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Synthesis and Characterization of Polymeric Thioxanthone
Photoinitatiors via Double Click Reactions
Burcin Gacal,†^ Hakan Akat,†,§^ Demet K. Balta,‡^ Nergis Arsu,‡^ and Yusuf Yagci*,†
Department of Chemistry, Istanbul Technical Uni V ersity, Maslak, Istanbul, 34469, Turkey, and
Department of Chemistry, Yildiz Technical Uni V ersity, Da V utpasa Campus, Istanbul, 34210, Turkey
Recei V ed No V ember 10, 2007; Re V ised Manuscript Recei V ed January 14, 2008
ABSTRACT: Macrophotoinitiators containing thioxanthone (TX) moieties as side chains were synthesized by
using “double click chemistry” strategy; combining in-situ 1,3-dipolar azide-alkyne [3 + 2] and thermoreversible
Diels–Alder (DA) [4 + 2] cycloaddition reactions. For this purpose, thioxanthone-anthracene (TX-A),
N -propargyl-7-oxynorbornene (PON), and polystyrene (PS) with side-chain azide moieties (PS-N 3 ) were reacted
in N,N -dimethylformamide (DMF) for 36 h at 120 °C. In this process, PON acted as a “click linker” since it
contains both protected maleimide and alkyne functional groups suitable for 1,3-dipolar azide-alkyne and
Diels-Alder click reactions, respectively. This way, the aromacity of the central phenyl unit of the anthracene
moiety present in TX-A was transformed into TX chromophoric groups. The resulting polymers possess absorption
characteristics similar to the parent TX. Their capabilities to act as photoinitiator for the polymerization of mono-
and multifunctional monomers, namely methyl methacrylate (MMA) and 1,1,1-tris(hydroxymethyl)propane
triacrylate (TPTA) were also examined.
Introduction
Recently macrophotoinitiators, which are considered as
macromolecules containing covalently bonded photoinitiating
groups, have been the subject to an increased research interest
since they may offer various advantages over the low molecular
weight photoinitiators such as greater reactivity, low volatility,
and low migration due to the well-known polymeric effect.
Macrophotoinitiators possessing chromophoric groups either in
the main chain or as pendant groups can be prepared in two
ways: (i) synthesis and polymerization of monomers with
photoreactive groups; (ii) introduction of photoactive groups
into polymer chains. In the latter case, macrophotoinitiators were
synthesized by using functional initiators and terminators in a
particular polymerization or by reacting functional groups of a
preformed polymer with other functional groups of low mo-
lecular weight compounds possessing also photoreactive groups.
Macrophotoinitiators, analogous to the low molecular weight
photoinitiators, are divided into two classes, according to their
radical generation mechanism, namely cleavage-type (type I)
and hydrogen abstraction-type (type II) macrophotoinitiators.^1
Typical type II photoinitiators include benzophenone and
derivatives, thioxanthone, benzil, quinones, and organic dyes,
while alcohols, ethers, amines, and thiols are used as hydrogen
donors. Among these initiators, thioxanthone (TX) derivatives
have recently received a revitalized interest because of their
absorption characteristics at near-UV range.2–11^ Moreover, their
simple synthetic procedure allows various modifications for
wavelength tunability or improved solubility by the incorpora-
tion of appropriate substituents on the thioxanthone structure.
Free radical generation process is H-abstraction reaction of TX
triplets from hydrogen donors such as amines and alcohols. The
radical derived from the donor can initiate the polymerization
while the radicals stemming from TX are usually not reactive
toward vinyl monomers due to bulkiness and/or the delocal-
ization of the unpaired electrons. Various structurally different
TX derivatives including dendritic,2,3^ polymeric,^11 and one-
component ones4–6,12,13^ possessing both light-absorbing chro-
mophoric group and hydrogen-donating sites in the same
structure have been synthesized, and their photochemistry has
been studied in detail.
