Synthesis and characterization of triarylsulfonium salts as novel cationic photoinitiators for UV-photopolymerization

Article abstract of DOI:10.1007/s11164-017-3009-1

Using diaryliodonium salts as the arylating reagent and copper as a catalyst, a series of triarylsulfonium salts bearing benzoyl or benzoheterocyclic moieties were synthesized and characterized by ¹H NMR, ¹³C NMR and high-resolution mass spectrometry. The photoinitiating behavior of the triarylsulfonium salts on photopolymerization of epoxy acrylate resin (UV6104) with hexane-1,6-diyldiacrylate (HDDA) as an activated monomer was investigated. The results showed that these triarylsulfonium salts could be utilized as cationic photoinitiators in the UV-curing process. A polymerization photoinitiation mechanism has been proposed.

Full text of DOI:10.1007/s11164-017-3009-1

Res Chem Intermed
DOI 10.1007/s11164-017-3009-1
Synthesis and characterization of triarylsulfonium
salts as novel cationic photoinitiators for
UV-photopolymerization
Bianxiang Zhang1 Tingting Li1 Yongqiang Kang1
•
•
Received: 5 April 2017 / Accepted: 12 June 2017
Ó Springer Science+Business Media B.V. 2017
Abstract Using diaryliodonium salts as the arylating reagent and copper as a
catalyst, a series of triarylsulfonium salts bearing benzoyl or benzoheterocyclic
1
moieties were synthesized and characterized by H NMR, 13C NMR and high-
resolution mass spectrometry. The photoinitiating behavior of the triarylsulfonium
salts on photopolymerization of epoxy acrylate resin (UV6104) with hexane-1,6-
diyldiacrylate (HDDA) as an activated monomer was investigated. The results
showed that these triarylsulfonium salts could be utilized as cationic photoinitiators
in the UV-curing process. A polymerization photoinitiation mechanism has been
proposed.
Keywords UV-curing Á Cationic photoinitiators Á Synthesis Á Triarylsulfonium
salt
Introduction
The ultraviolet (UV)-curing technology, owing to its remarkable and unique
properties such as slow energy consuming, high productivity, and low solvent
emissions, has been widely used in coatings, varnishes, adhesives, printing inks and
electronics [1, 2]. In normal conditions, a photopolymerization system is comprised
of an oligomer or a reactive monomer, a photoinitiator and optional additives. The
photoinitiator plays an important role in photo-curing and photopolymerization
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3009-1)
contains supplementary material, which is available to authorized users.
&
Bianxiang Zhang
zbxthh@sxu.edu.cn
1
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
123

B. Zhang et al.
since it determines the photopolymerization speed and affects the polymer
properties such as odor, yellowing and migration. Photoinitiators are divided into
a free radical polymerization photoinitiator and a ationic polymerization photoini-
tiator based on the reaction mechanism [3, 4]. Benzophenone (BP) and 1-hydrox-
ycyclohexyl phenyl ketone (Irgacure 184) are the most commonly used free radical
photoinitiators owing to their low cost, high photoinitiation efficiency, and good
solubility in the polymerization formulation. Compared with free radical polymer-
ization initiation systems, a cationic photoinitiating system has many advantages
such as rapid polymerization without oxygen inhibition, low shrinkage upon curing,
strong adhesive force, high hardness, and good performance of abrasion resistance,
etc. [5–7]. However, some limitations exist such as limited solubility of cationic
photoinitiator in monomers and resins, long curing time and high cost. Thus, there is
a great demand for the advantages of radical and cationic photoinitiators. Crivello
et al. [8–11] introduced a phenacyl substituent into the triarylsulfonium salt, a kind
of efficient cationic photoinitiator [12], in order to improve its compatibility with
resins. Xie et al. [13] reported the preparation of diaryliodonium salts which bear a
benzophenone group as free radical–cationic hybrid photoinitiators. Peter Pappas
et al. [14] also reported the synthesis and photoactivity of anthracene-bound
sulfonium salts, which efficiently absorb radiation between 330 and 410 nm due to
the strong absorptivity of anthracene in this region. In addition, several ferrocenium
salts have been synthesized and evaluated as novel cationic photoinitiators [15–18].
These compounds, however, usually involve complicated synthetic protocols
characterized by low yield. Accordingly, the development of easily prepared
photoinitiators with better properties such as low odor, low migration rate, low
benzene release and good solubility in the formulation continues to be a growing
area of interest. In the present work, a novel class of triarylsulfonium salts which
bear benzophenone or aromatic heterocyclic moieties have been prepared and
compared as cationic photoinitiators used in the UV-curing system.
Experimental
Materials
All starting materials for the preparation of triarylsulfonium salts were reagent grade
and used without further purification. The monomer 1,6-hexanediol diacrylate
(HDDA) was obtained from Specialty Chemicals (Jiangsu, China). The epoxy
acrylate resin (UV6104) was obtained from Jiangsusanmu Specialty Chemicals
(Jiangsu, China). 1-hydroxycyclohexyl phenyl ketone (Irgacure 184) was purchased
from Beijing Specialty Technology (Beijing, China). Scheme 1 depicts the chemical
structures and abbreviations of the materials used in the polymerization.
Instrumentation
1H NMR and 13C NMR spectra were recorded in CDCl3 and DMSO-d6 with a
Bruker DRX 500 MHz and 600 MHz spectrometer. High-resolution mass spectra
123

