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Synthesis and photopolymerization kinetics of polymeric one-component type II photoinitiator containing benzophenone moiety and tertiary amine.

INTRODUCTION

Photopolymerization science and technology has been obtaining much attention in recent years because of its various industrial applications in coatings, adhesives, printing inks, photoresists, and biomaterials [1-4]. A typical formulation consists of a vinyl monomer and a photoinitiator in photopolymerization. Benzophenone is by far one of the most widely used conventional low-molecular weight photoinitiators for UV-curing coatings as it has good surface curing, solubility, and been shown to initiate free radical polymerization of (meth)acrylic systems effectively in the presence of amines [5-9]. Conventional low-molecular weight photoinitiators have some disadvantages. For example, benzophenone is known for its relatively strong odor, yellowing, and exceptional ability to migrate and be extracted from cured products [10-14], it is clear that there is a need for a benzophenone derivative without these disadvantages.

One of the important ways to obtain high-performance photoinitiators is to develop polymeric photoinitiators. which have some advantages derived from polymer nature compared with low-molecular weight analogs [13, 15, 16]. Meanwhile, polymeric photoinitiator systems containing chromophore and tertiary amine have obvious advantages such as good solubility and compatibility in the curable formulations, low odor, and toxicity, due to the well-known polymer effect [17, 18]. If tertiary amine is introduced into the 4,4'-dihydroxybenzophenone to prepare a polymeric photoinitiator through chemical reaction, it may improve its solubility in (meth)acrylate-based formulation. At the same time, the incorporation of both benzophenone moiety and tertiary amine into the same molecule has obvious advantages that the benzophenone moiety is expected to be attached with (meth)acrylate monomers through amine-derived radical during the curing process, which makes the modified benzophenone derivative a potential polymeric one-component type II photoinitiator with low odor, good solubility, and limited tendency to migrate, and therefore can be used in UV formulations as a benzophenone alternative.

In this work, a polymeric one-component type II photoinitiator (PDABPP) was synthesized. The photopolymerization kinetics of the photoinitiator in different conditions was studied by real-time infrared spectroscopy (RTIR). The results indicated that PDABPP is an effective initiator.

EXPERIMENTAL

Materials

4,4'-Dihydroxybenzophenone (DHBP) was obtained from Wu Han Eastin Chemical Industry (Wuhan, Hubei, China) and used as received. Acryloyl chloride was purchased from Shanghai Chemical Reagent (Shanghai, China). Piperazine was purchased from Sinopharm Chemical Reagent (Shanghai, China). Tripropylene glycol diacrylate (TPGDA) was obtained from Sartomer (Warrington, PA). 2-(Dimethylamino)ethyl methacrylate (DMEM), triethanolamine (TEOHA), N,N-dimethyl ethanolamine (DMEA), N,N-dimethylaniline (DMA), and triethylamine (TEA) were purchased from Sinopharm Group Chemical Reagent (Beijing, China). Methylene chloride were dried and purified according to standard laboratory methods. All other reagents were of analytical grade and used as received unless otherwise specified.

Instrumentation

Polymer molecular weight was determined using a Waters 515-2410 gel permeation chromatography (GPC) (Waters, USA) on the basis of linear polystyrene (PS) standards. Tetrahydrofuran (THF) was used as eluent.

The [.sup.1.H] NMR spectra were recorded on a Bruker AV600 unity spectrometer operated at 600 MHz using CD[Cl.sub.3] as deuterated solvent.

FTIR spectra were recorded on a Nicolet 5700 instrument (Thermo Electron Corporation, Waltham, MA). All samples were photocured in 1.6-mm thick plastic molds with an 11-mm diameter central. The molds were clamped between two glass slides with spring-loaded binder clips [19]. The samples were irradiated with a UV spot source. Each spectrum was a signal of 1 scan with the resolution of 4 [cm.sup.-1] at room temperature. For each sample, the series RTIR runs were repeated three times.

UV spot source (EFOS Lite, 50W miniature arc lamp, with 5-mm crystal optical fiber, Canada) was used for photopolymerization.

Light intensity was recorded by the UV Light Radiometer (Photoelectric Instrument Factory, Beijing Normal University, Beijing, China).

