INDUSTRIAL AND ENGINEERING CHEMISTRY
850
from coal through calcium carbide. It has been reported that the entire war-time German production of synthetic rubber was made from acetone derived from acetylene. Similar considerations led Dr. Bolton to the choice of acetylene as the raw material for the synthetic rubber research in which we were engaged a t the time he made the acquaintance of Br. Nieuwland, With the practically unlimited domestic reserves in this country of coal and limestone for the production of acetylene and of salt for hydrogen chloride, obviously it will be possible, if a national emergency should arise, to equip ourselves quickly to produce a sufficient quantity of chloroprene rubber to supply the nation’s needs. Moreover, acetylene can be produced from petroleum, as well as from coal, although the process is not yet fully developed on a commercial scale. Unlike the synthetic rubbers that had preceded it, chloroprene rubber has its use in times of peace as well as in war. Because of its superior resistance to oils, heat, oxidation, and ozone and sunlight deterioration, as compared with natural. rubber, and because of its low permeability to gases, there are many places in our industries and arts where it can be used to much better advantage than the natural product. These distinctive properties are making it possible to create a synthetic rubber industry in times of peace which will serve as a nucleus for expansion Ff we should be so unfortunate as to be visited by another war. If it were not for these distinctive characteristics of chloroprene rubber, its peace-time production would be possible only through the aid of a government subsidy or import restrictions, since the cost of manufacture is still far greater than the cost of growing natural rubber.
CRELY no honor has ever been more richly deserved than this award to Dr. Nieuwland, who, after devoting the major part of his academic life to the study of the reactions of acetylene, has discovered this process of acetylene polymerization which has proved to be the key to the synthetic rubber problem and the means of creating an industry that
N ORGAKIC
chemistry the branch of analysis is more difficult and varied than in the realm of inorganic chemistry; yet we may conclude that its most important use is secondary. It is absolutely necessary first of all to know not only the elements and groups, but also their relationship to one another and to the basic structure of the molecule. This knowledge is necessary in order that the synthesis of a comparatively simple substance may be effected. Analysis with proof of structure of the many thousands of compounds is absolutely a prerequisite for synthesis, since practically no progress is otherwise possible in the field of organic chemistry in modern chemical procedure. The history of synthesis of organic dyes or colors and medicinals has shown that chemists must not only have in view the preparation of nature’s products but must iinprove them, Most of these are rather transient, useless, or at least, defective. We can now make better colors and more perfect drugs than Kature. The synthetic aniline, alizarin, and indigoid dyes are more perfect than any of the colors of plants. They not only rival the rainbow in beauty but are more varied.
VOL. 27, NO. 7
is giving employment to many American chemists and workmen and will undoubtedly employ many times more in the future-an industry, moreover, that not only contributes to our peace-time civilization but adds greatly to our national security. I am sure his work will prove an inspiration to other workers in pure science, will serve to teach the deairability of concentrating one’s efforts on a small portion of the unexplored realm of chemistry, and will emphasize t o those who are in the position to lend financial support to university research the great extent to which our industrial advancement depends upon further accretions to our store of fundamental scientific data.
9 9 9 The remarks of J. M. Weiss in presenting the Nichola Medal to Dr. Nieuwland were, in substance, as follows: The William H. Nichols medal is the highest honor within the gift of the New York Section of the AMERICANCHEMICAL So-. CIETY. Since 1903 it has been presented to many famous men, When I mention Noyes, Baekeland, Langmuir, Midgley, Roger Adams, Sherman, G. N. Lewis, and Conant of Harvard as some of those so honored, it is clear that it stands for the highest type of scientific achievement. Tonight we place upon that roster one who is no less worthy. In fact, I doubt if it has ever been our privilege to recognize work in pure organic chemistry which has been of more value to our national sufficiency and self-defense. Work in Lhe chemistry of acetylene derivatives i s dangerous. We all know the extreme explosion hazards incident to many of these compounds. This field of research requires courage greater than that of the battlefield. Nevertheless, our medalist devoted himself t o t’he field which less advent,urous investigators had avoided. He persist’edand taught the world how to control these hair-trigger rea,ctions. And all t’hrough these years of work it was devotion to science without thought of materia! gain. Father Nieuwland, Soldier of Science, the New York Section of the AMERICANCHEXICAL SOCIETY is honored by your presence. It is my great privilege on their behalf, as chairman of the Jury of Award for the year 1935, t o induct you into their Legion of Honor and to bestow their Croix de Guerre-the Xichoh Medal.
