Summary 1. A iiiechanism for the polymerization of urea-formaldehyde resins, based on the view that urea is an amino acid amide has been suggested. The polymerization is postulated as proceeding in two stages: first, the formation of trimethylenetriamine rings from methylene urea or the monomeric methylene methylol urea, and then the formation of methylene-bis-amide links to tie the rings together to form a cross-linked molecule. 2. As support for this theory, resins have been prepared from the reaction of glycinamide and
e-aminocaproamide with formaldehyt le. The composition of these resins indicates that they are composed of trimethylenetrianiine rings linked by methylene bridges between amide groups on different rings. 3. The trimers of the methylene-imine derivative of glycine methylamide and urethan have been prepared. 4. Sarcosinamide appears to react with formaldehyde to give a low molecular weight linear polymer. URBANA,ILLINOIS
KFCEI\.F.I)
IC'(JNIR1BCI'IOS FROM THE CCJNVERSE M E M O R I A L I,AHI)R,4IIIK\ OF f 1 A R V A R D UNIVERSITY
AI.,$\ 22, 1946
1
The Kinetics of Decomposition of Benzoyl Peroxide in Solvents. I BY KENZIENOZAKI'AND PAULD. BARTLETT Introduction.-Despite a considerable amount of able investigation of the thermal decomposition of acyl peroxides,2 one encounters many unanswered questions in the attempt to deal kinetically with peroxide-induced polymerization. Perhaps the simplest of these questions concerns the kinetic order of the decomposition. This decomposition is so nearly unimolecular that it is usually treated as but its rate definitely varies with c o n c e n t r a t i ~ n . ~ Brown6 ,~ showed that this variation could be accounted for on the basis of concurrent first- and second-order decompositions, and he assigned velocity constants to these processes. In view of our observation6that the decomposition of benzoyl peroxide can be strongly induced by free radicals, it seems likely that such decomposition is induced in part by the radicals normally present in a solution of decomposing benzoyl peroxide. The: result of this would be a reaction of higher order accompanying the unimolecular decomposition. All the observed decomposition products of benzoyl peroxide can be fitted into this scheme, the following example showing how one of these products, phenyl benzoate, might be formed as a product of the chain reaction. Every other product of the decomposition can be formulated as arising in a more or less similar fashion. CsH6CO-OOCCeHs zceHsco&
---t
----f
2ceH5Co-
COz f CsHjCOOCeH,
coo- -k CeH6C00-00CCeH~ +
(302 f CeHbCOOCaH, f CoHbCO-
(1) (2)
(3)
In general the benzoate, radicals will also attack the solvent, as they are known to do in the case of I ) Pittsburgh Plate Glass Fellow. (1) For historical references see McClure, Robertson and Cuthhertson. Cti?,..I, Research, 2OB, 103-113 (1942). ( 3 ) Karnennk:iya and Medvedev, Acta Physicochim., U.R . S . S . , f
13, 565 ( 1 9 4 l J i 4 ) B a r t l e t t and Altrchul, THISJ O U H X A L ,67, 816 (194:) f.7) D J . Brown. zbid , 62, 26.57 (1940) (ti) Bartlett and Nozaki. i h i d 63,in press (1946)
b e n ~ e n e . ~If such attack (chain transfer to the solvent) results in new free radicals which are more stable and less reactive than the benzoate radicals, the effect of the solvent should be to suppress the chain decomposition shown in equation 3. If, on the other hand, chain transfer t o the solvent yields new radicals comparable in activity to the old, this process will affect only the products and not the kinetics of the over-all reaction. The special case in which chain transfer to the solvent results in a change in the chain-terminating reaction will be dealt with in a forthcoming paper. In order t o test the reality of this picture of the decomposition of benzoyl peroxide, we have first established that a part of the decomposition of benzoyl peroxide in common solvents is of chain character (a) by showing that it responds to inhibitors, and (b) by inducing it with known free radicals. Next a survey has been made of the over-all rates of decomposition of benzoyl peroxide in a series of solvents. Finally, a kinetic equation derived from the chain mechanism has been rigorously tested in a number of solvents and velocity constants determined for the spontaneous and induced parts of the decomposition. The Effect of Inhibitors on Benzoyl Peroxide Decomposition.-If the decomposition of ben zoyl peroxide can be induced by free radicals, the addition of inhibitors for radical chain reactions t o benzoyl peroxide solutions should result in a decreased rate of decomposition. It is evident from Table I that such is the case. Oxygen, hydroquinone, p-t-butylcatechol, m dinitrobenzene and picric acid are all inhibitors for the reaction. This evidence not only supports induced decomposition but also renders unlikely Price's suggestionsb that the recombination of two benzoate radicals t o reform benzoyl peroxide is ( 7 ) Gelissen and Hermans, Ber., 69, 662 (1926); Wieland, Popper and Srefried,i b i d . , 65, 1816 (1922); Hey. J . Chein. Soc., 19fi6 (1934) ( 8 ) fa) Price, A n n . N . Y . A c a d S c r . . 44, 351 (19433: ih) ibid , p . 36.5; (c) Matheson. J . Chem. Phys 13, 584 (194.5).
