Phenothiazine Photosensitizers for Onium Salt ... - ACS Publications

Chapter 20

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Phenothiazine Photosensitizers for Onium Salt Photoinitiated Cationic Polymerization Zaza Gomurashvili and James V . Crivello* Department of Chemistry, New York Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8 Street, Troy, NY 12180 th

The syntheses of several different substituted phenothiazines are described and the ability of these compounds to photosensitize the photolysis of different onium salt cationic photoinitiators is described. These photosensitizers (PS) are generally operative in the mid- and long-range regions of the UV spectrum. The efficiencies of photosensitization of the cationic photopolymerizations of several typical epoxide and vinyl ether monomers bydifferentsubstituted phenothiazine compounds were evaluated and compared. In addition, a phenothiazine-containing monomer and polymer were prepared and were shown to be effective photosensitizers.

Introduction Currently, the most commonly used photoinitiators employed for photoinduced cationic polymerizations are diaryliodonium, I, and triarylsulfonium salts, Π, with the general structures shown below in which MtX " represents a weakly nucleophilic counterion. Another promising class of photoinitiators is dialkylphenacylsulfonium salts, HI. n

PT

© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

231

232 Although considerable synthetic efforts have been made to extend the spectral sensitivity of onium salt photoinitiators by substitution of appropriate chromophores onto the aromatic rings of these compounds, it has not been possible to significantly shift their sensitivity beyond the middle range of the U V spectrum. Fortunately, it is possible to photosensitize the decomposition of diaryliodonium and triarysulfonium salts photoinitiators with electron-transfer photosensitizers. We have reported extensively on our studies in this area ' and other investigators have also been active in this field. There are several mechanisms by which the photosensitization of onium salts take place, however, electron-transfer photosensitization appears to be the most efficient and generally applicable process. A generalized mechanism for the electron-transfer photosensitization of diaryliodonium salts is shown in Scheme 1. 1

2 3,4


5,6,7,8

9

10

PS

- ^ - ^ [PS1*

[PSl* + Ar I+Mt^|2

* {PS — Ar I+MtX,fl* 2

exciplei

+ +

[PS — Ar I MtX 1* 2

n

ΑΓ Ι· 2

[ PS ]*MtXn- + mM

» I PS f MtXn" + A r f 2

*

Arl + A r .

* polymer Scheme I

Unfortunately, at the present time, the number of electron-transfer photosensitizers operative in the mid- and long wavelength UV and visible regions are very limited Many of these photosensitizers suffer from various disadvantages, including toxicity and limited solubility in a wide variety of monomers. For these reasons, we have been conducting investigations directed toward the development of novel classes photosensitizers that mitigate the dissadvantages of the former compounds. The photosensitizing effects of 10H-phenothiazine (PT) for onium salt types Π and ΙΠ were reported and studied by several research groups. This compound is potentially useful for applications that require photosensitization in the middle region (300-400 nm) of the U V spectrum. This communication describes the discovery, that a wide variety of substituted phenothiazines are efficient photosensitizers for the photolysis of onium salts. Attempts were made to correlate the structure and spectral characteristics of the phenothiazines with their efficiency of sensitization in the cationic photopolymerizations of several typical epoxide and vinyl ether monomers. In addition, monomers and polymers containing phenothiazine moieties were prepared and evaluated as photosensitizers for onium salt photoinitiators. 11,12,13


233

Experimental Materials All organic starting materials, monomers and reagents, 10H-phenothiazine (PT), 10-methylphenothiazine, 2-chlorophenothiazine, 2-trifluoromethylphenothiazine and 2-acetylphenothiazine were obtainedfromcommercial sources. The photoinitiators, (4-n-decyloxyphenyl) phenyliodonium hexafluoroantimonate (IOC10), (4-n-decyloxyphenyl)-diphenylsulfonium hexafluoroantimonate (SOCIO) and S-dodecyl-S-methyl-S-phenacylsulfonium hexafluorophosphate (DPS-C1C12 PF ) were prepared as described previously. 14

