Polymer Journal, Vol. 7, No. 3, pp 366-371 (1975)

Photoreactions of N-Vinylcarbazole Induced by Metal Salts. V. Mechanistic Studies of the Cationic Polymerization Induced by Silver(I) Salts in Aromatic Solvents

Yoshihiko TAKEDA,* Michihiko AsAr,** and Shigeo TAZUKE***

Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan. (Received October 9, 1974)

ABSTRACT: Photosensitized cationic polymerizations of N-vinylcarbazole (VCZ) in­ duced by silver(!) perchlorate and tetrafluoroborate in various aromatic solvents were investigated under irradiation at 365 nm at 30°C. The polymerization rates decreased in the following order: nitrobenzene> benzene> toluene The specific in­ teraction between VCZ and silver which was observed spectroscopically only in the perchlorate-benzene system does not participate in the initiation processes. It was concluded that the effective absorption species leading to initiation is VCZ itself. Arguing from the kinetic behavior and the similarity in molecular weights of the poly­ mers, the polymerization in benzene and toluene proceeded via an identical mechanism. The poor photochemical reactivity in p-xylene was assumed to be attributable to the initial processes. Moreover, excitation of the charge-transfer band of the VCZ-nitroben­ zene complex in the presence of silver perchlorate enhanced the polymerization rate in comparison to the photopolymerization without silver perchlorate. The following initia­ tion mechanism was proposed: The electron transfer from excited VCZ* to solvent molecule produces a cation radical, vcz+, which in turn reacts with AgCI04 to pro­ duce the initiating species. KEY WORDS N-Vinylcarbazole I Cationic Polymerization I Silver(!) Salts I Photopolymerization I Charge Transfer Polymerization I

In the preceding paper/ kinetic features of (p-Xy), and nitrobenzene (NB) as well as Bz, the photosensitized cationic polymerization of using silver(!) tetrafluoroborate and tetra-n­ N-vinylcarbazole (VCZ) induced by silver(!) butylammonium perchlorate, as well as silver perchlorate in benzene (Bz) were investigated perchlorate, as catalysts. The molecular interac­ under irradiation at 365 nm at 30°C. The pre­ tion between VCZ and silver salt in the ground sent article describes the mechanistic studies of state was measured by absorption spectroscopy. the initiation processes; here two sorts of pro­ From these results, a possible mechanism of blems are examined: What is the effective light initiation was proposed. Kinetic data obtained absorbing species leading to initiation? And previously were interpreted in accordance with what is the role of silver(!) salt-silver cation this mechanism. or counter anion? To elucidate these problems, photopolymerizations were carried out in various EXPERIMENTAL aromatic solvents such as toluene (Tol), p-xylene N- Vinylcarbazole, silver(!) perchlorate, and * Present address: Mitsubishi Monsanto Co., Ltd., benzene were purified as described in the pre­ Yokkaichi, Mie 510. ceding article. 1 Toluene and p-xylene were ** Present address: Research Institute for Polymers purified by methods similar to that used for and Textiles, Sawatari 4, Kanagawa-ku, Yokohama, Kanagawa 221. To whom correspondence should benzene. Nitrobenzene was purified as reported be addressed. previously. 2 Silver( I) tetrafluoroborate was *** Present address: Research Laboratory of Re­ dried in vacuo for more than 10 hr at room sources Utilization, Tokyo Institute of Technology, temperature before use. Tetra-n-butylammonium Meguro-ku, Tokyo 152. perchlorate was prepared and purified using the

366 Photopolymerization of N-Vinylcarbazole: Mechanisms procedures reported by Fujinaga, et al. 3 All 100r------. other experimental methods were the same as described in the preceding article.

RESULTS AND DISCUSSION

Photopolymerization in Benzene As shown in the preceding paper/ the photo­ ..., 60 sensitized cationic polymerization of N-vinyl­ Ql u carbazole (VCZ) in the presence of silver(!) c: ..... perchlorate was induced under irradiation at ...... "' 30°C in benzene, whereas thermal as well as E 40 "'c: noncatalytic photosensitized polymerization were ... 1-"' negligibly slow. The problem of the photo­ absorbing species is not straightforward, just as 20 encountered in the photopolymerization system sensitized by tetrahaloaurate(III) in nitroben­ zene. 4 Under the polymerization condition

