May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
by Hindin and coworkers ( 7 ) ,requires modification and clarification in order t o account for the present results. It is proposed that the first step in the mechanism is the formation of a polarized complex between the protonic centers of the catalyst and a small amount of olefin, either present as an impurity in the eaturated hydrocarbon or formed by dehydrogenation, oxidation, or cracking under the conditions of the exchange reaction. The second step, which is probably the slowest step of the over-all mechanism, consists of the activation of the saturated molecules by extraction of a hydride ion by the carbonium ion portion of the initiating complex, releasing a stable saturated molecule corresponding t o the initial olefin and forming a new complex. The remaining steps are essentially the same as the last two steps of the Hindin, Mills, and Oblad mechanism-namely, rapid hydrogen interchange between the activated hydrocarbon complex and available hydrogen ions from the catalyst, followed by neutralization and release of the exchanged hydrocarbon cation by hydride ion extraction from another saturated molecule, thus propagating the activation process. ACKNOWLEDGMENT
The authors are indebted to B. S. Greensfelder for pointing out that the random distribution of hydrogen and deuterium among the nine exchangeable positions in a molecule of isobutane undergoing exchange may account for the observed peak of the curves in Figure 2.
1113
LITERATURE CITED
Beeck, O., Otvos, J. W., Stevenson, D. P., and Wagner, C. D., J . C h m . Phys., 17,418 (1949). Evans, A. G., and Polanyi, M., J . Chem. SOC..1947, 252. Greensfelder, B. S., “Advances in Chemistry,” Ser. 5, p. 3, Easton, Pa., Mack Printing Co., 1951. Greensfelder, B. S., Voge, H. H., and Good, G. M., IND.ENQ. CHEM.,41, 2573 (1949). Hansford, R. C., Conference on Catalysis, Gordon Research Conf., Colby Jr. College, New London, N. H., June 24, 1948. Hansford, R. C., IND.ENG.CHEM.,39, 849 (1947). Hindin, S. G., Mills, G. A., and Oblad, A. G., J. A m . Chem. SOC.,73, 278 (1951).
Honig, R. E., Anal. Chern., 22, 1474 (1950). Honig, R.E., preprint, Symposium on Use of Isotopes in Petroleum Chemistry, 118th Meeting AM. CHEM.SOC., Chicago, Ill., September 1950. Marisic, M. M.(to Socony-Vacuum Oil Co., Inc.), U. S. Patent 2.384.946 (19451.
Milbken, T.’H., Mills, G. A., and Oblad, A . G., Discussion Faraday SOC.,8,279 (1950).
Pines, H., and Wackher, R. C., J . Am. C h m . SOC.,68, 595 (1946).
Porter, R. W., Chem. & Met. Eng., 53, 94 (April 1946). Thomas, C. L.,IND. ENG.CHEM.,41, 2564 (1949). Wagner, C. D.,Beeck, O., Otvos, J. W., and Stevenson, D. P.,
J. Chem. Phys., 17,419 (1949). RECEIVED for review February 17, 1951. ACCEPTED January 31, 1952. Presented a8 part of t h e Symposium on Use of Isotopes in Petroleum Chemistry before the Division of Petroleum Chemistry a t the 118th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Chicago, Ill.
Thermal Polymerization of Drying Oils ISOMERS OF METHYL LINOLEATE R. F. PASCHKE, JOHN E. JACICSON, AND D. H. WHEELER General Mills, Inc., Minneapolis, M i n n .
