338
J. 0. THOMPSON
(16) SOLLNER, K.,A K D GREGOR, H. P.: J. Am. Chem. SOC.87,346 (1945). K , A N D GREGOR,H. P . : J . Phys. Chem. 60,170 (1946). (17) SOLLNER, (18) SOLLNER, K.,A N D GREGOR,H. P.: J. Phys. & Colloid Chem. 61, 299 (1947). K., A N D GREGOR, H . P . : J. Phys. & Colloid Chem. 64, 325 (1950). (19) SOLLNER, (20) SOLLNER,K.,A N D XEIHOF,R. A , : J. Phys. & Colloid Chem. 64, 157 (1950).
SEDIMENTATIOS EQUILIBRIA OF POLYDISPERSE S O S I D E A L SOLUTES. I11 THE DEGR.4DATION
OF
POLYSTYRENE
IN
SOLUTIOS’
J. 0. THOMPSON 8 Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received March 88, 184.9 I. IKTRODUCTIOK
The molecular size distribution existing in polymeric systems at equilibrium may be derived by statistical thermodynamics. This has been done for vinyltype systems by Tobolsky (12), who assumes that simultaneous polymerization and degradation may exist under certain conditions. Flory’s derivation (3), specifically for condensation systems, yields a similar result, but with reference to this result he states: “Vinyl type polymerizations could be included, although these processes generally are not reversible in the thermodynamic sense.” The attainment of thermodynamic equilibrium in vinyl systems under various conditions of heat, light, and catalyst has been suggested (6, 7, 9, 11). The dcpolymerization reaction was assumed to proceed by way of a radical chain mechanism and to compete with the polymerization reaction. On the basis of this simultaneous polymerization and depolymerization thermodynamics is introduced to determine the equilibrium distribution of molecular sizes. This equilibrium distribution turns out to be such that it may be regarded as being the result of a random scission process on an “infinite” linear polymer. For this size distribution the ratio of the -numbwaverage, weight-average, and z average molecular mights, approaches 1: 2 : 3 as the fraction of parent molecules decreases (4, 8, 10). It is the purpose of this paper to describe an investigation into the mechanism of attainment of this so-called equilibrium, and to show that the apparent approach t o a steady-state relative viscosity may not be construed as indicating an approach to equilibrium conditions. Some preliminary experiments (Section 111)
a,,:aw:Mz,
More complete details of this work are t o be found in the thesis submitted by J. 0. Thompson t o the Faculty of the Cniversity of Wisconsin in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1948. This work was supported in part by a grant from the Wisconsin Alumni Research Foundstion. Present address: The Institute of Paper Chemistry, Appleton, Wisconsin.
DEGRADATION O F POLTSTYREXE IS SOLUTION
339
are described which were performed with the view to clarifying the picture as to the effectiveness of each of the factors heat, light, air, and benzoyl peroxide in the degradation of polystyrene in toluene solution. A larger number of more detailed experiments concerned with the degradation of polystyrene in solution by the action of benzoyl peroxide were also performed (Section IV). In all of the experiments to be described no monostyrene was initially present, and the main effect observed was a random degradation of the polymer (polystyrene in various solvents) regardless of initial molecular weight. 11. E X P E R I M E N T l L DETAILS
All polystyrenes used in this investigation mere prepared in the oil phase in the presence of benzoyl peroxide as catalyst. The polymerizations were carried out in sealed tubes under nitrogen with dried vacuum-distilled styrene which had been freed of inhibitor by multiple washing with dilute sodium hydroxide. The polymer was removed from unreacted monomer by solution in benzene and triple precipitation with an excess of methanol. The final product was obtained by solution in benzene, shell-freezing, and sublimation. Solutions for degradation study were made up in Merck reagent-grade redistilled solvents. The benzoyl peroxide used was Eastman White Label grade, m.p. 103-104”C., and contained 100 per cent benzoyl peroxide by iodometric titration. The progress of polystyrene degradations in the preliminary experiments was followed in reversible viscometers of the type described by Mesrobian and Tobolsky (6). I n these viscometers the change in relative viscosity of the polymer solutions with time could be followed without opening the vessels for the extraction of samples. The flow time for toluene a t 30°C. was of the order of 35 sec. The more detailed degradation experiments were performed in sealed tubes, and the degraded polystyrenes were isolated from the reaction mixtures by precipitation with four volumes of methanol followed by solution in benzene, shell-freezing, and sublimation under vacuum. I n all of the experiments where air (oxygen) was to be excluded the vessels