June, 1958
OXIDATIVEDEGRADATION OF DEUTERATED STYRENE POLYMERS
to use oxygen as the oxidant. Acetylene flames are burnt with air. The results are listed in Table 11. For purposes of comparison, we estimate that [HI would be about 50 times the equilibrium concentration of hydrogen atoms in hydrogen flames a t 1600°K., and perhaps 10 times at 1850". It is remarkable that no such excess of hydrogen atoms is observed in hydrocarbon flames. With the chosen value of k2, [HI works out to be essentially the equilibrium concentration in each run. That [HI is 'the equilibrium concentration in the burnt gas from hydrocarbon flames is partly an imposed result. The value of kz derived from Avromenko and Lorentso was increased by a factor of 1.7, as explained earlier, in order t o achieve it. Even with a smaller value of k2, [HI works out to be very close to the equilibrium concentration. Since [HI increases markedly with temperature in hydrocarbon flames, but does not do so in hydrogen flames where it is present in great excess, we infer that there is no excess in the gas from hydrocarbon flames; and k2 is adjusted slightly to make the excess zero. The last six entries in Table I1 show that hydrocarbons are effective in decreasing excess [HI in hydrogen flames. At 1445"K., a pure hydrogen air flame has [H]/(H]e,, = 300 approximately. But if the fuel is 7.9% acetylene plus hydrogen, [HI/ [HI,,, is only 150; and for 19% acetylene it is only 65. At 1345"K., the poisoning action of acetylene is even more marked, and methane is similar in its action. I n these runs with mixed fuels, a little acetylene and methane always sur-
697
vives into the burnt gas. There is no evidence that acetylene actually exchanges hydrogen atoms with the substrate, however. Acetylene recovered from a rich flame of a 90% acetylene, 10% deuterium mixture gives a pattern on the mass spectrometer which does not differ significantly from that of the acetylene fed in the reactants. A steady rich hydrogen, acetylene, air flame cannot be burnt a t 1345°K. if the hydrocarbon is increased much beyond the percentages shown in Table 11. The flame burns with a pulsating noise if the hydrocarbon is increased. At the same time, the temperature jumps rapidly with increasing hydrocarbon. Dr. W. E. Kaskan of our group explains this as follows: the flame is quenched and locally extinguished a t some point on the burner surface. When the gas reignites above this point, .piloted by the surroundings, the igniting volume being farther from the burner possesses a burnt gas temperature nearer the adiabatic flame temperature. To this explanation we would add that hydrocarbon additions possibly quench the flames because they destroy hydrogen atoms or other radicals. It might be more nearly correct to suppose that fragments derived from hydrocarbons rather than the hydrocarbon itself catalyze the recombination of hydrogen atoms or of other radicals. Otherwise, acetylene might be partly deuterated after passage through an acetylene deuterium flame. If hydrocarbon fragments are the active catalysts, then one wonders whether C2 and CH emission may not derive its excitation energy partly from this catnlytic process. The suggestion is not original, but the decrease of excess [HI found by hydrocarbon addition has not been observed previously in flames.
OXIDATIVE DEGRADATION OF A SERIES OF DEUTERATED STYRENE POLYMERS1 BY MAXTRYON AND LEOA. WALL Rubber Section, National Bureau of Standards, Washington $6,D. C. Received January 86, 1968
The oxidation of polystyrene and a series of polymers prepared from specifically deuterated styrene monomers has been investigated in the presence of ultraviolet radiation and air a t 60". A ronounced isotope effect was obseryed when olymers contained deuterium in place of hydrogen in the a-position. All porymers in which the a-position was occupieaby deuthe post-effect terium or a methyl group showed a slower rate of increase in absorbance a t 340 mp wave length and about as compared with those with a hydrogen atom in the a-position. Values for the rate constants and apparent activation energies for both the fast and slow components of the, post-irradiation effect for each of the polymers are given. The results of a study of the photo-isomerization of trans-benzalacetophenone to the cis-isomer and thermal cis-trans isomerization of this material in a polystyrene matrix show that such an isomerization reaction is possible and may account for part of the post-irradiation effect.
