Synthesis and preliminary photopolymerization evaluation of novel photoinitiators containing phototrigger to overcome oxygen inhibition in the UV- curing system

Article abstract of DOI:10.1016/j.jphotochem.2019.112187

In this work, two types of novel photoinitiaors containing phototrigger were prepared to overcome oxygen inhibition in the UV- curing system in the absence of hydrogen donor. The structures of prepared novel photoinitiators were determined by nuclear magnetic resonance (NMR) and high resolution MS (HR[sbnd]MS) spectra data. The photo chemical behavior and photo-reactivity were also evaluated by ultraviolet-visible (UV–vis) spectroscopy and real-time Fourier transform infrared spectroscopy (RT-FTIR), respectively. The results show the prepared photoinitiators exhibit remarkable redshift compared to the commercial BP (benzophenone) and Irgacure 907 (2-methyl-1-(4-methylsulfanylphenyl)-2-morpholin-4-ylpropan-1-one), fast photolysis by C[sbnd]S bond, good photo initiation and significant overcoming oxygen inhibition for some compounds, which can be used as one-component photoinitiator candidates.

Full text of DOI:10.1016/j.jphotochem.2019.112187

Journal Pre-proof
Synthesis and Preliminary Photopolymerization Evaluation of Novel
Photoinitiators Containing Phototrigger to Overcome Oxygen Inhibition in
the UV- Curing System
Wenbin Chen, Lei Wang, Xinyue Liu, Bo Chen, Guofeng Zhao
PII:
S1010-6030(19)31421-2
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112187
JPC 112187
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
20 August 2019
16 October 2019
21 October 2019
Please cite this article as: Chen W, Wang L, Liu X, Chen B, Zhao G, Synthesis and Preliminary
Photopolymerization Evaluation of Novel Photoinitiators Containing Phototrigger to Overcome
Oxygen Inhibition in the UV- Curing System, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112187
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Synthesis and Preliminary Photopolymerization Evaluation of Novel Photoinitiators
Containing Phototrigger to Overcome Oxygen Inhibition in the UV- Curing System


Wenbin Chen , Lei Wang, Xinyue Liu, Bo Chen, Guofeng Zhao
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin, 300071, China
Graphical abstract:
Two novel kinds of photoinitiators containing photo-trigger have good overcoming oxygen
inhibition in the UV-curing system
1
2%
3%
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1%
2%
3%
4%
80
60
40
20
0
a)
b)
4%
60
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0
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100 200 300 400 500 600 700
0
100 200 300 400 500 600 700
Time（ s）
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Highlight
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Two novel kinds of photoinitiators containing photo-trigger were synthesized
The spectral properties show remarkable redshift compared to the references
The photo-initiating activity of the initiators was evaluated by real-time FT-IR (RT-FTIR)
The synthesized photoinitiators show good overcoming oxygen inhibition
The synthesized photoinitiators can be used as one-component initiators
Abstract
In this work, two types of novel photoinitiaors containing phototrigger were prepared to overcome
oxygen inhibition in the UV- curing system in the absence of hydrogen donor. The structures of
prepared novel photoinitiators were determined by nuclear magnetic resonance (NMR) and high
resolution MS (HR-MS) spectra data. The photo chemical behavior and photo-reactivity were also
evaluated by ultraviolet-visible (UV-vis) spectroscopy and real-time Fourier transform infrared
spectroscopy (RT-FTIR), respectively. The results show the prepared photoinitiators exhibit
remarkable redshift compared to the commercial BP (benzophenone) and Irgacure 907
(2-methyl-1-(4-methylsulfanylphenyl)-2-morpholin- 4-ylpropan-1-one), fast photolysis by C-S

Corresponding authors
E-mail addresses: wbchen@nankai.edu.cn (W. B. Chen) gfzhao@nankai.edu.cn (G. F. Zhao)

bond, good photo initiation and significant overcoming oxygen inhibition for some compounds,
which can be used as one-component photoinitiator candidates.
Keywords: Photoinitiator, Photopolymerization, Phototrigger, Oxygen inhibition
1. Introduction
Photoinitiated polymerization is widely used in numerous fields such as coatings, adhesives,
inks, dentistry, medicine, microelectronics and so on [1-3]. In recent years, UV-based 3D-printing,
being one of the most revolutionary and powerful tools, has attracted the public interest because of
the smart manufacturing on demand [1, 4, 5]. Photoinitiator is the most important component of
coating formulation, which absorbs UV radiation and generates active radicals to initiate the
polymerization [6]. There are two kinds of photoinitiators, radical photoinitiators and cationic
photoinitiators. The former include Type I or α-cleavage and Type II photoinitiators. Type I
photoinitiators undergo photo cleavage resulting in two radical species, both being capable of
initiating the polymerization, while Type II initiators generate radicals in the presence of a
co-initiator, typically hydrogen donating compounds, such as amines, thiols or alcohols[3]. The
cationic photo-polymerization also receives considerable attention in industry and academia
mainly due to its insensitivity to oxygen [7]. But because of the wide use of monomers and
photoinitiators, greater effort has been dedicated to free radical photoinitiating systems than the
effort to cationic photopolymerization.
Even though the UV-curing technique has great benefits of high efficiency, low energy
consumption, and being solvent-free and environmentally friendly, the free radical polymerization
has some shortcomings, such as odor, oxygen inhibition, yellowing, migration, and poor
compatibility with the UV-curable resin, which will lead to undesirable effects in the post-cured
materials. Because the UV-curing is operated in the air, the oxygen diffuses from the atmosphere
into the formulation and reacts with excited species and radicals to quench the UV-curing, and
results in diminished mechanical performance and tacky surface [8]. So the oxygen inhibition is
another major obstacle and challenge besides migration and yellowing. There are many tactics to
overcome oxygen inhibition of UV-curing procedure. These strategies include physical and
chemical methods [9]. The former is the oldest and most widespread strategy, which prevents
diffusion of oxygen from the atmosphere into the polymerizing resin by introducing inert gas
(nitrogen, argon, carbon dioxide) or physical barriers. On the other hand, increasing curing light
intensity and concentration of photoinitiator are known to be the easiest way to mitigate oxygen
inhibition, but it will result in light shielding, volume shrinkage, and especially higher industrial
cost [10].
In addition to the physical methods, a lot of chemical strategies have been developed over the
years to overcome oxygen inhibition. Chemical strategies are divided into the adding of additive
in the formulation and modification of the monomers, photoiniators or oligomers in the
formulation. The effective additive materials that can consume the oxygen during curing process
are tertiary amine [9], thiol [11] and triphenylphosphine (TPP) [12]. Thiol is usually used as
additive in the curing formulation to reduce the effect of oxygen inhibition by undergoing chain
transfer with the peroxide radical because of the high reactive thiol-ene click reaction. Beyond
reducing the oxygen inhibition, the thiols functional curing materials also exhibit many other

