S. MACHI,J. SILVERMAN, AND D. NETZ
930 y phase is most likely a CaFz type lattice with consider-
able deuterium-deuterium "atom" repulsion giving rise to high dissociation pressures and a high degree of nonstoichiometry. The results of a neutron diffraction investigation of VDo.5 and VD0.75 by Hardcast1el4 will soon be published. Preliminary calculations support the requirements of the structural models for the vari-
ous phases and are in overall agreement with the neutron-scattering results of Rush and Flotow. l1
Acknowledgments, Financial support for this study by the u. 8. Atomic E~~~~~commission is gratefully acltnowledged. The authors also wish to thank Mr. Thomas Nunes and Mr. Ralph Bowman for their help with the calculations.
Radiation-Induced Polymerization of Pure Styrene at Low Temperature by Sueo Machi,* Joseph Silverman, Laboratory for Radiation and Polymer Science, Department of Chemical Engineering, University of Maryland, College Park, Maryland .9074.9
and Donald J. Metz Brookhaven National Laboratory, Upton, New York
11979
Publication costs assisted by Brookhaven National Laboratory
The radiation-induced polymerization of styrene was carried out over a range of temperature from 0 to -78' using superdry styrene and wet styrene. Molecular weight distributions were determined by gel permeation chromatography. In the course of the rate studies, the time of contact between styrene and silica gel was found to be an important factor. Partially dried liquid styrene shows a bimodal molecular weight distribution which reflects the simultaneous operation of ionic and radical processes. Upon solidification, superdry styrene exhibits a sharply decreased rate of polymerization, while wet styrene shows a marked increase. The polymer from superdry styrene shows a unimodal molecular weight distribution when formed in the liquid phase and a multimodal distribution when polymerized as a solid. At -78', the rate of polymerization and the molecular weight of the product are rather low. In a sample irradiated a t -78' and then stored a t the freezing point of styrene (- 30.5') for a few hours, the polymer yield shows a large increase accompanied by the appearance of a very high molecular weight fraction in the distribution curve. Traces of water have no effect on the rate of polymerization and molecular weight distribution of frozen samples. A high yield of dimer and trimer, G = 2.5, is observed in wet liquid styrene. This can be attributed either to iiitraspur radical reactions or to water terminated ionic reaction in the bulk phase. The results in the solid state polymerization fit the assumption that a carbonium ion propagation predominates regardless of the water content.
Introduction Metz and his coworkers' discovered that styrene polymerizes extremely rapidly by y irradiation if it is exhaustively purified and dried. It has been well established by Metz, et aZ.,2-4 and Okamura, et aZ.,5-10 that the radiation-induced polymerization of superdry styrene proceeds via an ionic mechanism in the liquid state. Chen,ll Chapiro,l2,18 and Phalangas, et aZ.,14reported that the low temperature radiation-induced polymerization of styrene that was purified but not subjected to rigid drying procedures (wet styrene) showed a maximum rate slightly below the freezing point. In addition, Ueno, et u Z . , ~ reported that the polymerization rate of superdry styrene at -78' is almost the same as that of wet styrene at that temperature. The Journal of Physical Chemistry, Vol. 76, No. 6, 107.9
The present communication involves studies on the polymerization of styrene a t low temperatures in the
* Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma, Japan. (1) D. J. Met2 and C. L. Johnson, Polyn. Preprints, 4,440 (1963). (2) R. C. Potter, C. L. Johnson, R. H. Bretton, and D. J. Metz, J. Polym. Sci., Part A-1, 4,419 (1966). (3) R. C. Potter, R . H. Bretton, and D. J. iMetz, ibid., 4, 2295 (1966). (4) R. C.Potter and D. J. Metz, ibid., 9, 441 (1971). (5) K. Ueno, K. Hayashi, and S. Okamura, {bid., Part B , 3, 363 (1965). (6) K.Ueno, K.Hayashi, and S. Okamura, Polymer, 7,431 (1966). (7) K.Hayashi, H . Yamazawa, K. Ueno, K. Hayashi, K. Kamiyama, F. Williams, and S. Okamura, Abstracts of Symposium on Macromolecular Chemistry, Tokyo-Kyoto, Sept 1966. (8) K. Ueno, F. Williams, K. Hayashi, and S. Okamura, Trans. Faraday Soc., 63, 1478 (1967).
