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CHEMISTRY AND THE ORIGIN OF LIFE Academician A. I. Oparin Director, Bakh Institute of Biochemistry Academy of Sciences of the USSR According to modern ideas the beginning of life on Earth is by no means the result of a lucky chance (as assumed until quite recently); it is a completely regular event and an integral component of the general evolutionary develop- ment of our planet. The basis of this event is the process of successive increase of the complexity of carbon compounds and of the polymolecular systems formed therefrom. The whole process, which has lasted for thousands of millions of years, can be subdivided into the following stages: 1. The first appearance of hydrocarbons, cyanides and their nearest derivatives in cosmic space and after the planet Earth was formed, during the formation of the Earth’s crust, atmosphere and hydrosphere.2. The conversion, on the Earth’s surface, of the initial carbon compounds to increasingly more complex organic substances-monomers and polymers. The emergence of the so-called ‘primordial broth’. 3. Generation, in this broth, of the open polymolecular systems capable of interacting with the ambient external medium, and thus of growing and reproducing themselves (formation of the so-called ‘protobionts’). 4. Further evolution of protobionts, the improvement of their metabolism and of the molecular and supramolecular structure on the basis of pre- biological selection. Emergence of primordial organisms. It is readily seen that the first two stages referred to above are of a clearly pronounced chemical nature.Their knowledge is based completely on the study of physical and chemical phenomena wholly unrelated to life and of a purely abiogenetic nature. However, during the later stages the objects undergoing evolution attain a very high complexity of organization, and their subsequent development begins to be determined by regularities of biological nature. Consequently at a certain stage of the development of matter towards the origin of life there takes place the transition from chemical to biological evolution, the latter being based on the interaction between organism and medium and on the Darwinian principle of natural selection. Life is material in its nature, but inherent in it are peculiar qualities which distinguish the organisms from the objects of the inorganic world.In the first place there is the exceptionally perfect adaptability or, as it is often called, ‘expediency’ of the entire organization of living creatures, which is directed towards their constant self-preservation and self-reproduction under the given conditions of environment, and the adaptation of the structure of the individual parts of the living organism (molecules, organoids, cells, tissues and organs) to the functions they perform in the process of life. This expe- diency is an obligatory property of any living organism that does not exist outside the world of living creatures under the natural conditions of inorganic Oparin 1 nature. For a long time its essence seemed to be mystical and supernatural.It was considered to be a purposeful fulfilment, by the living creatures, of the designs of a spiritual principle which governs life. Darwin supplied a rational explanation of the formation of this ‘expediency’ on the basis of natural selection, which is a specifically biological regularity. However Darwin’s teaching applied in principle only to the fully formed and relatively highly developed living creatures. At present we consider that Darwinism is the resplendent top of an iceberg, almost nine-tenths of which is hidden under water. The pre-biological and the initial biological evolution, during which the fundamental features and properties characteristic of every living thing were evolved, had lasted for a much longer period of time and had abounded in no less dramatic events than the evolution of the objects usually investigated by Darwinists.The emergence of substances which are essential to this initial pre-biological evolution, viz. of hydrocarbons, cyanides and their nearest derivatives, took place many thousands of millions years ago, a long time prior to the forma- tion of the solar system. Formation of these materials was caused both by the exceptional predominance of hydrogen in the universe and by the fact that carbon was formed before the heavy elements, that are indispensable to the emergence of planetary systems, in the stable process of stellar emission of radiation. Carbon is detected in the spectra of all star classes, inchrding the most ancient generations, the age of which is reckoned as 15-20 x 109 years. For this reason carbon-hydrogen compounds are extremely abundant in the universe; they occur both on the surface of stars with a temperature of several thousand degrees and with a very high gravitation, and in the inter- stellar gas-dust matter, at an extremely low gravitation and a temperature near absolute zero.This can be confirmed both by studying the present clouds of interstellar matter themselves, and by investigating the spectra of comets, cosmic bodies which form under conditions closely resembling these of interstellar space. These investigations demonstrate that comets abound in hydrocarbons and cyanogen. Of special interest are the data obtained from the investigation of meteorites, firstly because they are the only non-terrestrial bodies that can be directly analysed at present, and secondly because, with respect to their composition, they are very similar to the clusters of cosmic-dust matter-the planetesi- mals-from which Earth and the terrestrial-type planets were formed.In the composition of some meteorites, the so-called ‘carbonaceous chondrites’, not only were the initial simplest compounds of carbon and hydrogen detected, but also their much more complex derivatives, diverse organic substances, which arose here abiogenetically, independent of life. Along with the high molecular weight hydrocarbon polymers, which are sometimes very similar to animal or vegetable fats, carbonaceous chondrites also contain substances characteristic of the animal world such as amino acids and the nucleotides.Consequently, not only prior to the emergence of life, but even a long time before the formation of the Earth, the second stage of the evolution of carbon compounds began-their conversion to increasingly more complex organic substances. These conversions occurred abiogenetically on the surface of R.I.C. Reviews 2 cosmic-dust particles and planetesimals as a result of the action of short- wavelength ultraviolet radiation and cosmic rays. Thus the Earth obtained a certain quantity of these substances in a finished form during the process of its formation as a planet, and later it was ‘nourished’ with these substances when meteorites and comet material fell on its surface.Nevertheless the bulk of organic substances, indispensable for the emer- gence of life, seem to have appeared on Earth endogenetically, when the Earth’s crust, the hydrosphere and the secondary atmosphere were formed. According to current thinking, our planet was formed by the accumulation of cold solids (planetesimals). Gases such as molecular hydrogen, helium etc. could be preserved only when adsorbed by solid rocks, where they formed part of the primary Earth atmosphere. However, that atmosphere was not long lived, because its component gases were not retained by terrestrial gravitation. The remaining dense mass of the Earth continued its evolution, which was principally determined by the thermal history of the planet.Owing to the heat produced by gravitational energy and by the energy of decay of the radioactive elements, the primary rocks were partly melted, and the terrestrial crust, hydrosphere and atmosphere formed. Most of the water was originally bound in hydrated rocks. Consequently the amount of water on the Earth’s surface was originally much smaller than it is today, and the formation of the oceans took place only very gradually, in conjunction with the formation of the Earth’s crust. The generation of the secondary terrestrial atmosphere was also closely related to crust formation. The secondary atmosphere differed in essence from the present-day atmos- phere. It was reducing, lacked free oxygen (02) and, in addition to water vapour, contained such hydrogen compounds as gaseous hydrocarbons, ammonia and hydrogen sulphide.Owing to the absence of molecular oxygen no ozone shield could form so the atmosphere was completely permeable to short-wavelength ultraviolet rays. Having reproduced the conditions of the surface at that time on the laboratory scale, numerous investigators in various countries demonstrated convincingly the inescapability of synthesis, in the secondary atmosphere and in the Earth’s hydrosphere, of various organic substances-amino acids, sugars, purine and pyrimidine bases, nucleotides, organic acids, and various polymers, including protein-like and nuclein-like substances. These, however, lacked any adaptability of their intramolecular structure to the fulfilment of biological functions, a characteristic of the present-day proteins and nucleic acids.One can, at present, imagine and reproduce to a certain extent the sequence of emergence and the nature of transformation of the complex organic substances referred to above in the waters of the Earth’s primary hydrosphere, the so-called primordial broth. Of course the nature of these transformations differed in essence from the highly organized metabolism which takes place in present-day living organisms. In general outline it only corresponded to the sequence of phenomena which occur in a simple aqueous solution of organic substances, and of course one would not find any of the ‘expediency’ of phenomena characteristic of life.However, life is not simply dispersed in space like the substances of the Oparin 3 primordial broth. Life is represented by organisms-discrete systems which are spatially isolated from the ambient external medium and yet interact with this medium as open systems. The stability of such systems, the duration of their existence, is determined not by their immutability-quiescence-but, on the contrary, by the constant transformation of substances, by the regular combination of synthesis and decomposition, which form, in their totality, the biological metabolism in living organisms. As has already been said, the characteristic feature of biological metabolism is its purposefulness with respect to the constant self-preservation and self- reproduction of the entire living system as a whole under the given conditions of the ambient medium.This feature could not have arisen by chance. It could have been formed only in the process of gradual perfection of the initial polymolecular open systems, which were more primitive than the organisms. One can not only imagine, but also reproduce in an experiment a large number of such systems (bubbles of Goldacre, microspheres of Fox, coacer- vates of Bundenberg de Jong, and many others). For the further evolution of these systems it was of importance that they should interact with the ambient external solution as open systems, and that their stability should not be of a static, but of a dynamic steady-state nature. From this viewpoint the coacer- vate drops appear to be the most convenient, but of course not the only models possible for the reproduction of phenomena which took place in the remote past.In the formation of the coacervate drops molecules of various polymers which were previously distributed uniformly throughout the homogeneous solution begin to accumulate at definite points to form entire molecular swarms and clusters, separating out of the ambient medium in the form of drops visible under the microscope and ‘swimming’ in the original solution from which, however, they are now separated by a sharp boundary-the surface. In the drops the concentration of polymers may be 50 per cent or more, whilst the ambient medium is almost free of them. Experiments conducted at our laboratory in the Bakh Institute of Bio- chemistry demonstrated that the coacervate drops form when one mixes aqueous solutions of even non-specific or completely homopolymeric poly- peptides and polynucleotides (for instance, polyadenine and polylysine).The intramolecular structure of these polymers, which is of such great importance to present-day organisms, is of no significance in the formation of coacer- vates. Only the size of molecules is essential. Therefore, during the simul- taneous synthesis of polymers of a disordered structure (which should also occur in the ‘primordial broth’) coacervate drops emerge without fail as soon as a definite degree of polymerization of the substances referred to above is attained. The coacervate drops are capable of absorbing selectively from the ambient broth various low-molecular weight substances-amino acids, sugars, mononucleotides, diverse salts etc.If only some of these substances are capable of accelerating catalytically the chemical reactions which take place in the drops, the drops become open systems which react specifically with the external medium. In our model experiments, by incorporating into the drops the most diverse simple and complex catalysts (organic substances and inor- 4 R. I . C. Reviews ganic salts), we induced in the coacervate drops the reactions of synthesis and of decomposition of the component polymers. As an example I should like to describe the layout of one of our experiments. In this diagram the rectangular outline represents the coacervate drop which contains a catalyst that converts adenine to a homopolymer (Poly-A).The source of adenine is adenosine diphosphate (ADP). ADP enters the coacer- vate drop and is polymerized here to Poly-A, causing the drop to increase in volume and weight-it grows before our eyes-while inorganic phosphorus (Pi) separates out into the ambient medium, which previously had contained no such phosphorus. We also produced in the coacervates more complicated systems of meta- bolite flow, in which not one, but several reactions were combined. According to the combination, one obtained a more rapid or a relatively slow growth of the drops and, in other cases, their decomposition and disappearance. Open polymolecular systems endowed with a primitive metabolism and resembling our models should have appeared readily in the Earth’s primordial broth.While increasing their volume and weight, such systems (let us call them ‘protobionts’) should, under the conditions prevailing in the primordial broth, grow and then break up as a ksult of external mech’anical forces (e.g. surf or wave shock), just as the drops of an emulsion break up upon shaking. The daughter protobionts which emerged in this process would have retained to a certain extent the original protobiont’s interaction with the ambient medium, absorbing all the time certain catalysts from this medium and thus preserving the constant rate ratio and constant concordance of reactions occurring in them.Of course such constancy was highly imperfect compared with the self-reproducibility of present-day organisms. Upon this basis the ‘competition’ of the protobionts with respect to the rate of growth and reproduction could have originated, followed by the peculiar ‘pre- biological selection’ during which only the protobionts, which became increasingly more adapted to the conditions of the ambient medium, were preserved and kept on growing. In the model experiments with coacervates we demonstrated the possibility of ‘selection’ of this kind. For this purpose we incorporated into some drops a complex of catalysts which, under the given conditions of the ambient medium, led to a relatively rapid synthesis of polymers and to the growth of the entire system as a whole.On the contrary, in other drops this complex of catalysts was less perfect. It can be seen in Fig. 1 that the drops of the first kind grow rapidly, whereas the other kind show a suppressed growth. Thus, even at the relatively early stage of evolution which we have been examining there arose a new, previously absent, regularity which determined completely the trend of development of protobionts and of the subsequent biological systems. We can, to a certain extent, form the idea of the subsequent stages of this evolution on the basis of the comparative biochemical study of metabolism and of structures in the most primitive contemporary organisms. Oparin 5 2.0 - 1.8 - 1.6 - - 1.4 / Time (min) Fig. I.In the evolutionary development of the protobionts, their catalytic appara- tus was the most important factor of the organization of metabolism, based on the rate ratio of the component reactions. Of course, at the analysed stage of evolution the catalysts available to the protobionts could only have been the inorganic salts and organic substances present in the primordial broth, the catalytic activity of which is very low. However, given suitable mutual combination, this activity can be enhanced by a factor of many hundreds and thousands. We can imagine a colossal number of various atomic groupings and their combinations, which to some extent were endowed with the ability of catalys- ing the reactions indispensable for the existence of protobionts.However, as a result of the continual rejection by natural selection of the less perfect com- plexes, only very few have survived till now, viz. the co-enzymes. Their number is relatively small, but they are extraordinarily universal bio-catalysts, which points to their formation very early in the process of the origin of life. The required constancy of concentration of the co-enzymes in the growing and reproducing protobionts could have been sustained by the entry of these 6 R. I. C. Re views compounds or their components from the ambient medium. We also find something similar in present-day organisms which are obliged to obtain from the ambient medium vitamins which play the role of co-enzymes in their metabolism. However, of necessity the protobionts must have gradually generated the ability to synthesize the co-enzymes by themselves.This freed the progressing systems from their too great dependence on the ambient medium. Yet the gradual complication of the metabolism of protobionts necessi- tated a very distinct combination of a larger number of reactions to form long chains and cycles, an entire coordinated network of biochemical reac- tions. For a coordination of this kind the catalytic activity and specificity of co-enzymes was no longer sufficient and, therefore, in the process of subse- quent evolution, the co-enzymes were supplemented by an entire arsenal of much more powerful catalysts-enzymes, i.e. proteins whose secondary and tertiary structures are highly adapted to the functions they are to fulfil.Thus the era of co-enzymes gave place to a new era in which the decisive role was to be played by protein substances with intramolecular organization. The initially produced protein-like polymers with their random arrangement of the amino-acid residues could serve as the material for the formation of the coacervate drops and protobionts, but they were either very poor or not catalysts at all. Of course, during the polymerization of the amino acids, there could also form in the protobionts combinations of functional groups able to play the role of enzymes. Hdvever, in disordered polymerization this advantage was rapidly lost in the growing protobiont. Therefore the emergence of an organization that would fix the constancy of the secondary structure of the newly-synthesized polymers was of great importance.In this organization an exceptionally important role fell to the polynucleotides. In present-day organisms the synthesis of enzyme proteins is effected by means of a highly complex and perfect mechanism, with the aid of which the amino acids are consecutively ‘threaded’ onto the polypeptide chain, viz. precisely in the sequence required by the specific, strictly regular combination of the mononucleotide groups in the DNA and RNA molecules. Of course, a mechanism of this kind could arise only in the process of a prolonged evolution of the protobionts and living systems, but even at the much earlier stages of development the polynucleotides in the protobionts could have had an effect on the polymerization of amino acids which took place in these systems.The intramolecular structure of the primary polynucleotides them- selves was quite imperfect; during the growth process of protobionts it underwent considerable variations. Each variant thus produced could be fixed to a certain extent in the particular growing system owing to the com- plementary nature of polynucleotides; at the same time it could influence the order of arrangement of the amino-acid residues incorporated in the poly- peptide system. Oparin If the combination of amino-acid residues thus generated was convenient from the point of view of increased catalytic activity of the polypeptides, the system which gave rise to this combination obtained preference in its more rapid growth and reproduction.Otherwise it was destroyed by natural selection. In this way the intramolecular structure of the protein-like poly- 7 peptides, and at the same time of the polynucleotides which took part in their synthesis, became increasingly more ordered and more adapted to the functions which these polymers performed in the growing and reproducing systems. Nevertheless, it should be clearly understood that selection was applied not to a particular polynucleotide capable of replication, or to the polypeptides which arose under their influence and were already endowed with a certain sequence of the amino-acid residues, but to the entire systems, protobionts, with a primitive, but more or less perfect metabolism which did or did not correspond to the given conditions of existence. The role of the nucleic acids was that they fixed spatially the constancy of synthesis of catalytically convenient amino-acid combinations in the growing and reproducing systems, and served as a stabilizing factor in the process of their evolution.Thus, at a fairly late stage of evolution, a new era began; living systems rose to an unprecedentedly high level of exact self-reproduction, which is charac- teristic at present of the entire world of living creatures. Further development of living systems, and the perfection of their meta- bolism and of the supramolecular structures can be followed on the basis of more profound investigations in the field of comparative biochemistry. The data obtained here show clearly that some forms of organization of the metabolism, and some combinations of biochemical reactions made their appearance at the very beginning of life and are therefore found in all present- day organisms without exception, whereas others were formed considerably later as supplementary superstructures of the earlier metabolic mechanisms. At the outset, the only source of nourishment for the primary organisms was the organic substances of the primordial broth.Correspondingly, the ability to feed on organic substances is built into the very principle of life and is characteristic of all living creatures without exception. The absence of free oxygen in the secondary terrestrial atmosphere and in the hydrosphere caused the anaerobic nature of the energy exchange in primary organisms. Indeed, the data of comparative biochemistry show convincingly that anaerobic exchange is the basis of the energy of absolutely all present-day organisms, including the higher animals and plants capable of respiration.During the development of life the reserves of organic substances on the Earth’s surface, which were formed abiogenetically, gradually became exhausted because the development of life progressed faster than the genera- tion of these substances. This change in the conditions of existence brought to the forefront of development organisms capable, owing to the acquired ability to absorb light, of building organic substances from carbon dioxide in the atmosphere. It is true that, by analogy with what happened in the primordial broth, the coacervates, protobionts or primary organisms were able to some extent to synthesize organic substances under the conditions of reducing atmosphere by utilizing the energy of short-wavelength ultraviolet light.However, in the Earth’s atmosphere there was a very slow, but gradual formation of free oxygen by abiogenetic processes. This was accompanied by the formation of the ozone shield which barred the access of short-wavelength R.I.C. Reviews 8 ultraviolet rays to the surface. Because of this, the process of selection was necessarily associated with a transition from the utilization of the short- wavelength radiation by primary organisms to the utilization of the long- wavelength light which is shed so abundantly onto our planet by the sun.This transition encountered a major hindrance, in that the individual quan- tum of visible light carried a relatively small amount of energy. For this reason, it was necessary to use photosensitizers in order to accomplish biologically important photochemical reactions. On the strength of data from comparative biochemistry and from model experiments it is possible to follow the genesis of such photosensitizers- porphyrins and their magnesium derivatives-and their incorporation into photosynthetic systems. The paths of successive perfection of these systems passed, on the one hand, through the selection of increasingly more effective pigments (porphyrins) and, on the other, through the greater complexity and functional adaptation of the supramolecular structure of the photosynthetic apparatus.Thus photosynthesis originated-a new, highly perfect method of synthesis of organic compounds, which replaced the previous, very slow and imperfect abiogenetic synthesis. Consequently, in the further development of life, photosynthesis acquired a predominant, monopolistic significance in the formation of organic substances on the surface of the Earth. Its beginning changed the entire set of conditions for life. Some organisms began to build for themselves indispensable organic substances, whereas others preserved the previous heterotrophic forms of nutrition, using organic substances now formed biogenetically by photosynthesis.Thus, two branches of the living world were formed : plants and animals. However, the origin of photosynthesis not only created an abundance of organic substances, but also resulted in the rapid formation of free oxygen in terrestrial atmosphere. This changed the entire nature of chemical processes on the Earth, and enabled most of the living creatures to rationalize con- siderably their energy exchange : by adding to the previous anaerobic mecha- nism the superstructure of the new supplementary systems of oxygen breath- ing, it became possible to utilize completely the energy hidden in organic substances. Along with the perfection of metabolism, an evolution in the spatial organization of living organisms also took place.Its origin and perfection were closely related to the evolutionary development of the functions performed by structures. Anaerobic fermentation is possible in homogeneous solution, while photosynthesis and respiration require very complex structures. Apparently the most primitive structural formations were protein-lipid membranes. These can be detected as early as the stage of formation of coacervates, and they are found in all living creatures without exception. However, the formation of structures such as chloroplasts, mitochondria and cell nuclei was accomplished only during the gradual evolution of living systems. Oparin Thus we see that, for a very long time, the evolution of primitive organisms was in principle related to the perfection of metabolism and of intramolecular and subcellular structure.The emergence of the cell, which is usually con- sidered to be the most primary indivisible element of life, required immense 9 time intervals and a sequence of untold generations of pre-cellular living creatures. Some modern authors consider it even possible that, during the initial periods of the existence of life, the individual structural formations developed as independent protobionts or primitive organisms (‘organoids’), and only later combined to produce the intricate biological complex which we find in the cell. With the emergence of unicellular and, later, multicellular organisms it became possible to study the evolution of life by palaeontological methods, on the basis of investigations of fossil remains with a definitely biological structure.We have seen, however, that the initial steps of biological evolution were, on the whole, related to the perfection of metabolism and intramolecular structure, and this could not easily leave direct traces in the palaeontological record. Considerably more information about this period can be gained on the strength of the data obtained from comparative biochemistry. Similarly the possibility of preservation until now of the initially generated pre-biological and biological structures, which had the nature of colloidal clusters of organic polymers, hardly exists. Only in exceptionally rare cases were structures detected in the most ancient deposits which might have been of biological origin (but which could have been artefacts resembling the ‘organized corpuscles’ found in carbonaceous chondrites). Consequently, one should use great caution when dealing with such very old findings.An example of such findings is the calcareous secretions detected in South- ern Rhodesia, whose age is estimated at 2.7 x 109 years. They are often called ‘algal limestones’ and are considered to be the most ancient manifesta- tion of life. However, we lack genuine fossils with the structurally-preserved remains of early organisms; what we find here is a lamellar structure of limestone, which of course cannot easily be treated as a purely inorganic formation. At the same time, the producers of the lime secretions of Rhodesia might have lacked a biological structure; they could have been our hypotheti- cal protobionts or even clusters of the organic substances of the primordial broth.The oldest genuine fossils are considered nowadays to be the remains of organisms preserved in the iron ores of South Ontario (age, 1.6 x 109 years). However, even in this case it is difficult to identify these organisms and the type of their metabolism. Much more certain and well-defined are the remains of organisms which lived during the later periods of the Proterozoic era. We do not find there the resplendent diversity characteristic of the present-day flora and fauna. Life in the course of that era was on the whole represented by unicellular (or even pre-cellular) bacteria and algae, and only at the boundary of the Cambrian system do multicellular plants and animals begin to appear.One can assume, therefore, that the evolution of living creatures took place during this era at a much slower pace than during the post-Cambrian periods when increasingly more perfect organisms rapidly succeeded one another. Figure 2, which is of course only highly approximate, takes in at a glance the entire path of evolution referred to before. In this figure the vertical line plots the time (in thousands of millions of years) from the present to the event in R.I.C. Reviews 10 I Emergence of aerobes I Beginning of photosynthesis anaerobes Era of the beginning of enzymes and of the nucleic acid code 1 Era of co-enzymes Coace rvates I question.To the right of the line are marked the fundamental landmarks of the evolution of our planet; on the left, data are given on the evolution of carbon compounds during the emergence and development of life. Analysis of the diagram enables us, first of all, to realize the immensity of our ignorance and the vast horizons which open up before us for the future. On the basis of data from comparative morphology and palaeontology, we have at present a fairly good idea of the trend of biological evolution from the late pre-Cambrian until now. But this is only a small section of the dia- 11 5 Fig. 2. Oparin - - Beginning of the Cambrian period Formation of the oxygen- containing atmosphere of Stromatoliths present-day composition Ontario fossils A End of the reducing and beginning of the transitional atmosphere - Rhodesian limestone secretions Formation of global ocean completed Formation of secondary reducing atmosphere 8 4 Formation of the Earth's crust Formation of the Earth with present-day mass Beginning of solar system gram.On the basis of astronomical, geological and chemical data we can picture the first stages of the evolution of carbon compounds. However, between this stage and the topmost section of the diagram there is an immense period of evolution which lasted for thousands of millions of years. In the course of this expanse of time the fundamental and essential changes in the organization of systems which gave rise to living creatures were slowly achieved.We can only advance hypotheses with respect to the sequence and, also to some extent, the nature of these changes. Yet we are still unable to indicate the exact time when life emerged. The difference between the inorganic world and the world of living creatures, which is so easy to establish today, came into being only because all of the intermediate organizational forms had been destroyed by natural selection. But the evolutionary emergence and the subsequent development of life passed through a number of intermediate stages; therefore, the question posed can be answered only by deciding which of these stages we consider to represent the beginning of life : the emergence of protobionts endowed already with a perfect metabolism, the emergence of proteins and of the nucleic code or, finally, the emergence of the cell. Scientific knowledge of the periods of the formation and organizational perfection of life, which are not very clear to us, depends in the first place on the future powerful development of evolutionary biochemistry, biophysics, cytology and physiology. Even before Darwin a tremendous amount of morphological material had been accumulated in biology ; this acquired its scientific importance after having been generalized by means of the single idea of evolutionary development. By analogy, there is today in biology, thanks to the application of physical and chemical methods, a rapid accumu- lation of information about the organization of metabolism and the structure of the vitally important molecules, membranes and organoids in living organisms which stand at various levels of evolutionary development. This material will attain its own new, exceptionally high significance for the know- ledge of the essence of life only when we have united and systematized it on the basis of evolutionary theory. Only by using such an evolutionary approach shall we be able to recognize not only what occurs in the living creatures and how it occurs, but also to answer these ‘hundred thousand whys’ which arise unavoidably in the path of the true study of the essence of life. In particular, there is the question of why the entire organization of every living thing, from the molecular to the organizational level appears so ‘expedient’ and so adapted to constant self- preservation and self-reproduction under the extant conditions of the ambient medium. R. I. C. Reviews 12

