Development of photobase generator with benzoin derivatives and its application to photosensitive materials

Article abstract of DOI:10.1246/cl.131148

We developed novel photobase generators (PBGs) that generate radicals and aliphatic amines simultaneously. We demonstrated that the PBGs induced radical polymerization and hydrolytic polycondensation. A film of poly[(methacryloxypropyl) silsesquioxane] (MA-PS) was cured in the presence of a PBG upon UV-light irradiation at 254 nm. This result showed that radical polymerization of methacryloyl groups of MA-PS with the photogenerated radicals and the crosslinking reactions based on the base-catalyzed condensation reactions of alkoxysilyl units of MA-PS proceeded. Furthermore, pencil hardness showed HB at 1.0 J cm2 and 4H at 21.6 J cm2 without heat treatment.

Full text of DOI:10.1246/cl.131148

CL-131148
Received: December 9, 2013 | Accepted: January 7, 2014 | Web Released: January 16, 2014
Development of Photobase Generator with Benzoin Derivatives
and Its Application to Photosensitive Materials
Nobuhiro Ishikawa, Koji Arimitsu,* Takahiro Gunji, and Yoshimoto Abe
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278-8510
(E-mail: arimitsu@rs.noda.tus.ac.jp)
We developed novel photobase generators (PBGs) that
Photogenerated aliphatic amines cause addition reactions
toward epoxy groups and sol-gel reactions of metal alkoxides.
In the meantime, hybrid UV-curing system is expected to be
a highly sensitive curing system to afford highly crosslinked
materials.13 Although hybrid UV-curing systems based on
radical and cationic polymerization were reported,14 hybrid
curing based on radical and anionic has not been reported.
If PBGs that generate amine and radical simultaneously are
available, selectivity of material designs should permit the use of
acrylate monomers or oligomers.
In this paper, we report a novel PBG that generates radical
and aliphatic amines simultaneously by photoirradiation
(Scheme 1). PBG 1 has benzoin structure, which is generally
known as the photo radical initiator with high quantum yield. It
is well known that the benzoin derivative undergoes the Norrish
Type I cleavage and generates radicals with high quantum yield
(Φ = 0.5) upon UV irradiation.15 PBG 1 generates radicals and
aliphatic amine by a similar reaction mechanism. Therefore,
PBG 1 is expected to afford a high quantum yield. Because
radicals and amine are produced simultaneously, the use of PBG
1 undergoes hybrid-curing based on radical polymerization and
base-catalyzed reactions such as addition reactions toward epoxy
groups and hydrolytic polycondensation of metal alkoxide. In
addition, we report the consequence on photohybrid-curing by
using a radical and an anion.
generate radicals and aliphatic amines simultaneously. We
demonstrated that the PBGs induced radical polymerization
and hydrolytic polycondensation. A film of poly[(methacryloxy-
propyl)silsesquioxane] (MA-PS) was cured in the presence of a
PBG upon UV-light irradiation at 254 nm. This result showed
that radical polymerization of methacryloyl groups of MA-PS
with the photogenerated radicals and the crosslinking reactions
based on the base-catalyzed condensation reactions of alkoxy-
silyl units of MA-PS proceeded. Furthermore, pencil hardness
¹2
¹2
showed HB at 1.0 J cm and 4H at 21.6 J cm without heat
treatment.
Most of the photosensitive materials are based on photo-
radical and photocationic polymerization. The technologies are
widely used in resists,1 coatings,2 printings,3 adhesions,4 and
so on, because many photoradical and photoacid generators,
monomers, and oligomers are commercially available. On the
other hand, the reports on photobase generators (PBGs) are rare
compared with photoacid generators (PAGs) because of its low
quantum yield and low basicity of generated base. Anionic UV-
curing has some merits compared with radical and cationic UV-
curing, because it is free from polymerization inhibition by
oxygen, termination by water, corrosion of metal substrates, and
volume shrinkage. Until now, several researchers have reported
PBGs and demonstrated the advantages of PBGs on photoli-
thography.5-11 The improvement in the quantum yield for PBGs
and the generation of strong bases for photobases are strongly
expected. Recently, we have reported photoreactive carboxylates
comprising ketoprofen and superbases as new PBGs that
undergo photodecarboxylation to generate free superbases.12
Synthesis of PBG 1 is shown in Scheme 2. A suspension of
paraformaldehyde in DMSO was treated with KOH at room
temperature. The reaction mixture was treated with a solution of
benzoin in DMSO. The solution was stirred at room temperature
for 30 min. After neutralization with HCl, ethyl acetate was
added to the solution. The solution was extracted with water and
saturated NaCl solution. The combined extracts were dried over
N
N
O
O
h
ν
HO
O
OH
O
C
O
CO2
+
+
NH2
+
Others
HN
O
O
radical
amine
PBG 1
n
O
O
O
O
radical polymerization
hydrolysis polycondensation
Si O
OMe
Si O
O
n
n
MA-PS
Scheme 1. Photobase generator 1 and this application to photopolymers.
612 | Chem. Lett. 2014, 43, 612–614 | doi:10.1246/cl.131148
© 2014 The Chemical Society of Japan