were flushed with dry nitrogen before sealing. Intrinsic viscosities were obtained from measurements made on benzene solutions at 30°C. by using standard extrapolation methods. The instrument used was an Ostwald-type viscometer of flow time 179 sec. for benzene. All solvent ujed was thiophene-free and redistilled. Viscosity-average molecular weights, M,, were obtained from a composite viscosity-molecular weight relation of Wales et al. (14). Osmotic pressure measurements were made on butanone solutions of the polystyrenes with a glass osmometer of the Zimm and Myerson (15) design. Membranes were prepared from du Pont 600 P.T. nonnaterproofed cellophane by treating with 30 per cent potassium hydroxide a t room temperature for 18 hr. The “period of half-life” of the osmometer for butanone at 30°C. vias 1 hr. The “semi-static” method of measurement was used throughout this work. Sedimentation equilibrium experiments were performed in butanone with a small Svedberg equilibrium ultracentrifuge with direct motor drive. The methods
340
J . 0. THOMPSON
employed in the calculation of the various molecular weight averages have been fully described elsewhere (13, 14). 111. PRELIYIX’ARY 0BSERV.LTIOh-S
The following preliminary results lyere obtained from experiments with toluene solutions of polystyrene : (1) In the complete absence of air (oxygen) and benzoyl peroxide, heating a t 100°C. for six months results in no detectable degradation. Supplementary intrinsic viscosity measurements made on polystyrenes jl-hich had been stored in benzene solution in air at room temperature for two years showed no evidence of degradation. (2) f n the absence of oxygen and benzoyl peroxide, ultraviolet light (\ > 3000 A,), though producing a blue fluorescence, produces no detectable chain scissioning even \\-hen maintained for one month at 80°C. ARROWS I N O I C I T E P E R O X I D E ADOITIOHS (GRAMS) 0.1
7.0
4.0 1
0
1
1
1
1
1
loa0
500
1
1
1
1
1
1500
TIME [HOURS)
FIG.1. Plot of relative viscosity against time for degradation experiment 3R; peroxide additions are indicated by the arrows (see table 8). (3) Heating at 100OC. in the presence of oxygen or benzoyl peroxide produces degradation, as indicated by a decrease in relative viscosity. For each addition of air or peroxide the relative viscosity remains constant after approximately 24 hr. (4) When repeated additions of benzoyl peroxide are made, though the decrements in relative viscosity decrease 11-ith each successive equal addition, the value of the relative viscosity has not reached a constant value even after the addition of 140 per cent by weight of peroxide has been made. One complicating factor here certainly is that the reaction mixture is continually changing as the peroxide additions are made. Initially there is present only polystyrene and toluene, but as peroxide additions are made and the degradation proceeds the decomposition products of the peroxide itself are accumulating. These latter products almost certainly interfere n-ith the scissioning ability of subsequent peroxide additions. This behavior is shown graphically for one case in figure 1. IV. BENZOYL PEROXIDE D E G R A D A T I O S O F POLTSTTREKE
-4. General experiments The preliminary experiments suggested that the degradation of polystyrene in solution depends on some sort of oxidation. S o steady-state relative viscosity
341
DEGRADATIOS O F POLYSTYRESE I S SOLUTIOS
was observed in these experiments. Further investigations into the degradation of the polymer were then conducted and the data are collected in a number of tables to facilitate discussion. Some physical constants for the polystyrenes used as starting materials are contained in table 1. TABLE 1 Collected physical constants for original polystyrenes*
~~,~~~ I
'C.
A
.
. .
I
C . .. Et ,. 2A-W. , . . . I 5-w. . . . 8-w.. . . . . . H ,... .
~
60 60 60
'
~
100
0.82 1.40 0.97 1.51 0.75 0.18 0.88
i
1 ~
202 456 267 507 185 30
' ~
~
i
109 226 144 400 155
219 459
I
I
i 320 (833)l
,
390
i ~
705
~
1
m:;,p
0.2 0.04 0.13
i I
'
cLnt
48.7 17.0 18.5
I I
145
0.025
,,
33.5
* Molecular weight data were obtained by using viscosity (GJ, osmotic pressure
(-v,,),
or sedimentation equilibrium methods. t This sample has been previously described (14); the fractions 2.4-W, 5-W, and 8-W were prepared from i t . t. The nonideality correction was so large for this sample that the values for M , and M,,l are doubtful (14). .II, would be expected to be about 700 X 103. TABLE 2 Reproducibility of degradation ezperiments Starting material, polystyrene E , [VI = 0.97 ( c j . table 1 ) ; 100°C. i 0,5'; 0.5 g. polystyrene in 11 ml. toluene; 0.1 g. benzoyl peroxide added; reaction time, 18 h r . (to constant relative viscosity)
x
10-1
EXPEPIXEST ~
I______,______#______
D-1A , , . . . , D-1B . , . . , . . # D-lC*.. . , . . D-1D.. . . .