Introduction Previous work in this Laboratory on the oxidation of styrene and or-deuterostyrene polymers2 indicated a pronounced isotope effect on the initial rate of increase in absorption at 340 mp due to oxidation in the presence of ultraviolet radiation and air at 60". The post-irradiation effect previously (1) Presented in part a t the 129th Meeting of the Amerioan Chemical Society in Dallas, Texas, April 9-13, 1956. (2) L. A. Wall, Mary R. Harvey and Max Tryon, THIBJOURNAL, 60, 1306 (1956).
observeda for polystyrene also was observed for the deuteropolymer. Studies of the reaction occurring during post-irradiation of polystyrene indicated two first-order reactions with activation energies of the order of 16 and 20-24 kcal. per m ~ l e . ~It. ~was suggested that these reactions were a hydroperoxide decomposition and a cis-trans isomerization. I n the present work the oxidation behavior of a (3) M. J. Reiney, M. Tryon and B. G . Achhammer, J . Raseorch Notl. BUT.Slonderds, 61, 155 (1953). (4) Leo A. Wall and Max Tryon, Nature, 1'78, 101 (1956).
MAXTRYON AND LEOA. WALL
698 0.5 I
0.4
0
traiismittance in this region but with a maximum decrease a t or near 340 mp as noted earlier.3 For this reason measurements of the change in absorbances in the polymers were made to follow both the radiation and the post-effect reactions in the polymers. In Fig. 1 the absorbance of films of each of the polymers studied, corrected for the original film absorbance, is shown as a function of time of exposure to ultraviolet radiant energy under the conditions described above. Group A represents the polymers'with structures in which there is a hydrogen atom in the a-position. Group B represents the polymers in which there is a deuterium atom in the a-position except for the upper curve in this group which is for a polymer that has a methyl group in the alpha position. Such clear separation of the effect of a-substitution was not evident in the recent work of Beachell and Nemphosg who measured oxygen consumption and carbonyl formation of a series of partially deuterated polymers. They found slower oxidation rates with the deuterated polymers, but the results did not fall into two distinct groups; the rates decreased roughly with the number of substituted deuterium atoms. However, a-substitution still appears to be most effective for decelerating the oxidation. The post-irradiation reactions were studied by the methods used earlierz and the rate constants were determined for each of ths polymers a t several temperatures. The results are shown in Table I. The over-all form of the rate equation for the posteffect is assumed to be the sum of two first-order reaction terms as4
I
1
120
240 360 480 600 Time, hr. Fig. 1.-Absorbance" of deuterated polyostyrenes z w w s time of ultraviolet irradiation in air at 60 : A: circles, poly-(8-deuterostyrene); triangles, poly-(p-deuterostyrene); squares, polystyrene; inverted triangles, poly-(&p-disquares, poly-( a-methylstyrene); deuterostyrene). B: circles, poly-( a-deuterostyrene); triangles, poly-( a,B,@-trideuterostyrene). Absorbance measured a t 340 mp corrected for absorbance of original polymer film.
series of deuteropolymers was compared with that of polystyrene. The purpose was to verify the site of radical attack and to evaluate the relative importance of the two post-irradiation reactions in the deuterated substances and thereby to elucidate in more detail the oxidation mechanism. Materials and Methods.-The polystyrene used in this work was a Sam le prepared by thermal bulk polymerization at 120". T i i s was the same highly purified polystyrene sample used in earlier w ~ r k . ~ The - ~ deuterostyrene polymers were prepared from the corresponding deuterated monomers synthesized by the methods reported previously.6 The poly-( a-deuterostyrene) was described in an earlier paper.2 All of the polymers were purified of monomer, dimer and similar materials by repeated solution in benzene, followed by precipitation in methanol. The final product, in each case, was dissolved in benzene, the solution jrozen, and the solvent removed by sublimation at reduced pressure. The extent of purification was determined by the ultraviolet absorption of chloroform solutions of these polymers after each cycle of solution and precipitation, as recently described.3 Films of these purified polymers were cast from benzene solutions by the methods described in references 3 and 6. The films were exposed to ultraviolet radiant energy from a sun-lamp in air on a rotating turntable 15 cm. from the lamp. All the s a m p h were exposed to give the same absorbance prior to the start of the dark reaction. The temperature of the table was 60". This equipment is described in method No. 6Q21 of Federal Specification L-P-406a.' The lamp gives a typical mercury emission spectrum and a peak intensity at 313 mp. The study of the rates of change of ultraviolet absorption after exposure to ultraviolet radiation and after storage in the dark a t a series of different temperatures was made as described in earlier work.* A Beckman Model DU spectrophotometer with ultraviolet accessories was used, with further accessories for temperature control of the sample compartment.
