Photopolymerization of Cyclohexene Oxide by Use of o-Nitrobenzyl Triphenylsilyl Ether / Aluminum Compound Catalyst. Dependence of Catalyst Activity on the

Structure of the Silyl Ether.

SHUZI HAYASE,* YASUNOBU ONISHI, SHUICHI SUZUKI, and MORIYASU WADA, Chemical Laboratory, Research and

Development Center, Toshiba Corporation, Komukai- Toshiba-ch, Saiwai-ku, Kawasaki 210 Japan

ATUSHI KURITA, Toshiba Silicone, 113, Nishishin-cho, Oh, Gunma, 373

Synopsis

o-Nitrobenzyl triphenylsilyl ether/aluminum compound has been previously shown by the authors to act as catalyst in the photopolymerization of epoxides. The dependence of the structure of the silyl ether on the catalyst activity was examined. There were two steps in the photopolymerization. The first step (“Step 1”) is photodecomposition of the silyl ether to silanol. The second step (‘‘Step 2”) is the initiation of polymerization by silanol and the aluminum compound. The introduction of an electron withdrawing group, C1, CF,, on the benzene ring bonded to Si made the quantum yield of Step 1 low, however, the rate of Step 2 was increased. The low quantum yield of Step 1 was explained in terms of the rate of electron transfer that is controlled by the relative electron density between the CH, and NO, in the o-nitrobenzyl group. The acceleration of Step 2 was explained in terms of an increase in silanol acidity that was promoted by the introduction of an electron withdrawing group. The overall rate of the photopolymerization depends to a greater degree on the rate of Step 2 than on that of Step 1.

INTRODUCTION

It is well-known that onium salts containing complex metal halide anions initiate photopolymerization of expoxides.’,2 For example, sulfonium salts and iodonium salts of AsF; and PF; were reported by Crivello.2 Recently, very sensitive sulfonium salts that have more than two cations in the same molecules were synthe~ized,~ and the mechanism of their catalytic activity has also been r e ~ o r t e d . ~ We call these catalysts ‘‘strong acids catalysts” because they photodecompose to form strong acids that initiate polymerization.

On the other hand, the authors have found a new “nonstrong acid” ~ a t a l y s t . ~ The catalyst consists of an aluminum compound and a photodecom- posable organosilane, o-nitrobenzyl triphenylsilyl ether. The photodecomposi- tion has two steps. The first step is the photodecomposition of the silyl ether to triphenylsilanol. The second step is the initiation of polymerization with the photodecomposed catalyst that consists of an aluminum compound and

*To whom all correspondence should be addressed.

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 25, 753-763 (1987) 0 1987 John WQey & Sons, Inc. CCC 03sO-3676/87/030753-11$04.00

754 HAYASE ET AL.

O B r - Mg P M g B r siHcr3- ( R g P H Rl

- NBS ( w 3 S i B r - aniline H20 ( R% Si OH CCL, R1

( R g 3 S i - O - C H 2 e R 2 NO

Scheme 1. Synthesis of Various Silyl Ethers

We call the first step “Step 1,” and the second step “Step 2” triphenylsilanol. in this paper. I t was not clear how the structure of the silyl ether affected the individual rates of Step 1 and Step 2. In this manuscript, the relation between the silyl ether structure and the rate of each step and the mechanism are reported.

EXPERIMENTAL

Materials

Cyclohexene oxide was dried over CaH, and then distilled and stored under nitrogen. Trigethyl 3-oxybutanoato)aluminum(Al(etaa),) and tris(acety1ace- tonato)aluminum(Al(acac),) were synthesized by reacting triisopropoxya- luminum with each ligand.6

Scheme 1 shows the synthesis of various silyl ethers. W(3-ChlOrO- phenyl)silane, tri(4-chlorophenyl)silane, tri(2-chlorophenyl)silane, tri(3-tri- fluoromethylphenyl)silane, and tri(4-trifluoromethylpheny1)silane were synthesized.’ The silanes were brominated with N-bromos~ccinimide,~ and the bromosilanes were reacted with o-nitrobenzyl alcohol in the presence of triethylamine. o-Nitrobenzyl alcohol, 5-methyl 2-nitrobenzyl alcohol, and 4-chloro 2-nitrobenzyl alcohol were purchased from Aldrich. 4,5-Dimethoxy 2-nitrobenzyl alcohol was prepared by the reduction of 4,5-dimethoxy 2- nitrobenzaldehyde.’

