Photoinduced redox initiation for fast polymerization of acrylaytes based on latent superbase and peroxides

Article abstract of DOI:10.1016/j.polymer.2012.05.031

The article presents a highly effective strategy for photopolymerization of acrylates via photolatent redox-accelerated reaction based on the synergistic photoinitiating systems containing photolatent superbase and readily available peroxides. Polymerization of acrylates could be instantly initiated with the effective interaction between the photogenerated amine and peroxides. Due to the persistent interaction of produced longeval amine with peroxides, remarkable post conversion after irradiation, which is significant for radiation crosslinking of photo-screened materials, was thus initially achieved in photoinitiated free radical polymerization. To explore the synergistic interactions of the photoinitiating systems, the effect of peroxide structures and QA-DBU:BPO ratios had been examined by RTIR, showing that all peroxides are applicable as the final conversion rate of acrylates is concerned. Further, BPO and CHP significantly accelerated the photopolymerization rate in air atmosphere. The synergistic efficiency of QA-DBU and BPO as a photopolymerization initiatiation system was close to that of the conventional D-1173 photoinitiator.

Full text of DOI:10.1016/j.polymer.2012.05.031

Polymer 53 (2012) 3172e3177
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Photoinduced redox initiation for fast polymerization of acrylaytes based
on latent superbase and peroxides
Minghui He a, , Xun Huang , Yugang Huang , Zhaohua Zeng b, Jianwen Yang a
b
a
a
a
,
,b,
*
a
Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, China
Key Laboratory of Designed Synthesis and Application of Polymer Material, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 February 2012
Received in revised form
The article presents a highly effective strategy for photopolymerization of acrylates via photolatent
redox-accelerated reaction based on the synergistic photoinitiating systems containing photolatent
superbase and readily available peroxides. Polymerization of acrylates could be instantly initiated with
the effective interaction between the photogenerated amine and peroxides. Due to the persistent
interaction of produced longeval amine with peroxides, remarkable post conversion after irradiation,
which is significant for radiation crosslinking of photo-screened materials, was thus initially achieved in
photoinitiated free radical polymerization. To explore the synergistic interactions of the photoinitiating
systems, the effect of peroxide structures and QA-DBU:BPO ratios had been examined by RTIR, showing
that all peroxides are applicable as the final conversion rate of acrylates is concerned. Further, BPO and
CHP significantly accelerated the photopolymerization rate in air atmosphere. The synergistic efficiency
of QA-DBU and BPO as a photopolymerization initiatiation system was close to that of the conventional
D-1173 photoinitiator.
6
April 2012
Accepted 16 May 2012
Available online 23 May 2012
Keywords:
Photolatent superbase
Peroxides
Photoinduced redox
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
with acetoacetates [19], etc. Nevertheless, these advances are not
satisfactory as a highly effective catalyst in many applications,
Photochemical generation of bases, utilized to catalyze versatile
types of organic reactions in order to offer new polymers with
special and outstanding properties [1e4], have been investigated
for a wide range of applications [5e7]. To date, photocatalytic ring-
opening polymerization (ROP) of epoxide resin by photolatent base
is one of the most common strategies [8,9]. However, the present
traditional photolatent base may have limited applications in
photosensitive materials due to the weaker basicity, lower catalytic
activity and quantum yields [10,11]. Photolatent superbase-
catalyzed polymerization, which has been recently explored,
provides an attractive alternative to conventional base-catalyzed
polymerization [12e14]. Thanks to these effective organocatalysts
because organocatalyst availability is still limited to subsequent
high temperature treatment for a long time and lower conversion
efficiency [14]. Indeed, there is no clear path for photolatent base to
initiate ambient radical photopolymerization with higher conver-
sion and polymerization rate. So further development in molecular
design and new concept of base-catalyzed polymerization is
desired [20].
