Photobase generators derived from trans-o-coumaric acid for anionic UV curing systems without gas generation

Article abstract of DOI:10.1002/pola.27552

Anionic UV curing systems of poly(glycerol methacrylate) and/or poly[3-(methacryloyloxy)propyltrimethoxysilane] are obtained using novel photobase generators (PBGs) derived from trans-o-coumaric acid. These PBGs decompose into amines and coumarins by UV i

Full text of DOI:10.1002/pola.27552

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Photobase Generators Derived from trans-o-Coumaric Acid for Anionic
UV Curing Systems without Gas Generation
Koji Arimitsu, Yuri Takemori, Atsushi Nakajima, Ayaka Oguri, Masahiro Furutani,
Takahiro Gunji, Yoshimoto Abe
Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278–8510, Japan
Correspondence to: K. Arimitsu (E-mail: arimitsu@rs.noda.tus.ac.jp)
Received 11 September 2014; accepted 27 December 2014; published online 3 March 2015
DOI: 10.1002/pola.27552
KEYWORDS: anionic UV curing; catalysts; crosslinking; curing of polymers; hardness; photobase generator; trans-o-coumaric acid
Anionic UV curing has been studied extensively because it
may overcome the serious drawbacks of conventional radical
UV curing such as volume shrinkage after photoirradiation
and polymerization inhibition by oxygen in air. Erosion of
metallic substrates observed in the case of cationic UV curing
could also be circumvented. In the anionic systems, photo-
base generators (PBGs) would be used as key reagents that
are photolatent catalysts for base-catalyzed polymerization
reactions and/or crosslinking reactions.
wavelength of the PBG. On the other hand, Wang et al.17,18
proposed the use of trans-o-coumaric acid to protect an
amino group in organic syntheses. Under UV irradiation,
photoisomerization from trans to cis isomers occurs, result-
ing in an intramolecular cyclization reaction to release
amines. In this work, we have applied this chemical structure
to design PBGs that could generate amines without gas gen-
eration and have applied the PBGs to photopolymer systems
for the first time (Scheme 1).19 After our proposal,19 substit-
uent effects of trans-o-coumaric acid as
a PBG were
reported.20,21
There are various kinds of PBGs that apply unique photogen-
eration reactions of base molecules. Tsunooka and
coworkers1–3 reported photochemically produced pendant
amino groups that were generated from acyloxyimino
groups. Oxime-urethane and phthalimido-carbamate groups
were also used as PBGs by Chae et al.4–7 Cameron et al.8
developed a-keto carbamates that provided an amine with
an accompanying intramolecular cyclization reaction. Pyridyl-
based aromatic polyureas developed by Papadimitrakopoulos
and coworkers9 provide pyridines by 365 nm light irradia-
tion. The o-nitrobenzyloxycarbonyl group and its analogs
have been commonly used to make PBGs that generate vari-
ous kinds of amines.10–12 Furthermore, our group has devel-
oped novel PBGs based on photocarboxylation reactions of
carboxylates derived from ketoprofen and xanthone acetic
acid that generate from aliphatic amines to strong organic
bases.13,14 However, the PBGs mentioned above have a cru-
cial drawback in that they generate not only base molecules
but also gases (carbon dioxide), which causes bubbles and
cracks in the product film.
PBG 1 was expected to be photodecomposed into a couma-
rin and a cyclohexylamine.17 A diamine-type of PBG 2 was
also designed and synthesized taking into account the fact
that the diamine is more effective for crosslinking reactions
toward epoxy resins.22 These PBGs were both synthesized in
a simple procedure from trans-o-coumaric acid and the cor-
responding amine (see Supporting Information Schemes S1
and S2). These PBGs showed good solubility toward organic
solvents such as dimethyl sulfoxide, alcohols, acetone, and
tetrahydrofuran (THF).
The photodecomposition behavior was examined using UV
spectral measurements with 37 mM of PBG 1 methanol solu-
tion (Fig. 1). The spectra show a decrease in the absorption
peaks around 270 and 320 nm accompanied with the
appearance of a peak around 210 nm under 254 nm light
irradiation. This new peak could be assigned to coumarin,
which implied photodecomposition of PBG 1 (Supporting
Information Fig. S1). Gas chromatography measurement was
also performed with the irradiated methanol solution of PBG
1, and coumarin was found to be generated as a byproduct
after 254 nm light irradiation (Supporting Information Fig.
S2). Next, the photodecomposition behavior of PBG 1 in a
As a PBG without gas generation, Wang and coworkers15,16
reported a bicyclic guanidinium tetraphenylborate that gen-
erates a strong organic base. However, it was difficult to
chemically modify it to regulate the maximum absorption
Additional Supporting Information may be found in the online version of this article.
C
V
2015 Wiley Periodicals, Inc.
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SCHEME 2 Crosslinking reaction of PGMA with photogener-
ated amines from PBG 1.
SCHEME 1 Photoreaction and chemical structures of PBGs 1
and 2.
of PBG 1 even in a polymer solid with 254 nm light irradia-
