COMMUNICATIONS TO THE EDITOR
3711
I
80
100
120
I
I
140
I6O/ L
i
KRYPTON
TEMPERATURE
OK
Figure 2. The self-diffusion coefficient along the , experimental temperature range; coexistence curve: I++-] , regression eq I ; - - - -, 95% confidence limits oneq 1; ---, PD extrapolation of experimental data;* - . - -,linear trajectory approximation.2
value) are obtained: argon (85OK) 167%; krypton (120°K) 167%; and xenon (200OK) 82%. The P D analysis does not seem a convincing verification of the LTA-RA theory. DIVISION OF CHEMISTRY AND CHEMIGAL ENGINEERING CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA 91109
A. F. COLLINGS C. J. PINGS
It has been tentatively proposed that the field-facilitated dissociation of ion pairs at growing chain ends may be responsible for the accelerating effect, on the assumption that the propagation rate constant of free ions (k,") is much larger than that of ion pairs (kp'). Though this interpretation appeared reasonable, it lacked clear support because of the intricate mechanism of the cationic polymerizations. I n the present paper, we give some experimental data for a living anionic polymerization of styrene initiated by n-butyllithium (n-BuLi) in a series of mixtures of benzene and tetrahydrofuran (THF), where the propagation process only needs to be considered. The possible influence of even small impurities was taken into account; our experiments were carried out using a high-vacuum line and CaH2, Na-K alloy, or S a mirror as drying agents. The n-BuLi was prepared from butylbromide and lithium Polystyryllithium, used as seed polymer, (degree of polymerization, about 1.5) was prepared with n-BuLi. The polymerizations were conducted at 2.5' in a glass vessel specially designed and equipped with an optical cell. A pair of parallel platinum electrodes, on which the de voltage was applied, was sealed into the cell. The polymerization rate was determined spectrophotometrically. Table I gives the main results. It is worth mentioning first that the observed propagation rate constant without field (kPJ agrees well with the value reported p r e v i ~ u s l y . ~Second, the rate constant in a
$
3.0
5 X
RECEIVED MAY19, 1967
I
1.0
0
Living Anionic Polymerization under an Electric Field
Sir: For a better understanding of the growing species in polymerization reactions, the influence Of an field on the rate of polymerization has been investigated. Our results show that the rates of cationic polymerizations are generally increased by the field, whereas free radical polymerizations are not affected.*
I
I
I
300
600
9nn
Time, sec.
Figure 1. Time dependence of the concentration of living ends for THF, 40 vol. %, 2.5': 0 , 5 kvlcm; 0,0 kv/cm.
(1) See, for example, I. Sakurada, N. Ise, and T. Ashida, Makromol. Chem., 9 5 , l (1966); I . Sakurada, N. Ise, Y. Tanaka, and Y. Hayashi, J . P o Z ~ ~ ZSW C ~ .A-1, , 4, 2801 (1966). (2) T. Fujim'oto, N. Oraki, and 11. Nagasawa, ibid., A - 3 , 2259 (1965). (3) s. Bywater and D. J. Worsfold, J . Phys. Chem., 7 0 , 162 (1966).
Volume 71, Number 11
October 1967
COMMUNICATIONS TO THE EDITOR
3712
field of 5 kv/cm (kD~)is larger than k,, except for 10% T H F mixture. It seems that the accelerating effect increases with the increasing dielectric constant (D)in agreement with our previous findings for cationic sysFinally, as shown in Figure 1, the concentration of living ends [LE] is practically time independent, with and without field. This result seems to indicate that no living ends are produced or consumed in the field and that the accelerating effects cannot be explained in terms of the so-called electroinitiation mechanism.‘
Table I : Kinetic Data of Living Anionic Polymerization of Styrene at 2.5’ Volume
fraction of THF, 7%
[LEI X lOS,
10
D
ilf
2.69
7.6
Field etrength, c - k , , M-1 sec-I---kv/cm Obsd Lit.4
0
5 30
3.58
1.9
40
4.05
3.9
44.5
4.27
1.0
0 5 0 5 0 5
2.07 2.07 14.1 20.4 2.0 38
...