([VCZ] 0 =0.25M) no change in absorption spectra 310 330 350 (Figure 1) was observed by the addition of Wavelength (nm) silver(!) perchlorate up to 2 x 10-4M. However, in absorption spectroscopy in the short wave­ Figure 2. Absorption spectra of relevant com­ length region under very dilute conditions, com­ pounds in benzene: I, benzene; 2, [AgCI04], I0-4M; 3, [VCZ]o, I0-4M; 4, [VCZ]o, IQ-4M, plex formation between VCZ and silver perchlo­ [AgCI04]o, I0-5M; room temperature, rate was observed, as shown in Figure 2. Therefore, the photoabsorbing species effective for the initiation is either VCZ itself or some Table I. Effect of (n-C4H9)4NCI04 (TBP) on the photopolymerization• kind of VCZ-AgC104 complex. Reference experiments using tetra-n-butyl­ TBP, M [VCZ]o, M Time, min Conv.,% ammonium perchlorate were performed as shown I X I0-2 0.25 120 in Table I. Since the complete inertness of the 5 0 0.25 120 7

1 00 .------, • Under irradiation of unfiltered light, 30°C.

;;:; 80 perchlorate anion as photosensitizer was proved, the active part of silver perchlorate must be Q) u c: Ag(I) itself and not the perchlorate anion. Also, .....ItS 60 silver(!) tetrafluoroborate as sensitizer was effec­ ..... e tive for the photopolymerization, although its c: activity was a little less than that of AgC104 , "'ItS 40 .....s.. as shown in Figure 3, indicating that the role of the anion is minor. However, the blue shift 20 in absorption spectra attributed to the complex formation observed in the VCZ-AgCl04 system was not detected in the VCZ-AgBF4 system. 365 370 The total amount of VCZ-AgC104 complex Wavelength (nm) is negligibly small in comparison with the free Figure 1. Absorption spectrum of the polymeriza­ VCZ under the present experimental conditions 4 tion system: [VCZ]o, 0.25M; [AgC104]0 , 1 X I0-4M; for polymerization ([Ag(I)]=l0- M, [VCZ]= room temperature, solvent, benzene. O.lM) even if the complex formation is nearly

Polymer J., Vol. 7, No. 3, 1975 367 Y. TAKEDA, M. ASAI, and S. TAZUKE

7

6

5 i. t c c 4 0 g Ill Q; > 5 <1> 3 c > 0 c u 0 u 2

0 50 100 150 0 40 80 120 160 time (min) time (min) Figure 4. Photopolymerizations catalyzed by Figure 3. Photopolymerizations catalyzed by AgCl04 in various aromatic solvents: [VCZ]o, AgCI04 and AgBF4 in benzene: [VCZ]o, 0.5M; 0.25M; [AgC104]o, 1 x 10-4M; )., 365 nm; 30°C: )., 365 nm; 30°C: -e-, [AgCI04]o, 1 x I0-4M; -0-, benzene; -e-, toluene; -0-, p-xylene; -0-, [AgBF4], 2x I0-4M. -•-, nitrobenzene. completed. Furthermore, the blue shift of the Table II. Thermal polymerization initiated absorption band of VCZ is brought about as a by AgCl04 in various solvents result of the complexation. 6 Consequently, the photoenergy at 365 nm is absorbed exclusively Solvent [VCZ]o, [AgC104]o, Temp, Rp, M M oc M/min by VCZ itself. The relation Rpw/0 o.s [ AgBF4]0 , 5 similar to the AgC104-Bz system/ was obtained. Benzene 0.25 1 X J0-4 30 3. 7 X J0- 1 X 10-4 X 5 Polymerization Induced by Silver(!) Perchlorate Toluene 0.25 30 3.1 J0- p-Xylene 0.25 1 X 10-4 30 2.9 X J0-5 in Several Aromatic Solvents (Figure 4) Nitro- 0.225 1 X 10-4 30 6.5 X J0-5 In toluene, which is chemically close to ben­ benzene zene, photopolymerization proceeded with a somewhat slower rate than in benzene; the 0.4 formation of VCZ-AgC104 complex was not observed, indicating light absorption by VCZ alone. Both thermal and noncatalyzed photo­ polymerizations were also negligibly slow. The d 0.2 kinetic investigation leads to the rate expression: a: 5 Rpw/0°" [AgC104]0 , which is again similar to 2'" that for the AgCl04-Bz system. 1 In contrast ' to benzene and toluene, however, photoirradia­ tion of the reaction mixture in p-xylene was almost ineffective, whereas thermal and non­ 0 0.2 0.4 0.6 catalyzed photopolymerizations proceed with a -log 10 (arb.) rate comparable to other solvent systems. The Figure 5. Dependence of Rp on lo in nitrobenzene: absorbance of the polymerization solution was [VCZ]o, 0.25M; [AgC104]o, 1 X I0-4M: -e-, )., nearly identical for 'AgC104-Bz, AgC104-Tol, 365 nm, 1ZOC; -0-, )., 436 nm, 23°C. AgBF4-Bz, and AgClO.-p-Xy systems. Con­ sequently, the marked differences in Rp in these styrene derivatives in benzene, toluene, or p• systems must be attributed either to the efficiency xylene proceeds with almost identical rates/ and of initiation or to the propagation andjor ter­ in addition, the slow dark cationic polymeriza­ mination processes. The latter possibility is tion of VCZ by silver perchlorate was hardly precluded since the cationic polymerization of affected by the kind of solvent (Table II).