0.77 0.ii 0,ii 0.76
190 190 190 187
'
P E R CENT RECOVERY OF DEGRADED PRODUCT
-I
102
201
1
297
362
~
I
99 97 96 96
* The toluene used as solvent here was washed with concentrated sulfuric acid, then with water, dried, and fractionally distilled. First of all, it is necessary to have some idea of the reproducibility of the degradation reaction as indicated by intrinsic viscosity measurements. Data for this purpose (table 2) are seen to be entirely satisfactory. The effects of variations in polymer and peroxide concentrations at 100°C. are illustrated by the data of table 3. The extent of degradation, as indicated by the decrease in intrinsic viscosity, depends on the concentrations of both polymer and peroxide. An increase in the concentration of either favors degradation. I n comparable experiments where the respective weights of polymer and peroxide
342
J. 0 . THOMPSON
are the same, the extent of the degradation decreases as the amount of solvent is increased. These results suggest that both polymer and solvent are competing for the peroxide. Degradation experiments were carried out with unfractionated polystyrene E to study the effects of method of peroxide addition (table 4), of temperature (table 5), and of nature of solvent (table 6 ) on the scission efficiency of the reactant. The data show that the amount of detectable scissioning produced by a given weight of peroxide depends upon the mode of addition of the reagent. The lower efficiency when the peroxide is added in one amount is not surprising, TABLE 3 Influence of reaction mixture composition on the scissioning eficiency of benzoyl peroxide at IOO°C.
A. Starting material, polystyrene E ( c f . table 1)
1 D-1A D-2A D-3A D-4A
I 1
mi
11 11 2i5
1
grams
1
0 5 0 5
~
‘3:
1 1 ~
grams
ll;
1
1
0 1 0 2 ~
0 77 0 72 083 0 71 I
1
per cent
190 178 220 175
~
1 1
102 201 95 120 I 95 1
362
1
1
98
I
B. Starting material, polystyrene C (6.table 1)
......i
D-7 11 D-8 . . . . . . I 11 D-16. . . , . 4.3* D-17. . . . .! 4.3 D-18. , . . . ’ 2.17* D-19. . , . 2.17.
.I
1
0 67 0 65 0 61
1 ’
162 155 1 140 0 7 0 ‘ 170 I ‘ 0 5 8 ’ 132 0 49 102
I
‘
1
I
87 84
75 90 ~
64 1
185 172
288
356
342
500
~
98 99 99 100
* Benzene as solvent. since the extent of peroxide “wastage” (2) increases with peroxide concentration. Temperature has only a slight effect on scissioning efficiency. I t will be observed that the nature of the solvent also plays a part in the degradation reaction. From the data in table 6 it can be seen that the extents of degradation in the solvents benzene and carbon tetrachloride are the same, and are greater than that which results when toluene is used as solvent. If a reversible polymerization-depolymerization process involving a radical chain mechanism were occurring, it would be expected that the molecular weights of the products would be related to the chain transfer constants of the respective solvents ( 5 ) . This is clearly not the case,
343
DEGRADATION OF POLYSTYRENE IN SOLUTION
To determine the influence of a readily oxidizable substance on the scissioning ability of benzoyl peroxide two degradation experiments were performed in the presence of hydroquinone. The results of these experiments are given in table 7 . TABLE 4 Influence of method of peroxide addition o n the scissioning eficiency of benzoyl perozide at 100°C. Starting material, polystyrene E ( c f . table 1) EXPERMNT
11
x
10-
~
-
MZTEOD OF ADDITION’
Mn
I
RECOVERY
OF DEGRADED
Mn
PRODUCT
per cent
I____-___ pef ccn1
D-6A TABLE 6 Influence of solvent nature o n the scissioning eficiency of benzoyl peroxide at 100°C. Starting material, polystyrene E ( c f . table 1); 0.1 g. peroxide added: concentration, 0.5 g. polymer in 11 ml. solvent EXPEPMNI
D -4 D-1A
D-5
1
-i
I
-
M,
SOLVENT
x
10-2
RECOVERY OF DEOPADED
PRODUCT
per cent
Toluene Benzene Carbon tetrachloride
1 0 7 7 0 61 0 61
1
190 140 140
98 90 97
It is seen that the hydroquinone competes for and is preferentially attacked by the peroxide. It cannot be stated with certainty that these results in themselves rule out a chain mechanism, but a t the same time they are in agreement with what would be expected on the basis of a simple oxidative mechanism. I n the attempt to obtain an equilibrium polystyrene mixture such as has been
344
J. 0. THOMPSON
suggested elsewhere (6,7), a number of degradation experiments were performed in which large percentages of benzoyl peroxide were added as degradation “catalyst” over an extended period of time. Both fractionated and unfractionated polystyrenes were used in these experiments. The concentration in each case vias 0.5 g. polymer in 11 ml. toluene. The total amount of benzoyl peroxide added and the methods of addition were as follows: Method (a)
5
x
0.1 g.; 1 x 0 . 2 g.