The “click”-type reactions, mainly exemplified by Huisgen
1,3-dipolar azide-alkyne,^14 [3 + 2], or Diels–Alder cycload-
ditions,^15 [4 + 2], have attracted much attention due to their
important features including high yields, high tolerance of
functional groups, and selectivity.^16 Thiol-ene chemistry^17 has
recently been introduced as an alternative click route that can
be performed at moderately low temperatures by using photo-
initiators. Huisgen 1,3-dipolar cycloaddition occurs between an
alkyne and an organic azide to give 1,2,3-triazole ring. The
reactions can be performed under mild experimental condi-
tions16,18^ when catalyzed by copper(I). Click reactions have been
extensively used in the synthesis of polymers with different
composition and topology, ranging from linear (telechelic, 19
macromonomer,^20 and block copolymer)^21 to nonlinear mac-
romolecular structures (graft,^22 star,^23 miktoarm star,^24 H-type,^25
dendrimer,^26 dendronized linear polymer,^27 macrocyclic poly-
mer,^28 self-curable polymers,^29 and network system^30 ). The
development and the application of click chemistry in polymer
and material science have recently been reviewed extensively.^31
As part of our continuing interest in the development of
photosensitive systems for various synthetic applications, the
present paper is devoted to synthesis of macrophotoinitiators
containing side-chain TX moieties by taking advantage of two
click reactions, namely Diels–Alder and 1,3-dipolar cycload-
dition reactions.
Experimental Section
Materials. Styrene (S, 99%, Aldrich) and 4-chloromethylstyrene
(CMS, ca. 60/40 meta/para isomer mixture, 97%, Aldrich) were
distilled under reduced pressure before use. 2,2′-Azobis(isobuty-
ronitrile) (AIBN, 98%, Aldrich) was recrystallized from ethanol.
N -Oxyl free radical (TEMPO, 99%, Aldrich) was used as received.
N , N , N ′, N ′′, N ′′-Pentamethyldiethylenetriamine (PMDETA, Aldrich)
was distilled over NaOH prior to use. Tetrahydrofuran (THF, 99.8%,
J.T. Baker) was dried and distilled over benzophenone-Na. Other
solvents were purified by conventional procedures. Triethylamine
(TEA, 98%, Aldrich) and dichloromethane (99.9%, HPLC grade,
Aldrich) were distilled from CaH 2. Dimethylformamide (DMF,
- Corresponding author. E-mail: [email protected]. † (^) Istanbul Technical University. ‡ (^) Yildiz Technical University. § (^) On leave from Egean University, Department of Chemistry, Bornova,
Izmir, Turkey.
Macromolecules 2008 , 41 , 2401- 2405 2401
10.1021/ma702502h CCC: $40.75 2008 American Chemical Society
Published on Web 02/28/
+99%, Aldrich) and 1,1,1-tris(hydroxymethyl)propane triacrylate
(TPTA, 95%, Aldrich) was used as received.
Instrumentation.^1 H NMR measurements were recorded in
CDCl 3 with Si(CH 3 ) 4 as internal standard, using a Bruker AC
(250.133 MHz) instrument. FT-IR spectra were recorded on a
Perkin-Elmer FTIR Spectrum One-B spectrometer. UV spectra were
recorded on a Shimadzu UV-1601 spectrometer. Differential
scanning calorimetry (DSC) was performed on a Perkin-Elmer
Diamond DSC. Molecular weights were determined by gel per-
meation chromatography (GPC) instrument equipped with a Waters
styragel column (HR series 2, 3, 5E) with THF as the eluent at a
flow rate of 0.3 mL/min-^1 and a Waters 410 differential refracto-
meter detector.
Photo-DSC. Photo-DSC was conducted on a modified Perkin-
Elmer Diamond DSC equipped with a homemade aluminum
cylinder. UV light (320–500 nm) was applied by a light guide
(OmniCure Series 2000) with a light intensity of 18.4 mW cm-^2
at the level of the surface of the cured samples. The mass of the
samples was 8 mg, and the measurements were carried out in an
isothermal mode at 30 °C under a nitrogen flow of 20 mL min-^1.