properties such as low polymerization shrinkage [13], red shift of absorption maximum [14], great
mechanical property [11] and photolysis reactions at the C–S bond as in a type I system [15].
Despite many potential desirable characteristics, there are a few drawbacks of thiols additive
UV curable systems, most notably odor and bad stability. It is a big challenge to combine the
advantages of overcoming oxygen inhibition and the disadvantages of odor and instability in the
UV curing materials. It is a good strategy to protect the thiol with a protective group to eliminate
the odor and instability, and then it will be released when exposed to light. Phototrigger is a good
protective group to eliminate the odor and instability of thiol, which will be removed under photo
initiation to release the thiol to overcome oxygen inhibition, and it has also been used widely in
organic synthesis and biology [16].
Herein, we report two types of novel photoinitiators containing phototrigger to protect thiol and
evaluate their photoreactivity and oxygen scavenging effect in photopolymerization process by
real-time Fourier transform infrared spectroscopy (RT-FTIR). The phototrigger
arylcarbonylmethyl group can undergo photoinitiated breaking of the C−S bond [17], which was
used as a protective to prepare the novel photoinitiators. The detailed synthetic route is depicted in
Scheme 1.
2. Experimental
2.1. Materials and instruments
All chemicals and solvents used for synthesis were purchased from commercial suppliers and
used directly in the experiments without further purification. The progress of the reactions was
monitored by thin layer chromatography (TLC) on pre-coated silica gel 60 F254 plates and spots
were visualized by UV light. Flash-column chromatography was performed using commercial
grades of silica gel 200-300 meshes. H NMR and 13C NMR spectra were recorded on a Bruker
1
4
00 spectrometer in CDCl . Chemical shifts are reported in parts per million relative to internal
3
standard tetramethylsilane (TMS = 0.00 ppm). Coupling constants (J) are reported in Hertz (Hz).
High-resolution mass spectra (HR-MS) were obtained on a Varian 7.0 T FTICR-MS. The
UV-visible spectra were recorded on
a
L5 spectrophotometer (INESA, China).
Photopolymerization kinetics studies were obtained on a Nicolet 5700 real-time infrared
spectrometer (RT-FTIR), (Thermo Fisher, USA). Melting point was recorded on a WRS-2 melting
point apparatus from Shanghai Shenguang Instruments Co., Ltd.
1, 6-hexanediol diacrylate (HDDA), 2-(2-ethoxyethoxy) ethyl acrylate (EOEOEA),
trimethylolpropane triacrylate (TMPTA), tripropylene glycol diacrylate (TPGDA) were obtained
from Tianjin Jiuri New Materials Co. Ltd.
The photosensitive formulations were deposited on a KBr pellet in laminate for irradiation with
-2
xenon lamp (XD 300, I = 28 mW cm ).

Scheme 1 The synthetic routes to photoinitiators BPT and 907T
.2 Preparation of BP-SH and 907-SH
2
2
.2.1 4-Mercaptobenzophenone (BP-SH) [18]
To the mixture of 4-chlorobenzophenone (20.0 g, 92.3 mmol) in 200 mL of DMF was added
S.9H
O (80.0 g, 333.1 mmol) with mechanical stirring. The mixture was heated to 110 C for
o
Na
2
2
12 h. The reactant was cooled to room temperature and poured into 200 mL of water, and adjusted
to pH 5-6 with 15% HCl aqueous. The light yellow precipitate was filtered and dissolved in 100
mL of toluene, washed with 5% NaOH aqueous (50 mL × 3). The organic phase was acidified to
pH 3-5 with 15% HCl aqueous, the precipitate was filtered and washed with water, dried to a
o
1
white solid, m. p., 72-74 C, 9.2 g (42.9 mmol), 46.5%. H NMR (400 MHz, CDCl
3
) δ/ppm 7.76 (d,
J = 7.3 Hz, 2H) , 7.69 (d, J = 8.3 Hz, 2H)，7.58 (t, J = 7.4 Hz, 1H) , 7.48 (t, J = 7.6 Hz, 2H) , 7.33
13
(
d, J = 8.3 Hz, 2H) , 3.65 (s, 1H) . C NMR (101 MHz, CDCl ) δ/pm 195.7, 138.1, 137.6, 134.4,
3
132.3, 130.9, 129.8, 128.3, 128.0
2.2.2 1-(4-Mercaptophenyl)-2-methyl-2-morpholinopropan-1-one (907-SH) [19]
In a 500 mL four-necked flask equipped with mechanical stirring, dropping funnel and
thermometer, was filled with Na
2
S.9H
2
O (22.0 g, 91.5 mmol), anhydrous Na
2
SO (13.5 g, 112
4
o
mmol), NMP (60.0 g). After the mixture was heated to 75 C, 1-(4-chlorophenyl)-2-methyl-
2-morpholinopropan-1-one (C-907) (10.0 g, 37.3 mmol) in 20 g of NMP was added to the above
o
mixture. After addition, the mixture was heated to 130 C for 2 h. The reactant was cooled to room
temperature, Na
2
SO was filtered off, and washed with NMP. The filtrate was poured into 200 mL
4
of ice-water, adjusted with 15% HCl to pH 6-7 and stirred at room temperature for 1 h, and
extracted with ether (50 mL × 3). The ether phase was concentrated to 25 mL, and washed with 5%