T
HERMAL polymerization of drying oils is quite old in practice, but the chemical studies of the reactions involved has
been a more recent development which has received considerable attention in the last 15 t o 20 years. The decrease in iodine value of oils as they polymerize was soon recognized, and attempts have been made to correlate the drop in iodine value with the molecular weight of the polymerized oil, assuming that two double bonds disappear for each joining of two oil molecules (I, 8). Generally, the molecular weight was found to be lower than would be expected from the degree of reaction as measured by drop in iodine vdue. While many such studies on the naturally occurring glycerides have given useful practical information as t o the effect of temperature and time of reaction on the bodying of drying oils, the beginning of an understanding of the nature of the chemical reactions involved has appeared largely as the result of studies on the polymerization of simple alkyl esters of the unsaturated fatty acids. The earlier studies of Kino ( I I ) , Kappelmeier (8), Steger and van Loon (ai),and of Brod, France, and Evans (6),followed by the more extensive studies of Bradley and Johnston (3, 4), have shown that carbon-to-carbon-linked dimers are the principal products of thermal polymerization of methyl ester of polyunsaturated acids, and that polymers higher than trimers are probably not formed. The generally proposed mechanism for the formation of these dimers and trimers is that the conjugated polyene esters dimerize
by a modified Diels-Alder addition (8), with one molecule of diene acting as dieneophile, and that the nonconjugated esters are thermally isomerized t o conjugated forms. These conjugated forms are assumed to dimerize by the same reaction. The principal evidence for these mechanisms may be summarized as follows: 1. The physical and chemical properties of the dimers are in general accord with the proposed possible structures. 2. The conjugated esters polymerize more rapidly than the nonconjugated esters. (The conjugation step is assumed to be the rate-controlling reaction.) 3. The conjugation of conjugated esters disap ears during polymerization, whereas nonconjugated esters deveyop a certain amount of conjugation. 4. Analogous dimerization of simple conjugated aliphatic dienes and trienes has been structurally proved. This general mechanism is the most satisfactory working hypothesis for postulating structures for the dimers and trimers of the polyene esters. Classical organic structural proof of the exact structure of any of these dimers or trimers is still lacking. The large number of possible isomers and the difficulty of isolation in a pure form no doubt account for this fact. In view of the recent development of convenient spectroscopic methods for estimation of conjugated and nonconjugated polyene acids (9,16),and of methods for determining monomeric, dimeric, and trimeric methyl esters ( 6 ) , it seemed desirable t o make detailed kinetic study of the thermal polymerization of pure methyl linoleate and its isomers. Working with pure material and with
1114
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
makes is t o show that the ''ISGIineate" previously observed in Iodine the earlier stages of polymerizaMonomer, Value, Dimer, Trimer, Dimer/ N", Cb, XC, Old, K , Hr.-le tion was due to trans, trans- or Hours % Monomer % % Trimer % % ' % 70 czs, trans-linoleate. The revised 300" C. data are shown in Table I. (The 0 100 172.1 .,. . .. ,... .., , ... 6 79.1 163.9 19.'6 i.'4 14,'O 66.0 7.0 6.1 7.6 0.064 values for .V, C, X, and K for 48 12 53.1 154.5 41.6 5.2 8.0 34.7 6.4 12.0 10.8 0.107 hours a t 300" C. and for 96 hours 0.084 24 30.2 131.7 59.9 9.9 6.1 12.7 3.2 14.3 14.0 48 19.4 100.5 62.2 18.1 3.4 (1.6) (1.6) (16.2) 16.1 (0.086) a t 290 C. were estimated by ex0,0861 trapolation, since these mono290" C. mers were not available for the 0 100 172.1 100 62, ., .. .. .. 12 74.9 163.5 23:6 i.'7 13:s 5.9 7.5 0.039 &hour i s o m e r i z a t i o n . ) El11.3 0.056 11.1 8.4 5.3 31.8 4 . 4 151.5 44.2 24 47.3 amination of the data by 14.1 15.3 0.048 59.6 13.2 4.5 10.1 3 . 1 129.8 48 27.3 (0.040) 63.5 18.4 101.6 3.5 (1.5) (1.5) (14.6) (14.3) 96 17.6 graphical means for total non0.047.f conjugated l i n o l e a t e s how eci a h T = nonconjugated linoleate from 6-hour isomerization. b C = conjugated linoleate by spectral before isomerization. them to be in closest agreement c X = monomers not analyzed as linoleate spectrally = M - ( N + C ) . with a first-order or monomolecd 01 = monomeric mono-olefin calculated ffiomjodine VGU:, of monomer. 7o'r 1 ular reaction for its disappcarE e K = monomolecular constant for N ; = t* n x log -%N2 ancr Conjugated linoleate, C, f Average from plot of log N versus time. rises rather rapidly to a fairly low value and remains more or less constant a t 5 to 7% during the period when most of N is disappearing, and then it drops off these newer methods of analysis, it was expected that such a study would give a clearer picture of the mechanism of thermal polygradually to a lower value. Dimer, D , increases fairly rapidly and then levels off, while trimer, Y', increases continuously. merization than previous studies based on naturally occurring X , which represents monomer which does not analyze spectrallmixtures of fatty acid esters. It is hoped that certain facts found as conjugated or nonconjugat,ed linoleate, increases regularly and in this study, and which may be found in a similar study of the trienoic acids, will be of value in the