Study of the Free-Radical Polymerization of Styrene by Differential Scanning Calorimetry John R. Ebdon and Barry J. Hunt Department of Chemistry, University of Lancaster, Lancaster, England
A comprehensive study of the kinetics of a simple freeradical polymerization by conventional techniques is usually time-consuming and requires the use of fairly large quantities of monomer, solvents, and initiators. Thus, any technique which can provide useful information rapidly and on small quantities of material is welcomed. The use of differential scanning calorimetry to follow the course of free-radical polymerization in bulk monomers has been demonstrated by Horie e t al. (1, 2) and more recently a similar technique has been used to study polymerization in emulsion (3). Haas e t al. (4) have measured overall activation energies for the bulk polymerization of several monomers by an analogous differential thermal analysis method, but some of the values they obtained are not in good agreement with those from conventional methods. This paper describes the examination of some kinetic features of the free-radical polymerization of styrene by isotherma] differential scanning calorimetry. The object of the work was to assess the usefulness of the technique for obtaining reliable quantitative information about freeradical polymerization kinetics. EXPERIMENTAL Apparatus and Reagents. The instrument employed was a Perkin-Elmer Differential Scanning Calorimeter type DSC-1B. Styrene monomer (B.D.H. Chemicals Ltd.) was used without further purification. If was unnecessary to remove the 10-20 ppm tert-butyl catechol stabilizer. Xylene (B.D.H., Analar grade) was used as a solvent in some polymerizations without further purification. Benzoyl peroxide (B.D.H.) was used as an initiator. I t was purified twice by dissolving in chloroform and precipitating with an equal volume of methanol, and was dried by pumping in a vacuum oven overnight at room temperature. p-Benzoquinone (Fisons Scientific Apparatus Ltd.) was used as an inhibitor without further purification. Procedure. Small quantities of styrene containing benzoyl peroxide and p-benzoquinone were encapsulated in standard PerkinElmer hermetically sealable sample pans (Part No. 219-0062). The samples were then placed in the DSC-lB, the temperature set manually to the desired value, and the polymerization exotherms recorded on a chart recorder. The benzoquinone was used to delay the onset of polymerization so that a steady base line could be established at the outset. It was used a t an overall concentration of 3.3 x mole per liter in all experiments. A similar procedure was adopted by Horie et al (11 It was unnecessary to remove dissolved air from the samples before encapsulation, although care was taken to exclude bubbles of air from the pans. After sealing, the pans were washed with a little acetone to remove external drops of styrene and were blown dry. The weight of sample used was recorded for each experiment. Also, the pans were checked for weight loss after polymerization. Leakage was not a serious problem provided that the pan sealing tool was not overtightened, and weight losses were usually less than 1%. (1) K. Horie, I. Mita, and H. Kambe, J. Polym. Sci., Part A - 7 , 6, 2663 1196AI I ___,.
(2) /bid., 7,2561 (1969). (3) K. E.J. Bartlett, Brit. Polyrn. J.. 2,45 (1970). (4) H. C. Haas, M. J. Manning, and S. A. Hollander, J. Polym. Sci., Part A - 7 , 8, 3657 (1970). 804
0
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
Complete exotherms were recorded for bulk styrene at 80, 90, 100, and 110 "C (expts 1, 2, 3, and 4), and initial portions of exotherms were recorded for polymerizations at 70, 75, and 85 "C (expts 5, 6, and 7). In these experiments, a benzoyl peroxide concentration of 0.10 mole per liter was employed. Initial portions of exotherms were recorded also at 80 "C for three samples of bulk styrene containing 0.05, 0.075, and 0.15 mole per liter of benzoyl peroxide, respectively (expts 8, 9, and lo), and for three samples of styrene dissolved in xylene to give overall styrene concentrations of 82.5, 75, and 66.7 mole 70,respectivly (expts 11, 12, and 13). The benzoyl peroxide concentration in the last three experiments was 0.10 mole per liter. Xylene was chosen as a solvent because it boils at a higher temperature than t h a t used in any of the experiments, and because its density and molecular weight are close to those of styrene. Thus volume fractions of styrene in xylene can be equated directly with mole fractions.
RESULTS AND DISCUSSION The use of standard sample pans proved quite satisfactow for these experiments, and it was not necessary to the Special encaPsulation Procedures used by Horie ,
1
'".