Results The transmittance progressively decreases on oxidation for all the polymers studied. The largest decrease is observed in the region from 290 to 360 mp. The post-effect results in a further decrease in (6) Leo A. Wall and D. W. Brown, THIBJOURNAL, 61, 129 (1957). (6) B. G. Achhammer, M. J. Reiney and F. W. Reinhart, J . Research Natl. Bur. Standards, 47, 116 (1951).
(7) "Plastics, Organic: General Specifications, Test Methods," Federal Specifioation L-P-406a (Government Printing- Office, Washington 26, D. C., Jan. 24, 1944).
Vol. 62
where a and 0 are the fractions of the total absorption due to the initial concentratioiis of the two species, A is the absorbance a t time t, Ao is the absorbance at zero time, and A , is the limiting absorbance. TABLE I RATECONSTANTS AS A FUNCTION OF TEMPERATURE FOR THE POST-IRRADIATION REACTION IN POLYSTYRENES Rate constant Temp., X 108, "C. sec.-l 25 ks
kf 60 70 80 00
Styrene 0.07 1.0
ks
kf k, kf k. kt k. kr
10 45 40 72
Polymers u-D
8-D
88-D
0.3 2.7 0.47 8.0 1.5 9.2
0.14 6.0 0.20 12
0.20 6.3 0.18 15
3.5
72
5.0 63
~ 8 8 - paraD D M%hyl
0.10
0.3 4.2 0 . 1 5 0 . 2 2 0.19 5.1 7.6 27 0.18 0 . 9 5 6 . 3 20 1.9 1.7 55 41
4.9
The apparent activation energies and frequency factors calculated from the data in Table I are shown in Table 11. As a further check on the hypothesisZ that part (8) Absorbance is defined as log 1/T, where T is the transmittance the ratio of the transmitted energy to the incident energy. (9) H. C. Beachell and S. P. Nemuhos. J . Polumer Sci.. 26, 173 (1957). 3r
(
c
OXIDATIVE DEGRADATION OF DEUTERATED STYRENE POLYMERS
June, 1958
699
Examination of the a and p factors of the entire TABLE I1 series of polymers indicated that they are reasonAPPARENT ACTIVATIONENERGIES CALCULATED FOR POSTably constant for all the deuterostyrene polymers IRRADIATION REACTION IN POLYSTYRENE Av.
Polymer
extent Blow of d a r k reaction reaction’ kcd./moie
As,b sec.-l
Fast reaction, kcal./ mole
and polystyrene.
Af?t
1.0
8ec.
10” 16 10’ 23“ 0.5 Styrene 20 109 25 10” .1 a-D 10” 19 109 27 .4 P-D 10” 20 109 26 .4 BB-D 108 13 106 21 .1 aB0-D 109 16 10’ 22 .4 para-D a-Methyl .1 (24)d (8)d (102)d ( A , - &)/A0 where A m is final absorbance of film at t = m and A . is the absorbance of the film at t = 0 for dark reaction. b Arrhenius frequency factor. A calorie equal to 4.185 joules was used in this work. Data in parentheses based on measurements at only two temperatures.
of the post-irradiation effect may be due to a cistrans isomerization of structures similar to benzalacetophenone, generated in the polymer under the ultraviolet radiation, a film of polystyrene was prepared in the same manner as the other films except that a small amount of trans-benzalacetophenone was added to the benzene solution used t o cast the film. The cyclic nature of the cis-trans isomerization of the benzalacetophenone even in a matrix of polystyrene is shown in Fig. 2. The film was exposed for only a short time, insufficient to cause any detectable change in absorbance of a similar polystyrene film not containing the benzalacetophenone.
The fast reaction accounts for
8 0.9
9 0.8 m
% 0.7 al
0.6
e
3 0.5 2
-4 0.2
0.1 Fig. 2.-Ultraviolet absorbance of bensalacetophenone in polystyrene film. Solid circles represent data for polystyrene film with approximately 1% benzalacetophenone added. Open circles represent pure polystyrene film. Solid lines represent exposure to ultraviolet light in air, Dotted lines represent storage of exposed film in dark a t 110” in air.
