose axis contained the molecular C-O of 112.8o, and a torsion angle of 64.5o axes, comparable with the cellulose fibres. along the chain skeleton (gauche Staudinger regarded the results as conformation). The distances between the transferable to cellulose and chose POM as a chains rest on lattice forces (secondary model for cellulose in his subsequent interactions) and have been correctly publications. No doubt, POM with a DP interpreted.66 The chemical molecular weight ranging from 50 to 100 shows polymer of the single fractions of POM agrees with specific properties, but nevertheless the the ones extracted from the chain length length of the chain or the size of the from X-ray data. However, X-ray macromolecules is relatively small in investigations of the molecular weight of comparison with those of native cellulose or of other synthetic polymers.

Figure 9: Representation of crystalline polyoxymethylene (POM) of orthorhombic modification: (a) Projection along the chain axes on the a, b plane, (b) projection of the chain on the b, c plane. Space group P212121 of sub-cell: a = 4.766(2) Å, b = 7.647(5) Å, c (chain axis) = 3.556(2) Å from a single crystal structure analysis67

Sauter70 continued the X-ray of POM can be described approximately as a investigations with excellent data from single 9/5 helix, i.e. 9 monomer units in 5 turns – crystals of high molecular weight POM. To actually a 29/16 helix with a fibre repeat of grow such uniquely large polymer crystals 56.02 Å is still a better approximation - as was a most challenging task, seldom compared to the 2/1 helix of the first successful for polymers, but it was achieved modification. The projection of a monomer for POM by O. Schweitzer.71 Another unit on the c-axis of the ca. 9/5 helix is polymorphic modification with a pseudo- somewhat longer and amounts to 1.93 Å, as hexagonal orthorhombic unit cell with compared to 1.77 Å of the 2/1 helix shown in almost the same base plane, but a longer Figure 9. fibre axis, was published by Sauter, with a = The investigations on POM proved that 7.74 Å, b = 4.46 Å, c = 17.35 Å. It had the macromolecules are formed by primary already been identified some years earlier by chemical bonds between the basic units Hengstenberg69 on the basis of fibre (monomers), i.e. that primary valence chains diffraction patterns. The longer c axis gives exist. Long linear macromolecular chains are rise to a longer fragment of the POM running through small unit cells. It cannot be molecule within the unit cell containing 9 necessarily concluded from small unit cells monomer units. There is a different twisting that they contain small molecules of the size of the chain skeleton, with C-O-C-O torsion of the unit cell. Further on, the POM angles of ca. 78o as compared to 64.5o of the macromolecule forms at least two shapes first POM modification. This chain structure (conformations) in the solid state with

360 Cellulose different total molecular length for the same X-ray analysis of 10.25 Å (the current value DP and neither of the two structures shows is 10.38 Å) and should account for the the most extended form. intensities of the strong reflections. They already knew from the work carried out by Structural studies on cellulose Irvine et al. and Haworth et al. that the X-ray diffraction represents a suitable question of the configuration of glucose, method for a complete determination of the pentagonal or hexagonal ring structure was structure of crystalline materials, if sufficient discussed on the basis of organic chemistry. intensity data are available, which is rarely Sponsler and Dore concluded that a single the case for diffractions on polymer fibres cellulose chain has to be formed of and powders. The lack of experimental data continuous β-glucose units, in the form of can be partially overcome by the introduction hexagonal rings in Sachse–Mohr chairs (Fig. of molecular models, today carried out by 10). The glucose units were connected by 1- computer-aided simulation and 1, 4-4 linkages to obtain a long primary simultaneously evaluated with X-ray data. valence chain. The fibre period of 10.25 Å The packing arrangements and the formation agrees with the length of two joined glucose of crystalline structures (crystallites) are units parallel to the fibre axis, permitting the determined by the interactions of atoms and observation that a continuous primary molecules and can be represented by valence chain agrees with a short unit cell potentials attributed to the single atoms of dimension (fibre period). The packing of the the molecules. The unit cell and the space cellulose molecules in Sponsler and Dore’s group, by which the structural unit of a unit cell is shown in Figure 11, each unit cell crystal is described, are nevertheless helpful containing four parallel chains without any mathematical quantities to reduce the symmetry elements. As shown in the figure, structural investigations to an acceptable this cell can be reduced to the current, level. Therefore, it is possible to introduce a generally accepted two-chain unit cell (space 72 larger “unit cell”, e.g. a multiple of the group P21), first determined by Andress (a smallest “correct” unit cell, if not referring to = 8.35 Å, b (fibre axis) = 10.28 Å, c = 7.96 symmetry elements of the proposed cell or Å, β = 84o) and the reduction proposed by introducing common symmetry elements to W. H. Bragg.73 both cells. In conclusion, a correct result The projection of the chains along a concerning the required structure will be thus diagonal of Figure 11, represented in Figure obtained. 12, shows the long primary valence chains of Sponsler and Dore68 remarked correctly, 10.25 Å period, parallel to the fibre axis. when performing a structure determination Between the chains, van der Waals of cellulose ramie fibres, that besides the interactions (secondary valences) occur. An chemical reactions of the cellulose chain, the attractive interaction between the hydroxyl physical properties of a fibre must be also groups is mentioned, shown by dashed lines, considered. For example, anisotropic which represents, from our current strength, thermal expansion and swelling in viewpoint, hydrogen bonds. the direction of the fibre axis must be The method introduced by Sponsler and explained by a proposed structure. It was Dore is nowadays generally accepted for the clear to them that the X-ray data alone would structure evaluation of crystalline polymers, provide only a rough conception on the with the simultaneous consideration of the distinct relationship between molecules. structural data by X-ray diffraction, on the Their proposed strategy consisted in creating one hand, and with the support of these data three-dimensional ball and stick or space- by molecular models, on the other. The filling models first for glucose and then for incorrect linkage of the monomer units of cellulose, on the basis of known average glucose was corrected by Freudenberg75,76 bond lengths and angles derived from and Haworth,77 who trusted the experiments organic molecules. These models for of organic chemistry more than the cellulose should correspond to the length of interpretation of the X-ray data made by the fibre axis (repeat distance) obtained by

361 PETER ZUGENMAIER Sponsler and Dore, and proposed exclusively to Polanyi,78 these reflections could be 1-4 linkages along the cellulose chain. caused by other materials, i.e. an additional The proposal of Freudenberg and crystal form. A simultaneously existing Haworth could not explain the existence of second modification was actually much later some odd-order meridional reflections of the discovered for ramie fibres, although unable X-ray fibre diagram of cellulose along the to explain the described phenomena. Further fibre axis, with the accepted monoclinic P21 ideas explaining the odd-order meridional space group of the crystal structure. The spots include a twisting of the microfibrils in structure favoured by Sponsler and Dore the fibres. does not exclude these reflections. According

Figure 10: Model of β-glucose, adapted from Sponsler and Dore.68 Hexagonal ring in the Sachse-Mohr chair conformation with numbering of the atoms

Figure 11: Reduction of the four-chain cell, proposed by Sponsler and Dore (a = 10.80 Å, b (fibre axis) = Figure 12: Representation of the packing 10.25 Å, c = 12.20 Å), to a smaller two-chain arrangement of the cellulose chains in diagonal monoclinic unit cell equivalent to the one of Andress72 direction of the cell in Figure 11, adapted from for native cellulose. A better agreement can be Sponsler and Dore68 obtained if the cell of Sponsler and Dore is also described with a monoclinic angle (adapted from Kiessig74)

A model with 1-4 linkages of the glucose was published by Meyer and Mark33 for units, according to the primary valence crystalline cellulose. The linear cellulose chains of a cellulose molecule proposed by chains had been placed in a monoclinic or in Freudenberg and Braun,75,76 or by Haworth,77 an orthorhombic two-chain unit cell,