9.,
A complete polymerization exotherm obtained a t 80 "C with bulk styrene is shown in Figure 1. It is similar to that obtained by Horie et al. ( 1 ) and illustrates the rapid onset of polymerization following an initial induction period, and the gel effect in the later stages of the reaction. Plots of percentage conversion to polymer us. time can be constructed from the exotherms if the areas under them a t various times are measured and if the total area is assumed to correspond to 1000/0 conversion to polymer. Such plots for bulk polymerizations a t 80, 90, 100, and 110 "C are shown in Figure 2. From these plots, it can be seen that the gel effect is less pronounced a t 90 "C than a t 80 " C and is absent a t 100 "C and above. Initial Rates and Activation Energies. Initial rates of polymerization have been estimated from the initial heights of exotherms and have been converted to specific rates (per gram of monomer) by dividing by the weight of styrene in the sample. These specific rates, although in arbitrary units, are self-consistent. A conventional Arrhenius plot of the specific initial rates at temperatures between 70 and 90 "C obtained from expts 1, 2, 5 , 6, and 7 is shown in Figure 3. It gives an overall activation energy for polymerization of 19 Kcal, which is in reasonable agreement with the literature value of 22 Kcal(5i. A second Arrhenius plot has been constructed from the reciprocals of the induction periods observed in expts 1, 2, 5 , 6, and 7 and is shown in Figure 4. The exact length of an induction period is difficult to establish, but the method of measurement indicated by Figure 5 was found to give reproducible and self-consistent values. The' activation energy obtained is 26 Kcal. If it is assumed that the rate determining reaction during inhibition is the decomposition of the- initiator, then t h i s activation energy should represent that for benzoyl peroxide decomposition. In fact, J . Fiory, "Principles of Polymer Chemistry," Cornell University Press, Ithaca. N . Y.,1953,p 124.
( 5 ) P.
0
40
120
80
I60
time (min)
Figure 1. Complete polymerization exotherm at 80 "C \/temp
x 10)
("K-')
Figure 4. Arrhenius plot of reciprocal induction periods for polymerizations between 70 ' and 90 OC
I /
trace
\
0
20
40
60
80
100 I 2 0
140
-induction
period
-
I
time (rnin)
Figure 2. Percentage conversion to polymer vs. time for polymerizations at ( a ) 80",( b ) go", (cj 100' and ( d ) 110°C
I
Figure 5. Method of estimating length of induction period
0.7
Figure 3. Arrhenius plot of specific initial rates of polymerization between 70 ' and 90 "C
log
0.9
1.1
peroxide concentration+2
(concentration in mole per l i t r e )
it is within 3 Kcal of the values normally quoted for this reaction in solution (about 29 Kcal) (6). Order with Respect to Benzoyl Peroxide and Styrene. A plot of loglo specific initial rate us. loglo peroxide concentration using the data from expts 1, 8, 9, and 10 is shown in Figure 6. It indicates that the order of reaction with respect to benzoyl peroxide a t 80 "C is 0.53 0.06. The order with respect to styrene can be obtained from the plot of loglo specific initial rate us. loglo styrene concentration shown in Figure 7. The plot is constructed from the data of expts 1, 11, 12, and 13 and indicates an order of reaction with respect to styrene at 80 "C of 1.2 f 0.3. Both these orders of reaction agree, within experimental error, with the theoretical values for a simple free-radical polymerization of 0.5 and 1.0, respectively (7). Heat of Polymerization. In principle, it should be possible to use the area under a polymerization exotherm to
Figure 6. Log-log plot of specific initial rate vs. benzoyl peroxide concentration
*
(6) "Polymer Handbook," J. Brandrup and E. H. Immergut. Ed., Interscience, New York. London, and Sydney, 1966, p 11-28. (7) F. W . Billmeyer, Jr., "Textbook of Polymer Science," Interscience, New York and London, 1st ed., 1962, p 274.
([MI = volume f r a c t i o n ot styrene) Figure 7. Log-log plot of specific initial rate vs. styrene concentration ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
805
obtain an estimate of the heat of polymerization. However, in practice, we have found that consistently low values of between 12 and 14 Kcal mole-I are obtained for styrene from our data, even after correcting for incomplete conversion to polymer ( I ) , compared with the literature value of 16.4 Kcal mole-I (8). Undoubtedly, this is partly due to the difficulty in calibrating the instrument for a reaction which takes plme over a period of one or two hours. Calibration is normally accomplished by melting a standard substance with a known latent heat of fusion, such as benzoic acid, and then measuring the area of the resulting endotherm. Obviously, such a melting process occurs very rapidly and a calibration based on this will not allow for heat losses during a lengthy polymerization. Nevertheless, realistic heats of polymerization have recently been obtained in this laboratory by this method for (8) F. S. Dainton, K. J. Ivin, and D. A G. Walmsley, Trans. Faraday S O C ,56, 1784 (1960).
monomers which polymerize more rapidly than styrene under similar conditions, e.g., methyl acrylate and acrylonitrile (9). Thus, it can be seen that differential scanning calorimetry can provide quantitative information quickly and easily on various aspects of the kinetics of a simple free-radical polymerization and uses only very small amounts of material. We suggest, therefore, that the technique could be used with advantage in a preliminary examination of the kinetics of a free-radical polymerization and also could form the basis of a useful undergraduate or postgraduate teaching experiment on polymerization. Further work on other aspects of polymerization and using other monomers is in progress. Received for review October 15, 1972. Accepted November 27, 1972. (9) J. A. L. Jemmett, Thesis, Lancaster University, England, 1972.
