Synthesis and Characterization of Polymeric Thioxanthone

Synthesis and Characterization of Polymeric Thioxanthone

(Parte 1 de 2)

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 UniVersity, Maslak, Istanbul, 34469, Turkey, and Department of Chemistry, Yildiz Technical UniVersity, DaVutpasa Campus, Istanbul, 34210, Turkey

ReceiVed NoVember 10, 2007; ReVised Manuscript ReceiVed 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-N3) 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 moietypresentin TX-A was transformedinto TX chromophoricgroups.The resultingpolymerspossessabsorption characteristics similar to the parent TX. Their capabilities to act as photoinitiator for the polymerization of monoand 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; (i) introduction of photoactive groups into polymerchains.In the lattercase, macrophotoinitiatorswere 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 molecularweight compoundspossessingalso photoreactivegroups. 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 I) macrophotoinitiators.1

Typical type I 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 characteristicsat near-UV range.2–1 Moreover, their simple synthetic procedure allows various modifications for wavelength tunability or improved solubility by the incorporation 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 delocalization of the unpaired electrons. Various structurally different TX derivatives including dendritic,2,3 polymeric,1 and one- component ones4–6,12,13 possessing both light-absorbing chromophoric 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 cycloadditions,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 chemistry17 has recently been introduced as an alternative click route that can be performed at moderately low temperatures by using photoinitiators. 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 conditions16,18 when catalyzedby copper(I).Click reactionshave 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 macromolecularstructures(graft,2 star,23 miktoarmstar,24 H-type,25 dendrimer,26 dendronized linear polymer,27 macrocyclic polymer,28 self-curable polymers,29and network system30). 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 cycloaddition reactions.

Experimental Section

Materials. Styrene (S, 9%, Aldrich) and 4-chloromethylstyrene

(CMS, ca. 60/40 meta/para isomer mixture, 97%, Aldrich) were distilled under reduced pressure before use. 2,2′-Azobis(isobutyronitrile) (AIBN, 98%, Aldrich) was recrystallized from ethanol. N-Oxyl free radical (TEMPO, 9%, Aldrich) was used as received. N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine(PMDETA, Aldrich) was distilledover NaOH priorto use. Tetrahydrofuran(THF,9.8%, J.T. Baker) was dried and distilled over benzophenone-Na. Other solvents were purified by conventional procedures. Triethylamine (TEA, 98%, Aldrich) and dichloromethane (9.9%, HPLC grade,

Aldrich) were distilled from CaH2. Dimethylformamide (DMF,

* Corresponding author. E-mail: yusuf@itu.edu.tr.

† Istanbul Technical University. ‡ Yildiz Technical University. § On leave from Egean University, Department of Chemistry, Bornova, Izmir, Turkey.

10.1021/ma702502h C: $40.75 2008 American Chemical Society Published on Web 02/28/2008

+9%, Aldrich) and 1,1,1-tris(hydroxymethyl)propane triacrylate (TPTA, 95%, Aldrich) was used as received. Instrumentation. 1H NMR measurements were recorded in

CDCl3 with Si(CH3)4 as internal standard, using a Bruker AC250 (250.133 MHz) instrument. FT-IR spectra were recorded on a

Perkin-ElmerFTIR SpectrumOne-Bspectrometer.UV spectrawere 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 permeation chromatography(GPC) instrumentequippedwith 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 refractometer 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:

where ∆Ht is the reaction heat evolved at time t and ∆H0theory is the theoretical heat for complete conversion. ∆H0theory ) 19.2 kcal mol-1 for an bond of acrylate.32 The rate of polymerization (Rp)i s directly related to the heat flow (dH/dt)b ye q2 :

Synthesis of Polystyrene-Azide (PS-N3). First, poly(styreneco-chloromethylstyrene) (P(S-co-CMS)) with two different chlo- romethylstyrene content (13 and 27 mol %) was synthesized as described previously.3 A typical procedure for the preparation of

PS-N3 from 13 mol % CMS containingP(S-co-CMS)isas follows: P(S-co-CMS) (2.0 g, 8.5 × 10-4 mol) was dissolved in N,N- dimethylformamide (DMF), and NaN3 (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 (1/1 by volume).The same procedurewas also appliedfor P(S-co-CMS)

Synthesis of N-Propargyl-7-oxynorbornene (PON). N-Propargyl-7-oxynorbornene (PON) as the click linker was synthesized according to the literatureprocedure.34 1H NMR (CDCl3): δ ) 6.50 (s, 2H), 5.28 (s, 2H), 4.2 (d,2H), 2.8 (s, 2H), 2.18 (t, 1H). FTIR

Synthesis of Thioxanthone-Anthracene (5-Thiapentacene- 14-one) (TX-A). Thioxanthone-anthracene (TX-A) (5-thiapentacene-14-one) was synthesized according to the literature proce- dure.12 1H NMR (250 MHz) in CDCl3: δ 8.86 (s, 1H), 8.61–8.64 (d, 1H), 8.42–8.45(t, 1H), 8.35 (s, 1H), 7.96–8.1(m, 2H), 7.82–7.91

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

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: 0.2 g (20%).

