Unsaturation in Isoprene-
Isobutylene Copolymers S . G. GALLO, H. K. WIESE, AND J. F. NELSON Standard Oil Development Company, Elisabeth, N . J.
‘A rapid, reproducible
method for determining relative unsaturation in raw isoprene-isobutylene copolymers has been developed, based on the reaction with iodine-in the presence of mercuric acetate and trichloroacetic acid. The total time per determination is approximately 1.5 hours compared to 2.5 to 100 hours or more for earlier methods. The new method is less sensitive to reaction conditions than other halogenation methods and employs a relatively stable reagent. In a cooperative test program involving four laboratories, the new reagent gave results which were twice as reproducible as those obtained with the Wijs iodine chloride reagent.
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N THE production of isoprene-isobutylene copolymers, a
rapid reproducible means of measuring the unsaturation in the raw product, a t least on a relative basis, is highly desirable, inasmuch as the curing rate of the polymer and to a large extent the properties of the finished product are a function of the degree of unsaturation (16). Examination of the existing methods showed that at best the available procedures are time-consuming; they require total time ranging from 2.5 hours to more than 100 hours, and for the most part are highly sensitive to reaction conditions. Rehner (14) has shown that nitrosobenzene, which was found to react quantitatively with natural rubber by Bruni and Geiger (3)requires more than 100 hours for complete reaction. The thiocyanogen procedure described by Kaufmann and Gaertner (7) requires in the neighborhood of 72 hours. Rehner ( I d ) found also that Marshall reagent (iodine monochloride in carbon tetrachloride) used by Boeseken and Gelber ( 2 ) requires a reaction time of a p proximately 24 hours to approach completion. The reaction with iodine monochloride in acetic acid (Wijs reagent), which has been most widely used in recent years (4, 8-11, 14) for the determination of unsaturation, has been found to be sensitive to reaction conditions, such as temperature, reaction time, light, and excess reagent. The iodine chloride procedure used in these studies was essentially the same as described by Kemp and Mueller (9) employing a 1-hour reaction time (in the dark) at room temperature. The sample size was 0.2 to 0.5 gram depending on the range of unsaturation; the volume of reagent, 5.0 ml. of 0.2 N Wijs solution. Titrations were made with 0.1 N standard thiosulfate to the customary starch end point. Although the Wijs iodine chloride method has been used with adequate precision by Kemp et al. (%IO), it was found to be somewhat lacking in reproducibility as applied by plant analysts on a routine basis. The ozone degradation method described by Rehner (14) is not easily applied in routine analysis, although it may offer the closest approach to the true unsaturation of isoprene-isobutylene copolymers. In the course of a study aimed a t improving the iodine chloride method, a new procedure was developed involving a reaction between the polymer and iodine in the presence of mercuric acetate and trichloroacetic acid. Depending on the range of unsaturation, the values obtained by this method are 10 to 35% higher than those obtained by ozone degradation. During the develop-
ment of the method and in the subsequent cooperative test program involving four laboratories, two additional methods (to be described later) were devised: one, a moderate version of the iodine-mercuric acetate procedure; the other, a bromination procedure. The former was found to be slightly inferior t o the more drastic procedure in reproducibility; the latter was found to be of about equal reproducibility. Like all the previous methods, however, the bromine procedure was found to be sensitive to reaction conditions. Moreover, the reagent (bromine in benzene) was found to produce appreciable substitution (Table 111) and its normality was found to change considerably on standing (18). For these reasons no exhaustive