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The thermal polymerization of styrene at temperatures up to 250°
European Polymer .Journal Vol. 14, pp. 889 to 894 © Pergamon Press Ltd 1978. Printed in Great Britain
0014-3057 781 IOI (18NgStI2111)I)
THE T H E R M A L P O L Y M E R I Z A T I O N O F S T Y R E N E AT TEMPERATURES
UP
TO
250 °
W. I. BENGOUGH and G. B. PARK Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, Scotland (Received 16 M a r c h 1978)
Abstract The kinetics of the thermal polymerization of styrene have been studied over the range 60-250:. The overall energy of activation was 86 ___2 k J/mole, a value identical to that obtained for the thermal polymerization of styrene in diethyl adipate. As expected, the molecular we_ight of the polymer decreases with increase in the temperature of the polymerization, and the ratio M,,'M,, becomes greater than 2 for polymer formed at above 140'. The plot of log (1,,M,,) against [absolute temperature)- ~ can be represented by two straight lines yielding 24.5 and 32,0 k J/mole for the activation energies at temperatures below 120 and above 140, respectively. The former value is in keeping with the molecular weight being controlled by chain transfer with monomer: the latter value would be that expected if the termination process controls the molecular weight of the polymer. Mark Houwink relationships between intrinsic viscosity and Mo and M~ have been found to apply to polymer samples when the molecular weight averages were determined by osmometry and by light scattering. However, deviations were found for low molecular weight material when measured using gel permeation chromatography. The K values were considerably lower, and the ~ values higher than reported in the literature.
tilled water: it was dried over calcium metal, and finally purified by distillation under reduced pressure in nitrogen.
Several recent papers have dealt with [1-5] the mechanism of the thermal polymerization of styrene, but they have been mainly concerned with the initiation process, and with the initial stages of the polymerization at temperatures below 100. Apart from the work of Hui and Hamielec [6] who attempted to fit their experimental results on the bulk thermal polymerization of styrene at temperatures between 100 and 2 0 0 to a kinetic model based upon a third order initiation process, relatively little has been published on this polymerization at temperatures above 100 t . In earlier studies [7 9] on the thermal polymerization of styrene in solution, it was found that at low m o n o m e r concentrations the rate of polymerization depends upon [ m o n o m e r ] 25 indicating a third order initiation process, but for [ m o n o m e r ] > 2M, a second order dependence of the rate on [ m o n o m e r ] gave a better fit. The latter dependence would be expected if the initiation reaction were second order with respect to m o n o m e r . F o r the thermal polymerization of styrene in diethyladipate, it was found [ I 0 ] that the rate of polymerization was directly proportional to [ m o n o m e r ] 2 for [ m o n o m e r ] ranging from 0.7 to 6.9 M at temperatures from 12(~160 :. In this paper the thermal polymerization of bulk styrene at temperatures up to 250: is reported, the results having been calculated on the basis of rate depending on [ m o n o m e r ] 2. The effects of temperature on the rate and on the degree of polymerization are reported, and a comparison has been made of the values of the molecular weight of the polymer determined by gel permeation chromatography, osmotic pressure and light scattering methods.
Procedure
For temperatures below 100, the polymerizations were carried out in ordinary dilatometers immersed in water thermostats: for temperatures of 10~250, sealed ampoules or specially designed dilatometers were used and immersed in a thermostatically controlled Tecam fluidized bed sand bath. The reaction vessels were filled and sealed in racuo as previously described [10]. After the polymerization had been taken to an appropriate stage, the polymer was isolated by dissolving the reaction mixture in carbon tetrachloride, and adding it dropwise to a stirred ten-fold excess of methanol. The finely divided precipitated polystyrene thus produced was filtered, dried in a vacuum oven at 60' and weighed. The molecular weight of the polymer in toluene solution was measured by osmometry, light-scattering, gel permeation chromatography and viscometry. The instruments used were a Mechrolab Model 502 Membrane Osmometer. a Sofica Photogoniometer, a Waters Gel Permeation Chromatography Apparatus, and an Ubbelhode suspended level viscometer modified to allow dilution of the polymer solutions in situ. RESULTS A N D
EXPERIMENTAL Materials
Commercial styrene was freed from inhibitor by repeated washing with 10°o caustic soda, followed by dis889
DISCUSSION
The effect of temperature on the thermal polymerization of styrene was first investigated over the range 60-200". At temperatures between 140 and 2 0 0 , some of polymerizations were carried out in sealed ampoules, taken to various conversions, the polymer precipitated, and the conversion estimated gravimetrically. The results are shown in Fig. 1, together with the continuous lines giving the results obtained with the specially designed dilatometers. Agreement between the gravimetric and the dilatometric results is satisfactory. The plot of log ( r a t e / [ M ] 2) against (absolute temperature) is a good straight line over the temperature
890
W.I. BENGOUGHand G. B. PA~K I00
,v 80
/
o/ oil •
60
I
/ !
o N
E 40 O-
20
Iff ~ 0
I
I
I
40
80
IO0
Time,
rain
Fig. 1. The effect of temperature on the thermal initiated polymerization of styrene. Continuous lines represent dilatometric measurements, points indicate gravimetric results. [] 140":O - - 170: 11--200°: @--250 °.
