Photo-crosslinking of polymeric photobase generator bearing O-acyloxime moieties with low eliminating by-products and high sensitivity

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.12.002

O-Acyloximes are a class of photobase generators (PBGs) that release a primary amine upon irradiation. The generated amine can be used as a crosslinker for polymers bearing epoxy groups, providing a novel photo-induced crosslinking system. However, O-acyloximes also release a ketone by-product that migrates and exudes from the crosslinked polymer. To reduce the migration and elimination of the ketone, 4-vinylacetophenone O-phenylacetyloxime (PaVO) was proposed as a monomeric PBG, and its copolymer with glycidyl methacrylate (PaVO-co-GMA) was compared to poly(glycidyl methacrylate) (PGMA) containing acetophenone O-phenylacetyloxime (PaApO) as a molecular PBG (PaApO/PGMA). Film thickness, infrared (IR), and ultraviolet (UV) spectral changes upon irradiation clarified the vaporization of photoproducts from the PaApO/PGMA films, while such behavior was not observed for the PaVO-co-GMA films. Furthermore, when PaVO-co-GMA was blended with PGMA, less irradiation energy was required for its crosslinking when compared to that of PaApO/PGMA, which contained the same molar ratio of the O-acyloxime unit.

Full text of DOI:10.1016/j.reactfunctpolym.2012.12.002

Reactive & Functional Polymers 73 (2013) 518–523
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Photo-crosslinking of polymeric photobase generator bearing O-acyloxime
moieties with low eliminating by-products and high sensitivity
⇑
Kanji Suyama , Satoko Ozaki, Masamitsu Shirai
Department of Applied Chemistry, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
O-Acyloximes are a class of photobase generators (PBGs) that release a primary amine upon irradiation.
The generated amine can be used as a crosslinker for polymers bearing epoxy groups, providing a novel
photo-induced crosslinking system. However, O-acyloximes also release a ketone by-product that
migrates and exudes from the crosslinked polymer. To reduce the migration and elimination of the
ketone, 4-vinylacetophenone O-phenylacetyloxime (PaVO) was proposed as a monomeric PBG, and its
copolymer with glycidyl methacrylate (PaVO-co-GMA) was compared to poly(glycidyl methacrylate)
(PGMA) containing acetophenone O-phenylacetyloxime (PaApO) as a molecular PBG (PaApO/PGMA). Film
thickness, infrared (IR), and ultraviolet (UV) spectral changes upon irradiation clarified the vaporization
of photoproducts from the PaApO/PGMA films, while such behavior was not observed for the PaVO-co-
GMA films. Furthermore, when PaVO-co-GMA was blended with PGMA, less irradiation energy was
required for its crosslinking when compared to that of PaApO/PGMA, which contained the same molar
ratio of the O-acyloxime unit.
Received 1 September 2012
Received in revised form 5 December 2012
Accepted 5 December 2012
Available online 19 December 2012
Keywords:
Photocrosslink
Photobase generator
By-product
Patterning
O-acyloxime
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
O-Acyloximes are known to release a primary amine upon irra-
diation, followed by hydrolysis [9–12]. As shown in Scheme 1, a ke-
Photo-induced crosslinking is widely used to enhance the
mechanical, thermal, and chemical-resistance properties of poly-
mers via the formation of network structure. By applying light,
spatial and dimensional control can be easily attained, often with-
out heating. The excellent process ability of photo-induced cross-
linking is advantageous in many fields, including imaging and
surface modification. Recently, further attempts to post-crosslink
engineering plastics [1,2], design degradable crosslinkers [3,4],
and use for biomaterials [5] have been reported.
Photo-induced crosslinking often utilizes active species such as
free radicals and acids that contribute to the bond formation in rad-
ical and cationic mechanisms. However, the radical mechanisms
suffer from oxygen inhibition, and the residual acids often induce
problems, such as metal corrosion in the cationic mechanism.
A photobase generator (PBG) is another photolabile active spe-
cies that releases active bases upon irradiation. Generated bases
can be used as catalysts for many reactions, such as polymerization,
crosslinking, and depolymerization [6–8]. The PBG units that are
introduced into polymer side-chains also induce crosslinking and
polarity alterations that lead to a drastic solubility change of the
polymers.
tone by-product is also produced in addition to the amine. In the
use of negative-type photoresists and coatings, by-products often
migrate in the macromolecular network and induce the reduction
of film thickness after irradiation. During UV curing, by-products
exude from the cured materials, which is a problem, especially in
packaging and food applications [7,13].
In this paper, we demonstrate the effect of ketone by-product
covalent bonding on the film reduction and photocrosslinking
behavior of polymers bearing O-acyloxime moieties. We propose
a novel O-acyloxime, 4-vinylacetophenone O-phenylacetyloxime
(PaVO), which is anticipated to generate an amine without releas-
ing a ketone after polymerization. Herein, PaVO was copolymer-
ized with glycidyl methacrylate (GMA), and its behavior was
compared to that of poly(glycidyl methacrylate) (PGMA), which
contains corresponding low molecular weight PBG, acetophenone
O-phenylacetyloxime (PaApO), in terms of spectral, film thickness,
and solubility changes.
2. Experimental
2.1. Instruments
NMR, IR, and UV spectra were recorded using Jeol JNM-LA300,
Jasco FT-IR4200, and Shimadzu UV2400 spectrometers, respec-
tively. Thermal decomposition temperatures (Td) and glass transi-
tion temperatures (Tg) were obtained using Shimadzu TGA-50 and
⇑
Corresponding author. Present address: Faculty of Liberal Arts and Sciences,
Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531,
Japan. Tel./fax: +81 72 254 9508.
E-mail address: suyama@las.osakafu-u.ac.jp (K. Suyama).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.12.002