RADIATION-INDUCED POLYMERlZATION OF P U R E
931
STYRENE
liquid and solid states. The effect of phase change on polymerization rate and molecular weight distribution is described. Postirradiation polymerization was found to take place in the solid state with the formation of extremely high molecular weight products.
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Experimental Procedures The preparation of superdry styrene has been described ear lie^.^ In essence the procedure consists of vacuum distillation of monomer, vacuum bake-out of glassware, degassing of monomer, and drying with activated silica gel. In previous papers reproducible data had been reported. However irreproducibility has been a chronic problem for the last few years. In the present investigation the contact time between styrene and baked silica gel has been extended to 200 hr instead of the 17-hr period previously e m p l ~ y e d . ~In systematic studies we have observed that the polymerization rate increases with prolonged contact time and reaches constant reproducible values reported in previous work. Wet styrene was prepared by distillation through a 2-m Heli-Grid packed column under a reduced pressure of helium. It was then degassed by freeze-thaw cycles and transferred to ampoules by vacuum distillation in a sealed system. The glassware was not baked and contact with silica gel was omitted. Irradiation of the samples was done in a 'Wo irradiation facility at BNL. Low temperature control was achieved by the use of liquid nitrogen slushes of organic compounds. Benzyl alcohol, styrene, furfural, benzyl acetate, and chloroform slushes were used to provide temperatures of -14, -30.5, -36, -49, and -64", respectively. Ice-water and mixtures of Dry Ice and methanol were used to achieve temperatures of 0 and -78". Styrene can be maintained as a supercooled liquid for several hours when it is cooled to its freezing point, -30.5". If styrene is frozen a t -78" for about 10 min and then raised to -30.5", it remains in the solid crystalline state for at least several hours. By use of these techniques, both solid and liquid phase polymerizations were carried out a t the melting point temperature. Polystyrene was precipitated from irradiated liquid styrene by an excess of methanol. The polymer was filtered and dried in a vacuum oven. Per cent conversion was calculated from the weight of the dried polymer. In the case of the solid state polymerizations, each sample was quenched to liquid nitrogen temperature promptly after irradiation, after which the ampoule was opened and the frozen monomer-polymer mixture was contacted with a large amount of benzene. This rapid dissolution reduced postirradiation effects to negligible 1 e ~ e l s . l ~In some cases polymer was separated from its benzene solution by methanol precipitation. In other cases, the freeze-dry method was used to minimize the loss of low molecular weight products
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S T Y R E N E MONOMER
E L U T I O N VOLUME ( C O U N T S )
Figure 1. Calibration curves for chromatographic columns A and B.
such as dimers and trimers; the dilute benzene solution was frozen at -78" and evacuated for several hours. The molecular weight distribution of the polymer collected was measured by gel permeation chromatography, using a Watets GPC Model 200 instrument. Two different sets of columns were used. Column set A was filled with polystyrene gel of pore sizes 5 X lo6, 1 X lo6, 1 X lo6,and 3 X lo4A; set B with pore sizes of 1.5 X lo6, 1.5-5 X lo4, 1 X lo3, and 350-700 A. Each column was calibrated with 13 standard monoM,/Mn 1.20) polystyrene samples disperse (1.06 I which were obtained from several sources (Pressure Chemicals Co., Arro Laboratories, Inc., and Instruments for Industry and Research). Tetrahydrofuran (THF) was used as the eluting solvent. A polystyrene solution of 0.3% by weight was prepared in T H F and filtered under pressure in a helium atmosphere. The polymer solution of 2 ml was injected for 120 sec. A T H F flow rate of 1 ml/min and an oven temperature of 25" were used. The number average molecular weight, J T n , was calculated from the chromatogram and the calibration curves (Figure 1). I n the subsequent paragraphs, illustrations, and Ta-
