JOURNAL OF POLYMER SCIRNCI: \'OL. XX, PAGES 133-136 (1936)

Decomposition of Benzoyl Peroxide in Dilute Solutions

C. A. BARSON and J. C. BE\-INGTON, The Chenzistry Department, The Unirersify, Birmingham 15, England

The mechanisms of the initiation and termination reactions in sensitized radical polymerizations have been studied using 14C-azoisobutyronitrile. l i 2

For this work it was necessary to study the decomposition of the azo com- pound in dilute solutions in lionpolymerizing solvei~ts.~ The initiation processes ill polymerizations sensitized by benzoj 1 peroxide are OH being studied, and in this paper some results obtained in the preliminary examina- tion of the decomposition of the peroxide are presented. Very dilute solutions have been uscd in order to reduce the importance of the radical- induced decomposition, 4 aiid to reproduce the conditioris under n hich the substat ice is used as a sensitizer for polymeriLations; under these circum- stances the use of labeled beiizojl peroxide is essential for accurate analyses for the various possible produrts.

Ta o samples of labeled benzoyl peroxide have been used; in one, all the rarbon atoms were labeled and in the other the labeling was confined to the carhoxyl group. The labeled peroxides were prepared by standard methods from labeled beuzoic acids supplied by Kadiochemical Centre, Amersham, England.

The primary step in the decoiriposition of benzoyl peroxide is the scission of the 0-0 bond to give t\i o benzoate radicals?

(1) C$H, co o o co CJI, - 2c61r, co 0.

These radicals may lose carboil dioxide to give phenyl radicals

CsH, co 0' - CSTI, + co, (2)

It has long been recognized that both benzoate and pheiiyl radicals are present in decoinposing solutions of benzobl peroxide; i t has now been shown that in benzene most of the benzoate radicals lose carbon dioxide before they can enter into other reactions. Figure 1 shows the weights of carbon dioxide produced at various stages in the decomposition a t 60 " of 1 mg. of berizoyl peroxide. In all experiments, the reaction mixture contained initially 1.0 rng. of the peroxide in 2.0 cc. of benzene a t 25" and was completely free from air. The maximum amount of carbon dioxide which could be produced from 1 mg. of the peroxide is 0.364 mg. ; the weight actually formed after about 630 hrs. a t 60" is about 0.33 mg.

133

134 C. A. BAHSON AND J. C . BEVINGTON

Fig. 1. Weight of carbon dioxide produced during the decomposition at 60" of 1 mg. of benzoyl peroxide in benzene.

C 0 2 e v o l v e d (mg.)

Fig. 2. Effect of diphenylpicrylhydrazyl upon the production of carbon dioside during the decomposition at 60" of benzoyl peroxide in benzene.

DECOMPOSITION OF BENZOYL PEROXIDE 135

In benzene, the benzoate radicals that do not lose carbon dioxide may be

(3)

converted to benzoic acid: CGHS.CO 0. + CsHs - CsT& CO OII + CsHs.

or to phenyl benzoate by combination with a phenyl radical CeH5.C0 0. + C6H5. - C& C0.0.C6H5 (4)

Analyses for benzoic acid and phenyl benzoate have been performed a t various stages in the decomposition ; small quantities of both substances are produced.

The quantity of carbon dioxide produced is depressed if the peroxide is decomposed in the presence of certain reactive substances, e.g., diphenyl- picrylhydrazyl, styrene, and cumene. Solutions of 1 .0 mg. of 14C-benzoyl peroxide and various weights of diphenylpicrylhydrazyl in 2.0 cc. of benzene were kept a t 60" for 96 hrs.; the results of analyses for carbon dioxide are shown in Figure 2. In all cases the hydrazyl was in excess and some remained at the end of the reaction. The results show that the hydrazyl can react with benzoate radicals preventing the liberation of carbon dioxide, but that a t fairly low concentrations of the hydrazyl the radicals can lose carbon dioxide before they react.

If benzoyl peroxide labeled with carbon-14 on the carboxyl carbon atom is used to initiate the polyrnerizatiorl of styrene at 60", the resulting polymers contain radioactive carbon; this shows that the monomer can capture benzoate radicals before they can decompose. From the molecular weights and the activities of the polymers, it is concluded that of the radi- cals which initiate polymerization, 85% are benzoate radicals and 15% are phenyl radicals. The same conclusion has been reached from experiments involving styrene and benzq 1 peroxide uniformly labeled with carbon-14.