Synthesis and characterization of triarylsulfonium salts…
Scheme 1 Chemical structures and abbreviations of the materials used in the polymerization
were obtained on a Bruker micrOTOF-Q III mass spectrometer. UV–vis absorption
spectra were recorded in dichloromethane on a Hitachi UH-5300 UV spectropho-
tometer (Hitachi High-Technologies, Tokyo, Japan). A portable pencil scratch
hardness tester (QHQ-A) was used, as were a manual coating machine (QTG) and a
portable UV-Curing machine (LT4003).
Synthesis
S-aryldibenzo[b,d]thiophenium salts 1
To a single-neck flask were added dibenzo[b,d]thiophene (1 mol), diphenyliodo-
nium triflate (1.2 mol) and Cu(OAc)2 (10 mol%) as catalyst. The reaction mixture
was stirred for 30 min at 130 °C. After cooling to room temperature, the precipitate
was filtered off. The crude product was purified by silica gel chromatography using
dichloromethane as the eluent to afford S-aryldibenzo[b,d]thiophenium salts 1.
1
S-phenyldibenzo[b,d]thiophenium triflate 1a White crystal, mp: 197–198 °C. H
NMR (CDCl3) d: 7.51–7.67 (m, ArH, 4H), 7.61–7.78 (m, ArH, 3H), 7.79–7.84 (s,
ArH, 2H), 8.30–8.60 (m, ArH, 4H). HRMS (ESI) clacd for C18H13S (M? OTf)?:
261.0738, found: 261.0732.
S-(4-tolyl)dibenzo[b,d]thiophenium triflate 1b Milky white crystal, mp:
1
179–181 °C. H NMR (CDCl3, 300 MHz) d: 2.32 (s, CH3, 3H), 7.37–7.40 (m,
ArH, 2H), 7.45–7.48 (m, ArH, 2H), 7.71–7.96 (m, ArH, 4H), 8.30–8.33 (m, ArH,
2H), 8.48–8.51 (m, ArH, 2H). HRMS (ESI) clacd for C19H15S (M? OTf)?:
275.0894, found: 275.0889.
S-(4-iodophenyl)dibenzo[b,d]thiophenium triflate 1c White crystal, mp:
1
253–254 °C. H NMR (CDCl3) d: 7.37–7.40 (s, ArH, 2H), 7.76–7.81 (m, ArH,
2H), 7.96–8.01 (m, ArH, 4H), 8.39–8.42 (m, ArH, 2H), 8.54–8.56 (m, ArH, 2H).
HRMS (ESI) clacd for C18H12IS (M? OTf)?: 386.9704, found: 386.9699.
S-arylsulfonium salts 2
Phenyl(4-(p-tolylthio)phenyl)methanone (1 mol), diphenyliodonium triflate
(1.2 mol) and 1,1,2,2-tetrachloroethane (2 mL) were placed in a 50-mL round-
bottom flask equipped with a mechanical stirrer, thermometer and reflux condenser.
After thoroughly mixing, the combination of CuI (5 mmol%) and Cu (5 mmol%)
was added as catalyst. The reaction mixture was stirred for 2 h at 110 °C. The
123

