Absolute rate constants for silylene reactions with ... - ACS Publications


Sep 1, 1987 - Jack O. Chu, David B. Beach, Joseph M. Jasinski. J. Phys. Chem. ... Ula N. Alexander, Keith D. King, and Warren D. Lawrance. The Journal...
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J . Phys. Chem. 1987, 91, 5340-5343

for stratospheric C1 and ClNO, may be useful to providing information concerning ClNO photochemistry, NO-catalyzed C1 atom recombination, and more accurate C1 calibrations in flow tube studies.

Acknowledgment. This work was supported by the NASA

Upper Atmospheric Research Program. We thank J . Margitan for communicating his unpublished results and D. Stedman for additional information on his published results. Registry No. CINO, 2696-92-6; C1 atomic, 22537-15-1.

Absolute Rate Constants for Sllylene Reactions with Hydrocarbons at 298 K Jack 0. Chu, David B. Beach, and Joseph M. Jasinski* IBM Thomas J . Watson Research Center, Yorktown Heights, New York 10598 (Received: April 1 , 1987)

We report absolute removal rate constants for silylene by reaction with alkanes, olefins, 1,3-butadiene,and acetylene at ambient temperature and 5-Torr total pressure. Rate constants were determined by laser resonance absorption flash kinetic spectroscopy. The results show that silylene is unreactive with alkanes and reacts rapidly and unselectively with unsaturated hydrocarbons.

Introduction Silylenes are widely recognized as important intermediates in silicon hydride and organosilicon chemistry. Evidence for their importance comes largely from classical mechanistic studies, and information on silylene reactivity comes almost exclusively from relative rate studies under pyrolysis, shock tube, or nuclear recoil Recently, two direct absolute measurements of SiHz reaction rate constant^^,^ and two relative rate s t ~ d i e s ,all ~ ,based ~ on generation of silylene by photolysis of phenylsilane, have appeared. The results of these studies do not agree well with previous estimates of relative or absolute silylene reactivity, for a number of reaction partners. These discrepancies require further investigation, and reliable direct measurements of absolute rate data are essential to conclusively unraveling the chemistry of these reactive intermediates. In this paper we present a survey of absolute rate constants for the reaction of SiH, with hydrocarbons. Direct measurements of absolute rate constants for the removal of silylene by methane, ethane, ethylene, propylene, 1,3-butadiene, and acetylene have been measured at 5-Torr total pressure and 298 K using laser resonance absorption flash kinetic spectroscopy (LRAFKS). The results demonstrate that SiH, is unreactive with saturated hydrocarbons but extremely reactive and rather unselective with unsaturated hydrocarbons. Since the anticipated products of these reactions are chemically activated organosilanes, the results may also provide some information on the decomposition chemistry of these molecules. Experimental Section A schematic of the LRAFKS apparatus has been published p r e v i ~ u s l y . ~Briefly, transient concentrations of silylene are produced in a flowing gas cell by excimer laser (Lambda Physik EMG- 102E) photodissociation of a suitable precursor. Silylene concentrations are monitored in real time by transient absorption of light from a single-frequency Rhodamine 6G ring dye laser 'A,(O,O,O) tuned to single rotational lines of the 'B1(0,2,0) electronic transition of SiH2. The only modifications of the apparatus for the present study were the addition of fast preamplifiers (LeCroy VVBl00, 2 4 s risetime) to the photodiodes and the addition of an automatic throttle valve to the exhaust port of the flow cell. The throttle valve is controlled by the output from a capacitance manometer (MKS Baratron Model 221A, 10 Torr full scale) which monitors the cell pressure. This allows operation of the flow system at constant total pressure and constant total flow as the composition of the gas mixture changes.

