POLYMERIZATION OF DRYING OILS Comparative Rates of Polymerization of Esters of Isomeric Octadecatrienoic Acids at 275" C. in Vacuum WALDO C. AULT, J. C. COWAN, J. P. ICASS, AND J. E. JACKSOS Northern Regional Research Laboratory, U. S. Department of Agriculture, Peoria, 111. merization of methyl eleoHE present shortage of Rates of polymerization of esters of pure stearate and of methyl esters tung oil has focused atoctadecatrienoic acids found in natural and of the unsaturated acids obtention on possible substisynthetic drying oils were determined at tained by dehydration of ricintutes for use in surface coatings. 275' C., and the influence of geometric oleic acid. The superior properties of tung configuration on the rates was noted. This As a part of the general oil have long been attributed program of the Oil Section to the presence of a high work gave additional evidence to support of the Northern Regional Reproportion of eleostearic acid, the observation that no uniform correlation search Laboratory concerned a conjugated trienoic acid. exists between the speed of drying and the with increased industrial utiliTwo processes which are now speed of gelation of certain synthetic oils. zation of semidrying oils such available for the introduction A simplified interpretation of the 1,4as corn oil, a study of the of conjugation into oils are the cause for the defects of syndehydration of castor oil and diene mechanism for the polymerization of thetic oils derivable from the alkali isomerization of the conjugated trienoic acids is presented, semidrying oils or oils closely linseed and soybean oils. Aland possible structures are suggested to related t o them was initiated. though conjugation undoubtaccount for the properties of the polySince it is evident that some edly improves certain propermerized products. processes for converting semities of these oils, extensiveuse drying oils to drying oils and testing have made it clear involve radical changes in that other factors in the structhe structure of their component acids, one phase of that study ture of the component acids may limit their utilization. is designed to correlate the drying characteristics of the isoTwo defects of these synthetic oils are the development of meric unsaturated acids with their specific structures. Since aftertack in their dried films (7A) and their slower bodying a previous study (16) was available on the oxidation rates of rate as compared with tung oil (1.9). Although the lower isomeric octadecatrienoic acids, the work in the present paper proportion of conjugated acids present in these synthetic oils was confined to a study of the rates of polymerization of the has been offered as one explanation of the longer times reesters of the isomeric octadecatrienoic acids. quired for bodying them ( I @ , no study has been published concerning the effect of the presence of positional or geometric isomers of the natural fat acids on the behavior of the synEsters of Isomeric Fatty Acids thetic oils. Methyl a-eleostearate was prepared from tung oil by isoIt is generally accepted that the individual acids present in lating the acids and recrystallizing them from petroleum ether naturally occurring drying oils usually have the same geountil a product was obtained melting a t 49" C. (15). The metric configuration regardless of the source. Since it has pure eleostearic acid was esterified with methyl alcohol, using been shown that both geometric and positional isomers may a few drops of sulfuric acid as catalyst. After washing with be formed by alkali conjugation of linoleic and linolenic acids water to remove the catalyst, the ester was finally purified by (8) and the dehydration of ricinoleic acid (7A, 16, 19), it repeated recrystallization from methyl alcohol. The melting seems likely that one cause of the undesirable properties of point of the ester was 6.5-7" C.; n3:, 1.5062; d:', 0.8939; synthetic oils may be the presence of isomeric acids possessing and iodine number (rapid Wijs method), 193 (6, 10). structures which do not favor the reactions leading to the Methyl p-eleostearate was prepared from tung butter by a formation of films free from aftertack. technique similar to that used for the alpha ester. The meltPrevious work (15) on the effect of geometric isomerism on ing point of the acid isolated was 71" C. The melting point the behavior of the octadecatrienoic acids showed that these of the ester was 12.5-13.2" C.; n3:, 1.5081; d?', 0.9001; and isomeric acids oxidize a t very different rates, and Spitzer, iodine number, 187.7. Ruthruff, and Walton (20) have suggested that certain Methyl pseudoeleostearate was prepared from alkalipeculiar properties of alkali-isomerized oils may be attributed conjugated linseed oil according to the method outlined by to the varying oxidation rates of these acids present in the Kass and Burr ( 8 ) . The melting point of the ester was 40.6oils. Studies have been reported on the rates of polymeri41' C.; nq, 1.4974; d:', 0.8824; and iodine number, 190.0. zation of natural and synthetic oils, of the esters of their Methyl linolenate was prepared by debromination of the acids, and of some pure esters of individual acids present in hexabromides isolated from linseed oil. The product from the natural oils (1-6,18, 14, 16, 18), but no extensive study of the debromination procedure of Rollett (17) was crystallized effect of geometric isomerism has been made. Bradley and three times from methyl alcohol and distilled under diminished co-workers (3, 4)converted the fatty acids of several oils to pressure, The iodine number was 254.7; n3:, 1.4669; d:', methyl esters and studied the polymerization of the latter; 0.8914. Brod, France, and Evans (5) reported a study of the poly-
T
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INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1942 260r
Ethyl elaidolinolenate was prepared according to the procedure of Kass et al. (9). The iodine number was 244.1; n3j, 1.4634; d3$, 0.8775.
