NOTES
3885 -
-.--
Direct Observation of Transient Intermediates in the Pulse Radiolysis of Isobutylene
by E. J. Burrell, Jr.' Radiation Physics Laboratory, Engineering Department, E. I . d u Pant de Nemours and Company, Wilmington, Delaware (Received J u l y 6, 1964)
An ionic mechanism has been postulated for the radiation-induced polymerization of liquid isobutyllene.2-4 This note presents the results of the direct observation of the transient ionic species in this reaction and the determination of the absolute rate constant for termination. Two principal methods of observation were employed : transnent ultraviolet absorption spectroscopy and transient dielectric constant measurements. The radiation source was 8 Varian linear electron accelerator (Lineac) which introduces about 3 joules of energy/ 5-psec. pulse of 6-Ptlev. electrons into the cold isobutylene sample. A transient absorption at 2970 8. in the isobutylene sample, which disappeared by a first-order process, was induced by the Lineac pulse. Trimethylcarboniuni ions are thought to absorb in this r e g i ~ n . ~The lifetime and intensity of the absorption depend on temperature and various additives, such as Oz and NzO. These results are shown in Table I.
Table I : Radiation-Induced Transients a t 2970 A. in Liquid Isobutylene
Temp.. OC.
Max. absorption,
tl/ e;
kterm,
%
Additive
880.
880. -1
- 60
10
- 15 - 59
10
None None
0.1 0.05 0.1 0.10 0.040
- 55 - 10
5
20 8
0 2
NzO Nz0
...
... ... 7.0 17.0
at 2970 8. This is in marked contrast to the very strong absorption with second-order decay observed in oxygen-saturated methylcyclohexane. The latter signal is due to the well-known termination of two peroxy radicals. Polymer was formed in the Lineac runs and typically had a molecular weight of 10,000 to 35,000 depending on temperature. These molecular weights were undoubtedly governed by chain transfer and are significant only in that polymer was formed. The second method of observation consists in ineasuring the change in dielectric constant of liquid isobutylene induced by a Lineac pulse. A 12-leaf capacitor was constructed in a small Pyrex irradiation cell. The cell was filled with isobutylene at -78" and the cell capacitance balanced by means of a General Radio capacitance bridge, Type 716-CS1, energized by a 1.000-Mc. oscillator. The unbalance in the capacitance cell induced by the Lineac pulse was amplified by a factor of 105 and mixed with a 1.0-v. signal from a 1.001RIc. oscillator, producing a 1-kc. beat. The resulting signal was fed through a tuned amplifier and presented on an oscilloscope whose sweep was triggered by the Lineac pulse. The shape of such a signal implies that the formation of polymer (of different dielectric constant) is being observed rather than the direct observation of ion pairs. However, the signal is a measure of the same termination rate constant, being of the form 1 - eckt. The first-order rate constant for termination measureld in this way is 0.07 set.-' at -78". This is qualitatively consistent with optical runs at higher temperatures. This variation in termination rate constant with temperature is characteristic of ionic polymerizations, whereas the lifetimes of the transients observed are much too long for radiation-induced free radical reactions. The dielectric constant cell could not be conveniently stabilized against temperature drift at any low temperature other than -78". When optical runs were made at this temperature an insoluble polymer formed which scattered light. However, the transient signal ~~
The best data were obtained when NzO was present at its saturation concentration. The NzO molecule is known to attach electrons very efficiently at any low energy and apparently allows the ionic polymerization to proceed more efficiently. On the other hand, no signal as long as 1 msec. was observed for a saturated solution of NzO in niethylcyclohexane. When oxygen-saturated isobutylene is irradiated, a rather weak signal with a first-order decay is observed
~~
~~
~
~~~
~~~~~
(1) Chemistry Department, Loyola University, Chicago, Ill. (2) W. H. T. Davison, S. H. Pinner, and R. Worrall, Chem. I n d , (London), 1274 (1957).
(3) E. Collinson, F. S. Dainton, and 1%.A. Gillis, J . Phys. Chem.. 6 3 , 909 (1959). (4) F. W. Lampe, ibid., 63, 1986 (1959). (5) J. Rosenbaum and M. C. R. Symons, Proc. Chem. SOC.,92 (1959); Mol. Phus., 3, 205 (1960); cf., however, N. C. Deno, D. B. Boyd, J. D. Hodge, C. U. Pittman, Jr., and J . 0. Turner, J . Am. Chem. Soc., 86, 1745 (1964). (6) R. L. McCarthy and A. MacLachlan, J . Chem. Phys.. 35, 1625 (1961).