70 to 80% of the absorbance produced by the postreactions. The temperature during the post-reaction does not produce any significant changes in the values of a or 6. In other words, the photo-reaction in all cases leads to approximately the same relative amounts of precursors for the dark reaction products regardless of the number or poDiscussion sition of deuterium atoms. However, the relative The results for the deuterated polymers show values for the two reactions appear to be reversed definite isotope effects on the photo-degradation for the cas? of poly-(a-methylstyrene). This may and subsequent “dark reaction.” As shown in indicate that though the total amount of the dark Table 11, the activation energies, as calculated for reaction is only one-fifth that of unsubstituted the two parts of the “dark reaction,” do not show polystyrene there is relatively more cis-trans isomany significant differences between the polymers erization than hydroperoxide decomposition durdeuterated in the a-position as opposed to those ing the dark reaction for poly-( a-methylstyrene). having hydrogen in this position. Poly-( a-methylThe ability of benzalacetophenone to undergo styrene), while not as extensively investigated, be- the cyclic cis-trans isomerization reaction in a polyhaves quite similarly to the a-deuterated polymers. mer matrix as indicated in Fig. 2 has not previously All the slow reactions have the larger activation been reported. It seems evident that such reacenergy. The activation energies for the slow reac- tions are important possibilities for explaining certion compare reasonably well with those for cis- tain chemical changes in polymers such as the posttrans isomerization.10 The activation energy for effect studied here. While an extended time of the fast reaction appears reasonable for that of the heating in the dark was necessary to cause an indecomposition of a quite labile hydroperoxide crease in absorbance in the example shown, it is structure. A product of this decomposition is, as not too unlikely that free acidic components of the we have suggested previously,2 an unsaturated spe- oxidative degradation or free radicals trapped in cies capable of a thermal cis-trans isomerization, the polymer might catalyze the isomerization. the trans isomer having a higher absorbance. The WymanlO reports that free acids and free radicals Arrhenius frequency factors show a somewhat have been observed to accelerate the thermal transimilar correlation. sition in some cases. The most pronounced effect of substitution in The over-all results, particularly their clean cut the a-position is shown in the column labeled “av- segregation into two groups depending on the posierage extent of dark reaction,’’ which was calcu- tion of deuteration, clearly confirms the imporlated as the amount of change of absorbance during tance of the a-position in the oxidation mechanism, the entire dark reaction ( A , - &) relative to the which was pointed out in a prior publication.2 The absorbance after the original photo reaction (Ao). present work gives a greater insight into the postHere it is seen that substitution in the a-position irradiation reactions in showing the correlation betends to reduce the total dark reaction by a factor tween extent of dark-reaction and ease of over-all of 4, even though the extent of the light reaction is oxidation as measured by the ultraviolet absorbthe same in both cases. ance. Since our previous work a report of a very inter(10) G. M. W y m a n , Chem. Revs.,66, 635 (1953).
700
B. F. FREASIER, A. G. OEERGAND W. W. WENDLANDT
esting investigation showing a similar post-effect in the oxidation of Nylon" has appeared. The dark reaction is ascribed to the formation of peroxy radicals which dissociate during the photo-reaction stage. While we do not exclude free radical intermediates as playing a role in the post-effect, the hypothesis of concomitant hydroperoxide decomposition and cistrans isomerization in polystyrene oxidation seems reasonable. The greater crystal(11) R. A. Ford, J . CoEloid Sci., 12, 271 (1957).
Vol. 62
linity in Nylon may account for the difference in mechanism of its post-effect. In ionizing radiation studies immobilized free radicals have been found to be more prevalent in crystalline polyethylene than in the more amorphous polyethylenes.12 Hence the different results in the post-effects may be ascribed to the amorphous character of polystyrene as opposed to the crystalline character of Nylon. (12) E. J. Lawton, paper presented at the 131et Meeting of the American Chemical Society, Miami, Fla., April, 1957.
THE 8-QUINOLINOL-5-SULFONICACID CHELATES OF SOME RARE EARTH METAL Ib BY BENF. FREASIER, A. G. OBERGAND WESLEYW. WENDLANDT Department of Chemistry and Chemical Engineering, Texas Technological College, Lubbock, Texas Received January 18, 1968
The formation constants for the 8-quinolinol-5-sulfonic acid chelates of lanthanum, cerium(III), praseodymium, neodymium, samarium, gadolinium and erbium were determined at three different ionic strengths and at three different temperatures. The three thermodynamic functions, AFO, AH0 and ASo, were calculated for the metal chelates at 25' and p = 0. The bonding in the complexes was shown to be ionic in nature.