ISSN:0035-8940    

DOI:10.1039/RR9690200001

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

INORGANIC POLYMERS B. R. Currell, B.Sc., Ph.D., A.R.I.C. and M. J. Frazer, BSc., Ph.D., F.R.I.C. 14 . . . . . . . . . . . . . . . . . . . . 16 17 . . . . . . . . * . Homopolymers, 17 Heteropolymers containing halogen and pseudo-halogen, 2 1 Heteropolymers containing oxygen or sulphur, 2 1 Heteropolymers containing nitrogen or phosphorus, 33 Northern Polytechnic, Holloway , London, N. 7. What are inorganic polymers ? . . Properties of inorganic polymers Classification of inorganic polymers Conclusions Acknowledgements. . References . . . . . . . . . . .. . . . . 38 . . . . . . . . . . . . . . 38 . . . . . . . . . . . . . . . . 38 A review entitled ‘Inorganic Polymers’ could not have been written 20 years ago. This is not because all the materials now described as inorganic polymers are new, but rather that a new idea has developed-polymers do not have to be organic.The term inorganic polymer was first widely used in the 1950s when the demand for new materials, in particular low density, high temperature resis- tant materials for use in the aircraft and aerospace industry, led chemists to investigate the possibility of preparing inorganic analogues of the well known organic polymers. The demand for new materials led to the provision of re- sources both in industrial and academic laboratories for the intensive study of inorganic polymer systems. There has been great activity, and although this has not yet produced much in the way of commercially exploitable materials apart from the silicones and the inorganic whiskers and fibres, it should be remembered that it may take as long as 20 years to bring the discovery of a new compound in the laboratory to the stage where there is a market for tonnage quantities. The use of the term inorganic polymer is not now restricted to materials with plastic or elastomeric properties; the concept of the inorganic macro- molecule has wide application to many materials bringing together studies of plastics, rubbers, glasses and ceramics.As an illustration that all the research activity in this field has produced a vast amount of new chemistry, one can refer to text-b~oks,l-~ reports of conferences and symposia,5~6~7 and review articless-ll all dealing with inor- ganic polymers and all having appeared since 1958.There are also many reviews dealing with specific inorganic polymers (the more important of these are mentioned in this review) and hundreds of original research papers. This review sets out first to answer the question : ‘What are inorganic poly- mers?’ and then to describe some typical inorganic polymer systems emphas- CurrelI and Frazer 13 izing, wherever possible, materials which have actual or potential technological importance. WHAT ARE INORGANIC POLYMERS ? The question is best answered by glancing at Table 1 where some typical inorganic polymers are listed. Other materials of less definite composition such as cements and glasses can also be cited as examples of inorganic polymers. There is no one satisfactory definition of an inorganic polymer, and not every reader will agree that all the materials in Table 1 should be given this description.For the purposes of this review we define inorganic polymers as macromolecules which do not have a ‘backbone’ of carbon atoms. Metals and simple ionic salts are clearly excluded; but the siloxanes (-RzSiOfn, which have a backbone of oxygen and silicon atoms with pendant organic groups, are clearly included. Table 1. Some typical inorganic polymers* Antimony (111) oxide Boron nitride C hrysotile asbestos Lithium methoxide Molybdenum disulphide Muscovite (mica) Nickel cyanide Pal lad iu m chloride Plastic sulphur Polydichlorophosphazene Polydimethylsiloxane Pol y (N- p h en y Is i I azan e) Silica Silicon nitride Sodium metaphosphate Su I p h u r trioxide Sb203 BN 3Mg0, 2Si02, 2H20 LiOMe MoS2 KA I2(OH)2A I Si3010 Ni(CN)Z PdC12 * The formulae represent the overall composition of the polymer.The macromolecules are usually electrically neutral, but there are also polycations and polyanions, e.g. (Ti02+)n and Graham’s salt, (Na+)n(POj)n. Macromolecules may be chains, as in polydichlorophosphazene, (NPC12)n ; sheets as in muscovite, a mica, KA12(0H)2AlSi30lo ; and three-dimensional networks as in the orthorhombic forms of phosphorus pentoxide, (P4010)n. There are variations on these basic structures ; for example antimony(rI1) oxide, Sb203, has a structure based on two Sb-0 chains cross-linked by oxygen.Such a linear 3-connected system is analogous to an organic ‘ladder’ polymer such as ‘Black Orlon’ (the pyrolysis product of polyacrylonitrile). S ClzPN MezSiO HzSiP h N SiO2 Si3N4 Nap03 so3 Antimony (111) oxide 14 ‘Black Orlon‘ R. I.C. Reviews We use the word ‘macromolecule’ in our definition advisedly, and there are two implications. Firstly, macromolecule means that it must be assumed that the atoms are bound by covalent bonds. For organic polymers this is largely true. However, difficulty arises with compounds such as wurtzite, ZnS, which can be described either as an ionic structure having hexagonal close packing of sulphide ions with half the tetrahedral holes filled by zinc ions, or as a three-dimensional macromolecule similar to diamond with every zinc atom surrounded tetrahedrally by sulphur atoms and every sulphur surrounded tetrahedrally by zinc atoms.Many inorganic materials including ceramics such as A1203 and ZrOz are of this type and can be classi- fied, either as ionic crystals or as inorganic macromolecules. It could be argued that the word ‘polymer’ should not be applied to materials such as ceramics even if the atoms are considered as being joined by covalent bonds, as the regularity of the structure removes all the characteristic properties such as elasticity and high viscosity normally associated with polymers. Our definition does not refer to ‘polymer-like’ properties, nor to applicability as a plastic or rubber, nor to regularity of bonds, and so the question whether ceramics are inorganic polymers is left open depending upon the formal labels we ascribe to the bonds.The question of bond type in macromolecular crystals, and also the question of the relation between crystal structure and polymer structure (i.e. the re- lation between the crystallographic unit cell and the polymer repeating unit or ‘mer’) is discussed in an interesting and thoughtful paper by Living- ston.12 This paper deals with macromolecules and is not concerned whether they are organic or inorganic. This general approach was also taken by Holliday in his Jubilee Memorial Lecture of the Society of Chemical In- dustry,l3 and the relation between macromolecular structure and physical properties was demonstrated.The second implication of using the word ‘macromolecule’ in our defini- tion is that small ring compounds and ‘clusters’ of atoms are excluded. Thus tris(dichlorophosphazene), (NPC12)3 is no more a polymer of the unknown NPCl,, than cyclohexane, C,H,, is a polymer of ethylene, C,H,. This is not to say that a study of small rings is unimportant in inorganic polymer chem- istry. Indeed it has been very necessary to study such compounds as models. A great deal of significant synthetic, mechanistic, stereochemical, structural and theoretical chemistry has been discovered, but this will only be mentioned here when it is directly relevant to a macromolecular system. One of the major problems in inorganic polymer chemistry has been to achieve the formation of high polymers rather than small rings.This problem has frequently been overlooked in many approaches to the synthesis of inorganic macromolecules. Gee,14 Carmichael15 and Allcock4 have discussed ring-chain equilibria. A well-known generalization is that the formation of organic compounds is kinetically controlled, whereas the formation of inorganic compounds is thermodynamically controlled, and this is certainly true in the field of polymer chemistry. Thus organic chemists have been able to select the most appropriate synthetic method to obtain a polymer of desired structure and therefore properties. In inorganic chemistry, however, the same (or approximately the same) equilibrium mixture may often be Currell and Frazer 15 2 obtained whatever the synthetic route.Equilibrium control of inorganic systems has been discussed by Van Wazer.l0J6 The importance of small ring- long chain equilibria cannot be overemphasized: upon them depend the possibility of making polymers from cyclic oligomers, and conversely the possibility that polymers once made will degrade by cyclization. PROPERTIES OF INORGANIC POLYMERS The incorporation of atoms other than carbon into a polymer network has given rise to a wide range of useful glass and ceramic materials. These mater- ials are characterized by their rigidity and in some cases by a high thermal stability, for example refractory bricks may be made of silica or a combina- tion of silica and alumina. The search for inorganic analogues of organic polymeric materials was based on the hope that if inorganic polymers with essentially a chain rather than a network structure could be prepared, they would combine high thermal stability with desirable plastic or elastomeric properties.Macromolecules which do not have a carbon-carbon bond in the backbone can be expected to have a higher resistance to oxidative degradation. It is also possible that the more varied types of skeletal atom stereochemistry in inorganic polymers will lead to a wider range of properties such as solubility, molecular flexibility and so on. There are some inorganic polymer systems which show these desirable properties, but it will be seen from the account which follows that many otherwise promising systems have not led to useful materials because of the ready degradation into simple molecules or small rings.There are Table 2. Classification of inorganic polymers JVpe HOMOPOLYMERS (a) pure elements (b) homopolymers with pendant side groups (c) anionic homopolymers H ETE ROPO LY M E RS heteropolymers containing hahgen or pseudo- halogen atoms in the ‘backbone’ heteropolymers containing oxygen or sulphur atoms in the ‘backbone’ ( i ) oxides, hydroxides and oxyacids ( i i ) anhydrides ( i i i ) mixed anhydrides ( i v ) polyanions ( v ) polyal koxides ( v i ) heteropol ymers containing su I p hur heteropolymers containing nitrogen or phos- phorus atoms in the ‘backbone’ 16 Example silicon, plastic sulphur polyp henyl boron polydifluorosilicon calcium boride nickel( I I) cyan id e, pal lad iu m( I I) chloride, niobium(1v) iodide, anti- mon y (V) f I uo r i d e antimony(ll1) oxide, sulphur trioxide polydimethylsiloxane boron phosphate sodium metaphosphate, muscovite I it h iu m met hoxide si I icon su I p hide, molybdenum disulphide boron nitride, polydichlorophos- p h azene R.I.C. Reviews problems of hydrolytic stability, and also many of the compounds described in the next sections have highly cross-linked regular network structures which give rise to a rigidity and brittleness not associated with useful plastic or elastomeric materials. CLASSIFICATION OF INORGANIC POLYMERS There are many ways of classifying inorganic polymers: by ‘key’ element, by type of structure (e.g.linear, sheet, network), by properties, by type of bonding (e.g. electron deficient polymers, coordination polymers), and by synthetic method (e.g. addition polymers, condensation polymers). The simple method adopted in this review is shown in Table 2. Black phosphorus Homopo ly m em These are polymers having atoms of the same element throughout the back- bone. There are three sub-divisions : (i) pure elements; (ii) homopolymer chains or networks with pendant side groups; and (iii) anionic homopolymer chains or networks. Pure elements can be classified into three types depending upon structure : (a) metals, (b) simple molecules (e.g. Nz; SS rings; P4 clusters) and (c) macro- molecules or homopolymers.Examples of the last class are found in periods 3,4 and 5 of the main Groups IV, v and VI of the Periodic Table. In Group IV, silicon, germanium and grey tin crystallize with the diamond structure. Black phosphorus, which is the most stable form of the element, can now be prepared quite simply by heating white phosphorus to 220-370” for eight days with a catalyst (Hg/Cu) in the presence of a seed of black P. It has a layer structure, which is a puckered hexagonal net in which each phosphorus is joined to three others. Red phosphorus is an amorphous material and is a product of random bond cleavage and rearrangement of the P4 molecule. It retains variable amounts of various impurities such as the halogens. Krebsll has suggested that it may be regarded as a random polymer network with end groups.The ‘metallic’ grey forms of arsenic and antimony have layer structures similar to the structure of black phosphorus. Plastic sulphur consists of very long helical chains with ten sulphur atoms every three cycles of the helix.l7 Polymeric sulphides and polysulphides Currell and Frazer 17 ' I \ P / \ I P - B r have been used as stabilizers and maintain sulphur in a polymerized form for an indefinite time.18 Sulphur has good mechanical properties and sur- prisingly has been used as a building material.lg Rhombic and monoclinic sulphur contain s8 rings. These forms melt at 113" and 119" respectively, and when the temperature is increased above 160°, the rings break homolytically forming diradicals which can attack other s8 rings to form long chains.The factors controlling the ring-chain equilibrium have been discussed.14J9 At about 200" the viscosity of molten sulphur is highest and at this temperature the chains have lo6 atoms. The 'metallic' or grey forms of selenium and tell- urium (which is not polymorphic) are isostructural ; having infinite helical chains.20921 Two other elements-boron and carbon-can also be considered as homopolymers. There are several polymorphic forms of boron, all based on B12 icosahedral clusters linked in various ways.Z2 Diamond and graphite are well-known examples of network and sheet macromolecules and although strictly not inorganic polymers the recent, important development of carbon fibres must be mentioned.The controlled pyrolysis of fibres such as poly- acrylonitrile gives rise to fibres with a graphite-like structure. The Young's modulus of these fibres depends on the conditions of preparation, longer and hotter heat treatments giving a higher modulus up to about 70 x 106 lb in-2. Because longer fibres have a greater chance of having a serious flaw there is a strong dependence of tensile strength on length of fibre tested, i.e. carbon fibres have a strength averaging about 300000 lb in-2 over a 10 cm length, but this may increase to 500000-600000 lb in-2 over millimetre lengths.23 The high modulus of carbon fibres is due to the preferred orientation of graphite flakes parallel to the fibre axis. This is in contrast with, for example, polyethylene which has a Young's modulus of about 0.3 x lo6 lb in-2, but if a tensile test could be performed on a single polymer chain a modulus (called lattice modulus) of 36 x 106 lb would be obtained.This dis- crepancy is due to the folding of the chain in the bulk polymer so that the bulk value has no more connection with the lattice modulus than the appar- ent stiffness of a spring has with the Young's modulus of steel. These fibres may be incorporated in other materials to give reinforcement. Carbon fibre reinforced laminates are used in the turbine blades of the new Rolls Royce RB211 jet engine.24 18 R. I.C. Reviews Fibres and whiskers of other materials (e.g. alumina, silicon carbide and silicon nitride) are also used for reinforcement.23 Average tensile strengths of about 1000 000 lb in-2 have been obtained for alumina whiskers over mm lengths; over similar lengths, tensile strengths of 1 400 000 lb in-2 have been shown by silicon carbide whiskers and 800000 lb in-2 by silicon nitride whiskers. Whiskers are stronger but denser than carbon fibres (specific gravities are 1.8 for carbon, 3.2 for silicon carbide and 3.9 for alumina).The high strength of whiskers is mainly due to their smooth crack-free surfaces. Whiskers have the advantage that they can be incorporated into metals. The prices of whiskers and fibres are at the moment high and they are there- fore only used for very specialized applications. Increased production and improvements in the methods of production will enable the price of these products to be considerably reduced; and it is to be expected that uses for these materials will increase rapidly in the near future.Other homopolymers contain elements which are not part of the skeleton of the macromolecule. There are numerous examples and only a few can be mentioned here. Calcium boride, CaB6, and many other metallic borides of the type MB6 have structures based on B6 octahedral units. Each corner of the octahedron has a boron atom joined to another octahedron, so producing a network of B atoms. There are large holes in the network (Fig. 1) and these hold the calcium or other metal i0ns.~5 3 Fig. I . The structure of CaB6 showing the network based on B6 octahedra. The structures of many other borides are based on boron chains (CrB), ladders (Ta3B4), layers (AlB2) or networks (UB4, B4C and UBl2).The chem- istry of borides has been reviewed recently.26 Another example of a homopolymer in boron chemistry is the material of composition (PhB),.27 Phenylboron dichloride heated with sodium or potas- Currell and Frazer 19 -Hg- I I I I I -Hg- I I I -Hg- I -Hg- Type Fig. 2. Structures of heteropolymers containing halogen. Table 3. Some heteropolyrners containing halogen I Examples SbFs BeC12 I I CHAINS Single halogen bridging Double halogen bridging SHEETS N ETWO RKS sium under reflux in toluene or xylene gives a solid of composition (PhB)n with n approximately equal to 10. The factors preventing the formation of longer chains are not yet understood, but this approach is potentially impor- tant for forming simple chain inorganic macromolecules.Similar examples are: (i) (MezSi),, with n approximately equal to 55, prepared from dimethyl- dichlorosilane and sodium;28 (ii) (F2Si)n a rubbery solid obtained by the condensation of the gas SiF2 (made by the interaction of silicon tetrafluoride and silicon at 1150" and low pressures) at temperatures below -80°, the solid catches fire in moist air and on heatingag generates all the perfluoro- silanes from SiF4 to Si14F30; and (iii) (BC12)n which was stated to be un- attacked by boiling aqueous sodium hydroxide and was prepared by y-irradia- tion of a boron trichloride-hydrogen mixture.30 Schmidt has made extensive studies of polysulphur systems :31 basically these are sulphur chains with 'end-stoppers' such as -X, -H and -SO3H.Compounds such as S100C12, S,OE- (n = 50-100) have been reported. Many binary silicides, phosphides, arsenides, sulphides and selenides can j3-cristobalite (see fig. 3) see fig. 2 MCl2 see fig. 2; tetrahedral arrange- 20 F F Structure ment about M square-planar arrangement about M see Fig. 2 see fig. 2 see fig. 2 R. I. C. Reviews also be considered as homopolymers. For details of the structures and for more information about many of the compounds mentioned in this review, the reader is referred to the excellent comprehensive work by Wells.32 Heteropolymers containing halogen and pseudo-halogen ,cU-cN Compounds of the halogens can be (a) ionic solids, with either simple halide ions (e.g.NaCl, CaF2) or halo-complex ions (e.g. PtCli-, PC1;); (b) small molecules (e.g. HCl, IF,) some of which are oligomers or clusters formed by halogen bridging (e.g. Al2C16, Mo&lt+, W2Cl”,) ; or (c) macromolecules. The macromolecules are classified in Table 3 and some structures are shown in Fig. 2. There are a number of substances which are borderline between ionic structures and macromolecules. For instance, although cadmium iodide has a layer structure it is probably rather extreme to cite it as an example of an inorganic polymer. The structures of the halides are described more fully elsewhere? The pseudo-halide ions CN-, SCN- etc.can also bridge between metals. There are chain structures (e.g. AgNCS and KCu(CN)2), sheet struc- tures (e.g. Pd(CN)2), and network structures (e.g. Prussian blue, KFe2(CN)6). d -Ag-NCS / \ \ CU-CN CN / Ag\ NC-CU NCS-Ag-NCS \ \ CN \ NC I C N I I I -Pd-CN-Pd- C N I I -Pd-CN-Pd- I I KFe,(CN)6 (K =o; Fe(Ii1) = 0 ; Fe(I1) =8) None of the compounds mentioned in this section find application as polymers, but they are described briefly here as an illustration of the range of inorganic macromolecular compounds. Many of the chain polymers dissociate at low temperatures and most of the halides are hydrolytically unstable. A number of halides (e.g. BeF2 and ZnClz) form glasses as well as crystalline forms.Heteropolymers containing oxygen or sulphur These are polymers containing either oxygen or sulphur atoms and atoms of at least one other element in the backbone. It is by far the largest class and most of the technologically important inorganic polymers are of this type. Currell and Frazer 21 I 0 0 \/ \ s / \ As 4\ / \ 0 00 0 0 I I 0 I I As As -o/ \o I1 I 0 0 I /o\s,/o\s,/ II 0 0 (d) PbO / \ 0- (e) p -cristobalite Fig. 3. Structures of macromolecular oxides. There are five types of heteropolymer containing oxygen : oxides, hydroxides and acids ; anhydrides ; mixed anhydrides ; polyanions ; and polyalkoxides. Oxides, hydroxides and oxyacids. Oxides are either ionic (e.g. CaO); simple molecules (e.g.N2O) and clusters (e.g. P 4 0 6 in phosphorus(II1) oxide vapour) ; or macromolecular. Table 4 lists some typical examples of macromolecular oxides and some structures are shown in Fig. 3. The formation of polymeric chains Of (SO& from molecular So3 by condensation of the vapour represents one of the few examples of inorganic addition polymerization by the opening of a multiple bond (cf. vinyl polymerization). With the exception of the hydrox- ides of the more electropositive elements (e.g. Kf OH-), most hydroxides and oxyhydroxides (e.g. lepidocrocite FeO.OH) may be considered as in- organic polymers. Sheet and layer structures involving OH - * * 0, hydrogen bonds, are common (see Chapter XIV in ref. 32). Boric acid is an example of a layer structure involving hydrogen bonds. The salts of many oxyacids 22 R.I.C. Reviews Chains asbestos-like so3, CrOs, SeO2 PdO, PtO Sbz03 Sheets Networks Table 4. Macromolecular oxides Type Examples As203, SnO, PbO MOO3 S i 0 2 , GeOz p4010 -0-Si(CH3)-0-- have polymeric structures involving bridging anionic groups such as nitrate, phosphate and sulphate. For example, anhydrous copper nitrate has a chain str~cture.~3 Compounds of this type are generally thermally unstable. Oxides have wide application as ceramics, the use of zirconium oxide as a reinforcing fibre has already been mentioned. Aluminium oxide is used as a refractory, solid articles may be fabricated either by cold compacting of the powder and sintering at suitable temperatures or by casting from the melt. Silica also has application as a refractory both by itself and in combination with other substances, for example mullite (3A1203.2Si02).and trifunctional or ‘cross-linking’ groups Anhydrides. These are systems containing M-0-M linkages where M represents not just an atom of an element, but a complex function such as -Ti(OR)z- The functionality of the M group can be con- or -SiR2--. trolled. For example, where M is silicon there are monofunctional or ‘end- stopping’ group, (CH&Si-O-; di-functional or ‘chain building’ groups, -0-Si(CH3),-0-; Fig. 3 ) PO4 tetrahedra sharing corners 0 I I Combinations of these groups produce the important class of anhydride polymers known as the siloxanes or silicones.The siloxanes have been known for many years. At the turn of the century Kipping and his coworkers studied many organosilicon compounds and noted that the hydrolysis of chlorosilanes, R,SiC14-, (n = 1,2 or 3), gave intractable solids and glues. It was not until the 1940s that these hydrolysis products were examined again and they now have application as fluids, resins and elastomers. The siloxanes represent the one class of inorganic polymers commercially exploited on a large scale as inorganic replacements for organic plastics and rubbers. The world consumption is currently estimated as approximately 50 000 tons/annum. There are many reviews and technical papers. 34 The normal method of preparation is by the condensation of silanediols.-2HCI Me,SiC I, f 2H,O - HO-SiMe2- OH -H20 - i!z,fSiMe,-O+, Currell and Frazer Structure see Fig. 3 PtS, see Fig. 5 see p. 14; (ladder polymer) see Fig. 3 MOOG octahedra sharing edges and corners various forms, e.g. /3-cristobalite (see 23 This process gives a mixture of long and short chain siloxane polymers, together with various small ring compounds, mainly octamethylcyclotetra- siloxane. A more homogeneous molecular weight distribution may be brought /Me 'Me Me Me about by 'equilibration', i.e. heating the mixture with a catalyst such as sulphuric acid, ferric chloride or potassium hydroxide. Under these condi- tions, cyclic compounds give straight chain polymers and a 'chain stopper' (e.g. hexamethyldisiloxane, MesSiOSiMea, prepared by the hydrolysis of MesSiCl) controls molecular weight by providing monofunctional end groups.aq. HISO, n(Me,SiO),+ (Me,Si),O Me I -SiMe2-O- * Me S i O( S i M e,0)4 nlS i Me Siloxane fluids are colourless, odourless, practically non-volatile and non- toxic. They are stable for long periods when heated in contact with the air at 150" and under anaerobic conditions there is no decomposition at 200". They are also inert to many chemical reagents. Siloxanes are used as polish additives (providing lubrication between wax crystals so facilitating formation of a uniform thin film or wax); as additives to paints, to assist grinding and dispersion of pigments ; as water-repellent finishes for textiles, leathers and paper; as lubricants and greases; as antifoams; and as stationary phases in gas chromatography.In siloxane resins, cross linking is provided by having the trifunctional MeSiCl3 present before hydrolysis. In some examples phenylchlorosilanes Si -O-SiMe2-O- Si-0-SiMe2-0- -SiMe2-0- 0 I I Me I are used. The resins are used as electrical insulators, surface coatings, and water repellents for brickwork and masonary. They are characterized by high thermal stability and resistance to chemical attack. Siloxane elastomers are linear polymers of high molecular weight which have been heated with benzoyl peroxide to form cross-links and thus develop elastic properties. R.I.C. Reviews 24 Me I Si-0-SiMe2--O- -SiMe2-O- CH2 CH2 S i -0-SiMe2-0- -SiMe2-O- I I I I Me The outstanding characteristics of silicone rubbers are their stability at high temperatures, retention of elasticity at low temperatures and excellent electrical properties.Physical properties are virtually constant in the range -80" to +250". At 150" they retain their physical properties for an unlimited time. An important recent development in polymer chemistry has been the inclusion of carborane clusters to give polymers of increased thermal stability. o -carborane HC-CCH \ / BIOHIO ( Hci!:)) Olin Mathieson Ltd has recently put on the market a polysiloxane contain- ing m-carborane clusters. The synthesis of these polymers is outlined below and involves the prior formation of the dilithio o-carborane which is then reacted with dimethyldichlorosilane.35 The isomerization to the m-carborane derivative is then carried out after the attachment of the chlorosilane groups.BULl - LiC \ / - CLi BIOHIO Me Me I (Me), SiCI, - C I - Si- C - C - Si-CI I 3 I0-35O0C * I \ / I B,,Hl0 Me Me Currell and Frazer m -carborane 25 Me I CI- Si -CBloH,,C- Me ~ I Me,SiCI,(FeCI,) -2MeCI \ -M-OH / \ -M-CI / \ -M-CI / These products are elastomers with useful properties up to about 400-500 '. Other examples of polymers containing carborane clusters have been pre- pared.36 Again the principal reaction involved the dilithio salt. In general, anhydride polymers are formed by elimination of small mole- cules such as water, hydrogen chloride, alkyl halide and alcohols.Indeed, condensation is the most common type of polymerization process in inorganic chemistry. There are numerous examples, in addition to the siloxanes. I / HO-M- \ / HO-M- \ RO-M- \ / RO-M- \ ---OH / M can be Ge, Sn, Pb, B, Ti, Zr and the groups attached to M can be alkyl, aryl, alkoxy, aryloxy, halogen etc. Properties of the materials depend upon these groups, and as systematic research proceeds we can hope to control hydrolytic and thermal stability. A quite different example of an anhydride polymer is NbOC13. This has a polymeric double-chain structure. It can be considered as an anhydride derived from the unknown NbC13(0H)~. Trans-octahedral Nb02C14 units are (c) - -M-0-M- \ / (d) / 26 Me I S i -CI 5 2MeOH MeO- Si-CB,,H,,C Me Me I I I Me I Si-CB,oHloC- S i -0- S i - 0 Me Me - H i 0 -HCI - RCI / - ROH \ Me I Me I Me Me I I Me / I Me - Si-OMe ( 0 ) \ / (6) / \ / I t - -M-0-M- \ / \ * -M-0-M- \ -M-0-M- / \ \ R.I.C.Reviews linked into double chains by sharing an edge (two Cls) and two opposite vertices (two 0s) as shown.37 CI, I /=I\ Nb Nb CI’ I ‘CI’ 0 0 I /CI I ‘CI Part of one NbOC13 chain. In aqueous solution, the formation of anhydrides (i.e. the formation of 0x0 bridges, also called oxolation) is usually preceded by the formation of hydroxo bridges (often called olation). The first stage can be represented as: This process can continue thereby forming chains and networks.With increase of temperature (loss of water from two -OH bridges) or increase of pH (loss of protons from -OH bridges), 0x0 bridges are formed. Examples have M = Al, Cr, Fe, Ti, Zr etc. The formation of gelatinous precipitates (‘hydroxides’) and, eventually, hydrated oxides by the hydrolysis of com- pounds of these and many other elements are good examples of inorganic polymerization processes. The subject of olation has been reviewed.38 A further example of an anhydride polymer is the polycation (Ti02+)*, present in the structures of many ‘titanyl salts’, e.g. titanyl sulphate has a structure involving a -Ti-0-Ti- chain.39 Mixed anhydrides. These are systems of the type -M-0-M’-.Andrianov40 has prepared many materials, adapting schemes (a), (b), (c), ( d ) above so that one M = M’. Thus polytitanosiloxane was obtained by the interaction of dihydroxypolydimethylsiloxane with tetrabutoxytitanium. These polymers are reported to be elastic, soluble substances with mole- cular weights up to 200 000. The rubbers obtained by vulcanization of these elastomers are said to have properties similar to polydimethylsiloxane based rubbers. Currell and Frazer 27 Another example is the room temperature polymerization of tetrapro,- poxyzirconium and diphenylsilanol, 4 1 a solid softening at 70" with molecular weight of about 5000 is obtained. Ph2Si(OH)2 + Zr(OPr), Ph OPr Gerrard and coworkers42 have made a detailed study of reactions of esters [M(OR)n] and covalent halides [M'Xm].Two types of reaction can occur : anhydride formation (cf. scheme (c) above), or halogen-alkoxy exchange. The mode of reaction depends on the nature of R and also on M and M'. Thus, most trialkylphosphates and boron halides eliminate alkyl halide giving a residue of boron phosphate, a network mixed anhydride polymer, but aryl phosphates merely form complexes. (RO),PO + BX, - 3RX + BPO, (ArO),PO i- BX, - (ArO), PO,BX, Boron trichloride and tetra-n-butoxysilane exchange alkoxy and chlorine, but boron trichloride and tetra-s-butoxysilane eliminate isobutyl chloride leaving a mixed anhydride polymer. (Bu"0lSi + BCI, - Bu"OBCI, + (BU"O)~S~CI (Bus O), Si + BCI, - Bu'CI + Si-0-B polyanhydride The preparation, properties and where known structures of mixed anhyd- ride polymers have been re~iewed.