OH
CH2OH
OH
HO(CH2O)nH
KOH / DMSO
O
O
2
Cl
O
NO2
O
O
NO2
O
HO
O
THF/ Pyridine
O
3
HN
NH
PBG 1
THF, reflux, 12 h
Scheme 2. Synthesis of PBG 1.
Wavelength/nm
¹5
MgSO4 and then filtered. The solution was concentrated in an
evaporator to remove all solvents, and a light-yellow solid was
obtained. The solid was recrystallized with THF and a white
solid 2 was obtained in 76% yield. Then, 2 and pyridine were
dissolved in THF, and 4-nitrophenyl chloroformate in THF was
dropped in an ice bath. After the completion of this process, the
reaction mixture was stirred at room temperature for 1 h. The
solution was concentrated in an evaporator to remove THF.
Dichloromethane was added to the residue. The solution was
extracted with water. The combined extract was dried over
MgSO4, filtered, and concentrated in a vacuum evaporator to
remove all solvents. The residue was separated by chromatog-
raphy on silica gel. Evaporation of the remaining eluent afforded
a viscous liquid. This viscous liquid was reprecipitated with
THF/hexane and a light-yellow liquid 3 was obtained in 92%
yield. A solution of 1,3-di-4-piperidylpropane in THF was
dropped to the viscous liquid of THF. The reaction mixture was
refluxed for 12 h and concentrated using an evaporator to remove
THF. Then, 40 mL of dichloromethane was added to the residue.
The solution was extracted with 5% sodium hydroxide solution,
water, and saturated NaCl solution. The combined extract was
dried over MgSO4, filtered, and concentrated in a vacuum
evaporator to remove all solvents. The residue was separated by
chromatography on silica gel. Evaporation of the eluent afforded
a white solid. The white solid was dissolved in a small amount
of THF. The solution was reprecipitated with hexane to afford
a white solid (PBG 1) in 31% yield.
Figure 1. UV-vis spectral changes of PBG 1 (4.1 © 10
mol L¹1) in acetonitrile during 254 nm-light irradiation.
0.00 molL-1
0.45×10-4
6.90×10-4
7.10×10-4
Wavelength/nm
Figure 2. UV-vis spectral change of phenol red solution upon
addition of a solution of PBG 1 during 254 nm-light irradiation.
Photodecomposition of PBG 1 was investigated in acetoni-
trile by UV-vis spectral measurements. Figure 1 shows the UV-
vis spectral change of PBG 1 in acetonitrile upon irradiation
with 254-nm light. Absorption of 250-nm light decreased, and
new bands appeared at 236 and 279 nm. These changes suggest
that PBG 1 was decomposed by 254-nm light irradiation in
acetonitrile.
The solution of PBG 1 in methanol was prepared in a stated
concentration. After the N2-bubbling treatment, the solution was
exposed to UV-light at 254 nm for 60 min. Upon adding the
irradiated solution of PBG 1 to the methanol solution of phenol
red, absorbance at 430 nm decreased, and a new band appeared
at 560 nm (Figure 2). With an increase in the concentration of
PBG 1, the peak intensity at 560 nm increased. The color of the
solution changed from yellow to red. These results show that
PBG 1 generated bases by UV-irradiation, affording deproto-
nated phenol red.
1
2: H NMR (500 MHz, CDCl3): ¤ 2.86 (br, 1H, OH), 3.64
(d, J = 11.5 Hz, 1H, CH2-OH), 4.50 (d, J = 11.5 Hz, 1H, CH2-
OH), 4.64 (s, 1H, CH2OH), 7.2-7.9 (m, 10H, ArH).
3: 1H NMR (500 MHz, CDCl3): ¤ 4.26 (s, 1H, OH), 4.59 (d,
J = 15 Hz, 1H, CH2-O), 5.15 (d, J = 15 Hz, 1H, CH2-O), 7.26-
7.56 (m, 10H, ArH), 7.86 and 8.24 (AA¤ BB¤, 4H) PBG
1: 1H NMR (500 MHz, CDCl3): ¤ 0.88-1.34 (m, 12H,
-CH2- and >CH-), 1.58 (s, 4H, -CH2-), 2.64 (br, 4H, N-CH2-),
3.89 (s, 2H, N-CH2-), 4.11 (s, 2H, N-CH2-), 4.11 (d, J = 10 Hz,
2H, CH2-OH), 4.88 (d, J = 10 Hz, 2H, CH2-OH), 5.49 (s, 2H,
-OH), 7.26-7.84 (m, 20H, ArH). 13C (300 MHz, CDCl3): ¤ 23.5,
31.9, 35.7, 36.5, 44.3, 71.0, 82.7, 122.2, 125.4, 128.0, 128.8,
130.3, 132.7, 134.5, 139.1, 156.4, 199.0. IR (KBr) (cm¹1): 3400
(¯-OH). HRMS (ESI)¢m/z: calcd for [M + H]+ 747.36; found
747.36. UV (CH3CN): ¾254 = 19000.
Chem. Lett. 2014, 43, 612–614 | doi:10.1246/cl.131148
© 2014 The Chemical Society of Japan | 613

