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Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 43– 49

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

ynthesis and evaluations of novel photoinitiators with side-chainenzophenone, derived from alkyl �-hydroxymethacrylates

zlem Karahana, Demet Karaca Baltab, Nergis Arsub, Duygu Avcia,∗

Department of Chemistry, Bogazici University, 34342 Bebek, Istanbul, TurkeyDepartment of Chemistry, Yildiz Technical University, Davutpasa Campus, Istanbul 34210, Turkey

r t i c l e i n f o

rticle history:eceived 5 June 2013eceived in revised form 7 September 2013ccepted 14 September 2013vailable online 18 October 2013

a b s t r a c t

A novel monomeric photoinitiator, tert-butyl 2-((4-benzoylphenoxy)methyl)acrylate (1), with a side-chain benzophenone (BP) group was synthesized from tert-butyl �-bromo methacrylate (TBBr)and 4-hyroxybenzophenone. It was thermally homo- and copolymerized with a coinitiator, N,N-dimethylaminoethyl methacrylate (DMAEM), using free radical polymerization to give the correspondingpolymeric photoinitiators (poly-1 and poly(1-co-DMAEM)). The polymeric photoinitiators showed

eywords:hotopolymerizationolymeric photoinitiatorenzophenoneynthesislkyl �-hydroxymethacrylates

similar UV absorption spectra to 1, which were red-shifted compared to BP. Photophysical studiessuch as phosphorescence and laser flash photolysis in addition to photopolymerization of methacry-lates were performed. The photopolymerizations of triethylene glycol dimethacrylate (TEGDMA) and1,6-hexanedioldiacrylate (HDDA) initiated by 1, poly-1, poly(1-co-DMAEM) and BP were studied byphoto-DSC. The polymeric initiator (poly-1) was found to have higher efficiency than BP and monomericinitiatior 1.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

In recent years, photopolymerizable initiators receive con-inuous interest due to their advantages in comparison withheir corresponding low molecular weight analogs [1,2]. Somef these advantages are compatibility improvement in the for-ulation, low odor, non-toxicity, and reduced migration to

he film surface which is important for the synthesis of envi-onmentally friendly materials. In addition, the efficiency ofhotoinitiation can be enhanced due to the energy migrationlong the polymer chain, or intramolecular reactions respon-ible for generating more reactive species. Also, the polymerhain can prevent coupling of the reactive species, thus favor-ng their reaction with the monomers. Further improvementsn the efficiency of photoinitiation, which result in reductionf exposure time and increased productivity, can be achievedy designing new polymeric structures containing photoinitiatorroups.

Polymeric photoinitiators contain side chain or main chain pho-oinitiator moiety which generates free radicals upon absorption ofV light and are able to initiate polymerization and crossinking

∗ Corresponding author. Tel.: +90 2123596816; fax: +90 2122872467.E-mail address: avcid@boun.edu.tr (D. Avci).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.09.010

of monomers and oligomers. A variety of polymeric photoini-tiators containing type I and type II free radical photoinitiatorsare described in the literature [3–9]. Most of the type II poly-meric photoinitiators are based on benzophenone, thioxanthone,anthraquinone, camphorquinone or benzyl moieties, and amongthem benzophenone-containing ones are more common [10–23].Also, type II photoinitiators require the presence of a coinitiator(usually an amine) in the solution to work, hence polymeric pho-toinitiators were tried that contain both benzophenone and aminegroups in the same macromolecule; and some of them were foundto show higher photoinitiation activity compared with the poly-meric photoinitiators without amine groups where the amines aresupplied externally. This result can be explained by the favorableexcitation energy transfer when both groups are in close proximity[11].