study of the method was made. The magnitude of the unsaturation values obtained by the moderate iodine-mercuric acetate method and the bromine method differed markedly, being about 25% lower and 25% higher, respectively, than those obtained by the method proposed. The dissolution of sample has been speeded up by substituting a brief refluxing operation for the overnight tumbling technique formerly employed. Refluxing with p-dirhlorobenzene has been previously proposed by Kemp and Peters (10)to effect more rapid solution of sample. Figure 1 shows the rate of solution of isoprene-isobutylene copolymers in boiling carbon tetrachloride as a function of viscosity average molecular weight (6). The Staudinger values corresponding to those plotted in Figure 2 are 27,000, 47,000, and 64,000. That no significant difference in unsaturation values results with the rapid method of dissolving the sample is illustrated in Table I. This is in agreement with observations made by Kemp and Peters (IO). PROCEDURE
A 0.5-gram sample of polymer (weighed to about 0.002 gram) is cut into small pieces and refluxed in 100 ml. of C.P. carbon tetrachloride for 30 to 45 minutes or until no discrete particles of rubber are visible on close examination in a good light. (For polymers possessing unsaturations greater than 1.25 mole %, a 0.25gram sample is used in order to maintain at least a tenfold excess of reagent.) A 500-ml. glass-stoppered Erlenmeyer flask (or iodine flask) is most convenient for refluxing. A reflux condenser with a ground-glass joint of corresponding size facilitates the operation. On cooling to room temperature, 5 ml. of 20% trichloroacetic acid in carbon tetrachloride are added. An equal amount of trichloroacetic acid solution is added to 100 ml. of C.P. carbon tetrachloride to serve as a blank. Twenty-five milliliters of 0.1 N iodine solution (12.5 grams of resublimed iodine dissolved in 1 liter of dry C.P. carbon tetrachloride) are then added, with gentle swirling, to both the sample and blank and followed by 25 ml. of mercuric acetate solution (30 grams of C.P. mercuric acetate per liter of glacial acetic acid). After allowing to stand for at least 30 minutes in a stoppered flask (longer standing will do no harm) 75 ml. of 7.5% potassium iodide solution are poured into each flask and the mixture is vigorously agitated. The excess iodine is then titrated with standard 0.1 N thiosulfate employing the customary starch end point (14)which is observed in the aqueous layer. 1277
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 40, No. 7
Table I1 shows that in the case of iodine-mercuric acetate reagent any desired excess above tenfold (up to 33-fold) yields essentially the same results. Excess below tenfold tends to give low results. m50w
Li
EFFECT OF REACTIOS TIME
z t
Reaction time is another important variable that has been given a great deal of attention. Rehner (14) showed that in a sample with a 2007, excess, the reaction with Wijs reagent a t room temperature was still in progress at the end of 72 hours although a similar sample maintained at 0 C. had stopped reacting at the end of 24 hours. At the end of 72 hours, the former sample showed appreciably higher unsaturation as might be expected.. I n the case of a control method, reaction time is of prime consideration not only because it affectsreproducibility but also because it is desirable to minimize the lag betn-een polymer produced and polymer analyzed. Figure 2 shows a comparison of the effect of reaction time on the iodine chloride method and on the iodinemercuric acetate method It is noted that the reaction with iodine-mercuric acetate is complete in about one half hour; no further reaction is shown on standing for an additional 42 hours. The reaction with iodine chroride was still in progress at the end of 8 hours although a t a considerably diminished rate after the first hour.
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The mole per cent unsaturation (defined as mole per cent of monomer units containing doublc bonds) is calculated as follows: Mole
7,unsatn.