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Silylene was generated by photodissociation of phenylsilane or disilane with ArF excimer laser radiation at 193 nm or by photodissociation of SiH31by KrF excimer laser radiation at 248 nm. The different silylene sources are necessary since not all of the hydrocarbon reaction partners are suitably transparent at 193 nm. Phenylsilane (Petrarch) was degassed prior to use. Disilane (Airco or Matheson) was used as received. Iodosilane was prepared by HI cleavage of phenylsilane or chlorophenyl~ilane.~Silylene transient absorption was monitored by using the RQo,J(5)or the RQo,J(7)rotational lines when phenylsilane or disilane was used as a precursor. The RQo,J(7)line was used exclusively when iodosilane was used as a precursor in order to avoid possible 5) complications owing to the near coincidence between the RQo,J( silylene transition and a molecular iodine absorption line. Gas flow rates were measured and controlled with calibrated electronic mass flow controllers (Vacuum General UV series, lo-, loo-, or 1000-sccm full flow) with the exception of phenylsilane and iodosilane, for which a needle valve was used. In a typical experiment a flow of silylene precursor in helium buffer gas at 300 sccm and 5-Torr total pressure was established. Hydrocarbon reactant flow was then added and the helium flow decreased to maintain a total flow rate of 300 sccm while the vacuum pump throttle valve maintained the total pressure at 5 Torr by maintaining a constant pumping speed independent of the gas composition. Under these conditions, the average residence time in the cell volume is 1-2 s. This procedure guarantees that the partial pressure of silylene precursor is constant. Phenylsilane flow rates through the needle valve were determined by measuring pressure rise in a fixed volume as a function of time. The partial pressure of phenylsilane in kinetic experiments was less than 1 X lo4 Torr. The mass flow rate of iodosilane was not determined. We estimate that the partial pressure of iodosilane in the kinetic experiments for which it was a precursor was less than 1 X low3 Torr. Hydrocarbon partial pressures were calculated from the total pressure by using the total gas flow rate and the flow rate of the hydrocarbon. The estimated absolute uncertainty in the hydrocarbon partial pressure is 1 5 % as determined from the stated accuracy of the capacitance manometer and the accuracy of the mass flow controllers.

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(1) Gaspar, P. P. Reocr. Inrermed. 1985, 3, 333 and references therein. (2) Davidson, I . M . Annu. Rep. Prog. Chem., Secr. C 1985, 82, 47 and references therein. (3) Inoue, G.; Suzuki, M. Chem. Phys. Lett. 1985, 122, 361. (4) Jasinski, J. M . J . Phys. Chem. 1986, 90, 5 5 5 . (5) Eley, C. D.; Rowe, M. A,; Walsh, R. Chem. Phys. Lert. 1986, 126, 153. (6) Frey, H. M.; Walsh, R.; Watts, I. M. J . Chem. SOC.,Chem. Commun.

1986, 1189.

*Author to whom correspondence should be addressed.

0022-3654/87/2091-5340$01.50/0

(7) Ward, G. L. Inorg. Synth. 1968, 11, 159.

0 1987 American Chemical Society

The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5341

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301

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PROPYLENE (C3Hs) PRESSURE (Torr)

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20

40

60 TIME ( p s e c )

80

100

Figure 1. Typical transient absorption signal for SiH2. Data (dots) are an average of 64 excimer laser pulses. Solid line is a single-exponential fit to the decay.

Bimolecular rate constants were determined from plots of the silylene decay rates as a function of hydrocarbon reactant partial pressure under pseudo-first-order conditions. Silylene decay rates were determined from single-exponential fits to the decay of the silylene transient absorption signal. The exponential fits were performed from near the maximum of the absorption to the base line. The risetime of the signal was rapid compared to the decay and, therefore, easily separated from the decay. A typical decay curve was the average of 64 excimer laser pulses at 2 s-l. When iodosilane was used, the repetition rate was increased to 10 SKI to conserve iodosilane. Hydrocarbon reactant gases, suppliers, and stated purities were as follows: methane (Matheson, 99.99%); ethane (Matheson, 99.99%) ethylene (Matheson, 99.99%); propylene (MG Scientific Gases, 99%); 1,3-butadiene (Matheson, 99.7%); acetylene (Matheson, 99.6%). Helium (Liguid Carbonic, 99.999% or Matheson U H P grade) was passed through a cartridge filter (MG Scientific Gases) to further limit oxygen and water contamination.

P

044

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040

METHANE (CH4) PRESSURE (Torr)

Figure 2. Pseudo-first-order rate data for reaction of SiH2 with propylene and methane. Solid lines are least-squares fits to the data.