The reaction heat liberated in the polymerization was directly
proportional to the number of acyrlate reacted in the system. By
integrating the area under the exothermic peak, the conversion of
the acrylate groups ( C ) or the extent of the reaction was determined
according to eq 1:
C ) ∆ Ht ⁄ ∆ H 0 theory^ (1)
where ∆ Ht is the reaction heat evolved at time t and ∆ H 0 theory^ is
the theoretical heat for complete conversion. ∆ H 0 theory^ ) 19.2 kcal
mol-^1 for an bond of acrylate.^32 The rate of polymerization ( R p ) is
directly related to the heat flow (d H /d t ) by eq 2:
R p ) d C ⁄ d t ) (d H ⁄ d t ) ⁄ ∆ H 0 theory^ (2)
Synthesis of Polystyrene - Azide (PS - N 3 ). First, poly(styrene-
co -chloromethylstyrene) (P(S- co -CMS)) with two different chlo-
romethylstyrene content (13 and 27 mol %) was synthesized as
described previously.^33 A typical procedure for the preparation of
PS-N 3 from 13 mol % CMS containing P(S- co -CMS)is as follows:
P(S- co -CMS) (2.0 g, 8.5 × 10 -^4 mol) was dissolved in N,N -
dimethylformamide (DMF), and NaN 3 (0.23 g, 3.6 × 10 -^3 mol)
was added. The resulting solution was allowed to stir at 25 °C
overnight and precipitated in excess methanol/water mixture (1/
by volume). The same procedure was also applied for P(S- co -CMS)
with 27 mol % CMS content. 1 H NMR (CDCl 3 ): δ ) 7.40–6.
(b, 9H), 4.25 (s, 2H) FTIR % T (cm-^1 ): 3060, 2923, 2096, 1681,
Synthesis of N -Propargyl-7-oxynorbornene (PON). N -Prop-
argyl-7-oxynorbornene (PON) as the click linker was synthesized
according to the literature procedure.^34 1 H NMR (CDCl 3 ): δ ) 6.
(s, 2H), 5.28 (s, 2H), 4.22 (d,2H), 2.88 (s, 2H), 2.18 (t, 1H). FTIR
% T (cm-^1 ): 3256, 2126, 1698, 1154, 1009, 880, 704, 623.
Synthesis of Thioxanthone - Anthracene (5-Thiapentacene-
14-one) (TX - A). Thioxanthone-anthracene (TX-A) (5-thiapen-
tacene-14-one) was synthesized according to the literature proce-
dure.^12 1 H NMR (250 MHz) in CDCl 3 : δ 8.86 (s, 1H), 8.61–8.
(d, 1H), 8.42–8.45 (t, 1H), 8.35 (s, 1H), 7.96–8.1 (m, 2H), 7.82–7.
(d, 1H), 7.44–7.72 (m, 5H). FTIR % T (cm-^1 ): 3050, 1672, 1622,
One-Pot Synthesis of Polystyrene - Tioxanthone (PS - TX). A
typical procedure for the synthesis of polystyrene-thioxanthone
(PS-TX) obtained from the precursor P(S- co -CMS) with 13 mol
% CMS content. PS-N 3 (0.10 g, 0.043 mmol), click linker ( 9 )
(0.019 g, 0.043 mmol), and TX-A (0.029 g, 0.043 mmol) were
dissolved in DMF (5 mL) in a Schlenk tube and purged with
nitrogen. CuBr (0.018 g, 0.14 mmol) and PMDETA (0.027 mL,
0.14 mmol) were added, and the reaction mixture was degassed by
three freeze–pump–thaw cycles and left under nitrogen and stirred
at 120 °C for 36 h. Polymer solution was then passed through
alumina column to remove copper salt, precipitated into methanol,
and finally dried in a vacuum oven at 25 °C. Yield: 0.22 g (20%).
1 H NMR (250 MHz) in CDCl 3 : δ 7.46 (m, 1H); 7.4–6.2 (b, 9H)
5.26 (m, 2H); 5.26 (m, 2H); 4.74 (m, 2H); 3.20 (m, 1H). FTIR % T
(cm-^1 ): 3020, 2922, 1709, 1625, 1601, 1493, 1452, 758, 699.