NaOH (15 mL × 3). The aqueous phase was separated and acidified to pH 6-7 with 15% HCl, and
extracted with CH Cl (25 mL × 3), dried over Na SO , concentrated and the residue was
2
2
2
4
separated by silica gel chromatography with hexane/ethyl acetate (V/V 12.5:1) as eluent to obtain
1
a white solid, 4.4 g (16.6 mmol, 44%). H NMR (400 MHz, CDCl
3
) δ/ppm 8.45 (d, J = 8.4 Hz,
1
3
2
H), 7.25 (s, 2H), 3.74 - 3.64 (m, 4H), 3.60 (s, 1H)，2.61- 2.53 (m, 4H), 1.30 (s, 6H); C NMR
(
101 MHz, CDCl ) δ/ppm 202.2, 137.9, 132.6, 131.1, 127.6, 68.4, 67.4, 47.0, 20.2
3
2.3 1-Bromo-arylcarbonyl methyl intermediate (ACM-Br) [20-22]
General procedure: (substituted) acetophenone or propiophenone (ACM) (1.0 eq) was
dissolved in the mixture solvent of chloroform and ethyl acetate (1:1) and then heated to reflux,
CuBr (1.6-2.0eq) was added in portions. The reaction was monitored by HPLC (mobile phase:
acetonitrile/H O = 40:60, 1 mL/min, 20min, 254nm). After reaction, the mixture was filtered and
2
2
washed with solvent, the filtrate was evaporated, and the residue was purified by flash column
chromatography on silica gel to give intermediates ACM-Br.
ACM-Br-a: 2-bromo-1-phenylethanone, white solid, yield 54.7 %.
1H NMR (400 MHz, CDCl
) δ/ppm 8.01 (d, J = 8.5 Hz, 2H), 7.64 (t, J = 7.4 Hz, 1H), 7.52 (t, J =
3
13
7
3
.7 Hz, 2H), 4.49 (s, 2H). C NMR (101 MHz, CDCl
3
) δ/ppm 191.3, 134.0, 133.9, 128.95, 128.9,
0.9.
ACM-Br-b: 2-bromo-1-(o-tolyl) ethanone, light yellow oil, yield 51.4 %.
1H NMR (400 MHz, CDCl
) δ/ppm 7.66 (dd, J = 5.6, 2.9 Hz, 1H), 7.44-7.38 (m, 1H), 7.29-7.24
3
(
m, 2H), 4.42 (s, 2H), 2.51 (s, 3H). 13C NMR (101 MHz, CDCl
3
) δ 194.2, 139.7, 134.4, 132.4,
132.3, 129.1, 125.8, 33.9, 21.5.
1
ACM-Br-c: 2-bromo-1-(4-hydroxyphenyl) ethanone, light yellow powder, yield 50.8 %. H NMR
(400 MHz, DMSO) δ/ppm 10.53 (s, 1H), 7.89 (d, J = 8.7 Hz, 2H), 6.87 (d, J = 8.7 Hz, 2H), 4.78 (s,
1
3
2
H). C NMR (101 MHz, DMSO) δ/ppm 189.8, 162.6, 131.4, 125.4, 115.4, 33.4.
1
ACM-Br-d: 2-bromo-1-(4-hydroxyphenyl) propan-1-one, white solid, yield 61.3 %. H NMR (400
MHz, DMSO) δ/ppm 10.54 (s, 1H), 7.93 (d, J = 8.8 Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 5.73 (q, J =
13
6
.5 Hz, 1H), 1.74 (d, J = 6.5 Hz, 3H). C NMR (101 MHz, DMSO) δ 191.8, 162.0, 131.1, 124.9,
15.2, 42.9, 20.2.
ACM-Br-e: 2-bromo-1-(4-hydroxy-3-methoxyphenyl) ethanone, white solid, 50.9 %. H NMR
1
1
(400 MHz, DMSO) δ/ppm 10.18 (s, 1H), 7.57 (d, J = 8.3 Hz, 1H), 7.49 (s, 1H), 6.89 (d, J = 8.3 Hz,
13
1
H), 4.81 (s, 2H), 3.84 (s, 3H). C NMR (101 MHz, DMSO) δ 189.9, 152.4, 147.7, 125.7, 124.0,
1
15.1, 111.91, 55.7, 33.4.
ACM-Br-f: 2-bromo-1-(3, 4-dimethoxyphenyl) ethanone, pale yellow solid, yield 41 %. H NMR
1
(400 MHz, DMSO) δ /ppm 7.71 (d, J = 8.5 Hz, 1H), 7.49 (s, 1H), 7.09 (d, J = 8.5 Hz, 1H), 4.88 (s,
13
2
H), 3.87 (s, 3H), 3.84 (s, 3H). C NMR (101 MHz, DMSO) δ 190.2, 153.6, 148.7, 126.7, 123.8,

1
10.9, 110.8, 55.8, 55.6, 33.5.
ACM-Br-g: 1-(benzo[d][1,3]dioxol-5-yl)-2-bromoethanone, white solid, yield 53.9 %. H NMR
400 MHz, CDCl ) δ/ppm 7.58 (dd, J = 8.2, 1.8 Hz, 1H), 7.44 (d, J = 1.7 Hz, 1H), 6.87 (d, J = 8.2
Hz, 1H), 6.07 (s, 2H), 4.38 (s, 2H). C NMR (101 MHz, CDCl
1
(
3
13
3
) δ 189.6, 152.6, 148.5, 128.6,
125.6, 108.5, 108.1, 102.2, 30.7.
2.4 Preparation of photoinitiator BPT and 907T [23-26]
General procedure: To a stirred and refluxed solution of thiol compound BP-SH or 907-SH (1.0
eq), K CO (1.2 eq) in acetone (20 mL), was added dropwise bromo-arylcarbonyl methyl
2
3
intermediate ACM-Br (1.0 eq) in acetone (10 mL). After completion of the addition, the reaction
was allowed to reflux until there was no starting material monitored by TLC. Then the reactant
was cooled to room temperature, filtered, and concentrated. The resulting residue was purified by
flash column chromatography on silica gel or recrystallized to afford the desired product.
o
BPT-a, 2-((4-benzoylphenyl)thio)-1-phenylethanone, white metallic glossy solid, m.p.108.6 C,
yield, 65.8 %.
1H NMR (400 MHz, CDCl
) δ/ppm 8.00 (d, J = 7.4 Hz, 2H), 7.76 (d, J = 7.2 Hz, 2H), 7.73 (d, J =
.4 Hz, 2H), 7.60 (dt, J = 14.5, 7.4 Hz, 2H), 7.53 - 7.47 (m, 4H), 7.42 (d, J = 8.4 Hz, 2H), 4.42 (s,
H).
3
8
2
13C NMR (101 MHz, CDCl
28.7, 128.3, 127.5, 99.9, 39.6
HR-MS (ESI, m/z) Calcd. for C21
BPT-b, 2-((4-benzoylphenyl)thio)-1-(o-tolyl)ethanone, light yellow powder, m.p. 96.5 C, yield
6.7 %.
1H NMR (400 MHz, CDCl
3
) δ/ppm 195.7, 193.4, 141.6, 137.6, 135.2, 133.8, 132.4, 130.7, 129.9,
1
+
+
H
16
O S 332.0871(M ), found 332.0866(M )
2
o
5
3
) δ /ppm 7.79 (d, J = 7.4 Hz, 2H), 7.75 (d, J = 8.3 Hz, 2H), 7.70 (d, J =
.7 Hz, 1H), 7.62 (t, J = 7.4 Hz, 1H), 7.51 (t, J = 7.6 Hz, 2H), 7.43 (t, J = 8.6 Hz, 3H), 7.32 (t, J =
.7 Hz, 2H), 4.40 (s, 2H), 2.48 (s, 3H).
) δ/ppm 196.9, 195.7, 141.7, 139.3, 137.6, 135.8, 135.0, 132.3, 132.2,
32.0, 130.7, 129.8, 128.7, 128.3, 127.3, 125.7, 41.9, 21.2.
7
7
13C NMR (101 MHz, CDCl
3
1
+
+
HR-MS (ESI, m/z) Calcd. for C22
H
18
O S 346.1028(M ), found 346.1021(M )
2
o
BPT-c, 2-((4-benzoylphenyl)thio)-1-(4-hydroxyphenyl)ethanone, light yellow solid, m.p. 138.2 C,
yield 66.7 %.
1H NMR (400 MHz, DMSO) δ/ppm 10.51 (s, 1H), 7.97 (d, J = 8.6 Hz, 2H), 7.71 (d, J = 7.3 Hz,
2
2
H), 7.69 – 7.64 (m, 3H), 7.56 (t, J = 7.5 Hz, 2H), 7.49 (d, J = 8.3 Hz, 2H), 6.90 (d, J = 8.6 Hz,
H), 4.77 (s, 2H).
13C NMR (101 MHz, DMSO) δ/ppm 194.78, 191.8, 162.5, 143.0, 137.2, 133.4, 132.4, 131.2,
30.1, 129.3, 128.5, 126.7, 126.2, 115.3, 38.6.
1

