Polymeric Michler's ketone photoinitiator containing coinitiator amine.INTRODUCTION
Photopolymerization as a convenient synthesis method is used in many kinds of fields, such as coatings, printing inks, adhesives, and photoresists (1-3). One of the most important components of polymerization system is photoinitiator, which can absorb the appropriate light and then generate radical species to initiate polymerization. Nowadays, most of the commercial used photoinitiators are low-molecular weight molecules, which have bad compatibility with polymer and migrate to the surface of curing-system leading to yellowide and toxicity, thus confine their application. So many researchers focus on the polymeric photoinitiators (4-9), which are considered as macromolecules containing covalently bonded photoinitiating groups because of their obvious merits over the low-molecular weight photoinitiators. As for hydrogen abstraction chromophores (type II) system, introduction of photoinitiator and coinitiator into the same polymeric chain is an effective way to enhance the photoinitiation efficiency (10-13) because the activity of photoinitiators can be improved as the result of energy migration between excited- and ground-state photosensitive moieties along the polymeric chains or of intramolecular reactions (14-19).
Most of type II macrophotoinitiators are based on benzophenone (20-27), probably because of its relatively low cost. Michler's ketone is the derivative of benzophenone, which has good photoinitiation efficiency and is widely used (28-30). Introduction of the coinitiator amine and Michler's ketone (MK) into the same polymeric chain have some merits, such as increasing photoinitiation efficiency, good compatibility between photoinitiator and monomer, reducing the migration of MK and coinitiator, also avoiding the odor and toxicity of coinitiator amine.
In this context, new polymeric photoinitiator (PMKPR) based on MK was synthesized by introducing MK and coinitiator amine into the same polymeric chain. In comparison with low-molecular weight model compounds, PMKPR is more efficient for initiating polymerization of 2,2-bis[4-(acryloxypolyethoxy)phenyl] propane (A-BPE-10) and trimethylopropane triacrylate (TMPTA), which will be very potential in application of coatings, printing inks, and microelectronics.
4.4'-Difluorobenzophenone (from Apollo), Resorcinol diglycidyl ether (RDE) and 4,4'-bis(diethylamino)benzophenone (MKE) (from Aldrich), 1-methylpiperazine (MP) and N.N'-dimethylpiperazine (DMP) (from Acros), N',N-dimethylformamide (DMF), hexahydropyridine and piperazine (from Chinese Medicine Group), A-BPE-10 and TMPTA (from Shin-nakamura Chemical Co.) were used as received. Other chemicals are of analytical grade except as noted.
Synthesis of PMKPR and Model Compounds
4,4'-bis(piperazino)benzophenone [MK[(pipaz).sub.2], and 4,4'-bis(piperidino)benzophenone [MK[(pip).sub.2], and 4,4'-bis(methylpiperazino)benzophenone [MK[(mpz).sup.2]] were synthesized by a modified literature (31) procedure described for synthesis of MK series according to Scheme 1. PMKPR was synthesized by addition polymerization between epoxy and amine group. The general process of synthesis was shown as follows. RDE (0.444 g, 2 mmol) was added to DMF (10 ml). After dissolved completely, MK[(pipaz).sub.2] (0.684 g, 2 mmol) was added under nitrogen. Then, the mixture was heated to 80[degrees]C for 12 h. After cooling to room temperature, the mixture was poured into anhydrous ether (100 ml) and filtered, dried in vacuum at 40[degrees]C to get yellow PMKPR (1.09 g, 96.7%).
MK[(pipaz).sub.2]: EIMS (70 eV) m/e: 350. [.sup.1]H NMR ([CDC1.sub.3]): [delta] = 7.74 (4H, ArH-2,6.2',6'), 6.90 (4H, ArH-3,5,3',5'), 3.30 (8H, [NCH.sub.2]), 3.02 (8H, [CH.sub.2]NH), 1.68 (2H, NH). Anal. Calc. for [C.sub.21][H.sub.26][N.sub.4]O: C 71.97; H 7.48; N 15.99. Found: C 71.83; H 7.45; N 15.87.
MK[(pip).sub.2]: EIMS (70 eV) m/e: 348. [.sup.1]H NMR ([CDC1.sub.3]): [delta] = 7.74 (4H, ArH-2,6.2',6'), 6.90 (4H, ArH-3,5,3',5'), 3.30 (8H, [NCH.sub.2]), 3.04-1.64 (12H, [CH.sub.2] [CH.sub.2] [CH.sub.2]). Anal. Calc. for [C.sub.23][H.sub.28][N.sub.2]O: C 79.27; H 8.10; N 8.04. Found: C 79.13; H 7.96; N 7.93.
