[acs symposium series] photoinitiated polymerization volume 847 || photoinitiation of acrylates via...

Chapter 4

Photoinitiation of Acrylates via Sensitized Phthalimide Derivatives

T. Brian Cavitt1, Brian Phillips 1, Charles E. Hoyle 1, Chau K. Nguyen 1, Viswanathan Kalyanaraman 2,

and Sonny Jönsson3

1School of Polymers and High Performance Materials, Department of Polymer Science, University of Southern Mississippi, 2609 West 4th Street,

Hattiesburg, MS 39406 2Becker-Acroma, Hattiesburg, MS 39406

3Fusion UV-Curing Systems, Inc., 910 Clopper Road, Gaithersburg, MDd 20878-1357

When isopropylthioxanthone is used as a coinitiator for substituted N-phenylphthalimide photoinitiators, rapid rates of acrylate polymerization are attained if a tertiary amine is present as a hydrogen source. N-Phenylphthalimide with electron withdrawing substituents on the N-phenyl ring in the presence of a combination of isopropylthioxanthone and N-methyl-N,N-diethanolamine results in an increase in the maximum rate of polymerization of 1,6-hexanedioldiacrylate by a factor of as much as twice that attained when only the coinitiator and N-methyl-N,N-diethanolamine are present.

© 2003 American Chemical Society 41

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

42

Introduction

As the EPA stipulates increasingly stringent regulations for coatings usage, industry has been forced to develop more "environmentally friendly" coatings (1). This has promoted steady growth in the UV-curable market over the last twenty years yielding novel technologies compliant to government regulations and consumer demand (2). Each of the major components of a UV-curable coating, monomers, reactive diluents, additives, and photoinitiators, have thus been investigated in detail to further the growth in this market.

Being a vital component in photocurable formulations, photoinitiating systems are the subject of particularly extensive research. Most of this research has focused on Type I photoinitiators, which undergo an alpha cleavage process to form two radical species. An example of a Type I photoinitiator is 2,2-dimethoxy-2-phenylacetophenone (DMPA), which undergoes cleavage alpha to the carbonyl to form a benzoyl radical and a tertiary carbon-centered radical. The benzoyl radical can subsequently initiate polymerization or abstract a hydrogen to form benzaldehyde. The second radical species rearranges to form methylbenzoate and a highly reactive methyl radical as an initiating species.

Type II photoinitiators are a second class of photoinitiators and are based on compounds whose triplet excited states are reduced by a labile hydrogen transfer process thereby producing an initiating radical. Type II photoinitiators are exemplified by the traditional benzophenone/N-methyl-N,N-diethanolamine system where a hydrogen is transferred to the triplet excited state of benzophenone from the amine via an electron/proton transfer mechanism to yield a carbon-centered radical adjacent to the nitrogen capable of initiation and the semipinacol radical serving only to terminate propagating species.

Recent work on new Type II photoinitiators has produced a system involving a maleimide or maleic anhydride, a tertiary amine such as N-methyl-Ν,Ν-diethanolamine, and a sensitizer (2, 3). Depending on the sensitizer and maleimide chosen for the initiating system, the resulting rate of polymerization can rival that of DMPA. The initiation process is probably based on a dual initiating mechanism involving an electron/proton transfer from the ketyl radical to the maleimide (e.g. chemical sensitization) and triplet energy transfer from the sensitizer to the maleimide. Phthalimide derivatives have electron affinities and reduction potentials that are comparable to those of maleimides, suggesting their use in three component photoinitiator systems (Table I). In this paper, phthalimide derivatives are evaluated for their viability in three component photoinitiators of acrylate polymerization.

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Table L Electron affinities and reduction potentials of several electron acceptors

Electron Reduction AffinityiA) Potential^)

Maleimide 1.16 — N-Methylmaleimide 1.12 — N-Phenylmaleimide 1.36 . . .

Phthalimide 1.01 -1.45 N-Phenylphthalimide 1.16 -1.31

NOTE: The units for electron affinity and reduction potential are electron volts and volts, respectively.

Experimental

Materials

N-methyl-N,N-diethanolamine (MDEA), N-phenylphthalimide (PPI), 4-aminophthalonitrile, and 4-aminoveratrole were obtained from the Aldrich Chemical Company. UCB provided 1,6-hexanedioldiacrylate (HDDA). The Albemarle Corporation supplied isopropylthioxanthone. The Stepan Company furnished the phthalic anhydride used in the synthesis of N-(3,4-dimethoxyphenyl) phthalimide, denoted DMPPI, and N-(3,4-dicyanophenyl) phthalimide, denoted DCPPI. Synthesis of DMPPI and DCPPI proceeded as given in the literature (d).

