Microporous thioxanthone polymers as heterogeneous photoinitiators for visible light induced free radical and cationic polymerizations

Article abstract of DOI:10.1021/ma501001m

Conjugated microporous polymeric networks possessing thioxanthone groups were reported to initiate free radical and cationic polymerizations of vinyl monomers and cyclic ethers, respectively, under visible light irradiation. These new classes of Type II macrophotoinitiators with high porosity having large BET surface area of 500-750 m² g^-1 were prepared through different cross-coupling processes. Polymerizations are successfully achieved in conjugation with several co-initiators benefiting from hydrogen abstraction or electron transfer reactions stimulated by either visible light or natural sunlight irradiation. Photopolymerizations conducted by using knitted photoinitiators show better conversion and rate of polymerization than those obtained via Sonogashira-Hagihara coupling. The heterogeneous nature of the photoinitiators makes them easily separable from the media and more importantly reusable for further polymerizations while retaining the photocatalytic activity.

Full text of DOI:10.1021/ma501001m

Article
pubs.acs.org/Macromolecules
Microporous Thioxanthone Polymers as Heterogeneous
Photoinitiators for Visible Light Induced Free Radical and Cationic
Polymerizations
†
‡
‡
‡
,†,§
Sajjad Dadashi-Silab, Hakan Bildirir, Robert Dawson, Arne Thomas, and Yusuf Yagci*
†
Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
‡
Department of Chemistry, Technische Universtat
̈
Berlin, Hardenbergstr. 40, 10623, Berlin, Germany
Center of Excellence for Advanced Materials Research (CEAMR) and Department of Chemistry, Faculty of Science, King Abdulaziz
University, 21589, Jeddah, Saudi Arabia
S Supporting Information
§
*
ABSTRACT: Conjugated microporous polymeric networks possessing
thioxanthone groups were reported to initiate free radical and cationic
polymerizations of vinyl monomers and cyclic ethers, respectively, under
visible light irradiation. These new classes of Type II macrophotoinitiators
2
−1
with high porosity having large BET surface area of 500−750 m g were
prepared through different cross-coupling processes. Polymerizations are
successfully achieved in conjugation with several co-initiators benefiting from
hydrogen abstraction or electron transfer reactions stimulated by either
visible light or natural sunlight irradiation. Photopolymerizations conducted
by using knitted photoinitiators show better conversion and rate of
polymerization than those obtained via Sonogashira−Hagihara coupling.
The heterogeneous nature of the photoinitiators makes them easily separable from the media and more importantly reusable for
further polymerizations while retaining the photocatalytic activity.
INTRODUCTION
applied to challenge some problems often encountered with the
low molecular weight photoinitiators such as migration, high
■
The use of light for green chemistry and energy applications is
an intriguing issue in today’s world. Thus, researchers are
focused on utilization of conversion of solar energy to usable
versions like electricity or production of green energy sources
23−30
1
volatility, and strong odor, etc.
As one step further,
heterogeneous semiconductor macrophotoinitiators have re-
31−36
cently been reported.
Porous materials with their high
2
,3
accessible pre/post-functionalized surfaces are mostly used
materials in heterogeneous catalysis in wide perspective from
like hydrogen via photocatalytic water splitting. Furthermore,
most of the photochemical reactions are useful to avoid
traditional high-energy consuming thermal methods to obtain
resulting products in green/sustainable way, which is crucial in
37−42
water splitting to chiral syntheses.
With respect to porous
metal oxides, porous organic polymers are favorable in this
aspect with their ease of functionality and higher surface
4
,5
industrial scale. In this aspect, photopolymerization is a
favorable method for low-cost production of polymers in milder
43
areas. Furthermore, covalently bonded microporous organic
polymers have the advantage of enhanced (hydro)thermal
stability in comparison to metal−organic frameworks effecting
constancy and reusability as a fundamental matter of
6
,7
conditions and relatively controlled media.
Photoinitiators are mainly classified into two categories: Type
I (also known as α-cleavage) and Type II (or hydrogen
abstraction type) initiators. Thioxanthone (TX) compounds
are well-known as efficient Type II photoinitiator in free radical
44−47
heterogeneous catalysis.