Cocaine has a five-carbon ring with a toxic curare-like action Chemists have built up a molecule with the anesthetic properties of cocaine, but they have left out the poisonous part of the structure, or have replaced it with valuable, or a t least harmless, groups. Hence, we have novocaine: and other similar compounds. A. peculiar tint or shade of color or a desirable niediciiial property can now be introduced into a compound almost a t will, and its characteristics can be predicted before the preparation of the substance. Along some linea of research, however, because of slight prejudices of the investigators, lack of progress has been due to the fact that they have for decades been sat,isfied to make “something just as good” instead of starting with the ideal that the world always wants “something better.” We may cite an example in the case of the attempts to prepare a. synthetic rubber. The structure of the fundamental unit, isoprene, was sufficiently known, but chemists could not cast aside the idea that rubber is a hydrocarbon. Someone had made that definition and it would have been heresy to contradict it. Therefore, we have had numerous attempts a t imitating nature in the preparation 0% butadiene, dimethylbutadiene, and other
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrocarbon rubbers which at best were poor imitations. The brilliant as well as painstaking work of a group of du Pont chemists with imagination finally resulted in the preparation of chloroprene rubber where a chlorine atom takes the place of a hydrogen or methyl group; the result was Duprene, a better product than the natural rubber with all the characteristics of the latter. Some chemists still maintain that Duprene is not a rubber because it is not a hydrocarbon, but a hydrocarbon derivative, although it apparently has the structure of rubber. Two things are fundamental to investigation for the success of synthetic chemistry. First, it is more logical to cast off a worn-out principle or definition that no longer applies than t o argue oneself into an inconsistency. Secondly, it is surprising how easily men of science persist in overlooking the simply obvious. Chemists have for some time felt that future progress of the science depends upon the discovery of new catalysts. Probably the idea that reactions in the vapor phase at high temperature must be used with catalysts is too much overemphasized. Plants and animals synthesize substances at moderate temperatures. There is no reason why the laboratories or the industries should not do likewise. The great difficulty consists in finding substances that are efficient catalysts. Little is known about the nature of catalysts, and we are still wasting too much time in disputing the definition of something we know imperfectly. Since few or no consistent standards of selecting catalysts are available, it seems that for quite a long time we shall have to select them empirically. Nevertheless, we may assure ourselves that the synthetic chemistry of the next two generations will be largely catalytic. The all-important vegetable plant which with a minimum of energy produces substances in ways we cannot now imitate, will undoubtedly in the future be replaced by an industrial plant which will with a machine catalytically take in simple products and, without loss of time and motion and without the aid of extreme heat, discharge the complex substances we desire. When we consider the industrial processes of a generation ago that today are being replaced with catalytic ones, we are assured that our prophecy is warranted.
T
HERE are not many direct reactions of the acetylene or ethylene series. Most of them are catalytic with the formation of easily decomposable intermediates. A substance, A , which does not directly combine with B may do so by forming with C as the compound AC. This, in turn, reacts with B to form BA, at the same time being changed or reduced back to C In completing the cycle, substance C, which may be called the catalyst, is continuously reformed. In case of a great many such simple catalytic reactions, it is possible and often easy to predict what the nature of substance C should be in order to perform and promote the reaction. It has been asserted that the preparation of compounds by means of these easily reversible intermediates is not, strictly speaking, catalysis. It is not necessary to force the issue by theoretical argument, for whatever point of view we may take, we are liable to be little less than eager purveyors of inconsequential opinions. When actual results are obtained, even when we cannot perfectly explain how we obtained them, we may a t least be gratified that successful results are what we seek, We have a good example in the case of the chlorination of both ethylene and acetylene by means of antimony in the form of trichloride or pentachloride. For example, it is well known that ethylene reacts readily with bromine but not easily with chlorine. Ethylene, moreover, may be chlorinated easily with antimony pentachloride. Therefore, if chlorine and ethylene a t ordinary temperatures are passed into antimony trichloride or pentachloride, the following series of reactions take place:
SbCls SbCls
85 1
++ CzH4 + CzH4C12 + SbCla Clz + SbCls
I n this way, a comparatively small amount of the antimony chloride may continually and indefinitely be made to combine the two gases. The dichloroethane may be distilled off, and the process goes on indefinitely. Although acetylene reacts so violently with chlorine as to produce explosions, the two gases may be combined without danger if the excess of acetylene in gaseous form or solution is removed from the vessel before the chlorine is passed in. The reactions are analogous to those of ethylene just given. Berthelot and Jungfleish in 1872 obtained tetrachloroethane by passing acetylene into antimony pentachloride. It was known for years before that antimony trichloride takes up chlorine to form the pentachloride. It seems perfectly obvious, therefore, that, after pawing acetylene into the mixture of antimony trichloride and tetrachloroethane, the antimony trichloride will be changed into the higher chloride which, in turn, will form more tetrachloroethane with acetylene. Nevertheless, forty years elapsed before anyone combined the idea of running the reaction in this manner. Four years more passed before we applied the same reactions to ethylene. This application seemed so simple that we did not believe it worth while even to publish a note in the journals. Curiously enough, when cheap ethylene became: available, someone patented this process in 1923. Twenty-one years more had passed before so simple an idea was made available in its application to the preparation of ethylene chloride. The obvious here was often hard to see, because chemists refused to be “catalysis-minded.” I may call attention here to the fact that the same catalyst that united ethylene with chlorine (because alone they did not react readily) also united acetylene and chlorine successfully even though the latter reacted too violently. Some of the energy of combination was divided u p by stage reactions to form the intermediate antimony pentachloride. Predictions may be made as to the choice of catalysts in such simple processes. Antimony is not the only nonmetallic element that forms two chlorides of which the lower one may easily with excess of halogen go to the one of higher chlorine content. Since we know or must assume that one can easily be changed to the other, we may select others also. For example, selenium forms a mono- and a tetrachloride, both easily obtainable from the other by addition of either selenium or halogen. As far as I know, this has not been experimentally shown; yet I should venture to prophesy that, although the intermediates are or are not known in this case to be organic complexes (probably easily affected by halogen), if both acetylene and ethylene were passed into either of the chlorides of selenium,*the di- and tetrachloroetlianes would be formed continuously and as easily as with antimony. In the case of catalysis by way of easily reverting intermediates, one should seek for catalysts in which a valency change takes place. In other words, the intermediate should easily reduce from a higher to a lower valence and the material of higher valence should react with the substance in question.
0
WING to the reactivity of unsaturated hydrocarbons, we find them apt substances for “stage-reaction” catalysis. Some unsaturated hydrocarbons in their intermediate stages as complexes cannot be isolated in pure enough form for analysis. Hence, it is difficult at times to explain a mechanism of reaction. This is true of mercury salts in the preparation of aldehydes, a process that has revolutionized the acetic acid industry. It is well to remember, however, that if these intermediates were sufficiently stable to analyze, they would probably be too stable to perform the process of cataly1 This ha8 since been found to be t h e case [ J . Am. Chem. Soc., 44, 395 (192211.
852
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