40
30
u~
0
20
x
rO
0
I
I
50
JO0
% Conversion
Fig. 2. The effects of temperature of reaction and extent of conversion on the molecular weight of the polymer formed. [ ] - - 140°; O - - 170°; 11--200°; 0 - - 2 5 0 c'.
range 60-200 c, and yields an overall activation energy of 86 ± 2 k J/mole, identical to that obtained for the thermal polymerization of styrene in diethyladipate [10]. The slight deviation from linearity at a temperature of 190-200 ° is believed to be due to the temperature of the polymerizing monomer being a little higher than that of the thermostat bath. At very high rates of polymerization, the heat generated by the exothermic reaction cannot be dissipated to the surroundings sufficiently quickly, and self-heating [11] of the system occurs. The high viscosity of the monomer-polymer mixture at high conversions will increase the magnitude of the effect. The effect of temperature and conversion on the molecular weight of the polymer formed is shown in Fig. 2, in which the weight average molecular weight M~ is plotted against conversion, for polymer produced at temperatures between 140 and 250 °. The polymer formed in the early stages of the reaction (i.e. <20% conversion) has significantly higher molecular weight than that produced at later stages. This probably arises because polymer is being continuously formed during the heating-up period. It may well take 5 m i n for the monomer to heat up to within a degree of the thermostat temperature: polymer produced during this period will have a wide range of molecular weight since it has been formed under continually changing temperature conditions and consequently under a variable rate of initiation. The average molecular weight of this initial polymer will, of course, be greater than that produced at the steady higher temperature of the thermostat bath. Figure 2 shows that over the mid-conversion range (i.e. 20-80% conversion) the value of Mw is reasonably constant but falls off towards the end of the polymerization. This fall-off is particularly marked at 200 ° and may be due to a rise in temperature at later stages of the reaction because of the self-heating previously mentioned. However, it seems unlikely that a sufficiently large temperature rise would occur, to explain the substantial fall in M~ at 200L The fall-off in M, with conversion was less marked at this temperature, indicating a wider distribution in molecular weight towards the end of the reaction. One possible explanation is that chain scission reactions may occur at such temperatures and some of the polymer molecules undergo thermal degradation, which is known to occur in this temperature region. The results at 250 ° are probably unreliable since the reaction had gone to about 800/0 conversion in less than 10min; consequently, the polymerization would have occurred over a wide temperature range up to, and possibly above 250 ° if self-heating effects were significant. The plot of (1/M,) against (absolute temperature)-x is given in Fig. 3. The values of M. used were obtained from osmotic pressure measurements and were taken for the mid-conversion range, since there is relatively little change in molecular weight in this region. The plot can be represented by two straight lines giving reasonable fits for the temperature ranges 60-14@" and 120-250°~espectively. Also plotted in the figures are values of M, calculated from DPo values obtained by extrapolation, in a study of the polymerization of styrene in diethyladipate [10]. From the gradients of the two lines in Fig. 3, values of 32.0
Thermal polymerization of styrene and 24.5 k J/mole are obtained for the overall energy of activation associated with the reactions controlling the molecular weight of the polymer. The relationship between the degree of polymerization and the rate constants for propagation (kp), chain transfer with monomer (k,,) and termination (k,) is given in Eqn (1),
l
v/kikt kp
-
DP
k,,
(1)
+
kp
assuming the rate of initiation is given by k~ [M] 2. The effect of temperature upon ~ will thus depend upon the values of the energies of activation for initiation, propagation, chain transfer with monomer, and termination. Under conditions where transfer with monomer controls the molecular weight of the polymer, the overall activation energy for (1/DP) will be (E,~ - Ep) whereas, under conditions where chain transfer can be neglected, it will be given by (½E~ + ½E~- Ep). A value for ( E , r - E,p) can be obtained from a plot of log (k~/kp) against (absolute temperature) ~: this plot (curve 2) is also given in Fig. 3 using published values [12] of the transfer constant. Although these values show substantial scatter, the line drawn through them gives a value of about 24 k J/ mole for (E,, - Ep) which is quite close to the 23.6 k J/ mole obtained by Hui and Hamielec [6] and also to that obtained from the lower part of curve 1 of Fig. 3. The higher value of 32 kJ/mole might therefore represent the system when termination largely determines the molecular weight of the polymer. Thus in these circumstances we have )E~ + ½E, -
Ep
891
and also, from the overall energy of activation of 86 + kJ mole
½E, +
Ep-
32
(3)
From Eqns (2) and (3) we obtain a value of 118 _+ 2kJ m o l e - i for E i, which agrees well with the value of 121 kJ m o l e - ~ found from a study of the rate of initiation using D P P H as a free radical scavenger [13]. It seems reasonable to conclude that at the lower temperatures the molecular weight of the polymer formed is controlled by transfer with monomer whereas at the higher temperatures the termination reaction mainly controls the molecular weight. it is possible, however, that the chain transfer reaction may alternatively involve the Diels-Alder intermediate I. This compound was originally suggested by Mayo [7] as an