( R - S j ( N ) ( I M ) (100) (grams of sample) (3000)
=
where B = ml. of thiosulfate for the blank, S = ml. of thiosulfate for the sample, N = normality of thiosulfate, and M = molecular weight of the structural unit of the polymer. For this calculation, the average molecular m i g h t of the monomer units in the case of Butyl rubber samples is taken to be that of isobutylene (56.1) as it is by far the major const,ituent of the polymer. Phenyl-&naphthylamine, which is commonly used to stabilize these polymers, is known to consume small amounts of halogen in this and other halogen procedures. I n this program, however, the correction was found to approach the limit of reproducibility of the method where the phenyl-0-naphthylamine content was in the usual range of 0.1 to 0.2 weight 7 0 . Since the completion of this program, it has been found that the subtractive mole Yo unsaturation correction is approximately 0.58 X weight yo phenyl-@naphthylamine in the polymer; the latter was best determined by ultraviolet absorption ( 5 ) . Small amounts of zinc stearate which are often added in the preparation of these polymers may be centrifuged out of the carbon t,etrachloride solution prior to absorption measurements. Zinc stearate does not interfere in the analysis for unsaturation EFFECT OF EXCESS REAGENT
In nearly all halogen methods for determining unsat,uration, the amount of excess reagent has been a matter of concern. Pummerer and Stark (IS) using Marshall reagent, minimized substitution by maintaining the excess reagent below 20%. Kemp (8) showed that if the reaction with Wijs reagent is carried out at 0 C., substitution is minimized even with a large excess of reagent. O
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EFFECT O F TEMPERATURE
The third important variable which has required careful control in most of the halogen methods is that of reaction temperature. This is amply illustrated in many of the foregoing references. Most investigators have cndeavored to strike a balance between the reaction temperature and the amount of excess r e agent, adjusting them so that values close to theoretical are obtained with natural rubber or known olefinic compounds. However, the temperature variable is aln ays ready to exert its influence whenever control lapses. Figures 3 and 4 show the effect of temperature on the unsaturation values of B-2 and B-8 polymers (higher B numbers denote higher unsaturations) as determined by the iodine chloride and iodine-mercuric ac-tate methods. The effect of temperature on both methods is greater in the case of the more unsaturated polymer. IT'ith iodirie chloride this effect is so great that the method is practirally useless at high unsaturations without careful temperature control. A log, difference could readily result from normal variations of laboratory temperatures. In the case of the iodine-mercuric acetate method the temperature effect is found to be negligible.
TABLEI. EFFECTO F REFLUXTEIIPERBTURES ON M O L E 70 UNSATURATION OF BUTYL RUBBER SAMPLES
1
Sample 1Q 2
3 4 5
0 IODINE CHLORIDE 8 IODINE MERCURIC ACETATE
Mole % Cnsaturation by IC1 Sample Sample dissolved dissolved by by refluxing tumbling 16 hr. 0.06,0.09 0.09,0.06 0.79.0.79 0 . 7 8 , O .79 1.08,l.OO 1.08.0.99 1 9 0 , l 94
1.90,1.90
3.76,3.50
4.10,3.77
31ole 3 ' % Unsaturation by 1%-Hg(0Ac)z Sample Sample dissolved dissolyed by by refluxing tumbling 16 hr. 0.09,0.06 0.04,0.04 0 . 7 0 , O .68 0.68, 0 . 6 7 0 . 6 8 , O .70 0 . 6 4 , O . 08 1.44 3.10,3:00
1.44,1.45
3.05,..
a The unsaturation obtained with this sample approaches the reproduoi bility of the method.
z,
EFFECTOF SAMPLESIZEA X D EXCESS REAGESTON U N S A T U R A T I O X D E T E R M I N E D B Y I O D I S E 1\IERCURIC ACETATE
TABLEI1
Weight of Sample I
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Figure 2. Effect of Reaction Time on Measured Unsaturation of a B-2and a B-8 Copolymer
1
?dl. of 0.1 3' Iodine
Nolar Ratio In/Douhle Bond
Bleasded Cnsaturation, Mole %
July 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1279
EFFECT OF LIGHT