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230
Polymerization Studies
200
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o 8-Eleostearate
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Elaido- Linolenate (Calc. For Methyl Ester)
140
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E F F E C T OF POLYMERIZATION T I M E UPON IODINE VALUE
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E F F E C T OF POLYMERIZATION TIME
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UPON REFRACTIVE INDEX i I I I x A-Eleostearate 0 8-Eleosteorate
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Pseudo-Eleostearate Linolenote anate Elaldo-Linoli (Ethyl Ester)
Because of the difficulty of preparing very pure compounds in quantity, the polymerizations have been conducted in small glass tubes or ampoules. Approximately 1 gram of the ester being investigated was sealed under vacuum in a small Pyrex ampoule which was then dropped into a small oil bath heated a t 275' * 1' C. The heating was continued for the desired length of time, after which the tube was removed and allowed to cool. When cool the tube was placed in a refrigerator, a t approximately 5' C., until the analysis was made. The tube was opened under carbon dioxide and flushed out with this gas, and a sample was transferred to a pycnometer for density determination; then the ampoule was flushed again with carbon dioxide and stoppered. A portion of the sample in the pycnometer was used for determining iodine value after the density had been determined, care being taken to discard the first and last portions of oil from the pycnometer which had been in contact with the air during determination of the density. Iodine value was determined in all cases by the rapid Wijs method, using mercuric acetate as catalyst (6, IO) and holding the temperature at 21' to 23" C. during the time of reaction. Molecular weight was determined cryoscopically, using benzene as a solvent. Approximately 4 hours elapsed between the time the ampoule was opened and the completion of the determinations. Experimental evidence indicated that the results obtained were capable of duplication within the usual limits of experimental error. Unfortunately the small quantity of available material limited the variety of observations which could be made on the physical and chemical properties of the polymerized samples. The results of the polymerization studies are illustrated graphically in Figure 1.
Discussion of Results I .46 600
0
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500 c rn .-
Elaldo
t
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- Llnolenate
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190 I
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4 5 T i m e In Hours
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FIGURE1. EBFECT OF POLYM~RIZATION Trim UPON VARIOUS PROPERTIES
The data shown for changes in iodine number, density, refractive index, molecular weight, and molecular refractivity with variations in duration of polymerization seem to warrant the following statements: The rates of polymerization of methyl a-,methyl p-, and methyl pseudoeleostearates a t 275' C. are approximately the same, with the pseudo form having a slightly faster rate than the other two. The rates of polymerization of methyl linolenate and ethyl elaidolinolenate are approximately equal, but the nonconjugated isomers (the linolenates) polymerize much less rapidly than the conjugated forms (the eleostearates). It should be noted that in the two pairs of geometric isomers tested, both members of a pair show approximately the same rate of polymerization. The conjugated acid esters polymerize rapidly to an apparent dimeric stage in 1 to 2 hours, and only relatively small changes in density, refractive index, iodine number, molecular weight, and molecular refractivity occur after this initial period of reaction. The Ending of almost identical rates of polymerization for the methyl a- and methyl /3-eleostearates is not surprising, since it has been shown that the alpha form will be rearranged to the beta form on heating (14). The pseudo form might be expected to polymerize somewhat more rapidly since the unsaturation is closer to the end of the chain. Similarly, while symmetrically disubstituted ethylenes do not readily polymerize, unsymmetrically disubstituted ethylenes do. With the pseudo form, the nearness of unsaturation to the end of the chain may reduce the steric hindrance, and this is probably
INDUSTRIAL AND ENGINEERING CHEMISTRY 0
Vol. 34, No. 9