2.1 15
28 34
110
a Estimated using k, values and activation energy data given by Bywater arid Worsfolds at 20”.
The second and third findings support the important role of free ions and ion pairs proposed previously16 and simultaneously our own interpretation of field effects on cationic po1ymerization.l Furthermore, by using benzene-THF mixtures we were able to exclude the specific role of 1,Bdichloroethane and nitrobenzene where the large field effects had been observed. Because of the large difference between kP” and k,‘ reported earlier,5 it is worth mentioning finally that we were able to observe the accelerating effect even for very weak electric fields, where the increase of conductivity of weak electrolyte solutions is, due to the second W e n effect, hardly detectable. ~~
(4) For electrolytically initiated polymerizations, see, for example, B. L. Funt and F. D. Williams, J . Polymer Sci., A-2, 865 (1964). ( 5 ) D. N. Bhattacharyya, C. L. Lee, J. Smid, and M. Szwarc, Polumer, 5, 54 (1964);H . Hostalkn, R.V. Figini, and G. V. Schulz, Makromol. Chem., 71, 198 (1964).
Radiation Chemical Studies with Heavy Ion Radiations’
Sir: To date, except for a few experiments with fission fragments, liquid-phase radiation chemical studies with heavy ions have been confined to particles of low mass and charge (Le,, protons, deuterons, and helium ions) where the linear energy transfer (LET) of the radiation is in the range of 0.5-25 ev/A.2 While reliable measurements of the dependence of differential radiation yields on LET can be made up to a few ev/A with these radiations, measurements at higher LET’s are more difficult because of the short range at the necessarily low particle energies. I n any event, for protons the maximum LET available in water is of the order of 7 ev/A and for helium ions of the order of 25 ev/A, and it becomes necessary t o use more massive highly charged particles to obtain information at higher LET'S.^ The experiments briefly reported here indicate that practical radiation chemical experiments at high LET’s can be carried out with the heavy ion beams made available by the Yale University Heavy Ion Linear Accelerator (HILAC). Preliminary experiments on the ferrous sulfate system (0.01 M Fe2+in aerated 0.8 N H2S04,no chloride) have been carried out with 120-hlev C6+ions from the HILAC. The range of these ions is 65 mg/cm2, so that window problems are of relatively minor importance. The irradiation arrangement was very similar t o that used in earlier studies with cyclotron radiations. 2a Absolute radiation chemical yields were determined from a knowledge of the amount of chemical reaction, the particle energy, and the integrated beam current. The ferric ion concentration was measured spectrophotometrically at 305 mp and the energy of the undegraded beam determined from magnetic measurements in the beam analyzing system. Measurement of the integrated current presents the most serious of the problems associated with this type of experiment since difficulties due to secondary electron emission, charge exchange in the ion beam itself, and current displacement effects in the window system analogous t o those previously observed with protons and helium ions4 are all present. Secondary electron ~
~
~~
~~
(1) Supported in part by the U. S. Atomic Energy Commission.
(2) For typical experiments with cyclotron and Van de Graaff radiations, see: (a) R. H. Schuler and A. 0. Allen, J . Am. Chem. Soc., 79, 1565 (1957); (b) E.J. Hart, W. J. Ramler, and S. R. RockDEPARTMENT OF POLYMER CHEMISTRY ICHIRO SAKURADA lin, Radiation Res., 4, 378 (1956); (c) W.G.Burns, R. A. Holroyd, and G. W. Klein, J . Phys. Chem., 70,910 (1966). KYOTO UNIVERSITY NORIOISE KYOTO,JAPAN HIDEOHIROHARA (3) For a discussion of the energy loss by heavy ions, see H. A. TETSUO MAKINO Bethe and J. Ashkin in “Experimental Xuclear Physics,” Vol. I, E. Segre, Ed., John Wiley and Sons, Inc., Xew l o r k , N. Y., 1953, RECEIVED JUNE 30, 1967 Chapter 11; L. C. Northcliffe, Ann. Rea. S u c l . Sci., 13, 67 (1963).
The Journal of Physical Chemistry