+
+
HR-MS (ESI, m/z) Calcd. for C21
H
16
O S 348.0820 (M ), found 348.0814 (M )
3
o
BPT-d, 2-((4-benzoylphenyl)thio)-1-(4-hydroxyphenyl)propan-1-one, white solid, m.p. 119.3 C,
yield 60.2%
1H NMR (400 MHz, DMSO) δ/ppm 10.52 (s, 1H), 7.95 (d, J = 8.8 Hz, 2H), 7.75 – 7.64 (m, 5H),
7
1
.57 (t, J = 7.6 Hz, 2H), 7.53 (d, J = 8.4 Hz, 2H), 6.87 (d, J = 8.8 Hz, 2H), 5.34 (q, J = 6.8 Hz, 1H),
.51 (d, J = 6.8 Hz, 3H).
13C NMR (101 MHz, DMSO) δ/ppm 194.8(ds), 162.5, 140.2, 136.9, 134.7, 132.6, 131.3, 130.2,
29.4, 129.3, 128.5, 126.1, 115.3, 44.4, 17.7.
HR-MS (ESI, m/z) Calcd. for C22 S 362.0977 (M ), found 385.0872 ([M+Na] )
1
+
+
H
18
O
3
BPT-e,
2-((4-benzoylphenyl)thio)-1-(4-hydroxy-3-methoxyphenyl)ethanone,
white
solid,
o
m.p.141.1 C, yield 74.2 %.
1H NMR (400 MHz, CDCl
m, 3H), 7.48 (t, J = 7.6 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 6.98 (d, J = 8.2 Hz, 1H), 6.20 (s, 1H),
.37 (s, 2H), 3.95 (s, 3H).
13C NMR (101 MHz, CDCl
29.9, 128.3, 128.1, 127.3, 124.1, 114.0, 110.4, 56.1, 39.1.
3
) δ/ppm 7.76 (d, J = 7.1 Hz, 2H), 7.73 (d, J = 8.4 Hz, 2H), 7.61 – 7.54
(
4
3
) δ/ppm 195.7, 191.9, 151.1, 146.8, 142.1, 137.6, 134.9, 132.3, 130.7,
1
+
+
HR-MS (ESI, m/z) Calcd. for C22
H
18
O S 378.0926 (M ) found 378.0918(M )
4
o
BPT-f, 2-((4-benzoylphenyl)thio)-1-(3,4-dimethoxyphenyl)ethanone, white solid, m.p.143.6 C,
yield 67.4 %
1H NMR (400 MHz, CDCl
) δ/ppm 7.76 (d, J = 7.4 Hz, 2H), 7.74 (d, J = 8.4 Hz, 2H), 7.62 (d, J =
.4 Hz, 1H), 7.59 (t, J = 7.4 Hz, 1H), 7.55 (s, 1H), 7.48 (t, J = 7.6 Hz, 2H), 7.43 (d, J = 8.4 Hz,
H), 6.92 (d, J = 8.4 Hz, 1H), 4.38 (s, 2H), 3.97 (s, 3H), 3.94 (s, 3H).
) δ/ppm 195.8, 192.1, 153.9, 149.2, 141.9, 137.5, 134.9, 132.4, 130.7,
29.9, 128.3, 128.3, 127.2, 123.5, 110.6, 110.1, 56.2, 56.1, 39.1.
3
8
2
13C NMR (101 MHz, CDCl
3
1
+
+
HR-MS (ESI, m/z) Calcd. for C23
H
20
O S 392.1082 (M ), found 392.1077 (M )
4
BPT-g, 1-(benzo[d][1,3]dioxol-5-yl)-2-((4-benzoylphenyl)thio)ethanone, white solid, m.p.157.4
oC, yield 69.0 %
1H NMR (400 MHz, CDCl
.47 (m, 3H), 7.44 (d, J = 8.4 Hz, 2H), 6.91 (d, J = 8.2 Hz, 1H), 6.10 (s, 2H), 4.37 (s, 2H).
13C NMR (101 MHz, CDCl
) δ/ppm 195.8, 191.5, 152.5, 148.4, 141.8, 137.5, 135.0, 132.4, 130.7,
29.9, 128.3, 127.3, 125.2, 108.4, 108.1, 102.1, 39.4.
HR-MS (ESI, m/z) Calcd. for C22 S 376.0769 (M ), found 377.0843[M+H] , 399.0650
3
) δ/ppm 7.78 (dd, J = 12.9, 7.8 Hz, 4H), 7.65 – 7.59 (m, 2H), 7.55 –
7
3
1
+
+
H
16
O
4
[
M+Na]+
907T-a, 2-methyl-2-morpholino-1-(4-((2-oxo-2-phenylethyl)thio)phenyl)propan-1-one, yellow oil,
yield 75.3 %