TABLE I: Absolute Rate Constants for Silylene Removal

reactant CH4 C2H6 C2H4 C3H6 C4H6 C2H2

rate constant, cm3 molecule-' s-I (2.5 (1.2 (5.2 (5.5 (1.2 (1.9 (9.8

f 0.5) f 0.5) f 0.5) f 0.5) f 0.1) f 0.2) f 1.2)

X X X lo-'' X lo-" X lo-''

x lo-'' X

lo-''

silylene precursor PhSiH, PhSiH, SiHJ PhSiH3 PhSiH, SiH31 SiH31

photolysis wavelength,

nm 193 193 248 193 193 248 248

Results and Discussion A typical SiH2 transient absorption signal is shown in Figure 1. We have examined the signal risetime with faster temporal resolution for the photodissociation of phenylsilane and disilane. The rise is characterized by a prompt component, followed by a slower, pressure-dependent component. We attribute the prompt component to direct population of the rotational level we are probing by the photodissociation process and the slower component to relaxation of the nascent SiH, rotational distribution. This process occurs with a rate constant of 3.7 X lo-" cm3 molecule-' s-I for helium buffer gas. Thus, under our experimental conditions, rotational relaxation of SiH, occurs with a time constant of 170 ns. Furthermore, we have observed no dependence of the reaction rate constant on SiHz rotational level for reaction with D, at helium pressures as low as 2 Torr.4 Relatively little is known about the photochemistry of the silylene precursor molecules. We observe production of SiH2from phenylsilane and disilane at 193 nm and from SiHJ at 248 nm. No SiH2signal is observed with phenylsilane at 248 nm, although there is some electronic absorption at that wavelength. Baggott et aLs have recently characterized the steady-state photochemistry of phenylsilane at 193 nm and conclude that both silylene and phenylsilylene are primary products. Perkins and Lampeg have studied disilane photochemistry at 147 nm and conclude that silylene, silyl, and possibly other species are produced. Iodosilane photodissociation has not been studied. It is known from the broad-band flash spectroscopic study of Billingsleylo that silylene

and SiHI are produced, although SiHI may not be a primary photoproduct. Finally, we observe production of electronically excited SiH in the photodissociation of phenylsilane and disilane at 193 nm. This species was conclusively identified by spectrally resolving the 1-1 and 0-0 vibronic bands of the well-known A X transition." The emission intensity has an approximately quadratic dependence on excimer laser intensity, consistent with reasonable estimates of the minimum energy needed to produce this species. We believe that SiH* is formed by secondary dissociation of species such as C6H5SiHand SiH,SiH during the excimer laser pulse. Further characterization of the photodissociation dynamics of these silylene precursor molecules will be the subject of future work. Representative pseudo-first-order plots for some of the hydrocarbon reactants are shown in Figure 2. Absolute rate constants determined in this study are presented in Table I and are discussed on a case by case basis below. Error limits were estimated by taking the maximum possible error in the absolute concentration of the reactant molecule to be twice the estimated uncertainty in any one value, Le., taking the maximum error in the pressure scale to be *lo%, for all cases except methane and ethane. This source of error is significantly larger than any uncertainty introduced by scatter in the pseudo-first-order decay rates for these cases. For the cases of methane and ethane, error bars were determined from the reproducibility of three separate rate constant determinations. The reported rate constants are the

(8) Baggott, J. E.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. Chem. Phys. Lett. 1986, 125, 2 2 . (9) Perkins, G.G. A.; Lampe, F. W. J.Am. Chem. Soc. 1980, 102, 3764.

(10) Billingsley, J. Can. J. Phys. 1972, 50, 531. ( 1 1 ) (a) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand: New York, 1979; pp 598-599. (b) Douglas, A. E. Can. J. Phys. 1957, 35, 71.

