Bug., 19-16
COPOLYMT3RIZATION OF L\T,LYL ACETATE WIT11
[CONTRIBUTION FROM
THE
hf A L E I C
i\NIIYDRTDE
1495
CONVERSE MEMORIAL LABORATORY OF HARVARD UNIVERSITY ]
The Polymerization of Allyl Compounds. 111. The Peroxide-induced Copolymerization of Allyl Acetate with Maleic Anhydride1 BY PATJI, D. BARTLETTZND K ~ x z r eN O Z A K I ~ Introduction.- 'Uie theory of copdymerizaLion has been thc subject of a number of recent paper~.~-lOThe authors of all of these have recognized four chain-propagating rate constants, governing the addition of monomer molecules -4 and B to growing chains ending in A and B units. I n terms of the free radical mechanism of chain growth, if RA and RB are free radicals ending in monomer units X and B, respectively
Wall8 has explored the theoretical possibilities attendant upon many kinds of variation in the relative values of the constants kAA, k A B , k B A , and k B B , and has provided graphical representations of a number of such cases, showing the dependence of the rate of polymerization and of polymer composition upon the composition of the mixture of monomers. Jencke13 made an experimental study of the products of polymerization of four monomer mixtures by fractionation and recognized that three of them represented true copolymers, while the fourth-the product of reaction of styrene and vinyl acetate-more closely resembled a mixture of separate polymers. Mayo and Lewis7 derived an elegant general method for the evaluation of the ratios k A A / k A B and k B B / k B A from quantitative determinations of the composition of the copolymers relative to that of the initial monomer mixtures, and found that for the copolymerization of styrene and methyl methacrylate a t 60' under a variety of conditions both these ratios were near 0.50. These authors opened the way by which such ratios can be evaluated for many copolymerizing monomer pairs. From the present literature, the actual existence of a number of the cases envisaged by the theory can only be surmised. This paper concerns the existence and characteristics of a rather extreme (1) P a r t s I a n d 1 1 : Barllett a n d Altschul, THISJOURNAL, 67, 812,
816 (1945). (2) Pittsburgh Plate Glass Fellow. (3) Jenckel, 2. ph,ysik. Chem., 190A, 24 (1941). (4) Wall, THISJ O U R N A L , 63, 1862 (1941). (5) Simha a n d Branson, J . Chem. P h y s . , 12, 253 (1944). (6) Alfrey a n d Goldfinger, ibid., 12, 205 (1944). (7) h l a y o a n d Lewis, T a r s JOURNAL, 66, 1594 (1944). (8) Wall, ibid., 66, 2050 (1944). (9) Stockrnager, J . Chem. Physics, 13, 199 (1945). (10) Norrish and Brookmnn. PI-OL.R o y . SOC.(London), A171,
117 (1939).
raw of cntry o f the two monomers into the polymer in a 1: 1 ratio, ie., KAB >> ~ A and A KBA >> k B B . This case acquires heightened interest in view of evidencei1 that such selectivity in many copolymerizations is absent or slight. Wagner-Jaui-eggl2 cliscovered that maleic anhydride has the property of forming copolymers with stilbene, benzalfluorene, and anisalfluorene, and remarked upon the tendency toward a 1: 1 ratio of the components in the copolymer. Observations have also been reported on the somewhat selective copolymerization of styrene and dimethyl maleate or dimethyl fumarate1' and there is increasing reference in the patent literature to copolymerizations involving maleic anhydride.13 The copolymerization of allyl acetate and maleic anhydride was found especially suitable for the present investigation for three reasons. First, a convenient method was devised for the quantitative determination of these two monomers present in a mixture; second, we had for comparison a background of information about the polymerization of allyl acetatel; and, third, the two monomers polymerize separately with low chain lengths of similar magnitudes which can be accurately determined for comparison. Experimental Method of Following Copolymerizations.-Allyl acetate reacts quantitatively with bromine from a standard bromate-bromide mixture.' Maleic anhydride and maleic acid are unaffected by this treatment, but maleic acid is titrated quantitatively in the presence of mercuric wlfate.I4 This was made the basis of a method by which either allyl acetate alone or the two monomers together could be titrated. The course of a copolymerization and the relative rates of consumption of the monomers could thus be followed without purification and analysis of the polymeric product. Table I summarizes the experiments carried out in developing the analytical method. Surprisingly, the mercuric sulfate, which promotes the bromination of maleic acid, inhibits that of allyl acetate (run 3 ) . Run 3 shows that mercuric sulfate does not adversely affect the results with allyl acetate if added after its hromination is complete. Run 12 illustrates the complete procedure for the mixed monomers. In the procedure adopted, the reaction titration vessel was first evacuated on the aspirator and an acetic anhydride solution of the mixture to be analyzed was added, followed by 5 cc of (iV ' sulfuric acid and 10 cc. of water. The mixture was heated to 50" for five to ten minutes, after which the broniide(11) Marvel and Schertz, THISJOURXAL, 66, 2054 (194R),and previous papers of t h e series. (12) Wagner-Jauregg, Be?., 63, 3213 (1930). (13) Arnold, Brubaker and Dorough, U. S. Patent 2,301.3.X (1942); E. I. d u P o n t , British P a t e n t 549,682 (1942); C e r h a r t U. S . Patent 2,297,331 (1943); Van Melsen, U. S. Patent 2,297,039 (1943); Swan, U. S. P a t e n t 2,299,189 (1943); Stoops a n d Denison, U. S. Patent 2,324,740 (1944); D'Alelio, U. S. Patent 2,340,110 (1944); Halbig, Matthias a n d Treibs, U. S. Patent 2,344,085 (1944). (14) Lucas a n d Pressman, I n d . Eng. Cheiri., A?znl. Ed., 10, 140
(1938).