1H NMR (400 MHz, CDCl
) δ/ppm 8.49 (d, J = 8.5 Hz, 2H), 7.99 (d, J = 8.5 Hz, 2H), 7.62 (t, J =
.4 Hz, 1H), 7.50 (t, J = 7.7 Hz, 2H), 7.34 (d, J = 8.6 Hz, 2H), 4.41 (s, 2H), 3.68 (t, J = 4.0 Hz,
H), 2.56 (t, J = 4.0 Hz, 4H), 1.30 (s, 6H).
) δ/ppm 202.26, 193.4, 141.5, 135.2, 133.8, 133.1, 130.9, 128.8, 128.6,
26.7, 68.4, 67.3, 47.0, 39.4, 20.2.
HR-MS (ESI, m/z) Calcd. for C22
3
7
4
13C NMR (101 MHz, CDCl
3
1
+
+
H
25NO S 383.1555 (M ), found 384.1631[M+H]
3
907T-b, 2-methyl-2-morpholino-1-(4-((2-oxo-2-(o-tolyl)ethyl)thio)phenyl)propan-1-one, light
o
yellow solid, m.p. 106.0 C, yield 88.3 %.
1H NMR (400 MHz, CDCl
) δ/ppm 8.48 (d, J = 8.6 Hz, 2H), 7.68 (d, J = 7.9 Hz, 1H), 7.42 (t, J =
3
7.6 Hz, 1H), 7.31 (d, J = 8.6 Hz, 2H), 7.28 (t, J = 7.6 Hz, 1H), 7.27(s, 1H), 4.36 (s, 2H), 3.69 (t, J
=
4.0 Hz, 4H), 2.56 (t, J = 4.0 Hz, 4H), 2.43 (s, 3H), 1.30 (s, 6H).
) δ/ppm 202.2, 197.0, 141.5, 139.4, 135.8, 133.1, 132.3, 132.1, 130.9,
28.7, 126.6, 125.7, 68.4, 67.4, 47.0, 41.8, 21.3, 20.2.
HR-MS (ESI, m/z) Calcd. for C23 S 397.1712 (M ), found 398.1787([M+H] )
07T-c, 1-(4-((2-(4-hydroxyphenyl)-2-oxoethyl)thio)phenyl)-2-methyl-2-morpholinopropan-1-one,
13C NMR (101 MHz, CDCl
3
1
+
+
H
27NO
3
9
o
yellow solid, m.p. 200.4 C, yield 70.0 %
1H NMR (400 MHz, DMSO) δ/ppm 10.52 (s, 1H), 8.41 (d, J = 8.5 Hz, 2H), 7.96 (d, J = 8.7 Hz,
2
1
H), 7.39 (d, J = 8.6 Hz, 2H), 6.88 (d, J = 8.7 Hz, 2H), 4.75 (s, 2H), 3.57 (m, 4H), 2.46 (m, 4H),
.22 (s, 6H).
13C NMR (101 MHz, DMSO) δ/ppm 202.07, 192.4, 163.0, 143.1, 132.2, 131.7, 130.8, 127.2,
26.0, 115.7, 68.2, 67.0, 47.1, 38.9, 20.3.
HR-MS (ESI, m/z) Calcd. for C22
07T-d, 1-(4-((1-(4-hydroxyphenyl)-1-oxopropan-2-yl)thio)phenyl)-2-methyl-2-morpholino-
1
+
+
H
25NO S 399.1504 (M ), found 400.1579([M+H] )
4
9
o
propan-1-one, white solid, m.p.155.7-156.4 C yield 64.5 %
1H NMR (400 MHz, DMSO) δ/ppm 10.50 (s, 1H), 8.39 (d, J = 8.6 Hz, 2H), 7.93 (d, J = 8.8 Hz,
2
4
H), 7.44 (d, J = 8.6 Hz, 2H), 6.85 (d, J = 8.8 Hz, 2H), 5.31 (q, J = 6.7 Hz, 1H), 3.57 (t, J = 4.0 Hz,
H), 2.45 (t, J = 4.0 Hz, 4H), 1.49 (d, J = 6.8 Hz, 3H), 1.22 (s, 6H).
13C NMR (101 MHz, DMSO) δ/ppm 201.8, 194.9, 162.4, 139.9, 133.0, 131.3, 130.3, 128.5, 126.1,
15.3, 67.7, 66.4, 46.6, 44.2, 19.8, 17.8.
HR-MS (ESI, m/z) Calcd. for C23
1
+
+
H
27NO S 413.1661 (M ), found 414.1733 [M+H] , 436.1557
4
[
M+Na]+
907T-e, 1-(4-((2-(4-hydroxy-3-methoxyphenyl)-2-oxoethyl)thio)phenyl)-2-methyl-2- morpholino-
o
propan-1-one, light yellow solid, m.p. 157.8 C yield 55.7 %
1H NMR (400 MHz, DMSO) δ/ppm 10.16 (s, 1H), 8.40 (d, J = 8.6 Hz, 2H), 7.65 (d, J = 8.3 Hz,
1H), 7.53 (s, 1H), 7.40 (d, J = 8.6 Hz, 2H), 6.90 (d, J = 8.3 Hz, 1H), 4.76 (s, 2H), 3.84 (s, 3H),