~3 There are several promising systems, but much more detailed work remains to be done.Block and his coworkers44 at Pennsalt Chemicals Corp. have studied the preparation and properties of polymers containing transition metals such as titanium and chromium in the backbone. They have been classed as co- ordination polymers, because the transition element is 'protected' by co- ordination to bidentate ligands such as acac (where acacH = acetylacetone). These ligands are not part of the polymer backbone and so the materials are better considered as mixed anhydride polymers. An example is the reaction of chromium(ii1) acetylacetonate with diphenylphosphinic acid which releases acetylacetone forming a mixed anhydride polymer.Cr(acac)3 + Ph,P(O)OH - A [Cr(acac)20PPh,0k + acacH (anions of heteropolyacids) are Polyanions. Negatively charged species containing the units -M-0-M- (anions of isopolyacids) or -M-0-M'- R.Z. C. Reviews 28 polyanions. Important examples are where M is boron, silicon and phos- phorus. Less important are examples with M = As, Sb and V; S, Se and Cr. The polyanions where M is a transition metal such as Zr, Nb, Ta, Mo and W can usually be looked upon as discrete clusters of atoms (e.g. Mo?O&, PW120&) but there are some examples of polymeric species. For example, K2Mo3010 has recently been shown to consist of infinite chains formed from Moo6 distorted octahedra and MOOS square pyramids sharing edge~.~5 Boric acid gives rise to three series of salts: the orthoborates and pyro- borates involving the discrete ions Bog- and BzOi-; the polymeric borates such as the metaborates which are small rings or infinite chains; and borates such as (B$O?-)n.Both 3-coordinate and 4-coordinate boron are found in polymeric borates and some of the structures are complex. Many materials are found as glasses and not as crystalline phases. The chemistry and struc- ture of the borates has been described ;32@ the technological applications are as fluxes and special glasses. The silicates represent another large class of inorganic polymers. Detailed knowledge of the many different types of structure has been obtained and is well described elsewhere.32 There are chain, sheet and network structures.Asbestos is a naturally occurring inorganic fibre, e.g. chrys0tile,~7 3Mg0,2Si02,2Hz0, is a two dimensional layer structure rolled about an axis to form a narrow tube. This gbre has a Young’s modulus of about 23 x lo6 lb in-2. Asbestos fibres are incorporated in thermoplastics in order to increase strength and rigidity. An interesting approach to the synthesis of inorganic polymers is the improvement of properties of a naturally occurring polymer by modification of the structure. Frazier and coworkers48 have changed the diameter of the chrysotile ‘tube’ by reaction with trimethylchlorosilane. The Me3Si.O- group is then linked to silicon atoms of the asbestos, and steric interaction between these groups produces a ‘tube’ of smaller diameter.Examples of other silicates with layer structures include talc, Mgs(OH)zSi4Olo, and one of the mica minerals, phlogopite KMg3(0H)2AlSi3010, in which of the silicon atoms in talc have been re- placed by aluminium, giving rise to negatively charged layers interleaved with K+. The electrostatic attraction between the layers and potassium ions is weak and micas possess well pronounced cleavage planes parallel to the layers. Replacement of silicon by aluminium is very common and clays, felspars, zeolites and ultramarines can all be considered as examples of network inorganic polymers. These are all minerals and possess ordered structures.Gla~ses,~g on the other hand, are characterized by lack of long- range order. The immediate environment of each silicon is a tetrahedron of oxygen atoms. In vitreous silica each corner of the tetrahedron is shared with another but in other glasses in which the network is negatively charged (the charge is balanced by cations such as Na+ and Ca2+) some tetrahedra will share only three or two corners. These are true inorganic polymers and borosilicate glasses can be quoted as good examples of inorganic copolymers. The application of oxides as ceramics has already been mentioned. Silicate networks occur in a wide range of ceramics, of which porcelain is a typical example. Porcelain is obtained from a mixture of clay (A1203,2Si02,2H20), Currell and Frazer 29 flint (Si02, nH2O) and felspar (K20, A1203,6SiO2).Clay imparts plasticity to the mixture which can be formed into various shapes which are then heated in the range 1100-1400 ’. The final product consists of a glass phase and two crystalline phases, mullite and quartz. Portland cement50 is a hydraulic cement (i.e. one which sets and hardens as a result of chemical reactions that occur on mixing with water). It is made by heating a mixture of limestone and clay to a temperature at which partial fusion occurs. It consists of phases of 2Ca0,SiOz and 3CaO,A1203. The setting process involves solution of these phases in water, followed by crystallization. Hardening involves the intergrowing of the crystals and current theory suggests that Si-0 and A1-0 networks are formed.Polyphosphates have been known since 1827. If either sodium ammonium hydrogen phosphate or sodium dihydrogen phosphate are heated, the solid obtained has the composition Nap03 and can be called sodium metaphosphate. The solid is not a single phase however; part is soluble in water and part insoluble. Many different forms of sodium metaphosphate have been recog- nized-the early date of much of the work is shown by the names. Graham’s salt is a glass made by rapidly cooling molten sodium metaphosphate. The insoluble sodium metaphosphates are Kurrol’s salt (insoluble in water, but soluble in alkali metal salt solutions) and Maddrell’s salt (insoluble in water and salt solutions). It is assumed that these and several other modifications of sodium metaphosphate are based on infinite PO, chains.This has been confirmed by X-ray diffraction in certain examples, i.e. (RbP@),.5l In \A 0- /O\ 0- 1 /O\ 0- 6 P\ I I I I 0 0 0 II addition to structural investigations, the polyphosphates have been examined by many methods generally applied to polymers. These include viscosity, sedimentation, diffusion and light scattering measurements. They have also been studied by pH and conductance measurements and separations have been effected by paper chromatography. Molecular weights of 250 000 have been found. Predominant among recent workers are Van Wazer and Thilo, and there are several good re~iews.5~ Related to the polyphosphates but not actually examples of polyanions P P P are polymers such as the polyphosphoryldimethylamides, I I I1 I I 0 0 0 R.I. C.Reviews 30 and the polyphosphorylhalides ii II II 0 0 0 obtained from mixtures of phosphorus pentoxide with hexamethylphosphory- lamide [OP(NMe&] and phosphoryl halides [OPX3]. In these systems and in the polyphosphates themselves, the chains and networks are built from the following units 0 I Y I Y I Y-P-O- Y-P-Y -0- 0 II neso Y I P-O- 0 II difunctional middle units 0 II monofu nct i on aI end units POC I, I -0- 0 II trifunctional branching units where Y = -NMe2 or halogen respectively. The importance of equilibrium control in inorganic polymer systems was mentioned earlier and these phosljhate structures are perhaps the most thoroughly investigated.l O ~ 6 If phosphorus oxychloride and phosphorus pentoxide are heated at 200" for a few hours complete statistical redistri- bution occurs (Fig. 4a) the relative amounts of each building unit, neso, end, middle or branching depeqds on the original proportions of oxychloride and pentoxide. The study of these 'scrambling reactions' has been mainly carried out by Van Wazer and co-workers and provides valuable information on the intrinsic stability of possible polymer systems. Information is also provided 100 80 60 40 20 0 - 0 - P 2 I 3 0 R = Me2N/P R = C l / P (a) (b) Fig. 4(a) Distribution of building units in the phosphorus pentoxide-phosphorus oxy- chloride; (b) in the phosphorus pentoxide-orthophosphoryldimethylamide systems.Currell and Frazer 31 3 on the preparation of polymers by ‘melt methods’. For example, melts of phosphorus pentoxide and orthophosphoryldimethylamide do not show random statistical redistribution (Fig. 4b), the melt where the value of NMeZ/P is in the region of 1.0 being entirely composed of middle units (this polymer has elas tomeric properties). The polymetaarsenates and polymetavanadates have been much less investigated but there is structural evidence showing infinite chains of the type with M = As and V for NaAs0353 and KVO~,HZO.~~ 0- 0- 0- I l l M M M I \o’ I I \ \o’ Il‘o’l 0 0 0 Polyalkoxides. These have been reviewed by Bradley.55 Most examples are oligomers containing a few metal atoms linked by alkoxy bridges.The tet- ramer of Ti(OEt)s is a typical example. However lithium methoxide has a layer structure,56 and there are a number of examples of polymeric materials formed by hydrolysis of alkoxides. These contain M-0-M bridges in addition to alkoxy bridges and so can be considered as anhydride polymers. Hydrolysis of [Ti(OEt)4]4 led eventually to material of composition [Ti304(OEt)4Ix.57 Heteropolymers containing sulphur. A number of compounds can be consid- ered as macromolecules having sulphur atoms in the backbone. Binary sulphides which could be written with the S2- ion are better considered, due to the polarizability of this ion, as infinite three-dimensional covalent net- works.There are also layer and chain structures. Macromolecular binary sulphides are classified in Table 5 and some structures are shown in Fig. 5. The weak bonding between SMoS layers in molybdenum disulphide gives a pronounced cleavage and this compound has application as a lubricant. Glasses based on the elements sulphur, selenium and tellurium are able to transmit to longer wavelengths in the infrared than the oxide glasses, a number are semi-conductors and are opaque in the visible region of the spectrum. For example, As& glass may be made 58 by distilling the technical grade sulphide in a stream of H2S. The glass formed on cooling the condensed liquid transmits in the infrared up to 12pm. Structure Table 5. Some binary sulphides Type Chains Examples SiS2 Sheets Networks PtS Sb2S3 MoS2 FeS, TiS, N b S see Fig.5 MoS2 structure NiAs structure (Fig. 5) Wurtzite (Fig. 5) ZnS, MnS R.I.C. Reviews 32 s =O; Zn =Q (b) Wurtzite (d) PtS N i = O ; A s = @ (a) NiAs (c) SiS, Fig. 5. Structure of some macromolecular sulphides. He teropo Iymers con tain ing nitrogen or phosphor us These are macromolecules containing either nitrogen or phosphorus atoms and atoms of at least one other element in the skeleton. Boron-nitrogen and other Group mB-nitrogen polymers. There has been tremendous effort to make polymers containing boron-nitrogen bonds. The idea behind this effort is that B-N is isoelectronic with C-C. Impetus was given in the 1950s by the research on organoborons and boron hydrides in connection with rocket fuels. A further attraction for making polymers containing boron is that natural boron contains 20 per cent lOB which has a high cross-section (a = 4017 barns) for the (n, a) reaction.A low density, polymeric neutron shield would of course be highly desirable. It must be stated however, that despite all the effort of the last 15 years, there is very little to report in the way of useful boron-nitrogen polymers. There are two reasons. Firstly, boron-nitrogen systems have a propensity Currell and Frazer 33 to form small ring systems, mainly the trimeric ring borazine, (XBNY)3. B X I B x/ \ - * / N Y I 'X The kinetic and probably more important thermodynamic factors favouring ring as opposed to long chain formation have not been fully elucidated, and it may be that a physical rather than a preparative approach will bring success.Secondly, most boron-nitrogen compounds are easily hydrolysed. This is usually explained in mechanistic terms as nucleophilic attack by water on the boron atom. There have been many attempts to avoid ring formation and most of these have involved placing bulky substituents on the boron atom (or more usually on the nitrogen) whereby it was hoped that steric interaction would prevent ring formation. For example dehydrohalogenatior15~ of t-butylamine-trichloroborane gives the s-tetraza-tetraborine and not the borazine. Similarly dehydrohalogenation60 of i-butylamine-phenyldichloro- borane gives a mixture of the six- and eight-membered rings.4 Bu'N%BCI, - (CIBNBU')~ + 8 HCI + ;(PhBNBu'),+ I 2 Bu"NHfPhBC1, 2 HCI n = 3 and 4 The action of heat on bis(i-buty1amino)phenylboron has been reported61 to give a linear polymer. The formation of this polymer appears to be easily affected by small changes in experimental conditions and this would not appear to be a route to the preparation of a useful polymer. 2 (Bu' NH), BPh molecular weight of 7000.62 H PhN t I - 9, NPh BH Ph - (PhBNBu')l+ 2 Bu'NH, Perhaps the most promising work has been with B-N ring systems joined together. For example, N-triphenylborazine on heating above 200" gives a B-B linked polyborazine which is a transparent, brittle product having a r Ph N - B B / \ NPh \ / N - B The ultimate in fused B-N rings is boron nitride. This is a white solid with a layer structure similar to graphite.32 The stacking of the planes of 1 Ph H FH2 R.I.C. Reviews 34 hexagonal rings of BN atoms is different however. In graphite a carbon atom of one sheet lies over the centre of a ring in the sheet below; in boron nitride atoms in one layer lie over atoms in the layer below. Boron nitride is an insulator and it also differs from graphite in other physical properties. A new and convenient63 method of making boron nitride is to heat an alkyl polyborate-ammonia complex at 800". & (ROBO)m+ NH, - A (BN),+ ROH + H,O Boron nitride can be made into fibres similar to the graphite fibres mentioned on p.18. These fibres have a tensile strength of 50-200 x lo3 lb in-2 and an elastic modulus of 4-12 x 106 Ib in-2 with an upper temperature limit, of use in an oxidizing atmosphere, of 850 0.G4 Although expensive, boron nitride has potential for the future. It has been suggested as a white filler for natural and synthetic rubbers, and for some purposes it may be superior to carbon blacks. When boron nitride is heated at 1800"/60 000 atm it is converted into the polymorph with a diamond-like structure. This material called 'borazon' was claimed to have a hardness grdter than diamond.65 There are a number of good accounts of boron-nitrogen chernistry.G6 A number of research groups have examined aluminium-nitrogen sys- t e m ~ .~ ~ As with boron-nitrogen compounds, the materials are easily hydro- lysed and there is a propensity to form small ring compounds. The general approach has been via the loss of hydrocarbon from amine complexes of alanes. -RH -RH I (RAINR')~ R,A I ,NH,R' - A (R,AINHR~" usually dimers Boron-phosphorus polymers. Boron-phosphorus polymers may be prepared6 * by the dehydrogenation of phosphine-borane adducts and the dehydro- halogenation of phosphine-boron halide adducts, e.g. n = 3 and 4 with only a trace of higher polymers 5 I (Me,PBMe,), + Et,NHBr Me,HP,BBrMe, + Et,N In these systems small ring formation is again predominant; linear poly- mers have, however, been obtained69 by carrying out the above dehydro- genation in the presence of, for example, triethylamine.It has been suggested that triethylamine acts as a catalyst for the polymerization of the monomer units. This approach, using tertiary bases, has yielded plastics. Their tech- nological application, however, is limited by the fact that depolymerization to the trimer and tetramer occurs on heating above about 300 '. This subject has been reviewed.70 Currell and Frazer 35 Silicon-nitrogen polymers. These have been reviewed recently.71 Other accounts of silicon-nitrogen chemistry should also be c ~ n s u l t e d . ~ ~ The general approach has been to consider the -N(R)- group as isoelec- tronic to -0- and therefore to prepare analogues of the siloxanes with imino groups replacing oxygen. However as with boron-nitrogen and boron- phosphorus systems there is a strong propensity to form rings rather than long chains.For example the reaction of dialkyl-or diaryl-dihalosilanes with ammonia proceeds as follows to give six- or eight-membered silazane rings : R,SiCI, 4- 3NH, - jj I (R,SiNH),,+ 2NH,CI n = 3 o r 4 If the R group is bulky or if primary amines R’NH2 are used instead of ammonia the reaction proceeds differently and the main product is RZSi(NR& (R’ = H, R = Bui; R’ = Me, Et etc., R = any alkyl or aryl group); (R’NHSiR2)zNR’ is also found as a product. The equilibrium between chains and small rings for the systems tSiMezNMef, and -(-SiMe20fn have been investigated by nuclear magnetic resonance,73 and as far as silazanes are concerned, rings are always thermodynamically favoured over linear chains.There have been a number of attempts to overcome ring formation. For example if alkyl or aryl on silicon is replaced by hydrogen, thermally stable, solid silazanes are obtained with degrees of polymerization of 15-30. SiH21, 4- 3PhNH2 - A f-SiH,NPhh 2PhNH31 More recently solids, (SiFzNMez),, which are resistant to alkaline hydrolysis and thermally stable to 400” were obtained from silicon tetrafluoride. Very recently Fink74 has prepared a series of thermally stable polymers Me Me \ / and S i / \ N \ /N - I containing the cyclodisilazaone group. Examples are : (Ph,SiNH), Si / \ Me Me By heating the small ring compounds decomposition and loss of hydro- carbon leads to cross-linking and formation of thermally and hydrolytically stable materials which, however, are usually insoluble and infusible, e.g.- (PhSiN)n + 3PhH Ultimate cross-linking in the silicon-nitrogen system produces silicon nitride. A novel method for producing silicon nitride as a coating has recently been described :75 gaseous silane, SiH4, and ammonia are subjected to a glow R. I. C. Reviews 36 I discharge as they pass over a glass, plastic or metal former. The ratio of silicon to nitrogen in the solid deposited can be accurately controlled by altering the relative amounts of the two reactants in the gas stream. By using a glow discharge the temperature of the former is not raised so that there is no destruction of the material being coated.The silicon nitride so formed is a hard, chemically resistant material with high dielectric constant. Silicon nitride has been obtained in the form of whiskers by workers at the Explosives Research and Development Establishment, Waltham Abbey.76 These whiskers are grown from the high temperature interaction of nitrogen and silicon monoxide which is formed by heating silica and silicon in an atmosphere of argon. The physical properties and technological applications of whiskers have been previously discussed (p. 18). Phosphorus-nitrogen polymers. The phosphazenes or phosphonitrilic com- pounds were among the first synthetic inorganic polymers to be studied. The compounds (PNC12)n although then not so formulated were discovered by Liebig in 1834.Intensive research was undertaken in the late 1940s and 1950s with the hope of producing thermally stable elastomers but the early promise has not been fulfilled. Ammonium chloride and phosphorus pentachloride are heated in sym- tetrachloroethane. The product is a complex mixture of composition (PNC12)n. These are trimers, tetramers, and other ring compounds with n = 5, 6 and 7 together with smaller amounts of oligomers of formula (PNC12)n, Pc15, i.e. Cl(PC12N)nPC14. The elements of Pc15 provide the end groups. The exact composition of the mixture depends on the conditions, the various components can be separated by fractional crystallization. On heating to 250-300°, all the lower members of the series polymerize to give a solid high polymer with high elasticity.Molecular weights as high as 200 000 have been reported. The kinetics of the polymerization and the ring-high polymer equilibria have been extensively investigated. Results are discussed in the key references.77 In the absence of air the polymers retain their elastic properties, but in the presence of air the elastic modulus and brittleness of the samples increases. This susceptibility to atmospheric degradation (mainlv by hydrolysis resulting in cross-linking) has meant that the phosphazene polymers have not had application as rubbers. The susceptibility to thermal degradation is roughly the same as for natural rubbers. CI I P = N - (C I, PN) - - (C I, PN), +H,O * -2HCI -(C I, PN), I ' I C I C I I -(C lz PN)x- P = N - (C I, PN), CI I =N -(C12PN)x- -(CI,PN)x-P C I \ \ / P = N -(C I, PN) - I 0 CI I Sulphur-nitrogen polymers.Compounds of the type N4S4, N&H4 have been studied as possible precursors to polymers containing nitrogen and sulphur. Cirrrell and Frazer 37 This chemistry is well reviewed.78 There are as yet no macromolecules of technological importance. Amido and imido derivatives of metals. Many compounds which formally contain NH, and NH2- groups are polymeric. For example, amidochloro- mercury(@, HgNHzCl, known as an infusible white precipitate, contains infinite cationic chains of the type : Mercury-nitrogen chemistry also provides examples of sheet and network structures. The cation HgzNH2+ in HgNHBr2 is an infinite sheet and the cation in NHg2N03 (which is the nitrate of Millon’s base) has the /3-cristobalite structure (see Fig.3). There are many other examples of -NH2 and -NH- acting as bridging groups, but none of the polymers so formed are of immediate technological importance. CONCLUSION We have attempted to demonstrate that inorganic polymers are not merely curiosities. Leaving aside the numerous small rings systems and metal cluster compounds, inorganic macromolecules are formed by many elements. However as far as useful polymers are concerned, the cogener of carbon, silicon, is predominant (asbestos, mica, glass, cement, siloxanes), and boron, phosphorus and sulphur show potential; oxygen and nitrogen, which are of course, also involved in many organic polymers, are usually present.The potential of inorganic polymers has by no means been fully exploited. There is still a paucity of relevant kinetic and thermodynamic information and much needs to be done in these areas before there are major advances in the preparation of technologically applicable materials. ACKNOWLEDGEMENTS We thank Mr J. Cook of the Explosives Research and Development Estab- lishment, Waltham Abbey and Dr W. Gerrard of Northern Polytechnic for many useful comments; also the Clarendon Press, Oxford for permission to reproduce illustrations from ref. 32. REFERENCES 1 F. G. R. Giniblett, Inorganic Polymer Chemistry. London: Butterworths, 1963. 2 D. N. Hunter, Inorganic Polymers. New York: Wiley, 1963.3 F. G. A. Stone and W. A. G. Graham (eds), Inorganic Polymers. New York: Academic Press, 1962. 4 H. R. Allcock, Heteroatom Ring Systems and Polymers. New York: Academic Press, 1967. 5 Inorganic Polymers. London: Chemical Society, Special Publication No. 15, 1961. Amsterdam : Elsevier, 1962. 7 High Temperature Resistance and Thermal Degradation of Polymers. London : Society 6 M. F. Lappert and G. J. Leigh (eds), Developments in Inorganic Polymer Chemistry. of Chemical Industry, Monograph No. 13, 1961. 8 W. Gerrard, Trans. J. Plast. Inst., 1967, 509; J . Oil Colour Chem. Ass., 1959, 42, 202; Repts Prog. appl. Chem., 1960, 45, 394. 38 R . I. C. Re views 9 D. B. Sowerby and L. F. Audrieth, J. chem. Educ., 1960,37,2, 86, 134.10 J. R. Van Wazer, J. macromol. Sci. (Chem.), 1967, Al(l), 29. 11 H. Krebs, Angew. Chem., 1958, 70, 615. 12 H. K. Livingston, J. Polym. Sci., 1967, C18, 105 13 L. Holliday, Chemy Ind., 1967, 970. 14 G. Gee, ‘Rings and Chains in Inorganic Polymers’, reference 5, 67. 15 J. B. 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London : Royal Institute of Chemistry, Lecture Series, 1965, No. 2. 27 W. Kuchen and R. D. Brinkmann, 2. anorg. allg. Chem., 1963, 325, 225. 28 C. A. Burkhard, J. Am chem. SOC., 1949, 71,963. 29 P. L. Timms, R. A. Kent, T. C. Ehlert and J. L.Margrave, J. Am. chem. SOC., 1965, 87, 2824; Nature Lond., 1965, 207, 1872 30 A. Levy, J. E. Williamson and L. W. Steiger, J. inorg. nrrcl. Chem., 1961,17, 26. 31 M. Schmidt, ‘Sulphur Polymers’, reference 3, chapter 3. 32 A. F. Wells, Structural Inorganic Chemistry, third edn. Oxford : Clarendon Press, 1962. 33 S. C. Wallwork, Proc. chern. SOC.. 1959, 311. 34 J. S. Hughes, ‘Some Recent Advances in Silicone Chemistry’, reference 6, chapter 6; A. J. Barry and H. N. Beck, ‘Silicone Polymers’, reference 3, chapter 5; V. Bazant, V. Chvalovsky and J . Rathousky, Organosilicon Compounds. New York : Academic Press, 1965, 1, 45; H. W. Post, Silicones and other Organic Silicon Compounds. New York: Reinhold, 1949; E. G. Rochow, An introduction to the Chemistry o j the Silicones, second edn.New York: Wiley, 1951. 35 S. Papetti, B. B. Schaeffer, A. P. Gray and T. L. Heying, J. Polym. Sci., (A-1) 1966, 4, 1623; H. Schroeder, 0. G. Schaffling, T. L. Larcher, F. F. Frulla and T. L. Heying, Rubb. Chem. Technol., 1966,39, 11 84. 36 S. Bresadola, F. Rossetto and G. Tagliavini, Chem. Commun., 1966, 623. 37 D. E. Sands, A. Zalkin and R. E. Elson, Acta crystallogr., 1959, 12, 21. 38 J. C. Bailar Jr., The Chemistry of the Coordination Compounds. New York: Reinhold, 1956, chapter 14; reference 1, chapter 3. 39 G. Lundgren, Ark. Kemi, 1956, 10, 397. 40 K. A. Andrianov, ‘Methods of Synthesis of Organometalloid Polymers’, reference 5, 90. 41 T. P. Avilova, V. T. Bykov, V. Yu. Glushchenko and V. P. Marinin, Vysokornolek.Soedin, 1966, 8, 11. 42 W. Gerrard, in The Chemistry of Polymerization Processes. London: Society of Chemical Industry, Monograph No. 20, 1966, 341. 43 J. Idris Jones, ‘Polymetallosiloxanes, Parts I and 2’. reference 6, chapters 7 and 8. 44 B. P. Block, ‘Coordination Polymers’, reference 3, chapter 8. 45 B. M. Gatehouse and P. Leverett, J. chem. SOC. (A), 1968, 1398. 46 P. H. Kemp, Chemistry of Borates, London: Borax Consolidated, 1956; reference 26, chapter 22. 47 A. A. Hodgson, Fibrous Silicates. London : Royal Institute of Chemistry, Lecture Series, 1965, No. 4. 48 S. E. Frazier, J. A. Bedford, J. Hower and M. E. Kenney, Inorg. Chem., 1967, 6, 1693. 49 H. Rawson, Inorganic Glass Forming Systems. London : Academic Press, 1967. 50 H. F. W. Turner, The Chemistry of Cements. London: Royal Institute of Chemistry, Lecture Series, 1966, No. 2. 51 D. E. C. Corbridge. Acta crystallogr., 1956, 9, 308. 52 P. G. Arvan, C. F. Callis and J. R. Van Wazer, Chem. Rev., 1954, 49, 58; Van E. Thilo, reference 5, 33; J. R. Van Wazer and C. F. Callis, reference 3, chapter 2; J. R. Van Wazer, Phosphorus and its Compounds, I and 11. 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London: Academic Press, 1961 ; M. F. Lap- pert, ‘Polymers Containing Boron and Nitrogen’, reference 6, chapter 2; K. Niedenzu and J. W. Dawson, Boron-Nitrogen Compounds. Berlin: Springer, 1965; H. Steinberg and R. J. Brotherton, Organoboron Chemistry, 2. New York: Wiley, 1966; A. L. McCloskey, ‘Boron Polymers’ reference 3, chapter 4. 67 A. W. Laubengayer, ‘Boron-Nitrogen and Aluminium-Nitrogen Polymeric Frame- works’, reference 5, 78; reference 4. 68 A. B. Burg and R. I. Wagner, J. Am. chem. SOC., 1953,75, 3872. 69 A. B. Burg, J. inorg. nucl. Chem., 1959, 11, 258. 70 A. B. Burg, ‘Types of Polymer Combination among the Non-metallic Elements’, reference 5, 17. 71 B. Aylett, Organometallic Chem. Rev., 1968, 3, 151. 72 U. Wannagat, Adv. inorg. Chem. Radiochem., 1964, 6, 225; R. Fassenden and J. S. Fessenden, Chem. Rev., 1961,61, 361 ; W. Fink, Angew. Chem. Intern. Ed., 1966,5,760; K. A. Andrianov and L. M. Khananashvili, Organometal. Chem. Rev., 1967, 2, 141. 73 J. R. Van Wazer and K. Moedritzer, J . chem. Phys., 1964, 41, 3122; K. Moedritzer, Organometallic Chem. Rev., 1966, 1, 179. 74 W. Fink, Helv. chim. Acta, 1968, 51, 954. 75 H. F. Sterling, J. H. Alexander and R. J. Joyce, Vide, No. Spkial A.V.I. Sem.-October 1966,SO. 77 J. R. Van Wazer and C. F. Callis, reference 3, chapter 2; N. L. Paddock, Quart. Rev. chem. Soc., 1964, 18, 168; H. R. Allcock, Chem. Engng. News, 1968, 68. 76 J. Cook, private communication. 78. M. Becke Goehring, ‘Polymeric Sulphur and Phosphorus Compounds’, reference 6, chapter 5 ; reference 4. R.I.C. Reviews 40

