JOURNAL OF POLYMER SCIENCE VOL. XXXIX, PAGES 493-499 (1959)

The Decomposition of Benzoyl Peroxide in Polystyrene

HOWARD C. HAAS, Polaroid Corporation, Chemical Research Laboratories, Cambridge, Massachusetts

INTRODUCTION

As a result of several attempts to decompose residual diisopropyl per- carbonate in a solid polydimethacrylate, we observed that peroxide decom- position at 60°C. occurred at a rate which was somewhat less than might be expected from published solution decomposition kinetics data. Since we could not locate any information on decomposition rates of peroxides in solid polymers, it was decided to make some preliminary measurements on a system of this type. The benzoyl peroxide-polystyrene system was chosen for study.

The decomposition of benzoyl peroxide in solution has been studied in detail. Several references very adequately summarize the current state of our knowledge regarding this rather complex reaction.2 Briefly, rate con- stants have been found to increase with peroxide concentration and the reaction has been shown to be the sum of spontaneous decomposition plus a radical-induced chain decomposition reaction. The rate of disappearance of peroxide, P, may be expressed by

--d[P]/dt = k,[P] + k,[P]"

where kl and k, are, respectively, the rate constants for spontaneous cleav- age and induced decomposition, and in which x may vary from 0.5 to 2.0. Values of kl obtained by the inhibitor method3 and by kinetic analysis4 show reasonable agreement. The values of both kl and ki are solvent- dependent, the solvent effect being much more pronounced in the case of Ici. Recently, Bevington and Toole have published some interesting results on the stability of the benzoyloxy radicaL5

PROCEDURE AND EXPERIMENTAL DATA

Our study consisted of preparing polystyrene films containing benzoyl peroxide, heating these films for various time intervals, and then analyzing the films for residual peroxide to obtain the rates of peroxide disappearance. Films containing three different peroxide concentrations were prepared by the usual solvent-casting techniques. The films were analyzed by iodome- try3b to establish accurately the initial peroxide contents. Narrow strips

493

494 11. C . IiAhS

of film were than placed in a glass chamber which was heated by the vapors of a refluxing liquid. Decomposition runs were made at 70.9, 80.1, and 89.5"C., the reflux temperatures of purified n-propyl bromide, benzene,

TABLE I Decomposition of Benzoyl Peroxide in Polystyrene

log log log Time, [PI, [Po]/ Time, [PI:. [Pol/ Time, [PI, [pol/

hr. wt.-% [PI" hr. wt.-S7, [PI hr. wt.-% [PI

0 2.5 6 . 3

22.0 26.5 30.5 45.7 52.5 71.7 94.0

166.1

0 1 .o 2 .0 4 .2 5 . 0 6 .0 7.0 8 . 0

15.7 20.4 26.8 39.7

0 2 4 6 7.8

15.5

0.431 0.412 0.406 0.341 0.326 0.307 0.255 0.246 0.196 0.157 0.075

0.431 0.408 0.409 0.364 0.344 0.347 0,324 0.307 0.220 0.188 0.128 0.101

0.431 0.348 0.277 0.211 0.174 0.065

0 0.020 0.026 0.101 0.121 0.147 0.228 0.243 0.342 0.438 0.761

0 0.024 0.024 0.075 0.097 0.094 0.124 0.146 0.295 0.364 0.528 0.633

0 0.097 0.192 0.310 0.394 0.821

T 0

21 .0 44.8 53.5

117.0 117.0 141.2 141.2 165.0 165.0

= 70.9"C.

0.933 0 0.788 0.073 0.613 0.182 0.567 0.216 0.277 0.528 0.325 0.458 0.250 0.572 0.236 0.597 0.201 0.667 0.198 0.670

0 2.0 4.0 6 .0 8 . 0

23.0 29.8 47.5

T = 80.1"C. 0.933 0.930 0.823 0.777 0.747 0.383 0.292 0.166

0 0 0.054 0,079 0.097 0.387 0.504 0.750

T 0 1 .1 2.0 3.7 6.0 8 . 0

15.8

= 89.5OC.

0.933 0 0.818 0.057 0.742 0.100 0.637 0.166 0.466 0.302 0.409 0.358 0.132 0.849

0 1 .2 2.2 4.2 6.2 8 .2

32.0 47.8

0 1 . 5 3 . 0 5.0 7 . 0

16.0

4.28 4.12 4.06 3.61 3.12 2.79 1.07 0.61

4.28 3.71 2.74 2.21 1.56 0.54

0 0.017 0.022 0.074 0.137 0.186 0.602 0.846

0 0.062 0.195 0.287 0.439 0.899

[Po] is the initial peroxide concentration and [PI the concentration after time t .

and triethylamine. Samples were removed periodically and analyzed. No effort was made to exclude air during these experiments. Polystyrene films, initially containing 0.431, 0.933, and 4.28% benzoyl peroxide by weight, were employed. The data are presented in Table I.