3
.58 (t, J = 4.0 Hz, 4H), 2.45 (t, J = 4.0 Hz, 4H), 1.22 (s, 6H).
13C NMR (101 MHz, DMSO) δ/ppm 201.5, 192.0, 152.2, 147.5, 142.6, 131.8, 130.3, 127.0, 125.6,
23.8, 114.9, 111.7, 67.7, 66.5, 55.6, 46.6, 38.4, 19.8.
HR-MS (ESI, m/z) Calcd. for C23 S 429.1610 (M ), found 430.2000 ([M+H] )
07T-f, 1-(4-((2-(3, 4-dimethoxyphenyl)-2- oxoethyl)thio)phenyl)-2-methyl-2-morpholino-
1
+
+
H
27NO
5
9
o
propan-1-one, orange solid, m.p.105.5 C, yield 91.8 %
1H NMR (400 MHz, CDCl
) δ/ppm 8.49 (d, J = 8.5 Hz, 2H), 7.63 (d, J = 8.4Hz, 1H), 7.54 (s, 1H),
.35 (d, J = 8.6 Hz, 2H), 6.91 (d, J = 8.4 Hz, 1H), 4.38 (s, 2H), 3.96 (s, 3H), 3.93 (s, 3H), 3.69 (t, J
3
7
=
4.0 Hz, 4H), 2.56 (t, J = 4.0 Hz, 4H), 1.30 (s, 6H).
) δ/ppm 202.2, 192.1, 153.8, 149.2, 141.8, 132.9, 130.9, 128.2, 126.5,
23.5, 110.6, 110.1, 68.4, 67.3, 56.1, 56.0, 47.0, 38.9, 20.1.
13C NMR (101 MHz, CDCl
3
1
+
+
HR-MS (ESI, m/z) Calcd. for C24
H
29NO
5
S 443.1766 (M ), found 444.1842 ([M+H] )
907T-g,1-(4-((2-(benzo[d][1,3]dioxol-5-yl)-2-oxoethyl)thio)phenyl)-2-methyl-2-morpholino-prop
o
an-1-one, white solid, m.p.138.2 C yield 60.9 %
1H NMR (400 MHz, CDCl
) δ/ppm 8.49 (d, J = 8.5 Hz, 2H), 7.60 (dd, J = 8.2, 1.6 Hz, 1H), 7.46
d, J = 1.3 Hz, 1H), 7.33 (d, J = 8.5 Hz, 2H), 6.88 (d, J = 8.2 Hz, 1H), 6.07 (s, 2H), 4.34 (s, 2H),
3
(
3
.69 (t, J = 4.0 Hz, 4H), 2.56 (t, J = 4.0 Hz, 4H), 1.29 (d, J = 8.3 Hz, 6H).
) δ/ppm 202.2, 191.6, 152.4, 148.4, 141.6, 133.1, 130.9, 129.9, 126.6,
25.2, 108.3, 108.1, 102.1, 68.4, 67.4, 47.0, 39.2, 20.2.
13C NMR (101 MHz, CDCl
3
1
+
+
HR-MS (ESI, m/z) Calcd. for C23
.5 UV–vis spectral characterization of photoinitiators
The synthesized photoinitiators were dissolved in acetonitrile to prepare 1×10 mol/L stocking
H
25NO
3
S 427.1453 (M ), found 428.1528 ([M+H] )
2
-4
-5
-5
-5
-5
solution. The stocking solution was diluted to 8×10 mol/L, 6×10 mol/L, 4×10 mol/L, 2×10
mol/L samples, respectively. The UV-vis absorption spectra of the above samples were recorded
on the L5 spectrophotometer using acetonitrile as blank. According to the Lambert-Beer law,
equation (1), the standard curve was made and the molar absorption coefficient was calculated
from the curve.
A=εbc
(1)
where C is the molar concentration of initiator, A is the absorption, ε is the molar absorption
-1
-1
coefficient of initiator in acetonitrile (M cm ), and b is the optical path length, (here 1 cm).
.6 UV-Visible light photolysis experiment
The synthesized photoinitiators were dissolved in acetonitrile to prepare 2×10 mol/L
2
-5
solution. The solution was charged in a quartz cell and oxygen was removed by argon. The cell
was sealed and exposed under a medium pressure mercury arc lamp (80mW/cm with output light
2
2
intensity of 30mW/cm -40 mW/cm , measured with a UV radiometer capable of broad UV range
coverage) for different time intervals at 0s, 2s, 5s, 10s, 20s, 30s, 40s, 50s. The photolysis
fragments in solution were recorded on a HRESI-MS.

2.7 Photopolymerization kinetic experiments
The photopolymerization reactions of the multifunctional monomers (i.e., EOEOEA, HDDA
TPGDA, TMPTA) in the presence of synthesized photoinitiator upon exposure to the UV point
source OmniCure SERIES 1000 were monitored using the Nicolet 5700 real-time infrared
(RT-FTIR) spectrometer. The prepared photocurable formulation was transferred on an infrared
KBr salt plate to give a film thickness of 10 μm, and another KBr salt plate was used to cover the
above KBr plate for preventing oxygen inhibition, while the evaluation of the overcoming oxygen
inhibition for the synthesized photoinitiators were used only one KBr salt plate. The
polymerization experiments were carried out by using a Nicolet 5700 RT-FTIR spectrometer to
monitor the conversion percent of double bonds at intervals. The conversion of double bond
-
1
(
DC %) was calculated according to the equation (2) by measuring the peak area of 790-830 cm .
(
퐴 −퐴 )
0 푡
DC(%) =
represents the initial peak area before illumination, and A
double bond absorption peak at time t.
× 1ꢀꢀ …………… (2)
퐴0
Where: A
0
t
represents the area of the
Rates of polymerization were calculated according to the following formula (3) [27]:
푑(퐷퐶)
Rp =
…………… (3)
푑ꢁ
3
. Results and discussion
.1. Synthesis and characterization
The synthetic pathways to generate photoinitiator BPT and 907T are depicted in Scheme 1. The
3
thiolated BP-SH and 907-SH are the key intermediates for the desired product. There are many
methods to generate BPT-SH and 907-SH, sodium sulfide used as thiolated reagent is undoubtedly
the best method from chlorinated C-BP and C-907, respectively. The appearance of the signal at
3
1
.6 ppm was considered to be evidence of the introduction of mercapto group.
-Bromo-arylcarbonyl methyl intermediate (ACM-Br) was obtained by CuBr in refluxed mixture
2
of chloroform and ethyl acetate, and monitored by HPLC in 40-60% yield, while NBS and Br
active brominated reagents to give a complex.
2
are
As can be seen from 13 CNMR results, the presence of carbonyl peaks in the range of 190-195
ppm for BPT and 192-200 ppm for 907T in two singlets, respectively, indicates successful
attachment of the BP-S and 907-S moiety. In addition, the high resolution MS data are in good
agreement with the proposed structures.
3.2 Spectroscopic properties of photoinitiators
UV-vis absorption spectra of the prepared photoinitiators BPT and 907T were measured in
acetonitrile with BP and 907 as the references. The corresponding values for the maximum
absorption wavelength (λmax) and the molar absorption coefficients (εmax) at λmax in CH CN are
3
summarized in Table 1. The typical UV absorption spectra of BP, C-BP, BP-SH, BPT-b and 907,
C-907, 907-SH, 907T-b are shown in Figure 1. From Figure 1 and Table 1, the maximum
absorption of C-BP and C-907 are about 250-260 nm, while the mercapto BP-SH and 907-SH

exhibit significantly red-shifted absorption to about 290 nm. This result can be addressed to the
strong electron-donating ability of the thiophenyl group and the key important n-π*-type
transitions [28]. Similarly, the prepared photoinitiators BPT and 907T exhibit similar absorption
characteristics as C-BP and C-907 or BP-SH and 907-SH but with a markedly red shift in solvent.
The wavelengths for maximum absorption (λmax) are above 300 nm due to increased electron
donation via the sulfur atom and transannular conjugate effect via the benzoyl group. This would
certainly account for the increased photoinitiation activity during photocuring [15].
Table 1 UV absorption properties of synthesized photoinitiators in acetonitrile
Photoinitiator
λ
max (nm)
ε
max
.
Photoinitiator
λ
max (nm)
ε
max
.
(
L/mol cm)
(L/mol cm）
BP
251
257
294
308
308
287
283
307
310
311
1.520×104
2.1240×104
1.5390×104
1.0895×104
1.1561×104
1.0085×104
0.9240×104
0.9535×104
1.0140×104
1.0745×104
907
304
251
285
300
300
288
283
305
302
306
6.063×104
C-BP
C-907
1.2810×104
1.0810×104
1.4095×104
1.2415×104
1.0140×104
1.8160×104
0.7595×104
1.0170×104
0.5035×104
BP-SH
BPT-a
BPT-b
BPT-c
BPT-d
BPT-e
BPT-f
BPT-g
907-SH
907T-a
907T-b
907T-c
907T-d
907T-e
907T-f
907T-g
9
07-Cl
1
.6
b)
907-SH
907T-b
BP-Cl
BP-SH
BPT-b
BP
2.5
2.0
1.5
1.0
0.5
0.0
1.4
a)
907
1
.2
.0
1
0.8
0
.6
.4
0
0.2
.0
0.2
0
-
200
250
300
350
400
450
500
200 250 300 350 400 450 500
Wavelength(nm)
Wavelength(nm)
-4
Fig. 1 UV-vis absorption spectra of some BP and 907 derivatives in CH
BP derivatives; b): 907 derivatives
3
CN (1×10 mol/L), a):