The typical reaction conditions are as follows. Synthesis of tri(4-chloropheny1)silane: All solvents were distilled and stored

under N,. There /was 191.6 g] of 4-bromochlorobenzene. in 500 mL of diethyl ether added dropwise’to 100 mL of diethyl ether in which 24.3 g of magnesium was dispersed, and the mixture was refluxed for 1 h. Then a solution of 135.6 g trichlorosilane in 100 mL of diethyl ether was added dropwise to the reaction mixture and refluxed for 3 h. The mixture was washed with 5% HC1 aqueous solution and extracted with ether. After ether was removed, the tri(4-chloro- pheny1)silane was distilled under reduced pressure (yield 89%).

Synthesis of tri(4-chloropheny1)silylbromide: There was 184 g of the silane and 89.0 g of N-bromosuccinimide dispersed in 500 mL of carbon tetrachlo-

PHOTOPOLYMERIZATION OF CYCLOHEXANE OXIDE 755

ride, and the mixture was refluxed for 3 h. The precipitate was filtered, and the filtrate was concentrated under reduced pressure in dry N,.

Synthesis of tri(4-~hlorophenyl)( o-nitrobenzy1)silyl ether (4C1-H): A solu- tion of 76.5 g o-nitrobenzyl alcohol in 100 mL toluene and a solution of 50.5 g triethylamine in 50 mL dry toluene were added dropwise at the same time to a solution of 222 g of the unpurified silyl bromide in toluene. After the mixture was refluxed for 5 h, the precipitate was filtered, and the filtrate was concentrated under reduced pressure. There was 2 g of the crude (4C1-H) in toluene and petroleum ether (1 : 2, volume/volume) chromatographed using a 70 mm diameter column packed with 700 g of silica (Merck). The eluent was concentrated and recrystallized from petroleum ether. Total yield, 40.1%. 'H NMR (CDCl,): delta value 5.25 ppm (2H, s, CH,), 7.27-8.19 (16H, aromatic protons); IR: 3100 cm-', 2870, 1618, 1585, 1520, 1490, 1440, 1380, 1338, 1060-1100; UV (CH,CN): 260 nm (molar absorptivity, 5554), 265 (5662), 269.5 (5162). Elemental analysis, found: C, 57.98; H, 3.78; N, 2.72; C1, 20.52; calcd for C,,H,,Cl,NO,: C, 58.32; H, 3.52; N, 2.72; C1, 20.66.

The other silyl ethers except for tri(4-trifluoromethylphenyl)( o-nitroben- zy1)silyl ether were synthesized in the same way.

Tri(3-chlorophenyl)( o-nitrobenzy1)silyl ether (3C1-H): total yield, 25.4%. 'H NMR (CDCl,): delta value, 5.27 ppm (2H, s, CH,), 7.27-8.16 (16H, aromatic protons); IR: 3050 cm-', 2925,1540,1350,1140,1100,1075; UV (CH,CN), 260 nm (6021), 265 (6465), 272 (6485), 278 (5267). Elemental analysis, found: C, 58.53; H, 3.45; N, 2.81; C1,20.28; calcd for C,5H,,C13N03: C, 58.32; H, 3.52; N, 2.72; C1, 20.66.

