DIPHENYLETHYLENE WITH FREEK.4DIC.A.I-S
July 3 , 19.59 [CONTRIBUTlOX FROM T H E
DEPARTMENT OF
3405
CHEMISTRY O F T H E UNIVERSITY O F SOUTH CAROLINA]
The Reactivity of Diphenylethylene and Related Olefins toward Free Radicals. Evidence for Anomalous Reactions of Radicals from Diphenylethylene BY J O H N L. KICEAND FATEMEH TAVMOORIAN RECEIVED DECEMBER 8, 1958 The reactivity toward methacrylate radicals of diphenylethylene ( D P E ) , methylenedibenzocycloheptadiene ( I ) and diienzoheptafulvene (11)has been studied both by measuring the retardation of methacrylate polymerization caused by these olefins and the composition of the “copolymers” formed when methyl methacrylate is polymerized in their presence. Of greatest interest is the fact t h a t the ultraviolet spectra of the methacrylate-DPE and methacrylate-I copolymers show a n intense maximum in the 290-310 m+ region which vanishes on short treatment of the copolymer with dilute base. This behavior is strongly suggestive of the presence in the copolymers of 1,4-cyclohexadiene type units of structures VI1 and X, resulting from anomalous reaction of the olefin free rsdicals a t the p-position of one of the aromatic rings. The quantitative variation of the intensity of this maximum with total olefin content suggests 1‘11 and X arise a t least predominantly from termination reactions of radicals from D P E or I. Comparison of the reactivity data for I and I1 shows no indication of any enhanced aromaticity for I1 or the radical derived therefrom
A recent study’ has shown that dibenzofulvene about 265 1n1.1,~but it is consistent with VII. (9-methylenefluorene) is extremely reactive toward Since VI1 should be readily isomerized to VIII, free radicals, being about 200 times more reactive additional evidence for VI1 is the observation than styrene toward methyl methacrylate radicals. that when the copolymer was briefly treated with We were interested in obtaining data on the re- 0.15 ;If sodium methoxide in methanol-benzene activity of related olefins and therefore undertook and then reisolated the ultraviolet absorption a study of the behavior of diphenylethylene, 1- spectrum was radically changed t o one similar met hylene-2,3,6,7-dibenz-2,6-cycloheptadiene(I) ,Ce& ,C&L and 1-methylene-2,3,6,7-dibenzcycloheptatriene -CH*C-CH=C (11). I n I as in diphenylethylene the two aromatic \ \
vfJ=JJa - 2 % ~ v
4
/
CsHa
CaHs
3
-
CHi I
CH2 I1
I11
0 \
C&S
\7I
VI1
c
H
~
2 ~ H
e
VI11
to those expected for structures such as V and VIII. Similar behavior was also exhibited by the methyl met hacrylate-I copolymers.
Results and Discussion rings cannot be simultaneously coplanar with the 1-carbon, but unlike diphenylethylene, I is a crystalline solid which can be more readily purified. Dibenzoheptafulvene (11) was included because studies of several ionic reactions2b3 and the ultraviolet spectra3 of certain compounds related to I1 have yielded somewhat conflicting evidence concerning the possibility of enhanced aromaticity of I1 or the related carbonium ion 111. We hoped our work would provide information on this point for both I1 and the radical IV. After obtaining the reactivity data using the retardation-of-polymerization we attempted to check some of our results by determining the composition of “copolymers” produced when methyl methacrylate was polymerized in the presence of diphenylethylene. I or 11. I n the course of this work we happened to examine the ultraviolet spectra of the diphenylethylene-methacrylate copolymers and found to our surprise that they exhibited a strong maximum a t 290 mp. This maximum is inconsistent with structures such as V or VI which should not have maxima beyond (1) J. L. Kice, THISJ O U R N A L , 8 0 , 348 (1958). (2) G . Berti, J . O r g . Chem., 22, 230 (1957). (3) (a) E. D. Bergmann, e! al., Bull. SOC. chim. Fvance, 18, G84 (1951); (b) E. D. Bergmann a n d D. Ginsburg, Chemistry b Induslry, 4 5 (1954). (4j J. L Kice, T H I S J O U R K A L , 76, ($274 (1954).