It is suggested that the choice of polymeric backbone mightalso affect the photoinitiation activity. While polymeric photoini-tiators were prepared with different polymeric backbones, noalkyl �-hydroxymethacrylate (RHMA)-based ones are reported inthe literature. The purpose of this work is to develop such newpolymeric photoinitiators and investigate their activity for UV cur-able coatings. We report synthesis and characterization of one

monomeric and two polymeric photoinitiators bearing side-chainBP groups. Their photopolymerization efficiencies were comparedwith non-bound BP in the polymerizations of TEGDMA and HDDAusing photo-DSC.

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4 Ö. Karahan et al. / Journal of Photochemistry

. Experimental

.1. Materials

Tert-butyl �-bromomethacrylate (TBBr) were synthesizedccording to literature procedures [24,25]. TEGDMA, HDDA,,N-dimethyl para toluidine, 4-hyroxybenzophenone, N-methyl-iethanolamine (MDEA), DMAEM, 2,2′-azobis(isobutyronitrile)AIBN) and all other reagents and solvents were obtained fromldrich Chemical Co. and used as received.

.2. Characterization

1H and 13C NMR spectra were taken on Varian Gemini (400 MHz)pectrometer. Elemental analyses were obtained from Thermolectron SpA FlashEA 1112 elemental analyser (CHNS separationolumn, PTFE; 2 m; 6 × 5 mm). The photopolymerizations werearried out on a TA Instruments Q100 differential photocalorime-er (DPC). Gel permeation chromatography (Viscotek) was carriedut with THF solvent using polystyrene standards. UV–vis spectraere taken on a Varian UV-Visible Carry 50 Spectrophotometer.

hosphorescence spectra were recorded on a Jobin Yvon–Horibaluoromax-P in cold finger at 77 K. A Nicolet 6700 FT-IR spectropho-ometer was used for recording IR spectra. Laser flash photolysisxperiments employed the pulses from an Applied PhotophysicsAG laser (355 nm, pulse, 5 ns) and a computer controlled system.olutions of the BP derivatives were prepared at concentrationsuch that the absorbance was ∼0.3 at the excitation wavelength355 nm).

.3. Synthesis of monomers

Tert-butyl 2-((4-benzoylphenoxy)methyl)acrylate (1): To a mix-ure of 4-hyroxybenzophenone (0.476 g, 2.4 mmol) and K2CO33.44 g, 24.9 mmol) in acetone (5 mL) under nitrogen, TBBr (0.575 g,.6 mmol) was added dropwise at room temperature. After stir-ing at 60 ◦C for 48 h, the solvent was removed under reducedressure. Dichloromethane (5 mL) was added and the solution wasxtracted with water (3 × 5 mL). The organic phase was dried overnhydrous sodium sulfate, filtered and the solvent was evaporatednder reduced pressure. The residue was purified by recrystalliza-ion from methanol. The pure product was obtained as a white solidmp 53–54 ◦C) in 68% yield.

1H NMR (CDCl3, 400 MHz, ı): 1.5 (s, 9H, CH3), 4.8 (s, 2H, CH2 O),.9 (s, 1H, CH2 C), 6.3 (s, 1H, CH2 C), 6.9 (d, 2H, Ar-CH), 7.5–7.8m, 7H, Ar-CH). 13C NMR (CDCl3, 400 MHz, ı): 28.0 (CH3), 66.5CH2 O), 81.5 (C CH3), 114.5 (Ar-CH), 125.7 (C CH2), 128.1 (Ar-H), 130.4 (Ar-CH), 131.9 (Ar-C), 132.5 (Ar-CH), 132.7 (Ar-CH),36.7 (Ar-C), 138.7 (C CH2), 161.9 (Ar-C), 164.5 (C O ester), 195.6C O ketone). FTIR (ATR, cm−1): 2976 (C H), 1713 (C O), 1680C O), 1142 (C O). Anal. Calcd for: C, 74.56; H, 6.51;. Found: C,4.27; H, 6.72.