MK[(mpz.sub.2]): EIMS (70 eV) m/e: 378. [.sup.1]H NMR [(CDC1.sub.3]): [delta] = 7.75 (4H, ArH-2,6.2',6'), 6.90 (4H, ArH-3,5,3',5'), 3.30 (8H, [NCH.sub.2]), 3.04-1.64 (12H, [CH.sub.2][CH.sub.2][CH.sub.2]. Anal. Calc. for [[C.sub.23][H.sub.28][N.sub.2]O]: C 79.27; H 8.10; N 8.04. Found: C 79.13; H 7.96; N 7.93.
PMKPR: UV: [lambda] = 349 nm, [epsilon] = 3.0 x [10.sup.4] L [mol.sup.-1] [cm.sup.-1]; [M.sub.n.] = 3.6 x [10.sup.4]. [.sup.1]H NMR ([[-d.sub.6]]DMSO, 400 MHz): [delta] = 7.68-6.44 (I2H, aromatic), 4.84 (2H, -OH), 4.04-3.80 (6H, -[OCH.sub.2], -OCH), 3.36-2.52 (16H, -[NCH.sub.2][CH.sub.2]), 2.43-2.32 (4H, -[NCH.sub.2]) FTIR (KBr): 3422 [cm.sup.-1] (O-H), 2937, 2831 [cm.sup.-1] (C-H), 1600 [cm.sup.-1] (C=O).
In this research, MKE was commercially available and also used as a model compound. From the molecular structure, we know that the molar concentration ratio of carbonyl and nitrogen atom in MK[(mpz).sub.2] and PMKPR is 1:4, while in MKE and MK[(pip).sub.2], the molar concentration ratio of carbonyl and nitrogen atom is 1:2. To compare the photoinitiation efficiency in the same condition, we added DMP to MK[(pip).sub.2] and MKE as a coinitiator to ensure [C= O]:[N] = 1:4. All the measurements were in terms of MK moieties. Four photoinitiator systems, MK[(pip).sub.2]/DMP, MKE/DMP, MK[(mpz).sub.2], and PMKPR were investigated, and their structures are shown in Scheme 2.
Gel permeation chromatography (GPC) measurement was performed at room temperature on a Perkin Elmer Series 200 apparatus on the basis of linear polystyrene (PS) standards using DMF as eluent.
[.sup.1]H NMR spectra were recorded on a Mercury Plus 400 Hz spectrometer with DMSO ([-d.sub.6]) as solvent.
UV-vis spectra were recorded in DMF solution by Shimadzu UV-2450 spectrophotometer.
FTIR spectra were recorded on a Perkin-Elmer Paragon 1000 FTIR spectrometer.
Elemental analysis was conducted on an Elementar Varioel apparatus.
Mass spectra were recorded on a HP5989A mass spectrometer at 70 eV.
Photobleaching Measurement (32), (33)
The DMF solution of sample was put into a 1-cm quartz cuvette with a cover slip to prevent evaporation. Subsequently, the irradiation was carried out under a high-pressure mercury lamp with a filter of 365 nm. The light intensity is 50 mW/[cm.sup.2]. The light exposure time intervals were set at appropriate ranges for different samples, depending on the rate of photobleaching. Then, the UV spectra of samples with different exposure time were recorded by Shimadzu UV-2450 spectrophotometer.
The photobleaching ratio can be evaluated through the profile of conversion versus exposure time. The conversion can be defined as follows (34).
C = [[[OD.sub.0] - [OD.sub.t]]/[OD.sub.0]] x 100% (1)
R = [C/T] (2)
where C is the conversion of the decrease ratio of the maximal absorbance of sample, [OD.sub.0] is the maximal absorbance at the initial time of exposure, [OD.sub.t] is the maximal absorbance at a given time of exposure and R is the photobleaching rate.
The photopolymerization of A-BPE-10 and TMPTA was carried out by DSC 6200 (Seiko Instrument) photo-DSC with incident light of 365 nm, whose intensity is 50 mW/[cm.sup.2]. Approximately 2.5 mg sample mixture was placed in the aluminum DSC pans.