In this paper, a highly effective method has been proposed to
eliminate the aforementioned difficulties under ambient condi-
tions via photolatent redox-accelerated reaction based on the
synergistic photoinitiating systems containing photolatent super-
base and readily available peroxides. Although not directed to the
photopolymerization area, a lot of studies on redox polymerization
have already demonstrated (i) the lower polymerization tempera-
ture and (ii) a quite high reactivity of the redox system to initiate
the polymerization of acrylates [21,22], thus it is of great interest to
apply this latent approach to light induced polymerization
reactions.
[15], several kinds of polymers are now accessible by employing
suitable reactions such as thiol-click reactions [16], trans-
esterification reaction [17], living anionic ring-opening polymeri-
zation of cyclic esters [18], and the Michael addition of acrylates
As part of our continuous interest in developing highly effective
photolatent base systems to prevail over the existing problems or
enhance the efficiency of UV-curable formulations, herein we
report the quaternary ammonium salt (QA-DBU) possessing two
*
Corresponding author. Key Laboratory for Polymeric Composite and Functional
Materials of Ministry of Education, School of Chemistry and Chemical Engineering,
Sun Yat-Sen University, Guangzhou 510275, China. Tel./fax: þ86 20 84111138.
E-mail addresses: cesyjw@mail.sysu.edu.cn, cedc30@zsu.edu.cn (J. Yang).
0
032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.05.031

M. He et al. / Polymer 53 (2012) 3172e3177
3173
photosensitive groups, 4-methylbenzophenone (MBP) and tetra-
phenylborate, in the structure. The covalent binding of MBP and
received unless otherwise noted. The chemical structures of QA-
DBU, TMPTA, BPO, DCP, TBPB and CHP are shown in Scheme 1.
1,8-diazabicyclo[5.4.0]unde-7-ene (DBU) was done to improve the
photoreactivity of the photolatent base compared to the physical
mixture. Our work has focused on the redox-induced propagating
behavior due to the self-enrichment effect of superbase toward the
unreacted areas during polymerization [23], providing a highly
effective technology platform based on photolatent superbase in
the fields of not only thin materials, but also thick samples that the
current photopolymerization fails to cure. Then, the synergistic
interactions of combining BPO with QA-DBU were further explored
in order to design a novel photoinitiator system that can undergo
efficient free radical photopolymerization. Interestingly, the pho-
tolatent redox couple on exposure can induce a remarkable accel-
eration of free radical polymerization of acrylates compared to the
immediate initiation by quaternary ammonium salt reported in the
literature [24e27], and also exhibit enhanced or specific properties
such as the self-propagating polymerization.
2.2. Photodecomposition dynamics of QA-DBU
First, QA-DBU in acetonitrile (1 ꢀ 10ꢁ mol/L) was put into
5
a quartz cell. Next, the photodecomposition was carried out by
optical cable-directed UV lamp (RW-UVA-F200U, Runwing Co.
China) over time (0 min, 1 min, 2 min, 5 min, 10 min), then UVeVis
Spectroscopy were measured. In these experiments, the optical
cable was placed at about 1 cm from the surface of the solution and
2
the light intensity was 20 mW/cm measured by a UV-radiometer
(type UV-A, Photoelectric Instrument Factory, Beijing Normal
University).
2.3. Quantum yield of photogenerated DBU
QA-DBU (1.96 ꢀ 10ꢁ mol) in DMSO-d
5
(0.7 mL) was put into
6
1
a quartz tube, after irradiation in air, then H NMR spectra were
ꢁ9
ꢁ2 ꢁ1
s
measured. The light intensitywasmeasuredas4.16 ꢀ10 Ecm
2
. Experimental
at 365 nm.