ISSN:0035-8940    

DOI:10.1039/RR9690200013

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

CHEMICALS AND THE WORLD ECONOMY . . . . .. . . . . . . . . 42 Developed Countries Output, 42 Capital investment, 44 Manpower, 44 Levels of production, 45 World trade, 48 Future growth, 51 Research and development, 53 54 A. C. H. CAIRNS, B.A., B.Sc., A.R.I.C. Chairman, Joseph Crosfield & Sons Ltd, Warrington, Lancs. Developing Countries Economic models, 56 . . . . .. . . . . . . . . Future Growth . . . . .. . . . . . . . . . . 58 Before the first World War, perhaps even before the Second, it would have seemed presumptuous to have thought of the chemical industry as having an immense effect on the economy of nations, for in the early days of this century, chemicals conjured up an image largely consisting of the alkali industry. Today it is quite clear that what we regard as the chemical and allied industries form a major part of the manufacturing industry and overall economy of what we call developed countries.When we talk of chemicals now, we tend to think first perhaps of the ever increasing range of organic chemicals derived from the mineral oil and natural gas industries, followed by the equally enormous growth of plastics materials. As some of the tables which follow show, the chemical industry today has a major share in overall national wealth and the prosperity of the chemical industry is an essential requisite of continued national growth rate and improvement in living standards. Even though, at present, its role may not be great in the economies of the undeveloped countries, it must take on increasing importance in these areas as they advance their living standards.It is one of the problems of the world today to ensure that the underdeveloped areas of the world make adequate advances so that they can at least keep pace with the growth of the developed countries and that the gap between ‘poor’ and ‘rich’ does not continue to widen. Any paper on the chemical industry must start by defining its scope and we must be clear what is generally regarded as the scope of the chemical industry in most of the statistical information available. However, if we adopt the standard international trade classification (SITC) which is the accepted United Nations definition, we will still find that no two countries appear to use precisely the same interpretation; hence comparisons on an international basis between countries have to be treated with great caution 41 Cairns and we must not read into such comparisons more than the data are worth.Chemicals as defined in the SITC fall into the following classes: 512 Organic chemicals. 513 Inorganic chemicals ; elements, oxides and halogen salts. 514 Other inorganic chemicals. 515 Radioactive and associated materials. 521 Mineral tar and crude chemicals from coal, petroleum and natural gas. 531 Synthetic organic dyestuffs, natural indigo and colour lakes. 532 Dyeing and tanning extracts, and synthetic tanning materials. 533 Pigments, paints, varnishes and related materials. 541 Medicinal and pharmaceutical products.551 Essential oils, perfumes and flavour materials. 553 Perfumery and cosmetics, dentifrices and other toilet preparations. 554 Soaps, cleansing and polishing preparations. 561 Fertilizers manufactured. 571 Explosives and pyrotechnic products. 581 Plastic materials, regenerated cellulose and artificial resins. 599 Chemical materials and products, not elsewhere specified. 23 1 Synthetic rubber and rubber substitutes. 862 Photographic and cinematographic supplies. It will be seen from this list that in addition to what we think of automati- cally as chemical products, such domestic materials as paints and varnishes, drugs, cosmetics, tooth pastes, toilet soaps and washing powders are also included. This means that we have to regard Unilever, Procter and Gamble and others as international giants in the chemical and allied field.For ex- ample, the inclusion of soaps and detergents puts Unilever about seventh in the order of chemical giants. DEVELOPED COUNTRIES It is convenient to look first at the place of the chemical industry in the more developed countries and it is fortunate that this is reasonably well docu- mented with statistics issued by the Organization for Economic Co-operation and Development (OECD). These statistics include most of Western Europe with Canada, Japan and the United States; taken together these countries represent about 76 per cent of the world industry. Of the remaining 24 per cent, 14 per cent is represented by the chemical industry of the USSR.To show the scale of influence on national economies we can look in turn at output, investment and manpower. output Table 1 shows the turnover and value added in the chemical industry for these major developed countries in comparison with their total gross national product. The last column of the total expresses turnover as a percentage of gross national product (GNP) varying from 2.5 to 8.5 per cent. This table also demonstrates the immense size of the US chemical industry which puts it in a class of its own. It is equivalent in size to the next eight developed countries (excluding the USSR) taken together and about equivalent to 37 per cent of the total estimated world turnover in chemicals in 1966. The UK now ranks only fourth in the Western World having recently fallen behind R.I. C. Reviews 42 Gross National Product ($m.) 10 073 Country Austria Belgium Canada Den mark Finland France W. Germany Ireland Italy Japan Net herlands Norway Spain Sweden Switzerland United Kingdom United States Table 1. The chemical industry and GNP, 1967 425 965 I975 280 * 250* (170) 385 nd nd I l5* 2390 4525 35" 5950 8215 85" 5230 2020 2720 580 145" 615 7055 I535 285* I725 695 (855) 6720 38 700 360 nd 3025 nd Total (%m.) nd 200 250 2690 3875 I 3 723 2368 nd 22 034 30 668 nd nd 510 925 nd 414 570* 349 534 I I 314 30 165 528 I 48 I52 - 70 nd 703 2970 2193 5405 4899 3949 I 8 470 128 600 18 110 53 191 I I 119 8604 101 076 I20 267 293 3 61 489 96 375 20 791 7588 24 570 21 300 I 4 901 104 944 756 490 * 1965 figures Figures in brackets are estimates only Table 2.Chemical investment 1966 Total Fixed lnvestment ($m.) Machinery & Equipment Other Country 141 I 2336 8617 I223 I279 I539 5106 I145 nd 8895 I 4 720 nd 13 139 I 5 948 na 7194 na 2952 I070 2615 na 4120 na 2329 I123 2790 I747 Austria Belgium Canada Denmark Finland France W. Germany Ireland Italy Japan Netherlands Norway Spain Sweden Switzerland United Kingdom United States I326 9100 52 098 3152 2623 9370 76 502 * Excludes SlTC groups 541 and 599 Cairns Turnover Value Turnover added ($m.) GNP ($m.) 5.7 as yo of 4.2 - 5.3 (2.5) (3.0) 5.9 6.8 (2 * 9) 8.5 7.3 7.4 (3 *8) 7.0 3.3 (5.7) 6.4 5.I Source: OECD lnvestment Chemicals as % of in Chemicals Machinery & Equipment - 13.0 4.9 - - 6.3 10.0 - 15.0 4.3 5.5 4.0 - 7.7 5.7 Source: OECD 43 Japan in total output. The West German chemical industry has ranked second for many years. If we include the Soviet bloc countries, then the USSR ranks second to the US in chemical turnover. Capital in ves tm en t The chemical industry is a very considerable user of capital and modern chemical industry is increasingly capital intensive.Although the fringe areas such as detergent manufacture, cosmetics etc. are not so capital intensive, manufacturers of direct chemical products are usually concerned with high capital investment, often turned over in sales at a relatively lower rate than in other industries. It is not surprising, therefore, to find that the chemical industry is one of the largest investors in the economies of developed countries. Table 2 shows, for the same countries as listed in Table 1, their invest- ments (where figures are available) in the chemical industry during 1966 together with their total investment on a national basis divided where pos- sible between manufacturing (machinery and equipment) and services. In the last column of the Table, chemical investment is given as a percentage of the total and it will be seen that this varies from 4 to 13 per cent.In the case of the major chemical countries-USA, Germany, UK and Japan-it is a very considerable part of total investment and, in this case also, the UK ranks fourth in the world in terms of chemical investment. Capital investment per employee is as high in the chemical industry as in any other industry, with the possible exception of the petroleum industry. In Western Europe it averages about $2000 per employee per annum with a US figure of over $3000 per employee per annum. Investment as a percent- age of value added ranges from 12.5 per cent in the US to 52 per cent in the Netherlands, with the UK at 21.7 per cent. Manpower The third parameter to consider in establishing the place of chemicals in the national scene is manpower.The chemical industry is not, relatively, a very large employer of manpower and modern processes and larger plants tend to reduce the numbers of operatives employed still further. However, in common with many other industries today, the numbers employed in administration and management generally tend to increase. In Table 3, again referring to the same group of developed countries, numbers employed in the chemical industry are compared with working population and the value added in terms of chemical output per employee. Differences in defini- tions make these figures difficult to compare. This is unfortunate since, if the split between operatives and others were accurate, one might be tempted to draw calculations as to the relative efficiency of the industry in different countries. For instance, if the figures are to be believed the ratio of operatives to management, administration etc.apparently varies between 1.3 : 1 in the case of France and 5 : 1 in the case of Italy. The UK seems to have a ratio of 1.7 : 1 while Germany has a ratio of 1.6. In total the numbers employed in the industry are generally about 1 .O-1.5 per cent of the working population. Switzerland, the Netherlands and Germany are highest, with Austria, Japan and Sweden towards the lower end. R. I . C. Reviews 44 Numbers employed in chemicals Table 3. Manpower (thousands) Country Austria Belgi u m Canada Denmark Finland France W.Germany Ireland Italy Japan Net herlands Norway Spain Sweden Switzerland United Kingdom United States * 1965 figures Working population Operatives 3400 3750 6900 2200 2100 20 000 26.0 36.7 nd 16.8 nd 147.6 286.2 4.7 27 150 I 100 I 9 900 49 000 4500 I450 12 200 170.5 244.0 46.9 12.7 117.5 21.1 36.5 257.0 570.5 3300 2800 25 300 78 400 - 7400 21 500" The figures relating to added value per employee are interesting and have been commented on before in many places. Perhaps the most detailed survey of manpower utilization in the UK chemical industry in comparison with America was carried out by the Economic Development Committee in the chemical industry and published in their report Manpower in the Chemical Industry.It would perhaps be more significant for us to compare ourselves with other European chemical industries and in this case the comparison is not so unfavourable and indeed the efficiency of our industry in terms of value added per employee is rising faster than in most other countries. Levels of production Before we turn to aspects of world trade it is perhaps of interest to go a little further into the national industries of the various countries to compare their importance as producers of certain basic chemicals. Tables 4 and 5 set out the production statistics for 1966 for the major inorganic products- sulphuric acid, ammonia, soda ash, caustic soda and chlorine-as these materials form an essential part of a large broadly-based chemical industry.In Table 5 similar data are given with regard to ethylene, dyestuffs, plastics materials, synthetic rubber, paints and varnishes. It is not possible in several cases to compare the position of the United Kingdom with regard to individual chemicals in comparison with our overall position because the figures are not declared. It will be noticed that some countries are not self-sufficient in basic materials but this is generally because of size on the one hand and availability from close neighbours on the other. Sources: OECD and FA0 ( $1 4800 6800 13 800" - 9100 9700 5300" 9900 6700 7600 7900" 3800 10 400 Cairns ~ ~____ Administration, technical etc.Total 9. I 19.7 nd 11.2 nd 116.4 180.4 2.3 35. I 56.4 72.9 28.0 nd 264.0 466.6 7.0 34. I 162.0 29.2 6.4 46.7 13.4 204.6 406.0 76. I 19. I 164.2 34.5 50.0 408.0 954.4 13.5 151 .O 383.9 ~~ working yo of Chemical workers as Value added per population employee I .o I .5 1 . 1 - I .3 I .3 I .7 0.6 I .o 0.8 I .7 I .3 I .3 1 .o I .8 I .6 I .2 45 Table 4. Production of major inorganic chemical products, 1966 Chlorine Soda ash Sulphuric acid Ammonia (1000 tons Caustic soda (1000 tons as (1000 tons as (1000 tons (1000 tons of Country Austria Belgium Canada Den mark Finland France W.Germany Italy Japan Netherlands Norway Spain Sweden Switzerland United Kingdom United States of&) 252 403 - 669 69 I282 I73 I I087 2000 738 340* 3 52 I12 nd nd 7960 100%) 229 I389 - (239) 480 299 I 3834 3343 6030 I059 I34 I796 600 I69 3168 26 133 * 1965 figures; -f including natural ash Table 5. Production of certain organic products, 1966 Country Austria Belgium Canada Denmark Fin land France W. Germany Ireland Italy Japan Netherlands Norway Spain Sweden Sw i tze r Ian d United Kingdom United States * 1965 figure Plastics Synthetic Paints, Ethylene Dyestuffs material rubber varnishes ( I 000 tons) ( I 000 tons) ( I 000 tons) (1000 tons) ( I 000 tons) - 2.0 - 278 18.I 71.4 - - 890 397 I065 11.5 44.9 4.3 - 60 - 20 11.7 - - 70 580 nd 25. I 40.4 107.0 94 I I 8 79 I 2272 - I066 201 I 287 69 I 47 I 45 I94 200 I 46 I037 6103 Short distances in Western Europe, now reinforced by the lowering of tariff barriers within the EEC and EFTA are helping to rationalize production facilities to an ever increasing extent. One fairly recent development in both the UK and Western Europe is the increasing co-operation between com- panies manufacturing ethylene. There is clear recognition of the fact that 51 .O 531 .O 3800.0 Figures in brackets are estimates only 46 Na2C03) NaOH) nd nd nd nd 704 - - I32 762 I303 750 I499 I I62 I I90 nd 834 I37 26 nd 67 I76 260 nd 6659 nd 6 I93t as Cl2) nd nd 597 - I18 664 I230 623 I469 nd 59 80 - 227 nd 6300 Figures in brackets are estimates only 71 .O 95.0 nd 5.3" 3.6 603 .O 846.0 13.0 255 .O 669.0 150.0 46.0 133.0 123.0 R.I.C.Reviews ethylene is a fundamental base material which, in everyone’s interest, should be readily and cheaply available. Plant sizes have now become so big that they require a high level of continuous output to make them worthwhile. It is increasingly recognized as foolish for companies to instal such large units simultaneously. Such conditions lead to alternative slumps due to overcapacity, followed by periods of shortage.(Phthalic anhydride is notori- ous in this respect.) Increasingly the large manufacturers of ethylene are being linked by an ethylene grid, for mutual support. What we accept as everyday for electricity is now spreading to chemical production. At least 20 producers or users of ethylene in Holland, Belgium and West Germany will be linked by the ethylene grid. Already over 60 per cent of the total West German ethylene capacity is linked and at least 80 per cent will be. In the future, conveyance of chemical raw materials by pipeline will become increasingly common. When considering the importance of chemicals as a whole, one should remember that the industry is involved in almost all other production areas. The chemical industry is, of course, its own biggest customer and every El00 of final chemical output in the UK requires an input of approximately E38 of chemical materials.In the US the corresponding figure appears to be 36 per cent of the chemical industries’ input drawn from its own sales. The other customers of the industry are very widespread : if we take the estimated UK input/output figure for 1963 as indicative of the spread of customers we see from Table 6 that, excluding the chemical industry itself as a customer, the UK industry exports 21 per cent. It sells 14 per cent direct to consumers and about 13 per cent into engineering and allied industries, but other areas such as agriculture and the construction industry, are also large consumers. There is no part of the economy into which products of the chemical industry do not enter and there are moderately strong forward linkages to many industries rather than exceptionally strong ones to a few.In some respects, one could regard the chemical industry almost as a service industry in its Table 6. The UK chemical industry’s customers, 1963 Consuming industry Agriculture etc. Coal mining and other mining Food, drink and tobacco Mineral oil refining Metal manufacture Engineering etc. Textiles, leather Other manufacturing Construction Gas, electricity and water Other services Public authorities Consumers Exports fm. output Cairns 4 % of 91 5 . 0 I 6 0 . 9 6 . 9 2.6 4 . 7 125 48 85 238 13.1 26 1.4 145 7 .9 I l l 6.1 1.2 22 161 8 . 8 109 6 . 0 262 14.4 382 21.0 47 relationship to its consumers. While this is true of the industry as a whole, certain sectors may be very strongly linked to a consuming industry. Thus Class 23 1-synthetic rubber-is strongly bound to the motor-car industry and depends on the latter's prosperity to a very great degree. 31 .O 1133.8 2414 0 19.2 656.7 669.4 763.4 137.0 77.4 160 5 650.0 1312 8 2675.9 * 1965 figures World trade If we now turn to world trade, we find again that chemicals play a large part. Table 7 shows exports and imports for 1966 for a number of countries and from this an indication of the balance of trade of each individual country in terms of chemicals. In the next column imports are expressed as a percent- age of exports and then exports and imports as a percentage of turnover.Most countries are net importers of chemical products. The strong exporting countries are the United States and Germany, with a very large gap separating these two from the remaining countries who have a positive balance of trade. The UK itself still has a considerable excess of exports over imports although the inward trade now amounts to over 60 per cent of the outward. Since 1966 the position has worsened and during 1968 up to October the value of imports represented 70 per cent of the value of exports. The export market position does, of course, vary from sector to sector. Thus, pharmaceutical products have the largest excess of exports over imports but we have a negative balance in organic chemicals and in fertilizers, the latter perhaps to be remedied when Shellstar is fully in operation. As a percentage of turnover the UK does not export an outstandingly high proportion compared with several European countries but it must be remembered that the relationship of the EEC countries now permits great Table 7.World trade in chemicals, 1966 (in $m.) Exports 90. I 422.3 346.0 126.0 Imports 204.5 467.4 513.9 265 5 172.9 840.5 876.6 93.7 578 497 590.9 189 8 312 8 361.8 367.3 825. I 745.2 Country Austria Belgium Canada Denmark Finland France W. Germany Ireland Italy Japan Netherlands Norway Spain Sweden Switzerland United Kingdom United States 48 Balance of trade - 114.4 - 45.1 - 167.9 - 139.5 - 141.9 293.3 1537.4 - 74 5 78.7 172.4 172.5 - 52.8 - 235.4 - 201.3 282.7 487 7 1930.7 Imports as Exports as /mports as % of % of % of turnover 48 48 26 94" 69 * 14 It turnover 21 44 18 45 * 12" 19 30 23 * 13 9 50 48 * 5 23 76 19 7 exports 226 Ill I49 21 I 558 74 34 493 88 74 77 I38 406 225 57 63 27 IlO* I I 7 38 67 * 18 52 43 12 2 Source: OECD R.I. C. Reviews freedom of movement of exports between them and the very large external trading both inwards and outwards carried on by such countries as Belgium, The Netherlands and Switzerland should be noted.In spite of the very large volume of exports from the United States this still represents a very low proportion of their total chemical turnover. The US is almost self-supporting in chemicals and imports represent less than 2 per cent of overall turnover. It must be remembered that the rate of development in chemical processing and the degree of novelty is considerable. Thus even an industry as large as that of the US still finds it necessary to import specialized products sufficiently new to be manufactured in only one or two countries. Chemical trade between developed countries is likely to continue to grow because of the increasing complexity of manufacture. Basic chemicals will always tend to be manufactured more generally. Their relatively low value means that freight plays a significant part in their cost and hence there is less tendency for them to figure in trade between developed countries.Rationalization of manufacture, economy of scale and emphasis on specialization will continue to foster international trade and indeed this is in the interests of efficiency. This rationalization of international chemical industry can be disturbing to politicians concerned with month to month scrutiny of balance of payments statistics. Nonetheless, to embark on processes of import substitution without bearing in mind the long-term factors would be penny wise and pound foolish. In the past tariff barriers played a large part in deciding the degree and direction of international chemical trade, Within the EEC and within EFTA such problems are no longer important but tariffs between the various groups of countries are still significant. They are reducing under the applica- tion of the Kennedy Round but until, for example, the US changes its method of calculation of the duty rates on chemical imports, there will still be con- siderable obstacles to trade between the US and Europe.Freedom brings additional worries to governments if young industries and new manufacture are to be allowed to take root. Much of the remaining UK chemical tariff structure is associated with the old key industry protection given after the first World War to safeguard, for example, the still small UK dyestuffs industry.The whittling away of such protective barriers places on governments the responsibility to keep a keen eye on dumping. It is an unfortunate consequence of the present development of the chemical industry in different countries that from time to time large excesses of capacity are created and there is very great temptation to keep these capacities filled by what amounts to dumping in other countries’ markets. In the UK, we ascribe much of the fall-off in chemical investment in and around 1961 in the newer plastics to the very damaging effect of dumped exports from excess capacities overseas. In general, however, the principal markets of the free enterprise world are controlled by well entrenched oligopolies. The fact that much of the output of the chemical industry is sold to other chemical companies has led to a complex formal and informal network of relationships which effec- tively regularize access to markets.Indeed, there are quite often international allocations and quotas. In many fields it is pretty well impossible for a small Cairns 49 new producer to enter a world market although a newcomer, backed by economic power such as an oil company, can decide to enter the petrochemical market and be successful. In general, the chemical industry does not like to compete on price and to squeeze profit margins. It has a desire for an orderly system and, perhaps more in the UK and European markets than in the US, a preference for an orderly existence. Competition in chemicals is at its keenest in the improve- ment of manufacturing processes and in the development of new and improved products.The growing emphasis on scale in manufacture, the need for ever larger plants, the insatiable appetite for capital, the importance of research and development are all factors which are leading more and more to the growth of larger chemical companies. This process has long been apparent in indi- vidual countries-in the United Kingdom the major chemical company, ICT, was formed as long ago as 1926. Amalgamations and mergers are still taking place in the UK, in the US, in Germany and in other countries. In the last year or two the emergence of larger groups has been most apparent in France. The stage is now set for the growth of truly international chemical companies operating on a world- wide basis.Some of the largest groups in the industry have been part of the international scene for a long period. ICI has always had considerable over- seas interests and today these represent over 50 per cent of group sales. Leading American companies, perhaps particularly those on the pharma- ceutical side, have for many years been established in the UK and other European countries. Today, most of the big 10-Du Pont, Union Carbide, ICI, Hoechst, Monsanto, Bayer, W. R. Grace, Dow Chemical, FMC Corpora- Table 8. Growth rate of the chemical industry Increase in production over the period Country /959/66 (%) 40 * I08 95 105 89 I37 200 I36 53 202 93 Austria 95 Belgium 83 Canada 65 Denmark Finland France W. Germany Ireland Italy Japan Netherlands Norway Spain Sweden Switzerland I I 6 United Kingdom 52 United States 108 * Over 4 years from I96 I 50 R.I.C.Reviews tion and sASF---have considerable overseas activities although it is unlikely that any company approaches the relative scale of ICI's overseas business." There is no doubt that over the next decade we shall see further growth in this direction. It is clear that while the great consumer goods companies have long realized the importance of international brands, so the major chemical companies with their growing involvement in the end use of their products are finding the importance of establishing themselves in world markets.From this it is a short and generally essential step to actual manufacture in different areas. It might be suggested that the extension of this inter- national aspect and the establishing of manufacture in widely separated marketing areas might lead to a diminution in export trade. This is generally not so. The establishment of a manufacturing unit in another country leads not only to the flow of intermediates, but the growth of that unit steadily involves it in the expanding export trade of the parent company and a constant flow of products which the subsidiary does not yet manufacture will find their way overseas. Future growth We have seen that the chemical industry forms a not inconsiderable part of the output and investment in most developed economies.Will this increase or decrease in proportion in the future? The economics of the developed countries depend on a number of dynamic sectors of which the chemical industry is one. It and a few other industries have expanded faster than the economy as a whole. The overall growth in the same selected countries is * More and more chemical companies are arranging to be listed in foreign stock exchanges. Table 9. Increase in industrial production and chemical production, 1966 over 1965 Country Austria Belgium Canada France Finland W. Germany Ireland Italy Japan Netherlands Norway Spain Sweden Sw i tzer Ian d United Kingdom United States Industrial Chemical production production (% increase) (% increase) 14.5 2 .8 8.3 Cairns 4.0 I .5 7 . 