Photoinitiated Polymerization - ACS Publications - American Chemical

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Chapter 3

A Mechanistic Description of the Sensitized N -Substituted Maleimide Initiated Photopolymerization of an Acrylate Monomer Chau K. Nguyen , T . Brian Cavitt , Charles E . Hoyle , Viswanathan Kalyanaraman , and Sonny Jönsson 1

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School of Polymers and High Performance Materials, Department of Polymer Science, University of Southern Mississippi, 2609 West 4 Street, Hattiesburg, MS 39406 Becker-Acroma, Hattiesburg, MS 39406 Fusion UV-Curing Systems, Inc., 910 Clopper Road, Gaithersburg, MD 20878-1357 1

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N-substituted maleimides have been investigated for their use as photoinitiators for free-radical polymerization. The mechanism of initiation in the presence of sensitizers and coinitiators is postulated to involve chemical and energy transfer sensitization processes. Photo-DSC and laser-flash photolysis results provide photophysical data supporting the proposed mechanisms.

© 2003 American Chemical Society

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

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28 Ultraviolet (UV) curing in the 1990s has experienced rapid development with the advent of new technologies (/). UV-curable resins usually consist of three major components: (1) functionalized oligomers, (2) mono and multifunctional monomers and (3) a photoinitiator (free-radical or cationic) to absorb the output of an appropriate light source and produce species capable of initiating polymerization. Free-radical photoinitiators can be classified into two basic categories: Type I, those that undergo unimolecular fragmentation (Norrish Type I, α-cleavage), and Type II, those that undergo bimolecular abstraction from a source of labile hydrogens (2). The first produces radicals by photocleavage. Examples of Type I photoinitiators include aromatic carbonyl compounds such as derivatives of acetophenone that undergo Norrish Type I fragmentation to form initiating radicals, e. g., 2,2-dimethoxy-2phenylacetophenone (DMPA). Abstraction type photoinitiators are typically diaromatic ketones such as thioxanthones and benzophenone derivatives. Upon irradiation and absorption of a photon, benzophenone (or thioxanthone) ultimately forms the triplet state in high yield. In the presence of transferable hydrogen sources (such as amines, ethers, or alcohols) a reaction from the benzophenone triplet state produces two radicals (Figure 1) by a direct hydrogen transfer process (alcohols, ethers) or an electron/proton transfer process (tertiary amines).

Figure L Reaction pathways for benzophenone photolysis.



29 The two radicals depicted in Figure 1 are the semipinacol radical and a radical centered on the hydrogen donor source. The semipinacol radical does not efficiently initiate polymerization, but rather couples with other radicals in the system. This radical-radical termination process severely reduces the overall polymerization rate. The radical formed on the hydrogen donor source adds to monomer and initiates polymerization. The low rates characteristic of abstraction type photoinitiating systems compared to cleavage type photoinitiators is unfortunate in view of the low cost and ease of synthesis of diaromatic ketones. Aromatic maleimides have been used extensively in polymer chemistry as crosslinkers for engineering thermoplastics, and for some three decades, it has also been shown that maleimides can be used to initiate photopolymerizations (5-7). Recently, the practical viability of using maleimides as efficient photoinitiators has been addressed (8Ί1). In the presence of an amine coinitiator, the mechanism involves an electron-transfer/proton-transfer to give a radical centered on the amine and a succinimidyl radical, both of which are capable of initiating polymerization (Figure 2). Ground state maleimides can then participate in free-radical homopolymerization (in the case of a neat maleimide sample) or copolymerization (in the case of having a primary monomer such as an acrylate present) depending upon the monomer composition. The maleimide is thus consumed by the very free-radical polymerization process that it initiates.