2.1. Materials
2.4. Thermal stability studies of peroxides
1
,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), sodium tetraphe-
A portion of peroxide in TMPTA (2.2 ꢀ 10ꢁ mol/g) was placed in
5
nylborate, dibenzoyl peroxide (BPO), dicumyl peroxide (DCP), tert-
butyl peroxybenzoate (TBPB) and cumene hydroperoxide (CHP)
were purchased from Aladdin-reagent (China). BPO was purified by
dissolving in CHCl
MeOH. DCP was recrystallized from 95% ethanol before use. 2-
Hydroxy-2, 2-dimethylacetophenone (Darocur 1173, abbreviated
an aluminum pan and completely sealed. The polymerization
profile was monitored by DSC under a nitrogen atmosphere, where
the temperature of the sample was raised rapidly from 40 C to
ꢂ
3
at room temperature and precipitated into
ꢂ
ꢂ
200 C at a heating rate of 10 C/min.
as D-1173, Ciba company), triphenylboron (BPh
3
, Alfa Aesar) and
2.5. Photopolymerization
trimethylol propane triacrylate (TMPTA, Sartomer Company) were
used as received. 4-(Bromomethyl)benzophenone was synthesized
according to the literature procedure [28]. All solvents were dried
over 4 A molecular sieves before use. Other chemicals were used as
Photopolymerization behavior of the QA-DBU under different
conditions was studied by real-time infrared spectroscopy (RTIR).
Typical procedure: QA-DBU (0.0146 g, 2.2 ꢀ 10ꢁ mol) and peroxide
5
Scheme 1. Chemical structures of QA-DBU, TMPTA, CHP, BPO, TBPB, DCP.

3174
M. He et al. / Polymer 53 (2012) 3172e3177
ꢁ
5
(
2.2 ꢀ 10 mol) were dissolved in acetone (0.5 mL) under ultra-
sonication, then TMPTA (1 g) was added to this solution. At last, the
mixture was injected into a mold.
2.6. Preparation of 4-(bromomethyl)benzophenone-(1,8-
diazabicyclo[5.4.0]undec-7-ene)ammonium bromide
A solution of 4-(bromomethyl)benzophenone (2.75 g, 10 mmol)
in toluene (200 mL) was added dropwise into a solution of DBU
(
1.67 g, 11 mmol) in toluene (30 mL). After the mixture was stirred
at room temperature for 12 h, the precipitated bromide was
l
filtered, and dried in vacuo to give white solid (yield: 94.8%). H
NMR (300 MHz, CDCl
3
,
d
, ppm): 7.83 (d, 2H), 7.76 (m, 2H), 7.58 (t,
Fig. 1. UVeVis spectral changes of QA-DBU in AN (1 ꢀ 10ꢁ mol/L) being irradiated
5
1
2
H), 7.47 (t, 2H), 7.38 (d, 2H), 5.00 (s, 1H), 3.8 (m, 6H), 2.96 (m, 2H),
.26 (m, 2H), 1.77 (s, 6H). C NMR (300 MHz, CDC1 , d, ppm):
3
l3
over time: 0 min, 1 min, 2 min, 5 min, 10 min.
195.89, 167.90, 138.97, 137.75, 137.27, 132.88, 131.10, 130.08, 128.58,
126.62, 57.27, 56.37, 50.09, 48.38, 30.06, 28.82, 26.36, 23.00, 20.68.
a distinct decrease in absorption bands at 196 nm and 224 nm was
observed with irradiation time, respectively, whereas the latter
exhibited red-shifted absorption maxima after irradiation and
a new absorption band appeared at 252 nm. These facts suggested
that photodecomposition reaction of QA-DBU proceeded in
acetonitrile.
ESI-MS was employed to detect the photogenerated bases. Fig. 2
shows that the detection of protonated DBU (m/z ¼ 153.3) after
30 min irradiation (Fig. 2a) was the direct evidence for the forma-
tion of DBU from irradiated QA-DBU. In comparison, the signal
intensity of MBP-DBU (m/z ¼ 347.4) was apparently smaller after
irradiation than before irradiation.
2
.7. Preparation of 4-methylbenzophenone-(1,8-diazabicyclo[5.4.0]
undec-7-ene)ammonium tetraphenylborate (QA-DBU)
To a solution of ammonium bromide salt (4.27 g, 10 mmol) in
water (500 mL) was added slowly a slight stoichiometric excess of
sodium tetraphenylborate (3.76 g, 11 mmol) solution in water
(
40 mL) with stirring at room temperature. Precipitate of the salt
was filtered, washed several times with water, and dried in vacuo.