7 6 . 2 na 9 . 2 5.6 9 . 2 2.7 6 . 0 6 . 4 7.7 6 . 2 6.0 6.0 I I . 3 3.7 11.6 I .5 5.2 I I . 4 11.5 6.0 4 . 4 12.6 3.5 3. I 0 . 9 9 . I Source: OECD 51 ~~~~ ~ Annual per cent change (compound) I964170 Product group Table 10. UK projections to 1970 6.0 1.2 Ferti I izers 7 . I Dyestuffs 4 . 5 Inorganic chemicals Organic chemicals 15.4 Miscellaneous chemicals 4 . 9 Pharmaceutical preparations 7 . 6 Toilet preparations 4 . 6 Soap and polishes Oils and greases 3 . 4 Paint and varnish 4 . 5 PI ast i cs materials 20.3 shown in Table 8 and in Table 9 the increase in overall industrial production 1966 over 1965 is compared with the increase in chemical production in the same series of countries. As a whole the chemical industry, because of the widespread distribution of its markets amongst many industries, tends to develop in step with the general development of the economy.The ratios of the production figures in Table 9, of course, vary considerably and indeed they vary from year to year but in general it can be said that the chemical industry, for example in the UK, grows at about twice the rate of growth of the gross national product. Growth rates in the industry will vary considerably from one sector to another Attempts at estimating an overall growth rate for the industry were given in the 1965 National Plan but for individual operators it is much more important to try to project growth rates for individual sectors of the industry.This is not so easily carried out, although an interesting attempt was made by Crum in the National Institute Chemical Review for August 1966. He gives a number of estimated growth rates shown in Table 10 for various sectors of the industry calculated in line with the output projections of the National Plan. The latter projection was, of course, substantially too high but at any rate the operative projection rates of the groups are perhaps relatively valid. It is interesting to compare these figures with those of the Chemical and Engineering News Computer Forecasts for American Development shown in Table 11. It will be seen that some parts of the chemical and allied industry are under- going much more dynamic growth than others It is clear that the most important groups in the English scene are organic chemicals, pharmaceuticals and above all plastics.In the American figures we again see the considerable growth rate expected in these areas, although in this case the plastics group is divided between plastics and man-made fibres. Rubber is also given separately. It is not surprising that these areas should be the ones of expected substantial growth and indeed one can only question whether sights have been set sufficiently high. The steady improvement in the properties of man- made fibres and plastics materials is leading to extending uses through the manufacturing industries of developed countries.While there is no doubt 52 ~ R .I. C. Reviews 1963173 12.3 13.9 21.8 17.0 7.6 4.5 5.4 Table 11. American projections to 1973 Annual per cent change (compound) Product group Organic chemicals Inorganic chemicals P last i cs Man-made fibres Synthetic rubber Fe rt i I izers Paints Soaps etc. 5 . 3 an upper limit set for the use of textiles for clothing etc. there must still be an unbounded future for plastics and reinforced plastics materials as substi- tutes for other materials commonly used today, e.g. metals. The use of plastics materials in building and construction has barely started. The use of reinforced plastics etc. in motor-car manufacture is still minute. In these two areas alone there is great room for expansion.It is important to consider the availability of raw material for these in- dustries. The whole of this side of chemistry has been revolutionized by petrochemical developments in recent years and petroleum as a raw material for the organic chemist is now of supreme importance. In other parts of the world and now to an increasing extent in the United Kingdom natural gas has become available as a source of material. At present the world still has abundant supplies of oil and natural gas. The UK has been fortunate in its recent discoveries of gas, but these are still not an unbounded source of raw materials. It is understandable that the gas industry should seize avidly on supplies of natural gas as a means of improving its service as a supplier of fuel to domestic consumers and industry, but of much greater importance is the preservation of sufficient natural gas as a raw material for the products of the chemical industry. It is encouraging to note the recent agreements to supply North Sea gas to ICI (for ammonia, methanol and steam-raising) and to Shellstar.The former will take up to 250 m.ftS/day, the latter up to 35 m.ft3/ day. Proximity in the former case and high load factor have enabled the industry to negotiate lower prices than those offered to the British Steel ,Corporation, which finally settled on oil as its major supply for bulk fuel needs. Nonetheless, a price even lower than 4d per therm will probably be needed to make ICI properly competitive in the future against other natural- gas-based ammonia manufacturers overseas.Although reserves of natural gas and petroleum oils are at present bound- less, demands on them are ever increasing. Success in the preparation of cheap but really usable protein from mineral-based oils would again create a demand of very high potential and our successors may find themselves much concerned with conserving the earth’s natural resources as raw material for the vastly expanded chemical industry of the future. Let us hope that such resources of materials are not squandered before their significance is realized. Cairns 53 Research and development Growth rate in the chemical industry must depend not only on the general growth rate of the economy but on the input of research and development.A considerable part of the R & D effort in a developed country is expended in the chemical industry. For example, in the UK the industry ranks third as a spender. In 1964/65 the expenditure on research and development in chemicals in the UK cost E59.5 m., ranking third behind aircraft and elec- tronics. Table 12 sets the international scene, showing R & D expenditure in 1963/64 and its relationship to turnover for a number of countries. In the next column the R & D expenditure in chemicals is expressed as a percentage of total national R & D expenditure. In some countries the aircraft industry is negligible so that the chemical expenditure looks correspondingly higher. In the final column, therefore, the aircraft industry is as far as possible excluded.Table 12. Chemical research and development expenditure R & D expenditure 1963164 ( $ m . ) over in 1966 Country R & D Expenditure in chemicals, I962 As % of total Expenditure as % of turn- As % of total excluding aircraft (approximate) R & D 39.6 24.4 16.8 32.9 I .4 2 . 2 2 . 4 3 . 8 13 43 144 315 53 40 26 23 33 - - - - (4.3) - I .o 2.4 2.6 3 .O 4.7 24.4 - 11.6 12.6 Belgium Canada France W. Germany Italy Japan Netherlands Norway Sweden United Kingdom United States I68 (65) - 18 200 1805 Figures in brackets are estimates only DEVELOPING COUNTRIES - 24 18 20 Sources: NIESR and OECD So far, we have been concerned with the part played by the chemical industry in the economy of developed countries.This, indeed, is by far the greater part of the chemical industry of the world. The 17 or more countries given in Table 1 constitute 89 per cent of the world chemical industries outside the Soviet bloc and China. The remaining small tonnage is largely made in other countries such as Latin America, South Africa, Australia, leaving a minute proportion to be assigned to the emergent nations. What is of interest to us today is how this situation will change. The chemical industry in the developed countries has had a long period of growth, continually accelerating in pace. The beginnings were usually in such products as saltpetre for gunpowder, alum for tanning hides, potash and later soda ash for soap and glass.The largest chemical company in the world-Du Pont-dates back to 1802 when Euth&re Trknke Du Pont set up 54 R . I. C. Reviews a powder mill near Wilmington, Delaware. Early chemical industry centred to a large extent on sulphuric acid manufacture (US 1793; UK 1730). The modern alkali industry was a later starter in America; it was not until 1584 that a Solvay plant was erected at Syracuse. In Europe the first Solvay plant was erected in Belgium about 1860 and the first UK plant at Winnington in 1873. On the other hand, electrolytic production of chlorine and caustic soda started in America in 1896 (UK 1920). Organic chemicals were very much a European and especially a German preserve until World War I (in 1914 Germany accounted for 87 per cent of the world’s dyestuffs).A new industry based on coal tar was erected over the years 1914 to 1916 both in the UK and the US. This time-table of development, spread over many years like development in other manufacturing processes, is not acceptable to developing countries eager to make rapid strides in improving their standard of living. On the other hand, it is very difficult for a developing country to plan the right route to expansion and to decide which task to tackle first. Should there be develop- ment in basic industry such as sulphuric acid, alkalis etc? Should advantage be taken of mineral oil deposits to proceed to the manufacture of petro- chemicals? On the other hand, should manufacture be developed first in the fringe industries such as detergents,paints, pharmaceuticals or indeed should the country concentrate first on improving its agricultural base before becoming a manufacturing nation? The last of these, expansion in agri- culture, may often be more sound financially but is not necessarily as attrac- tive to aspiring governments.An important factor in decision is, or should be, whether the country concerned can support an adequate scale of manufacture. In the chemical industry today, certainly where the commoner chemical products are involved, economy of scale is of paramount importance. In the manufacture of basic chemicals, sub-continental size markets may well be required for efficient operation.Size is not necessarily so significant in specialist areas-as Sir Paul Chambers said recently ‘where there are no technological advantages in having manufacture on a large scale, or having a great variety of products, there is generally a strong case for seeing that the size of a business and its complexity does not grow beyond what is ideal for the maintenance of simple efficient internal communication.’ Smaller countries determined to establish manufacturing industries will often find other alternatives more attractive than heavy chemical production with its requirement for large plants and large capital investment. The very large plants characteristic of modern chemical industry are run by a handful of operatives and can provide little attraction for developing countries with large excesses of untrained labour.Little can be expected of the chemical industry in directly mitigating an unemployment problem although, of course, secondary industries based on the chemical industry are generally bigger users of labour. The fringe areas-soaps, cosmetics etc.-are much more likely to be suitable greas of manufacture for smaller and perhaps less developed markets. It can even be feared that the establishment of too much chemical industry may itself lead to a bias in favour of certain types of consumption which do not help in the development of the country. It is Cairns 55 even possible for secondary industrial developments to be wrongly chosen because of the necessity to import intermediate materials to keep these in operation.This can produce an inflexibility of foreign exchange demand which may lead to an unbearable strain. The most important item in the economy of scale brought about by large plants is the reduction of unit fixed investment required as productive capacity increases. A secondary, generally minor factor, is a sharp reduction of unit labour requirements. Economies may also arise in raw material usage and procurement. There is also economy of scale when customers themselves grow larger and a not inconsiderable part of the apparent greater efficiency of the American chemical industry may be associated with economy of scale in distribution to large consumers.300 200 600 ~~ 12 1.66 3.39 6 90 18 136 44 1.35 2.40 6.45 73 27 128 344 130 611 0.16 0.44 I69 Econometric models The United Nations' planners have done a great deal of work in the prepara- tion of models for economies of smaller countries. In a model in Mexico, for example, 20 separate commodities were studied including soda ash, caustic soda, chlorine, fertilizers, sulphuric acid, ethylene and various polymers, carbon black etc. The optimal programme indicated that it would be ad- vantageous to produce each of these commodities domestically. Some calcu- lations of population size required at different income levels to support chemical plants of a given size have also been produced. Table 13 shows the necessary population in millions at each of four relatively low income levels necessary for the support of'an economic basis of the output of smaller and large sized chemical plants.A number of products have been covered from which caustic soda, soda ash, and butadiene have been selected. It can be seen that at the very low income of $100 per capita a very large population would be required for even a minimum size (by developed-country standards) soda ash plant. Even by the time we reach the income of $600, which is Soda Ash (Solvay) Consumption kg per capita per year Required population, 175 000 tons/year 825 000 tons/year Butadiene Consumption for rubber, kg per capita per year Required population, 27 000 tons/year Table 13. Population (millions) at various income levels needed to support mini- mum and maximum size chemical plants Income levels ($US per capita) 100 ~ Caustic Soda (electrolysis) Consumption kg per capita per year Required population, 20 000 tons/year I50 000 tons/year 230 000 tons/year 0.55 1.10 36 273 0.51 243 1617 1438 0.78 2.21 12 104 35 295 61 523 ~~ Source: United Nations R.I.C.Reviews 56 considerably greater than the per capita annual income in many parts of the world, something like 128 million people would be needed to support a European sized soda ash plant although a smaller sized plant could economi- cally supply a population of 27 million. Secondary processes could be shown to be more feasible than more basic ones.For example, a GRS-type plant of minimal size, say 18 000 tons/year, can be supported by a population of 18 million having a per capita income of $300. On the other hand, although butadiene may represent 80 per cent of the raw materials required for this plant, to manufacture butadiene economically requires a population of 35 million at the same income level. It follows that it is important to plan chemical processes in complexes as far as possible so that the scale problem can be attacked by the joint utiliza- tion by several secondary users of a common intermediate chemical. It also follows that grouping of markets on a regional basis may provide the neces- sary scale. For example, it has been demonstrated that production costs on a scale suitable for the Chilean market tend to be about 125 per cent higher than the production costs of the same product at the scale of demand of the whole Latin American market.Trading groups where tariff barriers may be eliminated may have a great significance in economic expansion of the chemical industry of the area provided care is taken to see that not every member manufactures the same product. This is a desirable practice but not always easily followed. Governments frequently tend to press for local production facilities for a product on a ‘me, too’ basis. For example, a few years ago the demand for modern washing powders in the Middle East probably amounted at most to 20 or 30000 tons a year. This is hardly enough to keep a single plant occupied in a developed country and there are many plants larger than this.Nonetheless, national pride and, of course, a desire to reduce the import bill led over a period of three or four years to the erection of 10 or 12 individual detergent powder plants scattered over the area in different countries each of which produced one or two thousand tons a year and operated over relatively short periods. Obviously, on an international basis this represented extremely poor operating efficiencies and economics. In such a case a single plant would have served the area admirably, whereas a lot of plants were put up, no doubt each including in their plans the probability of exporting to the others. It seems likely that many parts of the world, certainly where populations are smaller, will continue to be served by exports from the developed countries. As the smaller countries develop manufacture will probably, or should probably, be concentrated in the fringe areas which are less capital intensive and generally have higher labour usage.One exception should perhaps be chemical fertilizers. Tremendous scope exists today for raising crop yields, particularly in the Near East, Africa and Latin America. Indeed the growing populations of the world will make it essential for such an increase to be brought about in many areas. Much of this increase depends on the supply of chemical fertilizers. If demand expands quickly the scale of consumption could quickly rise to an economic production level and make it desirable to erect fertilizer plants and in this case quite large plants could be quickly justified.Cairns 57 FUTURE GROWTH Reference has already been made to the growth of international companies and, in common with other industries, we can foresee continuing growth in size by acquisition and merger in the chemical industry. The only limitations which can be expected on this development are those deriving from national legislation. In the US especially the anti-trust legislation can place a limit on growth by acquisition and merger and the US chemical majors will prob- ably grow largely by growth of the US market and by continued expansion overseas rather than by mergers at home. At the smaller end of the US industry-and there are still a large number of small companies-such mergers will continue. In the UK one can see the industry growing partly by mergers, partly by expansion overseas and partly by the continued growth of the petrochemical companies. The latter-Shell, BP, Esso etc.-are capable of presenting ICI with considerable competition in the organic field in the years ahead. This must be good for the UK industry. I have referred elsewhere (Chemistry in Britain, 1968,4, 544) to the fact that although ICI (UK) may be growing at 8.4 per cent, the rest of the industry must, on a weighted basis, be responsible for a rate of only 3.4 per cent, if the overall UK chemical industry has only achieved 5.3 per cent (over the years 1963-67). One of the UK industry’s handicaps is the size of the home market. It is essential for its prosperity to win a larger share of the European market. It does not matter very much whether this is done by UK entry into the EEC, or by greater investment by UK chemical companies in western Europe. Tbo much Government restriction on such investment will be extremely short-sighted. In the end the prosperity of the industry must rest on its ability to tap this larger market. Otherwise it will become progressively less resistant to the large European companies which grow rapidly stronger as they serve the whole EEC market. A further decline in strength must inevitably lead to a further growth of imports into the UK, without the ability to balance them by corresponding exports to the EEC. If direct investment proves impossible opportunities should be explored for joint ventures, Continental marriages, collaboration and so on. This is a second best and rationalization schemes for increased efficiency and economy in production might well not be in our favour and again not beneficial to the balance of trade. All this adds up to a belief in a continued remarkable growth rate for major sectors of the chemical industry, internationally. In particular the areas of plastics, pharmaceuticals and organic chemicals must continue to expand at a great rate. In undeveloped areas fertilizers should grow sub- stantially. The present considerable part played by chemicals in the world economy will tend to increase even further and the power of the great com- panies will become a still greater factor in international business. R. I. C. Re views 58