more complicated problem levelsoff a t maximum of about 15%. This material is apparently a monoethenoid product, since it's amount is in close agreement of interpreting the kinetics of polymerization of the naturally ocwith 01, calculated from the iodine value of the monomer on t h r curring triglyceride oils. assumption that the monomer is a mixture of linoleate and o1eat.e The results on normal linoleate are of interest in connection with the monomers recovered from polymerization of linoleic-rich isomers. Actual examination of the final monomers (48 hourp a t 300" C. and 96 hours a t 290' C.) showed that this mono-olefur acids or methyl esters for production of residual dimers. They was a mixture of an oleate isomer which hydrogenated to stearate, show that such monomers contain nonconjugated cis,trans-linoand a material, presumably a cyclic mono-olefin, which did not leate isomers which require special methods for spectral determination and that they contain a low percentage of conjugated linohydrogenate to stearate (18). leate. The source of the oleate isomer may be from hydrogenation of NORMAL LINOLEATE linoleate by hydrogen resulting frorn dehydrogenation of the The thermal polymerization of normal (debromination) methyl cyclohexene ring of t'he dimer. linoleate has been reported previously (18), and the methods were M E T H Y L LINOLELAIDATE reported for analysis for monomer, M ,dimer, D, trimer, T , and normal linoleate, N , on the reaction mixtures. The monomers, Linolelaidic acid (trans-, trans-9,12-linoleic acid j was prepared from debromination linoleic acid by heating for 6 hours a t 200' C. particularly in the earlier stages of polymerization, contained isomer8 of linoleate which were not determined by the spectral methwith 1% selenium (10). The crude elaidinized acids were vacuum distilled and crystallized three times from acetone (4 ml. per ods then used (15). Cnpublished studies discussed here show that these monomers gram) at -40' C., and four times at -20" C. (10 ml. per gram). contained nonconjugated trans-linoleate isomers which were not Yield was 14%, melting point, 28' to 29 C. The acid was estericompletely determined by the usual 25-minute isomerization fied with methanol and distilled. Two 1ot,swere made which diftime used for normal cis, cis-linoleate. These studies also showed fered only slightly. Batch A showed: acid value = 1.02: iodine value (rapid Wijsj that trans, trans-methyl linoleate (linolelaidate) required 6 hours = 166.5 (theory 172.4); refractive index, = 1.4560; meltto reach a maximum isomerization (potassium hydroxide-glycol, 180' C.) of specific absorbtion ( a ) = 88.7. Pure normal (cis, cis) ing point = -9.8" to -7.8" C.; specific gravity 20" = 0.8820: specific gravity, 30' = 0.8759; a 231 nip = 2.1. methyl linoleate showed a value of 84.8, and a cis, trans-methyl Batch B showed: acid value = 0.5; iodine value (rapid n'ijs; linoleate showed a value of 84.7 for 01 a t 6-hour isomerization Fortunately, samples of the nionomers from the polymeriza= 166.9; iodine value (hydrogenabion) = 175.5 (theory 172.4;; refractive index n g = 1.4562; melting point, -10" to - 8 " C . ; tion of normal linoleate had been preserved in vacuum-sealed vials under refrigeration These were reanalyzed; the values 01 231 my = 4.8. The low value for iodine value by the rapid Wijs method was checked by the reg-ular Wijs method a t 30- and were checked before isomerization, and after isomerization 60-minute reactions. Kass and Burr ( 1 0 )reported an iodine value periods of 25 minutes and of 6 hours. The values before isomeriof 178.3 for the acid (theory 181.1). However, the iodine value zation and after 25 minutes checked those previously obtained by hydrogenation of the material used in the present work was (18). The values at 6 hours were considerably higher than the slightly above theoretical. 25-minute values. The compositions of the polymerized methyl Linolelaidate was much slower than normal linoleate to isomerlinoleate were recalculated, using the determined &hour value, and ize to conjugated forms, under the conditions of alkali isomerizausing an average value of a = 86.6 for the nonconjugated linotion (potassium hydroxide-glycol, 180" C.) ( 1 5 ) . Xichols et a / . leate, whether cis-cis, cis-trans, or trans-trans. This average ( 1 7 ) and Riemenschneider et al. (19) have previously observed value (86.6) used for determining total nonconjugated linoleate, that linolelaidate isomerizes much more slowly than normal linoN , is about 2 units lower €han the value found for pure trans-trans leate. According to the present data, maximum absorption a t and two units higher than the value for nonconjugated cis, cis-, or 231 mp was reached only after a &hour isomerization, compared IB cis, trans-linoleate. The principal effect which this recalculation
POLYMERIZATION OF NORMAL METHYLLINOLEATE TABLE I. THERMAL
I
O
O
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
1115
NONCONJUGATED LINOLEATES
TABLE 11. THERMAL POLYMERIZATION OF METHYLLINOLELAIDATE Hours 6 12 24 48
12 24 48 96 a
Monomer,
%
66.6 43.6 24.5 16.2
64.2 41.0 23.8 15.1
Iodine
Value,
Monomer 158.0 146.5 122.8 95.8
157.1 145.1 122.0 96.4
Dimer,
Trimer,
32.1 53.0 65.3 68.3
1.4 3.4 10.2 15.5
34'. 4 53.3 fi7.9 72.2
2900 1.4 5.7 8.5 12.6
%
7%
Dimer/ Trimer 22.9 15.6 6.4 4.4
N.
C
51.2 25.6 6.7 1.1
7.5 7 . 9 10.9 6.111.912.9 3 . 5 1 4 . 3 14.0 1 . 5 14.6 1 4 . 3
0.11 0.12 0.11 0.075 0.11"
24.6 9.4 8.0 5.7
49.5 25.1 7.4 0.7
7.8 6.9 5 . 2 10.7 3.0 13.4 1 . 5 13.9
0.061 0.055 0.051 0.049 0.054a
%
'd
X
./d
01,
K,Hr.-1.
