Methyl Methacrylate Polymerization Peroxide Catalysis and the Oxidation
of Hydroquinine Inhibitor HUBERT N. ALYEA, JOHN J. GARTLAND, JR., AND H. R. GRAHAM, JR. Princeton University, Princeton, N. J. URING a n investigation D ($1 of the oxidation of sodium sulfite, it was observed
that hydroquinone, added as a n inhibitor, became slowly oxidized t o quinone. Apparently the oxidation involves a chain mechanism; the hydroquinone a c t s b y breaking r e a c t i o n chains and is oxidized in the process. These i n h i b i t i o n studies were extended to polymerization reactions-e. g., vinyl acetate and, in this report, methyl methacrylate.
with 0.1 N sodium thiosulfate in the presence of starch indicator.
Polymerization of methyl methacrylate at 80" C. in the presence of peroxide catalysts, with and without hydroquinone inhibitor, was followed b y density measurements. Polymerization is proportional to the square root of the peroxide concentration. The oxidation of hydroquinone to quinone, confirmed b y light transmission curves, is independent of the hydroquinone concentration and dependent on peroxide concentration. The results are consistent with the following mechanism: The peroxides initiate reaction chains; the hydroquinone breaks these chains and in so doing becomes oxidized to quinone.
Peroxide Catalysts Monomeric methyl methacrylate was freed of h droquinone inhibitor by shaking with 5 N sodium hydroxide anzdesiccating with 8-mesh calcium chloride. Hydroquinone and benzoyl peroxide were used as obtained from the manufacturers. Other peroxides were prepared by refluxing organic acids with thiongl chloride and treating the resultant chloride with sodium peroxide (&. p-CHLOROBENZOYL PEROXIDE. 22.4 grams of p-chlorobenzoic acid were refluxed 4 hours with 50 cc. of thionyl chloride. Excess thionyl chloride was distilled off at 78" C., and a 20-gram yield of p-dhlorobenzoyl chloride was added, bit by bit, to 10 grams of sodium peroxide (93 per cent pure) in 50 cc. of ice water. The peroxide, which floated, was atered off, dried, and recrystallized three times from ether. The yield was 15 grams (37 per cent of theoretical) of white stable crystals. Its peroxide content was found to be 30 per cent. P-TOLUYL PEROXIDE.25 grams of p-toluic acid were refluxed with 90 cc. of thionyl chloride; the reaction was practically instantaneous. Excebs thionyl chloride was distilled off a t 78" C. The p-toluyl chloride (yield 25 grams) was added bit by bit to 20 rams of sodium eroxide in 150 cc. of ice water. The er oxde was atered ofPand recrystallized three times from etgeri 17.6 grams (37 per cent of theoretical yield) of white crystals were obtained. Peroxide content was found to be 24 per cent. 25 grams of p-nitrobenzoic acid p-NITROBENZOYL PEROXIDE. were refluxed with 150 cc. of thionyl chloride for 17 hours. After the excess thionyl chloride was distilled off in a steam bath, a solid residue of p-nitrobenzoyl chloride remained. This was recrystallized from petroleum ether. The yield was 25 grams or 90 per cent of theoretical. Ten grams of this chloride were added to 5 grams of sodium peroxide in 50 cc. of ice water. The peroxide was atered off and recrystallized from ether. The yield was 4.5 rams or 25 per cent of theoretical. The peroxife content was found to be 21 per cent. It was ascertained by its liberation of iodine from potassium iodide (5). To do this, 0.1 gram of peroxide was dissolved in 50 cc. of peroxide-free dry ether; this was cooled to -5' C., and 2 to 3 cc. of freshly prepared 5 per cent sodium ethylate solution were added. The ether solution was extracted with 100 cc. of cold water. To half of the sample (i. e., to 0.05 gram of peroxide) were added 2 cc. of 5 per cent otassium iodide solution and 2 cc. of dilute hydrochloric acid. $he liberated iodine was titrated 458
Polymerization was carried out in a thermostated oil bath at 80' * 0.5' C. Reactants were mixed immediately before immersion in the bath. The course of the polymerization was followed by diluting samples of the polymerizing mixture with a n equal volume of glacial acetic acid and testing the solution in a simple Westphal balance. During polymerization in the presence of hydroquinone, a deep color developed. Its transmission was measured on a Soleman spectrophotometer in the light region 3500 to 6000 A. To follow the development of color during a run, 240. samples were removed from the polymerizing solution and the light absorption was observed in a photoelectric colorimeter (1). To increase sensitivity, a cobalt-blue glass filter was employed. As a color comparator a heat-resisting red-shade yellow No. 348 Corning glass, 3.49 mm. thick, was employed.