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mean values of the determinations, and the error bars reflect the largest deviation of any value from the mean. Methane and Ethane. The rate constants reported in Table I for the reaction of silylene with methane and ethane represent only upper limits. These limits are determined by our measured rate constants and the purity of the hydrocarbon reactants. A 0.01 % impurity in the methane or ethane, reacting gas kinetically with silylene, could account for all of the observed silylene removal. This interpretation is also consistent with the observation that ethane appears to react slightly slower even though more CH bonds are available. Although we are only able to establish an upper bound for the rate constant, the result is important, since Eley et found no observable reaction of silylene with methane in their competitive rate study while Inoue and Suzuki3 found a cm3 molecule-' s-') in relatively rapid reaction ( k = 1.0 X their direct measurement of the rate constant by laser-induced fluorescence. Our result agrees well with that of Eley et al. and with previous indirect evidence12and ab initio calculations,13 all of which conclude that silylene does not readily insert into CH bonds in saturated hydrocarbons. In an effort to understand the difference between our result with methane and that of Inoue and S ~ z u k iwe , ~ have made the following suggestive observations. If higher partial pressures of phenylsilane are used, addition of methane or ethane produces an increase in the decay rate of the silylene transient absorption signal until about 1 Torr of hydrocarbon has been added. Under these conditions, the silylene decay curves are not good single exponentials, but if they are analyzed as such, they produce an apparent bimolecular rate constant in the range (1-2) X lo-'* cm3 molecule-' s-'. Further addition of hydrocarbon reactant restores the silylene decay to a single exponential, but the apparent bimolecular rate constant drops to the range reported in Table I. Reduction of the phenylsilane partial pressure and the excimer laser power removes the artifact and gives a straight-line pseudo-first-order rate plot over the total range of hydrocarbon partial pressures. Since the same artifact can also be observed when disilane is used as the silylene precursor, we suspect that kinetic interference from some other silicon hydride species generated in the photolysis leads to the anomalous result. An effort to test for reactivity with strained carbon-carbon bonds was made by examining the reaction of silylene with cyclopropane. The attempt was unsuccessful in the sense that all observed reactivity could be accounted for by the 1% propylene impurity in the commercially available sample of cyclopropane. Ethylene and Propylene. Both ethylene and propylene react rapidly with silylene. In order to check for possible artifacts, we have measured the rate constant for the ethylene reaction using both phenylsilane photolysis at 193 nm and iodosilane photolysis at 248 nm and find no difference in the rate constants within experimental error. This is reassuring, since ethylene is only weakly absorbing at 193 nm. We estimate that under our conditions only 1% of the ethylene or propylene is excited by the ArF excimer laser. At 5 Torr, propylene reacts slightly faster with silylene than ethylene does. Since no difference in rate constant was found with different precursors and photolysis wavelengths in the case of ethylene, only phenylsilane was used as a precursor for the reaction with propylene. While it might be tempting to attribute the small difference between the rate constants for ethylene and propylene to enhanced reactivity of the substituted double bond, the effect is most likely due to the fact that the reaction of silylene with olefins is a three-body association reaction. In fact, the ethylene rate constant is moderately pressure dependent in the 1-10-Torr range. We measure k = (2.6 f 0.3) X lo-'' cm3 molecule-' s-l at 1-Torr total pressure and k = (8.0 f 0.8) X lo-'' cm3 molecule-' SKI at 9.5-Torr total pressure. This suggests that the reaction with propylene is simply closer to the high-pressure limit at 5 Torr, because of the larger number of degrees of freedom in the adduct,

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(12) Sawrey, B. A,; O'Neal, H. E.; Ring, M. A,; Coffey, D. Int. J . Chem. Kinef. 1984, 16, 31. (13) Gordon, M . S.; Gano, D. R. J . Am. Chem. SOC.1984, 106, 5421.

Chu et al. and that both reactions proceed at near gas kinetic rates at high enough total pressures. This conclusion is consistent with ab initio calc~lations,'~ which predict no potential barrier for the addition of silylene to a carbon-carbon double bond. Our rate constant for the reaction of silylene with ethylene at 1-Torr total pressure is approximately 3 times slower than Inoue and Suzuki's reported value of 9.7 X lo-" cm3 molecule-' s-l at a total pressure of 1 Torr. We conclude that there is a real discrepancy between our rate constant for reaction with ethylene and Inoue and Suzuki's. The difference is probably traceable to uncertainty in ethylene partial pressure or some systematic error other than reactant purity. The rate constant is already so close to gas kinetic that even fairly substantial quantities of impurities could not account for the difference. Butadiene. A direct measurement of the rate constant for reaction of silylene with butadiene is highly desirable, since a number of previous relative rate studies have employed butadiene as a silylene trapping agent.I5 Butadiene absorbs strongly at 193 nm but is transparent at 248 nm, so photolysis of iodosilane at 248 nm was used to generate silylene. The reaction proceeds rapidly, with a rate constant that is essentially gas kinetic. This result seems reasonable, given the results obtained for ethylene and propylene. However, the rate constant is 5 orders of magnitude larger than the value calculated from a tabulation of derived Arrhenius parameters recently suggested by Rogers et a1.16 Furthermore, comparison of our value for the reaction of silylene with butadiene with Inoue and S u ~ u k i ' s ~value . ' ~ of k = 1.1 X lo-'' cm3 molecule-' s-' for reaction with silane gives a relative reactivity 1.7, favoring butadiene. This value is in significant disagreement with the relative reactivity for the two substrates reported by Gaspar et al.ls They report a value of 13 f 4 favoring silane, upon extrapolation of their data to room temperature. Acetylene. Acetylene has also frequently been used as a silylene trap in competitive rate studies, silylacetylene being the isolated final p r o d ~ c t . ' ~ *This ' ~ product is presumably formed by addition of silylene to the triple bond followed by rearrangement of the silacyclopropene to the acetylene. Since acetylene has a significant absorption cross section at 193 nm, we used iodosilane photolysis at 248 nm to generate silylene. We measure a rate constant which is close to gas kinetic at 5 Torr and slightly faster than the rate constant for reaction with ethylene. This appears to contradict the findings of Eley et al.,5 who concluded that acetylene and ethylene react equally rapidly with silylene. However, their results were obtained at a total pressure of 200 Torr, and we have shown above that the ethylene reaction rate constant is pressure dependent at 5 Torr. This very likely explains the apparent disagreement. The rate constant for reaction with acetylene determined in our study is again IO5 times faster than the value calculated from Arrhenius parameters recently suggested by Rogers et a1.I6 The fact that the acetylene reaction rate constant is faster than the ethylene rate constant at 5 Torr is interesting. Since the initial adduct in the acetylene case is a smaller molecule than the ethylene adduct, the acetylene reaction should be farther from the highpressure limit than the ethylene rate constant if no product channels other than addition are available. The fact that the opposite result is found suggests that the initial, vibrationally excited acetylene adduct can decay by some processes other than collisional stabilization or loss of silylene. One possibility may be irreversible loss of molecular hydrogen from the chemically activated silylacetylene formed by rearrangement of the initial silacyclopropene. Some support for this possibility is found in