Tri(3-trifluoromethyl)( o-nitrobenzy1)silyl ether (3CF3-H): total yield, 9.9%. 'H NMR (CDCl,): delta value, 5.30 ppm (2H, s, CH,), 7.30-8.19 (16H, aromatic protons); IR: 3075 cm-', 3025, 2900, 2840, 1605, 1530, 1320, 1110, 1065; W (CH,CN): 262 nm (2899), 265 (2997), 272 (2587). Elemental analysis, found: C, 54.85; H, 3.03, N; 2.33; calcd for C,,H,,F,NO,: C, 54.60; H, 2.95; N, 2.28.

Tri(2-chlorophenyl)( o-nitrobenzy1)silyl ether(2Cl-H): total yield, 0.5%. 'H NMR (CDCl,): delta value, 5.13 (2H, s, CH,), 6.98-8.10 (16H, aromatic protons); IR: 3040 cm-', 2900,1590,1535,1435,1350,1105; UV (CH,CN): 250 nm (5020), 260 (6502), 265 (6996), 270.5 (7047), 290 (3189), 300 (2469), Tri(4-trifluoromethylphenyl)( o-nitrobenzy1)silyl ether (4CF,-H) was synthe-

sized as follows: There was 10 g of trichlorosilane in 50 mL dry ether added dropwise to a solution of 0.27 mol4-trifluoromethylphenylmagnesium bromide in 250 mL dry ether and refluxed for 4 h. The reaction mixture was poured into 5% aqueous HC1 solution. After washing with ether 3 times, the reaction mixture was distilled under reduced pressure: 4-trifluoromethylphenylsilane, yield 83.8%. There was 28.7 g of the silane added to the mixture of 170 mL of methanol and 0.5 mL of triethylamine, then the mixture was refluxed for 1 h. After the mixture was cooled, the crystals were washed with methanol: tri(4-trifluoromethylphenyl)methoxysilane, yield 64.6%. There were 19.5 g of the methoxysilane, 15 g of benzyl chloride, and 0.2 g of pyridine refluxed for 8 h and then distilled under reduced pressure. Tri(4-trifluoromethylphenyl)chlo- rosilane: yield, 60%. There were 3.3 g of the chlorosilane, 1.2 g of o-nitrobenzyl alcohol, and 10 mL of toluene refluxed for 8 h. The reaction product was chromatographed by using toluene as eluent through the column described before; yield, 90.9%. 'H NMR (CDCl,): delta value, 5.31 ppm (2H, s, CH,),

756 HAYASE ET AL.

7.30-8.16 ppm (16H, aromatic proton); IR: 3000 cm-', 2900,1530,1310, 1100, 1050; W (CH,CN): 263 nm (3442), 267 nm (3648), 273 nm (3225). Elemental analysis, found: C, 54.43; H, 3.09; N, 2.28; calcd for C,,H,,F,NO,: C, 54.60; H, 2.95; N, 2.28.

Synthesis of silanols: tri(4-chlorophenyl)silanol(4C1SiOH), tri(3-chloro- phenyl)silanol(3ClSiOH), tri(2-chlorophenyl)silano1(2C1SiOH), tri(4-tri- fluoromethylphenyl)silanol(4CF3SiOH), and tri(3-trifluoromethyl- phenyl)silanol(3CF3SiOH) were synthesized by reacting triarylsilylbromide or triarylsilylchloride with H,O in the presence of aniline in dry ether. Tri(4- chloropheny1)silanol was synthesized as follows. There was 4.4 g tri(4-chloro- pheny1)silylbromide in 50 mL of dry ether added dropwise to a solution of 0.93 g aniline and 0.18 g H,O in 200 mL of dry ether at -1O"C, and the reaction mixture was stirred at room temperature for 1 h. The precipitate was removed, and the filtrate was concentrated under reduced pressure a t room temperature. The residue was chromatographed over silica with dichloro- methane/petroleum ether (1:l) and recrystallized from ligroin, yield 21%. The other silanols were also synthesized in the same way. The following are newly synthesized. 3ClSiOH: 'H NMR (CDCl,): delta value, 1.42 (OH, s, lH), 7.24-7.55 (12H, aromatic protons); IR: 3200 cm-', 3030, 1555, 1468, 1387, 1133; 4CF,SiOH: 'H NMR (CDCl,): delta value, 2.99 (SiOH, s, lH), 7.70 (aromatic proton, 12H); IR: 3220 cm-', 1393,1320,1120,1060,1020. 3CF,SiOH: 'H NMR (CDCl,): delta value; 1.43 (SiOH, s, lH), 7.24-7.88 (12H aromatic protons); IR: 3220 cm-', 2900, 1602, 1415, 1330, 1122. 2ClSiOH: 'H NMR (CDCl,): delta value, 3.10 (SiOH, s, 1H); IR: 3600 cm-', 3430, 3040, 2990, 1578, 1555, 1413.