Method (b)
5
x
0.1 g.; 6
x
Total weight of peroxide = 0.7 g. Weight per cent of peroxide = 140 Reaction period = 12 days
0.2 g.
Total weight of peroxide = 1.5 g. Weight per cent of peroxide = 300 Reaction period = 20 days
I n all cases the minimum reaction period between peroxide additions \vas 48 hr., and the reaction vessel was purged with dry nitrogen after each addition TABLE 7 InJluence of added hydroquinone o n the scissioning eficiency of peroxide at IOO’C. in toluene Starting material, polystyrene H ( c f . table 1); concentration, 1.6 g. polymer in 10 ml. toluene ~
EXPERIMENT ~
D-56., . , . . . . . . , . , . , , . D-61’. . . . . . . . . . . . . . . . .! D-62*. . , , , . , . , . . . . , . . ’ ~
MOLES EYDBOQUINOSE
XOLES BEXZOYLPEROXIDE
0 0.3 0.6
’
-
[?I 0.61
~
~
Mn
102,000
0.76 0.82
* These reaction mixtures became deep red in color as the degradation proceeded, because of the formation of the quinoid structure. of peroxide and before sealing. The results of these experiments are given in table 8. The values of the intrinsic viscosities obtained for the degradation products demonstrate that equilibrium mixtures were not obtained even after 300 per cent by weight of benzoyl peroxide had been added. X similar experiment was performed on polystyrene of low molecular weight to test the validity of the assumption that both aggregative and disaggregative processes occur to yield the “steady-state” viscosity when polystyrene solutions are treated with benzoyl peroxide. If the assumption is to be acceptable, then it should be possible to demonstrate a net aggregative reaction by starting with a polystyrene having a viscosity well below that of the “steady-state” value, i.e., 3.0 ( 7 ) . Fraction 8W with an intrinsic viscosity of 0.18 was selected as the starting material. The viscosity-average molecular weight was 30,000, and the relative viscosity a t a concentration of 4 g./lOO ml. was 1.8. After the reaction the intrinsic viscosity was 0.17.
345
DEGRADATION OF POLYSTYRENE Ii% SOLUTIOK
Thus, the results from this group of experiments do not support the view that an equilibrium polystyrene mixture is obtained when a polystyrene solution is heated with even as much as 300 per cent of peroxide as catalyst. Indeed, the experiments suggest that the scissioning process may be caused to continue by peroxide addition until all the polymer as such has lost its identity. Degradation experiments were performed on unfractionated polymers having values between 100,000and 250,000. Predicted and observed number-average molecular weights for degraded polystyrenes have been collected to form table 9. The results show that vithin the molecular range covered (at least) the scissioning ability of benzoyl peroxide is independent of the molecular weight of the original polymer.
a,,
TABLE 8 Protracted degradations of polystyrene in toluene I
PEROXIDE ADDITION
EXPERrnSI
1
TEIPERATURE
__ 9cr ccml
I
Polystyrene C : D - 1 1 . .. . . . D-12. . . . . I Fraction 5W: 3R... . . . . . . . . Fraction 2A-W: 1 D-14 D-15 Fraction 8W. 8R 1
~
(b) (a 1 0 1 g added (b 1
(c)
1.40 0.60 0.44 0.75 100 0.45 1.51 100 1 00 100 0 38 0 18 111 0 17
456 138 89 185 89 507 280 72 30
226 75 48 155 61 400
459
*
82
1
(833) 161
,
134 230 114 136
169
96 97 186 88
~i
100
* M , expected t o be about 41,000. Note ratios of other moments. (a) Concentration, 0.5 g. polymer in 11 ml. toluene. Five additions of 0.1 g. peroxide spaced a t least 48 hr. apart; final addition of 0.2 g . peroxide. Weight per cent peroxide added (on polymer basis), 140 per cent. Total reaction period, 12 days. (b) Same as (a) except that five additions of 0.2 g. peroxide were made after the five additions of 0.1 g. Total weight of peroxide added = 300 per cent of polymer xveight. Reaction period, 20 days. (c) Concentration, 0.125 g. polymer in 0.78 ml. toluene. Seven additions of 0.05 g. peroxide; two additions of 0.1 g . Total = 440 per cent. Reaction period, 18 days.
A set of data relative to the influence of temperature of polystyrene polymerization on the degradation behavior in toluene at 100°C. is given in table 10. The temperature of preparation for the polystyrenes used in all of the other experiments was 60°C. Though the limited data suggest that the scissioning efficiency of the peroxide may be somewhat greater in the case of polystyrene prepared at 60"C., the order of the difference is insufficient to indicate that different scissioning mechanisms are operative in the two cases.