G,~
(I)
intermediate in the initiation process, a mechanism favoured by a number of other workers [4]. This compound could alternatively arise from the selftermination of a diradical which we believe [13] to be formed in the initiation of the spontaneous polymerization of styrene. A possible scheme for this process is given below. This readily accounts for the formation of compound ! and also diphenylcyclobutane which is known to be formed [4]. C H --~CH z
=
'E, = 86 + 2
CH
(2)
CH 2 -
CH 2
(~H
20
/ ~ C H
A A i5
~
~
,
,
N
~
°
• ~:a. I.O
~CH2~
~H2
CH
I CH - - C H 2
r,
CH 2 ,
"CH
•o
;a %
~.~o5 4- ÷
0 L
20
I
25
[
I
30
Recil~'OCol of obsolute temperoture
35 xlO8
Fig. 3. The effect of temperature on the number average molecular weight M° (©--curve 1) and on the chain transfer with monomer constant C m (O--curve 2) for the thermal initiated polymerization of styrene. A - - calculated values from polymerization in solution. E.PJ 1411- R
The ratio Mw/M. was determined both from comparison of light scattering and osmotic pressure measurements and from gel permeation chromatography. The effect of temperature upon this ratio is shown in Fig. 4. At temperatures up to 14if, the value of this ratio lies between 1.3 and 2.0, but at higher temperatures it tends to increase to above 2, and towards 3 or higher at around 250. A value of 1.5 would be expected for a normal free radical initiated polymerization of styrene with termination by combination. However, if the degree of p o l y m e r i z a tion were controlled by chain transfer with monomer
892
W.I. BENGOUGH and G. B. PARK
where a substantial conversion occurs during the heating-up period. The value of M~/M. obtained by gel permeation chromatography was found in general to be greater than that obtained from light scattering and osmotic pressure measurements for the same sample. Also the individual values of Mw and M, obtained by gel permeation chromatography were generally lower than 20 i • when determined by the other two methods, for polystyrene produced at 170° or higher. This difference might be due to more highly branched polymer being 0 i00 ~50 2oo z50 6O formed at the high temperatures giving rise to more Ternperofure, *C compact molecules which would tend to have longer Fig. 4. The effect of temperature on the value of the ratio retention times in the gel than linear molecules of M,,/M. for the thermal polymerization of styrene. © - - de- the same molecular weight: hence they would appear termined by gel permeation chromatography: • - - d e t e r to have lower molecular weights as determined by mined by light scattering and osmometry. gel permeation chromatography. Therelationships between the intrinsic viscosity [~/] or if termination were by combination, a value of and Mw and Mn are shown in the log-log plots in 2 would be obtained for M~/M,,. From Fig. 4 it Figs 5 and 6. The molecular weight averages given appears that the ratio lies between 1 and 2 for in Fig. 5, were obtained by osmometry and gel perpolymer formed at temperatures up to 140°: how- meation chromatography, whereas those in Fig. 6 ever, the fluctuation in the values is too great to were determined by light scattering and by gel perenable any firm deductions to be made regarding the meation chromatography. Both plots are reasonably molecular weight controlling reactions. At higher linear over most of the molecular weight range, but temperatures, the value of the ratio increases. This deviations from linearity occur for the low molecular might be due to branching as a result of transfer with weight polymer when determined by gel permeation polymer, or possibly to the occurrence of chain scis- chromatography. I t appears that polymers of sion reactions resulting in some thermal degradation M~, < 100,000 or M, < 50,000 show the greatest deof the polymer. The effect of conversion on the viations. These polymers were prepared at temperaM,./M. ratio did not show any definite trend: It seems tures of 200° and above, and might be expected to possible that, because of the high temperatures used contain branched polymers. Attempts to detect in thermal polymerizations, the variation of the mol- branching using carbon-13 NMR however, proved ecular weight in the early stages of polymerization negative. is substantially greater than for polymerizations at The Mark-Houwink K and ct values were deterlower temperatures, as a result of a significant amount mined from the linear portions of the plots in Figs of polymer being formed during the heating-up 5 and 6 by the method of least squares, and are listed period. This polymer will tend to have a molecular in Table 1. weight higher than that of polymer formed at the final Literature values [14] of K and a vary somewhat temperature. This effect will be very significant for depending upon the method used in their determinapolymerizations at temperatures of 200~ or greater tion and upon the solvent used, but for toluene as
4oI
,J
0.6 J
0.2
Ore-0.2
-0.6 o 0 0 -I .C
36
o I 40
/:.j I 44
I 4B
I 5,2
I '5.6
Log ( I~I. )
Fig. 5. The relationship between intrinsic viscosity and number average molecular weight for thermally polymerized styrene, determined using © - - gel permeation chromatography; • - - osmometry.
Thermal polymerization of styrene
893
1.6
_
o~
J
%
-02
-o 6
~i" o/
0
-~o 40
o
°
"
0
l
I
I
L
I
4.4
4.8
5.2
5.6
6.0
Log (~I,) Fig. 6. The relationship between intrinsic viscosity and weight average molecular weight for thermally polymerized styrene, determined by O - - gel permeation chromatography; • - - light scattering.
Table I. K and ~t values in the equation [q] = K M ~ for polystyrene produced by spontaneous polymerization at temperatures up to 200' Method used
Molecular weight average
Osmometry Gel permeation chromatography Light scattering Gel permeation chromatography
a solvent they tend to lie within the range (13 +_ 4) × 10-5 dl/g for K and 0.71 4- 0.02 for ~. The values of K are substantially higher and those of ~t lower t h a n reported in Table 1. The reason for the difference is not obvious but may be connected with possible differences in the structure and in the molecular weight distribution of the polystyrene produced by s p o n t a n e o u s polymerization at high temperatures, c o m p a r e d with the samples used in previous studies. It will be seen from Fig. 4 that the polymer produced at the higher temperatures tends to have a wider molecular weight distribution t h a n those produced below 100 t . Nevertheless, the above relationships have been based u p o n measurements from more t h a n 30 polymer samples, and might therefore prove useful in estimating the molecular weight of unfractionated commercial polystyrene produced by thermal polymerization. It is interesting, if somewhat surprising, that the value of the molecular weight of a polymer found using K and ct values of 13 × 10 -5 dl/g and 0.71 respectively agrees well with that obtained using K and ct values of 7.14 × 1 0 - 6 dl/g and 0.92 respectively, when the molecular weight of the polymer is of the order of 106, and with that obtained using K and ct values of 35 × 10 -6 dl/g a n d 0.85, when the sample has a molecular weight of a r o u n d 104 .
M, M, M~ M~
K
× 10 6
[dl/g) 32.9 35.0 7.43 7.14
0.83 0.85 0.91 0.92
To conclude, the results agree well with those found for the thermal polymerization of styrene in diethyladipate [10], in which the rate was found to depend upon [ m o n o m e r ] 2 and are also consistent with a diradical initiation process as suggested recently [13].
REFERENCES
1. W. A. Pryor and L. D. Lasswell, Polymer Preprints I1, 713 (1970a). 2. W. A. Pryor, J. H. Coco, W. H. Daly and K. N. Houk, J. Am. chem. Soc. 96, 5591 (1974). 3. K. R. Kopecky and S. Evani, Can. J. Chem. 47, 4041 (1969a) and 47, 4049 (1969b). 4. W. A. Pryor and L. D. Lasswell, Advances in Free Radical Chemistry, Vol. 5, p. 5. Elek, London (1975). 5. O. F. Olaj, H. F. Kauffmann and J. W. Breitenbach, Makrolek. Chem. 178, 2707 (1977). 6. A. W. Hui and A. E. Hamielec, J. Appl. Polym. Sci. 16, 749 (1972). 7. F. R. Mayo, J. Am. chem. Soc. 75, 6133 (1953). 8. R. R. Hiatt and P. D. Bartlett, J. Am. chem. Soc. 81, 1149 (1959). 9. C. Walling, E. R. Briggs, F. R. Mayo, J. Am. chem. Soc. 63, 1145 (1946). 10. W. I. Bengough and G. B. Park, Europ. Polym. J. 12, 431 (1976).
894
W. 1. BENGOUGH and G. B. PARK
11. W. 1. Bengough, Trans. Faraday Soc. 53, 1346 (1957). 12. Polymer Handbook (Edited by J. Brandrup and E. H. Immergut), 2nd Edn. Wiley-lnterscience, New York (1975).
13. N. J. Barr, W. 1. Bengough, G. Beveridge, G. B. Park, Europ. Po/ym. J. 14, 245 (1978). 14. M. Kurata and W. H. Stockmayer, Advances in Polymer Science, Vol. 3, pp. 1960. Springer-Verlag, Berlin ( 1961-64).
0014-3057 781 IOI (18NgStI2111)I)
THE T H E R M A L P O L Y M E R I Z A T I O N O F S T Y R E N E AT TEMPERATURES
UP
TO
250 °
W. I. BENGOUGH and G. B. PARK Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, Scotland (Received 16 M a r c h 1978)
Abstract The kinetics of the thermal polymerization of styrene have been studied over the range 60-250:. The overall energy of activation was 86 ___2 k J/mole, a value identical to that obtained for the thermal polymerization of styrene in diethyl adipate. As expected, the molecular we_ight of the polymer decreases with increase in the temperature of the polymerization, and the ratio M,,'M,, becomes greater than 2 for polymer formed at above 140'. The plot of log (1,,M,,) against [absolute temperature)- ~ can be represented by two straight lines yielding 24.5 and 32,0 k J/mole for the activation energies at temperatures below 120 and above 140, respectively. The former value is in keeping with the molecular weight being controlled by chain transfer with monomer: the latter value would be that expected if the termination process controls the molecular weight of the polymer. Mark Houwink relationships between intrinsic viscosity and Mo and M~ have been found to apply to polymer samples when the molecular weight averages were determined by osmometry and by light scattering. However, deviations were found for low molecular weight material when measured using gel permeation chromatography. The K values were considerably lower, and the ~ values higher than reported in the literature.
tilled water: it was dried over calcium metal, and finally purified by distillation under reduced pressure in nitrogen.
Several recent papers have dealt with [1-5] the mechanism of the thermal polymerization of styrene, but they have been mainly concerned with the initiation process, and with the initial stages of the polymerization at temperatures below 100. Apart from the work of Hui and Hamielec [6] who attempted to fit their experimental results on the bulk thermal polymerization of styrene at temperatures between 100 and 2 0 0 to a kinetic model based upon a third order initiation process, relatively little has been published on this polymerization at temperatures above 100 t . In earlier studies [7 9] on the thermal polymerization of styrene in solution, it was found that at low m o n o m e r concentrations the rate of polymerization depends upon [ m o n o m e r ] 25 indicating a third order initiation process, but for [ m o n o m e r ] > 2M, a second order dependence of the rate on [ m o n o m e r ] gave a better fit. The latter dependence would be expected if the initiation reaction were second order with respect to m o n o m e r . F o r the thermal polymerization of styrene in diethyladipate, it was found [ I 0 ] that the rate of polymerization was directly proportional to [ m o n o m e r ] 2 for [ m o n o m e r ] ranging from 0.7 to 6.9 M at temperatures from 12(~160 :. In this paper the thermal polymerization of bulk styrene at temperatures up to 250: is reported, the results having been calculated on the basis of rate depending on [ m o n o m e r ] 2. The effects of temperature on the rate and on the degree of polymerization are reported, and a comparison has been made of the values of the molecular weight of the polymer determined by gel permeation chromatography, osmotic pressure and light scattering methods.
Procedure
For temperatures below 100, the polymerizations were carried out in ordinary dilatometers immersed in water thermostats: for temperatures of 10~250, sealed ampoules or specially designed dilatometers were used and immersed in a thermostatically controlled Tecam fluidized bed sand bath. The reaction vessels were filled and sealed in racuo as previously described [10]. After the polymerization had been taken to an appropriate stage, the polymer was isolated by dissolving the reaction mixture in carbon tetrachloride, and adding it dropwise to a stirred ten-fold excess of methanol. The finely divided precipitated polystyrene thus produced was filtered, dried in a vacuum oven at 60' and weighed. The molecular weight of the polymer in toluene solution was measured by osmometry, light-scattering, gel permeation chromatography and viscometry. The instruments used were a Mechrolab Model 502 Membrane Osmometer. a Sofica Photogoniometer, a Waters Gel Permeation Chromatography Apparatus, and an Ubbelhode suspended level viscometer modified to allow dilution of the polymer solutions in situ. RESULTS A N D
EXPERIMENTAL Materials
Commercial styrene was freed from inhibitor by repeated washing with 10°o caustic soda, followed by dis889
DISCUSSION
The effect of temperature on the thermal polymerization of styrene was first investigated over the range 60-200". At temperatures between 140 and 2 0 0 , some of polymerizations were carried out in sealed ampoules, taken to various conversions, the polymer precipitated, and the conversion estimated gravimetrically. The results are shown in Fig. 1, together with the continuous lines giving the results obtained with the specially designed dilatometers. Agreement between the gravimetric and the dilatometric results is satisfactory. The plot of log ( r a t e / [ M ] 2) against (absolute temperature) is a good straight line over the temperature
890
W.I. BENGOUGHand G. B. PA~K I00
,v 80
/
o/ oil •
60
I
/ !
o N
E 40 O-
20
Iff ~ 0
I
I
I
40
80
IO0
Time,
rain
Fig. 1. The effect of temperature on the thermal initiated polymerization of styrene. Continuous lines represent dilatometric measurements, points indicate gravimetric results. [] 140":O - - 170: 11--200°: @--250 °.
40
30
u~
0
20
x
rO
0
I
I
50
JO0
% Conversion
Fig. 2. The effects of temperature of reaction and extent of conversion on the molecular weight of the polymer formed. [ ] - - 140°; O - - 170°; 11--200°; 0 - - 2 5 0 c'.
range 60-200 c, and yields an overall activation energy of 86 ± 2 k J/mole, identical to that obtained for the thermal polymerization of styrene in diethyladipate [10]. The slight deviation from linearity at a temperature of 190-200 ° is believed to be due to the temperature of the polymerizing monomer being a little higher than that of the thermostat bath. At very high rates of polymerization, the heat generated by the exothermic reaction cannot be dissipated to the surroundings sufficiently quickly, and self-heating [11] of the system occurs. The high viscosity of the monomer-polymer mixture at high conversions will increase the magnitude of the effect. The effect of temperature and conversion on the molecular weight of the polymer formed is shown in Fig. 2, in which the weight average molecular weight M~ is plotted against conversion, for polymer produced at temperatures between 140 and 250 °. The polymer formed in the early stages of the reaction (i.e. <20% conversion) has significantly higher molecular weight than that produced at later stages. This probably arises because polymer is being continuously formed during the heating-up period. It may well take 5 m i n for the monomer to heat up to within a degree of the thermostat temperature: polymer produced during this period will have a wide range of molecular weight since it has been formed under continually changing temperature conditions and consequently under a variable rate of initiation. The average molecular weight of this initial polymer will, of course, be greater than that produced at the steady higher temperature of the thermostat bath. Figure 2 shows that over the mid-conversion range (i.e. 20-80% conversion) the value of Mw is reasonably constant but falls off towards the end of the polymerization. This fall-off is particularly marked at 200 ° and may be due to a rise in temperature at later stages of the reaction because of the self-heating previously mentioned. However, it seems unlikely that a sufficiently large temperature rise would occur, to explain the substantial fall in M~ at 200L The fall-off in M, with conversion was less marked at this temperature, indicating a wider distribution in molecular weight towards the end of the reaction. One possible explanation is that chain scission reactions may occur at such temperatures and some of the polymer molecules undergo thermal degradation, which is known to occur in this temperature region. The results at 250 ° are probably unreliable since the reaction had gone to about 800/0 conversion in less than 10min; consequently, the polymerization would have occurred over a wide temperature range up to, and possibly above 250 ° if self-heating effects were significant. The plot of (1/M,) against (absolute temperature)-x is given in Fig. 3. The values of M. used were obtained from osmotic pressure measurements and were taken for the mid-conversion range, since there is relatively little change in molecular weight in this region. The plot can be represented by two straight lines giving reasonable fits for the temperature ranges 60-14@" and 120-250°~espectively. Also plotted in the figures are values of M, calculated from DPo values obtained by extrapolation, in a study of the polymerization of styrene in diethyladipate [10]. From the gradients of the two lines in Fig. 3, values of 32.0
Thermal polymerization of styrene and 24.5 k J/mole are obtained for the overall energy of activation associated with the reactions controlling the molecular weight of the polymer. The relationship between the degree of polymerization and the rate constants for propagation (kp), chain transfer with monomer (k,,) and termination (k,) is given in Eqn (1),
l
v/kikt kp
-
DP
k,,
(1)
+
kp
assuming the rate of initiation is given by k~ [M] 2. The effect of temperature upon ~ will thus depend upon the values of the energies of activation for initiation, propagation, chain transfer with monomer, and termination. Under conditions where transfer with monomer controls the molecular weight of the polymer, the overall activation energy for (1/DP) will be (E,~ - Ep) whereas, under conditions where chain transfer can be neglected, it will be given by (½E~ + ½E~- Ep). A value for ( E , r - E,p) can be obtained from a plot of log (k~/kp) against (absolute temperature) ~: this plot (curve 2) is also given in Fig. 3 using published values [12] of the transfer constant. Although these values show substantial scatter, the line drawn through them gives a value of about 24 k J/ mole for (E,, - Ep) which is quite close to the 23.6 k J/ mole obtained by Hui and Hamielec [6] and also to that obtained from the lower part of curve 1 of Fig. 3. The higher value of 32 kJ/mole might therefore represent the system when termination largely determines the molecular weight of the polymer. Thus in these circumstances we have )E~ + ½E, -
Ep
891
and also, from the overall energy of activation of 86 + kJ mole
½E, +
Ep-
32
(3)
From Eqns (2) and (3) we obtain a value of 118 _+ 2kJ m o l e - i for E i, which agrees well with the value of 121 kJ m o l e - ~ found from a study of the rate of initiation using D P P H as a free radical scavenger [13]. It seems reasonable to conclude that at the lower temperatures the molecular weight of the polymer formed is controlled by transfer with monomer whereas at the higher temperatures the termination reaction mainly controls the molecular weight. it is possible, however, that the chain transfer reaction may alternatively involve the Diels-Alder intermediate I. This compound was originally suggested by Mayo [7] as an
G,~
(I)
intermediate in the initiation process, a mechanism favoured by a number of other workers [4]. This compound could alternatively arise from the selftermination of a diradical which we believe [13] to be formed in the initiation of the spontaneous polymerization of styrene. A possible scheme for this process is given below. This readily accounts for the formation of compound ! and also diphenylcyclobutane which is known to be formed [4]. C H --~CH z
=
'E, = 86 + 2
CH
(2)
CH 2 -
CH 2
(~H
20
/ ~ C H
A A i5
~
~
,
,
N
~
°
• ~:a. I.O
~CH2~
~H2
CH
I CH - - C H 2
r,
CH 2 ,
"CH
•o
;a %
~.~o5 4- ÷
0 L
20
I
25
[
I
30
Recil~'OCol of obsolute temperoture
35 xlO8
Fig. 3. The effect of temperature on the number average molecular weight M° (©--curve 1) and on the chain transfer with monomer constant C m (O--curve 2) for the thermal initiated polymerization of styrene. A - - calculated values from polymerization in solution. E.PJ 1411- R
The ratio Mw/M. was determined both from comparison of light scattering and osmotic pressure measurements and from gel permeation chromatography. The effect of temperature upon this ratio is shown in Fig. 4. At temperatures up to 14if, the value of this ratio lies between 1.3 and 2.0, but at higher temperatures it tends to increase to above 2, and towards 3 or higher at around 250. A value of 1.5 would be expected for a normal free radical initiated polymerization of styrene with termination by combination. However, if the degree of p o l y m e r i z a tion were controlled by chain transfer with monomer
892
W.I. BENGOUGH and G. B. PARK
where a substantial conversion occurs during the heating-up period. The value of M~/M. obtained by gel permeation chromatography was found in general to be greater than that obtained from light scattering and osmotic pressure measurements for the same sample. Also the individual values of Mw and M, obtained by gel permeation chromatography were generally lower than 20 i • when determined by the other two methods, for polystyrene produced at 170° or higher. This difference might be due to more highly branched polymer being 0 i00 ~50 2oo z50 6O formed at the high temperatures giving rise to more Ternperofure, *C compact molecules which would tend to have longer Fig. 4. The effect of temperature on the value of the ratio retention times in the gel than linear molecules of M,,/M. for the thermal polymerization of styrene. © - - de- the same molecular weight: hence they would appear termined by gel permeation chromatography: • - - d e t e r to have lower molecular weights as determined by mined by light scattering and osmometry. gel permeation chromatography. Therelationships between the intrinsic viscosity [~/] or if termination were by combination, a value of and Mw and Mn are shown in the log-log plots in 2 would be obtained for M~/M,,. From Fig. 4 it Figs 5 and 6. The molecular weight averages given appears that the ratio lies between 1 and 2 for in Fig. 5, were obtained by osmometry and gel perpolymer formed at temperatures up to 140°: how- meation chromatography, whereas those in Fig. 6 ever, the fluctuation in the values is too great to were determined by light scattering and by gel perenable any firm deductions to be made regarding the meation chromatography. Both plots are reasonably molecular weight controlling reactions. At higher linear over most of the molecular weight range, but temperatures, the value of the ratio increases. This deviations from linearity occur for the low molecular might be due to branching as a result of transfer with weight polymer when determined by gel permeation polymer, or possibly to the occurrence of chain scis- chromatography. I t appears that polymers of sion reactions resulting in some thermal degradation M~, < 100,000 or M, < 50,000 show the greatest deof the polymer. The effect of conversion on the viations. These polymers were prepared at temperaM,./M. ratio did not show any definite trend: It seems tures of 200° and above, and might be expected to possible that, because of the high temperatures used contain branched polymers. Attempts to detect in thermal polymerizations, the variation of the mol- branching using carbon-13 NMR however, proved ecular weight in the early stages of polymerization negative. is substantially greater than for polymerizations at The Mark-Houwink K and ct values were deterlower temperatures, as a result of a significant amount mined from the linear portions of the plots in Figs of polymer being formed during the heating-up 5 and 6 by the method of least squares, and are listed period. This polymer will tend to have a molecular in Table 1. weight higher than that of polymer formed at the final Literature values [14] of K and a vary somewhat temperature. This effect will be very significant for depending upon the method used in their determinapolymerizations at temperatures of 200~ or greater tion and upon the solvent used, but for toluene as
4oI
,J
0.6 J
0.2
Ore-0.2
-0.6 o 0 0 -I .C
36
o I 40
/:.j I 44
I 4B
I 5,2
I '5.6
Log ( I~I. )
Fig. 5. The relationship between intrinsic viscosity and number average molecular weight for thermally polymerized styrene, determined using © - - gel permeation chromatography; • - - osmometry.
Thermal polymerization of styrene
893
1.6
_
o~
J
%
-02
-o 6
~i" o/
0
-~o 40
o
°
"
0
l
I
I
L
I
4.4
4.8
5.2
5.6
6.0
Log (~I,) Fig. 6. The relationship between intrinsic viscosity and weight average molecular weight for thermally polymerized styrene, determined by O - - gel permeation chromatography; • - - light scattering.
Table I. K and ~t values in the equation [q] = K M ~ for polystyrene produced by spontaneous polymerization at temperatures up to 200' Method used
Molecular weight average
Osmometry Gel permeation chromatography Light scattering Gel permeation chromatography
a solvent they tend to lie within the range (13 +_ 4) × 10-5 dl/g for K and 0.71 4- 0.02 for ~. The values of K are substantially higher and those of ~t lower t h a n reported in Table 1. The reason for the difference is not obvious but may be connected with possible differences in the structure and in the molecular weight distribution of the polystyrene produced by s p o n t a n e o u s polymerization at high temperatures, c o m p a r e d with the samples used in previous studies. It will be seen from Fig. 4 that the polymer produced at the higher temperatures tends to have a wider molecular weight distribution t h a n those produced below 100 t . Nevertheless, the above relationships have been based u p o n measurements from more t h a n 30 polymer samples, and might therefore prove useful in estimating the molecular weight of unfractionated commercial polystyrene produced by thermal polymerization. It is interesting, if somewhat surprising, that the value of the molecular weight of a polymer found using K and ct values of 13 × 10 -5 dl/g and 0.71 respectively agrees well with that obtained using K and ct values of 7.14 × 1 0 - 6 dl/g and 0.92 respectively, when the molecular weight of the polymer is of the order of 106, and with that obtained using K and ct values of 35 × 10 -6 dl/g a n d 0.85, when the sample has a molecular weight of a r o u n d 104 .
M, M, M~ M~
K
× 10 6
[dl/g) 32.9 35.0 7.43 7.14
0.83 0.85 0.91 0.92
To conclude, the results agree well with those found for the thermal polymerization of styrene in diethyladipate [10], in which the rate was found to depend upon [ m o n o m e r ] 2 and are also consistent with a diradical initiation process as suggested recently [13].
REFERENCES
1. W. A. Pryor and L. D. Lasswell, Polymer Preprints I1, 713 (1970a). 2. W. A. Pryor, J. H. Coco, W. H. Daly and K. N. Houk, J. Am. chem. Soc. 96, 5591 (1974). 3. K. R. Kopecky and S. Evani, Can. J. Chem. 47, 4041 (1969a) and 47, 4049 (1969b). 4. W. A. Pryor and L. D. Lasswell, Advances in Free Radical Chemistry, Vol. 5, p. 5. Elek, London (1975). 5. O. F. Olaj, H. F. Kauffmann and J. W. Breitenbach, Makrolek. Chem. 178, 2707 (1977). 6. A. W. Hui and A. E. Hamielec, J. Appl. Polym. Sci. 16, 749 (1972). 7. F. R. Mayo, J. Am. chem. Soc. 75, 6133 (1953). 8. R. R. Hiatt and P. D. Bartlett, J. Am. chem. Soc. 81, 1149 (1959). 9. C. Walling, E. R. Briggs, F. R. Mayo, J. Am. chem. Soc. 63, 1145 (1946). 10. W. I. Bengough and G. B. Park, Europ. Polym. J. 12, 431 (1976).
894
W. 1. BENGOUGH and G. B. PARK
11. W. 1. Bengough, Trans. Faraday Soc. 53, 1346 (1957). 12. Polymer Handbook (Edited by J. Brandrup and E. H. Immergut), 2nd Edn. Wiley-lnterscience, New York (1975).
13. N. J. Barr, W. 1. Bengough, G. Beveridge, G. B. Park, Europ. Po/ym. J. 14, 245 (1978). 14. M. Kurata and W. H. Stockmayer, Advances in Polymer Science, Vol. 3, pp. 1960. Springer-Verlag, Berlin ( 1961-64).