Visible and Long-Wavelength Cationic ... - ACS Publications

Chapter 21

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Visible and Long-Wavelength Cationic Photopolymerization 1

Marco Sangermano and James V .

2,*

Crivello

1

Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino, Italy Department of Chemistry, New York Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8 Street, Troy, NY 12180 2

th

A new photosensitizing system for efficiently carrying out cationic photopolymerization with visible and long -wavelength UV light is described. This system is based on the principle that certain onium salt cationic photoinitiators can be reduced byfreeradicals produced by the hydrogen abstraction reactions of photoexcited aryl ketones. The proposed mechanism was confirmed by the experimental results obtained and the effects of different photoinitiators, photosensitizers and the monomer structure were investigated.

Introduction In recent years, the use of onium salt initiated cationic photopolymerization has reached commercial significance as a wide variety of uses have emerged and the benefits of this technology have been realized. The advantages afforded by photopolymerization processes have led to their rapid growth in applications in different fields such as films, inks and coatings on a variety of substrates, including paper, metal and wood. In addition, a variety of high-tech and electronic applications of this technology, such as coatings for opticalfibersand the fabrication of printed circuit boards have emerged. The most important current developments in the field of UV-curing have been summarized in two recent reviews. Photoinitiated cationic polymerization has found its greatest application in 1,2

242

© 2003

American Chemical Society

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

243 the ring-opening polymerizations of epoxy monomars and other strained ring systems such as lactones and cyclic acetals. More recently, cationic UV curing technology has also been applied with considerable success to vinyl ether monomers and oligomers. " Unfortunately, aryl onium salt photoinitiators suffer from the disadvantage that they have their principal absorption bands in the short wavelength region of the UV spectrum. This has placed some limitations on the potential uses of cationic photopolymerization. For example, cationic UV curing is not widely used in reprographic, printing or dental applications that rely on the use of visible light emitting sources, such as laser, quartz halogen lamps and light emitting diodes. Downloaded by UNIV OF ARIZONA on December 5, 2012 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch021

3

5

Photosensitization of Onium Salts Although considerable synthetic efforts have been made to extend the spectral sensitivity of onium salts 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 UV spectrum. Fortunately, it is possible to photosensitize the decomposition of diaryliodonium and triarylsulfonium 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. " However, the number of photosensitizers operative in the 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 an investigation directed toward the development of novel methods of broadening the spectral sensitivity of onium salt photoinitiators into the long wavelength UV and visible regions, using alternative photosensitization techniques. 7 9

10

13

Experimental Materials Given in Table I are the structures and abbreviations for the monomers and photoinitiators used during the course of this work. The synthesis of diaryliodonium, triarylsulfonium, and dialkylphenacylsulfonium salts have been reported previously. A description of die apparatus and technique of Fourier Transform real-time infrared spectroscopy used to monitor the photopolymerizations has been reported in recent publications from this laboratory. 14

15

16

15


244 Table L Structures of Photoinitiators and Monomers Photoinitiators _ f /»3 SbF -

Η

6

6

IOC10 (4-ndecyloxyphenyl)pheny 1-iodonium SbF " 6


SbF " ^°12 25

SbF -

6

Ο

CHO cyclohexene oxide

SOC10 (4-ndecyloxyphenyl)diphen yl-sulfonium SbF " Monomers

DPS-C,C (S-methy-S-docecylS-phenacylsulfonium SbF "

VCHDO 4-vinyleyclohexene dioxide

VCHO 4-vinyl-l,2epoxycyclohexane

6

12

6

Results and Discussion Development of Long-Wavelength Photopolymerization

Photosensitizers

for

Cationic

Mechanistic investigations of the UV induced photolysis of diaryliodonium and triarylsulfonium salts were carried out in this laboratory as well as in several others. ' The mechanism involves first the photoexcitation of the onium salt and thai the decay of the resulting excited singlet state with both heterolytic and homolitic cleavages of the C-I or C-S bonds. Thus,freeradical, cation and cation-radicalfragmentsare simultaneously formed. This is depicted for diaryliodonium salts in equation 1 of Scheme 1. The cationic species interact with a proton source, usually the monomer (M), to generate the strong Bronsted acid, HMtX„. Initiation of polymerization takes place by protonation of the monomer (eq. 2) and this is followed by addition of additional monomer molecules resulting in chain growth (eq. 3). The aryl radicals interact with monomers via the pathways shown in equations 4 and 5. These species can either add to an unsaturated monomer (eq. 4) or alternatively, abstract a hydrogen atomfromthe monomer (eq. 5). In these mettons, secondary radical species are generated. Redox interaction of these radical species with the diaryliodonium salts gives rise to carbocations and the unstable diaryliodine radical (eq. 6). In a subsequent reaction (eq 7), the diaryliodine radical decays irreversibly to generate an aryl iodide and an aryl free radical. The reactions shown in equations 1-7 constitute a free radical chain reaction in which the 17