K. Suyama et al. / Reactive & Functional Polymers 73 (2013) 518–523
519
(1H, broad s, OH). 13C NMR (CDCl3): 12.17, 114.72, 126.17,
126.31, 135.71, 136.16, 138.43, 155.67. The 1H NMR peaks were as-
signed based on the literature [15].
2.4. 4-Vinylacetophenone O-phenylacetyloxime (PaVO)
In a 500 mL four-necked flask equipped with a drop funnel,
4.20 g (26.1 mmol) of 4-vinylacetophenone oxime and 3.63 mL
(26.1 mmol) of triethylamine were dissolved in 120 mL of chloro-
form. Then, 3.45 mL (26.1 mmol) of phenylacetyl chloride and
15 mL of chloroform were added dropwise. Maintaining the solu-
tion <ꢀ5 °C with an ice bath, a phenylacetyl chloride solution
was added dropwise over a period of 45 min. The flask was then
stirred at room temperature for 1 h 45 min and heated at 50 °C
for 10 min. The solution was subsequently washed with 1.2 N
HCl, sat. NaHCO3, and sat. NaCl aqueous solutions and dried over
sodium sulfate. After being concentrated, the resulting tar was
chromatographed using silica gel and chloroform to afford 6.42 g
of a colorless solid, which was further purified via recrystalliza-
tion twice from toluene/hexane = 9/1 (vol/vol). Finally, 3.50 g
(12.5 mmol) of a colorless solid was obtained: Yield: 47.8%, mp:
Scheme 1. The photoreaction of O-acyloximes.
DSC-60, respectively, with a heating rate at 10 K/min under N2. Ele-
mental analysis was performed using
recorder.
a Yanako MT-3 CHN
The molecular weights of the polymers were obtained after size
exclusion chromatography (SEC) using Tosoh TSKgel GMHHR-N and
GMHHR-H columns, a Jasco PU2080plus pump, a Jasco RI-2031plus
detector, and a Jasco DG2080-53 degasser at 40 °C with tetrahy-
drofuran (THF) as the eluent and polystyrene standards.
An Ushio ULO-6DQ low pressure mercury lump was used as a
254 nm light source. The films were coated with a Mikasa 1H-D7
spincoater and baked on a Koike Seimitsu Kikai HM-15G hot plate.
The thicknesses of the films on silicon plates were measured using
a Nanometrics Nanospec/AFT M3000 interferometer.
63.5–64 °C. Td (TGA onset): 224 °C. UV (CH3CN): kmax 276 nm (
e
2.45 ꢁ 104 L molꢀ1 cmꢀ1). 1H NMR(CDCl3): d (ppm) = 2.29 (3H, s,
CH3), 3.85 (2H, s, CH2), 5.32 (2H, d, @CH), 5.82 (1H, d, @CH),
6.72 (1H, dd, J = 17.6 and 10.9 Hz, gemCH), 7.25–7.36 (5H, m,
phenyl), 7.42 (2H, d, J = 8.3 Hz, meta), 7.70 (2H, d, J = 8.4 Hz, ortho).
IR (KBr):1755 cmꢀ1 (C@O). 13C NMR(CDCl3): d (ppm) = 14.16,
40.34, 115.43, 126.27, 127.19, 128.61, 129.29, 133.43, 133.84,
135.98, 139.75, 162.57, 168.86. Elemental analysis:Calcd. for
2.2. Materials
Phenylacetyl chloride (Tokyo Chemical Industry), propylene
glycol monomethyl ether acetate (PGMEA, Nacalai Tesque), chloro-
form (Wako Pure Chemicals, containing 150 ppm of amylene) and
2,20-azobisisobutylonitrile (AIBN, Nacalai Tesque) were used as re-
ceived. Triethylamine, THF, and GMA were distilled before use. 4-
Vinylacetophenone was prepared from 4-bromoacetophenone
and acetic anhydride as described in literature [14]: bp: 62–
64 °C/1 mmHg (lit [14]: 63–64 °C/1 mmHg), IR (neat): 1680 cmꢀ1
(C@O). PaApO was prepared from phenylacetyl chloride and aceto-
phenone oxime as described in the literature [9]: mp: 44.5–45 °C
(lit [9]: 42–43.5 °C).
C18H17NO2; C: 77.40, H: 6.13, N:5.01. Found C: 77.46, H: 5.56, N:
5.07.
2.3. 4-Vinylacetophenone oxime
In a flask, 6.87 g (47.0 mol) of 4-vinylacetophenone in 50 mL of
ethanol and 4.90 g (70.5 mmol) of hydroxyammonium chloride in
40 mL of methanol were mixed. The mixture was cooled in an ice
bath, and 24.3 g of sodium hydroxide was dissolved in 20 mL of
water and added. After stirring at room temperature for 75 min
and at 45 °C for 5 min, 1.2 N HCl was added until the pH became
4. The resulting solid was filtered, dried, and recrystallized from
a toluene/hexane = 9/1 (vol/vol) mixture to afford 4.16 g of a color-
less solid: yield 54.9%, mp: 119.5–120 °C, IR (KBr): 3325 cmꢀ1
(OH). 1H NMR (CDCl3): 2.30 (3H, s, CH3), 5.30 (1H, d, cis CH),
5.79 (1H, d, trans CH), 6.74 (1H, dd, J = 17.6 and 10.8 Hz, gemCH),
7.42 (2H, d, J = 8.4 Hz, meta), 7.60 (2H, d, J = 8.4 Hz, ortho), 9.46
Fig. 1. The 1H NMR (CDCl3) spectra of PaVO (top), PGMA (middle), and PaVO-co-
GMA (bottom).
Table 1
Polymerization conditions and characteristics of polymers.a
Yieldc (%)
Mn
Mw/Mn
Tg (°C)
Td (°C)
d
d
Polymer
PaVO (mol%)
In feed
In polymerb
PaVO-co-GMA
PGMA
10
0
21
0
22
60
5000
5400
1.54
1.64
55
50
234
337
a
b
c
Polymerization at 60 °C for 4 h.
From 1H NMR spectrum.
After reprecipitation.
From SEC.
d