Herein, we report TX-based microporous organic polymers
as heterogeneous macrophotoinitiator for visible light induced
free radical and cationic polymerizations. Synthetic methods for
the preparation of these macrophotoinitiators were Sonoga-
shira−Hagihara cross-coupling reaction with triethynylbenzene
polymerization in conjugation with various hydrogen donor
8
(
H-donor) groups. An interesting advantage associated with
specially designed TXs is their capability to act as one
component initiator if the H-donor group is chemically
incorporated into the TX structure. This diminishes any
probable problem regarding the use of additional H-donor co-
(
TX-CMP) in standard route for conjugated microporous
48
polymers (CMPs), and knitted random copolymerization
with benzene (TX-Ph) and triphenylmethane (TX-TPM).
49,50
9
−17
initiators.
Regarding this, Yin and co-workers reported various
macrophotoinitiators owning both TX and H-donor function-
Received: May 14, 2014
Revised: June 20, 2014
1
8−22
alities.
Aside from their initiation ability of photo-
polymerization process, these macrophotoinitiators can be
©
XXXX American Chemical Society
A
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Scheme 1. Synthesis of TX-CMP by Sonogashira−Hagihara Coupling
Resulting polymers exhibited a large specific Brunauer−
range. The measurements were carried out in an isothermal mode at
−1
2
−1
2
5 °C with a nitrogen flow of 20 mL min .
Emmett−Teller (BET) surface area (500−750 m g ). The
photopolymerization processes were accomplished with H-
donor or iodonium salt co-initiators under visible light and also
natural sunlight irradiation.
Synthesis of 2,7-Dibromo-9H-thioxanthene-9-one (TX-
5
1
Br2). TX (2.5 g, 0.012 mol) was dissolved in 20 mL of acetic acid,
and 5 mL of Br was added dropwise at room temperature and heated
2
to 130 °C. After 20 h of reflux, cooled mixture was poured over ice and
filtered. The precipitate was washed with saturated solution of
EXPERIMENTAL SECTION
NaHCO , 20% aqueous solution of NaHSO , and water, subsequently.
3 3
■
The solid part was dried in vacuum and recrystallized in toluene.
Materials. Thioxanthone (TX; 97.0%, Aldrich), formaldehyde
dimethyl acetal (98%, Alfa Aesar), 1,3,5-triethynylbenzene (TEB; 98%,
Alfa Aesar), dimethylformamide (DMF; anhydrous, 99.8%, Aldrich),
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh ) ; 99%, Aldrich),
copper(I) iodide (CuI; 99.5%, Aldrich), benzene (99.8%, Aldrich),
triphenylmethane (TPM; 99%, Aldrich), iron(III) chloride (FeCl3;
7%, Aldrich), and 1,2-dichloroethane (DCE; 99.8%, Aldrich) were
used as received without further purification. Methyl methacrylate
MMA, 99%; Aldrich) was purified by passing through a basic alumina
column to remove inhibitor. Cyclohexene oxide (CHO; 98%, Aldrich)
was vacuum distilled over calcium hydride. Triethylene glycol
1
Resulting bright yellow solid was yielded as 2.57 g (59%). H NMR
4
3
(
400 MHz, CDCl ) δ: 8.75 (d, J = 2.34 Hz, 2H), 7.75 (dd, J = 8.60,
3
4
3
J = 2.32 Hz, 2H), 7.48 (d, J = 8.60 Hz, 2H) (Figure S1, Supporting
Information). EI m/z for [M] calculated: 367.85006; found:
3
4
+
3
67.85089.
Synthesis of TX-CMP. TX-Br (0.200 g, 0.54 mmol), TEB (0.081
2
9
g, 0.54 mmol), CuI (0.015 g, 0.08 mmol), and Pd(PPh ) (0.030 g,
3
4
0
.026 mmol) were dissolved in 20 mL of dry DMF and 15 mL of dry
(
TEA in a glovebox. The sealed flask was stirred for 3 days at 100 °C.