I n many of the earlier halogen procedures where extended reaction periods were required to approximate complete reaction, light was an important variable to be controlled. It has lone been known that light catalyzes a variety of side reactions which tend to give high results in the determination of unsaturatibn. For this reason most workers have carried out the reactions in the dark and, wherever possible, minimized exposure even to ordinary light. Because the iodine-mercuric acetate reaction requires less than 1 hour, the light variable could not be expected to assume major importance. Moreover, any small differences encountered would likely fall within the limit of precision of the method. However, in order to obtain a comparison with the iodine chloride reagent, a n accelerated test was devised in which samples in admixture with the respective reagents were exposed to ultraviolet light for varying lengths of time prior to analysis. The results obtained are plotted in Figures 5 and 6. There is no noticeable increase in values obtained with iodine-mercuric acetate reagent whereas, on the other hand, values obtained with iodine chloride show marked increases with increasing exposure. Also, the effect of light is greater in the case of the more unsaturated polymer. DISCUSSION
TABLE111. UNSATURATION OF OLEFINSAS DETERMINED BY IODINE CHLORIDE AND IODINE-MERCURIC ACETATE Unsaturation, %
Olefin 2,3-Dimethylbutene-2 Isobutylene dimera Triisobutylene Tetraisobutyleneb Tetraisobutylene Tetraisobutylened Methallyl chloride a-and 8-Chloropropene *
Iodine chloride 95.0 102.0 72.5 76.0 92.0 115.0 116.0 0.0
Iodine: mercuric acetate CCIGOOH 100.0 100.0 90.0 76.0 86.0 90.0 95.0 25.0
Iodine; mercuric acetate 100.0 100.0
Bromine 108.0 105.5 107.5 110.0
+
45.5 20.0 25.0 37.0 95.0 0.0
145:o 160.0
0.0
Mixture of 2,4,4-trimethylpentene-land 2,4,4-trimethylpentene-2. b Prepared from dimer and HzSO4. C Prepared from dimer and activated clay. d Prepared from dimer and AlCla. e 50-50 mixture.
a
bon atom) and subsequently add two more atoms of iodine. This might account for the total of three atoms of iodine which are found to be consumed for each tertiary double bond. A second possibility would be simple dehydrohalogenation of a transitory diiodide and subsequent addition of a mole of iodine. This latter possibility could also account for the consumption of three atoms of iodine per double bond if i t is assumed that an approximately 50-50 split between allylic and vinyl type double bond results:
In the development of the iodine-mercuric acetate method, tests were made on a series of known olefins to investigate the quantitative aspects of the various methods under consideration. The results are given in Table 111. Although the extrapolation from simple compounds to a high molecular weight polymer is a large one, it is unlikely that a method would prove adequate for a H CH8H H polymer if it failed completely with simpler representative mole-c-c-c-ccules. I l l / Table I11 shows that the iodine chloride method gave low valH X X H ue$ for the first two tetramers and a high value for the third tetramer. Iodine-mercuric acetate reagent in the absence of trichloroacetic acid yielded results which were far from the theoretical, although no values in excess of theoretical were obtained. It is apparent, therefore, that iodine-mercuric acetate reagent alone is not a sufficiently potent reagent to halogenate certain types of double bonds. Trichloroacetic acid appears to render these resistant olefins more susceptible to halogenation. (This acid was selectdd because it is soluble in the nonaqueous medium and is not itself susceptible to further halogenaI I tion.) Bromine appears to be too acIO 20 30 TEMPERATURE, 'C. tive a reagent and is responsible for Figure 3. Effect of Temperature on considerable amounts of substitution. Measured Unsaturation of a B-2 Very little is known with certainty reCopolymer garding the nature of the reaction involved in the present procedure. Birkenbach and Goubeau ( 1 ) postulated the formation of iodoalkyl nitrate and a small amount of dinitrate in the reaction of iodine with ethylene in the presence of mercuric nitrate. This produces, a t the same time, a stoichiometric quantity of mercuric iodide which can be isolated. If an analogous reaction takes place in the presence of mercuric acetate and trichloroacetic acid, the iodoalkyl aceI 1 4 8 12 tate (or trichloroacetate) might in part TIME I N MINUTES split off a molecule of acid (dependFigure 5. Effect of Ultraviolet Light ing on whether the iodine or acetate o n Measured Unsatur.ation of a B-2 radical is attached to the tertiary carCopolymer