Photochemical change of the photoinitiator upon irradiation is an important issue for their use in
photopolymerization processes. BPT and 907T were expected to undergo photolysis in the
absence of hydrogen donors because of the hydrogen-donating thiol group in the structures. For
this purpose, BPT-b and 907T-b were selected to study the UV-vis spectral changes of the
initiators on irradiation in the absence of co-initiator and is shown in Fig. 2. The rapidly decreased
absorbance maxima with time indicate the efficient photoreactivity of of BPT-b and 907T-b, which
show that BPT-b and 907T-b are sensitive to irradiatation and have potential to be used as one
component photoinitiator.
Unirradiated
5s
Unirradiated
5s
2.5
2.0
1.5
1.0
0.5
0.0
b)
a)
1.6
1
2
0s
0s
10s
20s
30s
30s
1
0
0
.2
.8
.4
0.0
200
250
300
350
400
450
200
250
300
350
400
450
Wavelength(nm)
Wavelength(nm)
-5
Fig. 2 UV-vis absorption spectral changes of BPT-b and 907T-b (2×10 mol/L) on irradiation
under nitrogen in CH CN, a): BPT-b; b): 907T-b
3
To study the photolysis mechanism of the novel photoinitiators, we used HPLC and HR-MS to
analyze the photolysis product after irradiation. As can be seen from Fig.3 and Table 2, the content
of BPT-b and 907T-b in CH CN were decreased immediately after irradiation and the photolysis
3
rate of BPT-b was faster than that of 907T-b. BPT-b was photolyzed almost completely within 20
seconds and 907T-b was photolyzed about 90% within 30 seconds. They were all photolyzed by
breaking of the C-S bond after irradiation to form the thiol radical which was combined to
generate the disulfide compound, which was confirmed by HR-MS with a molecular ion peak at
+
+
4
27.0820 (M+H) for BPT-b and 529.2179 (M+H) for 907T-b (Fig. 4 and Scheme 2).
Table 2 The content of initiators in
CH CN under irradiated for different
time
1
20
00
BPT-2
07T-2
3
1
9
8
6
4
2
0
0
0
0
0
Time
5s
BPT-2 (%) 907T-2 (%)
56.5075
29.1246
0.8112
0
68.8053
57.3383
46.6549
12.4582
1
2
3
0s
0s
0s
0
5
10
Irradiation time (s)
20
30
Fig. 3 The contents of initiators in CH
3
CN under
irradiation for different time

Scheme 2 Photolysis products of the BPT-b and 907T-b after irradiation
Fig. 4 HR-MS results for photolysis products of BPT-b and 907T-b
3.3 Photopolymerization activity study
To evaluate the photopolymerization efficiency of novel photoinitiators on acrylate curing,
TPGDA was used as monomer, and mixed with photoinitiator content of 1.0 wt % using real-time
FT-IR (RT-FTIR) in the absence of an additional hydrogen donor. Because BP could not initiate
polymerization of TPGDA in the absence of hydrogen donor, the thiol photoinitiator BP-SH and
907-SH were used as references. The conversion-time and rate-time plots for the polymerization
of samples cured under oxygen-free conditions are presented in Figure 5 and Figure 6.

BP-SH
BPT-a
BPT-b
BPT-c
BPT-d
BPT-e
BPT-f
BP-SH
BPT-a
BPT-b
BPT-c
BPT-d
BPT-e
BPT-f
1
6
80
60
40
20
0
12
8
BPT-g
BPT-g
4
0
-
4
0
100 200 300 400 500 600 700
Time（ s）
0
10
20
30
40
Time（ s）
Fig. 5 Conversion-time and conversion rate (Rp) - time for the photopolymerization of TPGDA
and different BPT initiators 1.0 wt % relative to monomer, light intensity 30 mW/cm2
9
9
9
07-SH
07T-a
07T-b
9
07-SH
1
00
16
12
8
907T-a
907T-b
907T-c
907T-d
8
6
4
2
0
0
0
0
0
907T-c
907T-d
907T-e
907T-f
907T-g
9
07T-e
907T-f
07T-g
9
4
0
-
4
0
100 200 300 400 500 600 700
Time（ s）
0
10
20
30
40
Time（ s）
Fig.6 Conversion-time and conversion rate (Rp) - time for the photopolymerization of TPGDA
and different 907T initiators 1.0 wt % relative to monomer, light intensity 30 mW/cm2
As illustrated in Fig. 5 and Fig. 6, some BPT and 907T initiators are found to efficiently initiate
photopolymerization under oxygen-free conditions and in the absence of an additional hydrogen
donor. Although both types of initiators are effective in initiating the polymerization, 907T seems
to be more photoactive in the curing process. The conversion of double bonds reaches its
maximum within 10 s with a rapid first stage, but followed by a slow stage. This trend is also
reflected in the RT-FTIR data from the Rp value. These results can be attributed to following
reasons. First, the red-shifted maximum absorption would certainly account for its higher
photoefficiency. Second, more radicals may be generated from the photolysis reaction of the C-S
bond. Lastly, more radicals yielded could cause the monomers to polymerize rapidly [29].
Subsequently, the conversion did not increase due to the decrease in the concentration of
photoinitiator, rapid increase of viscosity and a significant reduction in the mobility of large
molecules because of gel effect and cage effect.
The conversion of TPGDA polymerization initiated by 907T-a, 907T-b and 907T-f are higher
than 70%, but they are lower than that of 907-SH in the absence of oxygen. Notably, the
conversion and rate of TPGDA polymerization initiated by BPT-a, BPT-b, BPT-c, BPT-d and
BPT-e are higher than those initiated by BP-SH. It is presumed that BP series are type II
photoinitiators and need co-initiators, and BP-SH needs one molecule as an initiator and one
molecule as a co-initiator to provide active hydrogen to form active radicals to initiate
polymerization, while other BP initiators can directly generate thiol radical to initiate