15


16

6

Preparation of 10-Decylphenothiazine A mixture of 20 g. (0.1 mol) of phenothiazine, 35 mL (-0.15 mol) of 1bromodecane, 9.0 g (—0.15 mol) of potassium hydroxide and 0.48g. (1% w/w) of tetra-n-butylammonium bromide in 300 mL dry toluene was stirred and refluxed for 12 h. The cooled reaction mixture was filtered, washed thoroughly with water, dried over anhydrous sodium sulfate and the solvent removed by evaporative distillation. The residue wasfractionallydistilled at 0.15 mm Hg. 10-Decylphenothiazine was obtained (yield: 27.5 g, 81%) as a pale yellow oil with a b.p. of 196-201°C (lit. b.p. 183-185°C at 0.5 mm). U NMR (300 MHz, CDC1 ): δ (ppm) 7.17 - 6.86 (m; Ar), 3.86 (t, N-C/£), 1.81(q, N-CH -C# ), 1.44 - 1.26 (q,m; (CH ) \ 0.90 (t, CH ) 17

l

3

2

2

2

7

3

Preparation of 10-Methylphenothiazine-5-oxide 18

The method of Schmalz and Burger was used for the synthesis of 10methylphenothiazine-5-oxide from 10-methylphenothiazine. An 85% yield of colorless crystalline 1O-methylphenothiazine-5-oxide with a m.p. of 193-194°C (lit. m.p. 193-194°C) was obtained. H NMR (300 MHz, DMSO-de): δ (ppm) 7.94 - 7.23 (m; Ar), 3.73 (s, N - C i £ ) lg

l

Preparation of 10-Methylphenothiazine-5,5-dioxide 10-Methylphenothiazine (1.5 g., 0.7 mmol) was dissolved in 40 mL of glacial acetic acid at 80°C to give a yellow solution. To the solution, 5.0 mL (4.8 mmol) of 30% hydrogen peroxide was added, causing the formation of deep red color. Stirring was continued for 1.5 h at 80°C. After evaporating the solvent, an impure red crystalline product was obtained. Recrystallization from 96% ethanol produced 1.2 g (68%) of orange colored crystals of 10methylphenothiazine*5,5-dioxide having a m.p. of 221-222°C (decomp.) (lit. m.p. 220-221°C, dec). The infrared spectrum showed characteristic sulfone bands at 1165 and 1126 cm . 19


234 l

H NMR (300 MHz, DMSO-d*): δ (ppm) 7.98 - 7.33 (m; Ar), 3.71 (s, N-Ci/,) In the same way, 10-decylphenothiazine-5,5-dioxide, m.p 95-96°C, was prepared.


Preparation of 10-(2-Vinyloxyethyl)phenothiazine (VPT) Combined into a 500 mL round bottom flask fitted with a magnetic stirrer, reflux condenser and nitrogen inlet were 5.2 g (0.13 mol) of NaH (60% dispersion in mineral oil), and 350 mL dry tetrahydrofuran. To this solution were added portion-wise 16 g (0.08 mol) of 10H-phenothiazine. The mixture was slowly heated and refluxed for 30 min. After cooling to room temperature, 16.8 mL (0.15 mol) of 2-chloroethyl vinyl ether and 0.40 g tetra-nbutylammonium bromide was added and refluxing continued for 30 h. The cooled reaction mixture was filtered and the solvent and excess vinyl ether evaporated under reduced pressure. Toluene was added to the residue and the precipitated unreacted 10H-phenothiazine was removed by filtration. Evaporation of the toluene and distillation of the resulting oil at 170-175°C (0.01T) afforded 5.4 g of VPT as a pale yellow oil. (lit. b.p. . 150-170 °C). H NMR (500 MHz, CDC1 ): δ (ppm) 7.29 - 6.92 (m; aromatic protons), 6.48 (q, -C/f=CH ), 4.26-4.20 (m, N - C / ^ C / ^ - N ) , 4.08 (m; CH=CJ/ ). Elemental analysis for C H N O S . Calc: %C, 71.34; %H, 5.61; %N, 5.20; %S, 11.90. Found: %C. 71.69; %H, 5.64; %N, 5.23; %S, 11.88. 20