368 Polymer J., Vol. 7, No. 3, 1975 Photopolymerization of N-Vinylcarbazole: Mechanisms

Table III. Photopolymerizations of VCZ catalyzed by silver(I) salts under irradiation at 365 nm

Ag(l) salt [Ag(l)]o, M [VCZ]o, M Temp, oc Solvent UV spectra X y

AgC104 0.3-2x 104 0.25 30 Benzene 1/2 Blue shift AgC104 0.5-5x 104 0.5 30 Toluene 1/2 No shift AgC104 1 X 104 0.5 30 p-Xylene No polymerization No shift AgC104 0.5-2x 104 0.25 23 Nitrobenzene 1/2 0 AgBF4 1 -5x 104 0.5 30 Benzene 1/2 No shift

In nitrobenzene, another type of aromatic Table IV. Degree of polymerization for solvent, a rapid photopolymerization resulted photopolymerization in various solvent under irradiations at 365 nm and 436 nm, al­ Solvent [VCZ]o, M [AgC104]o, M F though the problem of absorption species was considerably different from the above systems; Toluene 0.5 1 X J0-4 4.4xl02 this will be discussed later. RP was again pro­ Benzene 0.5 1 X J0-4 5.4x 102 Nitrobenzene 0.225 1 X 10-4 8.8x 102 portional to the square root of / 0 , as shown Benzene 0.25 1 X J0-4 3.4x 102 in Figure 5, whereas Rp was independent of

[AgC104]0 • Initiation Mechanism ki I+M The kinetic sequence and the degree of poly­ ----> Pi ( 6) merization in various photopolymerization + kp P n-t + M ----+ P! ( 7) systems induced by silver(!) salts are sum­ marized in Tables III and IV, respectively. kt P!+M ----+ Pn+l ( 8) The complex formation, which is observed as a blue shift of the absorption spectra of VCZ, where M, S, and AgX denote VCZ, benzene­ between VCZ and silver salt is a specific pheno­ like solvent, and silver salt, respectively. menon for the combination of VCZ-AgCl04- Another possible elementary process may be Bz. The fact that photopolymerization of VCZ the reaction of excited VCZ with silver salt proceeds equally in the systems AgCl04-Bz, leading to the initiation of polymerization, as expressed by (9): AgC104-Tol, and AgBF4-Bz, regardless of complex formation, provides further support vcz* +Ag1 ____.. vczt +Ag0 ( 9) that the efficient photoabsorbing species is VCZ itself. The silver salt themselves are transparent This mechanism is tempting since fluorescence to the irradiation at 365 nm and could not be of VCZ is quenched by AgCl04 with the quench­ 2 1 6 energy-absorbing species. ing constant of 10 M- in benzene. It is, how­ A mechanism of photopolymerization which ever, difficult to explain either the peculiar does not contradict the observed mechanistic solvent effects upon the initiation process or the and kinetic findings may be expressed as follows: kinetic expressions presented in Table III with this initiation mechanism. As discussed in the ( 1 ) preceding section, the sequence of Rp with re­ spect to solvent, benzene> toluene» ( 2) is presumably from the initiation processes. Moreover, the polymerization mechanisms in kd M"!- +S7 ----+ deactivation ( 3) AgC104-Bz, AgBF4-Bz, and AgC104-Tol systems will be the same as judged from the ( 4) identity of their kinetic behaviors (Table III) as well as their degrees of polymerization D"!-+AgX ( 5) (Table IV).