The fact that nonconjugated linoleate disappeared by a first-order reaction would agree with the previously proposed mechanism of polymerization
N
+C
C
+ C --+ D
c. 11.2 12.6 13.8 13.2
(slow, monomolecular or first-order reaction) (1) (rapid, consecutive bimolecular reaction) (2)
Average K from plot of log N versus time.
However, when this apparent firstorder reaction was more rigorously tested Iodine by dilution with inert methyl stearate, Monomer, Value, Dimer, Trimer, Dimer/ N, C, 01 i t was found that the apparent first-order Hours % Monomer % % Trimer % % %' K,Hr.-1 velocity constant, K , was not constant 24 53.7 149.5 39.0 6.8 5.7 43.0 1 2 . 0 14.2 0.034 as it should be for a true monomolecular reaction, but that with increasing dilution, it decreased to a limiting value of about one fourth that observed on the undiluted linoleate (18). 26 to 30 minutes for normal linoleate. The average value for a! These data indicated that the apparent firsborder reaction disap231 mp at 6 hours for the two preparations was 88.7 (corrected for pearance of normal linoleate must be due extensively to a second conjugated linoleate originally present, as indicated by a: before or higher order reaction which is a pseudomonomolecular reaction isomerization). Further details on the alkali isomerization and under the conditions which prevail without dilution. The reacthe infrared properties of linoleate isomers Rill be published seption might appear to be first order because normal linoleate is rearately ( 7 ) . The monomers were analyzed for total nonconjuacting with some other substance which is present in fairly congated linoleate, N , using a &hour isomerization time and 01 = stant concentrations during the main portion of the polymeriza$8.7 a t 231 m9. This would include any cis, trans- or cis, cistion reaction. Conjugated linoleate would appear to be the only linoleate which might be formed from the trans,trans-linolereasonable material which is so present. Thus, a modification of laidate by thermal isomerization. mechanism in Equations 1 and 2 would be The data on the thermal polymerization of methyl linolelaidate axe shon n in Table 11. The whole pattern of the reaction appears N +C (slow, monomolecular reaction) similar to that of normal (cis, cis) linoleate, The linolelaidate is (3) somewhat faster than normal linoleate as to disappearance of N C +D (rapid, consecutive bimolecular reaction) (4) nonconjugated linoleate and formation of polymer. The formation of X , oleate isomers, and the ratio of dimer to trimer are very The low concentration of conjugated linoleate, and the high similar for these two nonconjugated isomers of linoleic acid. The ratio of nonconjugated to conjugated linoleate during the main dimer-trimer ratio is appreciably higher for the linolelaidate than reaction should favor Equation 4 as the mechanism for dimerizafor the normal cis, cis- or the nonconjugated cis, trans-linoleates. tion. Quantitative tests made on mixtures with normal linoleThis must mean that the thermal isomerization to conjugated ate and alkali-conjugated linoleate did give rather definite eviforms, which occurs at a high temperature, and probably by a dence that Equation 4 can occur (18). Thus, the mechanism of Eree-radical mechanism, occurs as rapidly with the trans, transEquations 3 and 4 offered a t least akpalitative explanation of the elaidate as with the normal cis, cis-linoleate. This is in contrast apparent firstorder reaction velocity constant observed experito the alkali isomerization reaction which occurs at a lower tem-mentally. Complete quantitative description of the react,ion was perature by an ionic mechanism, and in which the normal linoleate not possible because of the complications of side reactions, such as conjugates much more rapidly. From infrared and ultraviolet the formation of X and trimer. studies of the various monomers there is evidence that there is a A detailed study of the rates of alkali isomerization of the three thermal cis-trans isomerization which precedes or is concurrent nonconjugated linoleates (cis-cis, trans-trans, and cis-trans) lead with the thermal conjugation and polymerization.. However, to a method permitting estimation of each in a mixture. This more detailed quantitative infrared data will be required to clarmethod was used to analyze the monomers recovered from the ify the relationship of cis-trans isomerization to the thermal connonconjugated linoleates ( 7 ) . jugation and polymerization reactions. It was found that monomers from cis, cis-linoleate showed rapid formation of cis-trans with very little trans-trans isomer, and CIS, TRANS-METHYL LINOLEATE rapid disappearance of cis-cis isomers as the cis-trans isomer was A material which is believed to be fairly pure cis, transformed. Monomers from trans, trans-linoleates showed a slower 9,12-methyl linoleate was prepared by extensive fractional crysformation of appreciable amounts of cis-trans isomers, but very tallization of the nonconjugated methyl esters of dehydrated caslittle cis-cis isomer. The monomers from cis, trans-linoleate tor oil acids. Details of preparation and evidence of identity showed very little cis-cis or trans-trans isomer. It thus appears will be published elsewhere ( 7 ) . The product had an iodine value that the cis-trans isomer is the form which most readily forms and accumulates during thermal polymerization. (rapid Wijs) of 170.2 (theory 172.4). The infrared absorption at 968 cm.-' (trans double bond) was It thus appears that cis, cis-linoleate rather rapidly isomerizes close to half that of methyl linolelaidate and about equal to that to cis-trans or trans-trans forms or both, concurrently with its of methyl elaidate. It was polymerized for 24 hours at 290' C. isomerization to the conjugated forms which react by polymerizaSufficient material was not available to make an extensive study. tion. The trans, trans-linoleate, also isomerizes to cis forms to a The data are shown in Table 111. This material polymerized lesser degree. If the cis-trans thermal isomerization is an equilibsomewhat more slowly than the cis-cia, or trans-trans isomers, but rium reaction favoring the trans forms, as is the case with catathe limited data preclude a detailed comparison. lyzed cis-trans isomerization of oleic-elaidic acids, the changes
TABLE 111.
