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Difficulties in the Emergence of the Polymer ConceptЧan Essay.

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Volume 26 - Number 2
February 1987
Pages 93-160
Difficulties in the Emergence of the Polymer Concept-an Essay**
By Herbert Morawetz*
1. Introduction
The inauguration of the Max-Planck-Institut fur Polymerforschung is a milestone in the long and tortuous road
over which the study of polymers emerged as a scientific
discipline and gained recognition by the scientific establishment.‘”’’ It is hard to believe that Partington’s monumental “History of Chemistry”“’ published in 1964 does
not refer to “polymers” in the index, refers to Staudinger
only in connection with his work on diazomethane and ketene, and still refers to polystyrene as “metastyrol.” Yet,
we should remember that students of polymers are not
unique in being victims of such glaring prejudice. In his
inaugural address as rector of the University of Bonn, a
little over a hundred years ago, Kekufi‘21complained that
“one-sided representatives of so-called humanistic studies,
who also confuse the application of chemistry with its
scientific work, tend towards the unjustified view that
chemistry ought to be taught at polytechnic schools and
not at universities.”
2. Three Early Experiments
What should we regard as the earliest significant polymer experiment? I shall suggest three that come to mind.
The first could be the demonstration by the blind John
Cough in 1805[31that natural rubber heats u p when
[*] Pof. H. Morawetz
stretched and contracts when a loaded sample is heated.
Cough interpreted his results in a rather fanciful way. For
instance, the contraction of a stressed rubber specimen on
heating “is occasioned by the absorption of caloric fluid in
the same manner that ropes are obliged to contract by absorption of water.” Yet, the phenomena he described were
real and it took more than a century to formulate their interpretation. The second choice might be Furaday’s analysis of rubber latex in 1826.‘41He showed that the aqueous
phase contains protein and he gave a reasonably accurate
figure for the elemental composition of the rubber. It is
little known that he also recorded in his notebook an experiment in which he heated rubber with sulfur. All he observed was hydrogen sulfide generation and he concluded
that heating organic compounds with sulfur might be a
novel way to reduce their hydrogen content!”] This may
surely be regarded as one of the great missed opportunities
in the history of science!
My third choice is the 1839 report by Simon, a Berlin
pharmacist, who first isolated styrene from a natural resin
and noted that styrene distillation left a residue whose
amount increased with the time the styrene was stored.[61
H e assumed that the product was styrene oxide. A few
years later, Simon’s experiment was repeated by Blyth and
Hofmann”’ who showed conclusively that no oxidation
was involved. To determine the nature of the transformation, they nitrated the styrene and the residue remaining
after distillation and characterized them by the N / C ratio.
They reported that this ratio changed from 118 to 1/7 and
concluded that the original substance, C8H8, had been
transformed into C7H,.
Polytechnic University
333 Jay Street, Brooklyn, NY I 1 201 (USA)
Lecture presented at the inauguration of the Max-Planck-Instttut fur
Polymerforschung o n March 1 I , 1986, in Mainz.
Note added by the editorial staff: The author of this article has written
a history of macromolecular chemistry ( H . Morawetz: Polymers. The
Origins and Growth ofa Science. Wiley, New York 1985), which was
reviewed in this journal (Angew. Chem. 98 (1986) 1039).
Angew. Chem. Int. Ed. Engi. 26 (1987) 93-97
3. The Origin of the Term Polymer
The origin of the term “polymer” is rather curious. It
was the result of the isolation, by Faraday, of butene for
which he found a gas density twice as high as that of ethy-
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lene, although the two substances had the same elemental
composition.lxl This puzzled Faruday, since he believed
that the elemental composition defines a chemical compound. He was clearly unaware of a paper by Gay-Lus.FUC,[”] who had pointed out as early as 1814 that “the composition of acetic acid does not differ significantly from
that of ligneous matter.“’. .. This is a new proof that the
atomic arrangement in a compound has great influence on
its character.”‘”’ But this paper was also unknown to Berzelius, who was astounded by Faraduy’s finding“” and
suggested that his new gas be referred to as a “polymer” of
ethylene. According to Berzelius’ definition, a polymer had
to have the same elemental composition as a reference
substance and this criterion was used rigorously a century
later by Staudinger, who objected, therefore, to the application of this term to the products of polycondensations.