362 Cellulose respectively. However, the monoclinic unit In a further investigation by Meyer and 81 cell of space group P21 was preferred. An Misch, they overturned the established orthorhombic or perhaps a monoclinic unit parallel packing of the cellulose chains in a cell was previously introduced by Polanyi22 microfibril, and proposed an antiparallel in 1921. A problem arises with the packing arrangement of the two basic chains introduction of the 1-4 linkage of the within the same sized unit cell and space glucoses. Since the linear cellulose chain group P21. This arrangement had been possesses reducing and non-reducing ends, justified by the assertion that native cellulose the chain has a direction. Adjacent parallel can be further transformed to cellulose II, by chains pointing in the same direction are a process (mercerisation) that preserves the called parallel running chains and those overall fibre structure, orientation and space pointing in the opposite direction are distribution. Besides mercerisation, cellulose antiparallel. The model of Meyer and Mark II is also obtained by regeneration and (1928) prefers the two chains in the unit cell crystallisation from solution. This to be parallel running chains, not because of regenerated cellulose would necessarily be the evaluation of X-ray data, but rather packed in an antiparallel fashion, according because the authors assumed that a parallel to Meyer and Misch, since the same number arrangement would arise from their of chains in solution point in opposite biosynthesis in plants. In a further directions, when they are aligned and publication of Mark and Meyer,79 as well as crystallised by fibre spinning. Since one by Andress,80 the relative chain shift regenerated and mercerised cellulose show (now often called “stagger”) between the the same structure, it was concluded that corner and centre chains in the unit cell was native cellulose is also arranged in an specified for the two parallel chains. In the antiparallel fashion. Further improvements of work of 1929, the relative shift of one the cellulose chain structure, as compared to glucose unit33 was reduced to a shift of half a the original proposals79,80, consist in the glucose unit,79,80 and the coordinates of Sachse-Mohr chair conformation of the Andress were used by Mark and Meyer to glucose ring, but some bond lengths and estimate the intensities of the reflections. angles are far from the desired standard This 1929 model for cellulose, in which the values. The small glycosidic bridge angle of basic units are continuous linked glucose ca. 105o leads to unreasonable short van der molecules, has to be regarded as a setback Waals distances between H1 and H4 of the regarding the chair conformation of the adjacent residues of 1.54 Å. The parallel glucose residue provided by Sponsler and chain model of cellulose elaborated by Dore (1926). In Mark and Meyer’s, as well Andress, which is almost identical with that as Andress’ proposals, all carbon atoms of of Mark and Meyer, was strongly criticised the glucose ring lie in one plane,79,80 only the on geometric grounds. However, the main oxygen atom of the ring deviating from this argument for the introduction of parallel plane. This plane also contains the oxygen packing, i.e. by the synthesis of the chains O6 of the primary hydroxyl group (Fig. 1). originating by the growth of plants, was not The bond lengths and angles deviate mentioned at all. considerably from the standard values The conformation of a glucose unit in the established at that time. Mark and Meyer cellulose chain was dealt with in details by adopted all coordinates from Andress, except Hermans,82 and the established standard those of the glycosidic oxygens. The angles geometry, as well as the Sachse-Mohr chair θ(C5-C6-O6) = 144o or θ(C4-C5-O5) = 134o conformation were introduced. He found that and all bond angles involving pendant O- the so-called virtual bond length between atoms are far from the standard values in two adjacent glycosidic oxygen atoms is both models. The glycosidic bridge angle can larger than half of the fibre period, and be calculated with the given coordinates to therefore a chain with tilted (bent) glucose 100o (Mark and Meyer79) and to 120o units must be considered. A rotation of two (Andress80). neighbouring glucoses then leads to a theoretically reasonable glycosidic bridge

363 PETER ZUGENMAIER angle and to intramolecular hydrogen bonds unit cell belongs to cellulose Iβ, while between the O3 and O5 of adjacent glucose cellulose Iα in pure form exhibits a triclinic residues (Fig. 1). one-chain unit cell. A detailed discussion of The antiparallel arrangement of native different cellulose structures, as well as their cellulose chains by Meyer and Misch has conversions, is provided in an overview by represented the basis of cellulose research in Zugenmaier.84 the solid state for decades. Only later in the Fibre diffraction of native cellulose has 1970s, with the introduction of computerized provided an important contribution to the molecular modelling with simultaneous clarification of natural and synthetic evaluation of X-ray data, was there a return polymers. Especially the cellulose chains in to the hypothesis of the parallel arrangement the crystalline state with primary valence of native cellulose molecules in the linkages along the chains, hydrogen bonds crystalline microfibrils. Meyer and Mark and van der Waals interactions to repeatedly insisted that the evaluation of the neighbouring chains can be determined with cellulose structure cannot depend on models high precision, according to the structural but rather only on X-ray data. It should be determination techniques pioneered by noted that the preference of the antiparallel Sponsler and Dore. The methods for packing arrangement of native cellulose, structure evaluation introduced by these two proposed by Meyer and Misch, relies only on authors are still used today with great ideas concerning the crystallisation process, success, with considerable improvement and the parallel arrangement of Meyer and however as to the representation of models. Mark (1928) relied on ideas concerning fibre A concise report on the development of growth in plants. The discrimination of the X-ray diffraction and a critical discussion of different packing arrangements by X-ray the results obtained for native cellulose data was not considered in those before the end of the 1930s is provided in a investigations and would not have led to the review by Schiebold.85 In this review, proposed goal because of the few reflections Schiebold criticised K. H. Meyer for quoting observed in their patterns. improperly or insufficiently essential Mercerised or regenerated cellulose II is scientific papers or results, and was still considered as an antiparallel supported by Staudinger – as revealed by the arrangement of chains, based on computer document reproduced in the Appendix. simulations and X-ray, as well as neutron Staudinger typed and glued this document on analysis. This proposal is supported by single the inside of the cover of Meyer and Mark’s crystal X-ray analysis of a longer fragment book found, in its last edition, at the library of cellulose, i.e. oligomer cellotetraose, of the Institute of Macromolecular Chemistry which exhibits a base plane projected down in Freiburg.86 Also, the term the molecular axis, almost identical with “macromolecule”, introduced by Staudinger cellulose II. The unit cell contains two in 1922, by extending the usual term cellotetraose molecules of different “molecule”, was first rejected by Meyer and conformation, packed in an antiparallel Mark. Therefore, Staudinger’s complaints fashion, which is also found in cellulose II should be considered as adequate. (of space group P21). Two somewhat different conformations of the two chains Size of cellulose macromolecules running through the unit cell are also In the investigations discussed so far, the possible for native cellulose I, as actually structure of cellulose could be established as established in a recent investigation.83 a linear primary valence chain with the By spectroscopic means, especially by monomeric unit of β-D-anhydroglu- high resolution solid state NMR, it was copyranose linked in 1-4 position. The size found that in the fibres of native cellulose and length of the native macromolecules there coexist, side by side, two could only be estimated by methods modifications, cellulose Iα and Iβ. They available at that time, since the precise exist in different proportions depending on assessment of molecular weights could not the species. The above-discussed two-chain be provided for large molecules until 1930.

