FOURREACTIONS BETWEEN IODINEAND STYRENE
KOV. 5, 1959
5573
ORGANIC AND BIOLOGICAL CHEMISTRY [CONTRIRUT ION FROM 1HE CONVERSE
MEMORIAL LABORATORY O F HARVARD UNIVERSITY AND THE
PLASTICS
LABORATORY OF
PRINCETON UNIVERSITY]
Iodine and Styrene. I. Four Reactions between Iodine and Styrene BY DANIELS. TRIFAN' A N D PAUL D. BARTLETT RECEIVED APRIL14, 1959 Iodine strongly inhibits the polymerization of styrene by the free radical mechanism. At t h e same time equilibrium is established rapidly in the dark, at and below room temperature, with styrene diiodide, the equilibrium lying on the side of styrene and iodine. Other products of the combination of styrene and iodine are formed slowly in the dark and much more has been isolated in 6.3% yield. The disrapidly in the light. One of these, identified as 1,4-diiodo-2,3-diphenylbutane, appearance of iodine from its solutions in styrene is greatly accelerated by the presence of oxygen, the product being bis-8iodo-a-phenylethyl peroxide. Solutions of iodine in styrene more concentrated than 20y0bring about, at room temperature, an exothermic polymerization of styrene which is shown to proceed by a cationic mechanism. From a mixture of styrene and methyl methacrylate only polystyrene is produced.
Introduction The four reactions to be discussed are the rapid, reversible formation of styrene diiodide, the slow formation of higher iodine-styrene products, the intervention of oxygen to yield a bis-phenyliodoethyl peroxide, and a cationic polymerization of styrene. These will be discussed in that order. 1. The Reversible Formation of Styrene Diiodide.-Styrene diiodide was first described by Berthelota2 It can be isolated as colorless needles by chilling a lOyc solution of iodine in styrene to 0' then making a rapid filtration. It is, however, very unstable, and any attempt to handle it a t room temperature results in its decomposition after a few seconds to a dark, sticky mass containing iodine and polystyrene of low molecular weight (see Section 4). The iodine content of styrene diiodide was determined by washing rapidly with ether and adding portions of the freshly washed crystals to tared flasks containing reagent benzene. These samples were titrated with 0.1 N sodium thiosulfate, showing an iodine content of 68.2 and G8,470 (calculated for dry styrene diiodide 7O.9yc).. In a sample which was allowed to decompose in a tared flask without solvent, 12.6570 of the iodine became bound in a non-titratable form in the decomposition product. The iodine color appears instantly on solution of styrene diiodide in styrene or benzene a t room temperature; the establishment of equilibrium in the iodine addition-elimination reaction occurs too fast for rate measurements using ordinary technique~.~ In addition to the colorless styrene diiodide, there is a complex between styrene and iodine whose presence is evident from the color of the solution. Solutions of iodine in styrene appear to the eye a winered color, intermediate between the violet of solutions in benzene and the more nearly brown color of solutions in alcohol. Indeed, the spectrum of a solution of iodine in styrene a t 2 5 O can be closely imitated over the range from 440 to 700 mp by the use of a mixed solvent containing 94.8% benzene and 5.2% ethanol by volume (see Figs. 1 and 2). (1) From P a r t I of t h e P h . D . thesis of Daniel S. Trifan. H a r v a r d University, August, 19-18. Present address' Plastics Laboratory, Princeton University, Princeton, N. J.. where a further part of this work was carried out. (2) M. Berthelot, Bull. POL. chim. France. 121 6, 294 ( 1 8 G G ) . (3) See part 11,THISJOURNAL, 81, 5582 (1959).