Photo-Differential Scanning Calorimetry

Exotherms were measured on a Perkin-Elmer Differential Scanning Calorimeter 7 modified to accommodate a Canrad-Hanovia medium pressure mercury lamp source from Ace Glass. Light intensities were measured at the sample pans via a black body absorber. A mechanical shutter was placed between the light source and the sample chamber to provide control of exposure time. Sample pans were specially crimped and injected with 2 \3JL samples, giving a film thickness on the order of 200 μχη (about 8 mils).

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Phosphorescence

Phosphorescence studies performed on a Spex Fluorolog 2 fluorimeter were used to estimate the triplet energies of the phthalimides for comparison to the sensitizers. An ethanol/methylene chloride glass (77 K) was used to observe phthalimide phosphorescence at 77 K. The triplet energies were taken as the energy corresponding to the phosphorescence intensity being 10% of the maximum peak intensity where no defined peak was discernable.

Laser Flash Photolysis

Laser flash photolysis lifetime quenching data were obtained by using a system based upon a Continuum Nd-YAG laser as the excitation source and a pulsed xenon probe and data acquisition/analysis system from Applied Photophysics. The ultimate time resolution of the laser flash instrument was approximately 15-20 ns. Each sample was purged with nitrogen for fifteen minutes in the sealed quartz cell. The excitation wavelength was 355 nm where only the sensitizer appreciably absorbed. The analysis wavelength was 630 nm for isopropylthioxanthone.

Results and Discussion

Photo-Differential Scanning Calorimetry (Photo-DSC)

In the past, we have used several diaryl ketone sensitizers with maleimides in the presence of tertiary amines to initiate acrylate polymerization. Three of these diaryl ketones, benzophenone, 4-benzoylbiphenyl and isopropylthioxanthone, were studied in great detail (2,3). All of these sensitizers were also investigated in conjunction with various phthalimides in the presence of an amine. In all of these investigations, systems that incorporated isopropylthioxanthone (ΓΓΧ) gave higher polymerization rates than benzophenone or 4-benzoylbiphenyl. Selected results are highlighted herein for several N-phenylphthalimides. The trends found for ITX are also found when benzophenone and 4-benzoylbiphenyl are used, although the rates are slower with the latter two sensitizers.

In Figure 1, results for the photopolymerization of 1,6-hexanedioldiaerylate (HDDA) in the presence of several N-phenylphthalimide derivatives and an ITX/MDEA mixture were compared to the standard ΓΓΧ/MDEA initiating

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system. The results are as expected and show that the phthalimides with electron withdrawing substituents [i.e. N-(3,4-dicyanophenyl) phthalimide -DCPPI and N-(3-trifluoromethylphenyl phthalimide - CF3PPI] yield the fastest rates of initiation as exemplified by the peak maxima. The N-phenylphthalimide substituted with an electron donating group [i.e. N-(3-methoxy phenyl) phthalimide - MPPI] as well as unsubstituted N-phenylphthalimide (PPI) resulted in only moderate rate increases compared to those achieved with ITX/MDEA alone. The results in Figure 1 certainly demonstrate that there is a correlation between the electron affinity of the phthalimide and its ability to initiate polymerization in the presence of an ITX/MDEA combination.

Time (min)

Figure 1. Photo-DSC exotherms of Phthalimide/ITX/MDEA in nitrogen purged HDDA. I0=64.3I mW cm . Full arc of medium pressure mercury lamp. A is 0.028wt% ΓΓΧ, lwt% MDEA; Β is 0.1wt% PPI 0.028wt% ΠΧ, Iwt% MDEA; C is 0.1wt% DCPPI, 0.028wt% ITX, lwt% MDEA; D is 0.1 wt% CF3PPI, 0.028% by weight ITX, lwt% MDEA; Ε is 0.1wt% MPPI, 0.028wt% ITX, lwt%MDEA.

Next we present results that describe the effect that the phthalimide concentration has on the rate of polmerization. This investigation is prompted

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by the effect that maleimide concentration has on the rate of acrylate photopolymerization where due to self quenching/inhibition reactions a maximum rate is achieved at only 0.1 wt% maleimide. Although we have measured results for several N-substituted phthalimides with increasing concentration, only results for PPI are presented herein for demonstration purposes. The effect of PPI concentration on the exotherm for HDDA polymerization with ITX as the sensitizer and MDEA as coinitiator is shown in Figure 2. At these higher concentrations of PPI, the rates are even greater than recorded when 0.1 wt% PPI was used (Figure 1). The rate increases even up to a concentration of 1 wt% phthalimide. This dichotomy between phthalimides and maleimides (increased maleimide concentration greater than 0.1 wt% results in a decrease in the polymerization rate) holds true for each of the other phthalimides in this present investigation (although as already mentioned, only results for PPI are given here). Apparently, the high self-quenching and polymerization inhibition characteristic of maleimides are not operative for sensitized phthalimides.