Photolysis

The photolyses were carried out using either a 400 W high-pressure mercury lamp (UVL 400H, Riko Kagaku Sangyo), or low pressure mercury lamps (UVL lOJA, UVL 30JA, or W L 60JA). The lamp was surrounded by a water-cooled quartz photolysis wall. Samples were placed in a "merry-go- round" holder that rotated about the lamp to provide even illumination. The entire apparatus was immersed in a thermostated water bath a t the specified temperature. The decomposition was followed by high pressure liquid chro- matography with a ODS silica column. CH,OH : H,O (5 : 1, volume/volume) was used as solvent. The silica was pretreated with octadecylsilane. An apparatus that produces collimated W light (400 W high-pressure mercury lamp) (USHIO-UI 501) was also used. In all cases, a WD-36A filter was used for transmitting the light of 365 nm. The combination of a high-pressure mercury lamp and W D 36A filter provided mainly 365 nm, with a small component of 313 nm (100 : 3 intensity ratio).

Photopolymerization

The catalyst was dissolved in cyclohexene oxide in a nitrogen atmosphere in a quartz tube with a glass stopper. The W irradiation apparatus is the same as the apparatus in the case of photolysis. After photopolymerization, a small amount of aniline was added, followed by the removal of the unreacted monomer under reduced pressure. Then the polymer was washed with acetone

PHOTOPOLYMERIZATION OF CYCLOHEXANE OXIDE 757

and dried. The catalytic activity was measured by the weight of polymer produced. Polymerization in dark was carried out in the same way as in the case of photopolymerization except for irradiation.

Quantum yields were measured according to the literatureg using a merry- go-round apparatus. There was 1.14 X lop2 M of the silyl ether in acetonitrile photodecomposed (365 nm light) in a 10 mm quartz tube. The loss of silyl ether was measured by HPLC.

NMR spectra were obtained with a FX 9OQ (JEOL). The extent of "Si-0-Al" products were determined by NMR. There were 2.1 x mol of silanol and 2.1 X mol of tris(ethy1acetoacetato)aluminum dissolved in 1 mL of CDC1,. The extent of Si-0-A1 bond formation was calculated by comparison of the amount of CH, in the aluminum complex and the amount of CH, in ethylacetoacetate. All the reaction was assumed to proceed to give the Si-0-A1 bond.

Measurement of Relative Acidity of the Various Silanols

mol/l mL) was determined in CDCl, and d,-acetone. The relative acidity was taken to be the difference between d,-acetone and CDCl,.

The chemical shift of the silanol proton (1.5 x

RESULTS AND DISCUSSION

Photodecomposition of Various Silyl Ethers

The mechanism of photogeneration of silanols from silyl ethers may be described as shown in Scheme 2.53'0 To begin with, the o-nitrobenzyl group is excited by W irradiation. According to the literature, the excited state would

hv Ph,SiO - C H , O -

NO, COOH COOH

+ Ph,SiOH + ( : i C G )