DECOMPOSITION O F BENZOYL PEROXIDE IN POLYSTYRENE 49.5

DISCUSSION

The rate data of Table I plotted as log [Po]/[P] vs. time show a linear behavior in accordance with first-order kinetics. Some scatter is present which appears random with respect to the first-order plots and is presum- ably associated with the semimicro character of the analyses. The first- order rate constants obtained from the slopes of the log [PO]/ [€'-time plots are presented in Table 11.

TABLE I1 First-Order Rate Constants (hi--*) for the Decomposition of Benzoyl Peroxide in

Polystvrene

[Pol, Temperature, "C. wt.-% 70.9 80.1 89.5

0.431 0.0107 0.0406 0.116 0.933 0.0098 0.0385 0.116 4.28 - 0.0414 0.129

- Average k , hr.-l 0.0103 0.0401 0.120

If the rate data of Nozaki and Bartlett4 for the decomposition a t 79.8"C. of 0.2 12 and 0.02 A2 solutions of benzoyl peroxide in the low dielectric aromatic solvent, benzene, are plotted in a first-order manner, there is a definite deviation from linearity a t higher reaction times. Furthermore if first-order rate constants are calculated from the initial slopes, a 20% increase in k is associated with a tenfold increase in peroxide concentration. These results have been explained in terms of higher order reactions result- ing from radical induced chain decomposition. Since we do not observe any deviation from first-order behavior or any dependence of rate on con- centration, it must be concluded that radical-induced decomposition is of no importance in the benzoyl peroxide-polystyrene system. This conclu- sion appears reasonable in view of the much smaller diffusion constants exhibited within a polymer as compared to a liquid. Presumably then, a radical generated in polystyrene undergoes some effective termination reaction before it can diffuse to the vicinity of a neighboring benzoyl peroxide molecule. (Kote that the 0.431 and 4.28% peroxide films corre- spond very closely to 0.02 and 0.2 M solutions of benzoyl peroxide in polystyrene if the reported density of 1.06 for polystyrene is used.)

A discontinuity is found in the Arrhenius relationship a t 80°C. near the apparent second-order transition temperature (81 "C.) of polystyrene.6 It should be realized that we are basing this statement on results obtained a t only three temperatures, and it is quite possible that the discontinuity does not occur a t exactly 80°C. or that E, may be changing continuously with temperature in this range. Energies of activation of 35.7 kcal. for the 70-80°C. range and 29.7 kcal, for the 8G9O"C. range have been calculated from the rate data, and corresponding Arrhenius relationships are

496 11. C. l lAAS

for 70-80°C.:

k (sec.-l) = 1.39 X lo’? e-36*700’RT

and for 80-90°C.:

k (see.-’) = 2.67 X 1013 e-29,700’RT

If one compares these equations with the published relationships of Hart- man, Sellers, and Turnbull,? k = 1 X 1014 e--29900’RT; and Bawn and Mel l i~h ,~” k = 3 X 1013 e-29600’RT; for the spontaneous decomposition of benzoyl peroxide in benzene, i t is seen that the rate of decomposition of benzoyl peroxide in polystyrene above the glass temperature of polystyrene corresponds fairly closely to the decomposition rates in benzene. The E, of 33.4 kcal. and the temperature dependence reported by Nozaki and Bartlett may be the result of erroneous assumptions in the kinetic scheme which does not take proper cognizance of the decomposition products.

At and below 8O”C., the rate constants observed in the polystyrene sys- tem are distinctly smaller than those obtained by various investigators for the spont,aneous decomposition of benzoyl peroxide in benzene. The values reported for spontaneous decomposition of benzoyl peroxide in benzene a t 80°C. are listed in Table 111.

TABLE I11

k , x 105, see.-1 Investigator Reference ~ ~~~

3.28 Nozaki and Bartlett 4 4 .28 Swain, Stockmayer, and Clarke 3b 1.58 B a r n and Mellish 3a 2 .5 Brown 8 3.63 Hartman, Sellers, and Turnbull 7 4.39 Barnett and Vaughan 9 1.11 Present work (polystyrene)

It is interesting to speculate about, the possible reasons for the low observed values of k a t and below 80°C. in polystyrene. One possibility is that, in the primary homolytic cleavage, not only is energy required to split the peroxide molecule into two benzoyloxy radicals, but an additional activation energy is required to push apart polystyrene chains in order to accommodate the two radicals which must now be separated by a van der Waals distance. It is estimated’O that the molar cohesion for polystyrene is about 4 kcal./g. mole per 5-A-chain length if a coordination number of 4 is assumed. This value is of the order of magnitude that could account for the observed E, of 35.7 kcal. in place of the usual 30 kcal. Above T, of polystyrene, where segment diffusion is no longer frozen in, the situation should revert to one which is more nearly analogous to that in a liquid.

