578
G. GOLDFINGER, I. SKEIST AND H. MARK
ON T H E MECHANISM U F INHIBITION OF STYRENE POLYMERIZATION G. GOLDFINGER, I. SBEIST',
AND
H. MARK
Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, New Y o r k Received M a y 19, 1948
The action of inhibitors (or stabilizers) for polymerization reactions is of scientific and practical interest. Two questions arise, mainly: (a) how effectively and for how long does a given concentration of inhibitor prevent polymerization of the monomer? and ( b ) what influence has the inhibitor originally added as stabilizer on t'he course of the eventual polymerization and the quality of the polymer? The first' question was dealt with by Foord (2), who showed that for a number of inhibitors of the benzoquinone type the length of the induction period of styrene polymerization is proportional to the amount of inhibitor added. The temperature dependence of the length of the induction period leads to an energy of activation of 28,000 cal. per mole for the initiation reaction, which indicates that the react,ion product of the first step ( E about 25,000 cal. per mole) is responsible for the consumption of the inhibitor. Raff (8) studied the weight (or viscosity) average molecular weights of the polymerization fractions of styrene produced during and a t the end of the induction period and found that while the rate of polymerization increases from zero to its maximum value, the average molecular weight of the polymer formed also increases. The present paper contributes new experiments on the inhibition of styrene polymerization by benzoquinone and attempts to correlate them with a simple theory which ascribes inhibition to two different effects: (a) deactivation of the activated monomer according to a reaction constant k3i and ( b ) termination of a growing polymer chain according to a react,ion constant ks,. (k3i > h.) EXPERIMENTAL
Commercial styrene was twice vacuum distilled and showcd the following characteristics: b.p. (760 mm. Ilg), 145-146°C.; b.p. (18 mm. Hg), 44°C.; density, 25"/4O, 0.9038; n?' = 1.5435. 1.t was mixed with 0.0025 per ccnt to 0.050 per cent (by weight) of benzoquinone and weighed samples of about 20 g. were sealed under nitrogen in glass tubes. In a series of orienting experiments a modification of the bubble method of Gardner and Holdt (cf. 3) was used to give a first estimate of the length of the induction period. Sarrow calibrated tubes cont,aining the monomer-inhibitor mixtures were submersed in thermostatically cont,rollcd bat,hs at 70.0", 100.7", and 130.0"C. The temperatures wcrc kept constant within f 0 . 1 "C. At certain 1 Research Chemist of the Celanese Corporation of America. The authors wish t o express their grotilude to the Research Director of this Corporation for his kind interest and for his assistance.
INHIBITION OF BTYRENE POLYMERIZATION
579
Em# (hourrl FIQ. 1. The amount of polymer produced by various inhibitor concentrations a t 70.0"C.is plotted versus time. The induction periods are proportional t o the inhibitor concentrationa.
TIM# Ihourb)
FIG.2. Same as figure 1, but fol' 100.7"C.
580
Q. GOLDFINGER, I. SKEIST A N D FI. MARK
times each tube was removed from the baths, cooled to 25"C., held in a vertical position, then inverted, and again held in a vertical position. Then the time required for the nitrogen bubble to ascend the distance between two marks on the tube was measured and the average of several individual observations was taken as a quantity proportional to the viscosity of the mixture. The tube was replaced in the bath and the same procedure was repeated after a certain time interval. A plot of the viscosities obtained in this way as ordinate against the time of reaction as abscissa showed a fairly sharp break a t the end of the approximate induction periods.
FIG.3. Same a8 figure 1, but for 13O.O0C.
After having determined in this qualitative way the length of the induction periods under various experimental conditions, the final experiments were carried out as follows: A number of samples were submerged in the thermostats and removed a t certain time intervals. The first set was taken out a t a time slightly shorter than the induction periods as determined by the preliminary runs. The others were removed a t regular time intervals (ranging from 10 min. to 3 hr.) thereafter. The tubes were opened and their contents were poured slowly and undcr stii~inginto methanol. The precipitates were washed, dried, and \\.(sighed. Thc rcsults are plotted on the ordinate in figure 1 for 7O.O0C., in figure 2 fCJr 100.7"C., and in figure 3 for 13OoC., as fraction of monomer polymerized versus time for various concentrations of inhibitor. From these data the overall ratcs of the polymerization reaction were computed,
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G . GOLDFINGER, I. SKEIST AND H. MARK
The polymeric samples obtained in the reaction under various conditions were dissolved in toluene and the viscosities of these solutions measured at various concentrations (0.25 to I .O per cent by weight) a t 25OC. in a CannonFenske viscosimeter. By plotting specific viscosity over concentration versus concentration the intrinsic viscosities at the limiting concentration zero were determined. The results are given in figures 4, 5, and 6 for 70.0°C., 100.7"C., and 130"C., respectively. They are used to compute the viscosity-average
96polymerized FIG.6. Same as figure 4, but for 130.0"C. polymerization degree (according to Flory) of the polymer produced under various conditions. DISCUSSION OF RESULTS
Table 1 shows that thew is a fair proportionality between the length of the induction period and the amount of inhibitor added at all three temperatures and confirms previous observations of Foord. I t can he seen that the inhibitor is used up much more rapidly at higher temperatures, which indicates a considerable activation energy for the formation of the active centers which consume
583
INHIBITION OF STYRENE POLYMERIZATION