18 19


245 diaryliodonium salt photoinitiator is consumed by a nonphotochemical process. At the same time carbocations are generated that ultimately initiate cationic polymerization. hv AijfMtX^

f

•

,1 I Ar f M t X ^ 2

lArfÎMtxv + Ατ· • J

»T H M t ^

Arl + Ar MtX„


+

Scheme 1 From Scheme 1 it may be concluded that the photochemically induced decomposition of the diaryliodonium salt photoinitiator can be greatly amplified by the redox cycle of equations 1-7. This results in the very rapid and efficient generation of a large number of initiating cationic species by a nonphotochemical process, which further produces an apparent increase in the rate of consumption of the monomer. ' * This mechanism has considerable precedence in the literature. " In principle, any photochemical source of free radicals can serve to start the above chain of reactions. It follows directly; therefore, that spectral broadening of an onium salt could be achieved if a chemical species is present in the photopolymerizable mixture that is able to generate radicals by absorption at long-wavelength light. It is also necessary that the radicals that are produced be capable of oxidation by the diaryliodonium salt photoinitiator. 20 2

22

28

Aryl Ketone-Sensitized Cationic Photopolymerization We decided to investigate the possibility of aryl and other types of ketones


246 Table IL U V Absorption Spectra of Photosensitizers Reference Photosensitizer λ,η,Οοβιοε)

%r

478 nm (4 )

30

260 nm (4.3) 480-486.5 nm (4)

31

256 nm (4.7) 325 nm (3.7)

32


CQ camphorquinone

coo oto " BZ benzil

CHi CHi

0

EAQ 2-ethylanthraquinone

as photosensitizers for onium salts photoinitiators. Table II shows the absorption characteristics of the ketones employed as photosensitizers in this work. Our initial work has focused on the use of camphorquinone (CQ) as a 1,2diketone photosensitizer. The choice of this compound was made on the basis of its high molar absorption coefficient and long-wavelength absorption as well as its excellent solubility and low toxicity. Furthermore, C Q is widely used, together with various amines, as a visible-light free radical photoinitiator in dental applications. Benzil (BZ), a 1,2-aromatic diketone, and 2ethylanthraquinone (EAQ) were also employed as photosensitizers and compared with CQ. Initial photopolymerization studies were conducted in the presence of IOC 10 as the photoinitiator. Homogeneous liquid samples of the mixtures of the monomer containing the photoinitiator and a photosensitizer were sandwiched between two layers of 12-μτη oriented and corona-treated polypropylene film and then irradiated with light to initiate polymerization. A polymer film filter with a cutoff of 290 nm was interposed between the sample and the light source. The decrease in the absorbance of the epoxy IR band (750 cm" ) was monitored using FT-RTIR during the polymerization. Compared, in Figure 1, are the photopolymerization curves few VCHDO in the presence and absence of CQ. 29

1


247

0

s

g

! Downloaded by UNIV OF ARIZONA on December 5, 2012 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch021

U

Figure L RTIR study of the photopolymerization of VCHDO with 4M mol% IOC10 alone (^)and in the presence of 4.0 mol% CQ(m). (Reproduced with permission from J. Polym. Sci. Polym. Chem. 2001, 39, 343-356. Copyright 2001 Wiley.)

Over the course of a 200 second irradiation period, no polymerization was observed in the absence of CQ. It is evident that the presence of the diketone is essential for the photosensitization of the iodonium salt. In contrast, polymerization proceeds rapidly and readily in the presence of the diketone. The proposed mechanism of photosensitization based on hydrogen abstraction by photoexcited ketones is shown in Scheme 2. hv

ArjC^O

|AT C==oJ

1

ISC

Ar?C=

2

(8)

[Ar C=o]

+ M-H

2

Ar C—OH

Μ·

2

(9) Μ·

+

Ατ Ι· 2

M-H

M+MtXn"

ArJ+MtX"

+

— Ar ·

Ατ·

ΗΜ·

+

Ατ Ι· 2

Αιί +

(10) (11)

