European Polymer Journal 47 (2011) 792–799

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Synthesis and characterization of a double photochromic initiatorfor cationic polymerization

Johannes Kreutzer, Kubra Dogan Demir, Yusuf Yagci ⇑Istanbul Technical University, Faculty of Science and Letters, Chemistry Department, Maslak TR-34469, Istanbul, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Available online 1 October 2010

Dedicated to Professor Nikos Hadjichristidisin recognition of his contribution to polymerscience.

Keywords:PhotopolymerizationCationic polymerizationPhotoinitiatorOnium salt

0014-3057/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.eurpolymj.2010.09.025

⇑ Corresponding author. Tel.: +90 212 2853241; faE-mail address: yusuf@itu.edu.tr (Y. Yagci).

A novel cationic photoinitiator namely, 2-benzyl-2-(N,N-dimethyl-2-oxo-2-phenylethyl)ammonium hexafluoroantimonate-1-(4-morpholinophenyl)-butane-1-one (BDMPP+ SbF�6 ),carrying two photochromophoric groups was synthesized and characterized. Theoreticalabsorption characteristics of the salt were studied and compared with those obtainedexperimentally. Photoinitiation activity of this salt was demonstrated by polymerizationof various monomers at k = 350 nm. Upon irradiation by UV light, cationic species formedfrom homolytic dissociation followed by electron transfer or directly by heterolytic scissioninitiate cationic polymerization.

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1. Introduction and co-initiators may create additional problems associ-

Photopolymerization is widely used for different appli-cations such as surface coatings, adhesives, and microelec-tronics [1–7]. Although the majority of the growinginterest deals with free radical systems [1,4], the corre-sponding cationic mode is also an important industrialprocess [7–17]. The growth of such an industry is depen-dent on the development of initiators, or initiator and co-initiator combinations, fulfilling requirements for specificapplications, e.g., wavelength selectivity, solubility, etc.[18]. Onium salts [19], such as iodonium [20–22], sulfo-nium [20,22–25] and alkoxypyridinium [26–29] salts arethe most widely used photoinitiators for cationic polymer-ization and their photochemistry has been well established[12]. These salts exhibit poor spectral sensitivity at longwavelengths which limits their use particularly in pig-mented systems. In order to extend their spectral response,various additives were combined with onium salts [29–31]. However, for industrial applications the use of onecomponent photoinitiators with long wavelength absor-tion characteristics is still advantageous since the additives

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ated with solubility, compatibility, migration and cost.Phenacyl onium salts like dialkylphenacyl sulfonium

[32–34] and phenacyl anilinium salts are one componentphotoinitiators for cationic photopolymerization reactionswhich undergo reversible and irreversible photolysis,respectively upon irradiation at relatively long wavelengths[34–38]. A number of phenacyl-based compounds withpyridinium [37,39], anilinium [40], and phosphonium [41]salt structures were shown to be appropriate photoiniti-ators for cationic polymerization [42]. Their photoactivitycan further be shifted to longer wavelengths by using pre-cursors with extended absorptivity in the synthesis [43].

Among many free radical photoinitiators, a-aminoalkyl-phenone derivatives are particularly useful for applicationsin pigmented systems where long wavelengths irradiationis crucial [44]. Upon photolysis, these photoinitiatorsundergo a-scission as shown in Scheme 1 for 2-benzyl-2-(N,N-dimethylamino)-1-(4-morpholinophenyl)-butane-1-one (BDMP).

This initiator can be modified by quaternization of thenitrogen atom present in the structure with a photoactive al-kyl halide such as phenacyl bromide. The resulting saltwould possess double chromophoric groups and thus exhi-bit enhanced photoactivity. Previously, an allylic type salt

Scheme 1. Photolysis of 2-benzyl-2-(N,N-dimethylamino)-1-(4-mor-pholinophenyl)-butane-1-one (BDMP).