Results and Discussion
As stated in the Introduction, our synthetic approach toward the direct preparation of polymers containing side-chain thiox- anthone moieties is based on “double click” chemistry strategy combining in-situ 1,3-dipolar azide-alkyne [3 + 2] and thermoreversible Diels–Alder [4 + 2] cycloaddition reactions. The overall process is represented in Scheme 1. According to this approach, first poly(styrene- co -chlorom- ethylstyrene), P(S- co -CMS), copolymers containing two dif- ferent chloromethylstyrene (CMS) units (13 and 27 mol %) were prepared via nitroxide-mediated radical polymerization (NMP). The compositions of copolymers as determined by using 1 H NMR spectroscopy are in agreement with the expected values and indicate the random copolymer structure. The resulting P(S- co -CMS) copolymers were then quantitatively converted into polystyrene-azide (PS-N 3 ) in the presence of NaN 3 /DMF at room temperature. In the 1 H NMR spectrum of PS-N 3 , while the signal at 4.50 ppm corresponding to - C H 2 - Cl protons of the precursor P(S- co -CMS) disappeared completely, a new signal appeared at 4.25 ppm was attributed to - CH 2 linked to azide groups. The FT-IR spectral analysis also supports this result. The other components of the double click reaction, namely thioxanthone-anthracene (TX-A)^12 and N -propargyl- 7-oxynorbornene^33 (PON), were synthesized according to the literature procedures. In the final step of the process, PS-N 3 , TX-A, and PON were reacted in one-pot to yield the desired PS-TX macro- photoinitiator. In this step, two independent click reactions occurred simultaneously. While CuBr/PMDETA catalyzed tria- zole formation was accomplished between the azide of PS-N 3 and the alkyne functional end group of PON, retro-Diels–Alder reaction proceeded concomitantly between PON and the an- thracene moiety of TX-A after deprotection of the maleimide group. Notably, PON acts as a “click linker” in the process, as it contains suitable functional groups for the two click reactions involved. The possible byproduct, i.e., furan, and excess TX-A or PON are completely soluble in the precipitating solvent methanol. Consequently, the side-chain modification was com- pleted with quantitative yields without additional purification steps.
Scheme 1. Side-Chain Functionalization of PS - N 3 with
Anthracene - Thioxanthone (TX - A) in the Presence of
N -Propargyl-7-oxynorbornene (PON) as Click Linker via
Double Click Chemistry
2402 Gacal et al. Macromolecules, Vol. 41, No. 7, 2008
information on the nature of the excited states involved. As can be seen from Figure 3, excitation and emission fluorescence
spectra in DMF of TX-A and PS-TX obtained by double click reaction are quite different. TX-A exhibits characteristic
emission bands of the excited (singlet) of anthracene moiety. In contrast, PS-TX has no significant emission of this kind,
and the spectrum shows a nearly mirror-image-like relation between absorption and emission again similar to bare TX.
Expectedly, the intensities are lower in the case of side-chain thioxanthone bound polymer.
Polymeric systems bearing side-chain TX groups can act as
bimolecular photoinitiators when used in conjunction with hydrogen donors analogous to the low molecular weight TXs.
PS-TX was used as a photoinitiator for the polymerization of methyl methacrylate (MMA) in the presence of triethylamine
(TEA) as hydrogen donor. The overall process is shown in Scheme 2.
Because of steric bulkiness and delocalization, the polymeric
ketyl radical is insufficiently reactive to initiate the polymeri- zation of vinyl monomers. Although not entirely elucidated,
presumably these radicals undergo bimolecular termination. The results are compiled in Table 1. For comparison, photopoly-
merizations in the absence of either PS-TX itself or TEA are also included. As can be seen, PS-TX is not an efficient
photoinitiator in the absence of a co-initiator. The presence of an amine such as TEA is important for effective photoreduction
and photopolymerization. In this connection, it should be pointed out that this result also specifies the change of the photophysical
properties of TX-A. Previously, it was shown^12 that TX-A generates initiating species without requirement of an additional
hydrogen donor. It is also interesting to note the effect of TX content in the macrophotoinitiator. A higher conversion was
attained when the TX content was higher, indicating that the rate of initiation is proportional to the absorbed light and
consequently to the amount of the TX functional side groups.