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K. Suyama et al. / Reactive & Functional Polymers 73 (2013) 518–523
2.5. Polymerization
In a tube, total 35 mmol monomers, 51 mg (0.31 mmol) of
AIBN, and 0.3 mL (1.3 mmol) of 1-dodecanethiol were dissolved
in 5.9 mL of toluene. After degassing with freeze-pump-thaw cy-
cles under N2, the solution was heated at 60 °C for 4 h. The mix-
ture was then cooled and poured into hexane. The obtained solid
was reprecipitated three times from methanol and twice from
hexane after being dissolved in THF. The polymerization condi-
tions and characteristics of the purified polymers are summa-
rized in Table 1.
Fig. 4. The UV spectral changes of (a) PaVO-co-GMA and (b) PaApO/PGMA films
upon irradiation at 254 nm. The numbers indicate irradiation energy. O-Acyloxime
unit/epoxy unit = 21/79 (mol/mol). Film thickness before irradiation: 0.3–0.4 lm.
Fig. 2. The IR spectral changes of (a) PaVO-co-GMA and (b) PaApO/PGMA films
upon irradiation at 254 nm. The numbers indicate irradiation energy. O-Acyloxime
unit/epoxy unit = 21/79 (mol/mol). Film thickness before irradiation: 0.3–0.4
The dotted lines demonstrate the spectra before irradiation.
lm.
Fig. 5. The changes in film thickness for (a) PaVO-co-GMA and (b) PaApO/PGMA
films upon irradiation, followed by heating at 140 °C for 10 min and soaking in THF
for 10 min. (s) After irradiation, (4) after heating, and (ꢁ) after soaking in THF
respectively, based on thickness after prebake. O-Acyloxime unit/epoxy unit = 21/
79 (mol/mol).
Fig. 3. The photolysis rates of the O-acyloxime units in the (s) PaVO-co-GMA and
(d) PaApO/PGMA films based on an IR peak intensity at approximately 1765 cmꢀ1
.