Comparison of Selective Ion Monitoring and Repetitive Scanning during Gas Chromatography-Mass Spectrometry B. S. Middleditch and D. M. Desiderio Institute for Lipid Research and Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77025
The use of the mass spectrometer as a highly selective detector during gas chromatography-mass spectrometry (GC-MS) has become increasingly popular in recent years ( I , 2). There are two methods of using a gas chromatograph-mass spectrometer in this mode. In the first, complete or partial mass spectra are scanned repetitively and "single ion" chromatograms are reconstructed later, either by hand (3) or with the assistance of a digital computer (4, 5). In the second method, selective ion monitoring (SIM) is carried out during GC/MS by tuning the mass spectrometer to detect ions of only one m / e value (6). More than one mle value per run may be monitored if an instrument with multiple detectors is employed (7), or if the accelerating voltage is alternated to permit sequential detection of the ions of different m / e value (8). The SIM method can only be used if the mle values of interest are known a t the start of the run. The repetitive scanning (RS) method is therefore far more flexible and would appear to be the method of choice, particularly if a suitable computer-assisted data acquisition and manipulation system is available. ( 1 ) C. J. W. Brooks and B. S. Middleditch. "Modern Methods of Steroid Analysis," E. Heftmann, Ed., Academic Press, New York, in press. (2) C. J. W. Brooks and B. S. Middleditch. "Specialist Periodical Report: Mass Spectrometry," Vol. 2, D. H. Williams, Senior reporter, Chemical Society, London, in press. (3) R. A. Appleton and A. McCormick, Tetrahedron. 24, 633 (1968). (4) R . A. Hites and K . Biemann, Ana/. Chem.. 42, 855 (1970). (5) R . Reimendal and J. Sjovall, Anal. C h e m . . 44, 21 (1972). (6) D. Henneberg. F r e s e n i u s ' Z Anal. C h e m . . 183, 12 (1961). (7) V . L. Tal'rose, V. A. Pavlenko, G. D . Tantsyrev, V. D . Grishin. L. N. Ozerov, A. E'. Rafal'son, and M. D . Shutov, Prib Tekn. Eksper.. 10, 130 (1965). (8) C. C. Sweeley, W. H . Elliott, I . Fries, and R. Ryhage. Anal. Chem.. 38, 1549 (1966).
806
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4 , APRIL 1973
The SIM method permits detection of picogram (9-11) or maybe even femtogram (12) quantities of certain samples. This is well below the level a t which acceptable mass spectra can be obtained using conventional mass spectrometers. However, it has been demonstrated that an LKB 9000 GC-MS instrument can provide partial mass spectra of 10 ng of prostaglandin derivatives (13) and that a Finnigan 1015C quadrupole instrument can be used to obtain mass spectra of chlorinated hydrocarbons in similar amounts (14). Because of the increasing applicability to biochemical problems, it is important to know the relative sensitivities, limitations, and versatilities of the RS and SIM methods, yet there is surprisingly little published information on this subject. We have accordingly investigated this problem using cholestane as a test substance.
EXPERIMENTAL Sample Preparations.
F i v e m g o f 5a-cholestane were dissolved
in 100 ml of N a n o g r a d e e t h y l acetate t o give a stock s o l u t i o n of c o n c e n t r a t i o n 5 mg p e r ml. A secondary stock s o l u t i o n of concent r a t i o n 1.5 mg p e r ml was m a d e by d i l u t i o n of t h e f i r s t . Aliqu,ots
of
each
of these
stock solutions were c a r e f u l l y d i l u t e d
by factors of
(9) C. J. W. Brooks and B. S. Middleditch, Ciin. Chim. A c t a . 34, 145 (1971). (10) L. Siekmann. H . - 0 . Hoppen, and H. Breuer, Fresenius. Z Ana/ Chem., 252, 294 ( 1970). (11) S . H. Koslow, F. Cattabeni. and E. Costa, Science. 176, 177 (1972) (12) H . Adlercreutz, Abh. Deuts Akad. Wiss. Berlin. K / M e d , 1968, 121. (13) C. J. Thompson, M . Los. and E. W. Horton. Life S c i . pt 7 . 9, 983 (1970). (14) E. J. Bonelli. Anal. C h e m . . 44, 603 (1972).