170T

0.0 0.2 0.4 0.6 0.8 1.0

Time (min)

Figure 2. Photo-DSC exotherms of ΡΡΙ/ΓΓΧ/MDEA in nitrogen purged HDDA. l0=69.52 mW cm . Full arc of a medium pressure mercury lamp. A is 0.028wt% ITX, lwt9c MDEA; Β is 0.5wt% PPI, 0.028wt% ΓΓΧ, lwt% MDEA; Cislwt%PPI, 0.028wt%TTX, lwtVbMDEA.

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Recalling the exotherm results in Figure 1, there is an indication that an electron donating group substituted on the phenyl ring does not result in an enhancement in the polymerization rate. It might be expected that two electron donating groups would result in a reduction in the ability of a phthalimide/ITX/amine system to intiate acrylate polymerization, since as will be shown in the next few sections the initiation is thought to involve an electron transfer process to the N-arylphthalimide molecule. In Figure 3, the results for a phthalimide/ITX/MDEA system where the phthalimide is N-(3,4-dimethoxyphenyl) phthalimide (DMPPI) clearly show that introduction of a second methoxy donating group on the phthalimide ring results in a system that exhibits little appreciable increase in polymerization rate over that achieved with the basic ITX7MDEA initiating system alone. From these results it is clear that the methoxy substituents decrease the ability of DMPPI to participate in the electron/proton transfer process that forms radicals capable of initiating polymerization.

In order to interpret the results in Figures 1-3, we next present photophysical results for the phosphorescence and transient specta of three of the phthalimides in this investigation with significantly different electron affinities. The electron affinities decrease in the order: DCPPI > PPI > DMPPI.

120t

Time (min)

Figure 3. Photo-DSC exotherms of DMPPI/ITX/MDEA in nitrogen purged HDDA. I0=61.21 mW cm'2. Full arc of a medium pressure mercury lamp. A is 0.028% by weight ITX, 1% by weight MDEA, HDDA; Β is 0.1% by weight DMPPI, 0.028% by weight ITX, 1% by weight MDEA, HDDA.

American Chemical Society Library 1155 IBth St., N.W. Washington, DC 20036

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Phosphorescence of Substituted N-Phenylphthalimide Derivatives.

The triplet energies of DMPPI, PPI, DCPPI were determined using low temperature phosphorescence as 61.1 kcal/mol, 67.6 kcal/mol, and 68.6 kcal/mol, respectively. From these data, it is obvious that efficient photosensitization of DCPPI and PPI by classical energy transfer from ITX (Et=64 kcal/mol as determined in our lab via phosphorescence at 77 Κ in a 1:1 ethanol/methylene chloride glass) is not feasible, suggesting that another process is responsible for enhancing the polymerization rate of phthalimides in the presence of ITX and an amine. Before proposing an exact mechanism, laser flash photolysis results are presented for DMPPI, PPI, and DCPPI.

Laser Flash Photolysis

Laser flash photolysis was used to determine the extent of the interaction of the phthalimide derivatives with the excited triplet state of ΓΓΧ. The lifetime decay of the ITX triplet state monitored at 630 nm was determined at increasing concentrations of the phthalimide derivative in order to calculate the quenching constant, kq, of ΓΓΧ (Ei=64 kcal/mol) by the phthalimide (Table II).

Table Π. Quenching constants of ITX by phthalimide derivatives

Sensitizer Quencher kq (Μ·1 s')

ITX DCPPI <107*

ITX PPI 7.48 χ 107

ITX DMPPI 3.66 x l O 9

limited by solubility in solvent.

Comparing quenching constants previously acquired for the quenching of ITX by MDEA (1.8 χ 109 M"1 s"1) to those obtained for the quenching of ΓΓΧ by the phthalimide derivatives, the kinetically preferred reaction is the electron/proton transfer reaction between ITX and MDEA for the two phthalimide derivatives (PPI and DCPPI) with triplet energies greater than that of ITX (7). Only in the case of DMPPI with a triplet energy markedly less than that of ITX is energy transfer efficient.