Ar-H

Scheme 2


(12)

248 Aryl ketones are well known to undergo η-π* excitation on irradiation to initially generate the excited singlet state that rapidly undergoes efficient intersystem crossing to the excited triplet (eq. β ) . Due to the diradical character of the excited triplet carbonyl, there is a strong tendency of these species to abstract hydrogen atoms from an appropriate donor. In equation 9, the indicated donor is the monomer in which a weak carton-hydrogen bond is broken. The usual fete of the resulting benzophenone ketyl radical is to dimerize. However, the onium salt can also oxidize this radical. The carbon-centered monomer


34

radical, M% can interact, as shown in Scheme 1 of the previous section, with the onium salt to induce its reduction (eq. 10). The generation of an aryl radical by the decomposition of the diaryliodidefreeradical (eq. 11) closes the cycle by providing a species that again can abstract a hydrogen atom from the monomer (eq. 12). To obtain further evidence of thefree-radicalinduced decomposition of die onium salt during long-wavelength photopolymerization, the addition of nitrobenzene as radical retarder was investigated. In the presence of the radical retarder a marked suppression of the rate of the photopolymerization of VCHDO was observed, indicating that the cycle of the radical induced chain mechanism is disrupted in die presence of nitrobenzene. A similar observation was made when a FT-RTIR study was conducted with samples containing CQ with and without a polypropylene cover film. With limited oxygen present (covered sample), the polymerization proceeded readily, but when the sample was run uncovered, polymerization was noticeably retarded. The effect of the concentration of the photosensitizer and photoinitiator was also investigated. There is a little change in either the rate of photopolymerization or its extent, with an increase in the concentration of CQ by a factor of 2 or 4. Similarly, an increase in the concentration of IOC 10 from 4.0 to 12.0 mol% had little effect on the rate of conversion of VCHDO sensitized with 4.0 mol% CQ. Different aryl ketones were employed as photosensitizers and compared with CQ. At the same molar concentration of IOC10 as photoinitiator, CQ was the most efficient photosensitizer, whereas benzophenone (BZ) was only slightly less reactive. 2-Ethylanthraquinone (EAQ) was substantially less active as a photosensitizer than either BZ or CQ.

Effects of Photoinitiator Structure Three different types of onium salt photoinitiators, IOC 10, SOCIO and D P S - C A 2 , were investigated in the C Q photosensitized ring-opening polymerization of VCHDO and the results are shown in Figure 2, A comparison of the principal characteristics of these onium salts is reported in Table III.


249 Table ΠΙ: Absorption characteristics and reduction potentials of onium salt photoinitiators Ref. Xmax Photoinitiator ε Eta 34 -0.2 V 15,000 247 nm IOC10 34 17,600 - 1.0 to-1.46 V 262 nm SOCIO 35 -0.7 V ~ DPS-C,C, 2

While the diaryliodonium salt is effective in the polymerization of this monomer, the triarylsulfonium salt is not. Although the structure of SOCIO closely resembles that of IOC 10, the reduction potential of the sulfonium salt is considerably higher than that of IOC10. As a result, SOCIO is not easily reduced byfreeradical species and consequently, does not undergo free-radical induced decomposition reactions. The reduction potential of DPS-CiC lies between IOC10 and SOCIO. Although DPS-CjC is not as efficient as photoinitiator as the iodonium salt, it is much more active than the SOCIO in the CQ photosensitized polymerization of VCHDO.


36

12

37

12

loo

,

Ί

ο

m man 0

50

i

>,...

100

150

à 200

Irradiation Time (sec) Figure 2. Comparison of VCHDO photopolymerization with 4.0 mol% CQ and2.0 mol%photoinitiator: IOC10 ( • ), SOCIO ( · ) andDPS-dCn ( ^ ) (Reproduced with permission from J. Polym. Sci. Polym. Chem. 2001, 39, 343-356. Copyright 2001 Wiley.) Equations 13 and 14 show the proposed pathway for the reduction of DPSC i C i by free radicals and the irreversiblefragmentationof the resulting free radical species. 2

(13)


250 CH

3

C 12^25

C

12 25 H

(14)


Influence of the Structure of the Monomer Since the above photopolymerizations were carried out in the absence of a solvent, the monomer provides the abstractable hydrogen. For this reason according to the proposed mechanism, the structure of the monomer should play an important role in the efficiency of the photosensitization and subsequent photopolymerization process. To test this conclusion, we compared the CQphotosensitized photopolymerization of three different epoxide monomers; CHO, VCHDO and VCHO. The results are shown in Figure 3. From this study one can observe that VCHO is by far the more reactive monomer and that VCHDO is considerably more reactive than CHO. 80 70

om

1

1

.