B. Sedimentation equilibrium studies Sedimentation equilibrium experiments were performed Tyith a selected number of degraded polystyrenes. The data from these experiments were intended
346
J. 0. THOMPSON
to show the behavior of the molecular weight distribution of the polystyrene systems a t various extents of degradation, and also to permit an assessment of the extent to which the viscosity-molecular weight relationship for the original polystyrenes remains valid for the degraded products. TABLE 9 Influence of molecular weight of original polymer on the scissioning eficiency of perozide i n toluene at 100°C. FINAL
CAICULATED ASSUYING PEROXIDE EFFICIENCY INDEPENDENT OF PEROXIDE CONCENTRATION
Pn x
PEROXIDE ADDED
D-7 D-1A D-9 D-2B D-8 D-2A D-10
-I1
,
I
‘
I
PRODUCT
per cenl
grams
108 144 226 102 108 144 226
RECOVERY
OF DEGRADED
0.1 0.1 0.1 0.1 0.2 0.2 0.2
87 102 155 85 85 100 145
109 150 83 106 144
73 88 117
96 99 100 96 98 96 97
* One experiment was selected from each series and the number of chain scissions computed from the change in nn. Then t h e B n t o be expected was calculated from the original value of M, for the other experiments in the same series, assuming this same number of chain scissions to be independent of molecular weight. TABLE 10 Influence of temperature of polymerization of polystyrene on degradation behavior at 100°C. i n toluene Concentration, 0.5 g. polymer in 11 ml. toluene EXPEPWNT
A. Polystyrene H, prepared a t 100°C. grams
Polystyrene H . . . . . . . . . . . . . . . . . . . . . . D-51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.1 0.2
I
,
::E
~
::;
~
0.66
B. Polystyrene E, prepared at 60°C. Polystyrene E . . . . . . . . . . . . . . . . . . . . . . . D-IA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
0.97 0.77 0.72
a,,:a,,,:az
1, i
per cent
145 118 107
98 98
144 102 95
As previously outlined the ratios of shoLld tend to 1:2 : 3 as degradation proceeds, whether the degradation process involves an approach to an equilibrium condition by way of a radical chain mechanism or whether the degradation process is one involving simple scissioning of “infinite” chains. Thus,
DEGRADATION OF POLYSTYRENE I N SOLUTION
347
if the original polymer has a molecular weight distribution such that the ratios are 1 :2:3, these ratios should be maintained asdegradationproceeds. For fractionated polystyrenes of sharper distribution, however, degradation should cause the distribution to broaden and the ratios should tend toward 1:2 :3. If a chain radical mechanism is the correct mechanism for the degradation reaction, with the peroxide acting as chain initiator, then it would be expected that for fractionated polystyrenes a small amount of peroxide would produce a relatively large change in distribution, even though the net number of detectable scissions was small. However, if the degradation proceeds by way of a simple irrerersible scissioning process, then the rate a t which the distribution broadens would be more closely related to the rate at which the number of detectable scissions is produced. A comparison of the weight- and viscosity-average molecular weights (table 1) of the unfractionated polystyrenes A, C, and E prepared in the oil phase a t 60°C. shows that these two molecular weight averages do not differ by more than 8 per cent. The viscosity-molecular weight relation may be expressed in the form [n] = k M a , where the values of the constants, k and a, depend on the temperature of polymerization (1). Thus it follows that if simultaneous polymerizationdepolymerization is involved in the peroxide degradation of the polystyrene then the viscosity-molecular weight relationship, which holds well for the unfractionated polystyrenes prepared a t 6OoC., should fail to hold for the products obtained upon degradation at 100°C. and 125°C. If, hoffever, the degradation process involves simple scissioning of the parent molecules, the viscosity relation would be expected to remain more nearly valid even after considerable scissioning and regardless of the temperature a t which the scissioning occurs. The scissioning must, of course, occur in solutions sufficiently dilute so that cross-linking does not interfere. The collected data for the degraded polystyrenes on which sedimentation equilibrium experiments were made are given in table 11, together with the corresponding data for the original polymers. The ratios of the molecular weights are given iz the form (where p takes the values 0, 1, and 2) instead of the form M,,:BW:az, for greater flexibility in handling the data. The values of the molecular weight ratios for experiment 3R show the broadening of the molecular weight distribution with degradation. The extent of this broadening, however, appears to be small, considering that 140 per cent by weight of peroxide had been used in the degradation. This result supports the simple scissioning hypothesis, for had a chain mechanism been involved greater spreading should have resulted. The data for experiments D-1A and D-6A in which the degradations were performed a t 100°C. and 126"C., respectively, with 20 per cent by weight of peroxide, show that the molecular weight ratios are in each case essentially the same as for the original polystyrene E. The results for experiments D-12 and D-17 also show no appreciable change in molecular weight distribution from the original polystyrene A. Strong evidence for the occurrence of cross-linking when the degradation is carried out at higher polymer concentrations is provided by the data for experiment D-19, where not only are the
an:a,o:#z
aQ+l/#Q
348
J. 0 . THOMPSON
molecular weight _ratios markedly higher than expected, but also the deviation between aV and M , shows the marked failure of the viscosity-molecular weight relationship. Though complete data were not available for the degradation product from experiment D-15 nor for the original polystyrene fraction 2 4 the results indicate a marked broadening of the distribution. From the sedimentation data alone for this experiment it would appear that the molecular weight distribution had broadened to the limiting function. A comparison of the viscosity-average and weight-average molecular weights (columns 7 and 8 of table 11) shows that the viscosity-molecular weight relationship holds as well for the degradation products as for the original polystyrenes. TABLE 11 Collected data for sedimentation e q u i l i b r i u m experiments on degraded polystyrenes