POLYMERIZATION OF
cis, trans-METHYL LINOLEATE A T 290' C.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
1116
TABLE 1v. THERMAL POLYMERIZATION llonomer,
7%
Hours
Iodine Valuea, Monomer
Dimer.
OF
ALKALI-CONJUQATED 1 I E T H Y I, LINOLEATE
Trimer,
'Z
Dimer/ Trimer
%
N,
C,
%
%
A',
R
l i b . Hr.-1
289' C
0 3 6 12
180.0 175.4 166.9 150.2
100,o 54.2 33.6 17. I1
179.5 6 65.0 162.7 24 20.5 48 10.2 139.1 a By Woburn method (14). b Monomolerular reaction velocity
TABLE v. Hours
1O:O 16.1 20.7 270' C .
3.6 3.1 2.9
3.08 75.2 20.8 4 . 2 37.6 12.4 2.3 20.6 10.7 1.6 8.1 8 . 2
25.5 58,s 63.5
9.3 20.7 26 0
2.7 2.8 2.4
2.7 1.6 0.9
Iodine Valuea, Monomer
Dimer,
m,
2900
0 1.5 3 6
100 32.8 24 1 19 3
175.2 159.5 148.5 134.3
4 7 . 2 15 1 11 2
3.7
7 7
5.6
0.231
0.200
0.156 0.0779
0.0801 0 0458
constant for diqappearanoe of conjugated linolcate.
THERMAL POI,YJnSRIZATION OF
Monomer,
%
.,.
35:7 50.0 60.1
Trimer
?&
~ ~ , ~ ~ - I L ~ ELISoIzF;lirr& THYL Dimer/ Trimer
C
..
s, 470
Kb
c;. ...
62.4 70.2 r4.1
4.7 5 5 6.9
13 11
96.7 25.6 16.7 11.9
1.9 3 2
23 19
46.4 30.3
1.5
3.3
7.2 7.4
7.4
O.Oi9l 0,0139 0.0081
270' C.
1 5 3
53 3 36 2
167 5 162 1
44 7 60 i
2500
gated monomeric products of lower iodine value, possibly cyclic, is evident again from the lower iodine values of the later monomers. Their formation may in part account for the lower order of reaction, assuming that they are formed by a firstorder reaction. Another explanation for this lower order of reaction may be that these isomeric conjugated form+ cistrans mixtures-are converted into tnore active forms, such as trans-trans forms, which dimerize with themselves or with other isomers of linoleate. Thus, in addition to the bimolecular dimerization of the cis, trans-linoleate which niag be fairly rapid, there may be a monomolecular isomerization to a trans-trans form which reacts very rapidly with itself or other forms C,,
+ C,t ---+
11 (rapid,
second order) 6.4 5.9
0 0074 0 0073
c.
179.1 2fi. 4 1.5 72.5 1.3 20 67.6 4.D 0.00207 176.5 40 0 3 68.3 1.7 24 52.6 3.7 0.00281 171.2 55.D 6 41.7 2.2 25 35.9 5.8 0,00295 12 28.6 166.4 68.8 2.4 29 22.6 6.0 0 00273 0 B y Woburn method (14). b Bimolecular reaction velocity constant f o r second-order reaction for d i s a p i ~ a r a n c cof conjugated linoleate.
would be expected to be more noticeable vith the c t s , cis-linoleate, as was observed. Since the polymerization of nonconjugated cis, cis-, cis, trans-, and trans, trans-linoleates were quite similar, it appears that the thermal cis-trans isomerization is incidental to thermal conjugation and polymerization, and is not an important rate-controlling factor.