Precipitation were filtered and washed with THF, CHCl , deionized
3
−1
water, and MeOH. A pale yellow material was purified via soxhlete
wash in THF and MeOH for 1 day each. Dried in vacuum at 100 °C
and yielded 0.181 g (94%). 13C CP/MAS NMR (cross-polarization
magic angle spinning nuclear magnetic resonance) δ: 176, 135, 128,
dimethacrylate (TEGDA; M = 286.32 g mol , Aldrich), triethyl-
n
amine (TEA; 99%, Aldrich), N,N-dimethylaniline (DMA, Fluka),
tetrahydrofuran (THF; 99.9%; Aldrich), diphenyliodonium hexafluor-
+
−
ophosphate (Ph I PF ; 98%, Alfa Aesar), toluene (Merck), and
2
6
1
22, 89. Elemental analysis: calculated: C 81.80%, H 2.62%, S 10.40%,
methanol (MeOH; 99.9%, Merck) were used as received without any
further purification.
and O 5.19%. Found: C 67.92%, H 3.44%, and S 6.66%.
Synthesis of TX-Ph and TX-TPM Networks. Using a 100 mL
two-necked flask TX (5 mmol, 1 equiv) and one of either benzene or
TPM (5 mmol, 1 equiv) were dissolved in DCE (20 mL) under
nitrogen. Formaldehyde dimethyl acetal (35 or 50 mmol) was added
Instrumentation. Nuclear magnetic resonance (NMR) spectra
were obtained from a Bruker Avance II 400 MHz (100.6 MHz for
1
3
C), and Bruker Avance 400 MHz cross-polarization magic-angle
13
spinning (CP/MAS) (100.6 MHz for C) for solution and solid state,
followed by FeCl (35 or 50 mmol). The reaction mixture was then
respectively. Diffuse reflectance UV−vis was recorded on a Varian
3
heated to 80 °C, after which the solution precipitated. Heating was
continued for 18 h, after which the precipitate was removed by
filtration and extensively washed with methanol until the filtrate
became clear. The product was then Soxhlet extracted with methanol
and chloroform for 18 h each after which the polymer was removed by
filtration and dried in vacuo at 70 °C.
Cary 300. N gas sorption measurements were performed using a
2
QUADRASORB SI and Autosorb 1-MP. The mass spectra were
recorded in Finnigan MAT95S.
Gel permeation chromatography (GPC) measurements were
performed on a Viscotek GPC max auto sampler system consisting
of a pump, a Viscotek UV detector, and Viscotek a differential
refractive index (RI) detector with three ViscoGEL GPC columns
TX-Ph. Yield 0.9326 g (55%). 13C CP/MAS NMR δ: 176, 134, 127,
34* (*overlaps with spinning sideband). Elemental analysis:
calculated: C 83.42%, H 3.74%, S 8.56%, and O 4.28%. Found: C
77.57%, H 4.47%, and S 2.12%.
(
G2000H HR, G3000H HR, and G4000H HR, 7.8 mm internal
diameter, 300 mm length) in series. The effective molecular weight
ranges were 456−42 800, 1050−107 000, and 10 200−2 890 000,
respectively. THF was used as an eluent at flow rate of 1.0 mL
TX-TPM. Yield 2.1404 g (74%). 13C CP/MAS NMR δ: 176, 134,
127, 51*, 35* (*overlap spinning sideband). Elemental analysis:
calculated: C 87.50%, H 4.17%, S 5.56%, and O 2.78%. Found: C
80.06%, H 4.58%, and S 2.25%.
−
1
min at 30 °C. Both detectors were calibrated with PS standards
having narrow-molecular-weight distribution. Data were analyzed
using ViscotekOmniSEC Omni-01 software.
Fourier transform infrared (FTIR) spectra were recorded on
PerkinElmer FTIR Spectrum One spectrometer with an ATR
accessory (ZnSe, PikeMiracle accessory) and cadmium telluride
Polymerizations. Visible Light Induced Free Radical Polymer-
ization. MMA (1 mL, 9.39 mmol) and TEA (10 μL, 0.07 mmol) (or
+
−
Ph I PF (30 mg, 0.07 mmol)) were dissolved in 2 mL of toluene,
2 6
(
MCT) detector.
and TX polymer (25 mg) was added to the solution. Then the content
was transferred to a Pyrex tube and degassed with highly pure nitrogen
gas for 20 min. The solution was irradiated using a photoreactor
equipped with six lamps emitting light at 400−500 nm. The light
Photodifferential scanning calorimetry (photo-DSC) measurements
were carried out by means of a modified PerkinElmer Diamond DSC
equipped with a Polilight PL400 Forensic Plus light source between
−2
3
20 and 500 nm. A uniform UV light intensity is delivered across the
intensity was 30 mW cm as measured by a Delta Ohm model HD-
9021 radiometer. After irradiation, the samples were dissolved in THF,
and TX polymers were separated by centrifugation. Then the samples
DSC cell to the sample and reference pans. The intensity of the light
−2
was measured as 40 mW cm by a UV radiometer covering broad UV
B
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were precipitated into methanol. The obtained polymers were dried
under vacuum for 24 h.