Volume 08,Y u m b e r 19
December. 198':
NOTES
3886
observed in this case is still consistent if one assumes a transient form, 1 - e--kt*, to take into account light scattering as a function of molecular weight of the growing polymer. It was also noted that a Lineac pulse induced ion pairs in the liquid isobutylene as determined by collecting charge between the plates of a capacitor under an electric field of 4000 v./cin. An oscilloscope triggered by the Lineac beam monitored the charge collected. The current flowed only during the Lineac pulse and therefore consisted of electrons which were collected in times short compared to 5 psec. and positive ions which did not move appreciably in this time. The charge collected from the sample was typically 6.5 x 10+ coulomb as compared to the Lineac beam current which gave only 2.0 X lo-' coulomb, thus establishing the presence of induced ion pairs as distinct from the Lineac beam current.
Acknowledgment. It is a pleasure to acknowledge P. C. Hoe11 and A. van Roggen for designing the apparatus for the transient dielectric constant measurements and V. F. Damme for constructing some of the equipment.
These give a total correction' of -0.148 A-unit. All solutions were made up by weight. The water used had a conductance of 1.65 x the maximum solvent correction was 0.65%. The cell had a constant equal to 1.0115 f 0.0001. Bridge, cell, and methods have been described previously.8 The conductance data (3 runs) are summarized in Table I. In order to obtain limiting conductances, Table I : Conductance of Cesium Bromide in Water at 25" 104~
82.824 65.955 41,442 24.924 14.540 78.639 56.192 40.152
A
10%
A
146.85 147.64 149.12 150,41 151.50 147.06 148.21 149.20
24.447 13,136 99,290 71.182 48.769 27.465 15.101
150.46 151.66 146.19 147.42 148.65 150.21 151.41
extrapolation was made on a A'-c plot by the method of least squares, using the equationsg
+ Scl" - EC log c = + JC
A' = A (obsd.) A'
Conductance of the Alkali Halides.
X.
The Limiting Conductance of the Cesium Ion in Water a t 25"
by Claude Treiner,' Jean-Claude Justice, and Raymond M. Fuoss2 Con,tribution N o . 176.6 f r o m the Sterling Chemistry Laboratory of Yale Unizersity. Mew Haven, Connecticut (Received J u l y 1.6, 1964)
Several recent determinations of the conductance of cesium salts in water lead to values for the cesium ion conductance which disagree by more than the estimated errors of measurement : 77.20 from cesiuni iodid@ and 77.33, 76.46, and 76.92 from cesium chloWe present here data for cesium bromide, which give Ao(Cs+)= 76.77, which is in excellent agreement with the weighted average of the other determinations. The cesium bromide was used as received from the Harshaw Chemical Co. ("random cuttings," from fused salt]). It was stored over phosphorus pentoxide in an evacuated desiccator. Analysis by the flame photometer showed trace impurities as follows: 0.0370/, LiBr, 0.076% NaBr, 0.00370 KBr, and 0.032% RbBr. T h e Journal of Physical Chemistry
A0
(1) (2)
For the three runs, the values A 0 = 155.157 f 0.013, 155.135 f 0.009, 155.143 f 0.0'17 and J = 180 f 2, 186 f 2, 186 f 3 were found. These average to d o = 155.15 f 0.02 and J = 184 f 2. Correcting for the impurities, Ao(CsBr) = 155.00 f 0.02. Using Longsworth's value'o of 0.4906 for the transference number of potassium in potassium chloride, and Lind's value8 of 149.89 for Ao(KC1), we obtain XO (E(+)= 73.54. From Kay's extrapolations" of the data for potassium b r ~ m i d e , ' ~we - ~have ~ Ao(KBr) = 151.77; (1) DuPont Postdoctoral Research Fellow, 1963-1964; on leave of absence from the University of Paris. (2) Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemlcal Society, for partial support of this work. (3) J. E. Lind, Jr., and R. M. Fuoss, J . Phys. Chem., 6 5 , 1414 (1961). (4) W. E. Voisinet, Thesis, Yale University, 1951. (5) J. C . Justice and R. M . Fuoss, J . Phys. Chem., 67, 1707 (1963). (6) F. Accascina and M. Goffredi, University of Palermo, private communication. (7) 12. IT'. Kunze and R. M. Fuoss, J . Phys. Chem., 67, 914 (1963); see eq. 11. (8) J. E. Lind, Jr., and R. M .Fuoss, ibid., 6 5 , 999 (1961). (9) R. M. Fuoss, J . Am. Chem. Soc., 81, 2659 (1959). (10) L. G. Longsworth, ibid., 54, 2741 (1932). (11) R. L. Kay, ibid., 82, 2099 (1960). (12) B. B. Owen and H. Zeldes, J . Chem. Phys., 18, 1083 (1950).