364 Cellulose Certainly, the molecular weight of the cellulose molecules. According to the current degradation products could be determined by view, macromolecules can be folded, as also the end group method. Osmosis and observed on small molecules with a flexible depression of the freezing point led to spacer between two stiff terminal groups, or molecular weight determinations of up to they might form fringed micelles, a 10000 (DP = 60 for cellulose) with high frequently seen packing model of cellulose precision. It was assumed that the values for (Fig. 13). This means that the required native cellulose might be far higher, but they correlation between crystallite size and could not be evaluated. The determination of molecule size does not exist. Both packing the size of macromolecules by X-ray models and some further proposals are not investigations was not possible, as expressed suitable for determining chain lengths. In the by many authors, since the necessary 1960s, especially Husemann and Bittiger correlation between crystallite size and dealt with problems of packing arrangements molecule size could not be provided. and chain folding of cellulose derivatives in Consequently, the values of the crystallite the solid state by electron microscopic size of ramie fibres, determined by Meyer investigations. and Mark, could not describe the length of

Figure 13: Schematic representation of fringed micelles of cellulose adapted from Fink and Walenta87

As to the size of the cellulose molecule, dissolved molecules, or by the end group in 1929, Fritz Haber considered that: “The method. However, those investigations are new arguments (Meyer and Mark33) that restricted to small molecules with MW up to favour large units linked by primary valences ca. 10000. For larger molecules, osmotic are no proof (for Haber), but must be pressure, increase in viscosity, as well as considered as more plausible than diffusion and sedimentation experiments in conclusive, while the possibility of a set-up an ultracentrifuge proved to be effective in by small chains cannot be neglected.” – cited determining molecular weights, such data according to Willstätter and Zechmeister.41 permitting to assess the size of At the same time (1929), Karrer expressed macromolecules. Later, more efficient the opinion that cellulose and starch exist as methods were added, such as static and colloidal micelles with associated molecules dynamic light scattering etc., or gel in solution. permeation chromatography, which, in The problems that researchers confronted addition, offered insight into the molecular around 1930 included two questions: weight distribution. These later introduced whether, in solution, there exist dispersed methods will not be discussed here. All single cellulosic molecules or micelle methods for determining chemical molecular particles composed of associated molecules; weight assume that the effective particles in and what is the size of the cellulose solution are actually single molecules, i.e. a molecules (molecular weight MW or DP). molecular disperse solution is present. The molecular weight as a measure for Although this assumption is often valid, the molecule size is usually obtained by sometimes and especially in investigations of colligative properties, such as vapour cellulosics, associations of molecules occur, pressure, freezing point depression etc., for which means that the existence of single

365 PETER ZUGENMAIER macromolecules in solution must first be 0.8, flexible chains are suggested, for a > 0.8 addressed. semi-flexible chains and for a = 2 absolutely stiff rods, such as those that occur in tobacco Viscosity mosaic virus. If molecular weight

Viscosity measurements played an distribution is present, then the so-called Mη, important role in the clarification of the the viscosity average value of M, may be constitution of particles in dilute solution. determined (c.f. Appendix eq. (A3)), which Investigations were carried out at various differs little from the weight average Mw for pressures, concentrations, temperatures and semi-flexible molecules. Formerly, Mw was with different solvents to exclude the determined by diffusion experiments and possibility for associated micelles to be today much simpler, by light scattering. present. Later studies included “polymer The intrinsic viscosity of a analogous conversions” that supported the macromolecular solution is a measure of existence of single molecules in dilute molecular weight, as long as macromolecules solutions. In this case, it must be verified increase the viscosity of a solution depending whether the DP is not changed by chemical on molecular weight. A relationship between conversions and whether measurements in the viscosity of a solution and the size of the different solvents lead to the same DP. Such soluble particles or micelles was concluded experiments are definitely qualified to rule many times before Staudinger and co- out the existence of micelles. workers (first published by Staudinger and Capillary viscosimetry is still the simplest Heuer89 for polystyrene). Fikentscher and method for determining molecular weights of Mark90 established a useful relationship for long-chain molecules. However, certain stiff rods. In a first step, Staudinger tested conditions must be observed (Marx-Figini this relationship for specific viscosity – eq. and Gonzalez88). For instance, the flow has (2) – at low concentrations for small to lie in the Newtonian regime – in the macromolecules, the molecular weight of 1930s, the fulfilment of the Hagen-Poiseuille which was available especially by colligative law was stated as a condition, which means properties, and extrapolated the validity of that the velocity gradient should be very low. the relationship with the same Km value, to High dilution with molecular disperse large molecules. Note that relationships (2) molecules is a precondition, which means and (1) are equivalent when a = 1, and with that the molecules should move extrapolation of ηspec to very low independently from each other. Between the concentrations of the solution c → 0, and limiting viscosity number, also called small shear rates → 0. intrinsic viscosity [η] and molecular weight η = K c M (2) M, there exists the following empirical spec m relationship as an approximation at certain The extrapolation c → 0 can be carried molecular weight intervals of a substance out by a number of graphical and (definition of intrinsic viscosity, c.f. computational methods. A very simple Appendix): computational method was published by Schulz and Blaschke.91 The extrapolation of [η] = K Ma (1) M* shear rate → 0 requires numerous Constant KM and exponent a depend on measurements. Therefore, a standardisation the solvent, temperature and spatial structure was proposed with the same low shear rate in of the polymer, and may in part represent normalised viscosimeters, and a choice of constants in a certain molecular weight concentration with little variation in the flow 4 region, which has to be larger than 2 x 10 . time was preferred (Schulz and Cantow92). Generally, the value of exponent a lies The validity of the linear ηspec–M relation between 0.5 and 1.0, and mostly between 0.7 eq. (2) assumed, as Staudinger did for low to and 0.8 for chain molecules. Experiments high molecular weights, the same constant have shown that, for poor solvents (theta Km is obtained according to eq. (3). Further solvents) a = 0.5 is to be expected and a ~ on, the same concentration c is required for 0.8 for good solvents. In the case of 0.5 < a < the polymers in solution, as well as similar

366 Cellulose polymer homologous products (equivalent viscosity is represented as a sum of two molecular weight distributions): terms αM + βM2, which leads, for small molecular weights, to Staudinger’s formula (ηspec/M)a = (ηspec/M)b =..(ηspec/M)x = Km (3) and, for high molecular weights, to the On the other hand, if Km and specific proportionality ∝ M2, which had been viscosities ηspec are known, molecular introduced by Fikentscher and Mark for stiff weights can be determined with eq. (3). rods, which means that linear In support of the viscosity–molecular macromolecules behave like stiff rods in weight relationship, Staudinger proposed a certain molecular weight regimes. model of stiff rods that move only in a plane. Staudinger’s response to the criticism of As to the form of chain molecules, Mark concerning molecular weight Staudinger was convinced that they have stiff determination is interesting:94 “At present, elastic structures. He wrote:93 “A chain the determination deals only with the molecule has to be regarded as a stiff elastic estimation of molecular weights to show that structure. The established relationship single large molecules are present in solution between viscosity and chain length is only and not micelles aggregated from small understandable by a stiff form of the chain macromolecules (Meyer and Mark), or from molecules, but not with the assumption that a few monomeric units (Karrer, Hess etc.).” these chain molecules in solution bend and He also rejected the doubts on the ηspec–M wind or twist helically, as sometimes law for large molecules. According to the assumed for rubber molecules. A chain present view, Staudinger overlooked that a molecule is more likely comparable to a stiff specific ηspec–M relationship according to eq. elastic glass thread than to a loose thread of (3) is only valid in certain regions with a = 1, wool, which can adopt any form. The chain and especially that the average Mn is used for molecule in solution has, on the average, the the calibration of different molecular weight same shape as in the crystal. The capability distributions (polydispersity). Generally, this of high molecular compounds to crystallize means that the same Km cannot be out of solution depends on this property. The transferred from small to large long chain molecules are collected as in a macromolecules or from a distinct molecular bundle of glass threads and form a weight distribution to another one. In macromolecular lattice.” comparison to the osmotic determination of Staudinger never abandoned this early molecular weight with regard to viscosity view of the shape of macromolecules, except measurements, it was clear to Staudinger that for a few compounds, such as polyester, and the molecular weight distribution had to be he correlated the exponent a = 1 in the considered. However, the exact relationships viscosity equation (1) with the stiff rod form between the various averages of Mn, Mw and of the chain molecules. In hindsight, he Mη were not known to him. overlooked the concept that dissolution and The essential pre-requisite for the crystallisation are driven by the change of determination of the molecular weight of Gibbs free energy. The shapes of molecules celluloses in dilute solution, i.e. the fact that depend on enthalpy as well as on entropy and they represent single macromolecules and temperature. In the course of time, exponent not micelles with associated molecules, was a was found ≠ 1 for many chain molecules. demonstrated by Staudinger and 94 Mark regarded Staudinger’s viscosity Schweitzer95 for cellulose derivatives. formula, eq. (2), as experimentally verified Further viscosimetric studies, especially for in the short chain regime, however rejected the estimation of DP were carried out by Staudinger’s model that assumed that stiff Staudinger and Freudenberger.96 They rod-like molecules move in a plane, arguing investigated acetolytically degraded cellulose that: “The viscosity formula of Staudinger triacetate with various chain lengths, can only be regarded as an empirical relation determined by the iodometric end group and can only be applied where it is method. Km in eq. (3) could be then obtained experimentally verified.” He himself for single macromolecules of different sizes proposed a relation in which specific in m-cresol and they demonstrated that Km =