0

1T

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3

2

2

16

l5

Polymerization of V P T The polymerization of VPT was carried out at -45°C by injecting boron triflouride etherate through a rubber septum into aflaskthat had been charged with a solution of 2.0 g. of the monomer and 25 mL dry dichloromethane. After stirring for 45 min., reaction was quenched by the addition of 3 mL of cold pyridine and then poured into cold ether. The precipitated polymer (PVPT) was collected byfiltration,washed with ether and dried overnight in a vacuum oven at45°C. Photopolymerization Studies Using Fourier Transform Real-Time Infrared Spectroscopy (FT-RTBR). Photopolymerizations of the monomers were monitored by FT-RTIR. A Midac M-1300 FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector was used. Experiments were performed at room temperature using broadband UV lightfroman Hg arc source with a light intensity of200 mJ/cm min. Photopolymerizations were carried out in monomer solutions containing 1.0 mol% of the photoinitiator. Thin films of the liquid monomer were sandwiched between two identical polypropylenefilmsand then mounted in 5x5 cm slide frames. Infrared spectra were collected at a rate of 1 spectrum per


235 second using LabCalc data acquisition software and were processed using GRAMS-386 software. During irradiation, the decrease of the IR band due to either epoxy group (780-915 cm' ) or vinyl ether double bond (1620 cm* ) of the monomers was monitored. 1

1

Results and Discussion


Synthesis and UV Characterization of Phenothiazine Compounds. PT has major UV absorption bands at 253 and 318 nm. A series of substituted phenothiazines bearing different alkyl, acyl and aryl substituents at both the nitrogen atom (10-position) and cm the aromatic rings (2-position) were either synthesized or obtained from commercial sources. Comparison of their UV-VIS data showed several general trends. The introduction of electronwithdrawing groups (particularly effective was the acyl group) onto the aromatic rings shifts the ^ absorption to longer wavelengths (281,397nm). Substitution of PT at the nitrogen atom (10 position) with alkyl or aryl groups produces little change in the absorption characteristics. Oxidation of die phenothiazine sulfur shifts the longest wavelength-absorption maximum to longer wavelengths ( λ ^ 1O-n-deeylphenothiazine, 297, 336 nm). However, the introduction of an acyl group (10-acetylphenothiazine) shifts the longest absorption to band shorter wavelengths.

Photosenstized Cationic Polymerizations Using Phenothiazines The ability of phenothiazine, low molar mass compounds, monomers and polymers to photosensitize the cationic photopolymerization of various monomers was evaluated using Fourier transform real-time infrared spectroscopy (FT-RTIR). RTIR has been proven to be an extremely useful method for monitoring the kinetics of very rapid photopolymerization reactions and for that reason, has been used in this laboratory as well as by many other investigators to probe these types of reactions. ' Cyclohexene oxide (CHO) was polymerized in the presence of 0.5% of PT, 10-methylphenothiazine and 10-n-decylphenothiazine. All three compounds display excellent activity as photosensitizers for onium salts. An acceleration of the rate of photoinitiated cationic ring-opening polymerization of the epoxide monomer was observed in all cases. A second, and more important effect is the considerable reduction in the induction period. Figure 1 shows the acceleration effect when the initiator DPS-C1C12 was used. 21,22

23,24



236 Whereas the two 10-alkyl substituted phenothiazines are essentially identical in their influence on the photopolymerizations of CHO, PT is clearly less effective as a photosensitizer for onium salts. An analogous comparison of 10-methyl and 1O-phenylphenothiazine with 1 O-acetylphenothiazine in the IOC 10 photoinitiated photopolymerization of CHO is portrayed in Figure 2. While 10-methyl and 10-phenylphenothiazines exhibit essentially the same activity as photosensitizers, as may be predictedfromthe similarity of their U V absorption spectra, 10-acetylphenothiazine appears to be an inhibitor. The reduced electron density on die acetylated nitrogen atom results in an increase in the oxidation potential of the compound, making it less likely to undergo photoinduced oxidation potential by onium salt photoinitiator. The introduction of the electron-withdrawing substituents into the 2position of the aromatic rings of PT produces a shift in the UV absorption bands to longer wavelengths. Figure 3 illustrates a study of the use of 2-acetyl-, 2chloro- and 2-trifluoromethylphenothiaziens in the IOC 10 photosensitized polymerizations of CHO. All three phenothiazine compounds display good photosensitizing activity. Oxidizing the sulfur atom of 10-methylphenothiazine also effects the photosensitization of the polymerization of CHO (Figure 4). Surprisingly, 10-methylphenothiazine-5-oxide appears to inhibit the polymerization. In contrast, the corresponding sulfone, 1Q-methylphenothiazine5,5-dioxide displays excellent response as a photosensitizer. In further experiments, 10-n-decylphenothiazine-5,5-dioxide exhibited similarly efficient behavior as 10-methylphenothiazine-5,5-dioxide, when used as a photosensitizer