EXPERIMXNI
Fraction 5W 3R
0.5 g . in 11 ml. toluene
Whole E D-1.4
0.5 g. in 11 ml. toluene 0.5 g. in 11 ml. toluene
D-6A Whole A D-12 D-17 D-19
I
0.5 g . in 11 ml. toluene 0 8 g in43ml toluene
~
I140
~
20
20
300 ~
12
I
l W ' 0 71)
841 1701 1851 2881 356l2 20 1 5511 24
benzene Fraction 2A D 15
I
0 5 g in 11 ml toluene
1
* M , too low for reliable osmotic measurement The only exception is in the data for experiment D-19, where the deviation is almost certainly the result of cross-linking, as previously mentioned. That the viscosity-molecular weight relationship holds as well for the degradation products as for the original polystyrenes is strong evidence supporting the simple scissioning hypothesis. C. Viscometry I n the viscosity equation
DEGRADATION O F POLYSTYRENE I N SOLUTION
349
the value of the constant k‘ for dilute solutions of polystyrene depends on the nature of the solvept and the temperature of preparation of the polymer (1). If a chain radical mechanism predominates in the peroxide degradation of polystyrene, the yalue of k‘ for degradation products should tend to an equilibrium value. Actually, the observed values for k’ in a number of degradation systems show no trend away from those for the corresponding original polystyrenes, the deviations which exist being always within the limits of experimental accuracy. So, again the evidence supports an oxidative scissioning mechanism rather than a catalyzed chain radical mechanism. V. GESERAL REXLRKS
Since the statement has been made that polymerization catalysts-such as benzoyl peroxide-which decompose to free radicals may also catalyze degradation” ( 7 ) ,it, is of interest to assess the efficiency of the peroxide in degradations of this type. Calculation of this efficiency from the number-average molecular weights before and after degradation shows that in all of the experiments performed more than 150 molecules of peroxide were required for each detectable scission. On this basis, therefore, benzoyl peroxide may not be regarded as a chain radical catalyst in the degradation process. It may be argued that the net number of detectable scissions is very small as compared with the actual number of scissions which would be detectable Tvere it not for an aggregative free-radical process. That there is no appreciable rearrangement or aggregation of free-radical fragments during the degradation process is indicated by the following experimental observations: ( 1 ) S o aggregative reaction was detected for polymers of very low molecular weight. (2) The intrinsic viscosity-molecular weight relation which holds well for the original polystyrene holds equally well for the degraded products. (3) The rate a t which the molecular weight distribution of a polystyrene fraction broadens \vith peroxide addition is very slow. (4)The value of the constant k‘ in the reduced viscosity equation s h o w no marked deviation from its original value as degradation proceeds. Hence, viscosity measurements alone and combined with the weight-average molecular \wights from sedimentation equilibrium measurements proride strong support for a simple scissioning mechanism. The evidence, however, is markedly more conclusive when \ve consider the higher molecular \wight averages determined by sedimentation equilibrium measurements. The ratios of the various average molecular weights of a polymer are functions of the molecular weight distribution of the polynier. When these ratios increase very slonly as degradation proceeds, it may be safely concluded that the molecular weight distribution also broadens very sloivly. further point which must not be overlooked, though it does not appear to be significant from the esperimental results obtained, is the possible occurrence of preferential scissioning of small segments from the ends of the polymer chains. A moderate amount of such scissioning would not be detectable by the physico-
3 50
J. 0. THOMPSON
chemical techniques employed; a large amount of such scissioning would be indicated by low values for “percentage recovery” in the degradation experiments. The experimental values for ‘the recovery did not indicate any such marked preferential chain end scissioning. From the statement , , catalysts of polymerization are also in general catalysts for degradation, and that in many cases steady states, or possibly equilibrium states, are reached for which the rates of polymerization and degradation become equal” (6),it is seen that the criterion which has been accepted as indicating the existence of simultaneous polymerization and degradation is the approach to a steady-state viscosity as degradation proceeds. An analysis of the validity of this criterion follows. Consider unfractionated polystyrene prepared in the oil phase at 60°C. with benzoyl peroxide as catalyst where the ratio = 1:2.* Assume that this polymer in solution is subjected to some random scissioning process the exact mechanism of which is immaterial. Then the value of the ratio M , : M , will remain 1:2 as the degradation proceeds. For simplicity of argument consider one polymer molecule and let it be scissioned once, Le., one molecule becomes two molecules and M , changes by 50 per cent. Let the scissioning process continue until the original molecule has become 100 molecules; then ae a result of the next changes by 1 per single scission the 100 molecules become 101 molecules and cent. Applying this_same argument to the whole polymer sample and ?membering that the ratio M,:M, remains 1:2, we find that the change in M , for the first cut per molecule is 50 per cent while it is but 1 per cent for the hu_ndredth cut, and so on. Thus, if we start with k molecules of polymer of = M,,, and if we distribute among them k scissions a t random, and repeat the scissioning a t equal time intervals (is., uniform rate of scissioning of k scissions per unit tirle), t i e mole_cule weight M1of the product will take on the successive values M,,, M , , / 2 , M , , / 3 , . . , or M , , / ( N l), where N is_the number of equal time intervals completed. Since M , is always equal to 2M,, takes on the successive values
.