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PAUL
n. BARTLETT AND KENZIENOZAKI
bromate solution was added. The addition of the mercuric sulfate solution was made three to five minutes later and the mixture was allowed to stand for thirty minutes. The excess bromine was then titrated iodometrically. The difference between the titers with and without mercuric sulfate added gives the concentration of maleic anhydride.
Vol. 68
tetrachloride, a few cc. of petroleum ether being added to decrease the solubility of the maleic anhydride. The crystals collected by filtration, washed with carbon tetrachloride, and dried in the air, melted at 53". The benzoyl peroxide was purified by dissolving in chloroform and precipitating by the addition of methanol. The product was dried in a vacuum desiccator over calTABLE I cium-chloride. Procedure for Po1vmerization.-Test-tubes, 10 X 70 ANHYDRIDEMIXANALYSIS OF ALLYL ACETATE-MALEIC mm., were carefully washed, dried and drawn out. TURES Known amounts of benzoyl peroxide and the monomer or Min- Minmonomers were added and the tubes were cooled in a Dry utes utes Ice-methanol bath. The tubes were evacuated 011 a Cenco M I . of for for Hyvac oil pump and then flushed with nitrogen which had 0.2 M hy- hroEquivalents Unsaturated HE SO^ drolv- minax 108 been passed through Fieser solution.15 This was repeated compound added sis tion Added Found Run six times, after which the tube was sealed off under 1 A l l y l acetate ') lo 1.885 vacuum. A check of the evacuation procdure on pure 2 Allyl acetate 30 1.855 allyl acetate indicated that no observahle loss of inoiiomer -.2ri 3a Allyl acetate "'i 30 1.836 resulted. The tubes were suspended in the thermostat in .I m'lYdride O () lo 2."4 O.O0 a rocking wire basket. At measured intervals tubes were Rfaleican'lYdride 7.3 13 30 2.04 o.25 removed, chilled in an ice-bath, opened, and the contents anllydride ''O 6o 2.04 1.76 dissolved in acetic anhydride, the total volume being made 7 n'aleic anhydride 2' 0 30 2.0' 1.34 up to 10 or 25 cc., in a volumetric flask. Known volumes 8* Maleic acid 2o 30 were pipetted out and the peroxide concentration, allyl gb Rlaleic acid 30 1.69 acetate concentration, and total utisaturation were deter10 Maleic anhydride 25 5 42 2.04 2.09 mined, 11 Maleic anhydride 25 2 30 2.04 2.07 Exploratory polymerizations were carried out in air. 12" A l l y l acetate f After conditions were chosen for the more careful experim a l e i c a n h y d r i d e 25 30 2.02 2.03 ments, all samples were sealed in evacuated glass tubes after repeated evacuation and flushing with nitrogen. a Sulfuric acid added after bromine and allyl acetate had The Conditions of Copolymerization of Allyl Acetate and been allowed to react for two minutes. Crude maleic Maleic Anhydride.-In the presence of 5LT0 of benzoyl acid. peroxide (in air) a t 80" the copolymerization of equimolar Titration of Benzoyl Peroxide.-The weighed sample quantities of these monomers proceeded violently, with containing the peroxide was dissolved in 5 cc. of acetic self-heating and with consumption of 97% of the peroxide anhydride and 0.5 cc. of 5070 potassium iodide was added. and 56% of the monomer in three minutes. The temperaThe mixture was allowed to stand in a 125-cc. glass-stop- ture was therefore progressively lowered in subsequent pered flask for ten minutes with occasional swirling. experiments. In a similar experiment, started a t 70" with About 50 cc. of water was then added and the iodine was one gram of each monomer, the temperature of the mixture titrated with 0.01 ,lithiosulfate, using starch indicator. rose in six minutes to over 105". These experiments were Materials.