is no uniform correlation between the speed of drying \ and the speed of 0 gelation. Spitzer, // Ruthruff, a n d CH-CH=CH-CH=CH(-CHz)-C-OCH3 Walton (20) reported certain peCHg- (CHz)3- H CH-CH=CH (-CHz) FC-OCH, culiarities (among them the observa\O tion that heat and oxygen convertibility are not CHa-(CH2)a-CH b H closely correlated) \/ of alkali-conj ugated soybean, linseed, a n d dehydrated castor oil, and suggested that +I the different rates of oxidation of the isomeric acids which are present may be one explanation for these peculiarities. CH 'CH-~H-(CH~)-C-OCH, However, it should \O be pointed out CHs-(CHz)sCH CH that we have had I I available for exCH~--(CHZ),-~ bH amination o n l y 0 five of the possible \ CHs(CHz)s CH=CH-CH=C CH -(CH~)~C-OCHS twenty-four geo\ / metric isomers of CH-CH the three positional isomers (9-, 12-, 15-; 9-, 11-, 13-; and lo-, 12-, 14-0 c t a d e c a t rienoic acids) , that we have made no study of the effects which metal containers or surfacevolume relations CH may have, and therefore t h a t a FIGURE 2. STRUCTURAL FORMULAS AND EQUATIONS definite statement concerning the relation between structure and drying characteristics is not yet possible. the reason that an increase in rate of polymerization is noted. Perhaps an investigation of the polymerization of the Then, too, it is possible that this slightly more rapid rate of esters of the various isomeric acids a t lower temperatures polymerization in the case of the pseudoeleostearate may be or a study of the behavior of the isomeric octadecadienoic connected with the fact that it has predominantly the trans acids may lead to some more definite correlations. Such form of molecular arrangement (8). investigations are being undertaken and will be reported The comparatively slow rate of polymerization of the isolater. meric linolenates is in agreement with observations of other Our data for the polymerized products of the eleostearates workers (3) on methyl linolenate. This observation is in agree closely (except for iodine number) with those of Bradley conformity with the isomerization theory advanced by and Johnston (5) for the nonvolatile dimerized methyl esters Scheiber (IQ)-i. e., that the polymerization of nonconjugated of tung oil acids. Working with polymerization products of acid esters proceeds first by rearrangement to a conjugated purified esters, we have confirmed their report of a density of isomer and secondly by polymerization of that conjugated polymerized octadecatrienoic esters considerably above that isomer. The reaction controlling the rate of polymerization of polymerized octadecadienoic esters. I n explanation of this at 275" C.may well be the rearrangement to a conjugated phenomenon these investigators considered that the physical isomer. and chemical constants indicated that dimers derived from The fact that geometrical isomers, in so far as we have the octadecatrienates have a bicyclic structure while those examined them, show the same thermal rate of polymerization from the octadecadienates have a monocyclic structure. Albut have widely varying oxidation rates, will readily account ternative explanations which may be advanced to account for for the observation of von Mikusch and Priest (IS)that there
// ~CH~-(CH~)~-CH=CH-CH=CH-CH=CH-(CHZ)~-C-OCH,
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September, 1942
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
this difference in density between polymerized octadecatrienoic esters and octadecadienoic esters are (a) a higher degree of polymerization in the octadecatrienoic esters to give a product relatively richer in trimers and (b) a reaction to give tricyclic dimers. I n support of the first, data publishbd by Bradley and Johnston (4) show that the trimeric fraction of polymerized methyl esters of dehydrated castor oil has a higher density than the dimeric fraction. I n support of the second, our data show that the molecular weight and density increase rapidly to the dimeric stage and that thereafter, although the density continues to increase slowly, the average molecular weight probably does not change appreciably. I n addition, the iodine value continues to drop slowly for a considerable period after molecular weight determinations indicate that a dimeric stage has been reached and that little additional intermolecular condensation is taking place. Bradley and Johnston (3) postulated that methyl eleostearate undergoes an initial self-condensation through l,.i-diene addition of the conjugated system of one molecule of ester to a double bond in a second molecule to give a monocyclic dimer and subsequent complex reaction of the monocyclic dimer to give a bicyclic dimer. This complex reaction was assumed to account for an iodine value of 130 (%hour Wijs) and for the high density of their material obtained by polymerization of methyl esters of tung oil acids. The reaction involves the shift of hydrogen and an unusual ring closure. As mentioned before, we confwm Bradley and Johnston's value for the density of the polymerized eleostearates but we obtain much lower iodine values. The methyl pseudoeleostearate polymerized on heating to give a product with an iodine value of 85.0 by the rapid Wijs method. This method (6,10) has been demonstrated to be comparable to the usual Wijs (30-minute) method for nonconjugated systems. A 2-hour Wijs method would certainly increase the possibility of halogen substitution in an alicyclic system and give high iodine values, while the short (3-minute) exposure used in the mercuric acetate method would minimize the halogen substitution. Therefore we believe that the rapid Wijs method is more suitable; but since the actual unsaturation present is not known, no definite statement can be made. Also, Bradley and Johnston apparently decided that the 2-hour Wijs method is unsatisfactory for the polymerized products since