B. Zhang et al.
resulting mixture was cooled, filtered and then excess solvent was evaporated under
reduced pressure. The crude product was purified by silica gel chromatography
(dichloromethane: ethyl acetate = 7:3) to afford S-arylsulfonium salts 2.
(4-Benzoylphenyl)(phenyl)(p-tolyl)sulfonium triflate 2a Light yellow viscous liq-
uid. 1H NMR (CDCl3) d: 2.53–2.58 (s, CH3, 3H), 7.53–7.62 (m, ArH, 4H),
7.67–7.73 (m, ArH, 3H), 7.79–7.87 (m, ArH, 9H), 8.04–8.07 (m, ArH, 2H). 13C
NMR (DMSO) d: 20.92, 119.98, 124.17, 128.21, 128.46, 129.44, 130.53, 130.72,
131.04, 131.27, 131.76, 133.05, 134.18, 135.28, 141.63, 145.77, 193.57. HRMS
(ESI) clacd for C26H21OS (M? OTf)?: 381.1347, found: 381.1303.
(4-Benzoylphenyl)diptolylsulfonium triflate 2b Light yellow viscous liquid. 1H
NMR (CDCl3) d: 2.44–2.50 (s, CH3, 6H), 7.56–7.63 (m, ArH, 6H), 7.70–7.72 (m,
ArH, 1H), 7.78–7.80 (m, ArH, 6H), 7.88–7.91 (m, ArH, 2H), 8.00–8.03 (m, ArH,
2H). 13C NMR (DMSO) d: 20.05, 120.46, 127.84, 128.69, 128.95, 129.88, 130.49,
131.00, 132.68, 134.77, 140.57, 144.65, 193.51. HRMS (ESI) clacd for C27H23OS
(M? OTf)?: 395.1463, found: 395.1469.
S-arylsulfonium salts 3
Compounds 3 were synthesized following the same procedure as compounds 2,
except that the reaction mixture was refluxed for 3 h.
S-diphenyl-(1-methyl-2-phenylindole) sulfonium triflate 3a White crystal, mp:
1
204–205 °C. H NMR (CDCl3) d: 3.83 (s, N-CH3, 3H), 6.96–7.19 (m, ArH, 4H),
7.22–7.44 (m, ArH, 5H), 7.45–7.64 (m, ArH, 5H), 7.67–7.73 (m, ArH, 5H). HRMS
(ESI) clacd for C27H22 NS (M? OTf)?: 392.1467, found: 392.1463.
S-phenyl-(4-tolyl)-(1-methyl-2-phenylindole) sulfonium triflate 3b White crystal,
1
mp: 174–176 °C. H NMR (CDCl3, 600 MHz): 2.47 (s, CH3, 3H), 3.84 (s, CH3,
3H), 7.00–7.22 (m, ArH, 4H), 7.23–7.45 (m, ArH, 4H), 7.46–7.51 (m, ArH, 5H),
7.55–7.65 (m, ArH, 5H). HRMS (ESI) calcd for C28H24NS (M? OTf)?: 406.1624,
found: 406.1627.
S-phenyl-(4-iodophenyl)-(1-methyl-2-phenylindole) sulfonium triflate 3c White
crystal, mp: 199–202 °C. 1H NMR (CDCl3, 600 MHz): 3.84 (s, CH3, 3H),
7.19–7.22 (m, ArH, 1H), 7.45–7.47(m, ArH, 3H), 7.60–7.61 (m, ArH, 4H),
7.63–7.68 (m, ArH, 10H); HRMS (ESI) calcd for C27H21NSI (M? OTf)?:
518.0434, found: 518.0437.
Coating preparation and film property test
Polymeric composition for use as coating material comprises 15 wt% monomer
(HDDA), 80 wt% epoxy acrylate resin (UV6104), 2 wt% triethanolamine and 3
wt% photoinitiator. The monomer and epoxy acrylate resin (UV6104) were mixed
at room temperature under stirring and then triethanolamine and photoinitiator
123