Polymerization Studies EFFECTOF BENZOYLPEROXIDE. This was followed by density measurements. A typical run for a solution containing 0.5 mole of hydroquinone and 1 mole of benzoyl peroxide per 1000 moles of methyl methacrylate follows: times, 0, 40, 65, 85, and 110 minutes; densities of acetic-acid-diluted samples, 0.9921, 0.9956, 0.9981, 1.0018, and 1.0065, respectively. The time for half the monomer t o polymerize in an uninhibited solution is inversely proportional to the square root of the benzoyl peroxide concentration (Table I). EFFECTOF DIFFERENT PEROXIDES. It is apparent from Table I1 that p-chlorobenzoyl peroxide is the best and benzoyl peroxide the poorest of the three peroxides.
TABLEI. EFFECTOF BENZOYL PEROXIDE ON RATITOF POLYMERIZATION
Concn., mole peroxide/moles methyl methacrylate Min. for half the monomer t o disappear (to density 1,0180) Ratio, l / d c o n o n . benzoyl peroxide
INDUSTRIAL AND ENGINEERING CHEMISTRY
OF VARIOUS TABLE 11. EFFECT Time Min.' 0 5 10 13 15 17 20 23 25 0 5 15 20 30 35
0 10 20 30 40 45
PEROXIDES ON RATEOF POLYMERIZATION Density Benzoyl p-Chlorobenzoyl p-Toluyl peroxide peroxide peroxide 1 Mole Peroxide/BOO Moles Methyl Methaorylate 0.9921 0.9921 0.9921 0.9964 0.9971 0.9956 1.0049 1.0127 1.0069 1.0242 1.0119 1.0402 i:oi& 1.0441 1:oiig 1.0286
.... .... ....
1:oiig 1 Mole Peroxide/2000 Moles Methyl Methaorylate 0.9921 0.9921 0.9966 0.9945 1.0008 1.0038 1.0078 1.0024 1.0178 1.0089 1.0257 1.0151 1 Mole Peroxide/4000 Moles Methyl Methacrylate 0.9921 0.9921 0.9982 0.9952 0.9992 1.0034 1.0053 1.0076 1.0102 1.0149 1.0219 1.0123
0.9921 0.9931 1.0030 1.0051 1.0161 1.0222 0.9921 0.9946 1.0018 1.0080 1.0186 1,0274
added to make it comparable in content with solution 1. Solution 3 was prepared by adding a saturated solution of quinhydrone in methanol to methyl methacrylate] and then adding peroxide and acetic acid. The light transmission of these three solutions was measured on a Coleman spectrophotometer (Table V).
TABLE V. Wavelength, Soln. 1 Soln. 2 Soln. 3
PERCENTAQE LIQHT TRANSMISSION OF SOLUTIONS 3500 3600 4000 4400 4800 5000 6000 1.1 1.1 4.7 7.5 31.6 56.1 90.7 3.3 10.0 12.8 36.8 1.8 59.7 93.3 33.0 36.4 47.9 60.0 66.9 70.0 86.2
RATEOF QUINONEFORMATION. Mixtures of benzoyl peroxide and hydroquinone were placed in the bath at 80" C., and the development of quinone was followed by measuring the light absorption of samples removed at regular intervals (Table VI and Figure 1). The galvanometer deflections increased with increasing quinone concentrations, but above120 the solution was darkening rapidly and the deflections increased only slowly.