J. Kreutzer et al. / European Polymer Journal 47 (2011) 792–799 793

of BDMP was prepared by a similar strategy and used asaddition fragmentation agent for cationic polymerization[45].

In the present work, the synthesis and photoactivity ofa-aminoalkylphenone type phenacyl onium salt (BDMPP+ SbF�6 ) possessing double photochromic groups inthe structure is described. As it will be shown below, thisphotoinitiator absorbs the light at longer wavelengths thanthe precursor a-amino alkylphenone and readily initiatescationic polymerization of appropriate monomers.

2. Experimental section

2.1. Materials

2-Benzyl-2-(N,N-dimethylamino)-1-(4-morpholinophe-nyl)-butane-1-one (BDMP), (Ciba Specialty Chemicals,phenacyl bromide (99%, Fluka) and sodium hexafluoroan-timonate (99% Acros) were used as supplied. Cyclohexeneoxide (CHO, 98%, Aldrich), n-butyl vinyl ether (BVE,>97%,Fluka), and styrene (S,P99%, Merck) were distillated overCaH2 in vacuo. N-Vinylcarbazole (NVC, 98%, Aldrich) wascrystallized from ethanol. Methyl methacrylate (MMA,99%, Aldrich) was passed through a column that containedbasic alumina and distilled over CaH2 under vacuum.

2.2. Characterization

UV spectra were recorded on a Shimadzu UV-1601spectrophotometer. 1H-NMR measurements were recordedin CDCl3 with tetramethylsilane as the internal standardusing a Bruker AC250 (250 MHz) instrument. FT-IR spectrawere recorded on a Perkin Elmer FT-IR Spectrum One Bspectrometer. Melting point was determined on a BüchiMelting Point B-540 with a heating rate of 20 �C min�1.Molecular weights were determined using a GPC instru-ment equipped with MZ-Gel SD plus columns (HR series3, 4, 5E) with THF as the eluent at a flow rate of 1 mL min�1

and a Waters 410 Differential refractometer detector.

2.3. Synthesis of 2-benzyl-2-(N,N-dimethyl-2-oxo-2-phenyl-ethyl)ammonium hexafluoroantimonate -1-(4-morpholin-ophenyl)-butane-1-one (BDMPP+ SbF�6 )

In a 25 mL round bottom flask, 761 mg (2 mmol) ofBDMP and 398 mg (2 mmol) of phenacylbromide were

Scheme 2. Synthesis o

dissolved in 5 mL acetone and stirred at room temperaturefor 4 days. The acetone was evaporated and the residuewas dissolved in 7 mL of hot water. The aqueous solutionwas extracted with 15 mL diethylether. Into the aqueousphase, 517 mg (2 mmol) of sodium hexafluoroantimonatewas added and the product was obtained as yellow precip-itates in 1 h (yield 23%); m.p.: 181 �C. IR: m = 2981, 2970,1659, 1597, 1497, 1450, 1383, 1261, 1244, 1196, 1099,1020, 927, 799, 707, 657, 628 cm�1; 1H-NMR (CDCl3,250 MHz): d = 0.96 (t, 3H), 2.49 (m, 2H), 2.75 (s, 6H), 3.38(m, 6H), 3.83 (m, 6H), 6.80 (m, 1H) 6.88 (d, 2H), 7.02 (m,2H), 7.18 (m, 7H), 7.73 (m, 1H), 7.90 (d, 2H); UV:kmax = 361 nm, e361 nm = 43.3 � 103 L mol�1 cm�1 (c = 5 �10�5 mol L�1, in DCM).

2.4. Photopolymerization

Appropriate solutions of monomer and BDMPP+ SbF�6 inDCM in a dry Pyrex tube were degassed with nitrogen priorto irradiation. Irradiations were applied through a RofinPolylight PL 4000 Light source at k = 350 nm (80 mWcm�2). After a given time polymers were precipitated into10-fold excess methanol, filtered and dried in vacuo. Con-versions for all samples were determined gravimetrically.