Polymer J., Vol. 7, No. 3, 1975 369 Y. TAKEDA, M. ASAI, and S. TAZUKE

The electron-accepting property of AgC104 should be a determining factor for (9). Reaction (9) should therefore be suppressed by charge­ 10 l transfer complex formation of AgC104 with c 0 solvent as a consequence of the decreased ele­ '§ ctron-accepting capacity of complexed Ag(I). "',. § 5 The sequence of the equilibrium constants of u complexes between alkylbenzenes and Ag(l) has been reported as toluene> p-xylene >benzene, 7 which could not explain the solvent effect upon 30 60 RP if we assume (9) as the initiating process. time (min) The mechanism expressed by (1)-(8) could explain the kinetic expression. The evidence Figure 6. Photopolymerization of VCZ catalyzed in support of this mechanism is circumstantial by AgCI04 in nitrobenzene: [VCZ]o, 0.25 M; 23°C: and given as follows. -e-, A, 365 nm, [Ag(I)]o, I x !0-4M; -0-, A, Since the strong donor character of VCZ in 436 nm, [Ag(I)]o, 1 x I0-4M; -L,-, A, 436 nm, the ground state would be enhanced by photo­ [Ag(I)]o, OM. excitation, an electron-transfer process (2) from absence of the silver salt is most likely to be excited VCZ to ground state solvent molecule expressed by (10); would be possible. Electron transfer from ex­ cited amine to aromatic hydrocarbon has been h" VCZ·NB ---+ vcz+ +NB-:- (10) demonstrated in the N,N-dimethylaniline­ biphenyl system8 and the N,N,N' ,N'-tetramethyl­ the role of silver salt will presumably enhance p-phenylenediamine-pyrene or naphthalene de­ the initiation efficiency after electron transfer rivatives system. 9 The sequence of Rv in these from VCZ to the solvent occurs. Although solvents agrees with the decreasing order of the the VCZ-NB system exhibiting charge-transfer acceptor strength of solvents. The expected interaction in the ground state is not strictly order of acceptor strength is: NB > Bz > Tol > comparable to the VCZ-alkylbenzene systems, p-Xy. This order is supported by the following these findings seem to be in favor of the pro­ facts: i) the ease of producing anion radicals posed mechanism for the VCZ-alkylbenzene­ when alkyl substituted benzenes react with AgC104 systems. metallic potassium is Bz, Tol »p-Xy; 10 ii) The The cyclodimer cation radical (D+) in process electron affinity decreases with increasing num­ (4) has been proposed by Ledwith13 as a inter­ ber of methyl groups introduced into the ben­ mediate in the cyclodimerization of VCZ, in zene ring. 11 Although this process proceeding which the following equilibrium has been con­ via the excited singlet state of VCZ would result firmed in the ring-opening reaction of the in a reduction in the fluorescence yield or the cyclodimer. This intermediate assumed from appearance of excimer emission, no such physical the chemical viewpoint was recently ascertained evidence was observed within the experimental from its direct observation in the pulse errors. These results might well be due to their extremely low efficiency via the triplet state of vcz. (11) Irradiation of VCZ-NB system at 436 nm excites the VCZ-NB contact charge-transfer complex exclusively and cationic polymerization radiolysis studies of VCZ in benzonitrile by is brought about even in the absence of the Imamura, et a/. 14 They have suggested that D"!" silver salt. 12 The addition of silver perchlorate is a precursor for both cyclodimerization and to this system enhanced the rate of polymeriza­ cationic polymerization of VCZ; moreover, the tion, as shown in Figure 6. Since the primary equilibrium of process (4) has boeen confirmed act of photoirradiation both in the presence and experimentally.

370 Polymer J., Vol. 7, No. 3, 1975 Photopolymerization of N-Vinylcarbazole: Mechanisms

Initiation species I is assumed to be produced to eq 14 is as follows: from some type of interaction of Dt with silver salt in process (5). A possible role of the silver (Rp)x= (15) ion is to stabilize n+ by formation of a charge­ transfer complex with a radical part, envisaged Acknowledgment. The authors express their as follows, and to prevent the backward reac­ thanks to Profossors S. Okamura and T. tion of (4): Higashimura for their encouragement and help­ ful discussion, and also to Mr. K. Kameoka for preparing the tetra-n-butylammonium per­ chlorate.