367 PETER ZUGENMAIER 1.6 10-3 is constant for all samples with DP = and by osmosis. Fitting exponent a of a 7-80. The DP of larger degraded cellulosic Kuhn’s formula ηspec = Km c M to the macromolecules determined with this experimental data, he confirmed an exponent constant exceeded 150 glucose units, much of a = 0.6. Further ηspec–M relationships larger than the crystallite lengths in ramie proposed by various authors will not be 33 fibres with 30-50 glucose units. discussed, since the viscosity formula of eq. The stiff rod shape of macromolecules in (1) is commonly accepted today. solution, proposed by Staudinger, was Wo. Ostwald102,103 generally expressed strongly criticised by many scientists. Kuhn97 doubts that a simple universal relationship opposed the statement of Staudinger that between viscosity and size of particles exists cellulosics represent elongated stiff threads and referred to the schematic graphic in solution, since the intrinsic viscosity [η] representation of this relationship (Fig. 14). should then be expressed by ∝ M2, The S-shaped curve can only be replaced by according to Kuhn’s own considerations, and a straight line (linear ηspec–M relationship), as proposed by Fikentscher and Mark. For as required by Staudinger, within very semi-stiff chain molecules of cellulose and narrow intervals. For small molecular their derivatives, a value a < 1 should be weights, the straight line D is obtained, obtained.97 Kuhn expected a = 0.5 in case of which considerably underestimates high an irregularly coiled chain molecule without molecular weights and can only lead to lower considering the molecular volume, with free limit values with a calibration using this low and also with limited free rotation around the value Km constant. Staudinger first stated a chemical bonds. The changes in the shape DP > 150 for degraded cellulose, using this and size of the chain molecules, considering low Km constant. Nevertheless, this DP value the volume of the molecule and especially lies by far higher than the one provided by hydrodynamic effects from the solvent, led Meyer and Mark with DP = 30-50. to values of 0.8 to 0.9, close to the empirical Considering the tangent on the S-shaped value of a = 1, obtained by Staudinger. Kuhn experimental curve, i.e. the straight line B, requested a change in the assumption about and assuming the molecular weight the shape of the particles.98 The specific determined by calibration with osmosis at viscosity increase according to ηspec = Km c high molecular weights, the molecular Ma led to a values of 0.5 < a < 0.9 for chain weights are accessible with sufficient molecules (Kuhn98). accuracy in the region considered. An Kuhn’s ideas were taken up by Mark,99 average straight line, e.g. curve A, shows the who proposed that inner molecular statistics ηspec–M relation for the total molecular represent a possible quantitative explanation weight region, as required by Staudinger. of the deviation from Staudinger’s empirical Consequently, Wo. Ostwald rejected formula. Although the quantitative views Staudinger’s viscosimetric evaluation for cannot be considered as settled, at a first taking the quantitative determination of approximation, the relationship ηspec = Km c molecular weights as a law governing over Ma can be established, i.e. the same the entire region. functionality preferred by Kuhn. Depending A double logarithmic plot of viscosity for on the assumption of the excluded volume cellulose derivatives (cellulose tricarbanilate and of the van der Waals forces within a CTC or cellulose trinitrate CTN) versus Mw, chain, exponent a takes values between 2/3 determined by light scattering, can prove the and 3/2, which deviate from the currently empirical relationship of eq. (1), [η] = a accepted values of Kuhn. Mark concluded KM*M , within limited regions of molecular that rigid rod-like molecules could not be weights. Sutter and Burchard104 obtained for valid. CTC the curve shown in Figure 15. For CTC 100 4 Houwink tested the viscosity dissolved in 1,4 dioxane for Mw > 10 , as relationship of Kuhn with variable exponent well as for ATC (amylose tricarbanilate) in a for a number of polyesters for which the same solvent, the experimental curve can 101 -3 Staudinger et al. observed discrepancies be described with a = 0.90 (KM = 3.9 10 ) at between DPs determined by viscosimetry higher Mw region for CTC, and a = 0.88 (KM

368 Cellulose

-3 = 2.36 10 ) at higher Mw region for ATC. the conclusion that macromolecules in Marx-Figini and Gonzalez88 decomposed the solution, as well as in the solid state, have a curve for CTN dissolved in acetone into two stiff rod-like shape, from the linear parts and obtained a = 1.0 (KM(DP) = 0.82) relationship with a = 1, and that he did not for DPw < 1000 and a = 0.76 (KM(DP) = consider Kuhn’s arguments on the existence 4.46) for DPw > 1000, which means that of coils caused by free or hindered rotation exponents a for semi-flexible chains deviate around single bonds. Staudinger declared this little from 1.0. The linear intrinsic viscosity– idea misleading and did not follow Kuhn, molecular weight relation, [η] = KM*M, who considered an exponent a of 0.9 to introduced by Staudinger is not generally almost 1.0, if the macromolecular coils are valid over the entire molecular weight disturbed. Staudinger ignored the regime, nevertheless, it represents an experimental results for many empirical approximation with sufficient macromolecular materials, among others, for accuracy within certain intervals. However, some semi-flexible cellulose derivatives that it is hard to understand that Staudinger drew exhibit a ≠ 1.

Figure 15: Intrinsic viscosity [η]–molecular weight Figure 14: Schematic representation of the M relationship in a double logarithmic plot for dependency of specific viscosity on molecular w narrow molecular weight distributions of cellulose weight, curve C, adapted from Wo. Ostwald and 103 tricarbanilate CTC (M /M = 1.10) and amylose R. Riedel w n tricarbanilate (ATC) in 1,4-dioxane (adapted from Sutter and Burchard104)

As soon as reliable molecular weights in Staudinger of a = 1. Staudinger and 105 the higher weight regime were available by Daumiller found for constant Km, eq. (2), -4 absolute osmotic measurements, Staudinger Km = 6.3 10 for cellulose triacetate with DP -4 and co-workers used these results for = 80-780 in m-cresol and Km = 5.3 10 in calibration purposes of their viscosity chloroform (only soluble until DP = 400). -4 measurements and corrected their earlier For cellulose trinitrate, Km = 11 10 was results. According to the plots represented in obtained, leading to a DP of nitrated cotton Figures 14 and 15, it is obvious that the linters of 3000 and of ramie of 3500 curves in the region up to M = 104 cannot be (Staudinger and Mohr106) with calibration by extrapolated for the determination of osmosis. Staudinger and Reinecke provided a molecular weight at higher values. comprehensive compilation of results for Staudinger and co-workers105-107 addressed methyl and ethyl derivatives, among other these problems from 1937 on. The results compounds.107 From all these investigations proved to be satisfactory for DP of semi-stiff can be concluded that cellulose trinitrates are cellulosic materials, since exponent a in eq. excellently suited for DP determination of (1) is close to the value assumed by cellulose, since derivatisation can be simply