Vol. 44, No. 5
C0i ('11
--+
Cit
(lapid, first order)
(j)
(6)
+ Ctt +D (very rapid, +
(TU
('"Ct
+0
second order)
(7)
(very rapid, eerond ordrr)
(8)
Slso. various conjugated forrris niav react with dimer to form trimer. This possibility would appear more reasonable if it were shown that a conjugated trans, trans-linoleate did polymerize very rapidly compared to the alkali-conjugated linoleate. For this rcxson, and because the analytical data would be more significant on a pure material, the methyl ester of a conjugated, pure C T ~ R talline (prewmably truns, tians-) 10, 12-linoleic acid was iiest studied.
ALKALI-CONJUGAIEI) LINOLEATE
METHY 1.-10, 12-L1XOLF:TE
The methyl ester of alkali-conjugated linoleic acid ( 9 % )was next examined. This material has tjhe advantage of being relatively easily prepared by alkali isomerization of normal linoleate. However, i t has a value of 01 a t 233 mp which is about 2501, lower than the pure crystalline conjugated acids which are usually used as standards. It is not certain to what extent this is due to the presence of nonconjugated isomers and to what extent it is due to a different value of 01 for the isomeric conjugated forms. If it, is assumed that the double bond which shifts may assume the cis or trans form and that the remaining double bond retains its original cis form, then the alkali-conjugated linoleate m-ould be a niixturr of cis-9- , cis- , and trans-11-linoleates and of cis- and trans-10, cis-12-linoleates. Recent work indicates that the cis-trans forme predominate (16). By the same considerations, the conjugated forms generated by heat during thermal polymerization would presumably he a similar mixture, whereas t,he crystalline 9, 11- , and 10, 12-linoleic acids used as spectral standards are presumably trans-trans forms. The course of the polymerization of alkali-conjugated linoleate is shown in Table IV. The much greater speed of polymerization in comparison with normal lincleate is evident>. The constant and low ratio of dimer to trimer is rather remarkable. Graphical examination of the data showed that the disappearance of conjugated linoleate was closer to a first-order than to a second-order reaction. The calculated order of reaction was 1.2 t o 1.5. This was somewhat unexpected since a second-order or bimolecular reaction was anticipated. The increasing amount of nonconju-
The 10, 12-linoleic acid (trans, trans) of von Mikusch (17') \+ its prepared from dehydrated castor oil acids by fractionally diatilling the methyl esters to obtain a nonconjugated linoleate fraction (prcsumably cis-9, and cis,trans-12). This ester was then alkaliisomerized, the acids were fractionally crystallized, and then ified Lvith methanol; 01 = 105.9 and melting point, 23.7' to 24.0"C. Tlic course of the polymerization of this ester is shown in Table V. The monomers from this Rerics of runs showed no ai)parent content of normal linoleate, that is, 01 a t 2320 A. did not, increase on treatment with alkali a t 180" C. under the analytical conditions. Actually, t'here was always a slight decreafie in a. The rate at 290" C. was so fast that it was necessary to work a t loiver temperatures in order t,o get accurate timing because of heatup period. It will be seen that a t 270" and 250" C., the disappearance of conjugated 1inoleat)efollows a second-order rea(:tion, t,he values for K being calculated for second-order reaction. At 290" C. the second-order reaction constant falls off, indicating appreciable side reactions. At 290' C. in the later stages there is the formation of nonconjugated isomers with lower iodine values which may be formed by a slower concurrent reaction, while a t 270' and 250' C., there is less of this by-product formed. The value of K a t 290" C. for the first period of 1.5 hours is probably fairly accurate since the monomer is still largely unchanged conjugated diene. The calculated order of reaction was 2.1 at 250' C., and 1.8 a t 270" C. At 290" C. it was 1.9 for the first part and 3.4 for the last part of the reaction. I
t
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
The very high ratio of dimer to trimer a t all stages of reaction is in marked contrast to the alkali-conjugated linoleate.
1117
100
80
INFRARED ABSORPTION DATA
It was hoped that infrared absorption Idata would give some useful information as 60 to the structures of the monomers and dic mers of the linoleates. Much of the earlier 0 W work was done by arrangement with Crawa 40 w ford of the University of Minnesota (Perkin-Elmer spectrograph Model 12-C with rock salt prism). Subsequently, a Beckman 20 spectrophotometer Model IR-2 (rock-salt prism with a special slit-drive mechanism) was used in this laboratory by Tolberg and Kerns. 0 The region of 900 to 1000 cm.-' proved to be the most useful in characterizing the isomeric linoleates. Four bands were found Figure 1. of value in this region, 948, 968, 982, and 988 cm.-l The 988 cm.-I band was characteristic of conjugated trans, trans-linoleate. The 968 cm. -1 band is known to be due to an isolated trans double bond. The 948 and 982 om.-' bands were a doublet due to conjugated cis, trans-linoleate. No band characteristic of conjugated cis, cislinoleate has been found in this study, either because it has no bands in this region, or because it is not present in appreciable amounts in the materials studied. A more detailed description of these studies on the infrared and ultraviolet properties of the linoleates will be presented elsewhere (7). The significant infrared data are shown in Table VI.