3. Results and discussion

The phenacyl salt of a-aminoalkylphenone was pre-pared by reaction of 2-benzyl-2-(N,N-dimethylamino)-1-(4-morpholinophenyl)-butane-1-one (BDMP) with thephenacyl bromide and subsequently bromide anion wasexchanged with a non-nucleophilic hexafluoroantimonateanion as depicted in Scheme 2.

The structure of the salt was confirmed by spectralanalysis. The characteristic aromatic and methylene pro-tons of the salt were between 6.8–7.9 and 1.0–3.8 ppmrespectively (see experimental section). The BDMPP+ SbF�6shows a strong absorption band around 361 nm (Fig. 1).

Interestingly, strong bathochromic shift was noted ascompared to the absorption bands of both precursors tak-ing part in the structure of the salt. Moreover, the phenacylsalt has much higher absorptivity compare to the structur-ally similar allylic derivative (Table 1) [45].

Absorption characteristics of the salt were also ana-lyzed. The spectrum in dichloromethane shows one transi-tion at 361 nm with an extinction coefficient of 43.3 �103 L mol�1 cm�1. Measurements in a polar solvent suchas acetonitrile show a hypsochromic shift of the absorptionband (Fig. 2). Although it gives a lower value than that indichloromethane, the extinction coefficient in this solventis still high (e= 22.0 � 103 L mol�1 cm�1). Both observa-tions related to the high extinction coefficient and the blue

f BDMPP+ SbF�6 .

Fig. 1. UV spectra of 5 � 10�5 mol L�1 dichloromethane solutions of thephotoinitiator BDMPP+ SbF�6 (black) and precursors phenacyl bromide(red) and BDMP (blue). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

Table 1Absorption characteristics of BDMP salts.

BDMP salt emaxa � 10�3 (L mol�1 cm�1) kabs

b (nm) Reference

Allylic 19.6 365 41Phenacyl 40.4 361 This work

a Extinction coefficient measured in dichloromethane.b Absorption wavelength measured in dichloromethane.

794 J. Kreutzer et al. / European Polymer Journal 47 (2011) 792–799

shift in polar solvent, allow us to attribute the band to a p–p* transition. An exact interpretation of the less intenseabsorption band at �250 nm could not be made due tothe cut-off window of the spectrophotometer.

Additional theoretical investigations on the photophys-ics of the salt were carried out. The E.01 revision of theGaussian03 [46] program was chosen to perform geometryoptimization, vibrational analysis and excitation calcula-tions. Density functional theory method in its time depen-

Fig. 2. Absorption spectra of BDMPP+ SbF�6 ; calculated (black), measuredin dichloromethane (6.6 � 10�5 mol L�1, blue) and in acetonitrile(2.5 � 10�5 mol L�1, red). (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of thisarticle.)

dent form (TDDFT) was used at the B3LYP/6-31G* level,except for the excitation calculations where the triple zetabasis set TZVP was employed. The less expensive 3-21G*type basis set developed by Dobbs [47] was used for theSb atom to cut down computational cost. It was shown thatall frequencies referring to an optimized structure are real.The calculated spectra, as shown in Figs. 2 and 3, weremodeled via Gaussian broadening of each transition linewith a half width of 0.18 eV for the BDMPP+SbF�6 salt and0.22 eV for the BDMP respectively. The values were mea-sured from the experimental spectra recorded indichloromethane.

The relative error of 2.5 kcal mol�1 for the energeticallylower band is in an acceptable range. The absorption bandat 380 nm in the calculated spectrum is formed due to twocontributions, namely the S0–S1 and S0–S2 excitation. Theless intense S0-S1 excitation at 398 nm (f = 0.0716) ismainly a HOMO–LUMO transition while the more intense(f = 0.3190) S0–S2 excitation at 377 nm is a HOMO–LUMO+1

transition. Both excitations are p�p* transitions which isin agreement with the experimentally observed hypsoshiftand the calculated higher oscillator strengths. The involvedorbitals are shown in Fig. 4 and 5.