H
H CHa I
t
-&-b=C-C-
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HCHsHH
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X H
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IO
20 TEMPERATURE,
30
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Figure 4. Effect of Temperature o n Measured Unsaturation of a B-8 Copolymer
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TIME IN MINUTES
Figure 6. Effect of Ultraviolet Light on Measured Unsaturation of a B-8 Copolymer
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INDUSTRIAL AND ENGINEERING CHEMISTRY
According to well established principles, the vinyl type double bonds would tend to resist further halogenation while the allyl type could add more halogen:
This would give a total of six atoms of iodine for two isoprene units or three atoms per double bond originally present. This does not, however, preclude the possibility of formation of organomercury compounds similar to those described by Whitmore (16). I n view of the uncertainty of the nature of the reaction, the present method is not recommended, without further verification, for the determination of unsaturation of polymers in which the unsaturation is derived from diolefins other than isoprene. I n order to establish the most reproducible of the several available methods, a coopcrative program involving four laboratories was organized and completed under sponsorship of a Rubber Reserve subcommittee. d series of representative samples was prepared, subdivided, and distributed to these laboratories for analysis. Four methods were tested in all, including a bromine procedure and a moderate version of the iodine-mercuric acetate procedure. The results obtained by the iodine chloride method and the proposed method are listed in Table IV. The bromine procedure was cariied out as follows: h I-gram sample of the polymer (cut into small pieces) was dissolved in 100 ml. of C.P. thiophene-free benzene either by refluxing or permitting i t to stand overnight. To the homogeneous polymer solution were added 25.0 ml. of bromine-benzene reagent (8 grams of bromine in 1 liter of benzene) and the mixture was allowed to stand for 30 minutes in the dark. At the end of that time, 20 ml. of 1274 potassium-iodide solution were added and the mixture was vigorously agitated. The liberated iodine was titrated n ith standard thiosulfate to the disappearance of the iodine color in the benzene layer. Care was taken to agitate thoroughly between increments as the end point was approached. A .blank was run at the same time, The calculation was made’as indicated for the iodine-mercuric acetate procedure except that the factor 2000 was employed in the denominator instead of 3000. The moderate iodine-mercuric acetate procedure was carried out in the same way as the more drastic procedure described above except that trichloroacetic acid was omitted.
The iodine-mercuric acetate procedure vas found to be superior to the other methods tested, in simplicity, stability of reagent, and relative insensitivity to temperature, light, reaction time, and large excesses of reagent. It mas found to be equal to the bromination procedure in reproducibility and definitely superior to the iodine chloride method. . Table IV shows that the iodine-niercuric acetate values indicate probable errors and average deviations about half as large as those for iodine chloride. The latter method was found to be rather errat,ic with extreme deviations as high as 0.8 mole % unsaturation. Although there is insufficient proof of the absolute accuracy of the foregoing method!, it