Table I11 indicates that p-nitrobenzoyl peroxide, on the other hand, is a decidedly less active catalyst than the peroxides in Table 11. MUTUALEFFECTS.Concentrations of 1 to 1000 benzoyl peroxide] of 1 to 1700 p-chlorobenzoyl peroxide, and of 1 to 1500 toluyl peroxide were used in the presence of hydroquinone. The rate of polymerization is given in Tables IV and
Formation of Quinone TRANSMISSION OF COLORED PRODUCTS. It was originally assumed that the red substance formed when the polymerization was carried out in the presence of hydroquinone was the oxidation product, quinone. T o confirm this, the transmission of light for various solutions was measured. Solution 1 was prepared by polymerizing, for 90 minutes at 80" C., 20 cc. of methyl methacrylate containing 1 to 100 peroxide and 1 to 100 hydroquinone. Solution 2 was prepared by shaking a methyl methacrylate-hydroquinone solution with a water solution of potassium permanganate until a considerable quantity of reddish quinone had been formed. The nonaqueous layer was separated and desiccated with calcium chloride, and then sufficient peroxide and acetic acid were
TABLE 111. EFFECT OF ~NITROBENZOYL PEROXIDTO ON RATEOF POLYMERIZATION Density at Following Ratio of Peroxide/Methyl Methacrylate: 1/500 l/ZOOO 1/4000 0.9921 0.9921 0.9921 0.9924 0.9947 0.9933 0.9992 0.9948 0.9930 1.0006 0.9964 1.0022 0.9980 0:9&3 0.9964 1.0029 1.0018 0.9984
Time, Min. 0 20 40 60 80 100 120
MUTUALEFFECTOF PEROXIDES AND HYDRORATEOF POLYMERIZATION
Density 1/1000 1/1700 Time, benzoyl p-ohlorobenzoyl Min. peroxide peroxide 0 0.9921 0.9921 30 0.9940 0.9949 60 0.9974 0.9986 80 0.9994 1.0014 120 1.0024 1.0048 5 Conoentration of hydroquinone, 1/1000.
1/1500 p-toluyl peroxide 0.9921 0.9950 0.9990 1.0018 1.0035
40 60 80 REACTION TIME, MINUTES
Effect of Hydroquinone and Benzoyl Peroxide on Color
Upper curve, l / l O O peroxide: 0 = 1/100 and c) = 1/50 hydroquinone Lower curve, 1/400 peroxide: €3 = 1/400 and X = 1/100 [hydroquinone
I n these and subsequent runs no quinone was formed from comparable hydroquinone-benzoyl peroxide mixtures in inert solvents, in the absence of methyl methacrylate; thus t h e process is not a direct oxidation of the quinone by the peroxide. OTHERCOLORSTUDIES.The formation of quinone wa9 measured in mixtures containing 1to 1000 benzoyl peroxide, 1 to 1700 p-chlorobenzoyl peroxide, and 1 t o 1500 p-toluyl peroxide (Table VII)
. Chain Reaction Mechanism Without postulating a specific mechanism, it will be assumed that the polymerization is a chain reaction, that the peroxide acts by initiating the polymerization of a number of monomer molecules, that the hydroquinone checks the polymerization of the monomer (i. e., breaks reaction chains), and that the hydroquinone is oxidized to quinone during the process. This assumes, in particular, that the hydroquinone does not act by combining with the peroxide. The purpose of this discussion is to show that the above results justify these assumptions. PEROXIDE EFFICIENCIES. The data in Tables I1 and 111 were calculated in another form (Table VIII) to illustrate the relative efficiencies of the different peroxides. "Half reaction" corresponds t o a density of 1.0180.
INDUSTRIAL AND ENGINEERING CHEMISTRY
HYDROQUINONE AND BENZOYL PEROXIDE CONCENTRATIONS ON RATEOF COLORATION Colorimeter Readings 1/400 Benzoyl Peroxide 1/100 Benzoyl Peroxide 1/100 1/50 1/400 1/400 1/100 hydrohydrohydrohydrohydroquinone quinone quinone quinone quinone 20.4, 17.0 21.6,21.4 ....... ..... ....... ....... 6.8, 6.9 ..... ....... ....... 1417 ....... 3.8,4.1 ....... ....... ....... lG,5 6.1,B.l ....... 22.0,21.s .. ..... 8.5, 8 . 7 2 8 . 2 , 26.9 ....... ....... ..... ....... 28.4, 26.1 12.5,12.3 ..... ....... 23: 1 ....... ....... 8.0, 7 8 30.8,28.1 ..,. ., . ....... ..... ...... 30.0, 3 0 . 2 14.1, 13.6 ..... 9.6,Q.i ....... .. ....... ., . . , .. .... ....... ,, , , ., , 30.9, 30.4 13.5 13.3 23:s 30.6,30.2 .......