θ

Figure 2. Electron/proton transfer mechanism for maleimide/amine. Unfortunately, due to a variety of reasons including the low yields of intersystem crossing to produce triplet state maleimides, the efficiency of initiation by



30 maleimides in the absence of a triplet sensitizer is low as reported by DeSchryver et al. (72). Recently, we reported that if an appropriate diaromatic ketone sensitizer is added (such as benzophenone or a substituted thioxanthone), the polymerization rate of multifunctional acrylates in the presence of an amine/maleimide system is greatly enhanced (75). Moreover, the rate of polymerization, when the three component initiating system is used, is much greater than that obtained with a sensitizer/amine mixture alone. Thus, the combination of a sensitizer, an amine and a maleimide presents a unique and efficient photoinitiator system that alleviates the problems associated with the low rates inherent to either the sensitizer/amine or the maleimide/amine initiator systems. A previously proposed mechanism for the three component photoinitiator involves triplettriplet energy transfer from the sensitizer (diaromatic ketone) to the maleimide, which then undergoes an electron-transfer/proton-transfer reaction with an amine (Figure 3). Herein, we will present evidence to suggest that a chemical sensitization process involving production and subsequent oxidation of a semipinacol type radical in the presence of a maleimide can also take place to generate two radicals capable of initiating radical polymerization.

ο Energy Transfer

Ο 1. Electron Transfer 2. Proton Trasnfer

Ο

Figure 5. Energy transfer mechanism.


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Experimental iV-methylmaleimide, benzanthrone, fluorenone, benzophenone and 4benzoylbiphenyl were purchased from Aldrich Chemical Company and further purified by sublimation. A^methyl-tyA^diethanolamine was also purchased from Aldrich and used without further purification. Isopropylthioxanthone was obtainedfromAlbemarle Corporation and used without further purification. 1,6Hexanedioldiacrylate was donated by UCB Chemicals Corporation. The sensitizers used in this experiment are presented in Table I, along with their respective triplet energies and acronyms (14).

Table I. Triplet Sensitizers

Sensitizer

Triplet Energy

Benzophenone (BP)

69

Triphenylene (TP) Isopropylthioxanthone (ITX)

66 a

64

Phenanthrene (PHT)

62

4-Benzoylbiphenyl (BBP)

61

Fluorenone (FLR)

50

Benzanthrone (BZT)

45

Anthracene (ANT)

42

Perylene(PRY)

36

NOTE: The units of the triplet energies are in kilocalories per mole 8

Determined in our lab via phosphorescence at 77 Κ (1:1 ethanol/methylene chloride glass)

Photo-Differential Scanning Calorimetry (Photo-DSC) was performed using a Perkin-Elmer DSC 7 modified to accommodate quartz windows. The sample cell was purged with nitrogen. The lamp source was a Canrad Hanovia mediumpressure mercury lamp from Ace Glass: specific spectral bands were isolated by the use of band-pass filters at 313 nm and 365 nm. The samples (2 μ ι ) were injected into specially crimped aluminum sample pans with corresponding film thicknesses of -180-200 μπι. Data were recorded and processed on a PC. Laser flash photolysis transients and lifetime quenching data were obtained using a system based on an excitation source of a Continuum Surelite Nd-YAG


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laser, and the UV-visible emission/absorption was monitored with a xenon probe and data acquisition/analysis system from Applied Photophysics. The main output wavelength of the laser at 1064 nm is converted with frequency quadrupling or tripling crystals to 266 nm or 355 nm. The monitoring system was used in the absorption mode, with a pulsed xenon lamp generating a monitoring beam. The intensity was then monitored by a photomultiplier tube connected to a Phillips digital oscilloscope.

Results and Discussion In order to define the effect that sensitizers have on the photopolymerization of a typical multifunctional acrylate, photo-DSC results are presented first. Laser flash photolysis results will then be presented and discussed in order to provide a mechanistic rationale for the polymerization observations.