.4. Synthesis of polymeric initiators

The thermal homo- and copolymerizations were carried out inulk at 60 ◦C using AIBN (0.5 wt%) with standard freeze-evacuate-haw procedures. After six hours, the viscous polymer solutionsere dissolved in methylene chloride and precipitated into a large

xcess of methanol. The precipitated polymers were repeatedly dis-olved in methylene chloride and precipitated again in methanol,ltered, dried under vacuum and stored in the dark. Poly-1 andoly(1-co-DMAEM) were obtained in 23 and 19% yields.

hotobiology A: Chemistry 274 (2014) 43– 49

2.5. Photopolymerizations

The samples (3–4 mg) were irradiated in isothermal mode at40 ◦C for 10 min with an incident light intensity of 20 mW/cm2

under nitrogen flow of 20 mL min−1. Rates of polymerization werecalculated according to the following formula:

Rp = (Q/s)Mn(�Hp)m

where Q/s is the heat flow per second, M the molar mass of themonomer, n the number of double bonds per monomer molecule,�Hp the heat released per mole of double bonds reacted and m themass of monomer in the sample. The theoretical value used for �Hp

was 13.1 kcal/mol for methacrylate double bonds [26].

3. Results and discussion

3.1. Synthesis and characterization of photoinitiators

A new polymerizable photoinitiator (1) with a side-chainbenzophenone group was synthesized through nucleophilic substi-tution reaction of 4-hydroxybenzophenone with TBBr under basicconditions using acetone as the solvent (Fig. 1). It was obtained asa white solid (melting point of 53–54 ◦C) in 68% yield after recrys-tallization from methanol. It was soluble in polar organic solventssuch as methylene chloride, acetone, ether, methanol and THF butinsoluble in water (Table 1). The spectral data are in agreementwith the expected structure of this initiator. For example, 1H NMRspectrum of this monomer showed characteristic peaks for tert-butyl protons at 1.5 ppm, methylene protons at 4.8 ppm, doublebond protons at 5.9 and 6.3 ppm and aromatic protons between6.9 and 7.8 ppm (Fig. 2). In the FTIR spectrum, characteristic of thestretching vibration of the two different C O bonds at 1713 (ester)and 1680 (ketone) cm−1 was observed. Satisfactory microanalysisresults were obtained for this monomer.

Photoinitiator 1 was homo- and copolymerized with DMAEM(50:50 mol%) using AIBN as thermal initiator to obtain the corre-sponding polymers, poly-1 and poly(1-co-DMAEM) (Fig. 1). Thehomo- and copolymer were obtained as white solids with 23%and 19% yield after repeated precipitations into methanol. 1H NMRof the homopolymer showed no peaks at around 5.9 and 6.3 ppmrelated to the unsaturated methacrylate protons. The peaks in the1.0–2.5 ppm region are due to tert-butyl and backbone protons,in the 3.5–4.5 ppm region due to methylene protons attached tooxygen atom as well as the peaks in 6.5–8.5 ppm the region dueto aromatic protons (Fig. 2). Also, the occurrence of polymeriza-tions were confirmed by the slight shift of the C O peak due tosaturated tert-butyl ester in the FTIR spectra (Fig. 3). The numberaverage molecular weight and polydispersity values of this poly-mer were found to be 17,360 and 1.4. DSC analysis gave a Tg valueof 138 ◦C. Poly-1 starts to lose weight around 175 ◦C due to decom-position of the tert-butyl groups. Poly-1 and poly(1-co-DMAEM)are well soluble in common organic solvents such as acetone, THF,methylene chloride and DMSO but insoluble in methanol, etherand water (Table 1). The copolymer composition was determinedby 1H NMR analysis by integration of aromatic protons of 1 withrespect to other protons in the copolymer and found to be 70:30(1/DMAEM) mol%. Tg and number average molecular weight valueof the copolymer were 71 ◦C and 54,995.

3.2. UV/vis spectral characterization of photoinitiators

UV–vis spectroscopy in dimethylformamide (DMF) was carriedout to investigate the absorption characteristics of the photoini-tiators (1, poly-1, and poly(1-co-DMAEM)) together with BP as the

Ö. Karahan et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 43– 49 45

Fig. 1. Synthesis of photoinitiators bearing a side-chain benzophenone group.

