Polystyrene-b-poly(2-vinyl phenacyl pyridinium) salts as photoinitiators for free radical and cationic polymerizations and their photoinduced molecular associations

Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 30–36

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Polystyrene-b-poly(2-vinyl phenacyl pyridinium) salts as photoinitiators for free radical and cationic polymerizations and their photoinduced molecular associations Omer Suat Taskin a , Irem Erel-Goktepe b,c , Muhammad Alyaan Ahmed Khan b,c , Stergios Pispas d , Yusuf Yagci a,e,∗ a

Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Department of Chemistry, Middle East Technical University, 06800 C¸ankaya, Ankara, Turkey c Department of Polymer Science and Technology, Middle East Technical University, 06800 C¸ankaya, Ankara, Turkey d Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., 116 35 Athens, Greece e Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 28 February 2014 Received in revised form 22 March 2014 Accepted 29 March 2014 Available online 13 April 2014 Keywords: Phenacyl pyridinium salt Photodecomposition Photopolymerization Cationic polymerization Free radical polymerization Photochemical switch

a b s t r a c t Polystyrene-b-poly(2-vinyl phenacyl pyridinium hexafluorophosphate) (PS-b-PVPP) was synthesized by reacting polystyrene-b-poly(2-vinyl pyridine) prepared by living anionic polymerization with phenacyl bromide followed by counter anion exchange reaction. The ability of PS-b-PVPP to act as a photoinitiator for both free radical and cationic polymerizations is demonstrated. In the free radical polymerization, the initiation step involves the decay of the excited state of the salt with homolytic bond rupture of the nitrogen–carbon bond. For the cationic polymerization, homolytic cleavage followed by electron transfer and/or heterolytic scission reactions are responsible for the generation of reactive species. Photoinduced switching behavior of PS-b-PVPP from ionic to neutral state was also demonstrated by surface analysis, particle size and film thickness measurements.

1. Introduction There is a continuous interest in photopolymerization processes of many different types of monomers and oligomers involving both radical and cationic mechanisms. This chemistry has found many applications [1–6] in areas such as adhesives, inks, coatings, printing plates, stereolithography, holography, optical waveguides, microelectronics, 3D objects and dental fillings [7]. Although the free radical photopolymerization is in more advanced state, the corresponding cationic mode also receives considerable attention mainly due to its insensitivity toward oxygen. Many photoinitiators [8,9] fulfilling requirement for specific applications have been developed. In the related work [10–16] from the authors’

∗ Corresponding author at: Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey. Tel.: +90 212 285 3241; fax: +90 212 285 6386. E-mail address: [email protected] (Y. Yagci). http://dx.doi.org/10.1016/j.jphotochem.2014.03.018 1010-6030/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

laboratory wide range of photoinitiators for both cationic and radical polymerizations acting at broad wavelength were reported. Photoinitiators capable of generating radical and cationic species simultaneously are particularly useful for hybrid systems consisting of epoxy and acrylate formulations [17–19]. Among them, phencyl pyridinium salts are effective in initiating free radical, cationic and zwitter-ionic polymerizations [20–24]. Electronically excited salt may undergo heterolytic cleavage resulting in the formation of phenacylium cations. Alternative pathway in which homolytic cleavage followed by electron transfer essentially yields the same species capable of initiating polymerizations oxiranes and vinyl ethers. Concomitantly liberated pyridine can initiate the zwitter ionic polymerization of monomers possessing strong electron withdrawing groups such as cyanoacrylates and methacrylate. The overall decomposition pathways of phenacylpyridinium salts are presented in Scheme 1 [25,26]. Polymeric photoinitiators containing photoreactive groups are steadily gaining interest in various fields because of the advantages offered by good compatibility with the monomers and solvents, and low migration tendency of the photoinitiator itself and

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* hV

N X

N X

O homolytic O

O heterolytic O

N

CH2

N

Scheme 1. General photolysis mechanism of phenacylpyridinium salt.

photoproducts. Many side chain, main chain and end chain functional polymeric photoinitiators [11] have been developed and successfully used. Herein, we describe efforts to employ a new class of polymeric initiator, polystyrene-b-poly(2-vinyl phenacyl pyridinium hexafluorophosphate (PS-b-PVPP) to initiate photochemically both free radical and cationic polymerizations of appropriate monomers. The precursor polymer was prepared by living anionic polymerization to yield well defined block structures in which polystyrene segment provided solubility in solvents and monomers, while poly(2-vinyl pyridine) segment facilitated photofunctionalization through pyridine rings. As will be shown below, the photochemical switch of the PS-b-PVPP from ionic to neutral form resulted in enhanced association among the PS-b-PVPP chains, as evidenced by intensityaverage hydrodynamic size measurements using dynamic light scattering technique, as well as thickness and rms (root mean square) roughness [27] changes of PS-b-PVPP monolayer films (prepared from irradiated PS-b-PVPP solutions), determined by ellipsometry and AFM imaging techniques, respectively.

