Synthesis of main chain polymeric benzophenone photoinitiator via thiol-ene click chemistry and Its use in free radical polymerization

Article abstract of DOI:10.1002/pola.24330

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using "Thiol-ene Click Chemistry" and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absor

Full text of DOI:10.1002/pola.24330

Synthesis of Main Chain Polymeric Benzophenone Photoinitiator via
Thiol-ene Click Chemistry and Its Use in Free Radical Polymerization
1
,2
2
2
GOKHAN TEMEL, NURCAN KARACA, NERGIS ARSU
1
Department of Polymer Engineering, Faculty of Engineering, Yalova University, Yalova 77100, Turkey
2
Department of Chemistry, Yildiz Technical University, 34210 Davutpasa, Istanbul, Turkey
Received 10 August 2010; accepted 19 August 2010
DOI: 10.1002/pola.24330
Published online 13 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Main chain polymeric benzophenone photoinitiator
PBP) was synthesized by using ‘‘Thiol-ene Click Chemistry’’
PBP was found as 250 ns. The photoinitiation efficiency of PBP
(
was determined for the polymerization of Hexanedioldiacrylate
(HDDA) with PBP and BP in the presence of a coinitiator
namely, N-methyldiethanolamine (MDEA) by Photo-DSC. The
initiation efficiency of PBP for polymerization of HDDA is much
higher than for the formulation consisting of BP. VC 2010 Wiley
1
and characterized with H NMR, FTIR, UV, and phosphores-
cence spectroscopies. PBP as a polymeric photoinitiator pre-
sented excellent absorption properties (e294 ¼ 28,300 molꢀ
1
ꢀ1
L
ꢀ
1
cm ) compared to the molecular initiator BP (e252 ¼ 16,600
ꢀ
1
ꢀ1
ꢀ1
mol
L
cm ). The triplet energy of PBP was obtained from
Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5306–
5312, 2010
the phosphorescence measurement in 2-methyl tetrahydrofur-
ane at 77 K as 298.3 kJ/mol and according to phosphorescence
lifetime, the lowest triplet state of PBP has an n-p* nature. Tri-
plet–triplet absorption spectrum of PBP at 550 nm following
laser excitation (355 nm) were recorded and triplet lifetime of
KEYWORDS: benzophenone; click chemistry; photopolymeriza-
tion; photochemistry; polymeric photoinitiator; radical poly-
merization; thiol-ene
INTRODUCTION Photopolymerization processes receive con-
tinuous interest because of their wide application in UV-cur-
able coatings, printing inks, photoresist, and microelec-
tronics. Free radical photoinitiators can be classed as a-
cleavage (Type I) and H-abstraction (Type II) initiators. Free
radical generation process is H-abstraction reaction of tri-
plet photoinitiator from hydrogen donors such as amines
and alcohols. The radical derived from the donor can initiate
the polymerization while ketyl radicals stemming from aro-
matic carbonyl compound are usually not reactive toward
vinyl monomers due to bulkiness and/or the delocalization
of the unpaired electrons. The overall process is depicted on
the example of benzophenone in Scheme 1.
meric photoinitiators and of photoproducts means that cured
coatings are less prone to yellowing.
1
2,13
In our previous studies, to overcome some of the problems,
we successfully synthesized novel side chain thioxanthone
and benzophenone containing polystyrene based polymeric
photoinitiators and they were used as polymeric photoinitia-
5,6,10,14–16
tors in free radical polymerization.
Over the last decade, efficient and versatile cross reactions such
1
4,17,18
5,16,19–32
as radical coupling reactions,
diels alder reactions,
click chemistry,
5
,33,34
35–37
thio-bromo click reaction,
etc.,
have been used to obtain functional polymers in quantitative
yield. Cu (I) catalyzed 1,3-Huisgen dipolar cycloaddition is the
most widely used reaction to synthesize telechelic poly-
2
6,27
23,38,39
There have been many new developments in synthesis and
photochemical studies of novel polymeric photoinitiators.
Polymeric photoinitiators have attracted much attention in
recent years for they combine the properties of polymers
mers,
functional dendrimers,
hyperbranched poly-
4
0
41
5,16
mers, biopolymers, polymeric photoinitiators
and so on.
High temperature conditions or the requirement of metal cata-
lysts, which generally need to be removed by various purifica-
tion methods after the reaction, are the only deterrents of the
click method. Recently, thiol-ene chemistry has appeared as a
new type of click chemistry method in the absence of a toxic
1
–11
with those of low molecular weight photoinitiators.