KINBTICS 08' DECOMPOSITION 01' BENZOYL ~RUXIDE
Sept., 1!346
TABLE I THE EFFbCT
OF' INHIBITORS ON THE DECOMPOSITION O F
BENZOYL PEROXIDE I N
ACETICANHYDRIDE AT 79.8'
Inhibitor
Concn., m./l.
None Air Oxygen Hydroquinone p-t-Butylcatechol nz-Dinitrobenzene Picric acid
..
.. .. 0.21 .21 .21 .21
% Dec. after 6.25 hr.
92.7 89.8 82 5 79.0 67.5 58.8 48.0
an important reaction in non-reactive solvents. I t also becomes probable that the interaction of the two benzoate radicals as a result of the "cage eff ect"8e leads chiefly to decomposition products rather than to regeneration of peroxide. The Effect of Added Free Radicals on Benzoyl Peroxide Decomposition.-When hexaphenylethane, pentaphenylethane or tetraphenylhydrazine was added to reaction mixtures, the rate of benzoyl peroxide decomposition was increased considerably (Table 11). The great efficiency of TABLE I1 THE DECOMPOSITION OF BENZOYLPEROXIDE AT 79 8" BENZENE( ( B Z ~ O = ~ )0.197 ~ M) Added Concn. % Dec. of BzrOt substances
m./L
None (CbHdXC(CaHJo 0.206 (CsHd&CH(CeW2 .206 (C&I&NN(C&JH~)~ .208
TABLE I11 THE DECOMPOSITTON OF BENZOYLPEROXIDE fir 79.8" Solvent
((BzzO2)o = 0.20 M )
IX
10 Min.
1 Hr.
4 Hr
50.4 84.5 68.7
47.6
15.5 58.8 32.1 94 0
tetraphenylhydrazine 4s a decomposition accelerator may be rlelated to its being an amine, and not due entirely to free radicals (see Table 111). The Decomposition of Benzoyl Peroxide in Different Solvents at 79.8'.-The rate of decomposition has been measured in over thirty different solvents and the data are summarized in Table 111. The results are listed as percentage decompositior! of a 0.197 M solution of benzoyl peroxide after heating a t 79.8' for the given time interval. All solvents were freshly distilled before use and the measurements were made under oxygen-free conditions. By way of a rough generalization, the order of increasing rates of decomposition appears to be as follows : highly halogenated solvents < most aromatics < m x t aliphatics < ethers, alcohols, monohydric phenols < amines. I n this paper studies on the decomposition of benzoyl peroxide in only the first three groups will be reported. Despite the high position of phenols as accelerating solvents, dihydric phenols such as hydroquinone and t-butylcatechol, which can lose a hydrogen atom to yield a stabilized semiquinone radical, are inhibitory, as is the highly nitrated picric acid.
1 (is7
10
Min.
Per cent. decomposition 30 1 2 Min. Hr. Hr.
Tetrachloroethylene 13.0 Carbon tetrachloride 13.5 Cyclohexeiie 14.0 Methyl benzoate 14.5 Anisole 14.0 Chloroform 14.5 Ethylhcnzene 15.0 Chlorobenzene 18.0 Nitrobenzene 15.5 Benzene 15.5 Toluene 17.4 Allyl acetate 17.0 Styrene 19.0 Cuinene 20.0 Iodobenzene 18.0 Carbon disulfide 19.0 32.6 Ethyl iodide 23.4 Methylene chloride 24.5 Ethyl chloride 26.0 Rromobenzene 26.3 28.5 t-Butylbenzene Acetone 28.5 43.0 20.3 Maleic anhydride Ethyl broniidc 33.6 Allyl bromide 37.2 69.0 3 2 . 5 48.5 64.0 Acetic anhydride Cyclohexane 34.0 51.0 Ethyl acetate 53.5 Acetic acid 59.3 Pyridine 57.0 77.3 Dioxane 67.6 82.4 75.2 Diethyl ether 82.2 Ethyl alcohol 87.7 m-Cresol Aniline, triethylamine, etc. Explosive reaction
4
Hr.