Heat flow versus time (DSC thermogram) curves were recorded in an isothermal mode under a nitrogen flow of 50 ml/min. The reaction heat liberated in the polymerization was directly proportional to the number of vinyl groups reacted in the system. By integrating the area under the exothermic peak, the conversion of the vinyl groups (C) or the extent of reaction could be determined according to:
C = [DELTA][H.sub.t]/[DELTA][H.sub.0.sup.theor] (3)
where [DELTA][H.sub.t] is the reaction heat evolved at time t, and [DELTA][H.sub.0.sup.theor] is the theoretical heat for complete conversion. For an acrylic double bond, [DELTA][H.sub.0.sup.theor] = 86 kJ/mol (35). The rate of polymerization ([R.sub.p]) is directly related to the heat flow (dH/dt) by the following equation:
[R.sub.P] = dc/dt = (dH/dt)/[DELTA][H.sub.0.sup.theor] (4)
RESULTS AND DISCUSSION
Synthesis of PMKPR and Model Compounds
Michler's ketone itself has good photoinitiation properties because its structure contains parent benzophenone and coinitiator amine. So, we can expect that the photo-initiation efficiency will be enhanced if more coinitiator amine can be introduced into the molecular chain. In this context, the polymeric photoinitiator PMKPR containing MK and coinitiator amine was synthesized according to Scheme 1, as well as the low-molecular weight model compounds MK[(pip).sub.2] and MK[(mpz).sub.2]. PMKPR was synthesized by addition polymerization between epoxy and amine group. The structure of the polymeric photoinitiator was characterized by [.sup.1]H NMR and FTIR. The occurrence of signals related to -OH in [.sup.1]H NMR and FTIR spectra of PMKPR were considered as the evidence of the polymerization of MK[(pipaz).sub.2] and RDE. The molecular weight of PMKPR was determined by GPC, and the [M.sub.n] is about 3.6 x [10.sup.4], which further confirmed the successful synthesis of polymeric photoinitiator PMKPR.
The structures of MK[(pipaz).sub.2] and model compounds, MK[(pip).sub.2] and MK[(mpz).sub.2], were confirmed by [.sup.1]H NMR, FTIR, UV-vis spectra, elemental analysis, and mass spectra. The results were in agreement with that reported in the literature (31).
Properties of PMKPR and Model Compounds
Figure 1 shows the UV-vis absorption spectra of PMKPR and model compounds in DMF solution. The wavelength of maximal absorption ([[lambda].sub.max]) and the extinction coefficient ([epsilon]) are summarized in Table 1. MK[(pip).sub.2]/DMP, MKE/DMP, and MK[(mpz).sub.2] were used as references.
[FIGURE 1 OMITTED]
TABLE 1. UV-vis data of the photoinitiators in DMF. PI [[lambda.sub.max]] [epsilon] (L [mol.sup.-1] (nm) [cm.sup.-1]) MK[(pip).sub.2]/DMP 354 35,700 MKE/DMP 363 41,880 MK[(mpz).sub.2] 348 33,180 PMKPR 349 29,960 [epsilon]: extinction coefficient.
The absorption peaks at 363 and 356 nm are assigned to the [pi]-[pi]* transition of MKE and MK[(pip).sub.2], respectively (31). The [pi]-[pi]* transition of MK[(mpz).sub.2] is 349 nm, which shows a little hypsochromic shift comparing to MKE and MK[(pip).sub.2]. This effect can be ascribed to the fact that the [pi]-[pi]* transition becomes more difficult due to the more steric demand by the substituent of MK[(mpz).sub.2] (36). The peak absorption of PMKPR is 349 nm and similar to that of MK[(mpz).sub.2], which indicates that the polymeric structure has no significant influence on the UV-vis absorption of MK moieties in polymeric photoinitiator. Compared with model compounds, the extinction coefficient of PMKPR is a little lower, which maybe due to the polymer hypochromicity.
Upon photolysis in the presence of hydrogen donor such as coinitiator amine, excited triplet MK moiety will abstract hydrogen rapidly and produce two kinds of radicals, which results in the destruction of MK moiety. Photobleaching behavior can be examined by the change of UV-vis absorption spectra with irradiation time (37).
Figure 2 illustrates the UV-vis absorption spectra of PMKPR and model compounds in DMF solution at different time under UV light. It is clear that the UV absorption bands decrease with increasing UV exposure time, which indicates that [pi]-[pi]* transition of MK series was photo-bleached by the UV light. The photobleaching time is defined as the time for the relative absorption intensity decaying to l/e of the initial intensity required (38). So, we can calculate the photobleaching time of the PMKPR and the model compounds. The photobleaching time and photobleaching rate of these photoinitiators are summarized in Table 2.