tion in the same manner.
PGMA has epoxy groups that react with amines to form
covalent bonds, and the photogenerated cyclohexylamine
from PBG 1 is expected to play a role as a crosslinker
(Scheme 2).
To examine the chemical states of a PGMA film containing 20
wt % of PBG 1, FTIR spectral measurements were performed
(Supporting Information Figs. S3 and S4). After UV irradia-
tion (254 nm, 60,000 mJ/cm2), the film was postbaked at
ꢀ
140 C. The time course of the FTIR spectra showed that the
peak intensity at 900 cm21 decreased with heating, which
implied a ring-opening addition reaction at the epoxy groups
with photogenerated amines. Furthermore, as shown in Fig-
ure 3, the postbaked film became insoluble in a developer
(THF), which indicated the formation of crosslinking net-
works. The photosensitivity was improved by increasing the
postbaking temperature.
FIGURE 1 UV spectra of PBG 1 in methanol with 254 nm light
irradiation. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Another anionic UV curing was examined with PBG 2 and
polymer matrix was examined using a poly(glycerol methac-
rylate) (PGMA, Mw 5 9.1 3 104, Mw/Mn 5 1.9) film. A film
containing 20 wt % of PBG 1 (ca. 0.8 mm in thickness) was
fabricated, and the UV spectral changes were observed (Fig.
2). Although the change was less obvious than in the case of
methanol solution, this result supported the decomposition
poly(3-(methacryloyloxy)propyltrimethoxysilane)
(PMAS,
FIGURE 3 Photosensitivity curves of PGMA films containing
PBG 1 with 254 nm light irradiation and subsequent heating at
100–140 ꢀC for 10 min. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 2 UV spectra of PBG 1 in a PGMA film with 254 nm
light irradiation. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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SCHEME 3 Crosslinking reaction of PMAS catalyzed by photo-
generated diamines from PBG 2.
Mw 5 1.2 3 104, Mw/Mn 5 1.5) tethering trimethoxysilyl
groups in its side chains that undergo a base-catalyzed poly-
condensation reaction (Scheme 3).
PMAS films containing 2.5–10 wt % PBG 2 were successfully
fabricated on glass substrates (Supporting Information Fig.
S5). The 365 nm light was used instead of the 254 nm light
because the isomerization equilibrium with the former could
increase the cis isomer when compared with the case of the
latter.17 The hardness of the coatings was monitored using
the pencil-scratch method based on JIS K5400.23 As shown
in Figure 4, increasing irradiation energy improved the pen-
cil hardness, which indicated that photogenerated diamines
contribute to form siloxane crosslinking networks as base
catalysts. Dark reactions did not proceed with low-
irradiation energies, and the maximum pencil hardness of
5H was accomplished with 10 wt % of PBG 2, 10,000 mJ/
cm2 of irradiation energy, and postbaking at 100 ꢀC for 90
min.
FIGURE 4 Pencil hardness of PMAS films containing PBG 2
after 365 nm light irradiation and subsequent heating at 100 ꢀC
for 90 min. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
PBG 2 is photodecomposed into a diamine and two coumar-
ins, with no accompanying gas generation during the curing
process. Figure 5 shows a comparison between PBG 2 and a
conventional PBG 3, when they photochemically decomposed
in a sealed PMAS film. A Hg–Xe lamp without any cutoff filter
was used as the light source, and the total irradiation energy
FIGURE 5 Gas generation observed in sealed PMAS films after UV irradiation (a) with PBG 2 and (b) with a conventional PBG 3.
The reaction scheme of the photodecomposition of PBG is shown beside the photo images. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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was 4000 mJ/cm2 in 365 nm light equivalents. In the case of
PBG 2, bubbles were not observed after adequate UV irradia-
tion. In the case of PBG 3, however, many bubbles were clearly
observed in the film under the same conditions. This would be
due to the generation of carbon dioxide gas as a byproduct
from PBG 3 in the photodecomposition reaction.
7 K. H. Chae, H. I. Cho, Y. H. Kim, U. C. Yang, Eur. Polym. J.
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ꢀ
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In conclusion, novel PBGs 1 and 2 were designed and syn-
thesized for anionic UV curing systems. PBG 1 was success-
fully photodecomposed both in a methanol solution and in a
PGMA film, and it could be integrated in the crosslinking
networks. PBG 2, which provided a diamine, was successfully
applied to a UV curing system of PMAS with neither dark
reactions nor gas generation.
11 T. Nishikubo, A. Kameyama, Y. Toya, Polym. J. 1997, 29,
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EXPERIMENTAL
For equipment and materials, syntheses of PBGs, and fabrica-
tion of PGMA and PMAS films, please see Supporting
Information.
16 C. B. Cao, C. Zhou, X. Sun, J. P. Gao, Z. Y. Wang, Polymer
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19 K. Arimitsu, Y. Takemori, T. Gunji, Y. Abe, Polym. Prepr.
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