a,,:a,
an
an
+
.
‘wo
N
T
a,
where N = 0, 1,2,
* *
aw
Thus, we may say that decays “harmonically” with the number of scissions, or with time if the rate of scissioning is uniform. Let us now consider the accompanying change in viscosity. Since the viscosity of a polymer in a given solvent a t constant concentration depends on (a) the size of this type of polymer molecules, and ( b ) the amount of “interference” between molecules, we see that the contribution to t_he viscosity due to size will decrease approximately “harmonically” along with M,, or, what is almost equivalent, the intrinsic viscosity decreases “harmonically” ; the “interference” factor varies approximately exponentially with particle size for constant concentration, and a Even where the ratio n,,:nw for the original polystyrene differs from 1:2 the argument remains valid in essence, for we have shown that the rate at which this ratio changes with peroxide addition is very slow.
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DEGRADATION OF POLYSTYRENE IN SOLUTION
for uniform decrease in mw the “interference” factor falls off roughly exponentially. From this it follows that the rate of change of relative viscosity with time-assuming a hypothetical uniform rate of scissioning-combines both the “harmonic” and exponential rates of decay; and on plotting relative viscosity against time for the hypothetical scissioning process a curve is obtained which has a sharp initial slope, and which rapidly tends to almost zero slope. We thus have what appears to be a “steady-state’J viscosity even though scissioning continues at a uniform rate (figure 2). The important conclusion to be drawn from this argument is that ordinary viscosity measurements will appear to indicate the existence of a steady-state viscosity; and it is true that the viscosity does remain very nearly constant as peroxide addition is continued. Yet the number of scissions, i.e., the amount of degradation, produced by each 0.1 g. of peroxide in the “steady-state” portion of the curve for the hypothetical scissioning process is exactly the same as that
-0.4 0
I
2
3
4
I TIME
6
7
0
.
9
10
FIG.2. Plot of viscosity against time for the degradation of polystyrene i n solution showing viscosity behavior for a hypothetical uniform-rate scissioning process, and weightaverage molecular weights corresponding t o equal time intervals.
produced by the first addition of peroxide where the hiwosity changes rapidly. Very accurate intrinsic viscosity measurements would be essential for the experimental detection of this fact. If the peroxide addition were carried on long enough it is apparent that the decrease in intrinsic viscosity produced by 0.1 g. of peroxide would lie well within the limits of experimental accuracy of the usual viscosity measurements. The above discussion has assumed that each addition of peroxide produces the same number of scissions. This has been shown to hold over the number-average molecular \\eight range 100,000 to 250,000 provided the degradation product is isolated after each peroxide addition. However, if the polymer is not isolated at each stage, it is clear that the degradation products of the peroxide itself will accumulate, and the detectable scissioning ability of the peroxide will fall off as the process continues, owing to the ever-increasing interference of these accumulating degradation products. Thus, in such an experiment the attainment of
352
J. 0 . THOMPSON
a “steady-state” viscosity would appear even more real than if the polymer product were isolated after each peroxide addition. The results of the degradation experiments which showed that the scissioning ability of benzoyl peroxide varies with temperature, peroxide and polymer concentration, and nature of solvent are in accord with the recent work of several authors (2), (although temperature variation is slight, as shown in table 5 ) . They have shown that, though the manner of decomposition of benzoyl peroxide under different conditions is not yet fully understood, the kinetics and also the stoichiometry of the decomposition in solution vary with these factors. Thus it is reasonable to expect that the scissioning ability of benzoyl peroxide on polystyrenes should also vary with temperature, peroxide concentration, and nature of solvent. The “peroxide vastage” factor of Barnett and Vaughan (2), i.e., dissipation of the catalyst