I No 4

QN="P COOH COOH

Scheme 2. Mechanism of Silyl Ether Photodecomposition

758 HAYASE ET AL.

TABLE I Quantum Yield of Silyl Ether Photodecomposition

Abbreviation Quantum yield

H-H H--5Me H-4C1 H-4,5Me0 4CF3-H 3CF3-H 4C1- H 3C1-H 2C1-H

H H H H 4CF3 3CF3 4c1 3C1 2c1

H Me 4C1 4,5Me0 H H H H H

0.20 0.26 0.22 0.04 0.14 0.12 0.13 0.13 0.15

"See Scheme 1.

be 3(a-n*)'0h or 3(n-7r*).10g An electron of the CH, group shifts from the CH, group to the nitro group to form a radical ion pair,log> loi then the proton shifts from the CH, group to the nitro group. The nitro group is changed to -N(O)OH. The OH group shifts from N(0)OH to -CH-, which is con- verted to -CHOH-. This product has a hemiacetal structure. The hemia- cetal smoothly decomposes to form triphenylsilanol. Nitrosobenzaldehyde is believed to be converted to azobenzene derivatives by further UV radiation.lob

It was anticipated that R, but not R , would affect the rate of Step 2, because Step 2 is influenced by the acidity of substituted arylsilanol." In addition, i t has been previously reported that R, affects the rate of Step 1,5 but the effect of R, on Step 1 was not. Five silyl ethers were newly synthesized by introducing C1 or CF, ( Rl).

Substitution on the o-nitrobenzyl moiety was also carried out. The quan- tum yields for photodecomposition of these silyl ethers at 365 nm (see experimental section) are shown in Table I. The quantum yields of those compounds with R, substituents were essentially identical (0.12-0.15) and were lower than that of triphenyl( o-nitrobenzy1)silyl ether.

The quantum yield was affected by R,, and, as described earlier, this could be explained by the assumption that the relative electron density between benzyl CH, and NO, determines the facility of electron transfer from CH, to NO,, which affects the rate of photodecomposition of the silyl ether.

Except for H-4,5 MeO, R, substitution had little effect on the quantum yield. For H-4,5 MeO, the 13C NMR chemical shift difference between benzyl- CH, and carbon where NO, is substituted largely decreased by about 8-10 ppm (75.34 ppm) compared with that of other compounds substituted with R, (82-85 ppm). This is explained by the assumption that the methoxy sub- stituents reduce the difference in the electron density between benzyl-CH , and NO, and retard electron transfer from benzyl-CH, to NO,. It is reported that methoxy substituents affected photoreduction rate of alkyl benzenes with nitrobenzene.la -lok

In case of the silyl ether substituted with R,, the substitution of an electron withdrawing group, such as C1 and CF,, on a phenyl group adjacent to Si would cause a decrease in the electron density of the CH, (benzyl) group. Cansidering the distance between the electron withdrawing group and benzyl-

PHOTOPOLYMERIZATION OF CYCLOHEXANE OXIDE 759

CH, or NO,, the electron density of the benzyl-CH, would get lowered more greatly than that of NO,, which made the electron shift from CH, to NO, become a little bit difficult compared with the triphenyl( o-nitrobenzy1)silyl ether.

Photopolymerization of Cyclohexene Oxide

The results from the photopolymerization of cyclohexene oxide with aluminum compound/silyl ethers with R, are shown in Figures 1 and 2. When Al(etaa), (see experimental section) was used, the catalyst activity decreased in the following order: 4Cl-H, 4CF3-H, 3Cl-H, 2Cl-H, H-H, 3CF3-H. On the other hand, the catalyst activity with Al(acac), decreased in the following order: 4CF3-H, 3Cl-H, 4CI-H, 3CF3-H, H-H.

100 - $ - c 0 In L 0) > C 0

.-

5 0 L 0)

- E 0 a

2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0

photopolymerization time ( min 1 Fig. 1. Photopolymerization of cyclohexene oxide (CHO) with Al(etaa),/silyl ether catalyst.

4OOC; 400 W high pressure mercury lamp, WD36A filter (365 nm); monomer : toluene = 4 : 1 (volume/volume); Al(etaa)3, 0.01 mol I; silyl ether, 0.02 mol I. Abbreviation, see Table I.