Introduction
forms water soluble metal chelates with the rare The trivalent rare earth metal ions, under the earth ions, thus eliminating the use of a nonappropriate conditions of hydrogen ion concen- aqueous solvent system and the inherent difficulties tration, readily form metal chelates with 8-quino- connected with such a system. Since it was delinol and substituted 8-quinolinols having the sired to determine the three thermodynamic funcgeneral formula M(0x)l; where M is the metal ion tions, AFO, AH" and ASo,for the metal chelates, of charge +3 and Ox is the 8-quinolinol ion. Since the formation constants were determined a t three the metal chelates with 8-quinolinol are slightly different temperatures and a t three different ionic soluble in water, previous work has centgred around strengths. In order to treat the data statistically, the three the gravimetric estimation of lanthanum,2cerium,384 praseodymium,6 neodymium,s samarium and gado- temperatures and three ionic strengths were chosen linium.6 The metal chelates with the substi- according t o a three-squared factorial design. l 3 tuted 8-quinolinols1 such as 2-methyl-6, 5,7- Thus, three evenly spaced ionic strengths of 0.138, dichlor0-,7~~ 5,7-dibromo,-8 5,7-diiod0-,8~~and 8- 0.276 and 0.414, were selected. The temperaquinolinol-5-sulf onic acid lo also have been de- tures, 25, 32.3 and 40°, were chosen, their reciproscribed. Little is known concerning the physico- cals being evenly spaced. chemical properties of these chelates. The forExperimental mation constants have been determined for only Reagents.-Double distilled water was used to prepare the lanthanum and cerium(II1) 8-quinolinol che- all of the solutions. Approximately 0.1 N sodium hydroxlates in dioxane-water solvent mixtures,11 while ide solution, carbonate free, was prepared from reagent the thermal properties have been studied on the grade sodium hydroxide. The solution was standardized against recrystallized, C.P. potassium hydrogen phthalate. thermobalance.~~8~Q~'2 Approximately 3 M potassium chloride solution was preIt is the object of this investigation to evaluate pared from C.P. potassium chloride. The chloride content the formation constants for the rare earth metal was determined by the Mohr method. chelates with 8-quinolino1-5-sulfonic acid. This The rare earth metal com ounds were obtained as the particular chelating agent was chosen because it chlorides or oxides of 9 9 . 9 k purity from the Lindsay (1) (a) Taken in part from the Ph.D. Thesis of Ben E". Freasier, August, 1957. (b) Presented before the 13th Southwest Regional Meeting of the American Chemical Society, Tulsa. Oklahoma. December 5-7, 1957. (c) Department of Chemistry, Texas A. and I. College, Kingsville, Texas. (2) T. I. Pirtea, 2.anal. Chem., 107, 191 (1936). (3) T. I. Pirtea, Bul. chim. aoc. romdne chim., 99, 63 (1937-6). (4) R. Berg and E. Becker, Z . anal. Che'hsm., 119, 1 (1940). (5) G.Mannelli, 20th Int. Cong. Chem., Rome, 11, 718 (1938). ( 6 ) W. W. Wendlandt. A n d . Chim. Acta, 17, 274 (1957). (7) T. Moeller and D. E. Jaokson, Anal. Chem., 22, 1393 (1950). (8) W. W. Wendlandt, Anal. Chim. Acta, 11, 428 (1957). (9) W. W. Wendlandt, i b i d . , 15, 533 (1956). (IO) W. J. Ramsey, D. L. Douglas and D. M. Yost, J . A m . Chem. SOC.,72, 2782 (1950). (11) H. Freiser. Analyst, 77, 830 (1952). (12) W. W. Wendlandt, A n d . Chim. Acta, 15, 109 (1957).
Chemical Co., West Chicago, Ill.; Research Chemicals, Inc., Burbank, Calif.; and Research Laboratories, Inc., Newtown, Ohio. The purity was that listed by the supplier. Solutions of the rare earth chlorides ranging in concentration from 0.003 to 0.01 M were prepared. The exact concentration of the solutions was determined by titration with standard sodium hydroxide. The 8-quinolinol-5-sulfonic acid was obtained from Eastman Organic Chemicals, Rochester 3, N. Y. The recrystallized acid had a melting point of 320" (previously reported 322-323"14). A 1.812 X.10-.8 M solution was rcpared and used in all of the titrations. The equivafent weight of the acid was determined by titration with standard (13) C. A. Bennett s a d N. L. Franklin, "Statistical Analysis in Chemistry and Chemical Industry," John Wiley and Sone, Inc., New York, N. Y., 1954. (14) K. Matsumura, J . A m . Chem. Soc., 49, 810 (1927).
I