All three olefins markedly retard the azobisisobutyronitrile (A1BN)-initiated polymerization of methyl methacrylate. It is thus possible to obtain data concerning their reactivity by the retardation-of-polymerization method.’.4 The kinetically /CH3
ki + CH?=-C(CfiH5)2+
wCH?C.
\
COOCHp
(R.) CH3 -XCH,C-CH,d‘ I
C6H5
(1)
I
I
COGCHI CeHj
(RZ.)
RZ.
+ CHz=C FH8+ R . k2
\
RZ.
(2)
CGGCHa
ka
+ R . +non-radical products
(3)
kd
2RZ.
+non-radical products
(4)
( 5 ) For t h e spectra of various model compounds closely related t o V a n d VI see R. A. Friedel a n d M. Orchin, “Ultraviolet Spectra of Aromatic Compounds,” John Wiley & Sons, Inc., h-ew York, N. Y., 1961 ; see also t h e spectra of styrene-methacrylate copolymers, Melville, et a i . , TraTts. Faraday Soc., SO, 279 (1954).
3406
JOHN
L. KICEAND
FATEMEH
TAYMOORIAN
Vol. 81
significant reactions involving the olefin (illustrated for diphenylethylenej are presumably 1-4, and from retardation studies it is possible to obtain values for k l , the reactivity of the olefin toward methacrylate radicals, and k2.”k3. The results of the individual kinetic runs are given in Table 11, the kl and kz’k3 values in Table I.
ultraviolet absorption spectra in chloroform of the methyl methacrylate-diphenylethylene copolymers revealed, instead of the expected maximum in the 260-270 mb region,%a much more intense peak a t 290 mfi (Fig. 1, curves X and B). A similar phenomenon was observed with copolymers from I, except that in this case Xmax was at even longer maw lengths (curves C and D, Fig. 1).8 LX‘ith TABLE I both copolymers, if the copolymer was refluxed for RATECONSTANTS FOR REACTION OF OLEFINSWITFI METH- a short while in a 0.15 31solution of sodium methACRYL.ATE RADICALS AT 5 0 ’ oxide in methanol-benzene and then reisolated the Compound ki X 10-2, 3 - 1 sec.-lb ( k ; . / k , ) X lO‘J ultraviolet absorption was radically changed (curves Diphenyleth ylene 7.2 2 A’ and C’, Fig. 1). These latter spectra in both I cases resemble those which would be expected for 3 .9 9 structures such as V and VI11 or IX and X I . I1 1.5 40
Dibenzofulvene 2500 4 Styrenea 13 104 Estimated from data for 60” given by C. ’Walling, “Free Radicals in Solution,” John W‘iley & Sons, Inc., New York, N. Y . , 1957, pp. 133, 146. kl values calcd. from d a t a in Table I1 using k , = 560 and k t = 1.6 X IO7 for methyl inethacrylate.
m-e had hoped the composition of the “copolymers]’ formed when methyl methacrylate was polymerized in the presence of the various olefins would provide a check on the accuracy of the kl values in Table I. Unfortunately the marked retarding effect of the olefins makes i t impossible to obtain polymers of respectable average molecular weight which also contain an appreciable weight percentage of olefin.6 As a result none of the Copolymers prepared contained over a few per cent hydrocarbon. Consequently the reactivity ratios (see Experimental) obtained from these copolymerization experiments must be regarded as only very approximate and cannot be considered accurate to better than *4070.7 Keeping this in mind, one may note that the reactivity ratios indicate the relative reactivity of diphenylethylene, I and I1 to be in the ratio 3.9:2.8:1 (as compared with 4.8:2.6:1 calculated from the k1 values of Table I). In view of the uncertainty in the copolymerization data the two sets of results appear to be in reasonable agreement. As indicated earlier the most significant results of the copolymerization experiments are the unusual ultraviolet absorption spectra of the copolymers. Before further consideration of the reactivity data in Table I we should like to consider the spectral data. Evidence for Anomalous Reactions of Radicals from Diphenylethylene and 1.-Examination of the ( G ) If t h e average molecular weight is too low a sizeable fraction of t h e polymer, particularly t h e lower molecular weight material, will be lost in t h e isolation procedure. I n a copolymerization of this t y p e where t h e added olefin causes marked retardation in t h e r a t e of polymerization most chains which contain greater t h a n t h e average ratio 3f added olefin t o methyl methacrylate will also be of lower t h a n average molecular weight. As a result a n y a t t e m p t t o increase t h e percentage of olefin in t h e copolymer will not improve t h e accuracy of -~ the derived reactivity ratio if a t t h e same time i t results in too low a n M n for t h e copolymer, since there will be a concomitant loss of low molecular weight olefin-rich polymer in t h e isolation procedure. (7) T h e olefin content of t h e copolymers was determined from duplic a t e C,H analyses on each polymer. T h e difference between t h e %C in t h e copolymers and t h a t in pure polymethyl methacrylate was only 0 . 3 - 1 . 0 ~ o , Considering t h e usual uncertainty in a C,H analysis there is obviously considerable uncertainty in t h e olefin content of t h e copolymers even with duplicate analyses. T h i s is particularly t r u e f o r I1 copolymer #I and DPE copolymer $1 which contain t h e smallest a m o u n t s of olefin