:
TABLE VI.
INFRARED ABSORPTION BANDSOF AT
900 TO 1000 CM.-'
Untreated monomers Debromination linoleate Recr stallized linoleate Linoglaidate Cis, trans-Q,l2-linoleate Elaidete Alkali-conjugated linoleate Trans, t~ana-10,12-linoleate Recovered monomers Normal linoleate Linolelaidate Cis, trans-Q,l2-linoleate Alkali-conjugated linoleate Trans,trans-10,12-linoleate Dimers
_ _ _ = no band. * = weak band. +++ = very strong band. d ++ = strong band. + = moderate band. a b c
e
___
+
I
IO
20
TIME
30
I
I
40
50
IN HOURS
Polymerization of Isomeric Methyl Linoleates at 290" C.
indicate that normal cis, cis-linoleate isomerizes with heat to nonconjugated trans forms, and also to cis-trans and trans-trans conjugated forms. The trans-trans conjugated Structures are apparently formed by a subsequent isomerization of the cis-trans conjugated structures. Linolelaidate monomers all showed a strong 968 em. -l band, characteristic of the original compound, but decreasing in intensity with increased reaction time. They also showed weak 948 and 988 em.-' bands similar to normal linoleate. These data indicate that the nonconjugated trans, trans-linoleate also isomerizes to cis-trans and trans-trans conjugated forms under conditions of thermal polymerization. The monomer from cis, transMETHYLESTERS linoleate was similar t o that from normal linoleate. Alkali-conjugated linoleate monomers showed a fairly strong 988 cm. -1 band, which was strongest in the earliest samples. The 948 cm. -I band persisted, but with decreasing intensity as polymerization increased. A weak 968 band appeared, with increasing intensity as polymerization increased, but never very strong. This could be due to a shift of the conjugated forms to nonconjugated trans-linoleates, or it might be due to a trans double bond in the mono-olefins, which are apparently formed in small but increasing amounts during polymerization. The monomers of the conjugated trans, trans-10, 1Zlinoleate all showed very strong 988 cm. bands, characteristic of the original ester. They also showed a weak to moderate band a t 948 --- _ _ _ cm. -1, which increased in intensity as the polymerization proceeded, being barely perceptible in the first monomers a t 270" and 250" C. These data indicate that the trans-trans conjugation is partially isomerized to cis-trans conjugation under conditions of thermal polymerization.
The closeness of the 982 and the 988 cm. -l bands caused some interference. If either one of these bands was very strong, a small amount of the other band would not be observed. If both bands were present in considerable intensity, a single broad band would be found between the two frequencies or the stronger band would be shifted toward the weaker band. The monomers from normal cis, cis-linoleate all showed a moderately strong 968 cm. -l band, indicating a rapid development of trans double bonds by thermal cis-trans isomerization. This was also indicated above by the alkali isomerization data. These monomers also showed very weak bands a t 948 and 988 om. -1, shifted somewhat by the 982 component of the 948 to 982 cm. -1 doublet and by the stronger 968 cm.-l bands. These bands were rather weak, as would be expected from the low concentrations of conjugated linoleate found by ultraviolet analysis. These data
COMPARISONS OF THE ISOMERIC LINOLEATE POLYMERIZATIONS
The most striking comparisons between various linoleates are in speeds of reaction and in differences in the ratio of dimer t o trimer. The relative speeds of polymerization are shown in Figure 1. The times required for 60% polymerization at 290' C. are approximately in the ratio of l(trans-l0,12-linoleate)-7(alkali-conjugated linoleate)+tO(nonconjugated linoleate). The slower rates of the nonconjugated linoleates are t o be expected if the thermal conjugation theory is accepted, since the thermal conjugation is the slow, rate-determining reaction. The slower rate of the alkaliconjugated linoleate compared to the 10, 12-linoleate is explainable if it is assumed that the former is a mixture of cis-cis and cistrans forms and that the latter is trans-trans. If the Fisher-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