In both cases the transition takes part from p-orbitalscentered on one side of the phenyl ring of the morpholinomoiety to the p* orbitals which are located in the center ofthe molecule. The orbitals located on the morpholino ringand the phenyl ring of the phenacyl moiety; however shownonbonding p orbital characteristic in both the LUMO andLUMO+1 orbital. The LUMO+1 orbital show a higher delocal-ized characteristic of the orbitals located on the phenacylmoiety compared to the LUMO orbital. The S0–S3 and S0–S4 transitions are mainly n�p* transitions from lower lyingoccupied orbitals to the LUMO and LUMO+1 orbital andgive raise to the energetically low lying absorption bandaround 250 nm. For the initial photoinitiator BDMP thefirst band is a superposition of the very weak S0-S1

(f = 0.0087) and the stronger S0-S2 (f = 0.0361) and S0–S3

(f = 0.5389) transitions. Excitations in all three transitions

Fig. 3. Absorption spectra of BDMP; calculated (black), measured indichloromethane (6.6 � 10�5 mol L�1, blue) and in acetonitrile(2.5 � 10�5 mol L�1, red). (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 4. Molecular orbitals of BDMP involved in the main absorption band.

Fig. 5. Molecular orbitals of BDMPP+ SbF�6 involved in the main absorption band.

J. Kreutzer et al. / European Polymer Journal 47 (2011) 792–799 795

Fig. 6. Photopolymerization of CHO (C = 9.88 mol L�1 in bulk) in thepresence of BDMPP+ SbF�6 (C = 4.5 mmol L�1) at 350 nm.

Fig. 7. Photopolymerization of CHO (C = 4.9 mol L�1) in dichloromethanein the presence of BDMPP+ SbF�6 at 350 nm for 90 min.

Table 3Photopolymerization of different monomers by using BDMPP+ SbF�6(7 � 10�3 mol L�1) in dichloromethane at 350 nm for 90 min.

Monomer (mol L�1) Conversiona (%) Mnb (g mol�1) Mw/Mn

b

CHO (4.94) 42 6300 1.54NVC (2.00) 62 2400 3.30MMA (4.71) 20 6800 1.81

a Determined gravimetrically.b Determined by GPC measurements according to polystyrene

standards.

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go from the HOMO and lower lying orbitals to the LUMOorbital respectively. While in the S0–S1 transition the maincontribution comes from the HOMO–LUMO excitation(around 61 %), in the S0–S2 the HOMO�4–LUMO (around59 %) and in the S0–S3 the HOMO�1–LUMO (59 %) excita-tion are dominant. The HOMO–LUMO transition is mainlyan excitation from the terminal phenyl moiety to the cen-tered carboxyl function and the central phenyl ring. TheHOMO�1–LUMO transition goes from the central phenylring to the direction of the carboxyl function and is mainlya redistribution of the charge in the conjugated system inthe center of the molecule. The HOMO�4–LUMO transitiongoes mainly from the nonbonding orbitals located aroundthe carboxyl group to the conjugate system in the centerand has therefore lower oscillator strength than theHOMO�1–LUMO excitation. However, the main contribu-tion of the absorption band at 321 nm comes from thep�p* transition of the HOMO�1–LUMO excitation. Thesmaller absorption band at 234 nm results mainly frommixed excitations from lower lying orbitals to the LUMO+1

and LUMO+2 and shows a bigger contribution of the termi-nal phenyl ring in the excitations. A summary of the exper-imental and calculated transitions and its attributions isgiven in Table 2.