demonstrated efficient incorporation of two monomers into the
polymer via ethynyl linkage. The peaks at 89 and 122 ppm
represent the signals of triple bonds and 3° carbons of
triethynylbenzene (TEB) within the structure, respectively. The
peak at 176 ppm corresponds to the carbonyl carbon of the TX
groups. Other peaks are assigned to both TX and TEB
moieties.
Sunlight Induced Free Radical Polymerization. The procedure for
sunlight induced free radical polymerization followed that of free
radical polymerization under artificial visible light irradiation, except
that samples were irradiated under natural sunlight rays. The samples
were placed in the campus of Istanbul Technical University in March
−2
2
014. The light intensity of sunrays was measured as 15 mW cm .
Free Radical Promoted Cationic Polymerization. CHO (1 mL,
The polymers yielded from random copolymerization of TX-
+
−
9
.88 mmol), toluene (2 mL), and Ph I PF (30 mg, 0.07 mmol) (or
Ph and TX-TPM via Friedel−Crafts alkylation process
2
6
DMA (10 μL, 0.07 mmol)) were charged to a Pyrex tube, and TX
polymer (25 mg) was added to the solution. The tube was filled with
nitrogen gas prior to irradiation. Then, the solution was irradiated
using a photoreactor equipped with six lamps emitting light at 400−
00 nm. After irradiation, the samples were dissolved in THF and,
after separation of the TX polymers by centrifugation, precipitated into
13
(
Scheme 2) were also characterized using CP/MAS
C
NMR spectroscopy.
The spectra exhibited a typical carbonyl peak at 176 ppm like
their analogue TX-CMP. Since TX-Ph and TX-TPM are
random copolymerized materials, comparison of these knitted
polymers with their homopolymer analogues without TX
clearly indicates the final structures composed of both TX and
aromatic comonomers (Figures S3 and S4). In the case of TX-
Ph, the slight shift from 134 to 133 ppm and significant increase
of the peak intensity at 127 ppm acknowledge the
copolymerization of benzene and TX. Similar changes have
been observed for the TPM samples as noted by the shift from
5
methanol. The polymers were dried under vacuum for 24 h.
Preparation of Photocurable Formulations for Photo-DSC. A
typical photopolymerization in photo-DSC is as follows: for 1 mL of
TEGDA was added 10 μL (0.07 mmol) of TEA or 30 mg (0.07 mmol)
+
−
of Ph I PF . Just before irradiation TX polymer (5 mg) was added
2
6
into the mixture. Then 6−10 mg of the reaction solution was
transferred to a photo-DSC pan and irradiated in an isothermal mode
−1
at 25 °C with a nitrogen flow of 20 mL min .
1
35 to 134 ppm and the increase of the peak intensity at
RESULTS AND DISCUSSION
around 127 ppm.
■
N gas sorption properties and pore size distribution of the
2
Microporous TX-based polymeric photoinitiators were pre-
pared by two coupling processes. TX-CMP was synthesized via
Sonogashira−Hagihara cross-coupling reaction according to the
procedure described by the Cooper group (Scheme 1).
The main structural characterization method of such carbon-
photoinitiators were also investigated. Typical N adsorption/
2
desorption isotherms of the TX polymers are presented in
Figure 2. The BET surface areas and pore sizes of the TX
photoinitiators are tabulated in Table 1.
Conventional TX photoinitiators usually have absorption in
the near-UV range. The microporous TX photoinitiators
exhibit a broad and strong absorbance peak in the visible
region enabling initiating activity even in the sunlight (vide
infra) (Figure 3).
48
1
3
rich insoluble polymers, CP/MAS C NMR (Figure 1),
To demonstrate the photocatalytic activity of the synthesized
TX polymers, free radical polymerization of MMA was
examined under visible light irradiation. For this purpose,
triethylamine (TEA) and diphenyliodonium hexafluorophos-
+
−
phate (Ph I PF ) were used as co-initiators. Table 2
2
6
summarizes the results of the photopolymerization of MMA
under visible light irradiation. Control experiments indicated
that both initiating components (TX and co-initiator) were
indispensable, and irradiations in the absence of each
component gave no polymer. In addition, no polymer was
resulted in the dark condition signifying a light-driven process.