sis which by its nature requires reactivity, changeability, or instability. Mercury salts have been used for the preparation of acetaldehyde from acetylene in dilute solutions of oxyacids such as sulfuric or perchloric. Hydrofluoric is the only halogen acid which can be used successfully. The aldehyde is catalytically changed to glacial acetic acid with air in the presence of cerium and manganese salts, notably the acetates. Many thousands of tons of product are thus made in America and abroad. In fact, the whole industry has been changed by this process since the Xorld War. Acetone may be catalytically made from the glacial acetic acid in the gas phase. Calcium hydroxide is the catalyst a t 580" C. The mechanics of this process are interesting: 2CHaCOOH CH3--C-CH, I1
+ Ca(@H)z+(CH3C0@)2Ca-tHOB 2HOH 4- CaC03 + CaO + COz +Ca(OH)2 -+=
0
Sincc -rater forms aldehydes, any hydroxyl-containing substance or any substance that has labile hydrogen, forms corresponding derivatives. Accordingly, from alcohols are derived acetals and from phenols p-ethylidene derivatives. Organic acids give two different substances in stage reactions. 2R-OH
HCsCH
R-0
+
R--O/
\CHCH3
+-
HC=CH
tetrafluoride in solution of the reagents, Dihydroxyfluoboric acid also gives good yields. When boron fluoride (BF3) is passed into methyl alcohol, a strong complex Auoroacid is formed of melting point --19' C. It has been used with both acetylene and ethylenic hydrocarbons for catalytic condensations, In the alkene series no mercury salt i s needed. Phenols, aryl acids, and oxyacids combine with propylene, butylene, etc., t o form aryl. ethers or esters. When a hydroxy group is present in the benzene residue, gentle heating rearranges these ethers, etc., to ring substitution derivatives, in the 2-, 4-, and 6-positions The ether compound (CzH&Q BFs of boron fluoride is used with mercury as a catalytic mixture, Aryl amines form alpha- and gamma- substituted and trisubstituted quinolines. Acids, phenols, and other substances with labile hydrogen can also be used, since the latter combines to form boron fluoride from which are derived strong catalytic acids. Boron fluoride or methoxy fluoboric acid is, therefore, a convenient catalytic reagent t o produce a vast number of derivatives of acetylene. Unlike sulfuric acid, it does not destroy ester or ether groups, so that ethylidene derivatives may be formed from chlorohydrins, the glycol ethers, or dihydroxy carboxylic esters. Diethyl tartrate, for example, forms ethylidene diethyl tartaric ester: COOCzHs
COOC,H,
bOOCzW6
COOCaHs
I
The phenols polymerize further to soluble synthetic resins of Bakelite €3 types used in the varnish industry. The final product i s an insoluble Bakelite analogous to that from formaldehyde and phenol. Vinyl acetate is the first product of acetic acid and acetylene: GHs(!!--OH
VOL. 27, NO. 7
--+ CHsCOOCH =
CHa
This i s the intermediate for the vinylite resins, since, when a t higher temperatures two molecules of acid react, ethylidene diacetate results. From this reaction acetic anhydride is obtained commercially:
I
The acid of this ethylidene derivative has peculiar physical properties. The salts have quite difeerent characters from those of tartaric, malic, and citric acids. Derivatives of glycerol (two compounds) and mannitol have been prepared. Pentaerythrite compounds are volatile and can be distilled Polyhydroxy substances can thus be prepared in pure form by means of this reaction. The aryl hydrocarbons and ethers also form ethylidene derivatives by means of this reaction with acetylene, concentrated sulfuric acid, and small amounts of mercury oxide, Ethylidene-bis-tetralin, a viscous liquid, boils at 384' C, without decomposition:
,
Very pure oxalic acid is obtained from nitric acid. Glycols form ring compounds called 2,3-dioxoles: CHI-OH
I
CH-OH
+CH=CH
CH2-0
1
CH2-0
)CHCH?
Oxyacids produce cyclic ether esters: +HC=CH--t II
8:
>c/
R
O
R
\C-@/ /I
CHCH,
Q
These substances and also the acetals are very stable to alkalies but are easily hydrolyzed by acids. There is but one alcohol (methyl) which gives satisfactory yields with the mercury salts and sulfuric acid. The higher alcohols, oxyacids, glycols, chlorohydrins, etc. , are prepared much more conveniently with methoxyfluoboric or silicon
CH,
Cuprous salts--e. g., cuprous chloride eithey with or without ammonium chloride -are effective catalysts for condensation of ethylenic and acetylenic hydrocarbons, The products vary, depending upon the other reagents used. In general, for the alkenes the chloride is not active because of its insolubility. With the oxide in sulfuric acid, ethylene forms diethyl sulfate. An appropriate mixture of acetylene and ethylene produces derivatives of sylvestrene. The table on the following page shows the reagents and products from acetylene with various mixtures. EPENDISG upon the conditions of experiment, either monovinyl- or divinylacetylene results when acetylene is passed into a mixture of three parts of cuprous chloride by weight, one part of water, and one part of ammonium chloride. The chloride of the alkali metals or alkaline earths may be used, but, although condensation takes place, the mixture is not effectively catalytic. A. complete solution