polymerization, and do not need the co-initiator.
We selected some photoinitiators with good photopolymerization activity under oxygen-free
conditions to evaluate the oxygen scavenging effect. As can be seen from Fig. 7, BPT-a, BPT-b,
BPT-c, BPT-d, BPT-e possess moderate oxygen scavenging activity, while 907T-b and 907T-f have
good ability to overcome oxygen inhibition. Usually, under air condition, the rate of
polymerization of TPGDA is very low and the final conversion hardly reaches 30% [12], however,
907T-b can reach about 50%. This result can be attributed to the photolysis reaction of the C-S
bond to afford thiol radical to consume some oxygen, which reduced the oxygen inhibition. If the
concentration of the photoinitiators increased, the overcoming oxygen inhibition of BPT-b and
907T-b show up more significantly. According to Fig. 8, when the concentrations of
photoinitiators are gradually increased from 1.0% to 4.0wt% relative to monomer, the conversions
of double bond vary from 40-60% for 907T-b and 30-70% for BPT-b under air, respectively.
BP-SH
35
30
25
20
15
10
5
0
BPT-a
BPT-b
BPT-c
BPT-d
BPT-e
907-SH
907T-a
907T-b
07T-f
50
a)
b)
9
40
30
20
10
0
0
100 200 300 400 500 600 700
0
100 200 300 400 500 600 700
Time（ s）
Time（ s）
Fig. 7 Conversion-time for the photopolymerization of TPGDA for different initiators at 1.0 wt%
2
under air, light intensity 30 mW/cm , a): BP-SH and BPT derivatives; b): 907-SH and 907T
derivative
1
2%
3%
%
1%
2%
3%
4%
80
60
40
20
0
a)
b)
4%
60
4
0
0
0
2
0
100 200 300 400 500 600 700
0
100 200 300 400 500 600 700
Time（ s）
Time（ s）
Fig. 8 Conversion-time for the photopolymerization of TPGDA at different concentrations of
2
initiators under air, light intensity 30 mW/cm , a): BPT-b; b): 907T-b
Under oxygen-free condition, the concentration dependence was more significant, and Fig. 9
shows the conversion variation with the increasing content of prepared initiators. It can be seen
that when the concentration of BPT-b is gradually increased from 1.0% to 4.0 wt% relative to
monomer, the final double bond conversion is increased. However, the concentration of initiator

had little effect on the final conversion rate for 907T-b and the final conversion rate reached a
higher percentage at 1.0 wt%.
1
2
3%
%
%
1
2%
%
a)
b)
100
3
4
%
%
100
4
%
80
60
40
20
0
8
0
0
6
40
20
0
0
100 200 300 400 500 600 700
Time（ s）
0
100 200 300 400 500 600 700
Time（ s）
Fig. 9 Photopolymerization efficiency of different concentration of BPT-b and 907T-b for
2
TPGDA under oxygen-free conditions, light intensity 30 mW/cm , a): BPT-b; b): 907T-b
Fig. 10 shows the conversion variation with the light intensity increasing. According to the Fig.
10, the maximum conversion rate rises from 50% to 80% with increased light intensity from 30
2
2
mW/cm to 60 mW/cm for BPT-b. It is easy to explain in that the higher light intensity can form
more free radicals to induce the photopolymerization. However, the light intensity has little effect
on the maximum conversion rate for 907T-b. This may be that 907T-b can generate more radicals
at lower light intensity and it will be a good initiator candidate with good photo-activity and good
ability to overcome oxygen inhibition, which can be used as type I and type II initiators.
1
00
a)
60
100
b)
60
45
30
45
8
6
4
2
0
0
0
0
0
30
80
60
40
20
0
-20
0
100 200 300 400 500 600 700
Time（ s）
0
100 200 300 400 500 600 700
Time（ s）
Fig. 10 Photopolymerization of TPGDA in different light intensity under oxygen free, BPT-b or
907T-b 1.0 wt%, a): BPT-b; b): 907T-b
Monomers not only play a role in dissolving and diluting oligomers and adjusting the viscosity
of the curing system, but also participate in the curing process. They also influence the curing rate
and the properties of the curing film. We evaluated the photopolymerization of different
monomers with the synthesized initiator BPT-b or 907T-b. As can be seen from Fig.11, it is clear
that the final double bond conversion of TPGDA initiated by 907T-b is much higher than HDDA
and TMPTA, and same as EOEOEA. Moreover, the final double bond conversion of TPGDA and
EOEOEA initiated by BPT-b is much higher than HDDA and TMPTA. Although TPGDA and
HDDA have the same functionality, TPGDA has better flexibility and the polymerization rate is

faster than HDDA. TMPTA is a trifunctional monomer, and its final conversion rate is
significantly lower than that of the difunctional methacrylate monomer because the chain growth
reaction of the polymer is controlled by diffusion which is decreasing with the rapid increase of
viscosity because of the gel effect [30-33].
EOEOEA
EOEOEA
HDDA
TPGDA
TMPTA
100
80
HDDA
TPGDA
TMPTA
100
b)
a)
80
60
40
20
0
60
40
20
0
0
100 200 300 400 500 600 700
Time（ s）
0
100 200 300 400 500 600 700
Time（ s）
Fig. 11Comparison of photopolymerization of monomer EOEOEA, TMPTA, HDDA and TPGDA
2
under oxygen free, BPT-b or BPT-b 1.0 wt%, light intensity 30 mW/cm , a): BPT-b; b): 907T-b
4. Conclusion
Here, two types of novel photoinitiators containing phototrigger were reported. The maximum
absorption wavelength of the prepared initiators are more than 300 nm with remarkable redshift
compared to BP (benzophenone) and Irgacure 907 (2-methyl-1-(4-methylsulfanylphenyl)-2-
morpholin-4-ylpropan-1-one). The UV curing behavior analysis indicated that the double
conversion rates were up to 50% under air conditions when the light intensity was 30 mW/cm2,
which is higher than for traditional photoinitiators. When the concentrations of photoinitiators are
increased to 4.0wt%, the ability to overcome oxygen inhibition can be further improved by the
conversion of double bond to about 60-70% in the absence of hydrogen donor, so the prepared
initiators can be used as one-component initiator candidates.
Conflict of interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgements
The authors thank Prof. Jun Nie and Prof. Xiaoqun Zhu from Beijing University of Chemical
Technology for the support of the real-time infrared (RT-FTIR) spectrometer for free and the
fruitful discussions with the group members on initiating systems. The authors are very grateful
for financial support from Tianjin Jiuri New Materials Company Limited (No. 2018120024001216,
No. 2017120024000913 and No. 2016120024001223).

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