Solu-
bility and miscibility problems, often observed with coatings
containing low molecular weight photoinitiators, do not
occur with the polymeric ones since polymers are easily mis-
cible with the resin to be cured as well as with the final
cured film. Moreover, odor and toxicity problems do not
occur with macrophotoinitiators owing to the low volatility
of the large molecules. The low migration tendency of poly-
2
0,42
metal catalyst.
Thiol-ene click chemistry has been success-
43,44
45
fully employed to synthesize biomaterials,
crosslinked materials,
star polymers,
42,46,47
42,48,49
and UV-curable coatings.
However, thiol-ene reactions can be initiated photochemically
2
0,42
or thermally in the presence of a photo or thermal initiator.
Correspondence to: G. Temel (E-mail: gtemel@yalova.edu.tr) or N. Arsu (E-mail: narsu@yildiz.edu.tr)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5306–5312 (2010) V
2010 Wiley Periodicals, Inc.
C
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ARTICLE
the basic alumina just before use. All other reagents were pur-
chased from Aldrich and Merck and used as received.
Synthesis of Diallyl Benzophenone (BP-Al)
0
A solution of 4,4 -dihydroxybenzophenone (1 g, 4.67 mmol)
and anhydrous potassium carbonate (1.6 g, 12 mmol) in 30
mL of acetone was stirred for 6 h. After that, allyl bromide
(
1.32 g, 11 mmol) was added and the mixture was refluxed
for 24 h. Acetone was removed on the rotary evaporator, and
the residue was diluted with dichloromethane and washed
with water. Then, it was dried over Na SO . The crude prod-
2
4
uct was recrystallized from ethanol (yield 75%).
þ
GC-MS (70 eV) m/e: 294 (M ). ELEM. ANAL. for C H O
1
9
18
3
ꢀ
1
(
294.13 g mol ): Calc., C, 77.53%; H, 6.16%; Found, C,
7.49%; H, 6.09%. IR (ATR): m (cm ) 3069 (CAH, aro-
ꢀ
1
7
matic), 2930 (CAH, aliphatic), 1636 (C ¼¼ O, ketone), 1599
SCHEME 1 Photoinduced radical polymerization mechanism of
ꢀ
1
(
C ¼¼ C, aromatic) cm
.
benzophenone in the presence of a coinitiator.
1
H NMR (CDCl 250 MHz): d ¼ 7.78-7.75 (d, 4H, aromatic),
3
In this study, we used the thiol-ene click chemistry method
to synthesize benzophenone based macromolecular photoini-
tiator and it was used in photoinduced free radical polymer-
ization as the photoinitiator in the presence of a coinitiator
namely N-methyldiethanolamine. The main chain polymeric
7.24-6.97 (d, 4H, aromatic), 6.13-5.98 (m, 2H, CH), 5.46-5.29
(m, 4H, CH ), 4.61-4.59 (d, 4H, CH ).
2
2
Synthesis of Polymeric Photoinitiator with Thiol-ene
Click Chemistry (PBP)
1
BP-Al (0.5 g, 1.7 mmol) and a catalytic amount of AIBN (1–2%
w/w) were dissolved in 3 mL of DMF in a Schlenk tube and
the solution was bubbled with argon for 15 min. 1, 2-ethanedi-
thiol (1.7 mmol, 143 lL) was added to this solution and the
solution was bubbled for another 5 min with argon. After that
photoinitiator was characterized by GPC, H NMR, UV, and
phosphorescence spectroscopy and the monomer conver-
sions and rate of polymerization of diacrylate monomer
were also determined by Photo-DSC experiments in air and
nitrogen atmospheres.
ꢁ
tube was kept under argon and stirred at 85 C for 24 h. At
the end of the period, polymeric photoinitiator was isolated by
two times of precipitation in excess methanol. The white prod-
EXPERIMENTAL
Materials
uct was filtered and dried in vacuo at room temperature.
0
ꢀ1
2,2 -Azobis(isobutyronitrile) (AIBN, 98%, Aldrich) and Benzo-
(
Yield: 60%, Mn,GPC ¼ 2300 g mol , M /M ¼ 1.28).
w
n
0
phenone (99%, Aldrich) were recrystallized from ethanol. 4,4 -
Dihydroxybenzophenone (97%, Aldrich), allyl bromide (97%,
Aldrich), 1, 2-ethanedithiol (>90%, Aldrich), and N-methyldie-
thanolamine (MDEA, 99%, Aldrich) were used as received.