Synthesis of 4,4'-Diacryloylbenzophenone

4,4'-Dihydroxybenzophenone (14.8 g, 0.069 mol), triethylamine (15.4 g, 0.152 mol), and 200 mL methylene chloride were added into a 500-mL three-necked round-bottom flask fitted with an overhead stirrer, a thermometer, and an addition funnel containing acryloyl chloride (13.8 g, 0.152 mol) and 50 mL of methylene chloride mixture. Under ice water bath cooling (0-5[degrees]C), the acryloyl chloride solution was dropped into the flask during 3 h. The mixture was allowed to stand overnight, and the precipitate was filtered off and washed twice with 30 g of methylene chloride. Then the organic layer was extracted twice with 1 mol/L hydrochloric acid, 1 mol/L sodium bicarbonate solution, and deionized water, and then dried overnight with anhydrous sodium sulfate. Subsequently, the methylene chloride was removed by rotary evaporation.

[.sup.1.H] NMR (CD[Cl.sub.3], 600 MHz): [delta] (ppm): 6.06-6.07 (2H); 6.32-6.36 (2H); 6.63-6.56 (2H); 7.25-7.28 (4H); 7.86-7.87 (4H).

Synthesis of PDABPP

6.45 g of DABP (0.02 mol) was dissolved in 30 mL of methylene chloride. 1.72 g of piperazine (0.02 mol) dissolved in 20 mL methylene chloride was dropped under magnetic stirring in [N.sub.2] atmosphere. The reaction was heated to 38[degrees]C for 48 h. Then, the reaction was cooled to room temperature and dropped slowly into vigorously stirred diethyl ether. Product was collected and dried under vacuum prior to analysis.

[M.sub.n] = 3.3 X [10.sup.3], [M.sub.W]/[M.sub.n] = 1.11 (determined by GPC using THF as eluent).

[.sup.1.H] NMR (CD[Cl.sub.3], 600 MHz): [delta] (ppm): 2.349-2.546 (4H); 2.784-2.845 (12H); 7.201-7.247 (4H); 7.824-7.836 (4H).

Photopolymerization Kinetics

The initiation efficiency of PDABPP was studied by RTIR which had become an important method for obtaining photopolymerization kinetics data [20, 21]. Conversion data were obtained by monitoring the decay of the (meth)acrylate double bond =C-H peak at about 6165 [cm.sup.-1]. Upon irradiation, the decrease of the =C-H absorption peak area from 6102 to 6248 [cm.sup.-1] accurately reflects the extent of the polymerization, since the change of the absorption peak area was directly proportional to the number of the (meth)acrylate that had polymerized. After baseline correction, conversion of the functional groups could be calculated by measuring the peak area at each time of the reaction and determined by the Eq. 1:

DC(%) = ([A.sub.0] - [A.sub.t])/[A.sub.0] x 100 (1)

where DC is the degree of (meth)acrylate double bond conversion at t time, [A.sub.0] is the initial peak area before irradiation, and [A.sub.t] is the peak area of the double bonds at t time.

RESULTS AND DISCUSSION

Synthesis

The polymeric photoinitiator was prepared according to a general synthetic route shown in Scheme 1 via Michael addition of piperazine to DABP. The reaction was carried out in C[H.sub.2][Cl.sub.2] at 38[degrees]C. Although the Michael addition is generally performed using alcohol or methanol as solvent and catalyst [4], we used anhydrous C[H.sub.2][Cl.sub.2] to minimize hydrolysis reactions during synthesis. The rates of Michael addition of primary amines are higher than that of secondary amines [22], therefore, Michael addition of the secondary amine (piperazine) to HABP was proceeded for 48 h at 38[degrees]C to ensure the completion of the reaction. The disappearance at about 3315 [cm.sub.-1] for N-H peak in FTIR spectra allowed monitoring of the course of the reaction. The structure of the photoinitiator was confirmed by GPC and [.sup.1.H] NMR spectroscopy.

[GRAPHIC OMITTED]

Photopolymerizations

The photopolymerization kinetics of the polymeric photoinitiator in different conditions was studied by RTIR.