Table 1Solubilities of synthesized photoinitiators in selected solvents.

Photoinitiator Acetone THF CH2Cl2 MeOH Diethylether DMSO

1 + + + + + +Poly-1 + + + − − +poly(1-co-DMAEM) + + + − − +

Fig. 2. 1H NMR spectra of 1, poly-1 and poly(1-co-DMAEM).

46 Ö. Karahan et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 43– 49

Fig. 3. FTIR spectra of 1 and poly-1.

Table 2�max and ε values of the synthesized photoinitiators and reference initiator in DMF.

Photoinitiator �max (nm) ε (L mol−1 cm−1)

1 289 19,323BP 267 13,220

rahn

Foc

poly-1 288 17,288poly(1-co-DMAEM) 289 17,086

eference (Fig. 4). The wavelength of maximum absorption (�max)nd molar extinction coefficient (ε) are summarized in Table 2. BPas �–�* transitions as a distinct maximum in 250 nm region and–�* transitions as a shoulder of �–�* transitions in the range of

ig. 5. (a) Photobleaching of 1 [7.4 × 10−5 mol L−1]/MDEA [1.0 × 10−2 mol L−1] at 280 nm, inf poly-1 [1 × 10−4 mol L−1]/MDEA [1.0 × 10−2 mol L−1] at 280 nm, inset: poly-1 [1.0 × 10−

o-DMAEM) [9 × 10−5 mol L−1]/MDEA [1.0 × 10−2 mol L−1].

Fig. 4. UV/vis absorption spectra of BP, 1, poly-1 and poly(1-co-DMAEM) in DMFsolution.

300–350 nm. The n–�* transitions have low extinction coefficientdue to the spin-forbidden transition. In all of the synthesized pho-toinitiators a significant red-shift of the �–�* transition (289 nm)was observed, probably due to the electron donating effect of oxy-gen directly attached to the BP unit. The n–�* transitions of theseinitiators were also observed as a shoulder around 350 nm.

Fig. 5 shows the UV spectral changes of photoinitiators in thepresence of an amine, MDEA upon photolysis. As the photolysisproceeds, the photoinitiators are consumed and their absorption

spectra change. At the end of the irradiation, the absorption bandsof both polymerizable and polymeric photoinitiators at 280 nmalmost completely disappeared and new bands at around 330 nmdue to formation of ketyl radicals appeared.

set: 1 [1.0 × 10−3 mol L−1]/MDEA [1.0 × 10−2 mol L−1] at 330 nm; (b) photobleaching4 mol L−1]/MDEA [1.0 × 10−2 mol L−1] at 330 nm; and (c) photobleaching of poly(1-

Ö. Karahan et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 43– 49 47

F7

2pmacas4cf[

4rsCIntri

dt(pa3

tpti

3

pitsmuma

Fig. 7. (a) Transient optical absorption spectra recorded at 0.27–2.47 �s follow-ing laser excitation (355 nm, 5 ns) of poly-1 (OD355: 0.3) in argon saturated DMF at

◦

cosity of the medium increases too slowly so that autoaccelerationdoes not have an important effect. On the other hand, the polymericinitiator’s high efficiency has probably been enhanced both by the

ig. 6. (a) Phosphorescence emission spectrum of 1 in 2-methyltetrahydrofuran at7 K (lexc: 340 nm).

Fig. 6 represents the phosphorescence spectrum of 1 recorded in-methyltetrahydrofuran at 77 K. The phosphorescence spectra ofoly-1 and poly(1-co-DMAEM) were given in SI. Phosphorescenceeasurements are useful to gain information on the triplet energies

nd electron configuration of the triplet states. The phosphores-ence spectra of photoinitiators displayed almost the same featurest 77 K and were similar to the parent BP compound. The (0, 0) emis-ion bands for 1, poly-1 and poly(1-co-DMAEM) occurred at 420,18 and 417 nm, corresponding to approximate triplet energies ofa. 285.8, 286.2 and ca. 286.8 kJ/mol, respectively. The triplet energyor BP parent compound is given as 285.5 kJ/mol in the literature27].