All new macrophotoinitiators could initiate the polymerization
of MMA. However, TX-TPM was more efficient than the
Figure 1. Solid-state 13C NMR spectra of thioxanthone polymers.
Scheme 2. Synthesis of TX-TPM and TX-Ph by Friedel−Crafts Alkylation Process (Knitting Method)
C
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Table 2. Visible Light Induced Polymerization of MMA
Using Microporous Thioxanthone Polymers in the Presence
of Amine or Iodonium Salt Co-initiators at Room
a
Temperature
co-initiator
+
−
TEA
Ph I PF
2
6
b
c
TX
type
conv
Mn
conv
Mn
(g mol−1) M /Mn
c
(%) (g mol−1) M /M
w n
run
1
(%)
w
TX-
TPM
26
151000
1.85
1.70
22
163550
2.35
2
TX-
15
202000
10
95200
1.80
CMP
TX-Ph
TX
3
4
21
13
141500
40540
2.10
1.45
16
10
154360
51660
2.15
1.75
a
Irradiation time = 4 h, [MMA] = 3.13 M, [TEA] = 0.023 M,
+
−
[
Ph I PF ] = 0.023 M, toluene = 2 mL, TX-polymer = 25 mg.
2 6
c
b
Measured gravimetrically. Number-average molecular weight (M )
n
and molecular weight distribution (M /M ) determined by GPC.
w
n
requirements. This can be achieved, for instance, by suitable
initiating systems working under visible light or more favorably
under natural sunlight irradiation. To investigate the versatility
of the so-prepared TX macrophotoinitiators, photopolymeriza-
tion of MMA was studied using the same procedure but under
sunlight illumination. Polymerizations were conducted by
irradiation directly under sunlight without use of any light
filter. Table 3 shows the results of these experiments.
Figure 2. (a) N sorption isotherms of BET surface area measurement
and (b) pore size distribution of the TX polymers.
2
Table 1. BET Surface Area Measurement and Pore Size
Distributions of Thioxanthone Polymers
Table 3. Sunlight Induced Polymerization of MMA Using
Thioxanthone Macrophotoinitiators in the Presence of
Amine and Iodonium Salt Co-initiators at Ambient
2
TX type
surface area (m g−1)
pore size (nm)
TX-CMP
TX-Ph
497
537
749
1.4
1.4
1.4
a
Temperature
TX-TPM
co-initiator
+
−
TEA
Ph I PF
2
6
b
c
TX
type
conv
Mn
conv
Mn
(g mol−1) M /Mn
c
(%) (g mol−1) M /M
w n
run
1
(%)
w
TX-
TPM
31
114700
126700
112850
2.62
2.25
2.45
23
14
18
86400
114700
109650
1.95
1.75
2.05
2
3
TX-
CMP
19
25
TX-Ph
a
Irradiation time = 4 h, [MMA] = 3.13 M, [TEA] = 0.023 M,
+
−
[
Ph I PF ] = 0.023 M, toluene = 2 mL, TX-polymer = 25 mg.
2 6
c
b
Measured gravimetrically. Number-average molecular weight (M )
n
and molecular weight distribution (M /M ) determined by GPC.
w
n
A bifunctional monomer, triethylene glycol diacrylate
TEGDA), was used to study the kinetics of the free radical
Figure 3. Solid-state UV−vis spectra of thioxanthone polymers.
(
polymerization initiated by TX polymers by means of photo-
DSC. Bifunctionality results in formation of polymeric films,
which have achieved immense industrial applications. Photo-
DSC is capable of measuring any liberating heat during the
photopolymerization process. This is directly proportional to
the number of acrylate double bonds reacted in the
photocuring process. Consequently, the rate of polymerization
others were, as it provided higher monomer conversion.
Furthermore, as co-initiator, TEA behaved more efficient than
the iodonium salt.