369 PETER ZUGENMAIER performed, little degradation occurs and the dimension of 30-50 glucose units as shown derivatives are easy to dissolve, which is also in Figure 6 could not accommodate a the case of cellulose tricarbanilate. cellulose chain with ~3000 glucose units, In the above-mentioned publications, the especially since chain folding can be state of dissolution was also addressed and excluded because the chains are arranged in “polymer analogous conversions” on isodirectional fashion (parallel packing). materials of various particle sizes were Instead, a cellulose chain may be passing carried out. The existence of macromolecules through several crystallites, as revealed in is extremely convincing, when studies are Figure 13. not only performed on a single material but Despite these convincing experimentally rather on various related compounds of a supported arguments for the presence of polymer homologue series,108 such as single macromolecules in dilute solutions, performed in the case of polysaccharides other conceptions as to the structure and size (first for cellulose, c.f. Staudinger and of cellulosic materials were presented around Schweitzer95). Since 1937, these polymer 1935 and later. Haworth et al.109 considered analogous conversions on cellulosics have that native cellulose is organised in been systematically improved.105-107 molecular aggregates of which a single Cellulose has been converted to polymer molecule contains no more than 200 glucose analogous triacetate, subsequently saponified units. McBain and Scott110 proposed that to cellulose or analogous 2.5 acetates, and cellulose in dilute solution appears as further to methyl and methyl acetyl association colloids, as found for soaps. celluloses. Cellulose was also converted to Also, in 1940, Mark commented on polymer analogous trinitrates. In all these Staudinger’s thesis on the existence of single conversions, a corresponding degree of molecules:111 “The macromolecular-isolated polymerisation was obtained (original state is only in part correct for the claimed material with DP up to 2000) and the cases. Especially for celluloses and their existence of single macromolecules in esters, another state is present in solution. In solution was assessed. The similarity of the recent years, extremely important reasons for Km constant in eq. (2) for the different the existence of micelle-like chain molecular derivatives was interpreted with an elongated aggregates have been brought forward by and unbranched shape of the cellulose Lieser.”112 Chain molecular aggregates derivatives in dilute solution, the same shape consisted, from the viewpoint of Mark, as that present in the solid state. Haworth et al., McBain and Scott, of primary The unbranched shape of the cellulose valence chains of limited length. P. H. derivatives was also experimentally Hermans113 wrote in 1942: “The opinions are confirmed by polymer analogous conversion still split for the often discussed problem if in comparing the molecular weight determined technical solutions for fibre spinning a by various methods, such as the end group dispersion to micelles or to chain molecules method with viscosity or osmosis occurs… As a consequence, almost all measurements. The agreement of DP values theoretical considerations about the spin and points towards a linear macromolecule107 deformation process prefer the micelle structure for cellulose, in contrast to theory… No decision is so far reached amylodextrins and glycogens, which are between the two extreme ideas.” branched macromolecules. For native cotton fibres, as well as for ramie, a DP > 3000 was Osmosis established. Exact values were difficult to The evaluation of osmotic pressure is an obtain, since the purification of the materials excellent method for determining particle or required a preliminary treatment with an molecular weights. It leads to absolute unknown effect for the size of the molecules. values, in contrast to viscosity The idea of a packing arrangement of the measurements, which allow only the fibres, as shown in Figure 6, can be determination of relative values. The osmotic eliminated when discussing the size of a pressure represents the difference in pressure cellulose molecule. A crystallite with a between a solution and a pure solvent, which

370 Cellulose are separated by a semi-permeable The extrapolation of the reduced pressure c membrane through which particles of a → 0 for various different solvents led to a certain larger size cannot permeate. Such constant value of molecular weight of M = membranes are difficult to produce for low 111000 (DP ~ 320), which represents the molecular weight substances, but easier to true weight and not associated micelles. In generate for macromolecules or large the presence of micelles, a dependency on M particles. The reduced osmotic pressure Π/c had to be observed for various solvents. is given by the following expression: Staudinger’s idea on disperse single macromolecules in dilute solution was Π/c = RT/M + A c (4) 2 confirmed again. Molecular weight where Π – osmotic pressure, c – determination was placed on a secure concentration of the solution, R – gas foundation with the introduction of the constant, T – absolute temperature, A2 – absolute osmotic method; equally, viscosity second virial coefficient, representing a measurements gained in reliability by the complex measure of the size and interaction calibration of the viscosity–molecular weight of the solute and the solvent. Further terms in relationship. the expansion play a minor role and are not The exact determination of referred to. For an ideal solution A2 vanishes macromolecular sizes caused difficulties (A2 = 0), as found especially for small even in the 1930s, since comparisons were molecules and eq. (4) is then called van’t rarely possible among the few readily Hoff’s law. Relation (4) contains the number available methods and because of different averaged value Mn of the molecular weight original materials and their purification. No distribution (c.f. Appendix). firm base was available for a theoretical 114 In the first publications of Schulz, the foundation. The idea of stiff molecular deviation from van’t Hoff’s law has been models hindered the viscosimetric taken care of with a relationship comparable determination of molecular weight, as well to the real gas law by introducing a co- as the [η]–Ma functionality, delaying an volume b: overdue development. An experimental Π/c = RT/ [M (1-b)] (5) discrimination between different geometric models was first obtained with the The virial expansion is to be preferred determination of the radius of gyration of since A also appears in the light scattering 2 macromolecules with light scattering formula and can therefore be independently measurements in the 1940s and 50s. That determined and compared. moved the concept of macromolecules very The extrapolation of c → 0, that is much to the awareness of science. limΠ / c in eq. (4), leads to the determi- 116 c→0 From today’s point of view, the a nation of particle or molecular weight. The empirical [η] = K M relation in eq. (1) can extrapolation may cause problems since the be extended to M = Mn (and K = Kn) or M = form of the experimental curves of the Mw (and K = Kw) with certain limitations reduced osmotic pressure as a function of (similar polydispersity, i.e. a similar ratio concentration is not linear and the Mw/Mn, especially when using Mn). If Mη is investigations must be carried out at very to be determined, molecular weight low concentrations. distribution has to be taken into account by a The deviation from van’t Hoff’s law was correction factor, which depends on a big obstacle to theoretical interpretation. exponent a, as well as on the Mw/Mn ratio. Here, as in viscosity studies, it appeared that These correction factors are especially large the experimental investigations provided in a conversion from Mn to M (i.e. Mη) or at conclusive results, whereas the theoretical different polydispersity Mw/Mn of the foundations often relied on incorrect materials investigated. They can be omitted assumptions (c.f. the above comment by John for standard measurements with the use of von Neumann concerning models). Mw for calibration purposes, e.g. for Mw/Mn 115 In 1937, A. Doby reviewed her osmotic = 2 and a = 0.7, the ratios Kn/K = 1.54 and investigations on cellulose nitrate (Fig. 16). Kw/K = 0.95 are obtained.