Recrystallized from a mixture of ethanol and acetone to give white
crystals (yield: 85%). H NMR (300 MHz, CDCl
l
3
,
d, ppm): 7.75 (m,
4
3
H), 7.60 (m,1H), 7.48 (m, 2H), 7.42 (m, 8H), 7.02 (t, 8H), 6.87 (t, 6H),
l3
.90 (s, 2H), 2.98 (s, 2H), 2.69 (d, 4H), 2.10 (s, 2H), 1.35 (d, 8H).
, ppm): 195.79, 167.47, 164.97, 164.31,
63.66, 163.01, 140.94, 137.56, 137.17, 136.22, 133.43, 130.81, 130.19,
29.26, 127.33, 125.97, 122.19, 56.36, 55.13, 49.48, 48.14, 28.63,
C
3
NMR (300 MHz, CDC1 , d
1
1
3.2. Quantum yield of photogenerated DBU
2
8.52, 26.14, 23.13, 20.27.
In order to further confirm the liberation of DBU from irradiated
QA-DBU, a series of H NMR spectra were measured (Fig. 3). The
1
2.8. Characterization
protons H
were disappearing after irradiation, meanwhile new protons H
2.61e2.58 ppm) and H (1.88 ppm, Fig. 3b), corresponding to the
a b
(2.87e2.85 ppm) and H (2.05 ppm, Fig. 3a) in QA-DBU
c
The NMR spectra were obtained on a Varian 300 MHz spec-
(
d
3 6
trometer with CD Cl or DMSO-d and TMS as the solvent and
same peak (2.50 ppm, 2.26e2.22 ppm) in DBU (Fig. 3d) appeared.
This fact shows that DBU was generated in the photolysis.
Quantum yield and conversion rate for amine formation in the
photolysis were calculated by integrating the area of the new peak
at 2.40 ppm, normalized by an internal standard peak at 2.50 ppm
internal standard, respectively. UVeS absorption spectra were
obtained on a Perkin Elmer Lambda 750 UVeVisible spectropho-
tometer. Differential scanning calorimetry (DSC) measurements
were recorded on a TA instrument (DSC 2910) at a heating rate of
ꢂ
1
0
C/min. Electrospray ionization mass spectra (ESI-MS) were
due to the solvent residual methyl proton of DMSO-d
Fig. 4, the quantum yield of photodecomposition ( ) of QA-DBU in
DMSO-d is 0.435, and the efficiency of photogenerated DBU
6
. As shown in
acquired on a Thermo Finnigan LCQ DECA XP ion trap mass spec-
trometer, equipped with an ESI source.
Real-time infrared spectra (RTIR) was conducted with a modi-
fied Nicolet 5700 instrument with a horizontal sample holder.
F
d
6
proved to be high, w90% conversions, was obtained after just
30 min irradiation.
ꢁ
1
Changes in the peak area from 6104 to 6222 cm attributed to the
stretching vibration of the acrylate carbonecarbon double bond,
were widely used to monitor acrylate polymerization kinetics for
thick films [29]. The compounds were injected into a mold from
two glass plates and spacers with 15 ꢃ 1 mm in diameter and
1
.2 ꢃ 0.1 mm in thickness, then irradiated with a UV-light source
(
Rolence-100 UV, Taiwan, China) by a light guide attached to the
ꢂ
RTIR unit in air at 22 C. The light intensity at the surface level of the
cured samples was measured to be 20 mW/cm .
2
3
. Results and discussion
3.1. Photodecomposition dynamics and detection of DBU from
QA-DBU
Photodecomposition of QA-DBU in acetonitrile (1 ꢀ 10ꢁ mol/L)
5
Fig. 2. Positive ion ESI-MS spectra of QA-DBU in DMSO (0.01 mM) (a) after and (b)
before irradiation.
was investigated by UVeVis Spectroscopy. As shown in Fig. 1,

M. He et al. / Polymer 53 (2012) 3172e3177
3175
Table 1
Thermal data and kinetic data of the radical initiators studied in this work.