-The exploratory work was carried out with performed with the impure allyl acctate referred to above. a batch of allyl acetate which had been standing for several All experiments described hereafter were performed with months and which was found to contain an impurity with the purest materials. A t BO" in the presence of air the active hydrogen to the extent of five mole per cent. Two temperature of a mixture during copolymerization renew prcparations of allyl acetate were made, one from mained nearly constant, but when this experiment was rcallyl alcohol, acetyl chloride, and pyridine, and the other peated in a stream of nitrogen the temperature rose to 71 from allyl alcohol, acetic anhydride, acetic acid, and in eighteen minutes. The inhibition by oxygen thus resulfuric acid. In the first case pyridine was removed by vealea proved to be very serious, and was never eliminated washing with ice-cold 2 N sulfuric acid and water; in the nor made entirely reproducible, even by the most careful second case acids were removed by careful washing with application of the procedure for degassing. Exploratory sodium carbonate solution and water, and drying over polymerizations a t 55" in sealed tubes showed that maleic potassium carbonate. In each case the product was dis- anhydride is polymerized by benzoyl peroxide under much tilled through an 8-inch Widmer column. Practically all the same conditions as allyl acetate, but the inhibitory the material distilled from 102.9 to 103.4' under 754 mm. effect of oxygen is more conspicuous in the case of maleic pressure. It showed no detectable active hydrogen, and anhydride. The copolymerization a t this temperature yielded a colorless copolymer with maleic anhydride. proceeded as far in eleven minutes as the separate polyThe maleic anhydride was obtained from the Eastman merization of either monomer in over a thousand minutes. Kodak Company and was purified by crystallization from The decomposition of the peroxide was also much more a inixtnre of equal volumes of chloroform and carbon rapid in the copolymerization (see Fig. l),and the results were so variable as t o suggest strong inhibition by traces of oxygen. Because of the rapidity of the copolymerization a t 55" the lower temperature of 30" was chosen for the most careful study of the reaction. This temperature carried with it the disadvantage that maleic anhydride, being solid, could not be polymerized separately for comparison. The Polymerization of Allyl Acetate at Different Temperatures.-At 30" the polymerization of allyl acetate is very slow. The benzoyl peroxide is only about 10% decomposed in a month. Nevertheless three points were obtaiiied which made possible a determination of k l , the 200 400 600 800 unimolecular rate constant for decomposition of the perTime in minutes. oxide, and of the quantity dM/dP, which is constant Fig. ].-Rate of decomposition of benzoyl peroxide at throughout the polymerization in the case of allyl acetate.' 55" in allyl acetate (upper), maleic anhydride (middle), (Here, as in Parts I and 11, Af denotes nionoiner concenand an equimolar inixture of allyl acetate and maleic tration, P is peroxide cmcentration, and all concentrations anhydride (lower). (15) Fieser, THISJ O U R N A L , 46, 2689 (19241
""
*
ilug., 1946
COPOLYMERIZATION OF
ALLYL A C E T A T E
are expressed in moles per kilogram of solution.) Table I1 compares these values as determined at 80,'55 and 20'. Representative polymerizations of allyl acetate have been described in Part I1 of this series.'
in allyl acetate, the extrapolation to 30 O is not trustworthy. No attempt is made to extrapolate dAd/dP, since this quantity for allyl acetate does not fit the Arrhcnius q u a tion. TABLE V
TABLE I1 POLYMERIZATIOX OF ALLYL ACETATEAT THREETEMPERATURES 1l'lTH 4.58$ B Y m;EIGHT O F BENZOYL I'EROXIDE
T %inp ., C.
80 55 30
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W I T H ~ ~ A T , E I hNHYDRIDF, C
ki for peroxide deconiposition Alinutes
Hours
dM / d P
3 5 x 10-3 1 . 2 3 x 10-4 2.17 X 10-6
2 . 1 x 10-1 7 . 7 x 10-3 1.3 X
19.3 30.7 36.4
EFFECT O F OSYCES O N C O P O L Y > f E X I Z . % T I 0 9 A T i)$)--:
