1016
DANIELW.BROWX ASD LEOA. WALL
Vol. 67
RADIATION INDUCED POLYMERIZATIOiV OF PROPYLESE AT HIGH PRESSURE1 BY DANELW.BROWN AND LEO A. WALL Xational B u r e a u of S t a n d a r d s , Washangton 25, D. C. Received September 22, 1962 The polymerization of propylene was investigated a t temperatures of 21, 48, and 83", and a t pressures between 5000 and 16,000 atmospheres. The effectof reaction conditions on the rate of polymerization and the molecular weight suggests that the polymerization proceeds by a radical mechanism with a large monomer transfer constant. The degree of polymerization was equal t o the ratio of propagation rate to transfer rate. It varied from 25 to 7 5 , decreasing with temperature increases or pressure decreases. The rate of polymerization increased by a factor of 100 a t room temperature between pressures of 5000 and 16,000 atmospheres. The activation energy was 8 kcal./mole a t 5000 and 12,000 atmospheres pressure.
Introduction A technique has been developed whereby materials can be exposed to ionizing radiation a t pressures at least as high as 16,000 atmospheres. Such pressures substantially affect the rate constants in vinyl polymerizations. propylene, for example, was found to polymerize readily with high values for the number, G( - M), of monomer units converted to polymer per 100 e.v. of energy absorbed. The kinetics of the polymerization are described herein. Experimental The pressure veesels were described previously.* Degassed propylene (10-12 g., Matheson, Reagent Grade) is condensed into the pre-evacuated, precooled bomb. -4 plug bearing a Bridgeman seal is inserted in the bore without admitting air. Enough force is applied to a piston bearing on the seal t o prevent leakage of propylene. The veesel is brought to temperature and the contents then are brought to the desired pressure by forcing the sealing plug into the cylinder. The vessel is exposed to radiation from cobalt-60, cooled, and opened. The unconverted propylene is measured by passing it through a wet test meter. The polymer is dissolved in benzene, and weighed after removing the solvent under vacuum. The polymerization rate is calculated by dividing conversion by time. The maximum conversion was 20%. The values of pressure are believed to be within 400 atmospheres of the true values; they have been corrected for deformation of the cylinder under load, packing friction, and the change in pressure that occurs when the bomb is removed from bhe hydraulic press. Heat is applied by use of an electrical winding around the bomb. The temperature is controlled to 0.5', by using a signal from a thermocouple located between wall and winding to operate the heaters. A relation a t atmospheric pressure was established between the temperature inside the bomb and the regulating temperahre. This relation was assumed to hold when the contents of the bomb were under pressure. Dose rates in the bombs were established t'hrough use of cobalt blue glass plates obtained from Bausch and Lomb Optical CO. Yumber average molecular weights were determined with a Mechrolab vapor pressure osmometer.
Results The essential kinetic data are presented in Table I and Fig. 1. Quantities listed in Table I are: R,, the fraction of monomer converted to polymer per hour, the dose rate in RIrad/hr., the square root of the dose rate, and ( D P N ) ~ the ' , reciprocal of the number average degree of polymerization. The results in this table were obtained from experiments performed a t constant pressure (14,600 atm.) and temperature (21'). Figure 1 is a plot of log lo5& and log (DPN) us. pressure. Ex(1) Based on research supported b y the U. S . Army Research Office, Durham, North Carolina. Presented a t 142nd National Meeting of the American Chemical society, Atlantic City, N. J., September, 1962. DiriPion of Polymer Chemistry. (2) L. A . Wall, D. W. Brown, a n d R. E. Florin, American Chemical Society National Meeting, Chicago, Illinois, September, 1961, Division of Polymer Chemistry, preprint booklet p. 366.
periments were performed a t 83, 48, and 21' and at constant dose rate (0.0031 Mrad/hr.). The thermal rate of polymerization (not shown) was measured a t 12,200 atmospheres pressure a t 83'; log 105R, was (-0.102). Each rate given in Table I or plotted in Fig. 1 was determined from the result of a single experiment. The inherent viscosities of many of the samples mere measured in decalin a t 135'. The viscosity average degree of polymerization (DPv) was calculated using the relation of Chiang. The maximum inherent viscosity of our samples was 0.11 whereas the minimum value in his calibration is 1.5, Qualitatively, the changes in D P v with reaction coiiditons parallel those in DPN,but the calculated DPv is as much as four times DPN. This ratio is not firmly fixed owing to the uncertainty of the extrapolation. TABLE I EFFECT O F RADIATION INTENSITY ON DPx AND Rp .4T 21" A N D 14,600 ATYOSPHERES PRESSURE R p X lo', I x 102 (1)1/2x 102 hr.-1
a
DPx-1
0 015 . . .32 0 015 .68 .016 1 08 .014 Thermal polymerization.
(Mrad/hr.)
0" 0.31 1 08 13 0
(Mrad/hr.)'/Z
0 5.6 10 4 36 0
Infrared spectra of the polymers are more like those of molten Ziegler-Natta polypropylene than the product of the low temperature halide-catalyzed polymerizat i ~ n . There ~ is evidence, however that propyl groups and vinyl groups are present in small amounts. This presumably reflects the relatively low molecular weights of our polymers. Terminal ethyl groups do not seem to be present. The polymers range from faintly colored materials with flow properties like those of low molecular weight polyisobutylene to opalescent gums so viscous that they show impressions for several days. The latter have higher molecular weights than the less viscous materials. All are soluble in benzene at room temperature; sometimes the solutions are opalescent but they pass through coarse frits without loss of polymer or opalescence. The specific volume of propylene was measured at pressures between 1,000 and 15,000 atmospheres a t 21, 48, and 83'. The values are accurate to 0.01 cc./g. and sensitive to about 0.002 cc./g. The accuracy of the pressure measurements was given above ; the sensitivity was 100 atm. The PVT data are in Table 11. They (3) R. Chian@,J . Polymer Sct., 28, 235 (1958). (4) A. D. Ketley a n d RI. C. Harvey, J . Org. Chem., 26, 4649 (1961).
RADIATION ~ S D U C E DPOLYMERIZATION OF PROPYLENE
May, 1963
1017
were used to get rates a t coiistant volume by interpolation of observed rates. 3
TABLE I1 PVT DATAFOR PROPYLEKE 15,000 14,000 13,000 12 ,000 11,000 10,000 9,000 8.000 7,000 6.000 5,000 4,000 3,000 2,000 1,000
83O
480
210
Specific vol., oo./g.
Pressure, atm.
1.164 1.175 1.186 1.199 1,214 1.234 1.255 1.276 1.301 1.333 1.364 1.406 1,460 1,537 1.659
1.166 I.178 1,190 1.203 1.219 1.238 1.258 1.281 1.307 I.339 1.375 1.420 1.475 1.558 1.688
1.175 1.187 1.198 1.213 1.232 1.251 1.271 1.296 1.325 1.357 1.396 1.439 1.500 1.588 1.725
Discussion Mechanism.-The kinetic data will be discussed in terms of the free radical mechanism Step
Reaction
1. Initiation
P.;I& 2R
2. Propagation
R, + >+ !I Rni 1
3. Transfer
Rn
+M+
I'n
4. Termination
+R
R, -k R, --+ F'n
+ Pm
Rate
~
5R n 1
m
dt
=
OR LOG(DPN),
2
I
I
I
5
IO
8
15
K I LOATMOSPHERES
Fig. 1.-Dependence of polymerization rate and number average degree of polymerization on pressure; radiation dose rate = 0.0031 Mrad/hr.; 0, 83"; A, 48'; 0, 21'.
-dM - k n J f dt
dc
LOG(105f$l
Rn
kaM 1
dt
I n these equations the concentrations of monomer, radicals, and polymer are M , R, and P , respectively. The quantities I and t are radiation intensity and time, respectively. The IC's are rate constants. If the steady-state assumption is applied to the over-all radical concentration, expressions can be obtained for the fractional rate of polymerization (R,) and DPN-'. These expressions a t constant temperature and pressure are
The measured thermal rate is so low that it can be ignored in comparison to the radiation-induced rate. Values of G ( - M ) , which can be calculated from information in Fig. 1 and Table I, range from 570 to 108,000. Such large G values indicate that the polymerization proceeds by a chain mechanism. That it is a free radical as opposed to ai2 ionic mechanism is indicated by the approximate square root dependence of rate on intensity below 0.01 Mrad/hr. (see Table I), and by the increase in rate with temperature (Fig. 1). Additional evidence against an ionic mechanism is the lack of infrared absorption attributable to ethyl groups. Such absorption was present in the product of the halide-catalyzed polymerization a t low temperature which presumably follows an ionic m e ~ h a n i s m . ~
The low values of DPN and high values of G(-M) suggest that transfer is an important part of this mechanism. Equation 2 offers a means of evalutiiig transfer relative to propagation. The quantity DPN-' should vary linearly with Rp. In fact DPN-' does not vary significantly with R,. I n terms of equation 2 this means that (2k4/IC22M)(R,) is much less than IC&z. Qualitatively, the DPNis regarded as being determined by the ratio lc3/kz. Therefore equation 2 can be replaced by equation 3 DPN
k2
= 3
(3)
The ratio has a maximum value of 7 2 in this work; molecular weights are 3,000 or less. At intensities above 0.01 Mrad/hr. the rate of polymerization increases less rapidly than the square-root of intensity. This effect is observed with other moiionzers and is attributed to terminatioii of an appreciable portion of the initiating radicals by other initiating radicals and by polymeric radicals; it is discussed by Chapiro and M ~ i g a t . ~A referee has commented that PDN a t this intensity should be less than at intensities at which the kinetics follow the square-root relation. A calculation by us indicates that the observable effect would be about 10% if the G-value for formation of radicals is 10 and about 1% if this G value is 1, since about 60 mole % of the polymeric products from the two reactions suggested above and all polymer formed by transfer reactions of the specific radicals formed by radiation would evaporate with the monomer. The sensitivity of individual DPN measurements is .t5% so no effect need be observed. The large ratio of DPv to DPN may indicate that there is transfer of radical activity to polymer. We do ( 5 ) A. Chapiro a n d M. Magat, "Actions Chimiques a t Biologiques Des Radiations," Masson e t Cie, 1958, p. 90.
DANIEL M 7 . BROWN AND LEO A. %‘ALL
1015
not consider this type of transfer since it does not affect DPN or R,. Moreover, the exponent iii the relation between inherent TTiscosity and molecular weight need only to be changed by 7y0to make DP\T/DPNless than 2 . A value less than 2 is compatible with thc proposed mechail ism. Equations 1 and 3 are applicable a t 14,600 atmospheres pressure and 21” at dose rates up to 0.01 Rlradi hr. If temperature and pressure are changed, do these equations still apply? The activation energy for propagation is greater than that for termination in vinyl polymerization. M’ith propylene that for transfer is greater than that for propagation since DPN decreases as temperature jncreases (see Fig. 1). Therefore, increases in temperature a t coiistant presxre should extend the applicability of both equations. Decreases in pressure a t constaiit temperature decrease DPN (see Fig. 1) and so suggest that equation 3 applies. Since R, decreases as the pressure is reduced, some reduction probably occurs i1i the range of equation 1. I t is not known whether it is sufficient to afi’ect interpretation of results obtained at 5,000 atmospheres pressure at 0.0031 ?Ilrad,’hr. We have assumed that equation 1 amlies under such conditions. Effect of Pressure.-Absolute rate theory offers the most straightforward interpretation of the effects of temperature alld pressure on R, and D P ~ ,If each rate constant is formulated in terms of this theory, i . e . , k j = V / ! T e x p ( - ~ ~ j * ~ ~ ~1; )is the molar volume, A is the ratio of the Boltzmalln to Planck constallt alld the other terms are defined belo\T, and the constants are combined in accord Tyith equation 3, the result for :Lreaction a t constant pressure is
log lO’R,
=
1-01. 67
[(-AI‘**
4
+ AV4*/2) -
(- AFZ*
$- AF3*)
(3’)
RT
I n this equation AF* is the change jn free energy in forming the transition complex from the reactants, the subscripts refer to the steps in the mechanism, R is the gas constant, and I’ is the absolute temperature. The variation of log DPN with pressure a t constant temperature can be shown to be6 log DPN =
+
(-AV2* AVs*)P 2.3RT
+
c1
(3”)
I n this equation P is the pressure, C1is an integration constant, and the AV*’s are volume changes that occur in forming the transition states. In getting this integrated form the sum of the AV*’s is assumed to be independent of pressure. In accord with this assumption log DPx varies linearly with pressure (see Fig. 1). The algebraic sums (AVz* - AV3*) are -1.57 and - 1.67 cc./mole at 21” and 83’’ respectively. I n arriving a t arialogous expressions for R, ai1 additional assumption is useful because it eliminates a molar volume term from the final equations. It is that the G-value for radical formation, G(R), is independent of temperature and pressure. This seems a reasonable assumption in view of t,he low activation energy exhibited by radiation-induced reactions that do not proceed by a chain mechanism. The expressions for E , analogous t o 3’ and 3” are, respectively (6) S. D. Hamann, “Physico-Chemical Effects of Pressure,” Academic Press. New York, N.Y., 1957, p. 162.
+ cz
(1”)
2.31-1T
I n these equatioiis the quantities not previously defined are: X,, the fractional rate of iiiitiation, IC, the Boltzmaim constant; h, Planck’s constant. The quantity log 105zZ, varies linearly with pressure (see Fig. 1). The sum (AV2* - 4Ve*’2) in cc./mole is: -9.62 a t 21°, -9.64 at 48”, and -12.22 at 83’. The value at 48” should probably be more nearly intermediate between the others. It certainly seems unlikely that the slope of the line at 48” should be less than at 21’ or a t 83’. Effect of Temperature.-The variation in log lO’R,, with temperature at constant pressure can be shown to he log 105Rp
‘/i
log T
- AHz* + (AH4*/2) + 2.3RT
c 3
(1”’)
- *
DPN = exp
P
111 this equatioll AH* is the challge in ellthalpy that occurs when the transition complex is formed. If the temperature is varied and the volume is kept constant, the expression for log 105R, is identical with 1’” except that the AH* is replaced by AE*, the change in internal energy. Thus the AH* term in equatioll (1”’) can be calculated from the slope of isobars of (1% 10’R~- 1, log T) vs. T-l. Similarly, AE* terms may be calculated if values of R, at constant volume are used. These were obtained by herpolating ordinates in Fig. 1 by use of the P V T data in Table 11. Since the pressure increases with temperature a t constant volume the slope of the latter isobars is greater than a t constant pressure. Consequently, values of the A E * term are greater than those of the AH* term. This is in accord with the PAV*. negative AV* since AH* = AE* Table I11 lists representative values of the AE* and AH* terms. They are both rather small compared with over-all activation eiiergies observed in thermal polymerizations; this suggests that the initiation is temperature independent as assumed. Both terms increase slightly with pressure. Since the rate increases \.T ith pressure the over-all AF* must decrease and as AH* increases with pressure the over-all AF* must decrease and as AH* increases slightly all the decrease in the over-all A F * comes from the increase in the entropy terms. Effect of Pressure on Entropy Change.-This call be put on a quantitative basis in several mays. We assume a value of G(R) ; this enables us to calculate 12, and therefore exp(- AFz* AF4*,’2)/RT by equation 1. From this can be calculated (APz* - AF4*/2). Values of (AS,*- AS4*/2) can be calculated from the relation A F = AH - TAS. Results of these calculations for G(R) = 1, G(R) = 10, and T = 294’K. are in Table 111. Between 5,000 and 16,400 atmospheres the quantity (AF2* - AF4*/2) decreases by about 2,500 cal./mole. All the decrease must result from the increase in (AS,*- A&*/2) since (AH,* - AH4*/2) increases slightly. Changes with pressure of the AF* and AS* terms are not affected by the value assumed for
+
+
RADIATIOS ISDUCED POLYMERIZATION OF PROPYLEXE
M a y , 1963
IO19
TABLE I11 QUASTITIESIN RATEEXPRESSIONS AT 21 SUMSOF TERXS IN RATEEXPRESBIOX
THERMODYXAMlC Pressure (k atin.)
AF*, cal./mole
Form of suma
AH*, cal./rnole
AS*, cal./mole deg.
AE*, cal./mole
AH* - AE*, cal./mole
PAT'*, cal./mole
15,860b 7800 -27.3 9050 - 1250 - 1160 14, 450b 8160 -21.4 9860 - 1700 - 2770 13 ,270h 8400d -16.6 16,540" 7800 -29.7 15,150" 8160 -23.8 16.4 13 960" 8400d -18.8 5.0 -2,070 - 855 4.13 - 676 - 179 - 190 12 0 -2,330 - 760 5.34 - 544 -216 - 453 16.4 -2,500 - 700d 6.12 Each thermodynamic function to the right is made up of elements having tfheform defined in this column. Assumed that G(R) = 1. Assumed that G(R) = 10. By extrapolation of values a t 5 and 12 katm. 5.0
A:* -- A4*/2
12.0 16.4 5 0 12 0
52* -. Aa*/2 A2* -. A4*/2 A2* - &*/2 A2* -. A4*/2 A2* -. A4*/2 Az* -. Aa* A?* -. A3* Az* -. As*
G(R). Assuming G(R) enables us to calculate the absolute value of the AF* and AS*. Indeed, the changes in the sums of the A,F* and AS* terms could have been calculated from any two rate constants. An alternative way of calculating the change in (ASa* - AS4*/2) is of some interest. Thermodynamically ( b X l b P ) equals ~ -(dV/bT)p. If we approximate (d(AVz*
;--AV4*/2)
>p
(AVz* -
by
AV4*/2)830
- (AV2* -
AV4*/2)210
83 - 21 we obtain -0.0419 >: l o w 3liter/mole deg. from values of the AB* terms given previously. According to our data this figure is independent of pressure. Direct integration is possible and gives A(AS2* - AS4*/2) = 11.5 cal./mole deg. betiveen pressures of 5,000 and 16,400 atmospheres. From Table IIJ by subtraction of AS* terms one obtains the value 10.8 cal./1moledeg. Difference between AH" Term a r d AE*.-The difference between the AH* term and AE* term sbould equal P(AV2* - A174*/2) since pressure is constant during formation of the transitio'i con 1) I < s Values of the difference between the eLit'-iZlw nd internal energy terms and also P(AVZ* - AV?*,'2) arc i n Table 111. The agreement is good at 5,000 atmospheres but the volume term becomes larger than the other term a t higher pressure. Probably the error js in the difference
between enthalpy and internal energy since the data from which these were calculated scatter considerably. Thermodynamic Quantities from DPN.--The lower portion of Table 111 gives results calculated from the variation of DPw with temperature a i d pressure. Smaller changes are obtained throughout since DPN changes relatively less than does R,. The negative AF* term indicates that propagation is faster than transfer, as it must be if polymer is obtained. The negative AH* term and positive AS* term show that propagation is favored by both enthalpy and entropy sums in the rate expression. PVT Data.-Coiisideration of the data in Table 11 indicates that propylene becomes more difficult to compress as the pressure is raised or the temperature is lowered. There are apparent point to point exceptions to this generalization, but they are not significant under the precision limits given in the Results section. At atmospheric pressure and room temperature isotactic polypropylene has a specific volume of 1.09 cc./g.' This is less than the lowest specific volume of the monomer that we obtained. This indicates that the average intermolecular distance in the monomer at 15,000 atmospheres pressure is still considerably greater than normal bond lengths. Acknowledgment.-The authors thank Mr. Edward P. Regalis, Department of Agriculture, for the use of the Mechrolab vapor pressure osmometer. (7) G. Natta, Anoew. Cham., 68, 393 (1968).