An alternate explanation is that, in a solid polymer particularly below T,, there exists an “enhanced cage effect” which allows recombination of benzoyloxy radicals to benzoyl peroxide. The cage effect, suggested years

DECOMPOSITION 01' BENZOYI, PEROXIDE IN POLYSTYRENE 497

kr S - R. + R (radicals isolated by diffusion)

ks

ago by Pricella and Matheson'lb for reactions in liquid media, was thought to be of limited significance for the decomposition of benzoyl peroxide in solution by Bartlett and Nozaki? and was also criticized by Flory'2 on the basis that diffusion out of the solvent cage is considerably more rapid than

' kz

cage

where P is the peroxide, X is an active cage containing two benzoyloxy radicals and k, is an average rate constant for all processes resulting in disruption of an active cage. Then

-d[P]/dt = kl[P] - k2IXl

and, on applying the Bodenstein steady state, the concentration of active cages a t any time is

The rate of peroxide disappearance becomes

-d[P]/d! = [kl - (k lk2 /k2 + k,jl[P] = K[PI

This treatment leads to first-order kinetics in agreement with the ob- served behavior and the rate constant K which we measure is then the composite rate constant kl - [klk2/(1c2 + k,)]. If a t 80°C. K has a value of 0.040 hr.-' and we assume that the rate of spontaneous cleavage kl is about the same as in benzene at 80°C. (0.118 hr.-l according to (Nozaki and Bartlett4) and also that in benzene X.1 is truly spontaneous cleavage without cage effect complications, then lcz/(k2 + k,) = 0.66, and k2 is about twice k,. This means that, in polystyrene a t 80°C., recombination of benzoyloxy radicals is occurring a t twice the rate of all processes which lead to disruption of the cage.

There remains a considerable amount of work which could be carried out on this and similar systems. It wouId be interesting to know the vari- ous decomposition products and their relative amounts in polystyrene for decomposition above and below T,. The presence of long lived free radicals from transfer reactions with polymer is probable and could be verified. Other benzoyl peroxide-polymer systems should be studied to determine the effect of polymer structure (if any) on rate and to verify the decom- position behavior above and below To.

498 11. C. llAAS

EXPERIMENTAL METHODS

Film Preparation

Commercially available polystyrene (Dow Chemical Company, Type K27-666) was purified by several reprecipitations from benzene into metha- nol and dried under vacuum a t 40°C. Benzene solutions containing 10% polystyrene by weight and about 0.5%, 1%, and 5% of benzoyl peroxide based on the weight of polystyrene were prepared and cast on glass. After the solvent had evaporated, the films were stripped from the glass and dried under vacuum a t room temperature for 1 week. The final thickness of the films was about 7 mils. I n spite of the prolonged drying there could still remain a small amount of residual so1vent.I3

Peroxide Analysis

Film samples weighing about 0.2 g. were dissolved in 5 ml. of purified methyl ethyl ketone in a small Erlenmeyer flask. Two grams of powdered Dry Ice was added slowly, after which the flask was warmed to room tem- perature. Saturated sodium iodide in acetone (0.5 ml.) was added, and this was followed by 20 ml. of distilled water saturated with Con. The liberated iodine was titrated with standardized 0.01 N sodium thiosulfate with the use of a Gilmont-Greiner 1-ml. capacity microburet. The end- point, the disappearance of the iodine color, was determined against a white background with the use of a bright lamp. A possible source of error in the method is the small amount of iodine trapped in the polymer which precipi- t.ates on the addition of carbonated water.

The author wishes to thank Mr. James L. Bailey for his help in carrying out a portion of the experimental work.

References 1. F. Strain, W. E. Bissinger, W. R. Dial, H. Rudoff', B. J . Dewitt, H. C. St,evens,

and J . H. Langston, J . Am. Chem. SOC., 72, 1254 (1950). 2. (a) A. V. Tobolsky and R. B. Mesrobian, Organic Peroxides, Interscience, X e a

York-London, 1954; (b) I). R. Augood and G. 13. Williams, Chem. Revs., 57, 123 (1957); (c) C. Walling, Free Radicals in Solution, Wiley, New York, 1957; (d) C. H. Bamford, W. G. Barb, A. D. Jenkins, and P. F. Onyon, The Kinetics of Vinyl Polymerization by Radical Mechanism, Butterworths, London, 1958.