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(14) Anwari, F.; Gordon, M . S. Isr. J . Chem. 1983, 23, 129. (15) Gaspar, P. P.; Konieczny, S.; Mo, S. H . J . Am. Chem. SOC.1984, 106, 424 and references therein. (16) Rogers, D. S.; O'Neal, H. E.; Ring, M. A. Orgunomernllics 1986, 5, 1467.

(17) We have also measured an absolute rate constant for silylene removal by silane and obtain a value which is within a factor of 2 of that reported in ref 3 at 1-Torr total pressure. Chu, J. 0.;Jasinski, J . M., to be submitted for publication. (18) Haas, C. H.; Ring, M . A. Inorg. Chem. 1975, 14, 2253. (19) Erwin, J. W.; Ring, M. A,; ONeal, H. E. Int. J . Chem. Kinef. 1985, 17. 1067.

J. Phys. Chem. 1987, 91, 5343-5341 shock tube studies of the decomposition of silylacetylene.*' The large discrepancies between our absolute rate constants and previous measurements or estimates of relative or absolute silylene reaction rates with butadiene and acetylene require explanation. At this point we can only suggest that the discrepancies arise because we directly measure silylene removal rates, whereas most previous studies have measured formation rates of stable products. Product yield studies do not have to give the same results as removal studies if all product channels are not accounted for. Conversely, our removal rate studies do not give accurate results if there are removal channels other than reaction or if the silylene molecules react because they contain excess internal or translational energy. Other possible removal channels are diffusion out of the probe beam or physical quenching of the silylene quantum levels we probe. Diffusion, artifacts related to the experimental geometry, and quenching processes which are not strongly dependent on the reactant molecule can be immediately ruled out as contributing to our observation of near gas kinetic reaction rates, since we can measure rate constants which are at least 4 orders of magnitude slower in the cases of methane and ethane. Physical quenching effects which might contribute to the silylene signal decay and which might depend on the physical properties of the added reactant molecules can also be ruled out. Electronic quenching is not possible, since we are probing the ground electronic state. Vibrational quenching cannot contribute to the signal decay, since we are probing transitions from the vibrational ground state. Rotational quenching is ruled out because we observe rotational equilibration in the rise of the signal and have demonstrated that it occurs at least 10 times faster than our fastest (20) Rogers, D. S.; Ring, M. A,; O'Neal, H. E. Organometallics 1986, 5 , 1521.

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pseudo-first-order decay rate under the experimental conditions (vide supra). Lack of translational equilibration is extremely unlikely under the experimental conditions, and high-resolution scans of silylene absorption lines give a line width which is consistent with the expected Doppler width at 300 K. Translational or internal energy effects due to the formation of excited silylene are further ruled out by the observation of identical reaction rate constants with different silylene precursors in the case of reaction with ethylene. It is extremely unlikely that the photodissociation of phenylsilane at 193 nm and iodosilane at 248 nm produces identical nascent silylene distributions. We therefore conclude that we are observing silylene removal which is due solely to reaction of rotationally and translationally thermalized, ground vibrational state silylene.

