706
COMMUNICATIONS T O THE
EDITOR
Mucrowwlecules
the Excitation of Its Charge-Transfer Complex Charge-transfer complexes between organic compounds have been extensively studied since the establishment of Mulliken’s quantum mechanical theory. Recently, excited-state properties of charge-transfer complexes have been studied21 a and the photochemical processes induced by the excitation of ; charge-transfer-
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COMMUNICATIONS TO T H E EDITOK 707
V ( J 3, ~ . No. 5 , Septetnber-October 1970
plex, namely, large contribution of dative structure in the excited state, is not enough to produce an initiating ionic species, but rather loosening or dissociation of the excited charge-transfer complex by solvation is necessary. A tentative mechanism is proposed (Scheme I). Only
R/S 0.51
3
1
;
:
Figure 3. EBect of the dielectric constant of solvents on the rate of photopolymerization: 1, benzene; 2, chlorobenzene; 3, 1,2-dichIoroethane; 4, nitrobenzene; 2 mol/l., V C Z ; 10-d rnol/l., TNF.
The quantum yield is almost constant, irrespective of water content. This result means that water is not involved in the formation of the initiating species of photopolymerization, but quenches the propagating chain ends. The influence of the dielectric constant of solvents on the polymerization rate is shown in Figures 3 and 4. The rate increases with the polarity of solvents as shown in Figure 3. I n Figure 4 the relative rate and the molecular weight of the polymer obtained in a mixed solvent, benzene and nitrobenzene, are plotted against the volume fraction of nitrobenzene. The relative rate of photopolymerization increases linearly with increasing volume fraction of nitrobenzene, although this relationship may not necessarily hold generally. The molecular weight increases slightly with nitrobenzene content and may be considered almost constant. The inhibition effect of water on the photopolymerization and effect of solvent polarity on the rate suggest that the photopolymerization is of ionic nature. Furthermore, from the consideration that VCZ can be easily polymerized by cationic catalysts and that nitrobenzene used as a solvent is a strong radical scavenger, it is concluded that the photopolymerization occurs in the cationic mechanism. The charge-transfer complex has 27 and 93z dative structure (b2,/(a2 b2)100)in the ground state and in the excited state, respectively, which are estimated from A H and / I V C T at 530 mp assuming the overlap integral S = 0.1. But photopolymerization could not be observed in a nonpolar solvent (benzene) even after 8-hr irradiation. Therefore, excitation of the charge-transfer com-
a n outer complex is given in this scheme and so far no inner complex has been detected. Further investigation o n the kinetics and the mechanism of the photochemical processes is being carried out.
* To whom correspondence should be addressed. M. Yamamoto,* S. Nishimoto, M. Ohoka, Y. Nishijima Drpurtnieiit of Polymer Cliemistry Kyoro Uiiicersity, Kyoro, Jupaiz Rrceiced Jirtir 12, 1970
Polymerization of 4-Vinylpyridinium Salts. Hydrogen-Transfer Polymerization
11.
It has been reported that when 4-vinylpyridine is quaternized with alkyl halides or protonated by monomeric o r polymeric acids, spontaneous polymerization occurs to give the corresponding poly(4-vinylpyridinium) sa1t.l-j The mechanism of initiation in these reactions has been considered to be the attack of the counterion (X-) on the p position of the double bond of the 4-vinylpyridinium salt t o yield a resonancestabilized zwitterion (I). Poly(4-vinylpyridiniurn) salts could then form by the addition of I to the double bond of a 4-vinylpyridinium ion t o reform an active CH,=CH
+
I
x- R
XCH,~H I
R
I, R = H or alkyl
zwitterionic end group which is capable of further reaction. I n a study of the quaternization of 4-vinylpyridine by methyl p-toluenesulfonate,6 we have found that the (1) V. A. Kabanov, K . V. Aliev, T. I. Patrikeeva, 0. V Kargina, and V. A. Kargin, J . Polj~m.Sci., Purr C, 16, 1079 (19671. (2) V. A. Kabanov, K . V. Aliev, and V. A. Kargin, Pol.j,m. Sci. USSR,10,1873, (1968). (3) V . A. Kabanov, T. I. Patrikeeva, 0. V. Kargina, and V . A. I