I00

c 0 .- $

- 5

2 50 0 0

W L

0 a

I0 20 photopolymerization time (min)

Fig. 2. Photopolymerization of cyclohexene oxide with Al(acac), silyl ether. 40OC; 400 W high pressure mercury lamp, WD36A filter (365 nm); Al(acac),, 0.01 mol I; silyl ether, 0.02 mol I%. Abbreviation, see Table I.

760 HAYASE ET AL.

CH3 0 0

\ n I I

/

0-c’ \ / \ + HO - SiPh3 - A l -0 -S iPh3 tCH3CCH,COC,H,

/CH /

Al / ro=c,

0 - H 4 ‘ A l -0-SiPh3

Scheme 3. B Interaction Between Al(etaa), and Silanol /

Step 2 is affiliated with the order. We have reported the reaction mech- anism of Step 2 as follows.” The polymerization behavior with Al(acac),/Ph,SiOH was different from that with Al(etaa),/Ph,SiOH, de- pending on whether or not the aluminum complex reacts with Ph,SiOH. The catalyst activity also depended on the acidity and steric environment of the silanol.

In the case of Al(acac),/triphenylsilanol, the H of the SiOH is detached from the oxygen when the SiOH interacts with Al(acac),, which is a weakbase. The polymerization is carried out cationically because the H of the SiOH is polarized and somewhat has characteristics of H’. There is no detectable reaction, such as ligand exchange, between the aluminum complex and the triphenylsilanol by ‘H NMR or gas chromatography. We want to call this interaction “(A) Interaction.”

In the case of Al(etaa),/triphenylsilanol, a reaction occurs to form a compound that contains an Si-0-A1 linkage, as shown in Scheme 3. This is a kind of ligand exchanging reaction and equilibrium reaction. Then the H of SiOH that was not consumed by the reaction of the aluminum complex is detached from the oxygen when the oxygen atom of SiOH interacts with the aluminum atom of the Si-0-A1 linkage. The H is polarized to form H’, which polymerizes epoxide cationically. The interaction is similar to that of a solid catalyst such as silica alumina. We want to call this interaction “(B) Interaction.” In the case of Al(etaa),/silanol catalyst, both (A) and (B) Interactions occur. However, the polymerization is initiated predominantly by (B) Interaction because (B) Interaction is stronger than (A) Interaction.

When an aluminum complex such as Al(etaa), reacts with the silanol, the extent of the reaction (formation of Al-0-Si linkage) increases, with an increase in silanol acidity, which causes catalyst activation, because (B) Interaction predominantly initiates polymerization. However, since the ratio of aluminum compound to silanol is determined to be 1, formation of the Si-0-A1 product reduces the silanol concentration, and if silanols over some acidity are used, the catalyst activity starts to decrease. Therefore, silanol that has appropriate acidity has maximum catalyst activity.

The relative acidity of various silanols shown in Table I1 was taken to be the difference between the ‘H NMR chemical shift value of silanol in d, acetone versus CDC1,. The acidity increased in the following order:

PHOTOPOLYMERIZATION OF CYCLOHEXANE OXIDE 761

TABLE I1 Properties of Triarylsilyl Ethers

Relative aciditya Si-O-db Silanol R,SiOH (ppm) linkage ( W )

4CF,SiOH 3.83 32 3CF3SiOH 3.89 26 4ClSiOH 3.52 22 3ClSiOH 3.66 26 2ClSiOH 3.01 8 HSiOH 3.29 4

a(ppm in d,-acetone)-(ppm in CDCl,). bAl(etaa)3, see experimental section. Abbreviation, see experimental section.

3CF3SiOH, 4CF3SiOH, 3ClSiOH, 4ClSiOH, HSiOH, 2ClSiOH. The ordering of the catalyst activity when using Al(acac), was almost the same as the ordering of silanol acidity that was formed by photodecomposition of silyl ether, except for 3CF3SiOH.