Hexandioldiacrylate (HDDA, 93%, ABCR) was passed through
ꢀ
1
IR (ATR): m (cm ) 3068 (C ¼¼ C, aromatic), 2926 (CAH, ali-
ꢀ
1
phatic), 1637 (C ¼¼ O ketone), 1599 (C ¼¼ C, aromatic) cm
.
1
H NMR (CDCl
3
250 MHz): d ¼ 7.76-6.91 (m, 8H, aromatic),
6.10-6.00 (m, 2H, CH), 5.46-5.29 (m, 4H, CH ), 4.61-4.59
2
SCHEME 2 Synthesis of bisallyl-
benzophenone (BP-Al) and poly-
meric photoinitiator (PBP) via
Thiol-ene click polymerization.
SYNTHESIS OF MAIN CHAIN POLYMERIC BENZOPHENONE PHOTOINITIATOR, TEMEL, KARACA, AND ARSU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1
3
FIGURE 1 H NMR spectra of BP-Al and PBP in CDCl .
(
d, 4H, CH ), 4.12 (t, 4H, CH ), 2.86-2.75 (m, 8H), 2.1-2.0
were taken on a Varian UV-Visible Carry 50 Spectrophotome-
ter. Gel permeation chromatography (GPC) analyses of the
polymers were performed with a set-up consisting of a
pump (Waters) and four ultrastyragel columns of different
porosities. Tetrahydrofuran (THF) was used as the eluent
(flow rate 0.3 mL min ), and detection was carried out
with the aid of a differential refractometer. The number-aver-
age molecular weights were determined using polystyrene
2
2
(m, 4H).
Instrumentation
1
H NMR measurements were recorded in CDCl3 with Si
CH ) as internal standard, using a Bruker AC250 (250.133
ꢀ
1
(
3
4
MHz) instrument. FTIR spectra were recorded on a Perkin–
Elmer FTIR Spectrum One-B spectrometer. UV-Vis spectra
SCHEME 3 Photoinitiated radical
polymerization mechanism of
PBP in the presence of MDEA as
a coinitiator.
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ARTICLE
ꢀ
5
FIGURE 2 UV absorption spectra of BP [9 ꢃ 10 ] M and PBP
6 ꢃ 10ꢀ ] in CHCl
5
[
3
.
FIGURE 4 Transient optical absorption spectrum recorded at
2
80 and 480 ns following laser excitation (355 nm, 5 ns) in ar-
standards. Gas chromatography-mass spectroscopy (GC-MS)
was performed using an Agilent 6890/ 5973 inert gas chro-
matograph/mass selective detector system in electrospray
ionization mode (70 eV), through an HP-5MS capillary col-
umn using helium as the carrier gas at a flow rate of 1.6
mL/min. Laser flash photolysis experiments employed the
pulses from an Applied Photophysics with YAG laser
gon saturated chloroform solution of PBP. Inset: Triplet decay
at 550 nm.
and the measurements were carried out in isothermal mode
at room temperature under a nitrogen atmosphere (nitrogen
or air flow: 50 mL min ). The samples were irradiated for
ꢀ1
4
min. at room temperature. The heat flow as a function of
(
355 nm, pulse, 5 ns) and a computer controlled system. A
reaction time was monitored using Photo-DSC under isother-
mal conditions, and both the rate of polymerization and con-
version were then calculated as a function of time. The heat
of reaction value DHp
retical heat evolved for acrylate double bonds. Rates of po-
lymerization were calculated according to the following
chloroform solution of polymeric benzophenone photoinitia-
tor (PBP) was prepared with a concentration of BP such
that the absorbance was 0.5 at the excitation wavelength
theor
¼ 86 kJ/mol was used as the theo-
(355 nm).
50
Photo Differential Scanning Calorimetry (Photo-DSC)
The photoinitiated polymerization of HDDA was carried out
by TA-DSCQ100 equipped with a medium pressure mercury
arc lamp. This unit emits radiation predominantly in the
equation; Rp ¼ (Q/s)M/n DH m where Q/s is heat flow per
p
second, M the molar mass of the monomer, n the number of
double bonds per monomer molecule and m the mass of
monomer in the sample.