[FIGURE 2 OMITTED]
TABLE 2. The photobleaching rate and photobleaching time of the photoinitiators. Parameter MK[(pip)sub.2]/DMP MKE/DMP MK[(mpz)sub.2] PMKPR R 0.018 0.053 0.095 0.485 t (s) 3429 1205 671 129
Figure 3 is the profile of conversion versus exposure time. From Table 2 and Fig. 3, we can see that the conversion is linear essentially with exposure time and the slopes of lines can represent the photobleaching rate. The slope of PMKPR is much steeper and the photobleaching time is much shorter than any other photoinitiator systems as references, which means that the excited triplet MK moieties of PMKPR can easily abstract hydrogen from the coinitiator amine in the same polymeric chain because of the high local concentration of coinitiator amine around MK moieties.
[FIGURE 3 OMITTED]
Among the three model compounds. MK[(mpz).sub.2] has the most similar structure with PMKPR, that is, both of them have two aliphatic tertiary amine corresponding to one carbonyl, which is more reactive than the aromatic tertiary amine. Although MK[(pip).sub.2] and MKE do not contain aliphatic tertiary amine group in structure. Figure 3 indicates that the photobleaching rate of MK in MK[(mpz).sub.2] and PMKPR is fast due to the efficient intramolecular hydrogen abstraction. It is clear that the photo-bleaehing rate of MK[(mpz).sub.2] is the second fastest but much slower than that of PMKPR because of the relative lower local tertiary amine concentration. We also can see that the photobleaching rate of MKE and MK[(pip).sub.2] is far slower than that of PMKPR and MK[(mpz).sub.2], both of which have aromatic tertiary amine. So, it is concluded that although the aromatic tertiary amine has reactivity and supply hydrogen atoms to hydrogen abstraction reaction of carbonyl, the aliphatic tertiary amine is more reactive and makes an important role in hydrogen abstraction reaction.
Photopolymerization of A-BPE-10
PhotoDSC is a convenient and rapid method to test the efficiency of photoinitiators and can get lots of useful information about polymerization. In this context, two different functionality monomer A-BPE-I0 and TMPTA were chosen to polymerize initiated by these photoinitiator systems. The structures of A-BPE-I0 and TMPTA are shown in Scheme 3.
Figure 4 shows the photopolymerization behavior of A-BPE-I0 initiated by these photoinitiators. Figure 4a shows the curves of heal flow versus time, which indicates that all the photoinitiator systems are effective. This polymerization behavior is similar to other multifunctional monomers (39), (40). At the beginning of photopolymerization, the radicals can move freely and initiate monomer polymerizing effectively, and both of the polymerization rate and termination rate of radicals are fast. With the process of reaction, three-dimensional gel structure is formed, which restricts the diffusion and mobility of macroradicals and pendant double bonds, slowing down the radical termination rate, which leads to autoacceleration. With the reaction going on, the increasing cross-linking will limit the radicals and monomer's mobility and result in the end of reaction eventually.
[FIGURE 4 OMITTED]
Figure 4b shows that the conversion corresponding to the maximal rate of polymerization ([R.sub.p,max]) is dependent on photoinitiator system. The data for [R.sub.p,max] and final conversion (DBC) of polymerization of A-BPE-10 are summarized in Table 3. From Fig. 4 and Table 3. PMKPR is the most efficient photoinitiator for polymerization of A-BPE-10) and the polymerization rate (R) is Fastest among these four photoinitiator systems during the whole polymerization process, which is different from other polymeric photoinitiators (8), (19), (20). Many of the polymeric photoinitiators have relative slower polymerization rate corresponding to the low-molecular weight analogues at early stage and faster at later stage. So, it indicates that the introduction of coinitiator and MK moiety into the same polymeric chain is a good method to increase the photoinitiation efficiency of MK. The reasons maybe as follows: First, the local amino concentration of PMKPR is higher, which makes the hydrogen abstraction reaction easily happen and produce more radicals to initiate polymerization, thus increases the polymerization rate. Second, the number of macroradicals is more than that of the low-molecular weight radicals because of the slow termination rate. With the progress of the reaction, the intramolecular hydrogen abstraction in PMKPR is less affected by gel -structure than that in low-molecular weight photo-initiator, which makes the PMKPR produce more radical species than other photoinitiators. Among these model compounds, MK[(mpz).sub2] has the most similar structure to PMKPR, but the photoinitiation efficiency is far less than that of PMKPR, which maybe ascribed to the difficult intermolecular hydrogen abstraction because of the steric hindrance of MK[(mpz).sub.2] stituent. MK[(pip).sub.2]/DMP has the better efficiency than any other model compounds, maybe due to the better compatibility with monomer, but not so good as PMKPR.