is known that none of them has given values greater than are possible from the known composition of the reaction mixtures. Material balances have from time to time shown good agreement with one or another of the above methods, but the results are uncertain because losses and serious analytical difficulties were encountered. It will remain for a more fundamental study, employing possibly radioactive isoprene, to establish which of the many available methods gives values in closest agreement with the actual imprenc content of these polymers. Except for this general limit,atiou, the iodine-mercuric acetate procedure for the determination of relative unsaturation of raw isoprene--isobutylene copolymers offers definite advantages in minimizing the more important laboratory variables and improving the o v e r d l reproducibility. ACKNOWLEDGMERT
The authors are indebted to Humble Oil and Refining Company, Polymer Corporation, Limited, and Standard Oil Company of Xew Jersey, Louisiana Division, for their participation in the test program and to R . M. Thomas and C. E. Morrell for their contributions and suggestions. The aut,hors wish also to express their apprecia,t,ion for helpful criticism by John Rehner, Jr. LITERATURE CITED
(1) Birkenbach, L., and Goubeau, J., Ber., 67, 1420 (1934). (2) Boeseken, J., and Gelber, E. T., Rec. trav. chim., 46, 158 (1927); 48, 377 (1929). (3) Bruni, G., and Geiger, E., Atti accad. nar. Lincei, [6] 5, 82.7 (1927): Rubber Chem. Tech.. 1. 177 (1928). (4) Cheyney, L.’E., and Kelley, E. J . IND. ENG.CHEM.,34,1323 (1942) Rubber Chem. Tech., 16, 280 (1943) (6) Eby, L. T., and Banes, F. W., IND WITH IODINECHLORIDE AKD TABLE IV. RESULTSOBTAINEDBY FOURLABORATORIES ENG. CHEM., ANAL. ED., 18, 634 IODINE-MERCURIC ACETATEMETHODS (1946) lodine Chloiide Method (6) Flory, P. J., J.Am. Chem. SOC.,65,372 .*v: 5% (19431. Laborator) Laborator3 Laboratory Laborator5 Probable Deliation (7) Kaufmann, H. P., and Gaertner, P., Samplea 1 2 3 4 I\ Error from Mean Bey., 57, 928 (1924). 1 0.00 0.24i. 0.00,O.OO 0.39,0.37 (8) Kemp, A. R., IND. ENG.CHEX., 19, a 0.83 0.83,0.74 0.53,0.53 0:69 0:ii aQ 3 1:07 1.25 1.24,1.24 1.33,1.31 1.24 0.05 a 531 (1927). 1.94,1.94 1.95 2.56,2.64 2.13 0.27 15 4 1.74 (9) Kemp, A. R., and Mueller. G. S..IND. 3.00,3.02 2.95 3.14,3.14 3.06 0.06 2 5 2.63c E ~ G . C H E M . , A N A L . E D .(1934); ,~,~~ 4.63.4.46 3.80,3.80 4.25 0.25 7 A 4.37 4 45 Rubber Chem. Tech., 7, 576 (1934). (10) Kemp, A. R., and Peters, H., IXD. Iodine-Mercuric Acetate Method E N G . CHEM.: ANAL. ED., 15, 453 (1943) : Rubber Chem. Tech., 17, 61 I 0.34,O.lOh 0.11,0.04 0.13 2 0.54,0.59 0.51,0.51 0.53 0.6s;O:~s 0.58 0:05 11 (1944). O.R8,0.94 0.88 0.10 12 0.68,0.77 0.89 3 0.83,1.08 (11) Ogg, R. A., J . Am. Chem. SOC.,57. 1.39 1.32,1.32 1.32 0.04. 3 4 1.38,1.36 1.25,1.25 2727 (1935) 2.32 2.42,2.40 2.46 0.08 4 5 2.87,2.64” 2.27,2.27 3.73 3.87.3.87 3.81 0.07 2 6 3.73.3.65 3.94.3.85 (12) Polymer Corp., Ltd., Sarnia, Ontario, Average 0 . 0 7 6 unpublished data. (13) Pummerer, R., and Stark, H., Ber., 0.08 0.07,0,04 0.12 0.04,0.04 76 0.14,O.13 b 64,826 (1931). 8 0.55,0.58 0.51.0.46 0.54 0.55,0.55 0.53 0103 6 0.75 0.87,0.87 0.81 0.04 3 9 0.77,0.85 0.77,0.81 (14) Rehner, J., IND. ENG.CHEM..36, 118 1.28,1.19’ 1.31 1.34.1.28 1.31 0.05 4 10 1.40,1.40 (1944) : Rubber Chem. Tech., 17, 2.22 2.45,2.43 2.31 0.07 3 11 2.28,2.26 2.27,2.24 679 (1944). 3.39 3.43,3.67 3.58 0.08 3 12 3.70,3.65 3.61.3.63 (15) Whitmore, ‘8. C . , “Organic ComAverage 0 . 0 5 4 pounds of Mercury,” New k’ork. Samples 1 t o 6 contained 0.1 to 0.2% phenyl-P-naphthylaniine and 2% zinc stearate; curreotions Chemical Catalog Co., 1921. for phenyl-@-naphthylamine were found t o be small a n d werr not applied. (16) Zapp, R. L., Paper 16, Division oi b Polymer thrown o u t of solution during analysis. 0 Single values deviating from the mean by more t h a n four times the average deviation f r o m the Rubber Chemistry, 110th Meeting. mean were discarded. AM.CHEX.SOC..Chicago, 1946. d Samples 7 to 12 contained no phenyl-0-naphthylamine or zinc stearate. ~
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