2
20–400 nm range, and provides light intensity of 50 mW
ꢀ
2
cm as measured by a UV radiometer capable of broad UV
RESULTS AND DISCUSSION
range coverage. The mass of the samples was ꢂ2 6 0.1 mg
0
Etherification reaction of the allyl bromide with 4,4 -dihy-
droxybenzophenone photoinitiator was preferred because it
has a reactive click component, and it can be easily obtained
in high yield. In addition, allyl ethers are more reactive than
styrenes, acrylates, and methacrylates in thiol-ene reac-
4
2
tions. Therefore, the main chain PBP was synthesized with
‘Thiol-ene Click Polymerization’’ method with diallyl benzo-
‘
phenone (BP-Al) and 1,2-ethanedithiol in the presence of cat-
alytic amounts of AIBN via thermal initiation (Scheme 2).
The resulting polymer was precipitated two times in metha-
nol to obtain a pure polymeric photoinitiator. According to
1
H NMR and FTIR studies, the desired polymeric structure
was entirely characterized. Allylic methine proton in 5.97–
6.13 and allylic methylene protons in 5.46–5.29 ppm showed
that a diallyl-end functional polymer was formed. Further-
1
more, a thiol signal could not be observed in either H NMR
1
or FTIR spectroscopic analyses (Fig. 1). According to
H
FIGURE 3 Phosphorescence spectra of PBP in 2-methyl tetrahy-
drofurane at 77 K (kexc ¼ 300 nm).
NMR spectrum, allylic protons at the end of the polymer and
aromatic protons of the benzophenone in the main chain
SYNTHESIS OF MAIN CHAIN POLYMERIC BENZOPHENONE PHOTOINITIATOR, TEMEL, KARACA, AND ARSU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
were used to determine the molecular composition of the
polymeric photoinitiator. PBP has five repeating units and
possesses diallyl end groups according to calculations from
1
H NMR spectrum and these results were in strong agree-
ꢀ
1
ment with GPC results (M ¼ 2300 g mol ).
n
The absorption behavior of the polymeric photoinitiator, PBP,
was compared with the low molecular weight analogue (BP)
which is used as a photoinitiator for many UV curing and
photopolymerization applications. As can be seen in Figure
2
, the absorption of PBP shifted to the red region wave-
ꢀ
1
ꢀ1
length (kmax ¼ 294 nm, e ¼ 28,300 mol L cm ) compared
to the low molecular weight analogue (k
¼ 252 nm, e ¼
max
ꢀ
1
ꢀ1
1
6,600 mol L cm ).
To gain more insight into the photophysical properties, PBP
fluorescence emission could not be observed as usual for most
of the BP derivatives. Therefore, phosphorescence measure-
ments were performed at 77 K in 2-methyl tetrahydrofurane
to get information about the triplet configuration of the photo-
initiator. PBP exhibited a phosphorescence emission spectrum
similar to the parent benzophenone. The (0, 0) emission band
occurred at 401 nm, corresponding to approximate triplet
energy of ca 298.3 kJ/mol (see Fig. 3). The phosphorescence
lifetime of PBP was calculated as 6.5 ms. Since the phosphores-
cence lifetime for nꢀp* triplets are significantly shorter (in the
order of several milliseconds) than pꢀp* triplets, we can com-
ment that the lowest triplet state of PBP has an nꢀp* nature.
FIGURE 6 Conversion versus time for the polymerization of
HDDA initiated by PBP/MDEA (0.1/1.0 w/w) and BP/MDEA (0.1/
ꢁ
1.0 w/w) systems cured at 30 C by UV light with an intensity
ꢀ2
of 50 mW cm in the presence and absence of an air atmos-
phere. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
nm) observed in the case PBP indicates a fast quenching
process due to the polymeric effect of the photoinitiator.
Photo-DSC is a unique method to obtain a fast and accurate
indication of the photoinitiator’s performance. It has been
well established that Photo-DSC experiments are capable of
providing kinetics data in which the measured heat flow can
be converted directly to the ultimate percentage conversion
and polymerization rate for a given amount of formulation.
Photopolymerization of HDDA formulations containing pho-
toinitiators (BP and PBP) with amine (MDEA) either in air
or nitrogen atmosphere was followed by Photo-DSC. To
Laser flash photolysis studies were performed to investigate
the triplet state behavior of the PBP. Figure 4 exhibits the
transient absorption spectra of degassed CHCl solution con-
3
taining polymeric photoinitiator after irradiation with laser
pulses of 355 nm. The maximum absorption is at 550 nm,
which is similar to the triplet–triplet absorption of parent
5
1,52
BP.