371 PETER ZUGENMAIER

Figure 16: Reduced osmotic pressure, p/c, as a function of concentration, c = gn/(1000 cm3), of a cellulose nitrate in various solvents, adapted from Dobry:115 1 – ethyl benzene; 2 – methyl salicylate + 20% methanol; 3 – acetophenone + 3% ethanol; 4 – cyclohexane + 5.8% ethanol; 5 – ethanol; 6 – glacial acetic acid; 7 – methanol; 8 – nitrobenzene

This means that, for the determination of cellulose was introduced as a linear chain of

Mη, the calibration of the viscosity constant variable length of 1-4 linked β-D- 76 with Mw is to be preferred. In this case, the anhydroglucopyranoses by Freudenberg corrections lie generally within the and Haworth77 in 1928, shortly after Haworth experimental error. An evaluation established β-D-glucose as a six-membered concerning the influence of physical ring in contrast to the formerly accepted five- quantities on constants K and a, i.e. the size membered ring.4 The length of the chain and shape of molecules, hydrodynamic could not be determined by means of organic effects etc. will be omitted here. Many chemistry. Sponsler and Dore68 investigated different models were introduced and are the structure of ramie fibres and introduced, discussed in reviews. Under certain with the help of molecular models and the conditions, the theoretical evaluation leads to interpretation of X-ray data, a linear chain an exponent a = 1 without introducing the model for cellulose and simultaneously the rigid rods of Staudinger. At this point, it conformation of the monomer units of should also be stressed that, at high dilution cellulose, i.e. glucose, as a chair in the of solutions of incompletely substituted Sachse-Mohr conformation. Two monomers cellulose derivatives, associations occur or form the repeat unit of the cellulose chain in so-called duplex structures may appear and the crystalline state. Their investigations the single molecules are connected by contradicted the general opinion of the 1920s stronger forces. that cellulose consists of aggregates of small molecules, e.g. anhydrocellobiose or even of CONCLUSIONS anhydroglucose. But the 1-1, 4-4 linkages of The application of today’s experimental the glucoses in their model were incorrect, and theoretical methods converged to well- the error being remedied by Freudenberg and characterised and well-defined structures for by Haworth. Mark and Meyer,79 as well as cellulosics, as well as for polymers in Andress,80 embodied this concept in a model general. The characterization and the for crystalline fibres of native cellulose. This determination of the cellulose structure are model, which rested on parallel packed inseparably connected with the historical chains pointing in one direction, was development of macromolecular chemistry, transposed to an antiparallel chain and have been extremely dependent on the arrangement by Meyer and Misch,81 which progress of the experimental methods was the generally accepted model until the available over time. The constitution of 1970s. Only then, with improved experi-

372 Cellulose mental methods and computer-aided model describe intrinsic viscosity for semi-flexible simulations, a data-based proposal was chain molecules. Kuhn’s coil model actually introduced for parallel chain arrangement of represented a simple solution for flexible and native cellulose in the fibres. The semi-stiff chain molecules with 0.5 < a < 1.0. clarification of the structure of native Further detailed information on the cellulose became more difficult, inasmuch as historical development of the two cellulose polymorphs were perceived to macromolecular character of cellulose until be simultaneously present in a fibre. Further, the middle of the 1930s can be found in the inter-convertible modifications of cellulose comprehensive publications of Haworth,1 have been discovered over the years, Freudenberg,23,117 Staudinger,93,118 Meyer and enriching the field of cellulose research. Mark,119 Saechtling,120 Marsh and Wood,121 The sizes of cellulose molecules and their Purves122 and Hess.123 size distributions, that is their molecular weight or DP, and, correlated with it, the APPENDIX concept of macromolecules, could not be Macromolecules resolved with X-ray investigations. Similar The term “macromolecule” was to the molecular weight determination of introduced by Staudinger and Fritschi in small molecules, the properties of 1922, to designate a compound with a macromolecules in solution were considered. molecular weight above 10000. The increase in viscosity with molecular Macromolecular compounds consist of weight represented the empirical quantity on molecules, similar in their composition, which Staudinger et al. concentrated a large chemical structure and size. In accordance part of their investigations. The initial, with this convention, a pure corpuscular inaccurate determination of molecular weight protein may be a macromolecular compound, of macromolecules by viscosimetry was but even a “pure” high polymer, other than a placed on a secure basis when the calibration fraction, which is precisely “molecularly with the absolute osmosis method was homogeneous”, is always a mixture of introduced from 1937 on. Average values for homologous polymeric compounds. DPn of cotton linters and ramie fibres of > The molecular weight distribution may be 3000 were obtained. The determinations of determined, e.g. by gel permeation sedimentation and diffusion coefficients with chromatography, and different moments the ultracentrifuge represent further (averages) of the distribution are accessible milestones in the clarification of molecular by various methods. weights and their distribution at considerable expenditure. At the beginning of this The number-average molecular weight development, the limited theoretical basis of Mn (eq. (A1)), obtainable from end group the viscosimetric method caused heated determination or by proper extrapolation to discussions about the applicability of these infinite dilution of cryoscopic, ebullioscopic, studies and only later led to sound results. or osmotic pressure data, is the ordinary kind After a broad palette of investigations, the of average, equal to the total mass of the size of cellulose and of its derivatives was molecules (in atomic weight units), divided ∞ finally determined in molecular disperse by the number (N = n ) of molecules solutions. ∑n=1 i With the linear [η]–M relationship, (ni – number of molecules with molecular Staudinger came very close to the description weight Mi): of semi-flexible cellulosics by the currently ∞ a accepted empirical formula [η] ∝ M (a = ∑ ni M i 0.8 - 1.0), but the stiff rod-like model he i=1 M n = ∞ (A1) introduced as an explanation was far from ∑ ni any reality. On the other hand, the stiff rod i=1 model of Fikentscher and Mark, which relied on sound theoretical considerations and provided the exponent a = 2, was unable to

373 PETER ZUGENMAIER

∞ ∞ Macromolecules”, J. Polym. Sci., 8, 257 2 ∑ni M i ∑ wi M i 1952) i=1 i=1 M w = ∞ = ∞ (A2) Viscosity ∑ni M i ∑ wi i=1 i=1 The viscosity of a liquid η describing the 1/ a ∞ fluidity of a liquid, can be determined by the  a  flow through a rod-like cylinder with the ∑ wi M i   i=1  application of the Hagen-Poiseuille law. To M = (A3) η ∞ determine the average molecular weight, the ∑ wi limiting viscosity number (intrinsic i=1 viscosity) [η] should be known. The a []η = K M ∗ M (A4) following relationships hold true: The weight-average molecular weight Viscosity of a solution: η Viscosity of the pure solvent: ηo Mw (eq. (A2)), obtainable, for example, by appropriate extrapolation of turbidity, light Relative viscosity: ηr = η / ηo scattering etc. data to infinite dilution, equals Specific viscosity: ηspec = (η – ηo)/ηo = ηr – 1 the sum of the squares of the masses of the Viscosity number (reduced spec. molecules, divided by the total mass (W = viscosity):ηred = ηspec / c = (ηr - 1) / c ∞ Limiting viscosity number (intrinsic w , wi – total mass of the molecules ∑i=1 i viscosity):[η] = lim ηspec /c = lim ηred with molecular weight Mi). with c → 0 and shear rate → 0 Polymer analogous reaction The viscosity-average molecular weight The transfer of a macropolymeric Mη (eq. (A3)) for a specified value of a, compound in a derivative of the same degree obtainable by extrapolation of solution of polymerisation is termed as a polymer viscosity η data to infinite dilution, if the analogous reaction. The size of the limiting viscosity number [η] depends on macromolecules does not change with molecular weight, according to relation (A4), temperature and concentration, or by KM constant. changing the solvent, only if a chemical It should be emphasized that this kind of reaction occurs, leading to degradation or to average is indefinite, unless the dependence molecular built-up of the macromolecules. of the limiting index on molecular weight is known. This dependence is a function of Micelle solvent and temperature. If a has the value of The term “micelle” is used in different unity, the viscosity-average is identical with ways: the weight-average. All such foregoing types According to Nägeli24 and Ambronn,7 the of averages, except the number-average, are term “micelle” describes particles “weighed averages”, differing in the extent (crystallites, small crystals) with definite to which the relatively high (or low) values boundaries in the solid state. The regular are “weighed” during averaging. repetition causes orientation birefringence and broadening of the X-ray reflections. The Degree of polymerization, DP term “micelle” does not provide any The degree of polymerization is defined information on the molecular structure of as “the (average) number of base units per crystallites. molecule”, if the molecules are composed of In contrast to the above definition, regularly repeating units, or as “the (average) McBain35 uses the term “micelle” for number of mers (monomeric units) per supermolecular complexes in aqueous soap molecule”, leading to DPn, DPw, DPη. solutions. The word “micelle” is less appropriate for M = DP ∗ M (A5) o crystallites; its use should be restricted only to colloidal particles or aggregates in (taken over from the “Report on solution. Nomenclature in the Field of 374 Cellulose Excerpt from the reference letter for wirkliche Neuerung einführen. Gegenwärtig Einstein’s election to the Prussian Academy arbeitet er intensiv an einer neuen of Sciences on July 24, 1913: Gravitationstheorie... Der eigenen reichen “Zusammenfassend kann man sagen, dass Produktivität gegenüber steht die besondere es unter den großen Problemen, an denen die Begabung Einsteins, fremden neu moderne Physik so reich ist, kaum eines gibt, auftauchenden Ansichten und Behauptungen zu dem nicht Einstein in bemerkenswerter schnell auf den Grund zu gehen und ihr Weise Stellung genommen hätte. Dass er in Verhältnis zueinander und zur Erfahrung mit seinen Spekulationen gelegentlich auch überraschender Sicherheit zu beurteilen.” einmal über das Ziel hinausgeschossen haben A number of famous scientists, among mag, wie z.B. in seiner Hypothese der them, Max Planck and Walther Nernst, Lichtquanten, wird man ihm nicht allzu regarded Einstein’s proposed law of the schwer anrechnen dürfen: denn ohne einmal photoelectric effect as deception (S. ein Risiko zu wagen, lässt sich auch in der Grundmann, Einsteins Akte, Springer, exaktesten Naturwissenschaft keine Berlin, Heidelberg, 1998, pp. 25-26).