At 100' the color is more violet, corresponding to the charge-transfer complex being formed with some evolution of heat. The optical densities a t 25 O of solutions of equal concentrations of iodine in benzene-alcohol and in styrene of similar spectrum are in the ratio of 10 to 9, showing that the equilibrium is quite unfavorable to styrene diiodide. A small but definitely reproducible shift in the position of equilibrium occurs on illumination of the solution. Large effects of this sort have been observed by Forbes and N e l ~ o nwho , ~ were able to shift the equilibrium in alkylene diiodide formation largely in the one direction or the other by suitable choice of wave length of irradiating light. I n general such effects arise from the microscopic irreversibility of the light absorption process, and may be observed whenever a rate constant which is specifically affected by light (in this case the rate constant for the dissociation Iz + 21.) enters into the expression for the position of the over-all dynamic equilibrium. Three sets of simple experiments on this effect were carried out with a Coleman junior spectrophotometer. Solutions of iodine in styrene, equilibrated in the dark, were suddenly illuminated and the small drop in absorbancy observed. Interruption of the light restored the initial iodine concentration and the cycle could be repeated a t intervals of about 20 seconds. The experiments were carried out in a dark room, using iodine concentrations less than 0.000735 M in pure styrene, to which hydroquinone had been added to retard the otherwise rapid reaction of iodine, styrene and oxygen (see Section 3). Table I summarizes the results of a series of such experiments. In each series the successive entries in the table represent repetitions of the cycle with the same sample. Controls of two types are available in testing for the reality of these small shifts in optical density. Table I1 shows the degree of constancy of the absorbancy observed in an identical set of experiments in which the iodine was dissolved in benzene or in ethanol instead of in styrene. Increases in the transmission of the order of 1% were consistently observed in the styrene experiments and constancy within =kO.l% was general in the experiments in (4) G. S. Forbes a n d A. F. Nelson, ibid.. 58, 182 (1936); 59, 693 (1937).
DANIELS.TRIFAN A N D PAULD. BARTLETT
5374
VOl. 81
illuminated state. These experiments, carried out a t 5-50mp, are summarized in Table 111. TABLE I1 COSSTAXCY OF TRANSMISSION OF 0.000T35 Ji SOLUTION OF IODINE IN BENZENE A N D IN ALCOHOL Solvent
Benzene
Wave Initial % length, m r transmission
500
I
400
450
500
550
600
650
A, m p . F i g 1.-Spectrum of iodine ( 7 . 3 5 X 10- 4 M) in styrene (&I), benzene ( B ) and ethanol (C).
1
550
700 Ethanol
500
8.8 8.8 8.8 25.5 25.3 29.4 29.4 29.3
Observation period, Shift in % sec. transmission
60 120 I80 60 60 60 60 60
0 .0 .0 f .1 .0 .0 f .1 i .1 + .2
The significance of this small but definite shift in composition of the reaction mixture with illumination is that the step or steps accelerated by light must enter into the expression for the over-all steady-state concentrations of reactants and product. This fact is incompatible with certain otherwise plausible mechanisms, as will be shown in Part 11. 2. The Slow, Irreversible Disappearance of Iodine. The Photochemical Reaction.-It was noticed during the experiments of Tables I and I11 that over a period long compared to the duration 400 450 500 550 600 650 700 of the experiments there was a slow and steady h , nip. fading of the iodine color on prolonged exposure to Fig. 2.