tion is the principal reaction and there is very little trimer formed. I4 The importance of the ratio of dimer to trimer in correlating molecular weight and 12 degree of reaction of triglycerides is obvious. It is generally true that the trienoic 10 acid esters give a lower dimer to trimer ratio than the dienoic esters. A similar detailed quantitative study of the trienoic esters 5 8 would be valuable. In spite of the fact that infrared data did 6 not differentiate the dimers derived from different isomeric monomers, the refractive 4 indices of the dimers did indicate a differ2 ence. Thus, the dimers from normal linoleate showed n g values of 1.4762 to 1.4772, the dimers from alkali-conjugated linoleate 0 20 30 40 50 60 70 80 90 were lower: 1.4743to 1.4753, and the dimers X POLYMER from 10, 12-linoleate were still lower: 1.4735 to 1.4740. Figure 2. Dimer-Trinier Ratio us. Per Cent of Polymer at 290” C. These refractive indices were taken a t the middle of the dimer plateau during analytical molecular distillation and usually Hirschfelder-Taylor models of cis-cis, cis-trans, and trans-trans showed a slight increase as the amount of polymer increased, but conjugated dienes are inspected, i t is found that the cis-cis form there was no overlapping in refractive indices between the dimers has the greatest interference to rotation around the central single derived from various isomeric monomers. bond. This form cannot assume the coplanar configuration of LITERATURE CITED its double bonds, which the diene must assume in the cyclohexene (1) Sdams and Powers, J . Applied Phys., 17,325-37 (1946). structure of the Diels-Alder adduct. The cis-trans diene shows ENQ.CHEM.,30,689-96 (1938). (2) Bradley, IND. much less interference, while the trans-trans diene shows no inter(3) Bradley and Johnston, Ihid., 32,802-9 (1940). ference a t all to rotation around the central single bond. (4) Ihid., 3 3 , 8 6 9 (1941). These effects of cis-trans configurations of the conjugated lino( 5 ) Brod, France, and Evans, Ibid., 31,114-18 (1939). (6) Cowan, Falkenburg, and Teeter, IND.ENG.CHEM.,BNAL.ED. leates on their reactivity as a diene in the Diels-Alder reaction 16, 9&2 (1944). with maleic anhydride have been similarly considered by von Mi(7) Jackson, Paschke, Taiberg, and Wheeler. J. Am. Oil Chemists’ kusch ( 1 2 ) . He showed that alkali-isomerized linoleic acid reacted Soc., in press. only slightly with maleic anhydride under the conditions of deKappelmeier, Farben-Ztg., 38,1018-20, 1077-9 (1933). Kass, in “Protective and Decorative Coatings,” Vol. IV, ch. 12, termining diene numbers, R hereas the trans, trans-10, 12- and the J. J. Mattiello, New York, John Wiley & Sons, Inc., 1944. trans, trans-9,ll-linoleates reacted completely. Kass and Burr, J . Am. Chem. SOC.,61,1062-6 (1939). The differences in the ratio of dimer to trimer are shown in Kino, Sci. Papers I n s t . Phys. Chem. Research (Tokyo), 15,127-9, Figure 2, Nonconjugated linoleates start out with a high ratio 130-6 (1931); 16, 127-32, 133-5 (1931); 18, 77-82 (1932); 20, 103-8 (1933); 21, 63-8 (1933); 24,25-32, 218-25 (1934); at low conversion, and drop to a relatively low ratio a t higher con26,91-7 (1935); 28, 140-5 (1935); 29,31-6 (1936). version, Alkali-conjugated linoleate is uniformly low, regardless von Mikusch, Angew. Chem., 62,475-80 (1950). of degree of conversion, .rc-hereasthe 10, 12-linoleate is uniformly von Mikusch, J . Am. Chem. Soc., 64,1580-2 (1942). quite high, regardless of degree of conversion. on Mikusch and Fraaier, IND.ENG.CHEM., ASAL. ED., 13, 7829 (1941). If trimer results from a Diels-Alder or other type of addition of Mitchell, Kraybill, and Zscheile, IND.ENG.CHEX.,ANAL.ED., an isolated double bond in a dimer n-ith a conjugated diene, these 15,1-3 (1943). results may be qualitatively explained. Iiichols, Herb, and Riemenechneider, J . Am. Chem. Sor., 73, K i t h normal linoleate in the early stages of polymerization, 247-52 (1951). Nichols, Riemenschneider, and Herb, J . Am. Oil Chemists’ Soc., there is only a very low concentration of conjugated linoleate 27,329-36 (1950). which can react n-ith dimer, 30 almost no trimer is formed. In Paschke and Wheeler, Ibid., 26, 278-83 (1949). the middle and later stages there is more conjugated linoleate and Riemenschneider, Herb, and Xichols, I b i d . , 26, 371-4 (1949). more dimer present, so more trimer is formed. Scheiber, Farheu. Lack, 1929,585--7; 1936, 315-16, 329-30, 341, 351-2; Fetle u. Seifeen, 43, 103-5 (1936). With alkali-conjugated linoleate, there is a high concentration of Steger and van Loon, Rec. trau. chim.,53, 769-78 (1934). conjugated linoleatp at the very beginning, which adds to dimer Terry and Wheeler, Oil & Soap. 23, 88-90 (1946). as it is formed, so the trimer content of the polymer is relatively RECEIVED for review August 13, 1951. . ~ C C E P T E DJanuary 11, 1952. high throughout the entire reaction. Presented in part before the Division of Paint, Varnish, and Plastics ChemisK i t h the 10, 12-linoleate the dimerization by addition to itself try a t the 116th Meeting of the AMERICAN CHEMICAL. SOCIETY, Atlantic City, is so very rapid compared to its addition to dimer that dimerizaN. J . Paper 104, Journal series, General 3Iills Research Laboratories. I6