Synthesis and characterization of triarylsulfonium salts…
were added dropwise. The resulting mixture was further heated for 30 min to
obtain a transparent coating material. The liquid formulation was coated onto a
clean glass using a coating machine and the coating thickness was 100 lm. Then,
the sample was exposed to UV radiation with a 400-W mercury lamp for a certain
time. The pencil hardness and adhesion of the UV-cured films were measured by
ASTM D 3363 and ASTM D 3359 test methods currently accepted by industry,
respectively.
Results and discussion
Synthesis and characterization of triarylsulfonium salt
Our previous work demonstrated that diaryliodonium salts are excellent arylating
reagents for the synthesis of S-phenylbenzothiophenium triflates [19–21]. In the
present work, three kinds of S-arylated products were prepared as shown in
Scheme 2. The reaction of dibenzo[b,d]thiophene with diaryliodonium salts were
performed under solvent-free conditions using Cu(OAc)2 as catalyst. Under mild
conditions, the target products were obtained in good yields. Synthesis of
triarylsulfonium salts 2 and 3 were attempted under analogous conditions but
failed. Under the optimized reaction conditions, compounds 2 and 3 were prepared
in tetrachloroethane in good yields using CuI and Cu as a cocatalyst. The
synthesized triarylsulfonium salts exhibited good solubility in common organic
solvents, such as ethanol, acetonitrile, acetone, ethyl acetate and dichloromethane,
suggesting that as new cationic photoinitiators they possess good compatibility with
other components of the polymerization system.
R
S
-C H 82%
1a: R=
1b: R=
1c: R=
6
5
-
S
Cu(OAc)2
130
OTf
R2I+OTf-
4-(Me)C H 77%
6 4
+
4-(I)C6H4 81%
1
-
OTf
Ph
O
R
Ph
O
CuI,Cu
110
-C H 76%
2a: R=
2b: R=
R I+OTf-
6
5
S
+
2
S
Tol
4-(Me)C H 75%
6
4
Tol
2
R
S
-
S
OTf
Ph
-C H 62 %
3a: R=
3b: R=
3c: R=
6
5
Ph
CuI,Cu
R2I+OTf-
+
4-(Me)C H 61%
Ph
6 4
N
110
Ph
N
4-(I)C H 63%
6
4
3
Scheme 2 Synthesis of the triarylsulfonium salts
123

B. Zhang et al.
1.0
0.8
0.6
0.4
0.2
0.0
1a
2a
3a
BP
Irgacure 184
200
225
250
275
300
325
350
Wavelenth(nm)
Fig. 1 Absorption spectra of 1a, 2a, 3a, BP and Irgacure 184
UV–Vis absorption studies
Figure 1 depicts the UV absorption spectra of 1a, 2a and 3a in dichloromethane. For
comparison, the absorption spectra of BP and Irgacure 184 are also presented. These
spectra were measured at an identical molar concentration (1 9 10-4 M), so that a
direct comparison of absorbance can be made. As can be seen from Fig. 1, the
absorption spectrum of the triarylsulfonium salts exhibits strong absorption in the
200–300 nm region, corresponding to p–p* transitions of the conjugated benzene ring.
The absorption bands observed at 316 and 294 nm for 1a and 3a, respectively, could be
attributed to n–p* transition of the thiophene and indole moieties. The maximum
absorbances of 1a and 3a are red-shifted with respect to the absorption (kmax260 nm) of
the triarylsulfonium salts previously reported [22]. In particular, the extended
conjugation of 3a due to the presence of the indole group induced a large shift of its
main absorption band toward the near-UV wavelength range, which makes it more
suitable for UV-curing with a UV high-pressure mercury lamp as a radiation source.
Photopolymerization studies
The synthesized triarylsulfonium salts were evaluated as photoinitiators for cationic
polymerization of epoxy acrylate resin (UV6104) and activated monomer hexane-
1,6-diyldiacrylate (HDDA). The triarylsulfonium salt was mixed with certain
amounts of epoxy acrylate resin (UV6104), monomer and additive to produce a
transparent coating. An experiment on UV-curing of the surface coating was carried
out and the properties of the cured film were tested.
As summarized in Table 1, compared with BP and Irgacure 184, the UV-cured
coatings obtained by the photoinitiation with 1a and 1b have good hardness possibly
due to the dibenzothiophene structure of these salts. Bearing the benzoyl moiety
with high photoinitiation efficiency, 2a and 2b provide excellent performance in
123