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Figure 4. Proposed photoinitiation mechanism involving phthalimide derivatives and ITX.

Proposed Initiation Mechanism

Based on the information acquired via photo-DSC, phosphorescence, and laser flash photolysis, an initiation mechanism can be proposed (Figure 4).

In the mechanism proposed in Figure 4, the sensitizer (ITX) is promoted by absorption of a photon of light to an excited singlet state that subsequently undergoes intersystem crossing to the excited triplet state of ITX. The ITX triplet state then preferentially reacts with the amine via an electron/proton transfer to form a carbon-centered radical on the amine and the corresponding

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ketyl radical. The ketyl radical will be formed in high yield in cases where the phthalimides have higher triplet energies than ITX (-64 kcal/mol) such as in the cases of PPI and DCPPI which have triplet energies greater than 64 kcal/mol (see Table II). The radical centered on the amine can initiate polymerization, whereas the ketyl radical can either terminate propagating polymer chains or interact with the phthalimide via an electron/proton transfer to give ground state ITX and the phthalimidyl radical. In the case of DCPPI, which has a relatively favorable electron affinity, the latter occurs readily. It has been reported that phthalimidyl radicals can add to double bonds, which in the present case would lead to acrylate polymerization. As shown in Figure 4, it is also possible that the phthalimidyl radical may form the benzoyl radical followed by reaction with an acrylate to initiate polymerization.

The mechanism in Figure 4 is certainly consistent with the high polymerization rate recorded in Figure 2 for DCPPI, which is electron deficient and would thus be susceptible to reduction by the ketyl radical formed from ITX and MDEA. The lower rate achieved when PPI is used no doubt results from a lower electron affinity than DCPPI. The low enhancement in the overall rate of polymerization for DMPPI/ITX/MDEA compared to ITX/MDEA may be due to a combination of factors. First, since as shown in Table Π, DMPPI can readily quench the triplet state of ΓΓΧ by energy transfer and thus prohibit the formation of the ITX ketyl radical, the efficiency of the resulting electron transfer process will be reduced. Furthermore, an electron/proton transfer process involving the excited triplet state of DMPPI and MDEA would not be favorable due to the reduced electron affinity of DMPPI resulting from the two dimethoxy substitutents. Likewise, any ketyl radical which did form from the ITX triplet and MDEA would not readily undergo an electron/proton transfer reaction to the electron rich DMPPI. All of this is consistent with the low polymerization rates noted for HDDA in the presence of the ITX/DMPPI/MDEA mixture as shown in Figure 3.

Conclusions

Electron withdrawing groups assist a facile electron/proton transfer between the ketyl radical of ITX and a phthalimide derivative yielding an enhancement in the polymerization rate, whereas electron donating groups diminish the phthalimide's ability to act as an efficient electron accepta". Comparison of the triplet energies of ITX and the phthalimide derivatives demonstrate that energy transfer between ΓΓΧ and DCPPI should be inefficient and thus not a factor in initiating the polymerization. The proposed mechanism (e.g. chemical sensitization) involving the electron/proton transfer

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between the ketyl radical of ITX and the phthalimide derivative is supported by laser flash photolysis data.

Acknowledgements

The authors would like to acknowledge Albemarle Corporation and Fusion UV-Curing Systems for support of this research.

References

1. Roffey, C. Photogeneration of Reactive Species for UV Curing; John Wiley and Sons: New York, 1997.

2. Hoyle, C.E.; Viswanathan, K.; Clark, S.C.; Miller,C.W.; Nguyen, C.K.; Jönsson, S.; Shao, L. Macromolecules 1999, 32, 2793.

3. Cavitt, T. B.; Hoyle, C. E.; Nguyen, C. K.; Viswanathan, K.; Jönsson, S. RadTech 2000 Proceedings 2000, 785.

4. Paul, G.; Kebarle, P. J. Am. Chem. Soc. 1989, 111, 464. 5. Leedy, D. W.; Muck, D. L. J. Amer. Chem. Soc. 1971, 93 (17), 4264. 6. Anzures, Ε. T. Photodegradation of Aromatic Polyimides; University of

Southern Mississippi: Hattiesburg, MS, 1991. 7. Nguyen, C. K.; Johnson, A. T.; Viswanathan, K.; Cole, M. C.; Cavitt, T.

B.; Hoyle, C. E.; Jönsson, S.; Miller, C. W.; Pappas, S. P.; and Shao, L. RadTech 2000 Proceedings 2000, 196.

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[acs symposium series] photoinitiated polymerization volume 847 || photoinitiation of acrylates via...

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