I initiates the cationic propagating species bear­ REFERENCES ing perchlorate ion as counter anion, as dis­ I. Y. Takeda, M. Asai, S. Tazuke, and S. cussed in the preceding paper. Interactions of Okamura, Part IV in this series, Polymer J., radicals with closed shell ions including silver 7, 359 (1975). (I) ion (d10) have been reported in the ESR 2. S. Tazuke, M. Asai, and S. Okamura, J. Polym. studies of various compounds. 15 Another possi­ Sci., Part A-1, 6, 1809 (1968). bility for the role of AgCl04 is the oxidation 3. T. Fujinaga, K. Izutsu, K. Umemoto, T. Arai, of geminate s-:- to prevent recombination of the and K. Takaoka, Nippon Kagaku Zasshi (J. cation radical (Mt or n+) with the anion radical Chern. Soc. Japan, Pure Chern. Sect.), 89, 105 (1968). as follows: 4. S. Tazuke, M. Asai, and S. Okamura, Kogyo 0 s-:- +AgC104 ----> S+Ag +Cl04 (13) Kagaku Zasshi (J. Chern. Soc. Japan, Ind. Chern. Sect.), 72, 1841 (1969); M. Asai, S. Takuke, and A similar role for a metal salt has been reported S. Okamura, J. Polym. Sci., Polym. Chern. Ed., in the pulse radiolysis of THF/pyrene in the 12, 45 (1974). presence of LiAlH4 , which removes the geminate 5. T. Higashimura and S. Okamura, Kobunshi cation and stabilize the pyrene radical anion Kagaku (Chern. High Polymers), 13, 397 (1956). with the innert lithium cation. 16 6. S. Tazuke, J. Phys. Chern., 74, 2390 (1970). By applying the assumptions, kct[M"t] » k2[M], 7. L. J. Andrews and R. R. Keefer, J. Arner. 1 L 2»k3[Ag ] and [M+]=[S-:-], the following rate Chern. Soc., 71, 3644 (1949); 72, 5034 (1950). expression is derived: 8. A. Weller, Pure Appl. Chern., 16, 115 (1968). 9. H. Tsubomura, N. Yamamoto, and Y. Nakato, (Rp)x= (14) Bull. Chern. Soc. Japan, 40, 451, 2480 (1967). 10. T. R. Tuttle, Jr., and S. I. Weissman, J. Arner. Chern. Soc., 80, 5342 (1958). where (Rp)x and denote the rate at a 11. G. Briegleb, Angew. Chern., 76, 326 (1964). distance x from the front face of the reaction 12. S. Tazuke, M. Asai, M. Ikeda, and S. Okamura, cell, as described in the preceding article, 1 and J. Polyrn. Sci., Part B, 5, 453 (1967). the amounts of light absorbed by monomer in 13. A. Ledwith, Ace. Chern. Res., 5, 133 (1972) and a unit volume at x, respectively. This equation references cited therein. explains the experimental results obtained in 14. S. Tagawa, S. Arai, M. Imamura, Y. Tabata,

AgCl04-Bz, AgClO,-Tol, and AgBF4-Bz and K. Oshima, 14th Symposium on Radiation systems very well. The detailed kinetic treat­ Chemistry, Osaka, 1972. 15. D. R. Gee and J. K. S. Wan, Can. J. Chern., ment in AgCl04-Bz system was described in 49, 160 (1971) and references cited therein; F. the preceding article. 1 Moreover, the rate ex­ Leh and J. K. S. Wan, Can. J. Chern., 50, 999 pression; Rpe0/0112 [AgC104] 0 as observed in ° (1972); P. C1opath and A. V. Zelewsky, Chern. nitrobenzene may be interpreted on the assump­ Comrnun., 1971, 47. 1 tion k_2 « k 3 [Ag ], which means rapid stabiliza­ 16. J. H. Baxendale, D. Beaumond, and M.A. J. tion reaction (12) in this highly polar solvent. Rodgers, Trans. Faraday Soc., 66, 1996 (1970). In this case the rate expression corresponding

Polymer J., Vol. 7, No. 3, 1975 371