The conversion versus time plots for the polymerization of TPGDA induced by PDABPP, BP/TEA, and BP are shown in Fig. 1. As they had the same BP moiety molar concentration, the final conversion of PDABPP was almost the same as that of BP/TEA. But BP moiety and tertiary amine were introduced into one macromolecule in PDABPP, which meant that radicals can be generated in the macromolecule and result in the low tendency to migrate from the cure products. The result showed that PDABPP almost had the same effect to BP/TEA and could be used as a BP alternative.

[FIGURE 1 OMITTED]

The plots of conversion versus irradiation time of TPGDA incorporating different PDABPP concentrations are showed in Fig. 2. The polymerization rate increased with increase of PDABPP concentration. The induction period was slightly shortened with increase of PDABPP concentration. Because the higher the PDABPP concentration, the more the free radical could be produced during irradiation resulting in the higher rate of polymerization. On the other hand, the volume shrinkage occurred at very fast rate of polymerization and resulted in an increase in free-volume formation, which increased the mobility of the residual double-bond and led to a higher final conversion [23].

Figure 3 showed the conversion versus time plots of TPGDA initiated by 0.3 wt% PDABPP at different light intensity. It had the similar trends to the effect of the concentration of PDABPP as showed in Fig. 2. The polymerization rate and final conversion increased with increase in light intensity. This was because the higher light intensity could yield more radicals which led to the increase in polymerization rate and final conversion. At the same time, the more radicals yielded by the increase of light intensity could overcome oxygen inhibition more efficiently, resulting in the shortening of the induction period.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Although PDABPP could initiate photopolymerization as a polymeric one-component type II photoinitiator, the effect of different amines was still investigated. The plots of conversion versus irradiation time of TPGDA incorporating PDABPP in the presence of different amines as the initiating system are recorded in Figure 4. The PDABPP/TEA combination in TPGDA exhibited the highest polymerization rates. DMEA and TEOHA were less efficient than TEA, it maybe because DMEA and TEOHA had higher molecular weight, which meant less radicals would be generated when they had the same mass concentration. DMA was not an efficient coinitiator in this system and just slightly more efficient than DMEM. DMEM is a copolymerizable amine which is used as a coinitiator in dental materials commercially [4].

Figure 5 showed the conversion versus time plots of TPGDA initiated by 0.2 wt% PDABPP with different TEOHA concentrations. The polymerization rate and final conversion increased with increase in TEOHA concentration. This was because higher coinitiator concentration yielded more radicals by the light irradiation, and thus led to the higher polymerization rate and final conversion. As shown in Fig. 5, the induction period was shortened with the increase of the TEOHA concentration. It is because photopolymerization carried out in the presence of air, the initiating radicals are scavenged by oxygen molecules dissolved in the formulation. As the oxygen is consumed, the monomer molecules become capable of successfully reacting with the initiator radicals, thus initiating polymerization. Thus, increasing the TEOHA concentration could produce rapidly large amounts of free radicals to overcome oxygen inhibition [24].

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

CONCLUSION

A polymeric one-component type II photoinitiator (PDABPP) based on 4,4'-dihydroxybenzophenone (DHBP), acryloyl chloride, and piperazine was synthesized, and its structure was confirmed by GPC and [.sup.1.H] NMR. The photopolymerization kinetics of the photoinitiator was studied by RTIR. It showed that PDABPP was a more effective photoinitiator than that of BP/TEA. Both rate of the polymerization and final conversion increased and the induction period was shortened with increase of PDABPP concentration, light intensity, and amine concentration. The structure of amine and its concentration had significant effect on the polymerization kinetics. The results showed that PDABPP was an effective photoinitiator.

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Pu Xiao, (1) Ying Wang, (2) Mingzhi Dai, (2) Suqing Shi, (3) Gangqiang Wu, (1) Jun Nie (1,2)

(1) Key Laboratory of Biomedical Polymers of Ministry of Education, Department of Chemistry, Wuhan University, Wuhan 430072, People's Republic of China

(2) State Key Lab of Chemical Resource Engineering, College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China

(3) Department of Chemistry, Northwest University, Xi'an 710069, People's Republic of China

Correspondence to: Jun Nie; e-mail: niejun@mail.buct.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50473024.
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Author:Xiao, Pu; Wang, Ying; Dai, Mingzhi; Shi, Suqing; Wu, Gangqiang; Nie, Jun
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:9CHIN
Date:May 1, 2008
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