Photo-DSC Exotherms Photoexotherms in Figure 4 show a marked rate enhancement for polymerization of 1,6-hexanedioldiacrylate (HDDA) when an amine and maleimide in low concentration is mixed with traditional diaromatic ketone sensitiers such as benzophenone (BP) or isoproplythioxanthone (ITX) (8-11). In this initial screening investigation, we have also included sensitizers with varying triplet energies and excited state configurations to determine any dependence of sensitization on energy level and sensitizer type. All of the sensitizers used have high quantum yields for intersystem crossing to the triplet upon direct excitation: some are diaromatic ketones and some are polycyclic aromatics. In each case, the concentrations of the sensitizers were normalized to have the same absorbance at the excitation wavelength of 313 nm as a sample with 1.0 wt% DMPA. Each sample included 0.1 wt% N-methylmaleimide (MMI) and 1.0 wt% #-methyl-#,W-diethanolamine (MDEA) with the corresponding concentration of sensitizer. Returning to an analysis of the results in Figure 4, it is obvious that the HDDA polymerization with the DMPA photoinitiator does not exhibit an exotherm that is much greater than that obtained for HDDA with the three diaromatic ketones ITX, 4-benzoylbiphenyl (BBP) or BP in combination with MMI/MDEA. Importantly, as shown in previous publications, the rates achieved with these three component sensitizer/maleimide/amine mixtures are far greater than can be


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100

Time (min)

Figure 4. Photo-DSC exotherms for nitrogen purged HDDA initiated by sensitized 0.1 wt% MMI in the presence of 1.0 wt% MDEA. Sensitizer absorbance adjusted to equal absorbance of 1.0 wt% DMPA. Light intensi was 0.52 mW/cm at 313 nm. (a) DMPA (b) ITX/MDEA/MMI (c) BBP/MDEA/MMI (d) BP/MDEA/MMl (e) TP/MDEA/MMI, PR Y/MDEA/MM1, PHT/MDA/MM1 and ANT/MDEA/MMI. 2

achieved with either of the two component sensitizer/amine or maleimide/amine systems alone at 313 nm, where both the maleimide and the sensitizer absorb (877). Probably one of the most interesting aspects of the photo-DSC exotherms in Figure 4 is the low (essentially non-existant) exotherms recorded for HDDA polymerization when the sensitizer was a polycyclic aromatic, i. e., triphenylene (TP), perylene (PRY), phenanthrene (PHT), or anthracene (ANT). While ANT and PRY with very low triplet energies might not be expected to sensitize the triplet state of MMI, it would be expected that TRY and PHT with triplet energies equivalent to or greater than BBP should also sensitize formation of the maleimide triplet state (in a later section the triplet energy of MMI will be shown



34 to be ~ 56 kcal/mol). However, both TRY and PHT have very long singlet lifetimes which will be quenched by MDEA at the concentration used, thereby preventing formation of their respective triplet states. BP, ITX and BBP all have very short singlet lifetimes which cannot be quenched by MDEA. Next, we confirm that results equivalent to that in Figure 4 can be obtained upon excitation at wavelengths longer than 313 nm. Accordingly, Figure 5 presents results for excitation at 365 nm with concentrations adjusted so that the absorbing chromophore in each system is equivalent to the absorption of 1 wt% DMPA. In the case of the sensitizer/amine/maleimide mixtures, excitation at 365 nm ensures that there is little competitive interference in the absorption of light by the maleimide.

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Time (min)

Figure 5. Photo-DSC exotherms for HDDA initiated by sensitized 0.1 wt%M in the presence of 1.0 wt% MDEA. For each sample sensitizer absorbanc adjusted to equal absorbance of 1.0 wt% DMPA. Light intensity was 0.7 mW/cm at 365 nm. All samples were thoroughly nitrogen purged, (a) DM (b) ITX/MDEA/MMI (c) BBP/MDEA/MMI (d) BP/MDEA/MMl. 2

The photo-DSC results in Figure 5 were obtained with the band-pass isolated 365-nm line of the medium pressure mercury lamp. [Since the molar extinction coefficient of MMI at 365 nm is around 60 L mol cm* , and since its concentration is relatively low (0.1 wt% in each sample), it does not 4