ꢂ
Initiator
T
onset
T
p
E
a
(kJ/mol)
t
1/2 ¼ 1 min ( C)
CHP
BPO
TBPB
DCP
65
87
87
130
175
73
108
125
144
167
e
254
130
167
175
e
96,133
95,114
149
Only TMPTA
187
T
onset is temperature at which the heat flow begins. T
p
is the peak temperature
ꢁ
5
which the sample releases the maximum heat flux in TMPTA (2.2 ꢀ 10 mol/g) at
ꢂ
a heating rate of 10 C/min under N
2
atmosphere. The activation energy (E
a
) and
half-life (t1/2 ¼ 1 min) are available from literature [30e33].
assigned to the high activity with DBU. The efficiency of QA-DBU and
BPO as a novel photopolymerization initiatiation system was close
to that of the conventional D-1173 photoinitiator (Fig. 6). Due to the
relatively lower decomposition activity and the higher absorption of
ultraviolet light, photopolymerization with TBPB or DCP (Fig. 5c or
d) occurred with a relatively long induction period approximately
Fig. 3. 1H NMR spectral changes of QA-DBU (a) before and (b) after 30 min irradiation
at 365 nm, and as well of (c) a mixture of Ph B and DBU and (d) DBU.
3
3.3. Thermal stability studies of peroxides
10 s. Furthermore, the bimolecular radical generation process were
generally slower than free radical photoinitiators (QA-DBU) which
formed radicals unimolecularly. However, a higher final conversion
rate about 85% was achieved (Fig. 5c).
In Table 1, DSC data (Tonset, T
energy (E
together with the definition of specific temperature parameters
that will be used hereinafter, are listed. As can be noticed, to our
surprise CHP started decomposition at temperature that was much
lower than the others. Actually, BPO and TBPB had no significant
p
) in TMPTA, as well as activation
), and half-life (t1/2 ¼ 1 h) concerning all four peroxides
a
3.5. The synergistic efficiency of QA-DBU and BPO
We further evaluated the effect of QA-DBU:BPO ratios on the
photopolymerization of TMPTA (Fig. 7). Fig. 7bef illustrated that as
differences both in Tonset and T
stable (Tonset equal to 130 C). In comparison, the exothermic peak
p
, while DCP turned out to be more
ꢂ
the QA-DBU:BPO ratios decreased, the time to reach the maximum
of TMPTA in the absence of initiator shifted to higher temperature
max
polymerization rate ðR
Þ was shortened, but all the final
ꢂ
p
at 187 C. This trend is confirmed by the corresponding E
a
values
conversion increased to above 85%. It is well known that either
quaternary ammonium salt or BPO can be separately used as a free
ꢂ
and half-life, which are equal to 108 kJ/mol and 254 C for CHP,
ꢂ
ꢂ
while they are 125 kJ/mol and 130 C for BPO, 144 kJ/mol and 167 C
radical photoinitiator. In comparison, QA-DBU alone had a lower
ꢂ
for TBPB, 167 kJ/mol and 175 C for DCP, respectively. These results
max
R
and final conversion of w75% (Fig. 7a). Likewise, BPO alone
p
indicate that peroxides are usable for acrylates at room
temperature.
didn’t have a remarkable effect on the final photopolymerization of
3.4. Photopolymerization initiated by QA-DBU coupling with
different peroxides
To investigate the effect of peroxide structures (BPO, DCP, TBPB
and CHP), the photopolymerizations initiated by QA-DBU coupling
with different peroxides were carried out by RTIR under constant
illumination (Fig. 5), showing that all peroxides are applicable as the
final conversion rate of acrylates is concerned. Further, BPO and CHP
(Fig. 5a and b) significantly accelerated the photopolymerization.