35.0 40.0 39.5 41.4 43.0 43.7 15.5 48.5 49.0 .io. 4
19.5 52.3
.. 53.3 55.8
..
61.2 62.2 64.7 t19.0 (i9 , -5
71 .s
84.8 85.2 87.4
Derivation of the Kinetic Equations. -The simplest kinetic equations for induced deconiposition of benzoyl peroxide are those which may be derived by assuming equations 1-3 or their equivalent. Equations 1-3 may be writteri ki P+2K k, 2 K --f RK ka K+I'+X+R
where P denotes peroxide, R any free radical and X the product or products of the chain decomposition. There is no kz in this scheme because we reserve this constant for the chain-propagating step in olefin polymerization. The concentration of free radicals a t the steady state is expressed by making the usual approximation dK/dt = klP
- k3R2E 0
R - d V 7 G
1688
Vol. 68
KENZIENOZAKI AND PAULD. BARTLETT '
The same equation would apply if kt ,had the significance of the rate constant for a benzoate radical's diffusing out of contact with its partner.
/I
/ "
I
0
5
I
10
15
Lffi Fig. 1 . -Benzoyl peroxide decomposition in solvents a t 79.8': E3, carbon tetrachloride (X = 8) ; 0-,t-butyl benzene (X = 6); 0 , ethyl iodide (X = 4 ) ; 0, cyclohesene toluene (X = 0 ) ; (X = 2 ) ; 6 ,benzene (-U = 0 ) ; -0, and 0 ,nitrobenzene ( X = 0).
The rate of decomposition of peroxide is then
0
5
10
15
Vdi% Fig. 2.-Benzoyl peroxide decomposition in solvents a t 79.8': 8 , ethyl acetate ( X = 8); a,acetic acid (X = 6) ; -0, cyclohexane ( X = 4); ?, acetic anhydride ( X = 2 ) ; and 0 acetic anhydride reaction products ( X = 0).
+
This eqiiation may be integrated to give
where a = kl//ki. The same equation can be derived on the assumption that every radical from the peroxide attacks the solvent, yielding a new radical by chain transfer which in turn may induce the decomposition of a peroxide molecule. Certain intermediate cases lead to more complicated kinetic equations. A description of the peroxide decomposition in terms of the "cage effect"& introduces further details of mechanism, but leads to a differential equation of the same form under certain assumptions. Thus, if all the benzoate radicals formed by dissociation of benzoyl peroxide either (a) react together as the original pairs with rate constant k, or (b) become separated by chain transfer to the solvent (with rate constant kt) then the peroxide disappears by the equation --dP/dt = k l P where ki = kl v / k l k t / k 4 ( k , k,)
+
+ kip'/%
The value of the ratio 4 in any solvent may be determined experimentally by using data from two runs with different initial concentrations of peroxide and with samples taken at the same time intervals in the two runs. After the same time, the value of the right-hand member of equation 4 is the same for both runs, and the logarithmic terms may be equated. If PI and PZare the peroxide concentrations a t equal times "in the two runs, one obtains
which may be converted to the form 1
c
c-I
z=z2+a From a plot of l/.\/Fl against 1/.\/% a straight line should be obtained from whose slope and Yintercept a may be calculated. Rate Measurements a t 79.8'.-Kinetic nieasurements of benzoyl peroxide decomposition in eleven of the solvents listed in Table I11 have been carried out under oxygen-free conditions and the equations derived above have been applied to them. The experimental results in
KINETICSOF DECOMPOSITION OF BENZOYL PEROXIDE
Sept., 10.18
THE
TABLE IV DECOMPOSITION O F BENZOYL PEROXIDE I N BESZENEBT 79.8"
Run 1
1 -
P
hr.
M/L
dF
0.01)
0.197
2.25 2.45 2.75 3.20 3.77 4.53 6.00 7.90
1,
1. 00 2.2.5 4.00 6.00 8.25 12.00 l i i . 25
.166 ,131 ,0975 ,0703 .0485 .(I277 ,0159
a
In-
P M/L
d P
t , hr.
1.004 1.058 1.135 1.241 1.361 1.500 1.723 1.953
0.00 1.00 2.25 4.00 6.00 8.25 12.00 10.25
ki, ( M I L )- ' / a
kI. hr.?
a
Carbon tetrachloride 0.563 Benzene ,767 Toluene .767 Nitrobenzene . 767 ,214 ,t-Butylberizene Cyclohexene .4t7 . 593 Ethyl iodide ,195 Cyclohexane Ethyl acetate ,245 Acetic acid ,159 Acetic anhydride ,245 .Acetic anhydride -t reaction products .753
Run 2
+