TABLE 3. Photopolymerization of A-BPE-10 and TMPTA initiated by MK[(pip).sub.2]/DMP, MKE/DMP, MK[(mpz).sub.2], and PMKPR, cured at 25[degrees]C by UV light with an intensity of 50 mW/[cm.sup.2]. A-BPE-10 Photoinitiator (a) [R.sub.p,max] (b) x [10.sup.3] [t.sub.max] DBC ([s.sup.-1]) (c) (s) (d) (%) MK[(pip).sub.2]/DMP 17.4 14.3 86.4 MKE/DMP 9.8 14.7 69 MK[(mpz).sub.2] 13.5 12.6 58.4 PMKPR 27.7 16.3 88.8 TMPTA Photoinitiator (a) [R.sub.p,max] x [10.sup.3] [t.sub.max] DBC (%) ([s.sup.-1]) (s) MK[(pip).sub.2]/DMP 19.7 11.8 59.1 MKE/DMP 15.4 11.7 49.6 MK[(mpz).sub.2] 13.0 11.8 37.1 PMKPR 18.1 11.7 40.5 (a) The photoinitiator concentration was 0.01 M, and DMP concentration was 0.01 M. (b) [R.sub.p,max]: the maximal polymerization rate. (c) [t.sub.max]: time to reach maximal heat flow. (d) DBC: final conversion.
Photopolymerization of TMPTA
Figure 5 illustrates the photopolymerization behavior of TMPTA initiated by these photoinitiator systems. The data for the maximal polymerization rate ([R.sub.p,max]) and final conversion of TMPTA are summarized in Table 3.
[FIGURE 5 OMITTED]
When compared with A-BPE-10, the double bond content of the trifunctional TMPTA is much higher, which leads to the different polymerization behavior of TMPTA. The maximal heat flow is much higher than that of A-BPE-10, and the gelation will occur at an earlier stage of the photopolymerization of TMPTA than that of A-BPE-10. The cross-linking density is also higher in photopolymerization of TMPTA, which confines the mobility of monomer and photoinitiator radicals leading to the termination of reaction earlier. So, the final conversion of TMPTA is much lower than that of A-BPE-10.
Figure 5a is the photoDSC profile of the polymerization of TMPTA initiated by these photoinitiator systems, which shows that all of photoinitiator systems can initiate the polymerization, but the photoinitiation efficiency is different from each other. PMKPR is an efficient photoinitiator for polymerization of TMPTA. The [R.sub.p.max] of TMPTA initiated by PMKPR is higher than model compounds except for MK[(pip).sub.2]/DMP, which maybe ascribed to the higher local amino concentration and efficient electron and proton transfer between excited MK moiety and coinitiator amine. When compared with A-BPE-10, the final conversion of TMPTA initiated by PMKPR is not high, which maybe ascribed to the higher double bond content of TMPTA. As for TMPTA. the photopolymerization is diffusion-controlled (41), (42) and the mobility of radicals is very important to polymerization due to high cross-linking density and viscosity of multifunctional monomer TMPTA. Although the high-molecular weight and relative rigid structure of PMKPR may restrict its mobility, which leads to the lower final conversion. although the hydrogen abstraction is the most efficient in PMKPR. Anyway. PMKPR is still a good photoinitiator due to the polymeric effect and the photoinitiation efficiency will be better if the polymeric chain is more flexible.
In this context, polymeric photoinitiator (PMKPR) was synthesized through introducing coinitiator amine and MK moiety into the same polymeric chain, as well as model compounds. PMKPR possesses the similar UV absorption spectra with MK, but has the much faster photobleaching rate than any other model compounds. The photopolymerization behavior of A-BPE-10 and TMPTA initiated by these photoinitiator systems was studied by PhotoDSC. The result shows that PMKPR can efficiently initiate the polymerization of A-BPE-10 and TMPTA. especially of A-BPE-10. A comprehensive study of this polymeric MK photoinitiator including effect of coinitiator amine, polymeric chain's flexibility, and molecular weight is currently under investigation.
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Yanna Wen, Xuesong Jiang, Jie Yin
School of Chemistry and Chemical Technology, State Key Laboratory for Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
Correspondence to: Jie Yin; e-mail: email@example.com
Contract grant sponsor: Science and Technology Commission of Shanghai Municipal Government; contract grant number: 06JC14041.
Published online in Wiley InterScience (www.interscience.wiley.com).
[C] 2009 Society of Plastics Engineers