1H NMR (250 MHz) in CDCl3: 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

Results and Discussion

As stated in the Introduction, our synthetic approach toward the direct preparation of polymers containing side-chain thioxanthone 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-chloromethylstyrene), P(S-co-CMS), copolymers containing two differentchloromethylstyrene(CMS)units(13 and 27 mol %) were prepared via nitroxide-mediated radical polymerization (NMP). The compositions of copolymers as determined by using 1H 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-N3) in the presence of NaN3/DMF at room temperature. In the 1H NMR spectrum of PS-N3, while the signal at 4.50 ppm corresponding to -CH2-Cl protons of the precursor P(S-co-CMS) disappeared completely, a new signal appeared at 4.25 ppm was attributed to -CH2 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-oxynorbornene33 (PON), were synthesized according to the literature procedures.

In the final step of the process, PS-N3,T X-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-N3 and the alkyne functional end group of PON, retro-Diels–Alder reaction proceeded concomitantly between PON and the anthracene 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 completed with quantitative yields without additional purification steps.

Scheme 1. Side-Chain Functionalization of PS-N3 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

Evidence for the occurrence of the double click reactions was obtained from 1H NMR, UV, and fluorescence spectroscopy. As can be seen from Figure1, where1H NMR spectraof TX-A and PS-N3 were recorded, the peaks between 7.4 and 8.5 ppm, characteristic for aromatic protons of anthracene, disappeared completely. This indicates the loss of the aromaticity of the central phenyl unit of anthracene by Diels–Alder cycloaddition click reaction. Furthermore, the two new signals corresponding to a bridgehead (-CH) proton of the cycloadduct and -CH proton on the fused maleimide ring appeared at 4.74 and 3.20 ppm, respectively. The appearance of the peak belonging to -CH proton of the triazole ring at 7.46 ppm is a typical indication for the successful completion of the other click reaction,1,3-dipolarcycloaddition.On the basis of a comparison of the integration of the peak intensities at 5.26 and 4.74 ppm, corresponding to the methylene protons linked to triazole ring and adjacent to the cycloadduct, respectively, the obtained ratio of 1:1 clearly indicates the efficiency of the process. The FTIR spectra also confirm quantitativereaction,as the azide stretching band at around 2094 cm-1 disappears completely and a new carbonyl band at 1709 cm-1 appears.

Photophysical characteristics of the obtained PS-TX were investigated by UV and fluorescence spectroscopy (Figures 2 and 3). As can be seen from Figure 2, TX-A displays characteristic five-finger absorbance in 300–400 nm range. On the contrary, the PS-TX macrophotoinitiator shows a different absorption signature with a maximum at 380 nm, which is similar to the absorption spectrum of pure thioxathone, indicating that the absorption regime of the precursor has changed as a result of the modification. Indeed, DA reaction between anthracene and deprotected maleimide moieties causes the loss of the aromaticity of central phenyl unit of anthracene. In other words, the TX-A group is transformedinto thioxanthonegroup by the so-called double click reaction.

Fluoroescence spectra of PS-TX may also provide further evidence for the efficiency of the modification process and

Figure 1. 1H NMR spectrum of TX-A, PS-N3 and PS-TX (27%) in CDCl3.

Figure 2. Absorption spectra of PS-TX (27%), TX-A, and TX in

Figure 3. Emission spectra of TX-A and PS-TX (27%) in DMF;

Scheme 2. Photoinitiated Free Radical Polymerization of Methyl Methacrylate (MMA) by Using PS-TX Macrophotoinitiator

Table 1. Photoinitated Polymerizationa of Methyl Methacrylate (MMA) with Macrophotoinitiator in CH2Cl2

(g/mol) Mw/Mn a [MMA] ) 9.28 mol L-1; irridation time ) 2h . b [PS-TX] ) 3.2 × 10-4 mol L-1. c Obtained from the precursor PS-co-PCMS with 13 mol % CMS content. d Obtained from the precursor PS-co-PCMS with 27 mol % CMS content.

Macromolecules, Vol. 41, No. 7, 2008 Polymeric Thioxanthone Photoinitatiors 2403

information on the nature of the excited states involved. As can be seen from Figure 3, excitation and emission fluorescence spectrain DMF of TX-A and PS-TX obtainedby doubleclick 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.

(Parte 1 de 2)

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