they used the Kaufmann method on other materials in the same (3) and subsequent (4) investigations. However, other investigators (6) reported an iodine value of 103 (4-hour Wijs) for polymerized ethyl eleostearate which agrees with our values. These observations and also unpublished data from this laboratory indicate that the difference in iodine values can be only partially attributed to the different methods of analysis and that the products of polymerization of pure esters of eleostearic acid and of esters of tung oil acids are not identical. Tung oil may contain some linoleic acid (11)which might affect the physical characteristics of the polymerized product. Also, polymerization at 300' instead of 275" C. may lead to a higher proportion of trimers such as illustrated by structural formula IV of Figure 2 (probable iodine value, based on calculation of both conjugated and nonconjugated unsaturation, 130). It should also be re-emphasized here that, due to the difficulty of preparing the highly purified esters in quantity, we have in no case removed the nonpolymerized portions before examination. Brod, France, and Evans (6) reported the formation of a distillable monomer with properties different from the original monomer. Such products were probably present in our materials when analyzed, since no steps were taken for their removal. Work now in progress may furnish a more adequate explanation. Our iodine value is in agreement with the theoretical value of 86.2 for the tricyclic dimer (formula 111). At least eight possible isomeric tricyclic dimers can be derived from the
1123
self-condensation of methyl eleostearate by an initial intermolecular 1,Cdiene addition and a subsequent intramolecular 1,Pdiene addition. The structure of one of these isomers (111) and of a possible trimer (IV) and the equations for their formation are given in Figure 2. The reaction of dimer I1 to form dimer I11 require$ a shifting of unsaturation if 1,4-diene addition requires two sets of conjugated systems, but a hypothesis involving such a shift cannot be objectionable since unsaturated bonds in simpler molecules undergo rearrangement a t elevated temperatures. I n addition, if the assumption of Rossman (18) that cyclization of the monomer may precede dimerization is accepted, these products could also yield tricyclic dimers. These dimers would account for the higher density observed for polymerized eleostearates and for the iodine values of our polymerized products. No unusual ring closure is necessary for the formation of molecules such as I11 and IV, and these cyclic structures are not unique since similar ones have been assigned to certain terpene hydrocarbons and their derivatives.
Conclusions The rates of thermal polymerization at 275' C. of the various conjugated trienoic esters investigated have the same order of magnitude, although methyl pseudoeleostearate polymerizes slightly faster than its positional isomers, methyl a- and methyl 8-eleostearate. These rates are much more rapid than those of the two unconjugated trienoic fatty acid esters examined. The fact that the geometrically isomeric forms of the trienoic acid esters have approximately the same rates of thermal polymerization, in so far as we have examined them, but at the same time have widely different oxidation rates may account for the anomolous behavior of certain oils in which no previously predictable correlation has been found between speed of gelation and rate of drying. Additional evidence in conformity with Scheiber's theory of isomerization and the Kappelmeier theory (7) of diene mechanism for polymerization of unsaturated fat acids or their esters has been presented. By a simplified interpretation of 1,4-diene addition of conjugated trienoic acids, structures for a bicyclic trimer and a tricyclic dimer have been suggested to account for the physical properties of the polymerized products.
Literature Cited (1) Bradley, T.F., IND. ENQ.C H ~ M29, . , 440,579 (1937). (2) Ibid., 30, 689 (1938). (3) Bradley, T.F., and Johnston, W. B., Ibid., 32,802 (1940). (4) Ibid., 33, 86-9 (1941). (5) Brod, J. S., France, W. G., and Evans, W. L., Ibid., 31, 114 (1939). (6) Hoffman, H. D.,and Green, C. E., Oil & Soup, 16,236 (1939). (7) Kappelmeier, C. P. A., Furben-Ztg., 38,1018 (1933). (7A) Kass, J. P.,Memphis meeting, Am. Chem. SOC., 1942. (8) Kass, J. P., and Burr, G. O., J. Am. Chem. SOC.,61,3292(1939), 62, 1796 (1940). (9) Kass, J. P., Nichols, J., and Burr, G. O., Ibid., 63,1060 (1941). (10) Kass, J. P., Norms, F. A., and Burr, G. O., Cincinnati meeting, Am. Chem. Soc., 1940. (11) Kaufman, H. P., and Baltes, J., Ber., 69,2676 (1936). (12) Mikusch, J. D.von, IND. ENO.CHEM.,32, 1061 (1940). (13) Mikusch, J. D. von, and Priest, G. W., Oil & Soap, 18, 50 (1941). (14) Morrell, R. S.,J . SOC.Chem. I d . , 37, 181T (1918). (16) Myers, J. E., Kass, J. P., and Burr, G. O., Oil & Soap, 19, 107 (1941). (16) Priest, G. W., and Mikusch, J. D. von, IND. ENQ.CHBM.,32, 1314 (1940). (17) Rollett, A.,2. physiol. Chem., 62,422 (1909). (18) Rossman, E., Fettchem Umschau, 40,96,117 (1933). (19) Scheiber, J., Furbe u. Luck, 1929,585. (20) Spitser, W.C., Ruthruff, R. F., and Walton, W. T., Am. Paint J . , 26,NO. 12,68-72 (1941).