Taking BPO as an example (Fig. 5a), within 20 s, almost 90% of the
acrylate double bonds were converted to polymer while just about
5
0% irradiated by QA-DBU alone under the same conditions,
Fig. 5. Photopolymerization kinetics of TMPTA initiated by QA-DBU coupling with
different peroxides in air: (a) BPO (-), (b) CHP (+), (c) TBPB (C), (d) DCP (:), and (e)
only QA-DBU (B).
Fig. 4. Quantum yield and conversion rate of photogenerated DBU.

3176
M. He et al. / Polymer 53 (2012) 3172e3177
Fig. 6. Photopolymerization kinetics of TMPTA initiated by: (a) QA-DBU and BPO (,),
b) D-1173 (C).
Fig. 8. Kinetics of polymerization of TMPTA initiated by the corresponding initiator in
the presence of cyan pigment (0.8 wt%). During first w15 s, terminated the photo-
initiation, then monitored the self-propagating characteristic: (a) QA-DBU and BPO, (b)
BPO and as well of (c) BPO without UV exposure.
(
TMPTA (Fig. 7g). For the ratio of 1.5:0.5 (Fig. 7b), the initial poly-
merization rate of the acrylates was roughly twice that of the
system initiated only by QA-DBU, indicating that redox reaction
significantly accelerate the photopolymerization in air atmosphere,
which can be explained below. The photolatent superbase system
on exposure could first undergo the immediate initiation by
quaternary ammonium salt which formed radicals unimolecularly,
accompanied with the liberation of DBU. Due to the physical
diffusion toward the unreacted areas during polymerization, lon-
geval superbase would persistingly interact with peroxides, and
thus actuate highly effective redox reaction, exhibiting a remark-
able accelerating effect compared to QA-DBU system, therefore
improve the rate and conversion.
3.6. Self-propagating polymerization initiated by QA-DBU coupling
with BPO
Fig. 9. Changes in the stretching vibration of the acrylate carbonecarbon double bond
at 6163 cm over time: 0 min, 10 min, 1 h, 3 h, 5 h.
ꢁ
1
The major challenge in photopolymerizing thick bulk materials
arises from the fact that the light is unable to penetrate into deeper
layers due to the absorption of the upper layer, which prevents its
use in a variety of applications. Fig. 8a demonstrates the kinetic
curve of photopolymerization of TMPTA initiated by QA-DBU and
BPO, in the presence of cyan pigment (0.8 wt%) in order to get
a lower initial conversion. Remarkably, the two-component initi-
ator system didn’t exhibit rapid termination of TMPTA polymeri-
zation when the irradiation source was turned off. Specifically,
a threshold value of conversion with first 15 s illumination was
about 13%. However, polymerization efficiently carried on in the
dark and the final conversion value attained was w40% at 5 h. The
ꢁ1
decrease of C]C stretching vibration at 6163 cm is observed in
Fig. 9. In comparison, BPO alone with UV exposure or not wouldn’t
have a remarkable effect on the final conversion of photo-
polymerization initiated by QA-DBU and BPO (Fig. 8b and c), which
further confirms the persistent interaction of produced longeval
amine with peroxides via the self-propagation of superbase toward
the unreacted areas. These desirable results for self-propagation
polymerization provide a promising future for their application to
high pigmented photopolymerizable compositions.
4. Conclusion
A highly effective strategy for photopolymerization of acrylates
via photolatent redox-accelerated reaction based on the synergistic
photoinitiating systems containing photolatent superbase (QA-DBU)
and readily available peroxides was presented. The results show
significant advantages over conventional photolatent base-catalyzed
ring-opening polymerization of epoxide resin as they exhibit: lower
temperature photopolymerization, higher polymerization rates and
Fig. 7. Polymerization kinetics of TMPTA irradiated at 20 mW/cm2 in air for different
QA-DBU:BPO ratios: (a) 2:0 (B), (b) 1.5:0.5 (-), (c) 1.34:0.66 (C), (d) 1:1 (:), (e)
0.66:1.34 (+), (f) 0.5:1.5 (A) and (g) 0:2 (,).

M. He et al. / Polymer 53 (2012) 3172e3177
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3177
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