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1935
853
H HzC=C-C=C=CH2
of these components may be used to make monovinylacetylene. Ammonium cuproso chloride is probably formed. When brought together, much heat is absorbed, since the cuprous chloride goes into solution partly or completely. Monovinylacetylene is apparently first formed and then condenses with another molecule of acetylene to give the complex copper compounds of the hydrocarbons, when heated or distilled. The mechanism of the catalysis is a t present unknown. The copper divinylacetylene is a lemon-
!b-C-CHI
6
This compound polymerizes spontaneously to a beautiful, transparent, flexible rubber-like substance. Divinylacetylene is SO active that it is difficult to apply successfullythe catalysts used for acetylene in aremration of derivatives. Sulfuric aiid -(BO to 70 per cent) with subREAGENTS AND PRODUCTS FROM ACETYLENEWITH VARIOUS MIXTURES sequent hydrolysis produces a very unMonovinylacetylene HzC=CH-CGCH Divinylacetylene A (3)CurCla (1)NHaCl (1)HOH+ saturated compound, vinyl allyl ketone. CH%=CH-CGC-CH=CH~ tetramer The mercury catalyst is immediately B Cu2Clz (trace) ("&I) HOH HC1 (coned.) Vinyl chloride= destroyed on contact with the divinvlMixture of cis- and transacetylene. Using calcium chloride with C Cu2Clz (NHdCl) HOHb Cuc12 { dichloroethyleneo a small amount of water as a catalvst and passing in air, formaldehyde is i b D CuzClz (NHhC1) HOHl HC1 (concd.) CuCIz eth Mixture lene ofand tms-dlch1orounsgm-dicxloroethylened tained together with a polymer of unknown structure. The methylene groups o Chloroprene (CHB= C - C H = CHz) is also obtained from vinylacetylene. L, are probably oxidized. BY controlled b Ammonium cuprichloride [(NHa)nCuCla]. polymerizatibn S. D. 0. (synthetic dryc Compounds with a system of double and triple conjugated bonds generally react, but not alkyl ing oil) is obtained; this has remarkably acetylenes. d Cuprichloric acid, and the ammonium salt or acid hydrogen cuprichloride. resistant properties as a lacquer film. It is not attacked by 60 per cent sulfuric acid, by any known organic solvents, or by bromine, chlorine, etc. yellow precipitate which partly dissolves in the catalytic mixBy spontaneous uncontrolled polymerization alone or in a ture and crystallizes in yellow elongated octahedral crystals. solution of solvents, divinylacetylene polymerizes to a white These crystals are especially instable in the presence of air, turn green, and yield other hydrocarbons. Although the or yellowish polymer that is spontaneously and highly explosive. In the presence of excess water, with small amounts copper complexes of divinylacetylene and perhaps monoof ammonia and some copper salt, the polymer when freely vinylacetylene are formed which on heating yield the hydrocarbons, the continued action of acetylene will finally produce divided resembles cuprene and is nonreactive. It may be a tritolylene. Hydrochloric acid with cuprous chloride gives free hydrocarbon, especially in the case of divinylacetylene. This floats on the aqueous solution. Monovinylacetylene a product that polymerizes so rapidly that, so far, it has not been controlled. Although divinylacetylene is prepared has a sharp sweetish odor and boils a t 5.8" C. Monovinylfrom acetylene by an exothermic reaction, it has probably acetylene combines with halogens to form derivatives. With halogen acids, chlorometliylallene is formed : the lowest heat of formation of any known hydrocarbon (about -160,000° C.) Still further progress in the chemistry of acetylene has been made possible because of the fact that a considerable number of substituted acetylenes can be prepared from acetylene itself through the Picon synthesis. The higher normal alkynes This may be isomerized in a hydrochloric acid solution of may be made from sodium acetylide in liquid ammonia in the cuprous chloride to 2-chlorobutadiene. When cuprous following steps from alkyl haloids: chloride or chloroprene is present in the hydrochloric acid, chloroprene is the main product. Chloroprene may be polyCH=CH Na +HaC=CH merized readily to Duprene, a synthetic rubber of unusually R-Br NaCECH --+R-C=CH NaBr useful properties: All of these compounds have been obtained up to a carbon 2 H C r C H +HC=C--CH=CHz HC=CH + content of 16 carbon atoms. Since aryl acetylenes are reH2C=CH-C~C-CH=CH~ duced to styrenes by sodium in