H
Comparison of curve A for the original diphenylethylene copolymer with the spectra of such model compounds as 1-benzylidene-2-cyclopentenegand l-benzhydrylidene-2-cy~lopentene~~ shows the extreme similarity in the spectra and strongly suggests t h a t a t least some of the diphenylethylene units in the copolymer have structure VII. Presumably the long wave length maximum in the methacrylate-I copolymers is due to structures such a s X. Although these results do not afford unequivocal proof of the presence of VI1 and X, such structures seem to offer the simplest interpretation of the spectral data consistent with all our data. The variation in the intensity of the long wave length absorption with total olefin content is also informative. The olefin concentration in the two solutions used to prepare the two diphenylethylene copolymers in Fig. 1 was in the ratio of 2.7:l. The total olefin content of the two copolymers is in the ratio of 2.4:1, in reasonable agreement with expectations in view of the inaccuracies of the C , H analyses. However, the concentrations of the units responsible for the 290 m p maximum are in the ratio of only 1.75:l. Similarly the total I content of the two methacrylate-I copolymers differs by a factor of 1.8 while the E’s differ by only a factor of 1.4. Therefore, while increasing the olefin concentration in the monomer feed results in approximately the expected increase in total olefin content of the copolymer, the concentration of the units responsible for the long wave length maximum does not increase as rapidly. This sort of behavior would be expected if the “special” units arise exclusively (or predominantly) from termination reactions involving the radicals from diphenylethylene or I : First, since both olefins act as effective retarders of methacrylate polymerization, an appreciable fraction of the diphenylethylene or I radicals produced disappear (8) T h i s disparity in ,A,, in t h e two cases would seem t o rule o u t all possible explanations involving t h e initiator, impurities in t h e monomer, impurities in diphenylethylene, etc. (9) E. A . Hraude and U’.F. Forbes, J . Chetn. Soc., 1755 (1951). (10) J . I,, Kice and F. M. P a r h a m , THISJ O U R N A L , 8 0 , ,3792 (1958).
July 5 , 1959
DIPHENYLETHYLENE WITH FREERADICALS
through termination reactions. Second, by reference to the kinetics of these degradative copolymerizations, it can be shown that if the termination processes involving the I or diphenylethylene radicals follow the course shown in eq. 5 and 6 the fraction of the total olefin present as VI1 or X will decrease quite markedly with increasing olefin concentration, and of course, the concentration of
3407
- 02
-0.6 h Q J
-E
3, c
\
CaH6
VI1 or X in the copolymer will not increase as rapidly as the total olefin content. A more quantitative examination of the present results, as outlined in footnote 11, certainly seems to suggest that the majority of VI1 or X arises from termination reactions involving the olefin radicals. (The uncertainties in the total olefin content of the copolymers inherent in the C,H analyses and our ignorance of the exact molar extinction coefficients for VI1 and X render any more exacting examination of the data of questionable value.) That VI1 or X should arise predominantly from termination reactions is also the most plausible theoretical interpretation of the experimental results. Only steric hindrance would seem likely to cause diphenylethylene radicals to undergo reaction a t the p-position of an aromatic ring rather than a t the 1-carbon. While there almost certainly is some steric hindrance to the addition of a diphenylethylene radical to the methacrylate double bond,]’ we doubt t h a t this could be important enough to cause the anomalous behavior observed. On the other hand, there is far more serious hindrance to any normal