-Spectrurn of iodine ( 7 . 3 5 X 31) in styrene light. I n bright sunlight an approximately tenth ( A ) , and in a mixture ( B ) consisting of 94.P:"c benzene and molar solution of iodine in styrene was bleached .i.2Cc ethanol by volume. in 18 hours of exposure. Removal of the styrene by distillation in an all-glass evacuated apparatus benzene or alcohol solvent. Separate experiments into a Dry Ice-cooled receiver left a residue of showed that the hydroquinone and iodine did not 5.52 g. of mixed colorless crystals and light brown react under the conditions of the experiments. viscous liquid from an original 2.76 g. of iodine. The calculated yield of diiodo-dimer is 5.04 g., of TABLE I diiodo-trimer, 6.17 g. These products could IS LIGHTTRANSMISSION o s SUDDEN ILLUMINAINCREASES TION OF SOLUTIONS OF IODINE IS STYRENE AT ROOM TEBI- be separated only with considerable losses; i t was possible to isolate 0.320 g. of 1,4-diiodo-2,3-diPERATURE phenylbutane (I), m.p. 199.0-199.5" (cor.) This Avvrox. time td ;each new compound was dehydrohalogenated readily to 2,3XTave Initial Sa of constant % S h i f t in diphenyl-1,3-butadiene (IT), crude m.p. 37-43", length, mu transmission transmission, sec. transmission which was brominated in h k h vield to the known 5on 15.5 30 +0 8 1,4-dibromo-2,3-diphenylbut&e~2(111),m.p. 14720 15 0 $1 1 148"(cor.), l i t 5 145-147'. 15 3 20 $1 2 -4ttempts to characterize the diiodide by the 20 18 0 $0 7 formation of a bis-trimethylammonium or dimethyl17 0 30 +12 sulfonium salt were unsuccessful, leading to the 15 43 0 +15 isolation of trimethylammonium iodide and tri42 9 20 +2 0 methylsulfonium iodide, respectively. The iso15 42 0 +2 8 lation of the tertiary sulfonium salt through easy 20 48 0 +I 0 methylation of dimethyl sulfide by the sought-for 30 48 0 4-1 6 sulfonium ion recalls the similar experience of 25 48 4 $1 2 Ray and Levine6 with dimethyl sulfide and phenyl20 48 4 +o 9 2-fluorylbromomethane. Finally, a type of experiment was carried out in The Thermal Reaction.-Even in the dark a t which the direction of the narrow cylindical light room temperature, the decline in color of degassed beam through the Coleman cell was changed solutions of iodine in styrene becomes appreciable periodically by rotating the cell 90' in its seat, after about 24 hours. At 100' complete reaction thereby sending the beam through parts of the of the iodine is evident after a few hours. At both solution which were formerly dark. On each such temperatures the reaction exhibits a reinarkable rotation the first reading corresponded to a lower C. I'. €1. Allen, C (',, 1,;lit)t : ~ n d.\ H v l l , ('(III. .I K t s c ~ , i v < l i17B, , transmission which quickly adjusted itself to the --, . I f.5)(l!J:W), higher transmission characteristic of the steady (1;) 17. l i 1 i ; i y rind I . 1,evine. .I, O T Z .('hi,iiz., 2 , 207 (l!J:37) 100
5575
FOUR REACTIONS BETWEEN IODINE AND STYRENE
Nov. 5. 1959
TABLE I11 TRANSMISSION BY SOLUTION OF TURE, X = 550 mp Rotation no. 1 2 3 4 5 % trans., steady 44.8 44.8 45.3 45.3 45.4 yotrans. after 90' rotation of tube 4 3 . 3 43.7 4 4 . 0 4 4 . 0 4 4 . 0 -1.5 -1.1 - 1 . 3 -1.3 -1.4 Shift, % trans. ' trans. 5 5 5 5 5 Time (sec.) for drop in % Time (sec.) for return to max. 20 ,.. 20 20 20 New const. max. % transmission 44.8 ., . 45.2 45.2 15.3 5 Between rotation 6 and 7 the tube was removed from the instrument.
EFFECT O F ROTATION O F COLEMAN
CELT, ON LIGHT
IODINE I N STYR.ENE AT
6 45.3 44.4 -0.9 5 25 45.9
7a 47.2 45.5 -1.7 5 20 47.2
ROOMTEMPERA-
8 9 47.0 46.7 45.5 45.3 -1.5 -1.4 c 5 25 25 46,7 46.3
10 46.7 45.8 -0.9 5
... ...