For sake of comparison a model reaction, employing low
molecular weight TX was performed for both TEA and
iodonium salt co-initiators under the same experimental
conditions (run 4, Table 2). Apparently, the polymeric
photoinitiators exhibit enhanced initiation efficiency probably
due to the better absorption characteristics arising from the
(
the number of monomers being consumed over total number
of monomers per time), R (1/s), can be determined according
P
to eq 1:
5
2−54
2D/3D structures.
R = (Q ) × 1000/[n × (E )]
P
t
db
(1)
From the point of view of energetic issues, many attempts in
the field of photopolymerization have been focused on
designing such initiating systems that need low energetic
where Q (J mol− s ) is the amount of released heat at time t,
1
−1
t
−1
n the number of acrylate double bonds, and Edb (kJ mol ) is
D
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Figure 4. Kinetics of the photopolymerization of TEGDA with TX macrophotoinitiators using (a) TEA and (b) iodonium salt as co-initiator
measured by photoDSC. Solid lines: rate of polymerization (R (1/s)); dashed lines: conversion (%).
P
Scheme 3. Proposed Initiation Mechanism of Free Radical Polymerization Using Thioxanthone Macrophotoinitiators in the
Presence of Amine or Iodonium Salt Co-initiators under Visible Light or Sunlight Irradiation
−
1
the energy of acrylate double bond (∼86 kJ mol ). By
An important advantage of heterogeneous photoinitiators is
their easily separable properties from the polymerization
mixture, which can be used further to initiate polymerization.
To clarify the reusability of these heterogeneous TX macro-
photoinitiators, polymerization of MMA was repeated three
times under the same conditions, which followed separation
and refining the TXs and reloading them for the next
polymerizations. Figure 5 shows the conversion of the
polymerization of MMA in three cycles in which remained
almost constant for all of TXs.
In order to gain insight into the possible effect of
photopolymerization on the TXs structures, FTIR analysis
was performed. Figure 6 shows typical FTIR spectra of one of
the TX photoinitiators (TPM-TX) before and after photo-
polymerization of MMA as well as that of PMMA. The FTIR
results showed no significant changes in the structure of TX
macrophotoinitiator without any detectable traces of PMMA in
the structure.
integrating the area under R −t graph, the conversion of the
P
acrylate monomer is obtained. Figure 4 depicts the kinetics of
the photopolymerization of TEGDA using TX polymers in the
presence of TEA or iodonium salt co-initiators, measured by
photo-DSC.
Kinetic results revealed photocatalytic activity of TX-TPM to
be the highest among all TX polymers. This was in agreement
with the UV−vis measurements and gas sorption results, which
had indicated higher photocatalytic activity of this knitted
polymer (higher absorbance and larger specific surface area).
Depending on the type of co-initiator, we propose two
distinct initiation mechanisms, namely hydrogen abstraction
and electron transfer reactions. In the case of TEA, photo-
excited TX species abstracts hydrogen to form initiating radicals
on the co-initiator. With iodonium salt, however, a different
situation is encountered. The photoexcited TX moiety forms an
exciplex with the ground state iodonium salts which is reduced
to Ph I . The subsequent decomposition of this unstable
•
To expand the applicability of TX macrophotoinitiators even
further, cationic polymerization of cyclohexene oxide (CHO)
was also examined using all of three TXs under visible light
irradiation. Irradiation of a mixture of CHO, TX, and iodonium
salt under visible light failed to form any polymer. However, by
addition of an H-donor compound, for instance N,N-
dimethylaniline (DMA) or tetrahydrofuran (THF), the system
2
species ultimately forms initiating phenyl radicals. Scheme 3
illustrates the general mechanism of photopolymerization using
TX compounds in the presence of amine or iodonium salt co-
initiators. The failure of the polymerization in the absence of
co-initiator indicates that intramolecular hydrogen abstraction
or electron transfer reactions are not probable.
E
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Scheme 4. Free Radical Promoted Cationic Polymerization
of CHO by Using TX Polymers and Iodonium Salt in the
Presence of a Hydrogen Donor Compound (DMA or THF)
under Visible Light Irradiation
Figure 5. Reusability of TX polymers in free radical polymerization of
MMA under visible light irradiation by applying the same conditions
(
irradiation time = 4 h, [MMA] = 3.13 M, [TEA] = 0.023 M, toluene
=
2 mL).