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.

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20 10 0 50

100

150

200

Irradiation Time (sec)

Figure 1. FT-RTIR study of cationic polymerization of CHO (cyclohexene oxide) using DPS-C,C alone (•) and in the presence of 0.5% PT (O), 10-methylphenothiazine (Δ), and 10-n-decylphenothiazine ( X ) . 12


237 90 80 70 60


§ « y

30 20 10 0

100

150

200


Figure 2. Photopolymerization of CHO in the presence of IOC 10 alone (•) and using 0.5 % of 1 O-acetylphenothiazine (O), ) and 10-methylphenothiazine ( X ) .

50

100

1 O-phenylphenothiazine

150

200


Figure 3. Study of the photopolymerization of CHO with IOC10 alone (•), and with 0.5%2-acetylphenothiazine ( • ) , 2-chlorophenothiazine (^ ) and 2-trifluoromethylphenothiazine ( X ) as photosensitizers.


238

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i

Ο ϋ

50

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200


Figure 4. Photopolymerization of CHO with IOC alone (•) and in presence of 0.5% 10-methylphenothiazine ( X ) , 10-methylphenothiazine-5-oxide (O) and 10-methylphenothiazine-5 5-dioxide (Δ). f

e

s ο

•RM

ε

Ι 50

100

150

200


Figure 5. Study of the photopolymerization of 2-chloroethyl vinyl ether in the presence (O) and absence (•) of 0.5% lO-decylphenothiazine-5,5dioxide using IOC10 as the initiator.



239 for CHO or limonene dioxide as well as for vinyl ether monomers (2-chlorœthyl vinyl ether; Figure 5). 10-Alkylphaiothizaine-5,5-dioxides exhibit extraordinarily photosensitive efficiency not only for diaryliodonium salt photoinitiators as well as for triarylsulfonium and dialkylphenacylsulfonium salts. On the other hand, oxidizing 10-acetylphenothiazine to the corresponding sulfone did not improve the efficiency of this compound as a photosensitizer, indicating that the electronwithdrawing effect of the 10-acyl group is the major effect in the determination of the photosensitizing ability of these two compounds. To establish whether the primary effect of photosensitization with phenothiazines results from their long wavelength absorption, a study was conducted in which filtered UV light was used. Using glassfilter,wavelengths below 300 nm were removedfromthe irradiation source. As the results show, the rate and extent of the photopolymerization of CHO using 10alkylphenothiazine-S,5-dioxides are unchanged in the presence and absence of the filter. Thus, the photoactive bands of this photosensitizer can be said to lie primarily at wavelengths greater than 300 nm. We have also prepared the monomer, 10-(2-vinyloxyethyl)phenothiazine (VPT) as shown in the following equation by the alkylation of PT with 2chloroethyl vinyl ether. This monomer was readily polymerized (PVPT) in the presence of BF etherate. Figure 6 shows the effect of VPT and PVPT on the photopolymerization of 4-vinylcyclohexene dioxide. The epoxide band at 794 cm* was monitored for this monomer. Again, both monomer and polymeric phenothiazines display very similar and excellent behavior as photosensitizers in the polymerization of this difunctional epoxide monomer. 3