The photoinitiation activity of the salt in cationic poly-merization was tested by using cyclohexeneoxide (CHO) asthe cationically polymerizable monomer. This monomer isunreactive towards radicals and but readily polymerizeswith cationic species. As can be seen from Figs. 6 and 7,the polymerization of CHO was initiated quite effectivelywith BDMPP+SbF�6 upon irradiation at 350 nm at roomtemperature.

The conversion of CHO increased rapidly up to a certaintime and leveled off therefrom. This behavior may be dueto the reaction of amino moiety in the morpholinocompound with the propagating ends. It is known that ami-no compounds are the most basic species in a cationicpolymerization system [27,48,49]. Thus, when theconcentration of photochemically released free amino con-taining morpholino compound is above a certain level,the propagating chains may be terminated. Similar behav-ior was also observed with pyridinium and aniliniumsalts [27,50]. Additionally, the high concentration ofBDMPP+ SbF�6 may cause efficient quenching to form anexcimer which is a sink for photochemical energy.

As presented in Table 3, in addition to CHO, the poly-merizability of some other monomers were also tested.The strong electron donating monomer, N-vinyl carbazole

Table 2Summary of the experimental and calculated transitions.

Compound kabsa (nm) emax

b � 10�3 (L mol�1 cm�1) Transition kcalcc (nm) f d Transition

BDMP 322 23.7 p�p* 368 0.0087 p�p*319 0.0361 p�p*312 0.5389 p�p*

BDMPP+ SbF�6 361 40.4 p�p* 398 0.0716 p�p*377 0.3190 p�p*

a Absorption wavelength measured in dichloromethane.b Extinction coefficient measured in dichloromethane.c Calculated transitions.d Oscillator strength in gas phase at B3LYP/TZVP level.

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(NVC) undergoes photoinduced polymerization more read-ily. Interestingly, although at lower rate free radical poly-merizable monomer methyl methacrylate (MMA) alsopolymerizes by photolysis of BDMPP+ SbF�6 indicating thatin addition to the cationic species, initiating radicals arealso formed.

Based on the previous studies and present results, amechanism for the initiation of cationic polymerization,consistent with the homolytic and heterolytic decomposi-tion of phenacyl based salts, is shown below (Scheme 3).The electronically excited salt may undergo heterolyticcleavage resulting in the formation of phenacylium cat-ions. Alternatively, homolytic cleavage followed by elec-tron transfer essentially yields the same species.

It is possible that the photoinitiator may also undergoa-cleavage analogous to the BDMP unit forming radicals.These radicals transfer one electron to another initiator

Scheme 3. Initiation of cationic polymerization by phenacyli

Scheme 4. Alternative mechanism for initiation of catio

and eventually form cationic species capable of initiatingcationic polymerization (Scheme 4).

The UV absorption spectrum of the poly(cyclohexeneoxide) obtained by using BDMPP+ SbF�6 also supports thatthese mechanisms may simultaneously be operative forthe initiation. As can be seen from Fig. 8, the spectrum con-tains absorption bands of the various chromophoric groupsattached to the polymer chain during the initiation.

4. Conclusion

In conclusion, we successfully synthesized a new photo-initiator possessing double photochromic groups in thestructure. The photoinitiator exhibits higher absorptioncharacteristics than the precursor compounds which makeit useful for photocuring application in the near UV region.

um cations formed by the photolysis of BDMPP+ SbF�6 .

nic polymerization by photolysis of BDMPP+ SbF�6 .

Fig. 8. Absorption spectrum of poly(cyclohexene oxide) obtained by thepolymerization initiated by BDMPP+ SbF�6 (C = 7.2 � 10�4 mmol L�1) indichloromethane.

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Initiation of both cationic and radical polymerization sug-gests that this photoinitiator can be used for practicalapplications involving hybrid monomers based on epox-ides and (meth)acrylates.

Acknowledgments

The authors thank Anne-Marie Kelterer from TU Graz,Austria for helpful discussions. One of the authors (J.K)gratefully acknowledges the financial support by OeAD.

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