ISSN:0035-8940    

DOI:10.1039/RR9690200041

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Dept of Chemistry and Applied Chemistry, University of Salford, Salford 5, lancs. lntroduction Experimental techniques . . Pulse apparatus, 65 Acoustic resonator, 66 Brillouin scattering, 66 . . . . . . . . SOME CHEMICAL APPLICATIONS OF ULTRASONIC ABSORPTION MEASUREMENTS IN 'THE LIQUID STATE E. Wyn-Jones, B.Sc., Ph.D., D.Phil. . . . . 59 . . 64 * . . . . . * . . . 68 79 . . . . . . . . . . 81 . . 78 . . . . 83 . . 83 . . 81 . . . . . . . . .. . . . . .. Applications to fast chemical reactions . . .. Electrolytes in water, 70 Non-electrolytes in water, 72 Rotational isomerism, 74 . . . . . . . . Molecular association, 77 Applications to reactions of biological interest Vibrational relaxation Critical point phenomena .. Shear and volume viscosity phenomena Recommended reading for the specialized reader References . . . . . . . . .. . . INTRODUCTION This article discusses the type of chemical information that can be obtained on the molecular level from measurements of the absorption and velocity of sound in liquids. Although many chemists are not acquainted with the application of the ultrasonic absorption method or, as it is sometimes called, the molecular acoustic technique, it should be mentioned that Professor Manfred Eigen, who shared the Nobel Chemistry Prize in 1967, uses this technique in his relaxation studies on the kinetics of fast reactions. In this review article a brief introduction to the principles of sound absorption and relaxation is followed by an account of the experimental technique and various chemical applications.The review ends with a list of text-books and general, articles on the subjects, which are recommended to the specialized reader. Experimentally it is possible to produce sound waves which can have a frequency from as low as a few cycles per second to as high as 10 Gigahertz ( 1O1O Hz). This acoustic spectrum can be divided roughly into three regions- the audible range (< 14 kHz), the ultrasonic range (14 kHz-800 MHz) and the hypersonic range (> 109 Hz). When a sound wave passes through a liquid the excess pressure at any point will alternate periodically because in . . . . . . . . . . . . . . Wyn- Jones .. . . 59 each cycle of the sound wave a given volume of the liquid is successively compressed and then decompressed.In most liquids (except water) the specific heat at constant pressure, Cp, is greater than the specific heat at constant volume Cv, thus y (= C,/C,) > 1 and the passage of a sound wave is automatically adiabatic. This means that a periodic variation in the tempera- ture will accompany the compression-decompression cycle of the sound wave. The pressure amplitude, p , of a one-dimensional sound wave travelling in the positive x-direction is given by: 1 p = poexp(- ax) exp iW(t - x/c) . . I . where po is the pressure amplitude at x = 0 and t = 0, a is the absorption coefficient related to the intensity of the sound wave, c is the phase velocity and w = 2nf where f is the frequency of the sound wave.In equation 1 the quantity poexp(- ax) represents the maximum amplitude of the sound wave after it has travelled a distance x in the liquid and is, therefore, a measure of the sound intensity. Thus the loss of sound intensity in a liquid depends on both the distance travelled by the wave and also the absorption coefficient a. The term exp io(t - x/c) is simply a function which describes the periodic motion of the sound wave. The real part of equation 1 is analo- gous to the Beer-Lambert law governing light absorption. The velocity of sound in a liquid is given by the following relationships: and c2 = 1 / p p . . . . .. . . . . 2 c2 = (y - 1) Cp/@TpV . . . . . . 3 where p is the density, /3 the adiabatic compressibility, 8 the coefficient of thermal expansion and V the molar volume.From ‘classical’ theory Stokes showed that the absorption coefficient a of a sound wave passing through a liquid is a function of the viscosity and the contribution aclass. from shear viscosity, qs is aclass. = (2n2/pc3) (4~,/3)f . . . . 4 The action of a sound wave in successively compressing and decompressing a given volume of liquid will cause this volume to alternate periodically about a given value. When these volume changes occur, the molecules of the liquid have to flow from a more compact to a less compact structure in the direction of the motion imposed by the sound wave. Since viscosity is defined as resistance to flow it follows that a liquid can have a ‘volume’ or compressional viscosity in addition to the more familiar shear viscosity.In deriving equation 4 Stokes assumed that viscous losses in liquids due to a motion of uniform compression are zero. Although recognizing the possibility that a liquid could have a volume viscosity, yv, Stokes had to assume qv = 0 because of a lack of any suitable method to measure this quantity. It is now known from hydrodynamic theory that the full expression for a is . . 5 R.I.C. Reviews 60 In addition to viscous losses, Kirchoff showed that a is also a function of the thermal conductivity, K, of the liquid according to equation 6: For all liquids except liquid metals the contributions to 1~ from thermal conductivity given by equation 6 are negligible.The excess absorption in any liquid is generally termed as the difference between the measured value CII and the classical value aclass. given by equation 4. Thus, the volume viscosity qv is 4 (a - aclass.) %lass. \ Coupling mechanism I Conformational energy ~ V v = 37" When this volume viscosity arises from a perturbation of a molecular equili- brium due to the temperature or pressure changes imposed by a compres- sional wave, ultrasonic relaxation will occur. Several different kinds of mechanisms are responsible for volume viscosity in liquids and within the limit of present knowledge all are relaxational in behaviour. This statement can be illustrated if we consider the total energy of a liquid in terms of the 'energy box' introduced by Litovitz.2 The total energy is the sum of many components such as translational, vibrational and the less familiar energy Vibrational energy Energy transfer Fig.I. A modified version of the energy box introduced by Litovitz. Wyn- Jones 6 7 . . . . . . 61 due to the degree of order which is brought about as a result of molecules aggregating in a quasi-crystalline state. In addition, a liquid can have energy due to the occurrence of a chemical equilibrium such as that between two or more conformers of a molecule. In Litovitz’s energy box these dif- ferent energies are considered separately as illustrated in the diagram Fig. 1 where each separate energy is contained in a segment. When the total energy of the liquid changes, the different energies in each segment will also change.Each energy segment is in contact with the other so that energy can flow from one to the other. This flow of energy is governed by some coupling mechanism which is relaxational in behaviour. This means that energy will flow from one segment to another at an exponential rate and with a finite time constant, T, known as the relaxation time. This relaxation time depends on the nature of the coupling mechanism which governs the energy transfer between segments. Jf the temperature of the system is raised, this will automatically increase the translational energy of the molecules and, in turn, some of this energy will be transferred to all the other energy modes until a new equilibrium is reached.It is well known that the mechanism of energy transfer between translational and vibrational modes occurs via molecular collisions. The local temperature rise accompanying the compression period of a sound wave will automatically increase the translational energy of the molecules and, in turn, all the other energies. After the sound wave has passed through the crest of the compression half cycle this energy begins to return to the sound wave. If all this energy returns while the sound wave is still in the same compression cycle there will be no net loss of energy during that cycle. On the other hand if some of this energy returns to the sound wave out of phase during the decompression period there will be a net loss in sound energy during that cycle and absorption in addition to the viscous contributions of equation 5 will occur.A factor which determines whether there is a loss in the sound energy per cycle is the rate of the energy transfer in relation to the frequency of the sound wave. Consider the passage of a sound wave through a hypothetical system which is composed of only a chemical equilibrium of the kind 8 A + B ( + AH) .. . . . . Because there is an enthalpy difference between two forms, the temperature fluctuations accompanying a compressional sound wave will impose a periodic change in the chemical equilibrium about a mean value. At very low acoustic frequencies the periodic changes in both the sound wave and the equilibrium will be in phase.This corresponds to the situation where enthalpy is taken and returned to the sound wave during the same cycle. If the acoustic frequency is progressively increased the equilibrium will not be able to respond instantaneously to the temperature fluctuations and will start becoming out of phase with the sound wave. This lag enables the equilibrium to accept enthalpy during the temperature crest of the sound wave and to give up enthalpy during the temperature trough with an inevitable attenu- ation of the sound wave intensity as the wave travels through the liquid. When the acoustic frequency is very high the equilibrium is unable to follow R.I. C. Reviews 62 \ \ /--q Velocity \ \ / / Frequency - _ _ _ _ - - l:.ig. 2.The variation of the ultrasonic parameters a/f2, p and c with frequency during a single relaxation process. the temperature fluctuations imposed by the sound wave, again resulting in a zero loss of sound energy. The maximum loss in sound energy will occur at an acoustic frequency fc given byfc = 1/27~T where T is the relaxation time of the chemical equilibrium. This time lag in the transfer of energy in rates for the two-state process equation 8, T = (k12 + kzl)-l where the k’s a chemical equilibrium is obviously related to the forward and reverse are the forward and reverse rate constants respectively. The simple processes described above are the basic principles involved in the relaxation techniques3 to study the kinetics of fast reactions.Two factors determine the magnitude of the loss of sound energy per cycle. These are the amount of energy shared and returned out of phase (in the chemical equilibrium 8 this is proportional to AH) and secondly the time constant T. The behaviour described above corresponds to acoustic relaxation in a liquid. In liquids there are several different molecular processes* that may give rise to ultrasonic relaxation such as a chemical equilibrium, vibrational relaxation and structural processes such as compressional and shear relaxa- tion. Provided the respective relaxation times are well separated then each process can be studied separately. The behaviour of the acoustic parameters during a single relaxation process is shown in Fig. 2.At any given temperature the quantity a / f 2 will vary with frequency according to the equation: . . 9 where A is a relaxation parameter, fc is the relaxation frequency (= 1/2m-) and B represents contributions to a/f2 which are not related to the relaxation. These contributions are shear viscosity (equation 4) and any excess absorption 63 Wyn- Jones 5 due to a relaxation process with a relaxation frequency much higher than fc. The loss per cycle or absorption per unit wavelength relating to the relaxation p is 10 p = afA . . . . . . . . where a' is the excess absorption for the relaxation process. Thus * - p = (a - Bf') h = ACf/(l + (flfc)') Whenf=f, (COT = I), p reaches a maximum value p m given by: pm = 4ACfc .. . . 11 . .. . . . 12 The dispersion in the sound velocity c is given by the expression 13 where the subscripts 0 and a: refer to the sound velocity at low and high frequencies. When ultrasonic relaxation occurs the sound velocity becomes frequency dependent in the relaxation region as shown in equation 13 (cf. Fig. 2). From equation 2 it also follows that P, the adiabatic compressibility of the liquid, must also be frequency dependent. This quantity varies with frequency according to the equation P = Pa3 + APl(1 + iwd where PO is the adiabatic compressibility at infinite frequencies and AP is the frequency independent part of the compressibility that arises from the relaxation process; this quantity is commonly referred to as the relaxing part of the adiabatic compressibility. From a consideration of the thermo- dynamic properties of a liquid where a single ultrasonic relaxation occurs it can be shown that 14 In the quantitative interpretation of ultrasonic relaxation processes the main problem is to relate AP/Po commonly referred to as the relaxation strength, and also T , to the properties of the actual process that is causing In the study of acoustic relaxation in liquids it is desirable to measure the velocity and absorption of sound over a frequency ranging from a few cycles per second to the hypersonic region.Experimentally this range can be more or less successfully covered and there are techniques available with which absorption and velocity can be measured in the range 102-1010 Hz.335 the relaxation.EXPERIMENTAL TECHNIQUES R.I.C. Reviews 64 The accuracy of these techniques varies from qualitative estimates to values considered accurate to within a few per cent. In this section a brief description of the principles of three versatile techniques covering the frequency range 105-1010 Hz is given. These techniques are also considered to give very accurate acoustic data. Pulse apparatus (106-109 Hz) The pulse technique is the most widely used for absorption and velocity measurements in the region 5-200 MHz. Figure 3 shows a block diagram of a typical apparatus. A train of pulses is produced by the pulse generator and fed into the transmitter which is set at the desired frequency. These pulses are used to excite this oscillator which in turn produces bursts of oscillations at the desired frequency, f.The resulting radiofrequency pulses are fed to a transducer which is acoustically coupled to a fused quartz delay line. The acoustic pulses produced by the transducer pass through the quartz delay line, through the liquid under test and into a second quartz delay line the upper end of which is attached to the detecting transducer. This second transducer retransforms the acoustic pulse into an electric pulse which is amplified and demodulated at the receiver and finally displayed on the screen of a cathode-ray oscilloscope. A second train of pulses is produced by the pulse generator and used to excite a comparison oscillator which is set at 0 Pulse generator Transmitter Detecting transducer n osci I lator Cathode-ray oscilloscope Fused quartz Fused quartz delay line u delay line Wyn-Jones Comparison Attenuator m 0 0 0 I Receiver Test liquid Fig.3. A block diagram of the pulse apparatus. 65 the same frequency, f, as the transmitter. These radiofrequency pulses, after suitable delay, are fed into an attenuator, then to the receiver and finally are displayed at the side of the acoustic pulse on the oscilloscope. The intensity and hence amplitude of the acoustic pulse is changed by varying the distance between the launching and detecting transducers. This change of amplitude can be measured by visually comparing the height of the acoustic pulse with that of the ‘comparison’ pulse which is attenuated by a known amount.By plotting the attenuation against path length, the absorption coefficient, 01, follows from equation 1 since The quantity ln(po/p) is directly proportional to the attenuator readings and x is the acoustic path in the liquid. The lower frequency limit of this technique is about 4.5 MHz. At frequencies below this value the sound beam travelling to excitation acoustic be corrected. By use of high frequency experimental thus in the liquid frequency starts of diverging - 4 x 1010 and Hz can the be produced. Using of measurements a quartz quartz resona- rod have an tors of this kind instead of transducers in the mechanical unit, absorption measurements at frequencies of 1 GHz have been measured with this tech- nique.The velocity measurements are done by using the apparatus as an acoustic interferometer. This is done by counting the number of beats between two pulses which have travelled respectively one and three paths in the liquid sample over a known change in path length. Acoustic resonator This is a relatively new technique6 which has been used successfully in the region 100 kHz-50 MHz giving very accurate velocity and absorption measurements. In principle, the liquid sample is kept in a cylindrical cell whose ends are sealed with transducers (quartz crystals) which are both matched in frequency and are accurately parallel. Acoustic waves can be generated in this system by exciting one of the transducers by means of an oscillator.When conditions are such that the length of the cell is an integral number of half wavelengths of the sound waves in the liquid, the system will resonate. The resonance frequency of the system is proportional to the velocity of sound in the liquid and the shape of the resonance peak is related to the absorption coefficient of the sound wave. In practice the resonance peak of the ‘resonator’ isnobtained by plotting the output voltage against frequency in the resonance region. This can be done graphically or on the screen of a cathode-ray oscilloscope. Brillouin scattering The discovery of laser action has been entirely responsible for the experi- mental advancement of this technique to study the acoustic properties of liquids at very high frequencies (109-1010Hz).In a liquid medium, an as- sembly of thermally excited sound waves (sometimes called Debye elastic waves) are present with a range of wavelengths varying from the dimensions of the container to interatomic distances. These thermally excited acoustic R.I.C. Reviews 66 Lens Test sample Fa b ry- Pe rot interferometer Lens Detector - Fig. 4. A block diagram of a Brillouin spectrometer. waves, sometimes called phonons, perturb the electrons and hence the polarizability of the liquid, i.e. they can be considered elastic and will com- press and rarify. This in turn produces a periodic fluctuation in the refractive index. Brillouin7 first showed that these phonons affect light scattering. If monochromatic light is incident on the medium, the Rayleigh scattered light contains, in addition to the incident frequency, the Brillouin components which arise from light scattered as a result of ‘reflection’ (or more correctly Doppler shifting) of these sound waves.The resulting spectrum consists of a central component which has the same frequency as the incident light and the Stokes and anti-Stokes Brillouin components which are separated from the central Rayleigh line by an amount A v given by Av = -J-- 2vo(v/c) sin 8/2 where v o is the frequency of the incident light, c is the velocity of light, v the velocity of the thermally excited sound waves of frequency Av (- 109- 101OHz) and 8 is the scattering angle. The width of the Brillouin line is related to the lifetime and thus absorption coefficient a of the sound wave involved in the scattering process.A block diagram of a simple arrangement is shown in Fig. 4. Monochromatic light from the laser source is passed through the liquid sample. The scattered light is then collected at the desired angle and analysed with a Fabry-Perot interferometer. The resulting triplet spectrum can be detected either photographically or recorded using a photo- multiplier. When recording, a pressure scanned Fabry-Perot interferometer must be used. This technique is still at a fairly early state of development and should prove very useful in the hypersonic range. There are several other techniques which have been used for measuring the absorption and velocity of sound in liquids.395J The operation of some of these techniques is very tedious, others require very large amounts of liquids and in many cases the accuracy of the absorption measurements can only be regarded as semi-quantitative. When acoustic relaxation occurs the dispersion in the quantity a/f2 is Wyn- Jones 67 -I 2200 0.6 c - 1800 h - N N 1 k U - 1400 ‘5 - 1000 - 0 - 600 200 - 8.0 7.0 log f .. .. 15 Fig. 5. Experimental data for the rotational isomeric relaxation in I ,2-dibromo-2-methyl- propane. often appreciable and in most instances the dispersion curve can be measured accurately. On the other hand the dispersion in the velocity is usually very small. For example, when COT = 1, equation 13 reduces to for most relaxation processes.This means that the velocity dispersion is less than .the experimental error in most techniques (acoustic resonator and Brillouin scattering excepted). Thus when investigating relaxation processes it is desirable to measure the absorption coefficient a over the frequency range which spans the relaxation region. When a single ultrasonic relaxation occurs the results (i.e. a/f2 at different frequencies) are fitted into equation 9 and the relaxation parameters p m found via equation 12. When a multiple relaxation occurs the results are fitted into an equation of the type f” i = n a = C (Ai/(I + (f/fd2> + B) A typical example of the single relaxation curves found for the rotational isomeric relaxation in 1,2-dibromo-2-methylpropane is shown in Fig.5. The velocity dispersion was negligible. These measurements were carried out with a pulse apparatus. APPLICATIONS TO FAST CHEMICAL REACTIONS The ultrasonic absorption method has been used widely as a relaxation technique to study the kinetics and thus mechanisms of very fast chemical R. I.C. Reviews 68 equilibria.334 In the simpler equilibria which involve a single acoustic relaxa- tion the relaxation times (T = 1/2.rrfc) are related to the forward and reverse rate constants, klz and kzl respectively, and also to the concentrations of the reactants C, as shown in the following model examples. A + B A +2B T = (kiz + kzi)-l . . . . . . . . 16 T = (kiz + 4 k z i C ~ ) ~ l .. .. . . 17 = (kiz(CA 4- CR) 4- kzi)-l . . . . 18 19 by: A + B e A B A + B + c + D 7 = (kiz(C~ + CB) + kzi(Cc 4- CD)>-' In the unimolecular reaction 16 which occurs in ideal solution the relaxa- tion strength Ap/po is related to the thermodynamic equilibrium parameters [TOZp Vcz/JCp2] 20 where AH and AY are the enthalpy and volume difference between B and A, J is the joule, V the molar volume of solution and K the equilibrium con- Ap/po = R[(AH/RT) - (AY. Cp/V. ORT)]' [K/(1 + K)'] stant. In an ionic equilibrium of the kind AB + A + + B- and the equilibrium constant K is .. . . . . 21 which usually occurs in dilute aqueous solution, Cp/Cv = 1, and thus the passage of the sound wave is isothermal. In these types of reactions AV is finite and thus the equilibrium will be able to impose a volume contraction during the compression (or pressure) cycle of the sound wave and a volume expansion during the decompression cycle.This will also lead to an extra attenuation of the sound wave at frequencies around 1/277T. For those reactions where VOAH = CpAY occurring in a solvent where C,/C, > 1 there will be no acoustic relaxation because the temperature rise and compression crest of the sound wave have equal and opposite effects on the equilibrium. For the equilibrium 21 occurring in dilute aqueous solution, the full relationship between the relaxation strength AP/Po and the equilibrium parameters is : d a . . 22 where a is the degree of dissociation, the y's are the activity coefficients and PO the adiabatic compressibility at low frequencies.In most examples of equilibrium 21 the concentration of the ions are sufficiently dilute to assume a value of unity for the activity coefficients and also dln y :/do = 0. Some examples of different types of fast reactions studied by acoustic methods are described below. 69 Wyn- Jones Electrolytes in water In the dissociation of the weak electrolyte ammonium hydroxide in dilute aqueous solutions : NH,OH + NHZ + OH- . . . . . . 23 the equilibrium constant K is given by:9 K = 02C/(l - a) (= 1.75 x 10-5 at 20 "C) where C is the concentration of the ammonium hydroxide and (T the degree of dissociation. The above equilibrium is characterized by a single relaxation time T given by 18: T = [klz + kzl(CNH$ -k COH-)]-' where the C's are the equilibrium concentrations of the ions.For reasonably dilute solutions : and thus CNH$ (= COH-) = C a % 1/KC T = [kiz + k2121/KC]-l . . .. . . 24 Tamm et ~ ~ 1 . ~ 0 observed an ultrasonic relaxation in this system which was attributedll to a perturbation of the above equilibrium. The relaxation frequency was found to increase from 22 to 55 MHz as the concentration of ammonium hydroxide increased from 0.5 to 2.5 molar. A plot of 7-l against 2 d K C (equation 24) was a straight line whose slope gave k21 = 3.4 x 1010 s-1 mol-1 From Debye's theoretical calculationslz on the rates of collisions of ions in solution a rate constant of the order 1010-1011 s-1 mol-l is expected for a diffusion controlled reaction showing that the recombination rate in equation 23 is diffusion controlled.From the relationship K = k12/k21 it is found that klz = 6 x 105 s-1. The value of this dissociation rate constant shows that the reaction proceeds as shown in equation 23 and not via the alternative route NH, + H2O $ NH, + Hf + OH- + NH; + OH- which involves the dissociation of water. The dissociation of such a reaction would be governed by the dissociation rate constant for pure water3 which has a value of 3.4 x 10-5 s-1. This is much lower than the observed value of kl2. By use of equation 22 the volume change AV was found to be - 28 cm3 mol-1. This is in excellent agreement with the value derived from partial molar volumes :I1 AV = VNH; - VOH- - V N H ~ - V N ~ O = - 28 cm3 mol-1 Further ultrasonic measurements carried out at different pressures13 showed that k21 was independent of pressure.The recombination of NH4f and OH- is diffusion controlled and has no activation energy. The effects of pressure on diffusion coefficients are relatively small and so the experimental observa- tion that k21 was independent of pressure is expected. An ultrasonic relaxation observed in aqueous solutions of potassium R.I.C. Reviews 70 cyanide was attributed14 to the hydrolysis reaction : CN- + H2O (excess) + HCN + OH- . . . . 25 and analysis of the experimental data gave: klz = 5.2 x lo4 s-l, kzl = 3.7 x lo9 mol-l s-l and A V = - 12.4 cm3 mol-I. From the temperature dependence of the ultrasonic relaxation times an activation energy barrier, AH$, of 25 & 8 J mol-1 was found.The rate constant for the reverse hydrolysis k21 is much less than that found for the corresponding diffusion controlled recombination rate constant in equation 23. This can be understood if we consider the simple mechanisms for the proton transfer processes which are respectively : and m 5 N T 0 X h 4.0 NH; + OH- -+ H,N***H+*..OH- -+ NH,OH NCH + OH- + NC-***H+***OH- -+ CN- + H20 . . 27 for equations 23 and 25. In 26 the proton leaves behind an uncharged species (NH,OH) as it bonds chemically to the hydroxyl ion. On the other hand, in 27 the proton leaves behind a negatively charged ion as it transfers to the hydroxyl ion to form water.On purely electrostatic considerations more energy is needed for the proton transfer in equation 27 and the rate is, therefore, expected to be slower. In multi-step chemical reactions of the kind: A1 s A2 + A3 + A4 etc. a spectrum of relaxation times and frequencies is found with the number of relaxation times corresponding to the number of independent ~ t e p s . 1 ~ Because of the coupling between each state the relaxation times will not be the same I I 7.0 e ref. 10,16 0 ref. 17 A ref. 18 A ref. 19 I 5.0 5.0 log f Fig. 6. Ultrasonic relaxation data for magnesium sulphate. Absorption cross-section (Qh) per wavelength (QX is proportional t o p, the absorption per wavelength). [from: Physical Acoustics Vol.II, part A (Academic Press, New York), 1965. Fig. 22, p. 430. J. Stuehr and E. Yeager by courtesy of the publishers.] 