the inhibitor. We arrive at a value of 27,000 cal. per mole, as calculated from the three available temperature intervals; it agrees with the order of magnitude as usually observed for the initiation reaction of styrene polymerization. Thus it appears that each active center formed while there is still an appreciable concentration of inhibitor present has no chance to grow out into a chain, but will more probably be deactivated by collision with a molecule of the inhibitor. Only after the conceqtration of the inhibitor has fallen to a sufficiently small value can chain propagation develop and polymerization take place. The overall rate of it incremes as more and more of the residual inhibitor is used up and reaches its full value only some time after the end of the induction period. Figures 4, 5, and 6 show that the viscosity-average molecular weight increases in the time following the induction period; this indicates that the residual inhibitor acts as a chain terminator and as a nucleus deactivator. However,
TABLE 1 Length of inhibition periods at ua~ioustemperatures and inhibator concentrations TEMPERATURE
I N l l l A L INEIBITOR CONCENTRATION I N GRAMS OF INELBITOR PER GRAM OF MIXTURE
L E N 0 1 6 OF INDUCTfON PERIOD I N MlNUTES
LENGTB OF INDUCTION PERIOD IN MINUTES OVEP INIIIAL INEIBITOB CONCBNTMTION
Xlo-6
Xloi
'C.
25
300 612 1320
"1 1224 1056
70.0 70.0 70.0
125
100.7 100.7 100.7
125 250 500
300
82.4 62.8 60.0
130.0 130.0 130.0
500 lo00 UMO
16 30 50
3.2 3.0 2.5
50
78 132
these figures also show that the amount of inhibitor originally added has a distinct influence on the average molecular weight of the polymer eventually formed. The more inhibitor present originally, the smaller is the average molecular weight of the polymer formed after complete removal of the inhibitor. It appears, therefore, that activated styrene and quinone react with each other to form a compound having the capability of terminating growing chains. As one varies the concentration of originally added inhibitor, one proportionally changes the concentration of this compound. More initial inhibitor produces more of this chain-breaking substance and consequently reduces the average molecular weight of the polymer produced also in the somewhat later stages of the reaction. This seems to contribute to the explanation of a fact which is known in commercial practice. If one wants to obtain high average degrees of polymerization and small fractions of low polymeric material and has to use inhibited monomers,
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G. GOLDFINGER, 1. SKEIST AND H. -MARK
it is necessary to remove the inhibitor (by distillation or washing) as completely &s poseible and it is not sufficient to dispose of it by an excess of catalyst or by a higher initial temperature. For the purpose of a more quantitative interpretation of the above results, we propose the following kinetic treatment : It seems appropriate to represent the uninhibited reaction by a sequence of three steps: initiation, propagation, and termination. Several authors (cf., e.g., references 1, 4, 5, 6, 7 , and 9) have recently shown that the normal polymerization of vinyl derivatives can be fairly well represented in this way.. Following this procedure, we obtain the following elementary processes and corresponding rate equations: Initiation of the active centers takes place according to a first-order reaction in respect to the monomer ml (either catalytically or by thermal activation)': Reaction
Rate equation
In the case of styrene polymerization it is known that this step requires an energy of activation of about 25,000 cal. per mole. Propagation or chain growth occurs as a bimolecular process if any active center (monomer m: or polymer mr) collides with a monomer: Reaction
Rate equation3
Termination takes place through mutual saturation of active centers of any kind according to Reaction
Rate equation
In order to include the action of the inhibitor as suggested by our experiments, we propose to introduce two new lermination processes, the deactivation of activated centers m* of any chain length by a collision with the inhibitor (the concentration of which is i ) and their deactivation by the addition product (the concentration of which is a ) of inhibitor and monomer. Altogether we now have three termination processes and consequently shall introduce three rate constants having the first index three: for normal termination, ka; for inhibition, and k3? for retardation. As usual, active centers on the monomer or on a growing chain are characterized by an asterisk. The rate constant of the initiation reaction is denoted by k,, of the propagation by kl, and of the termination by k s . * As usual, we denote here by m * the sum of the concentrations of all active centers including the monomer.
INHIBITION OF STYRENE POLYMERIZATION
585
Taking care of the two above expansions, we arrive at the following five rate equations: Initiation
+ dm* - = klml dt
Propagation
- dml - = ksmlm* dt
(2)
Termination
- dm* -- kSirn*P dt
(3)
- di =hm*i dt
(4)
- $ = ksm*a
(5)
Inhibition Retardation
(1)
Using the steady-state approximation (4. 4) we can compute the stationary concentrations of m* by equalizing the rate of production to the rate of consumption: klml = kStm*'
+ ks%m*i+ ka7rn*a
(6) From equation 6 the steady-state concentration of active centers becomes:
This equation, together with equations 1, 2, and 4, can'be used to compute three quantities, which can be directly compared with the experimental results: (a) the rate of disappearance of the inhibitor, (b) the overall rate of the polymerization, and ( c ) the number-average degree of polymerization. (a) According to equations 4 and 7 , the consumption of the inhibitor is given by
During the inhibition period when no polymer is yet formed and termination and retardation not yet important, k3,i is large as compared with k3,a, and the disappearance of the inhibitor becomes in first approximation
Integrating, we obtain
c
i ( t ) = io- klmlt
(10) which shows that the length of the inhibition period-time at which the concentration i of the inhibitor is zero-is proportional to i ~ the , initial concentra-
586
Q.