In the case of Al(etaa),, the order of the catalyst activity was not the same as the order of silanol acidity. The following two points have to be considered in order to explain the catalyst activity. (1) In Step 2, in the case of Al(etaa),, a part of the aluminum complex reacts with a part of silanol to form a Si-0-A1 linkage, and the amount of Si-0-A1 linkage increases with increases in silanol acidity, because it is an equilibrium reaction [(B) Interac- tion in Scheme 31. (2) In Step 2, the catalyst activity is affected by both the amount of the Si-0-A1 linkage and the amount of the remaining silanol that was not consumed by the reaction of the aluminum compound, because there is no catalyst activity when all silanols are consumed by the reaction with the aluminum complex. Figure 3 shows the polymerization results of cyclohexene oxide with Al(etaa),/silanol catalyst in dark, which corresponds to Step 2. The catalyst activity decreased in the following order: 4ClSiOH,

10 20 30 40 50 60 Polymerization time ( min)

Fig. 3. Polymerization of cyclohexene oxide with Al(etaa),/silanol catalyst. 40°C; monomer : toluene = 3 : 1 (volume/volume); Al(etaa)3, 0.01 mol %; silanol, 0.01 mol %. Abbrevia- tion, see experimental section.

762 HAYASE ET AL.

4CF,SiOH, 3ClSiOH, 3CF,SiOH, BClSiOH, HSiOH. As shown in Table 11, when silanol and Al(etaa), were added in the ratio of 1 : 1 (mol), the amount of Al-0-Si linkage almost increased with an increase in the acidity of SiOH except for 3CF3SiOH. There was no reaction between Al(acac), and silanol by ‘H NMR. The relation between the amount of A1-0-Si linkage and the catalyst activity supports the notion that complete reaction between the aluminum complex and the silanol decreases the catalyst activity because of a lack of silanol in the polymerization system. For example, although the acidity of 4CF,SiOH is higher than that of 4ClSiOH, the catalyst activity of the former is lower than that of the latter, because, in the case of 4CF3SiOH, almost all the SiOH was consumed by the reaction of the Al(etaa), and the concentration of the free SiOH that carries out polymerization actually is low.

The order of catalyst activity for photopolymerization is the same as that of the aluminum compound/silanol catalyst activity in Step 2, which proves that the photopolymerization ratio is affected by Step 2 more predominantly than Step 1.

The activity of 2C1-H was quite low despite substitution of the elec- tron withdrawing group C1, because the bulky C1 prevents the SiOH from interacting with the aluminum compound or epoxide. The reason for the low activity of Al(acac),/SCF,--H and the decreased reaction between Al(etaa), and 3CF,-H have not yet been explained.

CONCLUSION

The catalyst activity of aluminum compound/o-nitrobenzyltriarylsilyl ether was varied when the benzene ring bonded to Si was substituted with elec- tron withdrawing groups C1 and CF,. Al(etaa),/4Cl-H was the most active. The substitution on the benzene ring decreased the quantum yield for pho- todecomposition of the silyl ether compared with triphenyl( o-nitrobenzy1)silyl ether; however, there was no large difference of quantum yield between these derivatives. Substitution of R, varied the rate of Step 2, which changed the catalyst activity. Since the same silanol was formed in Step 1 even if R, was varied, the variation of R, had no effect on the rate of Step 2. On the other hand, when R, was varied, not only the rate of Step 1 but also the rate of Step 2 was varied, because silanols with various R,’s initiate polymerization. The substitution on benzene ring affected the rate of Step 2 more greatly than the rate of Step 1. These substituent effects on the catalyst activity were explained in terms of the electronic and steric factor owing to position and variety of these substituents.

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PHOTOPOLYMERIZATION OF CYCLOHEXANE OXIDE 763

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Received January 3,1986 Accepted April 25, 1986

Polym. Chem. Ed., 22, 1789 (1984).

(1984); ibid., 22, 69 (1984).