The phosphorescence lifetimes were found to be 29, 26 and2 ms for 1, poly-1 and poly(1-co-DMAEM), respectively. Phospho-escence spectra of ketones with n–�* nature of the lowest triplettate are usually structured due to the vibrational progression of the

O vibration, and �–�* triplets are mostly unstructured [28,29].n addition, the phosphorescence lifetime for n–�* triplets are sig-ificantly shorter (on the order of several milliseconds) comparedo �–�* triplets (more than 100 ms) [28,30]. The short phospho-escence lifetimes observed for the synthesized photoinitiatorsndicate a n–�* nature of the lowest triplet state.

The triplet states of 1, poly-1 and poly(1-co-DMAEM) in aegassed solution of DMF were investigated by laser flash pho-olysis at room temperature. After irradiation with laser pulses355 nm) the spectra of 1, poly-1 and poly(1-co-DMAEM) showedeaks at 540, 560 and 560 nm (Fig. 7 and SI), respectively. BP tripletsnd BP ketyl radicals are known to have some absorption between00 and 400 nm.

These transients decay kinetics with a first order contribu-ion show lifetimes of 19 ns, 422 ns and 380 ns for 1, poly-1 andoly(1-co-DMAEM), respectively. To confirm the triplet nature ofhe transients, oxygen quenching experiments were performed andt was observed that these transients were quenched by oxygen.

.3. Photoinitiating activity

The photopolymerization efficiencies of the initiators (1 andoly-1) bearing benzophenone groups and BP were investigated

n the photopolymerizations of TEGDMA using N,N-dimethyl paraoluidine as coinitiator. Fig. 8 shows photopolymerization resultsuch as time to reach the maximum polymerization rate (tmax),

aximum rate of polymerization (Rpmax) and conversion obtained,

sing photo-DSC. It was clearly seen that the synthesized poly-eric photoinitiator poly-1 exhibits much higher photoinitiation

ctivity with improved tmax, Rpmax and conversion compared to

25 C; Inset: Kinetic traces of the transient optical absorption spectrum at 560 nmrecorded at 0.27 ms–2.47 �s following laser excitation (355 nm; 5 ns) of poly-1 inargon saturated DMF solution at 25 ◦C.

the others, where the monomer 1 also performed slightly betterthan BP. For example, the maximum rate of polymerization forTEGDMA initiated with the polymeric photoinitiator was 4.4 and3.2 times greater than that of TEGDMA initiated with BP and themonomeric photoinitiator. The conversions reached were 45%,59% and 72% for BP, 1 and poly-1, respectively.

Part of the reason for the observed low efficiency of low molec-ular weight photoinitiators in polymerizing TEGDMA probably isthat the initiating radicals are consumed by the inhibitors presentin TEGDMA, delaying the onset of polymerization; also, the vis-

Fig. 8. Rate-time and conversion-time plots for the photopolymerization ofTEGDMA initiated by (-) BP, (- -) 1; (�) poly-1. Photoinitiator and amine (N,N-dimethyl para toluidine) concentration in monomer are 1 and 3 mol%.

48 Ö. Karahan et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 43– 49

Fig. 9. Rate-time and conversion-time plots for the photopolymerization of HDDAi(p

pt

bpisl

po(ilcpcmetctsmio

tdNia6h

Table 3Photoinitiated polymerization of MMA [4.68 mol L−1] in DMF in the presence ofMDEA at 350 nm in air.