Conclusion We have made direct measurements of silylene removal rates by a series of hydrocarbons. The results show that silylene is unreactive with alkanes and reacts at or near the gas kinetic rate with olefins, butadiene, and acetylene. Our results agree with recent relative rate studies5 for the cases of methane and ethylene but disagree with other relative rate studies for b ~ t a d i e n e . 'Our ~ measured rate constants for removal of silylene by butadiene and acetylene are lo5 times faster than values calculated from a recent tabulation of suggested Arrhenius parameters for silylene reactions.

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Acknowledgment. We thank Mr. Richard Estes for technical assistance. Registry No. CH4, 74-82-8; C2H6, 74-84-0; C2H,, 74-85-1; C3H6, 11 5-07-1; C4H6, 106-99-0; C2H2, 74-86-2; PhSiH,, 694-53-1; SiH,I, 13598-42-0;

disilane, 1590-87-0; silylene,

13825-90-6.

Pulse Radiolysis Study of the Formation of Aromatic Radical Cations Enhanced by Diphenyliodonium Salts Yukio Yamatnoto,* Xiao-Hua Ma, and Koichiro Hayashi The Institute of Scientific and Industrial Research, Osaka University, 8- I Mihogaoka, Ibaraki, Osaka 567, Japan (Received: March 24, 1987)

Further pulse radiolysis investigations of the formation of the biphenyl radical cation enhanced by diphenyliodonium salts in dichloromethane solution are reported. The slow formation of the radical cation in the presence of Ph21PF6(or Ph21AsF6) is also observed in 1,2-dichloroethanebut not in acetone, acetonitrile, and ethyl acetate. It was suggested that the free radicals generated from the chlorohydrocarbon solvents are largely responsible for the slow formation of the radical cation. Results with naphthalene, anthracene, and pyrene are also presented. The contribution of the excited states of the aromatic compounds, which have been proposed to be the precursors of the radical cations by a photolysis study, is not important in the pulse radiolysis. The effects of the salts in the radiolysis are compared with those in the photolysis.

Introduction Diaryliodonium and triarylsulfonium salts with complex metal halide anions are known as photoinitiators for cationic polymerization. Different photoinitiation mechanisms have been proposed for different systems. Upon direct irradiation with UV light, the salts undergo photodecomposition to yield Bransted acids capable of initiating cationic polymeri~ation.'-~It has also been (1) Crivello, J. V.; Lam, J. H. W. J . Polym. Sci., Polym. Symp. 1976, 56, 383-395. (2) Crivello, J. V.; Lam, J. H. W. Macromolecules 1977, 10, 1307-1315. (3) Crivello, J. V.; Lam, J. H. W. J . Polym. Sci., Polym. Chem. Ed. 1978, 16, 2441-2451. ( 4 ) Crivello, J. V.; Lam, J. H . W. J . Polym. Sci., Polym. Chem. Ed. 1979, 17, 911-999. ( 5 ) Crivello, J. V.; Lam, J. H. W. J . Polym. Sci., Polym. Chem. Ed. 1979, 17, 1059-1065.

reported that the salts promote cationic polymerization by oxidizing free radicals to corresponding carbenium ions in thermal and photochemical polymerization initiated by conventional radical initiators."12 Recently, it has been reported that in the presence ( 6 ) Crivello, J. V.; Lam, J. H. W. J . Polym. Sci., Polym. Chem. Ed. 1980, 18, 2617-2695. (7) Crivello, J. V.; Lam, J. H. W. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2691-2114. ( 8 ) Jones, R. G. J . Chem. Soc., Chem. Commun. 1985, 842-843. (9) Pappas, S . P.; Pappas, B. C.; Gatechair, L. R.; Schnabel, W. J . Polym. Sci., Polym. Chem. Ed. 1984, 22, 69-76. (10) Ledwith, A. Polymer 1978, 19, 1217-1219. (1 1) Abdul-Rasoul, F. A. M.; Ledwith, A,; Yagci, Y. Polymer 1978, 19, 12 19-1 222. (12) Yagci, Y.; Aydogan, A. C.; Sizgek, A. E. J. Polym. Sci., Polym. Leff. Ed. 1984, 22, 103-106.

0022-3654/87/2091-5343$01.50/0 0 1987 American Chemical Society