71 Wyn-Jones 26 . . I 8 .O as those found by treating each step as an isolated two-state equilibrium. A typical example of a multi-step chemical reaction occurs in magnesium sulphate solutions where a double ultrasonic relaxation10J6-19 was found experimentally as shown in Fig. 6. This double relaxation has been inter- preted by Eigen and Tamml5 as a perturbation of the multi-state dissociation which takes place in three steps, thus State 0 is the diffusion controlled approach of the two hydrated, dissociated ions and contributes to the electrical conductivity of the solution. These ions can associate into three different ion pairs states 0, 0 and @, which are dissolved in solution and are electrically neutral.In steps 11 and 111 these ions come closer together owing to the stepwise removal of the waters of hydration trapped between the ions in step I. In state @ the ions are in direct contact with each other. This four-step model has been successful in quanti- tatively describing the multiple relaxation found in several 2,2 electrolytes. In magnesium sulphate ultrasonic measurements at different pressures21 also support the above model. Many other examples of the application of the ultrasonic method to study ionic equilibria are given in several excellent review articles.3322-24 Non-electrolytes in water Ultrasonic absorption and velocity measurements have been made in several solutions of non-electrolytes in water such as amines, alcohols, ethers and ketones. Andrae and his collaborators25 were amongst the first to attempt a quantitative explanation of the experimental data.In these systems the experi- mental observations were as follows. As the concentration of the solute is increased from zero there is a sharp rise in the sound velocity until a certain concentration is reached when the sound velocity reaches a maximum and then falls off with further increase in concentration. An ultrasonic relaxation was also observed in these systems and the maximum absorption per wave- length was found to reach a maximum at a concentration termed the peak sound absorption concentration (PSAC). The solute concentration relating to the maximum in sound velocity is not the same as the PSAC.The sharp rise in the sound velocity at the lower solute concentrations shows that there is a sharp drop in the compressibility of the solution (cf. equation 2) although the solute that is added is more compressible than water. This is explained in terms of a breakdown in the hydrogen-bonded structure of water, which results from a solute molecule preventing a number of the water molecules around it from maintaining hydrogen-bonded contact with the highly orientated water structure nearby. These ‘free’ water molecules pack closely around each solute molecule with a subsequent decrease in their own compressibility and that of the solute. On the other hand the ultrasonic relaxation is attributed to a perturbation of the equilibrium 72 R.I.C. Reviews between solute-water complexes and solute and water. Several model equili- bria were used in an attempt to explain quantitatively the absorption data. These were respectively : and A + Bm +ABm A + B + A B A + B + AB + B + AB2 + B + AB3 + B etc. Model 3 Model 4 A + B + A B and B + B * where A represents the solute and B and B* ‘free’ and ‘bound’ water respec- t ivel y . The general agreement found between experiment and theory in the application of some of these model equilibria to explain the experimental data is a good indication of their basic correctness. However, several details Mole fraction acetone - Fig. 7. Experimental points and theoretical curves for model IV used by Andrae et a/. (refer- ence 25) Acetone-water.[ from : J. H. Andrae, P. D. Edmonds and J. F. McKellar. Acustica. 1965, 15, p. 74-88, Fig. 6 by courtesy of the publishers, S. Hirzel Verlag, Stuttgart.] Wyn-Jones Model 1 Model 2 73 have to be worked out in order to get a more complete quantitative explana- tion. One of the drawbacks in testing these models was the lack of data on thermodynamic quantities such as Cp and 8. In the system acetone-water where most data were available the agreement between experiment and theory for model 4 is shown in Fig. 7, where the solid lines are theoretical and the points experimental. In some aqueous solutions of aliphatic amines a double ultrasonic relaxa- tion was observed.The lower relaxation frequency (< 10 MHz) was only found in those solutions whose temperatures were fairly close to the critical solution temperature, and it was attributed to a phase separation process. The higher frequency relaxation was found to follow the pattern described above. In the system dioxan-water Hammes and Knoche26 observed a double relaxation in the frequency region 5-200 MHz. This was attributed to the two reactions : DW+W+DW2 and DW2 + D + D2W2 where D represents dioxan and W is water. The ultrasonic relaxation observed at very low amine concentrations in amine/water mixtures3 has been attributed to the reaction : RzNH + H2O + R2NH; + OH- which corresponds to model I used by Andrae and his collaborator^.^^ Rotational isomerism Ultrasonic relaxation also occurs in molecules that exhibit the phenomenon of rotational isomerism.27928 Most of the studies of these molecules have been carried out in the pure liquids. The ultrasonic relaxation is attributed to a perturbation of the equilibrium that exists between conformers that differ in energy.The equilibrium is disturbed by the temperature fluctuations accompanying the passage of the sound wave. The rotational isomers of 1,Zdibromoethane are shown in Fig. 8 together with the potential energy diagram for rotation. In the liquid phase the molecules of this compound exist as an equilibrium mixture of the trans and two optically active and energetically equivalent gauche isomers. This equilibrium can be represented as a two-state process A + B where A represents the more stable trans isomer and B the gauche isomers.B is a degenerate state owing to the optical activity of the gauche isomers. The relaxation time, T, and hence the relaxation frequency,f,, of a two-state equilibrium is related to the forward and reverse rate constants kl2 and k21 respectively by T = (k12 + k21)-l = 1/2nfc Thus k21 = 2nfc/(l + K) where K(= k12/k21) is the equilibrium constant. By measuring the temperature dependence of the relaxation time and also 74 R. I. C. Reviews t Potential energy 60 240 I80 I20 3 60 0 300 Azimuthal angle - Fig. 8. Rotational isomers and potential energy diagram for 1,2-dibromoethane (x = Br). making use of the Eyring first-order rate equation: kzi = K [y] exp (AH&/RT) exp (ASZIR) the slope of a plot of log (kzl/T) against 1/T will yield AHZ the potential barrier hindering rotation for the reverse isomerization B -+ A.In many cases K, the equilibrium constant, is not known, but a plot of log (fc/T) against reciprocal temperature29 will still give a very accurate value for AHZ. Thus by use of the ultrasonic technique the barrier height hindering rotation in molecules can be obtained directly from experimental data. The equilibrium thermodynamic parameters, AH, AS and A V, the enthalpy, entropy and volume differences between the isomers, are related to the maximum absorption per wavelength p m through equations 14 and 20. The manipulation of these equations to yield the equilibrium thermodynamic parameters requires many approximations29 and in many cases the resulting enthalpy differences are found to be incompatible with those from spectro- scopic methods.Ultrasonic relaxation studies leading to values for the activation energy barriers relating to conformational changes in other systems include ring inversion in cyclohexane derivatives30 : Axial Equatorial Wyn-Jones 75 cyclic sulphites :31 Axial Equatorial dioxans : Axial Equatorial 0 \ as well as cis .+ trans isomerism in aliphatic esters:33 R2 / R Trans 0 Cis In Table 1 a selected list of the potential energy barriers for conformational changes in different types of molecules is given. The n.m.r. technique can also be used to determine the exchange rates (kJ mol-1) 18.8 between different conformers of a molecule.For 1-fluoro-1 lY2,2-tetra- Table 1. Some energy barriers hindering rotation in molecules found from molecular acoustic studies Type of isomerism AH2 potential barrier hindering rotation in reverse isomerization Mo/ecu/e I ,2-dichloro-2-methyl propane I ,2-dibromo-2-methyl propane acrolein cinnamaldehyde methyl formate ethyl formate met h y I cyclo hexane ch lorocyclohexane 4-methyl- I ,3-dioxan 4-phenyl- I ,3-dioxan trimethylene sulphite 4-methyl trimethylene sulphite 23 .O 23 .O 20.9 23.4 32.6 25.5-33.5 axial-equatorial 45.4 chair inversion axial-equatorial 50.2 chair inversion axial-equatorial 37.2 chair inversion axial-equatorial 18.I chair inversion axial-equatorial chair inversion 19.2 trans-gauche trans-gauche cis-trans cis-trans cis-trans cis-trans axial-eq uatorial chair inversion R . I.C. Reviews 76 4 c 3.a t- Y M - & 2.c 1.c C - I .( Fig. 9. An Eyring rate plot for the gauche + trans isomerization in I-fluoro-I, ,2,2-tetra- chloroet hane. chloroethane both ultrasonic and n.rn.r. techniques have been used to study the temperature dependence of the exchange rates between the isomers. The different results are compared in the form of an Eyring rate plot in Fig. 9 and show that the agreement between these two different approaches is very g00d.34 .O Molecular association The first complete treatment of an ultrasonic relaxation process leading to the energetics of an equilibrium involving molecular association was carried out by Lamb and Pinkerton35 in a study of pure acetic acid.The relaxation process was attributed to a perturbation of the equilibrium between monomer and dimer molecules. The interpretation of the results for the pure acids, however, is not unique36 and there is also a possibility that internal rotation 3.0 Wyn- Jones 0 Ultrasonic data I 1 I 1 I I 6 .O 4 .O 0 n.m.r. data I f I O - ~ K- 77 about the C-0 bond (as found in esters) may contribute to the relaxation process.37 The monomer + dimer equilibrium in benzoic acid:3 has been measured in both carbon tetrachloride and toluene solutions.The rate constants for the dimerization reactions (cf. model equation 17) involving the formation of the hydrogen bonds are of the order 109 s-l mol-l i.e. almost diffusion controlled. Ultrasonic relaxation has also been observed in solutions of 2-pyridone,3* caprolactam and N-methyl acetamide39 in organic solvents. The mechanism of these relaxation processes is a perturba- tion of the following type of monomer-dimer equilibrium : APPLICATIONS TO REACTIONS OF BIOLOGICAL INTEREST While the examples discussed in the above sections are of interest because of their own intrinsic value they also form the basis of some recent work involving chemical reactions in the biological field. It has been shown that ultrasonic absorption and velocity measurements can be used as a probe to study microscopic solvent structure; in particular the behaviour of water in certain aqueous solutions (cf.non-electrolytes in water). Successive additions of urea, guanidine chloride, ammonium chloride and sodium chloride to water appear to have similar effects in causing a breakdown in the hydrogen-bonded structure of ~ a t e r . ~ O , ~ l On the other hand, an ultrasonic relaxation was observed in an aqueous solution of the synthetic polymer polyethylene glycol which was attributed42 to a hydrogen- bonded equilibrium between the polymer and water. When the behaviour of this relaxation was investigated in the presence of urea, guanidine chloride, ammonium chloride and sodium chloride it was found that the effects of urea and guanidine chloride were different from those of ammonium chloride and sodium chloride.Since urea and guanidine chloride are well known protein denaturants it was concluded that their distinctive effects on the relaxation process in affecting the local solvent structure around the hydro- phobic groups of the polymer could be related to their denaturing ability.41 these At molecules serve as simple models for solutions The the behaviour isoelectric of pH amino - 5-6, acids an in various ultrasonic the relaxation polypeptide is also was of chain interest observed43 in proteins. because in aqueous solutions of glycine, diglycine and triglycine. The relaxation time was independent of concentration and was also unaffected by the addition of protein denaturing agents and neutral salts.The relaxation process was R. I. C. Reviews 78 found to be a unique property of the zwitterion structure and was attributed to a perturbation of an equilibrium of the kind: where G represents glycine, n = 1, 2 or 3 and the number x was unspecified. aqueous d i g l y ~ i n e ~ ~ In another independent solution investigation, at pH - 11 was an ultrasonic attributed relaxation to the proton observed transfer in equilibrium : -0OCCHzNHCOCH2NH; + OH- + -0OCCH2NHCOCH2NHz + H2O which is equivalent to model equilibrium 18 when occurring in excess water. The rate constant klz was of the order 1O1O s-l mol-l which means that the recombination rate is diffusion controlled.In many biological systems, especially those involving macromolecules containing the -NHCO- group, the role of equilibria where hydrogen bonding occurs is important. In systems containing proteins there are several equilibria, both intra- and intermolecular in nature, competing for hydrogen bonds. In order to make an assessment of the importance of these equilibria it is desirable to study several simple equilibria which serve as model steps in these complicated systems. Progress in this direction is discussed in the section on molecular association. In addition, simple competitive reactions between molecules with purine and pyrimidine structures, such as that between caprolactam and 2-aminopyrimidine in various solvents, will serve as models for base-pairing reactions in nucleic a c i d ~ .~ 5 The kinetics of the conformational changes involving the helix-coil transi- tion has also been observed in the polypeptide poly-~-lysine.~~ This confor- mational change was one of the mechanisms discussed in the interpretation of the relaxation observed in poly-L-glutamic acid solutions.46 VIBRATIONAL RELAXATION An ultrasonic relaxation in liquids occurs due to the time delay in the transfer of energy from the translational to the vibrational degrees of freedom. This phenomenon is usually called vibrational relaxation and is associated with the vibrational specific heat of the molecule. The redistribution of energy between the vibrational and translational modes is induced by temperature changes and at constant temperature and pressure does not involve volume changes.The time delay in the transfer of energy occurs because it takes many collisions before a molecule loses one quantum of vibrational energy. This energy is coupled into the molecule through the lowest frequency vibrational mode and then spreads rapidly to all the other modes. Since this process is a thermal effect (i.e. it is initiated by the temperature changes accompanying the sound wave) the total specific heat becomes frequency dependent in the relaxation region. The relaxation contribution to the specific heat, AC, usually termed the vibrational specific heat, is related to the relaxation strength AS/&, by APlP = (Y - 1) ACPIC, .. . . 28 79 and thus p m = ~ ( r - 1) ACp/2Cp The quantity (y - 1) can be calculated from equation 3. The vibrational specific heat ACp can be calculated from the usual Planck-Einstein formula: ACp = C niRxi exp(- xi)/(l - exp(- xi)) i .. 29 where X i = hvi/kT and vi is the frequency of the ith vibrational mode of degeneracy ni. The relaxation strength AP/Po can be determined from experi- ment via equation 14 and can also be calculated through equations 3, 29 and 28. In some molecules the total vibrational specific heat is associated with a single relaxation time whereas in others a multiple relaxation is observed resulting from the different relaxation times of different vibrational modes. In liquid carbon disulphide a single ultrasonic relaxation was attributed47 to the relaxation of the total vibrational specific heat involving the two non- degenerate vibrational modes at 1523 and 657 cm-1 and also the doubly degenerate deformation mode at 397 cm-l.Analysis of the relaxation led to the following data: 0.165 0.085 25 "C -63°C 25 "C 25 "C 0 "C 0.167 0.085 In methylene chloride, on the other hand, a single relaxation in the ultra- sonic range48-50 was attributed to the relaxation of all the normal modes except the lowest at 283 cm-l. The following data were obtained to justify this conclusion. 0.065 0.069 0.055 0.022 0.066 0.066 0.055 0.022 -60°C The results of a spot absorption,5l and velocity measurements52 in the Gigahertz region using Brillouin scattering, indicate that the relaxation frequency associated with the deactivation of the lowest mode, the bending CCl2, is at approximately 10 GHz.The behaviour of the relaxation processes associated with vibrational specific heat is very similar for gases and liquids. This was demonstrated experimentally for carbon dioxide when the relaxation time was measured as a function of density in going from gas to Using some of the existing kinetic theories of the liquid phase and assuming that vibrational relaxation occurs due to binary collisions of molecules, Litovitz and his collaborators53-56 have been able to account quantitatively for the pressure dependence of the vibrational relaxation time of both CSz and COZ.In brief, an increase in R. I . C. Reviews 80 the pressure increases the number of binary collisions and thus makes the energy transfer from translational to vibrational degrees of freedom occur in a shorter time. This, in turn, increases the ultrasonic relaxation frequency. In the interpretation of the experimental data for COz a modified version of the ‘fixed wall model’ was proposed53 to calculate the mean free path of a molecule in the liquid state. In most liquids the binary collisions between the molecules are so efficient that the relaxation frequencies associated with the vibrational specific heat are well into the Gigahertz region. This means that it has only been possible to study a handful of molecules in the liquid phase. CRITICAL POINT PHENOMENA Ultrasonic relaxation also occurs around the critical points in liquids ; the critical point being the normal gas/liquid critical point in a one-component system or the point of critical phase separation in binary liquid mixtures.Most of the work reported on this subject has involved velocity and/or absorption measurements in binary liquid mixtures near the critical point or, as it is sometimes referred to, the consulate temperature.57358 In most of the studies involving sound absorption, the observed relaxation cannot be represented by a model involving a single relaxation time. Two mechanisms have been put forward to account for the anomalous behaviour of sound absorption and velocity near the critical point. In the first mode159 the relaxation is thought to arise from a scattering of the sound energy due to density or composition variation around the critical point.This theory predicts that alf2 should increase with increasing frequency. An alternative model has been proposed by Fixman60-62 and is associated with critical fluctuations in the liquid parameters that are induced by the temperature changes accompanying the sound wave. These fluctuations imposed by the sound wave affect the liquid parameters specific heat, compo- sition, the molecular level friction constant and the coefficients of the long- range correlation function. This theory predicts that the dependence of a/f2 with frequency is: alf2 cc f 514 in the critical region. Recent experimental data appears to be consistent with Fixman’s theory both for the gas/liquid@ critical point as well as the consulate temperature in binary liquid mixtures.G1363364 However, there are several details that still remain to be worked out before a full quantitative description of sound velocity and absorption around the critical point is obtained. SHEAR AND VOLUME VISCOSITY PHENOMENA In liquids where relaxation processes such as those described above are either absent or occur well outside the present available frequency range ( 104-1010 Hz) acoustic absorption and velocity measurements have been used to determine the volume viscosity of liquids by use of equations 4, 5 and, where applicable, 6.The values of the quantity rlv/rls for several classes of liquids are in Table 2.65 In liquids ranging from hydrocarbons, alcohols and Wyn-Jones 81 Table 2.Comparison of volume and shear viscosities for several liquids.65 Molecule Carbon tetrachloride Toluene Chlorobenzene Cyclohexanone Propane Cyclopentane n - Hexan e n-Heptane n-Octan e Acetone Benzene Dichloromethane Carbon disulphide lsobutyl bromide Hydrocarbon oil (MW m 300) Liquid polymers Associated I iq u ids Liquid metals Fused salts Liquid argon WIT8 I .27 long chain polymers to inorganic melts, molten metals and liquid argon the orders of magnitude of volume and shear viscosity are approximately the same. This result suggests that there is a close relationship between the mechanisms of shear and volume viscosity.In order to appreciate this relationship it is necessary to consider the general background of the theories of both liquid structure and viscosities. A liquid exists structurally in a quasi-crystalline form made up of submicro- scopic crystals containing from about five to 50 molecules. These crystalline groups are held together by ‘intermolecular bonds’ which are continually breaking up and reforming. At any temperature or pressure a degree of order exists in the liquid which describes its geometrical state. The liquid lacks long-range order and the short-range order that is present is imperfect result- ing in the presence of holes. A change in energy of the liquid by variation of temperature or pressure will affect the short-range order, hence the imper- fection, and also the number of holes.For example, a temperature rise will cause the short-range order to decrease and thus increase the number of holes. These changes, in turn, affect the thermal expansion coefficient, 8. On the other hand a pressure rise in the liquid will increase the short-range order, decrease the number of holes and thus affect the compressibility, ,8. In the current theories of shear viscosity of liquids the molecules under shear flow are involved in translational jumps from one lattice site to another. The mobility of the molecules that are invoIved in these translational jumps is determined by both the probability that a molecule has enough energy to break away from its neighbours (submicroscopic crystalline groups) and also the probability that there is sufficient free volume (holes) for the molecule to jump into.In a similar way volume viscosity arises as a result of the flow of molecules from one lattice site to another lattice position of closer packing 0 . 9 0.44 I I . 3 3 1-3 0.4-4 82 1-2.5 I .7 I .6 0.4 1 . 1 0.7 6 7 6 2 0.5 I .4 0.2 R. I . C. Reviews as a result of acoustic pressure. This flow takes a finite time and thus the volume changes are out of phase resulting in acoustic attenuation. In both shear and volume viscosity processes, molecules change their lattice positions and thus the same intermolecular ‘bonds’ must be broken in the molecular clusters, making these processes closely related as the data in Table 2 suggest.When a molecule changes from one lattice site to another an energy change is involved and since the molecules take a finite time over this movement this type of structural rearrangement will give rise to acoustic relaxation. In practice acoustic relaxation arising from shear and volume (or compres- sional) viscous processes have been observed. These relaxation processes have been studied by combining both longitudinal (i.e. ordinary ultrasonic) wave and also ultrasonic shear wave measurements at different temperatures and frequencies. The data are usually presented in the form of reduced plots of the liquid moduli.66 It has been found that the behaviour of both shear and volume relaxation data are very similar.With the exception of molten zinc chloride all the liquids studied give rise to a distribution of relaxation times which has been attributed to the co-operative behaviour of these structural decay processes, and the results have been used to test various theories of the liquid phase.66 RECOMMENDED READING FOR THE SPECIALIZED READER Books K. F. Herzfield and T. A. Litovitz, Absorption and Dispersion of Ultrasonic Waves. New York: Academic Press, 1959. V. F. Nozdrev, The Use of Ultrasonics in Molecular Physics. London: Pergamon, 1965. D. Sette (ed.), Dispersione ed assorbimento del mono nei processi rnolecolari (in English). New York: Academic Press, 1963. Warren P. Mason (ed.), Physical Acoustics. New York: Academic Press, 1965.Vol. I Part A and B: Methods and Devices, Vol. I1 Part A: Properties of Gases, Liquids and Solutions, Vol. I1 Part B: Properties of Polymers and Nonlinear Acoustics. A. B. Bhatia, Ultrasonic Absorption. Oxford: Clarendon Press, 1967. (Monograph on the Physics and Chemistry of Materials). Review articles R. 0. Davies and J. Lamb, Q. Rev. chern. Soc., 1957. 11, 134. G. Verma, Rev. mod. Phys., 1959, 31, 1052. G. Atkinson, S. Kor and S. Petrucci, Proc. Instn elect. Engrs, 1965, 53, 1355. J. E. Piercy, Proc. Znstn elect. Engrs, 1965, 53, 1346. M. Eigen and L. de Maeyer in Technique oforganic Chemistry, ed. S. L. Friess, E. S. Lewis and A. Weissberger, vol. 8, part 2, p. 895. New York: Interscience, 1963. T. A. Litovitz, J . acoust.SOC. Am., 1959, 31, 681. D. Sette in Handbuch der Physik, ed. S. Flugge, Sec. XI, Vol. I . Berlin: Springer, 1961. Aids to literature searches The Journal of the Acoustical Society of America publishes, every other month, an index of published papers on ultrasonic absorption in liquids under subject index 10.5. REFERENCES 1 G. Stokes, Trans. Camb. SOC., 1845, 8, 287. 2 T. A. Litovitz, J . acoust. SOC. Am., 1959, 31, 681. 3 M. Eigen and L. de Maeyer in Technique of Organic Chemistry, ed. S. L. Friess, E. S. Lewis and A. Weissberger, vol. 8, part 2, p. 895. New York: Interscience, 1963. 4 See, for example, Chapters 4, 5 and 6 in Physical Acoustics, ed. Warren P. Mason, Vol. 11, Part A. New York: Academic Press, 1965. Wyn-Jones 83 5 H. J. McSkimin in Physical Acoustics, ed.Warren P. Mason, Vol. I, Part A, Chapter4. New York: Academic Press, 1965. 6 F. Eggers, private communication. 7 L. Brillouin, Annls Phys., 1922, 17, 88. 8 K. Tamm, Z. Elektrochem., 1960, 64, 73. 9 W. A. Poth, quoted in Landolt-Bornstein, Physical Chemical Tables EG 111,1936,2818. 10 K. Tamm, G. Kurtze and R. Kaiser, Acustica, 1954, 4, 380. 11 M. Eigen, Z. phys. Chem. Frankf. Ausg., 1954, 1, 176. 12 P. Debye, Trans. electrochem. SOC., 1942, 82, 265. 13 E. Carnevale and T. A. Litovitz, J. acoust. SOC. Am., 1958, 30, 610. 14 J. Stuehr, E. Yeager, T. Sachs and E. Hovorka, J. chem. Phys., 1963, 38, 587. 15 M. Eigen and K. Tamm, Z. Elektrochem, 1962,66, 93 and 107. 16 G. Kurtze and K. Tamm, Acustica, 1953, 3, 33.17 0. Wilson and R. Leonard, Tech. Report 4, Office of Naval Research Contract N6 onr 27507, University of California, Los Angeles, California. 18 C . Mulders, Appl. scient. Research, 1949, B1, 341. 19 M. Smith, R. Barrett and R. Beyer, J . acoust. SOC. Am., 1951, 23, 71. 20 J. Stuehr and E. Yeager in Physical Acoustics, ed. Warren P. Mason, Vol. 11, part A, p. 351. New York: Academic Press, 1965. 24 G. Verma, Rev. mod. Phys.. 1959, 31, 1052. 21 F. H. Fisher, J. acoust. SOC. Am., 1958, 30, 442; 1963, 35, 805. 22 K. Tamm in Dispersione ed assorhimento del suono nei processi molecolari, ed. D. Sette, p. 175. New York: Academic Press, 1963. 23 G. Atkinson, S. Kor and S. Petrucci, Proc. Instn elect. Engrs, 1965, 53, 1355. 25 J. H. Andrae, P.D. Edmonds and J. F. McKellar, Acustica, 1965, 15, 74. 27 R. 0. Davies and J. Lamb, Q. Rev. chem. SOC., 1957, 11, 134. 26 G. G. Hammes and W. Knoche, J. chem. Phys., 1966,45,4041. 28 J. Lamb in Physical Acoustics, ed. Warren P. Mason, Vol. 11, part A, p. 203. New York: Academic Press, 1965. 29 E. Wyn-Jones and W. J. Orville-Thomas, Chem. SOC. Special Publication No. 20 (Molecular Relaxation Processes), 1966, 109. 30 J. E. Piercy, J . acoust. SOC. Am., 1961, 33, 198. 31 R. A. Pethrick, E. Wyn-Jones, P. C. Hamblin and R. F. M. White, J. mol. Struct, 1968, 1, 333. 32 P. C. Hamblin, R. F. M. White and E. Wyn-Jones, Chem. Communs, 1968, 1058. 33 J. Bailey and A. M. North, Trans. Faraday Soc., 1968, 64, 1497. 34 R. A. Pethrick and E. Wyn-Jones, J. chem. 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ISSN:0035-8940    