K. Suyama et al. / Reactive & Functional Polymers 73 (2013) 518–523
521
2.6. Film state evaluation
polymer matrix or supplied from moisture. Contrastingly, the
PGMA/PaApO film showed little change near this region. This result
suggests that the photo-generated acetophenone vaporized from
the thin film during irradiation.
The polymers and additives were dissolved in PGMEA, filtered
with 0.45 lm pore size PTFE disposable filters, and spin-coated
on silicone or quartz plates. The films were heated at 80 °C for
2 min on the hot plate to remove the PGMEA and were then
irradiated with the low-pressure mercury lamp with a light inten-
sity of 1.0 mW/cm2 at 254 nm. IR and UV spectral changes of the
films were obtained after repeating irradiation and spectral
measurement.
Fig. 4 presents the UV spectral changes of the PaVO-co-GMA
and PaApO/PGMA films upon irradiation at 254 nm in ambient
air. A peak at 258 nm for the PaVO-co-GMA film decreased slowly
with a small blue shift. This result is consistent with the formation
of benzylamine and a pendant acetophenone unit because benzyl-
amine and 4-methylacetophenone demonstrate maximum peaks
After irradiation, the films were post-exposure baked at 140 °C
on the hot plate and soaked in THF for 10 min at room tempera-
ture. The insoluble fractions were obtained from the thickness ratio
before and after soaking.
at 259 nm (in ethanol, e: 232) [16] and 251 nm (in methanol, e:
12,670) [17], respectively. In contrast, a peak at 247 nm for the
PGMA/PaApO film almost disappeared after irradiation P600 mJ/
cm2, suggesting vaporization of the generated acetophenone from
the irradiated films. The thickness changes of the PaVO-co-GMA
3. Results and discussion
PaVO-co-GMA was obtained via conventional free-radical poly-
merization. According to the 1H NMR spectrum of this copolymer
in Fig. 1, the molar ratio of PaVO and GMA was estimated to be
21: 79 with a feed ratio of 10: 90. The thermal stability of PaVO-
co-GMA was high, as demonstrated by its Td at 234 °C.
Fig. 2 shows the IR spectral changes of the PaVO-co-GMA and
PaApO-blended PGMA (PGMA/PaApO) films upon irradiation at
254 nm in ambient air. In both cases, a peak at approximately
1765 cmꢀ1 due to the C@O stretching of the O-acyloxime moiety
decreased upon irradiation. Based on the changes of peak intensity
at approximately 1765 cmꢀ1, the degree of O-acyloxime photolysis
is plotted in Fig. 3. The photoreactivity of the O-acyloxime unit in
both films was nearly identical.
For the PaVO-co-GMA films, a peak appeared upon irradiation
at 1680 cmꢀ1, which is assigned to an aromatic ketone group in
Scheme 1. This signal indicates that the hydrolysis in Scheme 1
immediately proceeded with water, which was involved in the
Fig. 6. The UV spectra of PaVO-co-GMA blended PGMA films. Film thickness:
0.4
lm. The inset shows the absorbances at 258 nm plotted against the PaVO molar
ratio in the films.
Scheme 2. The photo-induced crosslinking of PaVO-co-GMA and PaApO/PGMA.

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K. Suyama et al. / Reactive & Functional Polymers 73 (2013) 518–523
benzylamine and ketone units remained in the polymer matrix, de-
spite that the boiling point of benzylamine is 185 °C and lower
than that of acetophenone (202 °C). For the reaction mechanism
of epoxy-amine curing, the possibility of epoxy and amine complex
formation [18] and the catalytic role of added water [19] have been
recently reported. In the present study, such complex formation or
addition reaction between epoxy and amino groups likely pre-
vented the vaporization of the generated benzylamine. After soak-
ing in THF (ꢁ), the films were soluble in THF at low irradiation
energy, showing insufficient network formation. As the irradiation
energy increased, the remaining film thicknesses increased due to
crosslinking between the generated benzylamine and pendant
epoxy groups.
The PGMA/PaApO films in Fig. 5b demonstrated a difference in
behavior of thickness changes. The film thicknesses decreased
upon irradiation (s) with increasing irradiation energy, suggesting
the removal of the volatile acetophenone formed on irradiation.
The film thicknesses reduced upon heating at 140 °C (4), although
they slightly increased with increasing irradiation energy. This
trend suggests that PaApO sublimed upon this heating condition,
and the generated benzylamine stayed in the PGMA matrix as dis-
cussed above. The irradiated and heated films (ꢁ) became insolu-
ble in THF when the irradiation energy was >700 mJ/cm2.
However, the remaining films were thinner when compared to
those of PaVO-co-GMA.
From the above results, the photo- and crosslinking reactions of
both systems are proposed in Scheme 2. Although we could not ob-
tain spectral evidence, a linkage is likely generated between the
benzylamine and two epoxy groups due to its high reactivity. For
example, a near IR measurement clarified the formation of a ter-
tiary amino group after heating the mixture of 3-(amino-
methyl)benzylamine and diglycidyl ether of bisphenol
A for
10 min even at 80 °C [20].
The photo-induced crosslinking of the films containing different
feed ratios of O-acyloxime units was also investigated. The molar
ratios were controlled via blending PaVO-co-GMA and PGMA. No
haze was observed for all of the blended films, and the UV spectra
of the films indicate that the absorbance at 258 nm is proportional
to the PaVO molar ratio, as shown in Fig. 6. These results suggest
the absence of PaVO-co-GMA aggregation in the PGMA matrix.
Fig. 7 shows the changes of the insoluble fractions of the PGMA
films containing PaVO-co-GMA and PaApO with the same O-acy-
loxime unit. When the films contained 1.8% O-acyloxime unit
(Fig. 7a), similar solubility curves were obtained. With increasing
the molar ratio of the O-acyloxime unit (Fig. 7b–d), less irradiation
energy was required to induce crosslinking due to the increased
crosslinking points. However, the crosslinking of films containing
PaVO-co-GMA was initiated at lower irradiation energy.
The higher ability of PaVO-co-GMA/PGMA crosslinking can be
explained by the possibility of secondary photoreactions of photo-
products which were incorporated in the polymers. It is well
known that free radicals are produced via a-hydrogen abstraction
of the amino groups by the photogenerated ketone [21]. The result-
ing radicals can couples together to form crosslinking points, as
shown in Scheme 3.
Fig. 7. The insolubilization of PaVO-co-GMA blended with PGMA (open) and
PaApO/PGMA (solid symbols) films upon irradiation, followed by heating at 140 °C
for 10 min and soaking in THF for 10 min. O-acyloxime unit/epoxy unit = (a) 1.8/
98.2, (b) 2.6/97.4, (c) 10.5/89.5, and (d) 21/79 (mol/mol).
4. Conclusions
We have prepared copolymer PaVO-co-GMA from a new mono-
meric PBG and compared its behavior to that of PGMA/PaApO
films. The photoreactivity and thermal stability were similar for
both films. Removal of the acetophenone by-product from the irra-
diated PaApO/PGMA films was confirmed in terms of spectral and
film thickness changes, although such removal was not observed
for PaVO-co-GMA. Both films became insoluble in THF upon
and PGMA/PaApO films during irradiation, heating, and soaking
processes were monitored. As shown in Fig. 5a, the film thickness
of PaVO-co-GMA was almost constant even after irradiation (s)
and heating at 140 °C (4). This result indicates that the generated

K. Suyama et al. / Reactive & Functional Polymers 73 (2013) 518–523
523
Scheme 3. Plausible secondary photoreactions.
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