Figure A1 a, b: H. Staudinger and E. Husemann in Figure A2: E. Husemann (left) with assistant A. conversation and on the motor scooter Bauer-Carnap at the electron microscope

375 PETER ZUGENMAIER

Figure A3: Remarks of H. Staudinger on the revised edition of Meyer and Mark (1950), glued on the inside of the cover of a volume in the library of the Institute of Macromolecular Chemistry (K. H. Meyer und H. Mark, Makromolekulare Chemie, Zweite Auflage, völlig neu bearbeitet von K.H. Meyer unter Mitwirkung von A. J. A. van der Wyk. Akademische Verlagsgesellschaft Geest & Portig K-G, Leipzig, 1950)

REFERENCES 14 W. N. Haworth and E. L. Hirst, J. Chem. Soc., 1 W. N. Haworth, Ber. Dtsch. Chem. Ges., 65, 119, 193 (1921). A43 (1932). 15 W. N. Haworth, C. W. Long and J. H. G. Plant, 2 E. Fischer and G. Zemplén, Liebigs Ann. Chem., J. Chem. Soc., 2809 (1927). 365, 1 (1909). 16 G. Zemplén, Ber. Dtsch. Chem. Ges., 59, 1254 3 J. Böeseken, Ber. Dtsch. Chem. Ges., 46, 2612 (1926). (1913). 17 B. Tollens, “Kurzes Handbuch der 4 W. N. Haworth, Nature, 116, 430 (1925). Kohlenhydrate”, Third Edition, Barth, Leipzig, 5 W. Charlton, W. N. Haworth and S. Peat, J. 1914. Chem. Soc., 89 (1926). 18 A. Nastukoff, Ber. Dtsch. Chem. Ges., 33, 2237 6 H. Ost and L. Wilkening, Chem.-Ztg., 34, 461 (1900). (1910). 19 K. Freudenberg, Ber. Dtsch. Chem. Ges., 54, 7 H. Ambronn, Kolloid-Z., 18, 273 (1916). 767 (1921). 8 S. Nishikawa and S. Ono, Proc. Math. Phys. 20 H. Ost, Liebigs Ann. Chem., 398, 313 (1913). Soc., Tokyo, 7, 131 (1913). 21 J. Madsen, Diss., Hannover, 1917. 9 P. Scherrer, in “Kolloidchemie – Ein 22 M. Polanyi, Naturwissenschaften, 9, 288 Lehrbuch”, edited by R. Zsigmondy, Third (1921). Edition, Spamer, Leipzig, 1920, pp. 387-409. 23 K. Freudenberg, “Tannin, Cellulose, Lignin”, 10 R. O. Herzog and W. Jancke, Z. Phys., 3, 196 Julius Springer, Berlin, 1933. (1920). 24 C. v. Nägeli and S. Schwendener, “Das 11 R. O. Herzog and W. Jancke, Ber. Dtsch. Mikroskop”, Second Edition, Leipzig, 1877, p. Chem. Ges., 53, 2162 (1920). 425. 12 A. P. N. Franchimont, Ber. Dtsch. Chem. Ges., 25 P. Karrer and C. Nägeli, Helv. Chim. Acta, 4, 12, 1938 (1879). 169 (1921). 13 Zd. H. Skraup and J. König, Monatshefte 26 P. Karrer, Z. Angew. Chem., 35, 85 (1922). Chem., 22, 1011 (1901).