combinative termination reaction involving the present polymer radicals. Furthermore, a n important additional factor which should enhance the likelihood of reactions such as 5 and 6 i s the fact that termination i s a highly exothermic process of very low activation energy. For such a reaction Hammond’s Principle12 predicts the transition state will closely resemble the reactants. As a result the loss of aromaticity attending the formation of VI1 and X should have no significant effect on the energy of the transition states for reactions 5 and 6, and such reactions might easily occur to the exclusion of such disproportionative termina(11) Assuming the termination reactions involving the olefin radicals to occur as in eq. 5 and 6 , one can estimate from the results of the kinetic studies the fraction, 6, of the total olefin units in each copolymer which would be expected to have either structure VI1 or X. For diphenylethylene copolymer 2 (the one with the higher total diphenylethylene content) 6 = 0.23, while 8 = 0.30 for diphenylethylene copolymer 1. For the methacrylate-I copolymers 6 = 0.12 for 1 and 6 = 0.093 for 2. For either olefin multiplication of the ratio of the 6’s by the previously given ratio of total olefin content gives a predicted ratio for the concentration of V I 1 or X in the copolymers. l t is interesting to note that in both cases the calculated value of this ratio is quite close to the observed ratio of the extinction coefficients, being 1.9 for the diphenylethylene copolymers (found 1.8) and 1.4 for the I copolymers (found 1.4). (12) G . S. Hammond, THIS JOURNAL, 17, 348 (1955).
-1.0
- 1.8 Fig. 1.-All spectra in CHCla; E = optical density/g. polymer per liter of solution: curve A, diphenylethylene(DPE)-methyl methacrylate(MMA) copolymer 2, monomer feed ( M M A ) / ( D P E ) = 36; curve A’, same copolymer after treatment with dilute base; curve B, DPE-MMA copolymer 1, monomer feed ( M M A ) / ( D P E ) = 97; curve C, MMA-I copolymer 1, monomer feed ( M M A ) / ( I ) = 40; curve C‘, same copolymer after treatment with base; curve D, MMA-I copolymer 2, monomer feed ( M M A ) / ( I ) = 20; curve E, pure MMA polymer.
tion processes as 7, which might otherwise seem more likely to predominate. /CH, -CH,C;
‘COOCH3 ‘COOCH,
Discussion of the Reactivity Data of Table I.Clearly comparison of the results for I and I1 does not indicate any exceptional behavior for I1 or radical IV. The reactivity of I1 is somewhat less than I, and IV is somewhat more prone than the radical from I to add to monomer rather than undergo cross-termination. Our results do not seem to indicate any particular enhanced stabilization for IV, for were this the case kz/k3 for IV should presumably be much smaller than the value of the same quantity for the radical from 1.l This is in interesting contrast to Berti’9 conclusions regarding the carbonium ion 111. From com-
parison of the ~ K i I R d d of I11 with those of tropylium.16bbenzotropylium, 13c b e n ~ h y d r y l ' ~and " fluorenylIda carbonium ions he concluded there was substdntial additional stabilization of I11 beyond that found for a bemhydryl carbonium ion. concluded On the other hand. Bergmann, et from examination of the ultraviolet and infrared spectra of XII, 11. several benzylidene dibenzocvcloheptatrienes, and their dibenzocycloheptadiene analogs t h a t they could find no evidence indicating any particular aromaticity for 11, and they believe t h a t IT is correctly represented by a nonplanar structure.
Experimental Preparation and Purification of Materials.-Diphenylethylene was prepared by the usual method," b.p. 94-95' ( 3 mm.). T h e olefin was subsequently further purified b y either of two procedures. T h e first procedure consisted o f careful fractional distillation a t reduced pressure under a nitrogen atmosphere. The center cut, b.p. 128" (8 mni.), was retained for use. The second procedure involved frcwtional low temperature crystallization. The original di-lene was fractionally crystallized six tiincs, of the material being discarded each time. L l Tleiie purified by either method gal-e identical results in the reactivity and copolytnerization studies. The purified diphenylethylene was stored a t -2O0 under nitrogen until used. I-Methylene-2,3,6,7-dibenz-Z ,6-cycloheptadien prepared from benzal phthalide'8 by the fiiur-step of Cope and Fenton.'g The sublimed olefin was lized from methanol, m . p . 66-6;". I t was stored under nitrogen a t -20' until used.