1

PVPT

VPT

Conclusions Marked acceleration of the onium-salt induced photopolymerizations of epoxides and vinyl ether monomers were noted in the presence of small quantities of low molar mass phenothiazine compounds bearing a wide variety of types of functional groups. Particularly efficient photosensitizers are 10alkylphenothiazine-5,5-dioxides. Moreover, the ability to readily change the wavelength of absorption of these compounds in a very predictable manner



240

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Ι

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Irradiation Time (sec) Figure 6. Comparison of the photosensitized polymerization of 4vinylcyclohexene dioxide carried out with 1.0 %IOC-10 alone (•), in the presence of 0.5 % 10-(2-vinyloxyethyl)phenothiazine (•) and poly(10-(2-vinyloxyethyl)-phenothiazine) (Δ). (Light intensity 400 mJ/cmfmin.) makes this new class of onium salt photoinitiators potentially very valuable for photosensitization in the long wavelength UV-VIS regions of the spectrum. The discovery that monomers and polymers containing phenothiazine groups are also excellent photosensitizers for onium salt photoinitiators is also of considerable academic and industrial interest.

References 1. 2. 3. 4. 5. 6.

Crivello, J.V. Photoinitiators for Free Radical Cationic and Anionic Photopolymerization, Wiley: New York, 1998; p.329. Crivello, J.V.; Lam, J.H.W. J. Polym. Sci. Part A: Polym. Chem. 1976, 16, 2441. Crivello, J.V.; Lam, J.H.W. J. Polym. Sci. Part A: Polym. Chem. 1979, 17, 1059. Crivello, J. V.; Lee, J.L. Macromolecules 1981, 14, 1141. Gu, H.; Zhang, W.; Feng, K.; Neckers, D.C. J. Org. Chem. 2000, 65, 3484. Chen, Y.; Yamamura, T.; Igarashi, K. J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 90.


241 7. 8. 9.


10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Nelson, E.W.; Carter, T.P.; Scranton, A.B. J. Polym. Sci. Part A: Polym. Chem. 1995, 33, 247. Kampmeier, J.A.; Nalli, T.W. Macromolecules 1994, 59, 1381. Crivello, J.V. and K. Dietliker, in: Chemistry & Technology of UV and EB Formulation for Coatings, Inks & Paints, Vol. III; Bradley, G. Ed.; SITA Technology Ltd.: London, 1998; p.420. Crivello, J.V. in: Ring-Opening Polymerization; Brunelle, D.J. Ed.; Hanser: Munich, 1993; p. 157. Crivello, J.V.; Lee, J. L. Mcromolecules 1983, 16, 864. Kunze, Α.; Müller, U.; Tittes, Κ.; Fouassier, J.-P.; Morlet-Savary, F.J. J Photochem. Photobiol. A 1997, 110, 115. Neumann, M.G.; Rodrigues, M . R. J. Polym. Sc.i Part A: Polym. Chem. 2001,39,46. Crivello; J.V.; Lee, J.L. J. Polym. Sc.i Part A: Polym. Chem. 1989, 27, 3951. Akhtar, S.R.; Crivello; J.V.; Lee, J.L. J. Org. Chem. 1990, 55, 4222. Crivello, J.V.; Kong, S. Macromolecules 2000, 33, 833. Gillman, H.; Shirley, D.A. J. Am. Chem. Soc. 1944, 66, 888. Schmalz, A.C.; Burger, A. J. Am. Chem. Soc. 1954, 76, 5455. Gilman, H.; Nelson, D.R. J. Am. Chem. Soc. 1953, 75, 5422. Kamogawa, H. Japanese Patent 73 33,756, Oct 16, 1973; Chem. Abstr. 1974, 81, 26182h; J. Polym. Sci. PartA-11972, 10, 95. Decker, C.; Moussa, Κ. Makromol. Chem. 1990, 191, 963. Decker, C. J. Polym. Sci.: Part A: Polym. Chem. 1992, 30, 913. Müller, U. J. Macromol. Sci.; Pure and Appl. Chem. 1996, A33, 33. Bradley, G.; Davidson, R.S.; Howgate, G.J.; Mouillat, C.G.J.; Turner, P. J. Photochem. Photobiol. A: Chem. 1996, 100, 109.


Phenothiazine Photosensitizers for Onium Salt ... - ACS Publications

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