autocatalytic character such that the more con- served along the length of the tube, from the usual centrated the initial solution of iodine, the shorter dark wine-red a t the bottom to a colorless zone a t the time required for complete disappearance of the top. Moureu and Dufraisse' reported that the iodine color. This is a characteristic of iodine and many iodine compounds are catalysts in reactions whose product is a catalyst for the re- the autoxidation of styrene. Table V shows the action, and it was found that a completely reacted molar ratios of iodine, styrene and oxygen consolution of 0.217 -If iodine in styrene served to sumed in a series of experiments in which oxygen catalyze the reaction of another such solution, a t one atmosphere initial pressure was absorbed shortening the time to complete disappearance in styrene-iodine solutions until the iodine color of the iodine color a t 100" from 1.3 hours to half was replaced by a very pale yellow amber. In this an hour. Nevertheless, it did not appear to be the series of experiments the styrene concentration was identified product, 1,4-diiodo-2,3-diphenylbutane,varied over a sixfold range in dry benzene as solwhich was responsible for this autocatalysis. vent, and the iodine was varied over a sevenfold Two solutions of 0.109 M iodine in styrene, one of range in styrene as solvent. The oxygen absorbed which was made 0.0073 M in the diiodo compound, was determined both manometrically and gravimetrically and the styrene by titration with broreacted a t identical rates. Table I V illustrates the results of these experi- mate-bromide solutions, a process not interfered ments. with by the reaction product. ICHlCHCsHj
CH2=CCsH6
+ CH2=CCsHE I ICHJCHCeHE I
---+
I1
I
BrCH2CC6H5
II
CsHaCCH2Br I11
TABLE IT THERMAL DISAPPEARANCE OF IODIXE AT 100" FROM DEGASSED SOLUTIOXS I N STYRENE Sample
Concn. of iodine, M
Approximate total reaction time, hr.
Special conditions
1 2
0.434 0.5-0.7 ,434 0.25-0.33 HIa 3 .217 1.1-1.4 4 ,109 2.6-2.7 5 ,109 1.4-1.6 HIa 0.213 g. of concentrated aqueous hydriodic acid (50%) was degassed with 10 ml. of styrene, sealed off, and the tubes shaken vigorously. The concentration of H I in the styrene phase was unknown.
For reasons mentioned below i t was suspected that hydrogen iodide might be the autocatalyst in these experiments. The comparison of samples 1 and 2, and of samples 4 and 5, shows that even in the presence of water, which must keep most of the hydrogen iodide out of the styrene phase, there is a substantial catalysis. I n samples 2 and 5 final titration showed that 18.1 and 12.5%, respectively, of the acid introduced remained a t the end of the reactions. I n some control runs with hydriodic acid but without iodine, 29.2 and 27.9y0 of the acid remained after 1- and 2-hour heating periods a t looo, respectively. It is very unlikely that the degassing procedure removed so much of the acid. 3. The Reaction of Iodine and Oxygen with Styrene. Structure of the Product.-If a solution of iodine in styrene is allowed to stand in an open test-tube for a day, a color gradation can be ob-
TABLE 2: PROPORTIONS O F STYRENE, IODINE AND OXYGEX CONSUMED IN
Initial iodine concn., molal
DIFFUSELIGHT Molar reaction ratio for 100% reaction -0xygenGraviManoStyrene Iodine metric metric
Initial styrene concn., molal
0.2273 9.0484 2.193 1.000 1 , 0 5 3 ,2311 4.5967" 2.419 1.000 0.999 ,2320 2.4571" 2.336 1,000 0.832 ,2330 1.4843" 2.566 1.000 1.218 .4301 8.5539 2.474 1.000 0.717 ,2274 9.0489 2.150 1.000 ,823 ,1179 9.3150 2.157 1.000 ,959 1.000 ,792 ,0598 9.4567 1.866 Dry reagent benzene used as solvent.
1,049
... 0.804 .,.
... , ,
.
1.011 0.814
It is seen that the product contains styrene, iodine and oxygen in the molar proportions of 2 : l : l with deviations up to 0.566 mole (excess) in styrene and up to 0.283 mole (deficiency) in oxygen. This is consistent with the formation of a peroxidic product, bis-P-iodo-a-phenylethyl peroxide (IV), by intervention of oxygen which attacks the P-iodo-a-phenylethyl radical (SI.) in competition with iodine. ICH2-CHCeH5 f O2 -+ ICH2CHO0. I
ICH~CHOOCHCH~I IV
I
CsH5
\
CsHs
The product was characterized by conversion into a bis-trimethylammonium iodide (V) , m.p. (7) C. Moureu and C. Dufraisse, Chemistry & I n d u s l r y , 47, 819 (1928).