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35 competitively absorb light]. As with the results at 313 nm, the highest HDDA polymerization exotherm was recorded for the sample with the DMPA initiator. Interestingly, the HDDA maximum exotherm rate with the three component ITX/MDEA/MMI photoinitiator mixture approaches that for the HDDA sample with the DMPA photoinitiator. The HDDA polymerization rates with the BBP/MDEA/MMI and BP/MDEA/MMI systems are not quite as efficient as DMPA. Having established that equivalent results to those obtained using 313 nm light can be obtained at 365 nm, we present results in Figure 6 that highlights the use of sensitizers with very low triplet energies. If the energy transfer mechanism depicted in Figure 3 was the only method for sensitization in the maleimide based three component systems, then it would be expected that use of sensitizers with very low triplet energies (50 kcal/mol or less) would not lead to efficient initiation.

Figure 6. Photo-DSC exotherms for HDDA polymerization initiated by sensitized 0.1 wt% MMI in the presence of 1.0 wt% MDEA. Sensitizer absorbance adjusted to equal absorbance of 1.0 wt% DMPA. Light intens was 1.61 mW/cm at 365 nm. All samples were thoroughly nitrogen purge (a)BZT/MDEA/MMI (b) FLR/MDEA/MMI (c)BZT/MDEA (d) FLR/MDEA. 2

The results in Figure 6 using FLR (E = 50 kcal/mol) and BZT (E = 45 kcal/mol) clearly show that even compounds with low energy triplets are capable T


T

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of sensitizing the MMI/MDEA combination (though not by conventional triplet sensitization). Based upon these results, it appears as though the magnitude of the triplet energy is not the sole sufficient factor or the only factor in the sensitization process. What does appear to be essential is the structure of the sensitizer (i. e., it needs to be a diaromatic ketone). In the next section, data is presented that provides a basis for a mechanism to account for the polymerization results in Figures 4-6.

Laser Flash Photolysis To establish a mechanism for the sensitizer effect and confirm from basic photochemical/photophysical principals a rationale for the results in Figures 4-6, laser flash photolysis data were recorded. First, the maleimide triplet energy must be determined to confirm in which cases energy transfer sensitization may occur. Second, critical quenching rate constants involving the sensitizer and each of the components in the system must be measured to suggest a viable mechanism for the role of the sensitizer and each component in the mixture. The absorption spectrum of the excited state triplet of MMI has a peak maximum near 340 nm. The rate constant for quenching of the excited triplet state of a molecule (in this case either MMI or the sensitizer) may be calculated according to the Stern-Volmer Equation 1,

x /x=l + k x[Q] Q

q

Eq. 1

where τ is the triplet lifetime in the absence of quencher τ is the triplet lifetime at a given quencher concentration kq is the rate constant for triplet quenching [Q] is the concentration of the quencher 0

Table II summarizes rate constants for quenching the MMI triplet by MDEA, as well as each sensitizer triplet by MMI and MDEA. From the rate constants in Table II for quenching of each sensitizer by MMI (and additional constants not shown) the triplet energy of MMI was estimated to be ~ 56 kcal/mol. We note that BP, ITX, and BBP are all readily quenched by MMI at close to diffusion controlled rates, while the triplet states of FLR and BZT are not quenched by MMI to any appreciable extent. Hence, FLR and BZT cannot efficiently sensitize via an energy transfer process formation of the triplet state of MMI, thus ruling out the mechanism in Figure 3 for the generation of radical


37 species responsible for initiating the polymerization of HDDA in Figure 6. The triplet states of BP, BBP, ITX, FLR and BZT, however, are all readily quenched by MDEA at high rates indicative of an efficient electron/proton transfer process.