Synthesis and characterization of triarylsulfonium salts…
Table 1 Properties of UV-curable coating
Entry Photoinitiator Gloss
Curing timea (s) Adhesionb Pencil hardnessc
1
2
3
4
5
6
7
8
9
10
a
1a
1b
1c
Smooth and transparent 10
Smooth and transparent 11
Smooth and transparent 12
1
1
3
1
1
2
2
3
3
2
4H
4H
2H
4H
4H
5H
5H
2H
3H
2H
2a
2b
3a
3b
3c
Smooth and transparent
Smooth and transparent
5
5
Smooth and transparent 10
Smooth and transparent 10
Edge of coking
10
BP
184
Smooth and transparent 10
Smooth and transparent 11
The curing time of the UV-curing coating was measured by the finger contact method
b
The determination of curing film adhesion by the grid method, levels are classified into 0–5, with 0 the
best, and level 5 the worst
c
The determination of curing film hardness by using the pencil hardness
UV-curable coatings. The curing time was much shorter (5 s) and the pencil
hardness was greater than 4H (Table 1, entries 4 and 5). 3a and 3b also have good
pencil hardness due to the presence of indole ring (Table 1, entries 6 and 7). By
contrast, 1c and 3c exhibit poor pencil hardness possibly due to the iodo substituent
on the benzene ring, which may cause poor solubility of triarylsulfonium salts in the
resin. All the UV-cured films are glossy and transparent except when using 3c as a
photoinitiator. Overall, the synthesized triarylsulfonium salts could be utilized as
cationic photoinitiators for UV photopolymerization.
Photoinitiation mechanism
The photopolymerization mechanism was preliminarily discussed for the case of
photoinitiator 2a (Scheme 3). First, 2a absorbs the photons and thereby goes to the
*
hv
O
C
S
S
+
Tol
S
Tol
Tol
C
O
C
O
(II)
HR
(I)
Tol
S
H
Tol
S
+
H
Scheme 3 Possible mechanism of photo-initiated polymerization
123

B. Zhang et al.
excited state (I). Possessing benzophenone and triarylsulfonium salt groups in its
structure, the excited state (I) of 2a could undergo photolysis in two ways: the
homolytic cleavage of the C–S bond to form cationic radical (II) [23–26] since the
bond energy of the C–S bond is low, and the electron transfer in the molecule to
produce the benzoyl radical [27–30]. The cationic radical (II) extracts a hydrogen
ion to produce a protonic acid and a free radical. The protonic acid is further
deprotonated and the acidic proton generated ultimately initiates the oligomer
polymerization. Thus, 2a combines the advantages of radical and cationic
photoinitiators. i.e., rapid polymerization without oxygen inhibition and good
curing and post-curing ability.
Conclusions
Three kinds of triarylsulfonium salts have been synthesized and their activity as
cationic photoinitiators was studied. These triarylsulfonium salts could be utilized
as cationic photoinitiators in the UV-curing process. The triarylsulfonium salts 3a
and 3b showed strong absorptions at wavelengths above 294 nm and excellent
photoinitiating activity. 2a and 2b can photoinitiate free radical and cationic
polymerizations.
Acknowledgements We thank the Shanxi International Science and Technology Cooperation Program
(No. 201603D421039) for financial support. We also thank the Large Instrument Center of Shanxi
University.
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