liquid ammonia, sodium amide ~HCZK!R=CH~ +H~C=C-C~C--C=C-C=CHZ is used. The solution of sodium in liquid ammonia can readily H H H H 1,5,7-octatriene-3-yne be catalyzed to sodium amide by means of a small crystal of hydrated ferric nitrate. These higher alkynes, in general, can Monovinylacetylene can also be used as the intermediate for then be treated by means of the numerous analogous reacthe preparation of numerous products by reactions analogous tions and synthesis like acetylene itself. Whereas acetylene to these used with acetylene. Applying the reaction with forms acetals with alcohols, ketals are formed from the homo. mercury salts and acids (as in the case of acetylene itself) to logs. These ketals can be hydrolyzed to a great variety methyl alcohol, a trimethoxy compound results: of new ketones: OCHa R--.C=CH 2CHsOH +R-C CHs HOHZ.-CH2 H,C-C /OcHa
1
3
b,
++
+
+
+
\-
I
\
+
0
//
R-C-CHS Monovinylacetylene with mercury salts and methoxyfluoboric acid produces with organic acids-. g., acetic-2-acetoxybutadiene:
A variation of the reaction produces the higher vinyl ethers by distillation of the ketals with a trace of toluenesulfonic acid :
INDUSTRIAL AND ENGINEERIYG CHEMISTRY
854
These vinyl ethers are extremely reactive and combine directly with alcohols: /OCHa 3R-C=CHn f R’-OH
--+
/OR‘ R-C-CHI \OCHa /OR‘ R-C-CHs \OR‘
+ /OCH3 + R-C-CH, \OCHa
Other alcohols form ketals directly with substituted alkynes. In this way the higher ketals and mixed ketals are obtained. From these also the corresponding ketones are made by hydrolysis. ‘Vinylacetylene adds the methoxyl group to the double as well as the triple bond with mercury and aryl sulfonic, methoxyfluoboric or dihydroxyfluoboric acids: OCH,
H2C=CH-CdX
I + 3 CHIOH +H& -CHZ--C-CNI I
OCH3
I
OCH3
Although disodium carbide can easily be made from sodium acetylide, it is extremely nonreactive in synthesis, lodoacetylenes can also be made without trouble or difficulty with iodine in liquid ammonia, not, however, with t,he bromo or chloro alkynes. Whereas iodine in liquid ammonia reacts safely, chlorine and bromine explode violently. Fhenols form substituted Bakelite-like products. Using trichloroacetic acid as a promoter, the higher acetylenes give with the mercury acid catalyst, distyrenes with aryl hydrocarbons and ketylidene substances of the formula: R-OH R--OH
+ R’-C-CH
RO ---3
R 0 P C 3
Acetylene and its homologs behave like very weak acids. This property is evident from the formation of ketalic sub-
VOL. 27, NO. a
stitution derivatives, Ethyne is a weaker acid than alcohols Disodium acetylide or sodium carbide is decomposed by alcohol to form sodium ethylate. Salts of mercury, silver, and copper are formed in an aqueous system. Very little has been done to further our knowledge of these metal derivatives, and their study apparently is of great importance for the explanation of the role of mercury and copper in the condensations mentioned earlier in this discussion. Sew series of compounds result, one in neutral or slightly acidic media, the other in alkaline solutions of carbides where the metals usually more or less completely replace both labile hydrogens in acetylene itself. They are probably not monomeric in either case. Even organic metal derivatives such as alkyl or aryl mercuronium hydroxides react t o form very definite monomers according to the equation : BRHg(0H)
+ HC-CH ---+(RHg)C=C(HgR)
3. 2HOH
The melting points of these are definite and sharp and may serve as means of identification of alkyl or aryl mercuronium halides or other salts. It may serve also, perhaps, for the identification of substituted higher acetylenes, since these also form derivatives with mercuronium halides, OOKING back upon the numerous reactions of a c e t p lene itself, especially the catalytic transformations with a view of applying them to the substituted alkynes, it seems evident that the possibilities of further research in acetylene derivatives promise great and varied possibilities. Even in the investigation of acetylene itself, we are f a y from having covered the field in an adequate manner. I n regard to the substituted dienes alone, in a few years we may look for methoxy, acetoxy, acetyl, thio, nitrogeno, and other dienes that may be polymerized to peculiar “rubbers” of unusual or a t least interesting properties. Some of these have already been made, but the investigation of the polymers has not been satisfactorily pursued, It is not improbable that the nol far distant future will present us not only with cheaper and better, but even with innumerable substituted diene rubbers. The synthetic medicinals and dyes have replaced the natural, why should not the synthetic rubbers?