~i~~ in Bath, Min. 5 10 15 20 25 30
40 46 60
70 SO 95 100
PEROXIDES ON TABLEVII. EFFECTOF DIFFERENT FORMATION Time, Hours
1/1000 benzoyl peroxide
1/250 Hydroquinone 1/1700 p-ohlorobenzoyl peroxide Colorimeter Readings----
1/1600 p-toluyl peroxide 26.0 31.1 0.9921 1.0005 1.0011
TABLEVIII. PEROXIDE EFFICIENCIES AND RATE OF POLYMERIZATION
Peroxide Benzoyl p-Chlorobenzoyl p-Toluyl p-Nitrobenzoyl 0 Standard.
blin. for Half Reaction Canon. Canon. 1/800 l/ZOOO 18.5 39 13 29 15.5 32
1.05 1.4 1.2 0.17
Canon. Giving Equal Rates 1/1000 1/1700 1/1500
The time for half reaction appears to be inversely proportional to the square root of the peroxide concentration. I n column 4 (Table VIII) the value 1.4 is the ratio 18.5/13 and 39/29; the value 1.2 is 18.5/15.5 and 39/32. The value 0.17 is obtained from data in Tables I1 and 111,and indicates that p-nitrobenzoyl peroxide is only one sixth as efficient as benzoyl peroxide in catalyzing the polymerization of methyl methacrylate. On the other hand, p-chlorobenzoyl peroxide and p-toluyl peroxide are better than benzoyl peroxide. INHIBITION. The last column of Table VI11 represents a condition of three different peroxides all polymerizing a t the same rate. It was calculated by plotting concentration of peroxide against time for half reaction (columns 2 and 3). The plot shows that a concentration of 1 to 1000 benzoyl peroxide (chosen because it had certain experimental advantages) would be half polymerized in 25.3 minutes, and that in the same time solutions oontaining 1 to 1700 p-chlorobenzoyl peroxide and I to 1500 p-toluyl peroxide would be half polymerized. For three solutions of these respective strengths the same number of chains is being initiated per minute. Hydroquinone in this instance should exhibit equal inhibitory effect for it acts solely by breaking chains, whose number in the three solutions is equal. This consequence of the chain theory is satisfied in Tables N a n d VII. However, a t a lower concentration of hydroquinone (1 to 4000), the p-toluyl peroxide causes a more rapid reaction than the other two, possibly because of removal of hydroquinone by interaction with the peroxide, as explained in the last section. QUINONEFORMATION. Table V shows that the hydroquinone is oxidized to quinone. Presumably the hydrogen from hydroquinone adds onto the chain residue, breaking the
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polymerization chain and at the same time forming quinone. According to this, the number of quinone molecules formed would be a measure of the number of reaction chains initiated, QUANTITY OF QGINONE FORXED. According t o the chain mechanism, the rate of quinone formation should be dependent upon the peroxide concentration; for increasing the peroxide content increases the number of chains initiated and thereby increases the number of chains to be broken by hydroquinone. Agreement with this mechanism is seen by comparing, for example, the third and sixth columns of Table VI. Also, the values 13.3 and 30.2 at 100 minutes show that the quinone formation, like the rate of polymerization, is approximately proportional to the square root of the peroxide concentration. According t o this same chain mechanism, the rate of quinone formation should be independent of the hydroquinone concentration, provided the latter is present in quantity sufficiently large t o break all reaction chains. This independence of hydroquinone concentration is clearly indicated in the last five columns of Table VI where an eightfold variation of hydroquinone produces little change in the quantity of quinone formed. The fact that the hydroquinone concentration of 1 to 400 is slightly lower than the other two indicates that all of the chains are not being broken by hydroquinone a t this lower concentration. QUISONEWITH OTHERPEROXIDES. Uninhibited solutions containing different peroxides, but polymerizing a t the same rate, would be initiating the same number of chains. If inhibited by hydroquinone, these would produce equal quantities of quinone. This is the result obtained in Table VII, where solutions of benzoyl peroxide and p-chlorobenzoyl peroxide of the strengths given in column 5, Table VIII, underwent equal coloration. I n the case of p-toluyl peroxide, however, a deep coloration quickly developed as the result of a side reaction between the peroxide and the hydroquinone. Apparently the quinhydrone complex was formed. Acknowledgment We wish to thank A. K. Parpart and Louis Schwab for assistance in measuring light transmission. Literature Cited J. Chem. Education, 18, 57 (1941). (2) H. N., and Baokstrom, H. L. J., J. Am. Chem. SOC..51, . , Alma. (1) Alyea, H. N., 90 (1929).
(3) Gilman, Henry, “Organic Syntheses”, Collective Vol. 1, p. 423. New York, John Wiley & Sons, 1932. (4) Weinland, R. F., and Lowkowitz, H., Ber., 36,2702 (1903).