Berzelius’ definition paid no attention to molecular structure, so that all through the nineteenth century we may
find statements describing, for example, styrene, C8Hs, as a
polymer of acetylene, C2H2,or even lactic acid, C3H603,as
a polymer of formaldehyde, CH,O. Our current usage of
the term was defined by Carofhers,““who called polymers
substances whose “structure may be represented by
-R-R-Rwhere “-R-” are bivalent radicals which, in
general, are not capable of independent existence.”
4. The Second Half of the 19th CenturyImportant Experiments and Misconceptions
In spite of the mistaken characterization of “metastyrol”
by Blyth and Hofmann, this and similar products of transformation were soon considered to be “polymeric.” We
may well wonder how this concept could have arisen at a
time when gas densities were the only reliable measures of
molecular size. An increasing molecular weight was then
estimated from a rising boiling point or inferred from an
increasing density and the softening temperature of solids,
which was not differentiated from a melting point. Considering the crudity of such methods, it is remarkable that
Berthelot arrived at an understanding of the conversion of
vinyl compounds into polymeric chain molecules. He reasoned[’21that on addition of an olefin to a chain with a
terminal double bond, the unsaturation would be retained,
so that there was no reason why very long chains should
not be produced, although, as he stressed, the possibility of
the formation of such chains did not ensure that they
would be produced under some given conditions. Berthelot
isolated the dimer, trimer, and tetramer of pentene and estimated from their heat of combustion the heat of polymerization. His long lecture on “la polymerie” presented to
[*] His analysis was inaccurate. Cellulose was isolated from wood only 25
years later. However, he also pointed out that starch, which could be
obtained pure at that time, has the same elemental composition.
“Cette composition de I’acide acetique ne differe pas sensiblement d e
celle d e la matiere ligneuse qui ne jouit en aucune maniere des proprietes acides. Voila donc deux corps composes d e carbon, d’oxigene et
d’hydrogene, en meme proportions, dont les proprietes sont eminemment differentes. C’est une nouvelle preuve que I’arrangement d e molecules dans un compose a la plus grande influence s u r le caractere neutre,
acide ou alcalin d e ce compose. Le sucre, la gomme et I’amidon conduisent encore a la mPme conclusion ...”
the Chemical Society of Paris in 1863 may well be regarded as the first intensive treatment of the subject. We
should admire his insight and not stress unduly some of
his more extravagant theories on the nature of catalysis or
his cautious suggestion that the principle of polymerization may perhaps also apply to atoms, so that sulfur might
be viewed as the dimer of oxygen and tellurium its tetramer.
A considerable number of vinyl monomers were polymerized before the end of the nineteenth century although
the nature of the process was generally not understood. We
should not be misled by the expression “hochpolymer” in
the German literature of that time, which could refer to
association complexes of small molecular species as well
as to large molecules. The nature of polycondensation was
understood by lour en^^,"'^ who, between 1859 and 1863,
studied the condensation of ethylene glycol and described
the products up to the hexamer. He should also be given
credit for having understood the concept of copolymerization-something which seemed surprisingly difficult when
it was considered again sixty years later. There were, of
course, other results which obviously involved polycondensation, but they were misunderstood mainly for two
reasons: ( I ) Nothing was known about the constraints on
molecular geometry, so that there was no hesitation in assigning to the products of ester interchange of resorcinol
or hydroquinone with diphenyl carbonate“41the structures
1 and 2, respectively.
(2) There was a strong bias toward assigning a unique
structure to a reaction product, particularly if it appeared
to be crystalline. Thus, the condensation of salicylic acid
was represented as leading to a cyclic tetramer[’’] and the
product of the condensation of succinic acid with ethylene
glycol was thought to be the cyclic dimer.[I6’
5. The Early 20th CenturyNew Arguments against the Existence of Polymers
The strong resistance to the concept of long-chain molecules involving only normal chemical bonds had several
causes. The major opposition was due to the emergence of
the concept of colloids as aggregates of a large number of
small molecules. This “colloidal state” was supposed to be
responsible for a high solution viscosity, low diffusion
rate, gelation, and the failure of substances such as “metastyrol” to crystallize. Most important, it was frequently
stated that “colloid solutions” cannot be expected to follow the laws of physical chemistry which were supposed to
be restricted to “true solutions.” It was also widely believed that solubility decreases necessarily with molecular
size, so that the solubility of substances such as “metastyrol” seemed incompatible with a polymeric nature. Later,
with the introduction of X-ray crystallography, the small
Angew. Chem. In!. Ed. Engl. 26 11987) 93-97
size of the unit cell of substances such as cellulose and
Hevea rubber seemed to suggest a small molecular size. It
is true that P~lanyi“’~
wrote in 1921 that the unit cell of
cellulose allows two alternative interpretations: It could
contain either two disaccharide segments of long polysaccharide chains, o r two cyclic disaccharides. But he never
returned to this argument, which seems to have been generally overlooked at the time. Polanyi wrote ruefully forty
years later:“‘1 “Unfortunately I lacked the chemical sense
to eliminate the second alternative-I should have been
better occupied in establishing definitely the chain structure.” We may also refer to some rather quaint arguments
against the existence of long-chain molecules: The influential organic chemist Karrer ridiculed in 192 I l l 9 ] the notion
that starch could consist “of dozens o r hundreds of glucose molecules joined by glucosidic bonds” since “it is improbable that a plant in converting sugar to a reserve substance from which it might soon have to be recovered
would perform such complex work.” Finally, Herrnann
Mark recalls that at a tempestuous farewell lecture given
by Staudinger at the ETH in Zurich in 1925, one of the
speakers compared Staudinger’s championship of longchain molecules to the report of some traveler in Africa
that he had seen a zebra which was 400 meters long. Who
could believe such nonsense!
6. Staudinger’s Decision to Concentrate on Polymer
Staudinger’s conviction that “hochmolekular” compounds consist of covalently bonded long-chain molecules
was first expressed in a paper published in Switzerland in
1919[’01 and elaborated at some length in Berichte der
Deutschen Chemischen Gesellschaft during the following
year.l2’] Staudinger cited three examples, polyoxymethylene (then called paraformaldehyde), “metastyrol,” and
Hevea rubber, for which, as he acknowledged, Pickles[z21
had previously proposed a linear structure. It is fair to say
that at that time Staudinger’s view was largely intuitive
with little experimental evidence, but he was determined to
prove his point and he had the courage to concentrate
henceforth on a field which was viewed with undisguised
contempt by many of the leading organic chemists of that
time. As for this first communication on high-polymeric
compounds, four features deserve our attention: It contained a clear description of the concept of polydispersity.
It also contained a statement that “it makes no sense to
carry out molecular weight determinations on such substances,” justified by the observation that ebullioscopic
and cryoscopic data would only reflect the presence of
low-molecular-weight impurities. It raised the question of
the chemical nature of the ends of chains formed by the
addition of unsaturated monomers: Here Staudinger observed that with sufficiently long chains the concentration
of these reactive sites would be reduced to a point where
their presence would not be significant. Similar arguments,
claiming that reactivities of chemical structures decrease
with the length of chains to which they are attached may
strike us today as farfetched, but such arguments were
commonly accepted for a long time. Finally, Staudinger
believed that “hundreds of molecules will add to each
Angew. Chem. Int. Ed. Engl. 26 (1987) 93-97
other until equilibrium, which may depend on temperature, concentration, and solvent, has been attained.”
In view of Staudinger’s eminent role in establishing the
polymer concept, it is of some interest to inquire what may
have been the influences which induced him, a man with a
solid reputation in classical organic chemistry, to concentrate after 1920 on a field which appeared so unpromising
at the time. While the answer is necessarily speculative,
there are some pointers worth considering. In his mem ~ i r , [Staudinger
writes warmly about the “outstanding
personality” of Carl Engler, in whose chemical institute at
Karlsruhe he spent the five years 1907-1912 as “Extraordinarer Professor.” It is, therefore, of some interest that
Engler wrote as early as 1897,[241in speculating about the
origin of petroleum, that molecules with double bonds
may undergo “slow addition to complex structures” and
that “one need not assume that only similar molecules assemble.” (It may even be significant that Engler’s wording
“lagern sich zusammen” was adopted by Staudinger in his
1920 paper). Engler was well ahead of his time in speculating as early as 1904 about the relationship between autoxidation and polymerization,‘2s1and Staudinger became involved for the first time with polymerization when he advised Engler’s student Lautenschliiger on his thesis dealing
with “Autoxidation and polymerization of unsaturated hydrocarbons,”‘261Staudinger became involved for the first
time with polymerization processes.
7. 1920-1940: The Classical Age of Polymer
A number of experimental results that were incompatible with the view of the colloid school made surprisingly
little impression on the scientific community. Duclaux and
carried out a fractional precipitation of cellulose nitrate solutions and found that the fractions differed
in their solution viscosities. This result would have been
clearly incomprehensible if cellulose were an aggregate of
small molecular species. In 1925 Hock12X1
found that when
a stretched rubber specimen was frozen in liquid air and
shattered, it broke u p into fibers-a result which certainly
suggested a fibrous structure for the constituent molecules.
During the same year Katz[291published his finding that
Hevea rubber, amorphous in the relaxed state, yields a
sharp X-ray diffraction diagram when stretched. In reporting this sensational observation, he wrote that “molecules
or parts thereof’ are being oriented, but although he was
unhappy with the notion that the small unit cell signifies
that rubber is composed of small molecules, he did not
dare to conclude that it is composed of molecular chains.
In 1926, Sponsler and Dord3” published their interpretation of the X-ray diffraction by cellulose. It is important to
note that they started with the observation that “a satisfactory formula for cellulose must account for its physical
properties.” Thus, they were clearly predisposed to reject
the notion that cellulose represents an aggregate of small
molecules o r that “lattice forces are here comparable in
their magnitude and nature to the valence forces: The
whole crystallite appears as a large m o l e c ~ l e . ” [ ~They
proposed a chain structure on the basis of rather limited
data, but their result had the fateful flaw that it contra-
dicted solid chemical evidence that cellulose is composed
of cellobiose residues. This tended to reinforce the skepticism of organic chemists regarding the utility of the crystallographic method.
One of Staudinger’s most fruitful ideas was the concept
of model chain molecules, analogous to natural macromolecules but offering experimental advantages for their
study. He reasoned that since studies of Hevea rubber are
complicated by the chemical sensitivity due to its unsaturation, polystyrene, an amorphous synthetic polymer, would
be a suitable analogue. In the case of cellulose, a highly
crystalline material, he proposed to study the polymers of
formaldehyde, since here the properties of a continuous
series of oligomers could be compared with those of the
high polymer. The results of this study showed quite
unambiguously that the polymer is built u p on the same
principle as the oligomers which could be characterized by
standard chemical
It contained another
most important generalization: The ends of long-chain
molecules are randomly distributed in the crystallites, so
that X-ray diagrams cannot be used for an estimation of
molecular weights.
The paper on formaldehyde polymers by Staudinger and
his students may be said to have provided for the first time
incontrovertible evidence on the chain structure of a high
polymer. The crystallographic method was soon to be applied successfully by Meyer and Mark to cellulose, silk fibroin, chitin, and Hevea rubber.‘331In the case of cellulose,
a correction of the interpretation by Sponsler and Dore
showed that the X-ray data were fully compatible with the
chemical evidence that the chains are composed of repeating cellobiose units. In the case of rubber, the crystal structure was shown to indicate a cis-1,4 enchainment of isoprene residues, contrary to the trans structure championed
by Staudinger. It is remarkable that these papers were submitted from an industrial laboratory which was sufficiently
farsighted to realize that such structural studies would
form the basis for the development of successful artificial
At about the same time, Staudinger proposed a method
by which the molecular weight of polymers could be deduced from their solution viscosity.[341Although his theory
assumed a rigid rodlike structure for chain molecules and
was incorrect even on this basis, the use of the parameter
that became known as the “Staudinger index” provided
for the first time a cheap and convenient method for the
characterization of the molecular size of polymers. In that
way it advanced polymer science even though it led to incorrect values for molecular weights.
However, Staudinger’s solution viscosity theory had also
an unfortunate side effect in that it contributed to the bad
relations between the Staudinger school and the polymer
chemists emphasizing physico-chemical techniques, particularly Meyer, Mark, and Kuhn. These men listed incontrovertible arguments for the flexibility of chain molecules,
but Staudinger and his students chose to disregard them. It
is hard for us today to understand that even phenomena
such as rubber elasticity, the swelling of cross-linked polymer gels, and the dramatic increase in the viscosity of
poly(acry1ic acid) solutions when they are neutralized were
not considered by Staudinger as incompatible with the
model of rigid rodlike polymer chain molecules. On the
other hand, two of Staudinger’s arguments must have
seemed convincing:[351( 1 ) He felt that the easy crystallization of chain molecules could not be understood if randomly coiled chains had to assume extended shapes in the
crystal. (2) He believed that the small volume change when
crystalline polymers melt implied a packing efficiency in
the melt which could not be achieved with randomly
coiled chains. My conversations with Staudinger’s widow
have indicated to me that there was also possibly a deeper
psychological cause for these controversies: Staudinger
seems to have been haunted by the conviction that physical chemists looked down on organic chemistry and that
they expected, in particular, physical chemistry to provide
all significant progress in the understanding of polymers.
Since he believed that the study of macromolecules was
opening a vast new area of activity for organic chemistry,
he reacted to the perceived slight with passionate emotion.
8. Macromolecular Chemistry and Molecular
Biology -Little Interaction despite Common
Finally, I should like to comment briefly on the relation
between the development of synthetic macromolecular
science and the rise of molecular biology. The idea that
very large molecular structures are involved in the life
process was expressed already by Kekule in his inaugural
address at the University of Bonn, referred to above.[21He
cited P’iiger as believing that “these mass molecules ...
through constant change of position of polyvalent atoms
change the connection between individual molecules so
that the whole ... is in a sort of living state.” Later, colloid
chemists were fascinated by the idea that their field played
a central role in explaining the life process. This may be
exemplified by P ~ u l i [ ~writing
that “the colloid chemistry
of proteins provides an approach to the obscure region
which scientists contemplate today only with the silent
longing for the promised land-the physical chemistry of
living matter.” Staudinger expressed a similar sentiment
when he wrote in his memoir that he had chosen as a
young man “to study chemistry in order to enter more
readily into the problems of botany” and he concluded his
Nobel Prize lecture in 1953 with some speculations concerning macromolecules and life.[23’
Clearly, a number of experimental techniques are
equally applicable to the study of synthetic polymers and
the investigation of proteins and nucleic acids with which
molecular biology is concerned. Let us see then how the
developments of the macromolecular concept in these two
fields compared with each other.
For a long time, protein chemistry was just as influenced
by the skepticism concerning the possible existence of very
large covalently bonded molecules as the chemistry of synthetic polymers-particularly since the great Emil Fischerl3’]had expressed his doubt of the existence of proteins
with molecular weights exceeding 4000. It was only when
Svedberg’s ultracentrifuge study of hemoglobin showed
that it consisted of particles with a discrete molecular
weight, that he abandoned his preconception that proteins
Angew. Chem. Ini. Ed. Engl. 26 (1987) 93-97
are aggregates of small molecules with a variable particle
size, such as he had observed with inorganic colloidal dispersions.‘3x1Yet, Svedberg told his student, B. G . Rinby
that he was completely unaware of Staudinger’s existence
at the time he developed the ultracentrifuge. The study of
globular proteins, which have a uniquely defined sequence
of a relatively large number of comonomers and a unique
conformation in their “native state” and which exhibit
denaturation phenomena, raised obviously different problems compared with polystyrene or rubber. Crystallographic data on synthetic polypeptides were useful in confirming the structure of the ~ t - h e l i x las~ ~one
~ of the structural
elements of proteins, and studies of helix-coil transitions
in polypeptide solutions1401continue to be motivated by the
hope that they might lead to a full understanding of the
principles governing chain folding in globular proteins.
The utility of this approach is yet to be proved.
Nucleic acids were assigned for a long time a tetranucleotide structure.1411
The realization of their high molecular weight was probably delayed owing to difficulties in
their isolation, which led to various degrees of degradation. This made Feulgen1421
refer in 1935 to the “bad reputation” of DNA. He showed that an enzyme can convert a
gelling preparation “a” into a non-gelling product “b.”
Study of these two materials in the ultracentrifuge led
Schmidt and L e ~ e n e Ito~ ~conclude
that the preparation
“a” with a molecular weight of 200000-1 000000 was depolymerized by Feulgen’s enzyme to the tetranucleotide.
But what did they mean by depolymerization? As late as
1938 they referredla] to the “dissociation of the tetranucieotides of high molecular weight to those of lower molecular weight,” suggesting that they viewed the heavy particles as loose aggregates. Only the flow birefringence
study of Signer, Caspersson, and H a r n m a r ~ t e n ,pub~~~~
lished during the same year, established the rodlike nature
of the DNA particle.