If cumene is used as solvent in place of benzene, the yield of carbon dioxide is reduced. An air-free solution of 1.0 mg. of 14C-benzoyl peroxide in 2.0 cc. of cumene was kept a t 60" for 96 hrs.; the weight of carbon dioxide found was 0.12 mg., nhereas in a similar experiment with benzene as solvent 0.19 mg. of carbon dioxide was formed. The molecule of cumene contains a hydrogen atom that is easily abstracted by free radicals,6 and it is concluded that in pure cumene the rate of the abstraction reaction:

c & . c o 0. + CsH, C(CH3)pH - CjHj.CO OH + C,fI,.C(CH,). ( 5 )

is comparable with the rate of decomposition of the benzoate radical according to equation ( 2 ) .

The results reported here show that isotopic techniques can be used in quantitative studies of the reactivities of various compounds toward benzoate radicals. The reactions of the benzoate radicals that do not lose carbon dioxide and of the phenyl radicals can be traced by using labeled benzoyl peroxide. Details of the experimental techniques and the results of further experiments will be published later.

Dunlop Rubber Company for financial assistance to C. A. B. The authors thank Professor I f . W. Melville, F. R. S., for his interest in this work, and

136 C. A. BARSON AND .I. C. BEVINGTON

References

1. J. C. Bevington, H. W. Melville, and R. P. Taylor, J . Polymer Sci., 12, 449 (1954.);

2. J. C. Bevington, Trans. Paraday Soc., 51, 1392 (195.5). 3. J . C. Bevington, J. Chem. Soc., 1954,3707. 4. K. Noxaki and P. D. Bartlett, J . Am. Chern. Soc., 68,1686 (19,166). 5. D. 11. IIey and W. A. Waters, Chem. Revs., 21, 169 (1937). 6. R. A. Gregg and F. R. Mayo, Disc. Furuday Soc., 2, 328 (1947).

14, 463 (1934).

Synopsis

A study of the decomposition of benzoyl peroxide in very dilute solutions is reported. This work is preliminary to a detailed investigation of the mechanism by which the per- oxide initiates radical polymerizations; “C-benzoyl peroxide has been used. In dilute solutions in benzene, almost all the benzoate radicals generated from benzoyl peroxide lose carbon dioxide before they can enter into other reactions; various substances e.g., diphenylpicrylhydrazyl, styrene, and niimene, which can react with free radicals, de- crease the yield of carbon dioxide.

RdsumC

On a 6tudi6 la d6composition du peroxyde de henzoyle en solution trhs diluCe. Ce travail constitue le dChut. d’une Ctude dCtaillte du nGcanisme suivant lequel les perox- ydes iriitient les polymCrisations radicalaires. On a utilisC du peroxyde de berizoyle tnarquC 1 C . En solution diluCe d a m le henzhne, tous les radicaux benzoates issus du peroxyde de benzoyle perdent I’anhydride carbonique avant d’entamer une autre rC- action. De nombreuses substances telles le diphbnylpicrylhydrazyl, le styrkne et le cuinbne, qui sorit toutes capables de rkagir avec des radicaux libres, diininuent la production d’acide carbonique.

Zusammenfassung

Ks wird eine Untersuchung des Abbaus yon Benzoylperoxyd in sehr verdunnten Ws- urigen berichtet. Diese Arheit ist prsliminar zu einer genauen Untersuchung des Mechanismus der Initiation von Radikalpolyrrierisationen durch das Peroxyd; es nurde ‘4C-Benzoylperoxyd verwendet. In verdiinnten Lijsungen von Berizol verlieren fast alle Benzoatradikale, die aus Benzoylperoxyd hervorgegangen sind, Kohlendioxyd, bevor sie andere Reaktionen eingehen kijnneri; verschiedene Substanzen, 2.B. Di- phenylpikrylhydrazyl, Styrol und Curnol. welche mit freien Radikalen reagieren konneri, vertnindern die Ausbeute an Kohlendioxyd.

Received December 28, 1955