Among the several solvents tested in our experiments (Table
2), dimethylformamide (DMF) seemed to be the most suitable solvent for the photopolymerization initiated by PS-TX.
Obviously, the situation is complex and two effects are combined. First, PS-TX dissolves in DMF better than the other
solvents. Second, although radical polymerizations are not sensitive to the polarity of the solvent,^34 triplet-state lifetime
of photoinitiators involving electron transfer such as TX derivatives may depend on some polarity effects.^35
We have also tested the polymerizability of styrene (S)
monomer with PS-TX. In complete contrast to TX-A, polymerization of S with this macrophotoinitiator in the presence
of TEA did not proceed. Although aromatic carbonyl/amine combinations represent an effective photoinitiator system for
the polymerization of (meth)acrylates, they appear to be less reactive toward styrene monomers due to the high quenching
rate of the monomer and the low reactivity of R-amino radicals with S.^36 This behavior is in accordance with the spectral
findings indicating that PS-TX exhibits photochemical char- acteristics of typical aromatic carbonyl compounds.
The efficiency of the PS-TX in the photocuring of formula- tions containing multifunctional monomers was also studied. In Figure 4, photo-DSC exhoterm referring to the polymerization of 1,1,1-tris(hydroxymethyl)propane triacrylate (TPTA) contain- ing PS-TX and TEA under polychromatic light is shown. Figure 5 displays a plot of the conversion vs irradiation time derived from Figure 4. The shape of this “conversion-time” kinetics curve indicates two stages: a rapid first stage followed by a slow stage. At the second stage, gelation and vitrification of the polymerizing trifunctional acrylate most likely render the diffusion of the components more difficult. In conclusion, we have successfully combined 1,3-dipolar azide-alkyne [3 + 2] and thermoreversible Diels–Alder (DA) [4 + 2] click reactions for the synthesis of polymers bearing side-chain TX photoactive groups. One of the consequences of the method is that such modification causes a dramatic change in the photochemistry of the precursor TX-A. The obtained polymeric photoinitiators were shown to efficiently initiate the free radical polymerization of mono- and multifunctional monomers via type II mechanism. Generally, the in-situ double click chemistry strategy described here is simple and quantitative and may permit a wide range of derivatives of polymers with various functional groups to be prepared in high yields. Currently, this concept is being transferred to other functional groups, and results will be presented in the near future.
Acknowledgment. The authors thank Istanbul Technical Uni-
versity, Research Funds, and Tubitak for financial support. H.A.
thanks Tubitak for the financial support by means of a postdoctoral
fellowship.
References and Notes
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Table 2. Effect of Solvent on the Photoinitated Polymerization a
of Methyl Methacrylate (MMA) with PS - TX b^ at Room
Temperature
solvent conversion (%) DMF 21. THF 1. CH 2 Cl 2 13.
- c^ 19. a (^) [MMA] ) 9.28 mol L- (^1) ; [PS-TX] )3.2 × 10 - (^4) mol L- (^1) ; [TEA] )
15 × 10 -^4 mol L-^1 , irridation time ) 2 h. b^ Obtained from the precursor PS- co -PCMS with 13 mol % CMS. c^ Polymerization was performed in bulk.
Figure 4. Photo-DSC profile for polymerization of TPTA in the
presence of TEA and PS-TX (27%) macrophotoinitiator, cured at 30
°C by UV light with an intensity of 180 mW/cm^2.
Figure 5. Conversion vs time for polymerization of TPTA in the
presence of TEA and PS-TX (27%) macrophotoinitiator, cured at 30
°C by UV light with an intensity of 180 mW/cm^2.
2404 Gacal et al. Macromolecules, Vol. 41, No. 7, 2008
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MA702502H
Macromolecules, Vol. 41, No. 7, 2008 Polymeric Thioxanthone Photoinitatiors 2405