1-Methylene-2,3,6,7-dibenzcycloheptatriene(I1 1
0 XI1
In comparing the results in Table I for I and diphenylethylene with those for dibenzoful\-ene the most striking feature is the fact t h a t although dibenzofulvene is about 300-600 times more reactive toward methacrylate radicals than diphenq-lethylene or I the RZ.radicals from the three olefins do not differ greatly as regards their relative propensities to add to monomer or cross-terminate (k2 ' k 3 ) .
In the past k 2 'k3 has often seemed an excellent measure of the stability of the RZ.radical.' being larger the less stable the radical. *kt first glance, therefore, the present results seem to rule out Szwarc's hypothesis'? attributing the high reactivity of the dibenzofulvenes to the stability of the fluorenyl radical. However, as there would seem sound theoretical basis for expecting the fluorenyl radical to be a t least somewhat more stable than the diphenylmethyl radical, we feel it is rather more likely t h a t other factors are important in the present instance, factors which cause both k l and k? ,'ka for diphenylethylene and I to be considerably smaller than would otherwise be the case. These factors, which are envisaged to be primarily steric in origin, are outlined in footnote 1.5. Acknowledgments.-The authors gratefully acknowledge the financial support of National Science Foundation Grant #NSF-G4203. (13) (a) S . C. Deno, J. J . Jaruselski and A. Schriesheim, THIS J O U R K A I . , 77, 3047 (1955); (b) W. von E. Doering and L. H. Knox, ihid.,76, 3203 (1951); (c) H. H. Rehnharrl, E. Heilbronner and E. Eschenmoser, Chemislvy 3 I u d u s l u y , 415 ( 1 9 5 3 ) . (14) M.Szwarc a n d F. L e a v i t t , ibiii.. 78, 33CJO (195G). (15) A previous estimate' f r o m Szwarc's "methyl affinities"'6 predicted diphenylethylene should be about four times more reactive than styrene toward methacrylate radicals. Since it is now f o u n d t o be less reactive this seems clear indication t h a t t h e addition of a methacr! late radical t o diphenylethylene or I involves significant steric hindrance. Examination of molecular models suggests t h e steric strain in reaction 2 is even greater. I t can also reasonably be argued t h a t t h e rate constant of t h e anomalous termination reaction 5 will not be too much smaller t h a n t h a t expected for K n in t h e absence of steric hindrance. T h u s i t is possible f o r botli 121 and h / k a for I or diphenylethylene t o he smaller t h a n would be t h e case in t h e absence of these steric considerations. Provided t h a t such steric effects are of much less importance for diIienzofulvene t h a n for I t h e results in Table I may be explained without alianiioning Szwarc's'l hypothebir. Alodels of t h e intermediates inv,ilved do indeed suggest there is considerably less hindrance in t h e dibenzofulvene reactions. (It:) 1:. I . e a \ i t t , 51. I x v y , A T . Sizw:irc a n d V S t a n n e t t , i h u l . , 77, ,ji!I:i (193.5).
,:j, -
ii,~-DiSenz-'7,9-c~-cloheI,tndien-i-I-o1ie K:LS preparid frotit benzal phthalide'8 by the rnethod of Cope a u d I'eirti~n . I u T h e ket(:ne was then coiiverted to dibenzcyclolicpt:ttrieiii)tie (XI1 j by Triebs and Kliiiklianimer's proceclure.2g This ketone was treated with methyl magnesium iodide atid the resulting tertiary carbinol tlehj-drated to tlie desired olefin by Triebs arid Klinkhamnier's inetlii~d.zgYpoii rccrystullization from methanol I1 riidted a t 110-120°. I t w;is stored in the same fashion as I . Methyl methacrylate wis purified as prcvic~usly tiescribed. T h e purification of azobisisobutyronitrile has also been pre\-iousll- meiitioiied. Procedure for Kinetic Runs.-For diphcnj-letli~.lenctllc procedure was t h a t previously described for dibenzofulvene.' For the other olefins the procedure folloired t h a t used for the substituted tlibenzofulvenes.' Xll of the kiiietic runs were carried out at 50'. The results of the individual r u x wit11 tlie three compounds are gi\-en in Table 11.
I