Table 4. Visible Light Induced Free Radical Promoted
Cationic Polymerization of CHO Using Microporous
Thioxanthone Polymers and Iodonium Salt in the Presence
Figure 6. FTIR spectra of TX-TPM (before and after photo-
polymerization) and PMMA.
a
of DMA or THF as H-Donors at Room Temperature
could successfully polymerize CHO. TX ketyl radical undergoes
electron transfer reactions with iodonium salt to yield protonic
acids capable of initiating cationic polymerization. DMA was
deliberately chosen as the amine co-initiator in the cationic
system so as to prevent possible termination reactions. Because
of the higher basicity, aliphatic amines are known to terminate
H-donor
DMA
THF
b
c
TX
type
conv
Mn
conv
Mn
(g mol−1) M /Mn
c
(%) (g mol−1) M /M
w n
run
1
(%)
w
TX-
TPM
37
4960
2140
3250
3.25
2.70
2.90
31
15
28
5120
2610
3100
2.80
2.15
3.10
55
cationic polymerization. The radicals formed from the co-
initiator DMA as described for the free radical polymerization
are readily oxidized to corresponding carbocation. Similarly,
THF generates oxidizable radicals by hydrogen abstraction. The
polymerization mechanism involving all possibilities is
presented in Scheme 4. The polymerization results are
collected in Table 4.
2
3
TX-
CMP
18
29
TX-Ph
a
I+PF −] = 0.023 M,
Irradiation time = 4 h, [CHO] = 3.30 M, [Ph
[DMA] = 0.023 M, [THF] = 0.023 M, toluene = 2 mL, TX-polymer =
5 mg. Measured gravimetrically. Number-average molecular weight
2
6
b
c
2
(
M ) and molecular weight distribution (M /M ) determined by
n
w
n
Reusability was also examined in the cationic pathway. As
illustrated in Scheme 4, oxidation of ketyl radicals results in
regeneration of the TX functionality facilitating the reusability
of the TX macrophotoinitors. Figure 7 shows the conversion of
CHO polymerized by TX macrophotoinitiators in the presence
of DMA co-initiator after three times recycling, which verifies
no decay in the photocatalytic activity.
GPC.
CONCLUSION
In summary, new classes of TX macrophotoinitiators were
synthesized via different cross-coupling reactions resulting in
high porosity (500−750 m g ) and excellent optical
characteristics. The obtained conjugated microporous TX
networks readily initiate free radical and cationic polymer-
izations of vinyl monomers and cyclic ethers, respectively. In
the polymerization processes, hydrogen abstraction and
electron transfer reactions of the photoexcited TX play a
major role for the generation of the reactive species. In the free
radical mode, excited TX abstracts hydrogen from the amine
■
2
−1
Figure 7. Reusability of TX-polymers in free radical promoted cationic
polymerization of CHO under visible light irradiation applying the
same conditions (irradiation time = 4 h, [CHO] = 3.30 M,
+
−
[
Ph I PF ] = 0.023 M, [DMA] = 0.023 M, toluene = 2 mL).
2 6
F
dx.doi.org/10.1021/ma501001m | Macromolecules XXXX, XXX, XXX−XXX

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Article
co-initiator or directly reduces iodonium salt to initiate the
polymerization. In the corresponding cationic polymerization,
the oxidation of the photochemically formed electron donor
radicals is responsible for the successful initiation. Facile
separation of these heterogeneous photoinitiators from the
polymer mixture, retaining photocatalytic activity after several
uses in both free radical and cationic polymerizations,
microporosity, capability of being carried out under visible
light or even sunlight irradiation, applicability to different
polymerization modes, eliminating any possible problem
associated with the low molecular weight compounds make
them high potential photoinitiators for widespread applications.
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ASSOCIATED CONTENT
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Supporting Information
Scope, Reactivity and Efficiency; Wiley-VCH: Weinheim, 2012.
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H NMR and CP/MAS 13C NMR spectra of thioxanthone
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photoinitiators. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Tel +90-212 285 3241; Fax +90-212 285 6386; e-mail yusuf@
itu.edu.tr (Y.Y.).
■
*
Macromolecules 2008, 41 (7), 2401−2405.
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Author Contributions
S.D.-S. and H.B. contributed equally.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors thank the Istanbul Technical University Research
Fund and European Research Council (ERC) (Grant:
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78593_ORGZEO) and People Programme (Marie Curie
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no. PIEF-GA-2011-300534 for financial support.
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