Table II. Laser flash quenching constants in acetonitrile solution

Compound/Quencher MMI/MDEA BP/MDEA BP/MMI BBP/MDEA BBP/MMI ITX/MDEA ITX/MMI FLR/MDEA FLR/MMI BZT/MDEA BZT/MM BP ketyl radical/MMI

(Lmo/"' sec') 9

4.3 χ 10 1.1 χ 10 5.7 χ 10 1.6 χ 10 1.4 χ 10 1.8 χ 10 6.8 χ 10 2.5 χ 10* No quenching 8.8 χ 10 No quenching 3.7 χ 10 9

9

8

9

9

9

7

7

Based upon the triplet quenching results in Table II, we suggest that the scheme in Figure 7 may be operative and in certain cases help account for the exotherm results in Figures 4-6. For example, when using a diaromatic ketone sensitizer such as BZT (50 kcal/mol) or FLR (45 kcal/mol) with a triplet energy below that of MMI (56 kcal/mol) there is no observable quenching of the triplet state, and hence the mechanism in Figure 3 does not contribute to initiation of HDDA polymerization in Figure 6. As for BP, ITX and BBP, their triplet energies are all greater than that of MMI. Even though the rate constants for quenching of these sensitizers by MMI are calculated to be anywhere from -2.5 to 10 times that of quenching by MDEA, the MDEA is in almost ten fold excess in the samples polymerized in Figures 4 and 5. Hence, MDEA would account for a substantial portion of the quenching of the sensitizer. Thus, it might be expected that the initiation process of an amine/maleimide system where the triplet energy of the sensitizer is greater than that of the maleimide would proceed to initiate polymerization according to the mechanisms depicted in both Figures 3 and 7, the contribution of each depending upon the relative energy transfer and electron transfer rates of each process.



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Figure 7. Semipinacol reduction mechanism. Evidence for the mechanism in Figure 7 is obtained from the quenching rate constant for the semipinacol radical (derived from BP triplet and MDEA) by MMI of 3.7 χ 10 L mol" sec". This rate constant, indicative of electron transfer (although a little slower than for the electron transfer process involved in quenching of BP, BBP, or ITX by MDEA) is consistent with the scheme in Figure 7. In principal, the semipinacol radical could react with another radical. However, since the radical concentration would be several orders of magnitude lower than the MMI concentration, the chemical sensitization process involving the electron/proton transfer sequence of reactions in Figure 7 would be favored. Interestingly, in both the energy sensitization process depicted in Figure 3 and the chemical sensitization process depicted in Figure 7, two initiating radicals 7

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1


3?


are produced and the semipinacol radical is rendered inoperative (in Figure 3 it never forms and in Figure 7 it is converted back to the diaromatic ketone). No doubt the exact contribution of each pathway is determined by the lifetimes of the triplet sensitizers and relevant competing quenching rate constants for each step involved.

Conclusions A chemical sensitization process was found to be operative for diaromatic ketone/maleimide/amine photoinitiator systems. In the presence of a sensitizer with a triplet energy above that of MMI and where higher concentrations of MMI would be employed, both an energy and a chemical sensitization process are probably operative: we project that chemical sensitization is more prevalent at lower MMI concentrations (i.e. 0.1 wt% as used in this paper). In systems where the sensitizer has a triplet energy less than that of the maleimide, the chemical sensitization process probably takes place exclusively.

Acknowledgements The authors would like to acknowledge Albemarle Corporation and Fusion UVCuring Systems for support of this research.

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40 10. Miller, C.W.; Hoyle, C.E.; Jönsson, E.S.; Hasselgren, C.; Haraldsson, T.; Shao, L. RadTech North America Proceedings 1998, 182-188. 11. Nguyen, C.K.; Johnson, A.T.; Kalyanaraman, V.; Cole, M.C.; Cavitt, T.B.; Hoyle, C.E.; Jönsson, E.S.; Pappas, S.P.; Miller, C.W.; Shao, L. RadTech North America Proceedings 2000, 197-210.


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Photoinitiated Polymerization - ACS Publications - American Chemical

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