1

0

50

100

150

200

Irradiation Time (sec) Figure 3. Comparison of the photosensitized polymerization of VCHDO ( M) CHO (Φ) and VCHO (*) with 2.0 molHIOCWand4.0 mol%CQ. (Reproduced with permission from J. Polym. Sci. Polym. Chem. 2001, 39, 343-356 Copyright 2001 Wiley.) t

The results can be explained by taking into account that VCHDO is difunctional. In addition, VCHDO has a readily abstractable tertiary proton located at the juncture of the cyclohexane ring and the side chain, while CHO has only less reactive secondary protons. The monomer VCHO bears a labile hydrogen at the juncture of the ring and a vinyl group that is both tertiary and


251 allylic. On the basis of this analysis, VCHO would be expected to participate most efficiently in mechanism of Scheme 2 resulting in a higher polymerization rate and conversion. This is what was observed.


Conclusions When ketone photosensitizers such as camphorquinone, benzil and 2ethylanthraquinone are irradiated with long-wavelength U V light, in the presence of a monomer that can serve as a hydrogen donor, the resulting monomer-bound radicals rapidly reduce diaryliodonium or dialkylphenacylsulfonium salts. The monomer-centered cations that are formed initiate the polymerization of epoxides. Camphorquinone was the most efficient photosensitizer and onium salts with higher reduction potentials, such as triarylsulfonium salts, do not undergo photosensitization by this new pathway. We have further demostrated that the structure of the monomer as a hydrogen donor plays a key role in the photosensitization process.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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252 13. 14. 15 16. 17. 18.


19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31. 32. 33. 34. 35. 36. 37.

Kampmeier, J.A.; Nalli, T.W. Macromolecules 1994, 59, 1381. Crivello, J.V.; Lee, J.L. J. Polym. Sci. 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, 825. Crivello, J.V. Makromol. Chem. Macromol. Symp. 1998, 13/14, 15. Devoe, R.J.; Sahyum, M.R.V.; Serpone, N.; Sharma, D.K. Can. J. Chem. 1987, 65, 2342. Dektar, J.L.; Hacker, N.P. J. Org. Chem. 1991, 56, 1838. Rajaraman, S.K.; Mowers, W.A.; Crivello, J.V. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 4007. Crivello, J.V.; Liu, S.S. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 1199. Ledwith, A. Polymer, 1978, 19, 1217. Abdoul-Rasoul, F.A.; Ledwith, Α.; Yagci, Y. Polymer 1978, 19, 1219. Bi, Y.; Neckers, D.C. Macromolecules 1994, 27, 3633. Crivello, J.V.; Bratslavasky, S.A. J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 2755. Muneer, R.; Nalli, T.W. Macromolecules 1998, 31, 7976. Jönsson, S.; Sundell, P.-E.; Skolling, O.; Williamson, S.E.; Hoyle, C.E. Aspects ofPhotoinitiation, Radcure Coat. Inks, 1993, 79, 81. Pappas, S.P. Prog. Org. Coat. 1985, 13, 35. Crivello, J.V.; Dietliker, K. Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Wiley, New York, 1998, Vol. III, p. 266. Cook, W.D. J. Dent. Res. Β 1982, 1, 1438. Hulme, B.E.; Marron, J. J. Paint Resin 1984, 1, 31. Sadler Standard Spectra Spectrum No. 4588, Sadler Research Labs,, Philadelphia, 1970, Vol. 19. Fouassier, J.P.; Rabek, J.F. Radiation Curing in Polymer Science and Technology, Elsevier, New York, 1993, Vol. II, p. 181. Gilbert, Α.; Baggot, J. Essentials ofMolecular Photochemistry, Blackwell Science, London, 1995, p. 177. Pappas, S.P.; Gatechair, L.R.; Jilek, J.H. J. Polym. Sci. Part A: Polym. Chem. 1984, 22, 77. Kunze, Α.; Müller, U.; Tittes, Κ.; Fouassier, J.-P.; Morlet-Savary, F.J. J. Photochem. Photobiol. A: Chem. 1997, 110, 115. Sundell, P.E.; Jönsson, S.; Hult, Α.; J. Polym. Sci., Part A: Polym. Chem. Ed., 1991, 29, 1535.


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