The transient was recorded at 250 ns, after the pulse
for PBP. However, a short triplet lifetime (s ¼ 250 ns at 550
FIGURE 5 Rate of polymerization versus time for the polymer-
FIGURE 7 Rate of polymerization versus time for the polymer-
ization of HDDA initiated by PBP/MDEA (0.1/1.0 w/w) and BP/
ization of HDDA initiated by PBP/MDEA (1.0/10.0 w/w) and BP/
ꢁ
ꢁ
MDEA (0.1/1.0 w/w) systems cured at 30 C by UV light with an
MDEA (1.0/10.0 w/w) systems cured at 30 C by UV light with
ꢀ
2
ꢀ2
intensity of 50 mW cm in the presence and absence of an air
atmosphere. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
an intensity of 50 mW cm in the presence and absence of an
air atmosphere. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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ARTICLE
TABLE 1 Photo-DSC Results of BP and PBP in the Absence and
22
Presence of Air (Light Intensity 5 50 mW cm
)
Photoinitiator
wt %)
Rpmax
Final
ꢀ
1
(
N
2
t
max (s)
(s
)
Conv. (%)
BP (0.1)
BP (0.1)
BP (1.0)
BP (1.0)
PBP (0.1)
PBP (0.1)
PBP (1.0)
PBP (1.0)
ꢀ
þ
ꢀ
þ
ꢀ
þ
ꢀ
þ
4.05
3.05
4.90
1.25
0.006
0.013
0.020
0.052
0.010
0.045
0.050
0.153
12
38
35
59
12
52
27
65
10.55
7.85
2.45
2.25
MDEA reached 38% conversion in nitrogen atmosphere. The
effect of oxygen inhibition on the photopolymerization reaction
is dependent on various parameters, one of which is the type of
initiator which was employed in the formulations. In our case
the molecular initiator BP and polymeric initiator PBP belong
to the type II class and as clearly seen from Figures 5 and 6;
the inhibition effect of oxygen is extremely detrimental.
FIGURE 8 Conversion versus time for the polymerization of
HDDA initiated by PBP/MDEA (1.0/10.0 w/w) and BP/MDEA (1.0/
ꢁ
1
0.0 w/w) systems cured at 30 C by UV light with an intensity
ꢀ2
of 50 mW cm in the presence and absence of an air atmos-
phere. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
The influence of initiator concentration on the rate of poly-
merization was also determined and the results are given in
Figures 5 and 7. When the initiator concentration was
evaluate the initiation efficiency of photoinitiator PBP, two
different concentrations of initiator were used and the poly-
merization profiles obtained for the formulations cured in
air or nitrogen atmospheres are shown in Figures 5–9 and in
Table 1.
increased from 0.1 to 1.0 wt % (under nitrogen) the rate of
ꢀ1
polymerization jumped from 0.045 to 0.153 s
for PBP.
Even in air atmosphere, the value of the rate of polymeriza-
tion of formulations consisting of PBP (1.0 wt %) (0.050
ꢀ
1
s
(
) was much higher than the formulation containing PBP
Under air, for both of the initiators even with MDEA, the rate of
polymerization is very low and the final conversion reaches
ꢀ
1
0.1 wt %) (0.010 s ). However, the effect of increasing the
photoinitiator concentration for BP was not as effective as
PBP (Fig. 9). As stated above, the inhibition effect of oxygen
is suppressed by increasing the initiator concentration from
1
2%. When air is omitted, under nitrogen atmosphere the poly-
merization rate is increased, especially for initiator PBP/MDEA
by a factor of 4. However, the formulations consisting of BP/
0.1 to 1.0% for PBP. The highest polymerization rate was
also obtained at high initiator concentrations for both initia-
tors and for both conditions (air and nitrogen).
CONCLUSIONS
The synthesis of PBP using thiol-ene click polymerization
was successfully achieved. Since PBP contains BP chromo-
phore groups, the initiation efficiency of PBP was compared
to low molecular weight analogue. Since the new polymeric
initiator PBP presents type II initiator characteristics (see
Scheme 3), it showed very good performance with N-methyl-
diethanolamine for the polymerization of HDDA possibly due
to the high molar absorptivity of PBP when compared to the
low molecular weight analogue BP.
The authors would like to thank to TUBITAK and Yildiz Techni-
cal University Research Fund for financial support.
FIGURE 9 Conversion versus time for the polymerization of
HDDA initiated by PBP/MDEA systems in different concentra-
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