DOI:10.1039/RR9690200059

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Index to Volumes I (1968) and 2 (1969) Cairns, A . C. H., Chemicals and the World Economy . . . . 2, 41 Chemical Applications of Ultrasonic Absorption Measurements in the Liquid State, Some . . 2, 59 . . 1, 205 . . 2, 41 . . 2, 143 . . . . .. . . .. . . .. .. . . .. 1, 1 . . 2, 1 . . 2, 117 . . 1, 135 .. 2, 13 . . 2, 117 . . 2, 41 .. 2, 87 . . 2, 117 . . 2, 87 .. 2, 143 .. 2, 163 . . 2, 163 207 . . .. . . . . .. .. .. Chemical Education : Problems of Innovation Chemicals and the World Economy . . . . .. .. Chemistry and Nutrition Chemistry and the Consumer . . .. . . .. .. Chemistry and the Origin of Life Chemistry and Physics of Enzyme Catalysis, The . . . . Chemistry of Tribology, The . . .. .. . . . . Currell, B.R. and M. J. Frazer, Inorganic Polymers Doonan, S., The Chemistry and Physics of Enzyme Catalysis Economy, Chemicals and the World . . .. . . . . Electrochemistry, Organic Enzyme Catalysis, The Chemistry and Physics of . . . . Fleischmann, M. and D . Pletcher, Organic Electrochemistry Frazer, Alastair, Chemistry and Nutrition . . . . .. Frazer, M . J.-see Currell, B. R. .. . . .. .. . . .. . . . . Frost, B. R. T., Nuclear Fuels . . .. . . . . .. Fuels, Nuclear . . .. .. . . .. .. .. Gowenlock, B. G. and C. A . I;. Jolinson, Techniques of Physical Measurement: Vacuum Technique . . .. .. .. .. 1, 107 Hullam, H. E., Infrared and Raman Spectra of Inorganic Com- pounds . . . . 1, 39 .. . . . . . . .. . . 2, 13 . . . . . . . . .. . . . . . . 1, 39 Halliwell, H. F., Chemical Education: Problems of Innovation . . 1, 205 Infrared and Raman Spectra of Inorganic Compounds . . Inorganic Polymers Ives, D. J. G. and T. H. Lemon, Structure and Properties of Water 1, 62 Johnson, C. A . F.-see Gowenlock, B. G. Lemon, T. H.-see Ives, D. J. G. Life, Chemistry and the Origin of . . . . . . .. 2, 1 . . . . . . 2, 87 . . . . . . . . . . . . 2, 13 . . . . .. 1, 1 . . . . . . 1, 135 .. .. . . 1, 62 . . .. 1, 13 . . .. .. 2, 1 . . . . . . 2, 163 . . . . .. 2, 143 Nutrition, Chemistry and Oparin, A . I., Chemistry and the Origin of Life Organic Electrochemistry Pletcher, D.-see Fleischmann, M. Polymers, Inorganic Roberts, Eirlys, Chemistry and the Consumer Rowe, Geofrey W., The Chemistry of Tribology Structure and Properties of Water . . . . Transition Metal Ions in Biological Processes, Role of Ultrasonic Absorption Measurements in the Liquid States, Some .. . . . . Chemical Applications of . . . . . . . . . . Nuclear Fuels . . .. . . . . . . Vacuum Technique . . .. . . . . . . 2, 59 . . .. . . . . . . .. .. 1, 107 Williams, R. J. P., Role of Transition Metal Ions in Biological Processes . . . . . . .. . . . . .. .. 1, 13 Wyn-Jones, E., Some Chemical Applications of Ultrasonic Absorp- tion Measurements in the Liquid State . . . . . . . . 2, 59 208

ISSN:0035-8940    

DOI:10.1039/RR9690200207

出版商:RSC    

年代:1969

数据来源: RSC