3. (a) C. E. H. Bawn and S. F. Mellish, l'.rans. Faraday Soc., 47, 1216 (1951); (b) C. G. Swain, W. Stockmayer, and T. Clarke, Jr., J . Am. Chem. SOC., 72, 5426 (1950).

4. K. Nozaki and P. D. Bartlett, J . ,477~. Chem. SOC., 68, 1686 (1946). 5. J. C. Bevington and J. Toole, J . Polyme7 Sci., 28, 413 (1958). 6. R. F. Boyer and R. S. Spencer, Advancec in Colloid Sn'., 2, 2 (1946). 7. P. F. Hartmari, H. G. Sellers, and D. Turnbull, J . Am. Chem. Soc., 69, 2416

8. D. J . Brown, J . Am. Chem. Soc., 62, 2657 (1940). 9. B. Barnett and W. E. Vaughan, J . Phya. & Colloid Chem., 51, 926, 942 (1947).

(1947).

10. H. Mark, Znd. Eng. Chem., 34, 1343 (1942). 11. (a) C. C. Price, Ann. N . Y. Acad. Sci., 44, 351 (1944); (b) M. S. Matheson, J .

Chem. Phys., 13, 584 (1945).

DECOMPOSITION OF BENZOYL PEROXIDI.: I N POLYSTYRENE 499

12. P. J. Flory, Principles of Polymer Chemislry, Cornell Univ. Press, Ithaca, Sew

13. E. Merz, L. Xielsen, and R. Buchdahl, J . Polynzcr Sci., 4, 605 (1010). York, 1953, p. 121.

Synopsis The rate of decomposition of benzoyl peroxide in polystyrene has been measured a t

70.9, 80.1, and 89.5"C. The kinetics are strictly first order. There is no dependence of rate on concentration, which indicates that radical-induced decomposition is absent. At 7&80°C., the Arrhenius relationship is k = 1.39 x 101ie-35,i00/RT and at 80-90°C., k = 2.67 X 1013e-29i00/RT. Above 8O"C., the apparent second-order transition tempera- ture of polystyrene, the rates of decomposition correspond fairly closely to the rates of spontaneous decomposition of benzoyl peroxide in benzene. Below 8O"C., the rates of decomposition of benzoyl peroxide in polystyrene are considerably smaller than those in benzene. Two possible explanat,ions of this behavior are presented.

R6sum6 La vitesse de decomposition du peroxyde de benzoyle dans le polystyrkne a @tB mesuree

h, 70,9, 80,l et 89,5"C. La cinetique est. rigoureusemcnt de premier ordre. La vitesse ne depend pas de la concentration, ce qui indique qu'il n'y a pas de decomposition induitc par des radicaux. De 70 6, SOT, la relation d'Arrhenius est k = 1,39 X 101ie-z517W/RT et de 80a 90°C, k = 2,67 X AU del6, de 80°C, temperature de transi- tion de second ordre apparente du polystyrkne, les vitesses de decomposition corre- spondent sensiblement aux vitesses de decomposition spontanee au peroxyde de benzoyle dans le benzene. A moins de 8O"C, les vitesses de decomposition du peroxgde de benzoyle dans le polystyrkne sont plus faibles que celles dans la benzhe. Deux explications pos- sibles sont. presentees.

e-29f i00/RT.

Zusammenfassung Die Geschwindigkeit der Zersetzung von Benzoylperoxyd in Polystyrol nurde bei

70,9, 80,l und 89,5"C gemessen. Sie gehorcht streng dem kinetischen Gesetz erster Ordnung. Es besteht keine Abhangigkeit der Geschwindigkeit von der Konzentration, was dafur spricht, dass keine radikalinduzierte Zersetzung auftritt. Znischen 70 und 80°C gilt die Arrheniusgleichung k = 1,39 x 10'1 e-35i00/RT und zwischen 80 und 9O"C, k = 2,67 X loL3 e--297W/RT. Oberhalf von 80"C, der scheinbaren Umwandlungstempera- tur zweiter Ordnung von Polystyrol, entspricht die Zersetzungsgeschwindigkeit ziemlich genau der Geschwindigkeit der spontanen Zersetzung von Benzoylperoxyd in Benzol. Unterhalb 80°C ist die Zersetzungsgeschwindigkeit von Benzoylperoxyd in Polystyrol betrachtlich kleiner als die in Benzol. Znei moglich? Erkliirungen fur dieses Verhalten werden angegeben.

Received December 22, 1958