We can see from the examples detailed above that the
interaction between polymer chemists and the practitioners of molecular biology was minimal. Most of the time
they went their separate ways with little awareness of each
Received: June 26, 1986 [A 605 IE]
German version: Angew. Chem 99 (1987) 95
Anyew. Chem. I n t . Ed. Engl. 26 (1987) 93-97
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[4] M. Faraday, Quart. J . Sci.Arts 21 (1826) 19.
[5] T. Martin (Ed.): Faraday Diary. Bell, London 1932.
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(1833) 63.
[ l l ] W. H. Carothers, J. Am. Chem. SOC.5 / (1929) 2548.
[I21 M. Berthelot: Leconsde chimieprofessees en 1864 et 1865, Societe Chimique d e Paris, Paris 1866, pp. 18-65 and 148-167.
[13] A. V. LourenGo, Ann. Chim. Phys. (3) 67 (1863) 257.
[I41 C. A. Bischoff, A. von Hedenstrom, Ber. Dtsch. Chem. Ges. 35 (1902)
[l5] R. .4nschiitz, Justus Liebigs Ann. Chem. 273 (1893) 73: R. Anschiitz, G.
Schroter, ibid. 273 (1893) 97.
1161 D. Vorlander, Justus Liebigs Ann. Chem. 280 (1894) 267.
[I71 M. Polanyi, Naturwissenschaften 9 (1921) 288.
[IS] M. Polanyi in P. P. Ewald (Ed.): Fyty years ofX-ray diffraction. N.V.A.
Oosthoek Uitgeversmaatshapij, Utrecht 1962, p. 631.
1191 P. Karrer, C. Nageli, Helu. Chim. Acta 4 (1921) 263.
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1211 H. Staudinger, Ber. Dtsch. Chem. Ges. 53 (1920) 1073.
[22] S. S. Pickles, J . Chem. SOC.97 (1910) 1085.
[23] H. Staudinger: Arbeifserinnerungen, Hiithig, Heidelberg 1961.
[24] C. Engler, Ber. Dtsch. Chem. Ges. 30 (1897) 2358.
[25] C. Engler, J. Weissberg: Kritische Studien iiber die Vorgange der Autoxydation, Vieweg, Braunschweig 1904, p. 179.
[26] L. Lautenschlager, Dissertation. Technische Hochschule Karlsruhe
[27] J. Duclaux, E. Wollman, Bull. Soc. Chim. Fr. 27 (1920) 414.
[28] L. Hock, Z . Elektrochem. 31 (1925) 404.
I291 J. R. Katz, Kolloid-Z. 36 (1925) 300; 37 (1925) 19.
[30] 0. L. Sponsler, W. H. Dore, Colloid Symp. Monogr. 4 (1926) 174.
[31] H. Mark, Ber. Dtsch. Chem. Ges. 59 (1926) 2982.
[32] H. Staudinger, H. Johner, H. Signer, G. Mie, J. Hengstenberg, Z . Phys.
Chem. Stochiom. Verwandtschafsl. 126 (1927) 425.
[33] K. H. Meyer, H. Mark, Ber. Dtsch. Chem. Ge.7. 61 (1928) 593, 1932, 1936,
[34] H. Staudinger, W. Heuer, Ber. Dtsch. Chem. Ges. 63 (I93 I ) 222.
[35] H. Staudinger, Z . Elektrochem. 40 (1934) 434.
[36] W. Pauli, Kolloid-Z. 3 (1908) 2.
[37] E. Fischer, Ber. Dtsch. Chem. Ges. 40 (1907) 1754.
[38] T. Svedberg, Kolloid-Z. 51 (1930) 10.
[39] L. Pauling, R. B. Corey, Proc. Natl. Acad. Sci. USA 37 (1951) 235,
[40] M. Sueki, S. Lee, S. P. Powers, J. B. Denton, Y . Konishi, H. A. Scheraga,
Macromolecules 17 (1984) 148.
[41] P. A. Levene, J . Biol. Chem. 40 (1919) 415; 48 (1921) 119.
[42] R. Feulgen, Hoppe-Seyler’s Z . Physiol. Chem. 227 (1935) 261
[43] G. Schmidt, P. A. Levene, Science 88 (1938) 172.
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[45] R. Signer, T. Caspersson, E. Hammarsten, Nuture (London) 141 (1938)
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