without forming free radicals, appears to be related to our observation that the scissioning efficiency of the peroxide decreases as its concentration is increased. Further, since the mode of peroxide decomposition varies with the nature of the solvent, it follows that its mode of decomposition will also vary with the nature of other solutes, e.g., monostyrene, polystyrene, hydroquinone, products from its own decomposition, etc. Reactions involving the decomposition of benzoyl peroxide are undoubtedly very complicated-some of them are known to involve free radicals and others not-hence it appears judicious a t this time to reserve speculation on the exact nature of the reaction involved in polymer scissioning until more detailed information on the manner of benzoyl peroxide decomposition under simpler conditions is available. Free radicals may play a part in the degradative process, but certainly not in the same manner as they do in the polymerization of the monomer. The mechanisms of the two reactions are quite different; the polymerization reaction involves a chain radical mechanism, whereas the depolymerization reaction is probably one of simple random oxidation. Therefore the conclusion from this work must be-as stated by Flory (3) for vinyl-type polymerizations-that “these processes generally are not reversible in the thermodynamic sense.” VI. SUMMARY
The degradation of polystyrene in solution under a variety of conditions has been studied by the techniques of viscometry, osmometry, and sedimentation equilibrium. I n the absence of oxygen and benzoyl peroxide at 100°C. no detectable degradation occurs within a period of six months; at room temperature and in the presence of oxygen no measurable degradation occurs within a period of two years. I n the presence of oxygen or benzoyl peroxide the extent of the degradation depends on the concentration of the polymer and peroxide (or oxygen), and the nature of the solvent, There is little temperature dependence. For conditions otherwise constant the scissioning efficiency of benzoyl peroxide is independent of the molecular weight of the initial polystyrene. The scissioning efficiency of the peroxide for all of the conditions investigated was very low; each detectable scission required more than 150 molecules of peroxide. S o evidence for the occurrence of an aggregative reaction in dilute solution was obtained.
DEGRAD-ITIOS OF POLYSTYREXE IS SOLUTION
353
Sedimentation equilibrium, viscosity, and osmotic pressure studies made on the isolated degradation products support a simple oxidative mechanism for the degradation reaction rather than a chain mechanism. The approach t o a steadystate viscosity as degradation proceeds is shown to be an insufficient criterion for establishing the existence of simultaneous polymerization and depolymerization. The author wishes to express his appreciation to Drs. M. Wales and J. W. Williams for their interest, suggestions, and encouragement. REFERESCES (1) ALFREY, T . , B.ARTOVICS, .h., ASD ARK, H . : J. Am. Chem. soc. 66, 2319 (1943). (2) BARSETT, B . , A S D V A U G H A S , Fv, E Phys. Br Colloid Chem. 61, 926 (1947); see also CASS,W.E . : J . Am. Chem. SOC.68, 1976 (1946); KHARASCH, M. S., JESSEN, E . V., ASD U R R YW. , H . : J . Org. Chem. 10, 386 (1945); NOZAKI, K., ASD BARTLETT, P. D.: J. Am. Chem. SOC.66, 1686 (1946). (3) F L O R YP. , J.: J. Chem. P h p . 12, 435 (1944). (4) KUHS,W . : Ber. 63, 1503 (1930). (5) MAYO,F. R . : J. .im. Chem. SOC.66, 2324 (1943). (6) MESROBIAS, R . B . , ASD T O B O L S KAY. ,i’.: J. Am. Chem. SOC.67, 785 (1945). (7) > f E S R O B I A S , R . B . , A S D T O B O L S KAY. ,v.: J. Polymer Sci. 2, 463 (1947). (8) SCATCHARD, G., OSCLEY,J . L., WILLIAMS,J. W., AND BROWB,A . : J. Am. Chem. SOC. 66, 1980 (1944). (9) S P O D H E I M H ., R., BADGLEY W. , J., ASD MESROBIAS, R. B.: J. Polymer Sci. 3, 410 (1948). (10) S T O C K ~ ~ A W. Y EH R ., : I n Adaancing Fronts in Chemistry, Vol. 1. Reinhold Publishing Corporation, S e w York (19451, (11) T A Y L O R H,. S., A S D T O B O L S K Y , .h. v.: J. Am. Chem. Soc. 67, 2063 (1945). (12) T O B O L S KAY. ,V.:J. Chem. Phys. 12,402 (1944). (13) WALES,31.: J. Phys. 8; Colloid Chem. 62, 235 (1948). (14) WALES,X , WILLIAMS,6.W,, T H O M I P S JO. X O,. , ASD EWART, R. H . : J. Phys. & Colloid Chem. 62, 984 (1048). (15) ZIMM, B. H., ASD MYEXSOS, I . : J. Am. Chem. SOC.68, 911 (1946).