376 Cellulose 27 P. Karrer, “Einführung in die Chemie der 56 S. Raymond, A. Heyraud, D. Tran Qui, Å. polymeren Kohlenhydrate”, Akademische Kvick and H. Chanzy, Macromolecules, 28, 2096 Verlagsgesellschaft, Leipzig, 1925. (1995). 28 R. O. Herzog and W. Jancke, Z. Angew. Chem., 57 C. Priesner, “H. Staudinger, H. Mark und K. H. 34, 385 (1921). Meyer: Thesen zur Größe und Struktur der 29 R. O. Herzog, Ber. Dtsch. Chem. Ges., 58, Makromoleküle”, Verlag Chemie, Weinheim, 1254 (1925). 1980. 30 M. Bergmann, Ber. Dtsch. Chem. Ges., 59, 58 H. Staudinger, Ber. Dtsch. Chem. Ges., 53, 2973 (1926). 1073 (1920). 31 K. Hess, “Die Chemie der Zellulose und ihrer 59 S. S. Pickels, J. Chem. Soc., Trans., 97, 1085 Begleiter”, Akademische Verlagsgesellschaft, (1910). Leipzig, 1928. 60 H. Staudinger and J. Fritschi, Helv. Chim. Acta, 32 K. Hess, C. Trogus, L. Akim and I. Sakurada, 5, 785 (1922). Ber. Dtsch. Chem. Ges., 64, 408 (1931). 61 H. Staudinger, Ber. Dtsch. Chem. Ges., 57, 33 K. H. Meyer and H. Mark, Ber. Dtsch. Chem. 1203 (1924). Ges., 61, 593 (1928). 62 H. Staudinger and M. Lüthy, Helv. Chim. Acta, 34 K. H. Meyer, Kolloid-Z., 53, 8 (1930). 8, 41 (1925). 35 J. W. McBain, J. Phys. Chem., 30, 239 (1926). 63 H. Staudinger and M. Lüthy, Helv. Chim. Acta, 36 A. Stamm, J. Am. Chem. Soc., 52, 3047 (1930). 8, 65 (1925). 37 R. O. Herzog and D. Krüger, Kolloid-Z., 39, 64 H. Staudinger, Helv. Chim. Acta, 8, 67 (1925). 250 (1926). 65 H. Staudinger, H. Johner, M. Lüthy, R. Signer, 38 M. Bergmann and H. Machemer, Ber. Dtsch. G. Mie and J. Hengstenberg, Chem. Ges., 63, 2304 (1930). Naturwissenschaften, 16, 379 (1927). 39 K. Hess and K. Friese, Liebigs Ann. Chem., 66 H. Staudinger, H. Johner, R. Signer, G. Mie 450, 40 (1926). and J. Hengstenberg, Z. Phys. Chem., 126, 425 40 W. N. Haworth and H. Machemer, J. Chem. (1927). Soc., 2270 (1932). 67 V. Gramlich, ETH Zürich Techn. Chem. Lab., 41 R. Willstätter and L. Zechmeister, Ber. Dtsch. Conference Report. Chem. Ges., 62, 722 (1929). 68 O. L. Sponsler and W. H. Dore, Colloid Symp. 42 W. S. Denham and H. Woodhouse, J. Chem. Monogr., 4, 174 (1926). Soc., Trans., 111, 244 (1917). 69 J. Hengstenberg, Ann. Phys., 84, 245 (1927). 43 J. C. Irvine and E. L. Hirst, J. Chem. Soc., 70 E. Sauter, Z. Phys. Chem., B 18, 417 (1932). Trans., 123, 518 (1923). 71 O. Schweitzer, Diss., Freiburg i. Br., 1930. 44 L. Zechmeister and G. Tóth, Ber. Dtsch. Chem. 72 K. R. Andress, Z. Phys. Chem., 136, 279 Ges., 64, 854 (1931). (1928). 45 L. Zechmeister, H. Mark and G. Tóth, Ber. 73 W. H. Bragg, Nature, 125, 633 (1930). Dtsch. Chem. Ges., 66, 269 (1933). 74 H. Kiessig, Z. Phys. Chem., B 43, 79 (1939). 46 K. Freudenberg, W. Kuhn, W. Dürr, F. Bolz 75 K. Freudenberg and E. Braun, Liebigs Ann. and G. Steinbrunn, Ber. Dtsch. Chem. Ges., 63, Chem., 460, 288 (1928). 1510 (1930). 76 K. Freudenberg, Liebigs Ann. Chem., 461, 130 47 R. E. Reeves, J. Am. Chem. Soc., 72, 1499 (1928). (1950). 77 W. N. Haworth, Helv. Chim. Acta, 11, 534 48 J. Lehmann, “Chemie der Kohlenhydrate”, (1928). Georg Thieme Verlag, Stuttgart, 1976. 78 M. Polanyi, Naturwissenschaften, 16, 263 49 W. G. Ferrier, Acta Cryst., 13, 678 (1960). (1928). 50 W. G. Ferrier, Acta Cryst., 16, 1023 (1963). 79 H. Mark and K. H. Meyer, Z. Phys. Chem., B 2, 51 S. C. Chu and G. A. Jeffrey, Acta Cryst., B 24, 115 (1929). 830 (1968). 80 K. R. Andress, Z. Phys. Chem., B 2, 380 52 R. A. Jacobson, J. A. Wunderlich and W. N. (1929). Lipscomb, Acta Cryst., 14, 598 (1961). 81 K. H. Meyer and L. Misch, Helv. Chim. Acta, 53 C. J. Brown, J. Chem. Soc., A, 927 (1966). 20, 232 (1937). 54 S. Raymond, B. Henrissat, D. Tran Qui, Å. 82 P. H. Hermans, Kolloid-Z., 102, 169 (1943). Kvick and H. Chanzy, Carbohyd. Res., 277, 209 83 Y. Nishiyama, P. Langan and H. Chanzy, J. (1995). Am. Chem. Soc., 124, 9074 (2002). 55 K. Gessler, N. Krauß, T. Steiner, C. Betzel, C. 84 P. Zugenmaier, “Crystalline Cellulose and Sandmann and W. Sänger, Science, 266, 1027 Cellulose Derivatives – Characterization and (1994). Structures”, Springer, Berlin Heidelberg, 2008. 85 E. Schiebold, Kolloid-Z., 108, 248 (1944).

377 PETER ZUGENMAIER 86 K. H. Meyer and H. Mark, “Makromolekulare 110 J. W. McBain and D. A. Scott, Ind. Eng. Chemie”, Second Edition, Geest & Portig, Chem., 28, 470 (1936). Leipzig, 1950. 111 H. Mark, in “Hochpolymere Chemie”, Band 1, 87 H.-P. Fink and E. Walenta, Papier, 48, 739 Akademische Verlagsgesellschaft, Leipzig, 1940, (1994). pp. 296. 88 M. Marx-Figini and F. R. Gonzalez, Papier, 112 Th. Lieser, Liebigs Ann. Chem., 528, 275 42, 660 (1988). (1937). 89 H. Staudinger and W. Heuer, Ber. Dtsch. 113 P. H. Hermans, in “Fortschritte der Chemie, Chem. Ges., 63, 222 (1930). Physik und Technik der makromolekularen 90 H. Fikentscher and H. Mark, Kolloid-Z., 49, Stoffe”, Band 2, edited by W. Röhrs, H. 135 (1929). Staudinger and R. Vieweg, Lehmanns Verlag, 91 G. V. Schulz and F. Blaschke, J. Prakt. Chem. München, Berlin, 1942, pp. 17-35. N. F., 158, 130 (1941). 114 G. V. Schulz, in “Fortschritte der Chemie, 92 G. V. Schulz and H.-J. Cantow, Makromol. Physik und Technik der makromolekularen Chem., 13, 71 (1954) Stoffe”, Band 2, edited by W. Röhrs, H. 93 H. Staudinger, “Die hochmolekularen Staudinger and R. Vieweg, Lehmanns Verlag, organischen Verbindungen”, Springer, Berlin München, Berlin, 1942, pp. 49-74. Göttingen Heidelberg, 1932 (Reprinted 1960). 115 A. Dobry, Kolloid-Z., 81, 190 (1937). 94 H. Mark, Kolloid Z., 53, 32 (1930). 116 M. Kurata and Y. Tsunashima, in “Polymer 95 H. Staudinger and O. Schweitzer, Ber. Dtsch. Handbook”, Fourth Edition, edited by J. Chem. Ges., 63, 3132 (1930). Brandrup, E. H. Immergut and E. A. Grulke, J. 96 H. Staudinger and H. Freudenberger, Ber. Wiley & Sons, New York, 1999, pp. VII 1-83. Dtsch. Chem. Ges., 63, 2331 (1930). 117 K. Freudenberg, Ber. Dtsch. Chem. Ges., 100, 97 W. Kuhn, Kolloid-Z., 68, 2 (1934). 172 (1967). 98 W. Kuhn, Z. Angew. Chem., 49, 858 (1936). 118 H. Staudinger, Sonderdruck aus 99 H. Mark, in “Der feste Körper”, edited by R. Papierfabrikant, 36, 373, 381, 473, 481 (1938). Sänger, Hirzel, Leipzig, 1938, pp. 65-104. 119 K. H. Meyer and H. Mark, “Hochpolymere 100 R. Houwink, J. Prakt. Chem. N. F., 157, 15 Chemie”, Band 1, 2, Akademische (1940). Verlagsgesellschaft, Leipzig, 1940. 101 H. Staudinger and H. Warth, J. Prakt. Chem. 120 H. Saechtling, “Hochpolymere organische N. F., 155, 261 (1940). Naturstoffe”, Fiedr. Vieweg & Sohn, 102 Wo. Ostwald, Kolloid-Z., 81, 195 (1937). Braunschweig, 1935. 103 Wo. Ostwald and R. Riedel, Kolloid-Z., 70, 67 121 J. T. Marsh and F. C. Wood, “An Introduction (1935). to the Chemistry of Cellulose”, D. van Nostrand 104 W. Sutter and W. Burchard, Makromol. Company, New York, 1939. Chem., 179, 1961 (1978). 122 C. B. Purves, in “Cellulose and Cellulose 105 H. Staudinger and G. Daumiller, Liebigs Ann. Derivatives”, edited by E. Ott, Interscience, New Chem., 529, 219 (1937). York, 1946, pp. 27-68. 106 H. Staudinger and R. Mohr, Ber. Dtsch. Chem. 123 K. Hess, Naturwissenschaften, 22, 469 (1934). Ges., 70, 2296 (1937) 107 H. Staudinger and F. Reinecke, Liebigs Ann. Chem., 535, 47 (1938). 108 H. Staudinger and H. Scholz, Ber. Dtsch. Chem. Ges., 67, 84 (1934). 109 W. N. Haworth, E. L. Hirst and A. C. Waine, J. Chem. Soc., 1299 (1935).

378