5576
DANIELS. TRIF.\NAND PAULD. RARTLETT
108-100", in Olyo yield, and into the corresponding bis-triethyla~n~rio~~iu~ri salt, 1 1 1 . p . 134.0-156.5 '. The bis-triiiietliyl3mnloriiuni s:d t was reduced with sodium borohydride to 2-hydroxy-2-phenethyltrimethylammonium iodide (VI), m.p. 226.4227", identical by infrared spectrum (Nujol mull) and mixed melting point with the synthetic salt from the iodohydrin VI1 made by the action of iodine and yellow mercuric oxide on styrene,8 and different from the salt IX, m.p. 95.5-100°, from the iodohydrin VI11 made from H I and styrene oxide.
+
(CH~)~L\CH~CMC~H~ I
I\'
accelerated termination shortens the kinetic chains and therefore lowers the over-a11 rate of formation of styrelic diiodide under non-equilibriuin conditions. The degassed solution required 18 hours for complete reaction, compared to 0.17 hour for thc oxygen-saturated solution. On sudden exposure of a flask containing the vapor of styrene, iodine and oxygen to bright sunlight, a white mist was formed immediately and became quite dense within five seconds, corresponding to the occurrence of the reaction in the vapor phase. Retardation by Antioxidants.-The addition of hydroquinone (excess, in suspension in the solutio.1) caused a period of retardation during which shiny black crystals of quinhydrone could be seen to grow on the hydroquinone crystals. On consumption of the hydroquinone the absorption of oxygen and disappearance of iodine color proceeded a t the normal rate. The antioxidant 2,B-di-tbutyl-p-cresol (Ionol DBPC), being soluble in the reaction mixture, was a still more effective retarder. Table VI shows the results of a series of experiments in each of which 10 ml. of an iodine solution in styrene was shaken with air in a closed 250-ml. erlenmeyer flask which contained 4 moles of oxygen per mole of iodine present. TABLE VI
(CH3)3N HOCH?CI-IC6I-T,
I
RETARDA rIos
I'fciifler
Anii
EBPC)
STYRENE AT
AND I ~ T ~,~-DI-~-BuTYL-~~-
O F THE
IO1)OIJEROXII)AI'IOS 01'
ROOMTEMPERAPURE
1-
The course of this reaction is similar to that observed by Bockemuller and Pfeuffer in the reaction of a number of olefins with bromine and ~ x y g e n but , ~ the 2 : l : l ratios maintained in the iodine system over a range of reactant concentrations were not observed in the bromine system, where chain propagation with bromine is far less reversible (see Discussion below). Effect of Light, Oxygen and Temperature on Bleaching Rate.-Like the reaction in the absence of oxygen, the styrene-iodine-oxygen reaction is strongly promoted by light. Both with and without strong illumination the decolorization is more rapid in the presence of oxygen than without it. A 0.109 M solution of iodine in styrene, mounted in a shaker in diffuse laboratory light a t room temperature in a slow stream of oxygen, required 20.5 hours for decolorization; in bright sunlight the time wqs reduced to 0.17 hour. At 100' in dim light, the corresponding times were 2.5-3 and 0.23 hours, respectively. In bright sunlight two 0.109 -11solutions of iodine in styrene were compared a t O D , one with and one without saturation with oxygen a t one atmosphere. This hundredfold acceleration of a chain-terminating process by oxygen, under conditions where the iodination of styrene is already a t equilibrium, stands in apparent contrast to the equally great retardation by oxygen of the styrene-iodine addition reaction itself (see Part 11). The cause of each is the same; ( 8 ) c Gc,lumbic and D 1. Cottle T H I S T O 1 7 R N 4 1 61 rlofi (1q7')) ( 0 ) JT nockemiiller ~ n ( 1l,
D Y HYDROQUINONE
CKBSOI, ( I O X O L ,
+S(CH3)3 IX
Vol. SI
637, 178 (111