5H
4H
3H
2H
H
Table 1. Results of the photoradical polymerization of MAS
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Upside down
C=CH2
Si-OMe
PBG 1
/mol %
Irradiation energy
Mw
Mw/Mn
¹2
/J cm
1.0
2.5
5.0
5
5
5
10000
6800
6300
2.0
2.0
2.0
F
HB
B
2B
3B
The (3-methacryloyloxypropyl)trimethoxysilane (MAS) so-
lutions were prepared by dissolving 2.4 g of MAS and PBG 1
(1.0, 2.5, and 5.0 mol % relative to MAS) in 4 mL of THF. After
N2 bubbling, the solutions were exposed to UV-light from a
Hg-Xe lamp through quartz glass under stirring. Thereafter,
GPCs of the solutions were measured. As shown in Table 1, the
molecular weight was increased by UV-light irradiation. These
results show that photoinduced polymerization proceeded by
photogenerated radicals from PBG 1.
4B
5B
6B
liquid
0
2
4
6
8 10 12 14 16 18 20 22
Irradiation energy/J cm−2
The above-mentioned facts suggest that radicals and amines
were generated simultaneously by UV-light irradiation. To
demonstrate the generation of radical and amine simultaneously,
we tested for a radical and base-sensitive resin poly[(methacryl-
oxypropyl)silsesquioxane] (MA-PS). MA-PS is a polymer
obtained by the hydrolytic polycondensation of MAS, which
has methacryloyl groups and residual alkoxysilyl groups.
The solutions were prepared by dissolving 0.5 g of MA-PS
(Mw = 2000, Mw/Mn = 1.3) and 1 (10 mol % relative to mono-
mer units of MAS) in 0.5 g of chloroform. The solution was spin-
coated on a silicon wafer and baked at 100 °C for 1 min to afford
a thin film. The film was exposed to 254-nm light under nitrogen
atmosphere. Figure 3 shows the area ratio of the absorption
bands in FT-IR spectra due to C=CH2 and SiOMe of MA-PS. On
increasing UV irradiation, the peaks due to C=CH2 and SiOMe
decreased in parallel. This result shows that the radical and
amine generated from PBG 1 caused radical polymerization of
methacrylate groups and hydrolytic polycondensation of alkoxy-
silyl groups simultaneously. Pencil hardness also increased with
Figure 3. Peak intensity of C=CH2 and Si-OMe and pencil
hardness of MA-PS film on 254 nm-light irradiation.
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¹2
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614 | Chem. Lett. 2014, 43, 612–614 | doi:10.1246/cl.131148
© 2014 The Chemical Society of Japan