GOLDFINGER, I. SKEIST AND H. UARK *
tion of the inhibitor, This is what was found by Foord ahd in the experiments presented in table 1. ( b ) The overall rate of polymerization is obtained by combining equations 2 and 7 :
As long as there is inhibitor present in any appreciable concentration, the term kBCiwill keep the overall rate of polymerization at a very lorn value. While the inhibitor is used up and retarder is formed, the term ka,a becomes more and more predominant in the denominator and the orerall rate of polymerization is in first approximation inversely proportional to the concentration of the retarder a . Figures 1, 2, and 3 show in fact that the curve indicating the total amount of polymerized material &s function of time rises more slou4y after a longer inhibition period than it does after a shorter one. (c) The number-average degree of polymerization is given by the weight of polymer formed over the number of molecules of polymer formed. Since the number of polymer molecules can be counted as the number of nuclei formed, we have
E
kz ml _.-
ksr a
We should, therefore, expect that the number-average degree of polymeiization of the material formed immediately after the induction period is inveisely proportional to the amount of inhibitor originally added. Figures 4, 5 , and 6 show that, in fact, the intrinsic viscosity of the polymer formed a t that time rises more slowly in the cases when more inhibitor was initially added. Thc difference is most noticeable a t 70°C. and becomes less distinct at higher temperatures. This may be due to the fact that at higher temperatures both kl and kJ1 assume larger values and the second twin in the bracket of equation 12 cannot be neglected. SUMMARY
1. The polymerization of styrene was carried out at 70°, loo", and 130°C. with various amounts of benzoquinone as inhibitor. 2. The length of the inhibition period is proportional to the amount of inhibitor originally added. 3. The overall rate of polymerization immediately after the induction period and the number-average degree of polymerization of the polymer formed are
ANION ROTATION IN CRYSTAL LATTICES
587
approximately inversely proportional to the amount of inhibitor originally dded. 4. The activation energy of the initiation reaction is 27,000 cal. per mole. REFERENCES (1) BREITENBACH, J. W.: Monatsh. 71,275 (1938). (2) FOORD, S. G.: J. Chem. SOC.1840,48. (3) GARDNER, H. A., AND SWARD,G. G.: Phyaical and Chemical Examination of Paints, Varnishes, Lacquers and Colors, 9th edition, p. 216. Institute of Paint and Varnish Research, Washington, D. C. (1937). (4) GINNELL, R., AND SIMEA,R.: J. Am. Chem. SOC.66, 706, 715 (1943). (5) MARK,H., AND RAFF,R . : High Polymeric Reactions. Interscience Publishers, New York (1941). (6) MOORE, J. K., BURK,R. E., AND LANKELMA, H. P . : J. Am.Chem. SOC.68, a954 (1941). (7) PRICE,C., AND KELL,R. W.: J. Am. Chem. SOC.89, 2798 (1941). (8) RAFF,R . : Unpublished results. (9) SCHULZ, G, V.: Z. physik. Chem. BS4, 246 (1938).
ANION ROTATION IN CRYSTAL LATTICES OF AzBXr COMPOUNDS M. A. BREDIG Vanadium Corporation of America, New York, New York Received June 9, 1948
In a previous paper (l), the high-temperature alpha modifications of both potassium and sodium sulfates were shown to be isomorphous with glaserite, hexagonal form, observed a t room temperature, of the solid solutions (K,Na)2SOd. However, some evidence in the literature seems to contradict this finding: a comparatively strong thermal effect was found in (K,Na)&304 by R. Nacken (10) and by E. .J&necke (6), in the range between 25 and 75 mole per cent potassium sulfate, with a temperature maximum of 470°C. at the composition KNaS04. This effect was recently confirmed by Perrier and Bellanca (13), and was also observed by the writer. The heat evolution in cooling indicates a transition of some kind in the concentration-temperature range in which the writer had assumed the existence of one crystal phase only, namely, that of hexagonal glaserite (figure 1 of reference 1). The previous investigators also have made the conspicuous observation that both above and below the transition temperature the solid solutions (K,Na)$Op were of hexagonal symmetry. For this apparent discrepancyoccurrence of a thermal transition effect, but no change in crystal symmetry-the following explanation is advanced: It seenis highly probable that we are dealing here with a second-order transition which, by analogy with sodium nitrate ( 8 ) , ammonium chloride, and ammonium bromide (14), is most likely due to the transition from oscillation to rotation (Pauling (12)) of the SO4-- anions.