Conversion (%)Photoinitiator (mol L−1) 1 poly-1 Poly(1-co-DMAEM)

1 × 10−2 7.3 10.3 12.6a

1 × 10−3 4.6 30.6 10.5b

1 × 10−4 1.6 − <1

nitiated by (-) BP/DMAEM; (- -) poly(1-co-DMAEM) 1/1; (··) poly-1/poly-DAMEM;�) poly-1/DMAEM; and (�) 1/DMAEM. Photoinitiator and amine (N,N-dimethylara toluidine) concentration in monomer are 1 and 3 mol%.

olymeric effect and the higher viscosity of the medium caused byhe polymer, which in turn contributes to autoacceleration.

We also investigated photopolymerization of HDDA initiatedy BP/DMAEM, 1/DMAEM, poly-1/DMAEM, poly-1/poly-DMAEM,oly(1-co-DMAEM) at 40 ◦C (Fig. 9). The tmax values observed for all

nitiating systems were found to be lower than those of TEGDMAystems. This is due to higher reactivity of HDDA which is a diacry-ate compared to the methacrylate monomer, TEGDMA.

It was observed that there are remarkable differences amonghotoinitiating systems. The systems constituted by combinationsf polymeric BP with both low and high molecular weight aminepoly-1/DMAEM and poly-1/poly-DMAEM) showed higher activityn terms of polymerization rates and conversions compared to allow molecular weight counterparts (BP/DMAEM, 1/DMAEM) andopolymer poly(1-co-DMAEM)). The copolymeric photoinitiator,oly(1-co-DMAEM), showed similar efficiency with the monomerombinations (1/DMAEM) and BP/DMAEM system. Among theore active systems, poly-1/DMAEM was found have the high-

st efficiency with the maximum rate of polymerization about 2.4imes greater than that of BP/DMAEM system. Almost completeonversion (>95%) was reached for this initiator system comparedo BP/DMAEM system (60%). The high efficiency of poly-1/DMAEMystem can be explained by a decrease of coupling reactions ofacromolecular radicals due to steric hindrance. The lower initiat-

ng ability of the copolymer can be explained by the lower amountf DMAEM (1/DMAEM ratio of 70:30 mol%) in the copolymer.

In addition, the initiation efficiency of the synthesized photoini-iators (1, poly-1 and poly(1-co-DMAEM)) were also investigateduring polymerizations of MMA (Table 3). Tertiary amines, such as-methyldiethanolamine (MDEA) are often used as co-initiators

n photopolymerization formulations. MMA, MDEA and variousmounts of photoinitiator dissolved in DMF were irradiated for0 min under air atmosphere. Polymeric initiator (poly-1) gaveigher conversion compared to that of its low molecular weight

tirr = 60 min.a 3.2% without amine.b 7.8% without amine.

counterpart and copolymeric initiator (poly(1-co-DMAEM)) withMDEA. Polymerization experiments were also performed in theabsence of MDEA, as it was expected no polymer was obtained forphotoinitiators 1 and poly-1. However the conversions obtained forcopolymer at 1 × 10−3 M concentration were 10.5% and 7.8% withand without MDEA, respectively. This indicates that intramolecularhydrogen abstraction by BP from the amine on the side chain leadsto radical generation and hence results in formation of polymer.

4. Conclusions

The presence of the photosensitive BP group covalently attachedto the side chain of an RHMA polymer in combination with lowmolecular weight tertiary amine result in photopolymerization ofTEGDMA and HDDA with a higher rate of polymerization comparedto all low molecular weight combination (monomeric photoinitia-tor and BP with low molecular weight amine). Higher efficiencyof the polymeric initiator might be attributed to the improvementof compatibility and the polymeric effect. The beneficial effect ofphotoinitiator poly(1-co-DMAEM) is that it initiates photopolymer-ization reactions successfully without an additional amine.

The materials we report in this article constitute the first exam-ples of RHMA-based monomeric and polymeric photoinitiators. Thework suggests that an entire class of such materials, with goodefficiency, is possible.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2013.09.010.

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