2. Experimental 2.1. Materials Bromoacetophenone (phenacyl bromide, 98%, Aldrich), cyclohexene oxide (CHO, 98%, Aldrich) methyl methacrylate (MMA, 99%, Aldrich) potassium hexafluoroantimonate (KSbF6 , 99%, Aldrich), acetone (99.8%, Aldrich), dicholoromethane (CH2 Cl2 , 99%, Aldrich) methanol (CH3 OH, 98%, Aldrich) and hexanes (95%, Aldrich) were used as received. N-vinyl carbazole (NVC, 98%, Aldrich) was recrystallized from ethanol. Styrene (St, 99%, Aldrich), 2-vinylpyridine (2VP, 97%, Aldrich), and tetrahydrofuran (THF, 99.9%, Aldrich) used

for the synthesis of the diblock copolymer were purified to the standards for anionic polymerization, using well-established high vacuum techniques [28]. n-Butyl lithium (n-BuLi, 1.6 N in hexanes, Aldrich) was diluted with hexane on the vacuum line using an all glass dilution apparatus with ampoules having break-seals [29]. 2.2. Synthesis of polystyrene-b-poly(2-vinyl pyridine) (PS-b-PVP) PS-b-PVP diblock copolymer was synthesized following procedures reported elsewhere. Briefly, purified styrene (7.2 g, previously distilled over CaH2 and dibutylmagnesium) was polymerized first in THF (100 mL) at −78 ◦ C, using n-BuLi (6.3 × 10−4 mol) as initiator, in an all glass reactor equipped with appropriate ampoules with break-seals. After one hour of polymerization purified 2-vinylpyridine (2.9 g, previously distilled over CaH2 and sodium mirror) was distilled from its ampoule into the polymerization mixture and left to polymerize for another hour, at −78 ◦ C, before termination of the polymerization reaction with degassed methanol. After opening the reactor about 70% of THF was evaporated in a rotor evaporator and the concentrated solution of the diblock was poured into hexane resulting in precipitation of the product. The copolymer was isolated by filtration and dried under vacuum for two days before use. 2.3. Synthesis of polystyrene-b-poly(2-vinyl phenacylpyridinium) hexafluoroantimonate (PS-b-PVPP) A solution of bromoacetophenone (phenacyl bromide, 1.54 g, 7.78 × 10−3 mol) in 10 mL of acetone was added drop-wise to a solution of PS-b-PVP (110 mg, 6.11 × 10−3 mmol) in 10 mL of acetone with cooling. After the mixture was stirred at 56 ◦ C for 24 h and then, the solvent was evaporated from the solution to yield

Scheme 2. Schematic representation of polystyrene-b-poly(2-vinyl phenacylpyridinium) hexafluoroantimonate synthesis.

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Fig. 1. FT-IR spectra of PS-b-PVP and its phenacyl pyridinium salt.

a brownish solid. The obtained phenacyl pyridinium salt with bromide counter anion was poured into 50 mL of distilled water and washed with 20 mL of dichloromethane. Upon addition of potassium hexafluoroantimonate (50 mg, 1.81 × 10−4 mol) to the aqueous layer, white crystalline powder was precipitated. This powder was collected by filtration, and successively washed with distilled water. Finally it was reprecipitated from methanol and dried at 40 ◦ C.[30] 2.4. Photopolymerization Similar procedures were employed for photoinitiated free radical and cationic polymerizations. Thus, depending on the polymerization mode appropriate solutions of MMA (500 ␮L, 4.69 mol L−1 ) and CHO (500 ␮L, 4.91 mol L−1 ) or NVC (0.05 g, 2.58 mol L−1 ) containing known amount of PS-b-PVPP were mixed in Pyrex tubes and degassed with nitrogen prior to irradiation by a merry-go-round type reactor equipped with 16 Philips 8 W/06 lamps emitting light at  > 300 nm and a cooling system. At the end of a given time, polymers were precipitated into methanol, filtered, dried and weighed. Conversions were determined gravimetrically.

Fig. 3. UV spectral changes of PS-b-PVPP salt on irradiation at  < 350 nm under nitrogen in CH2 Cl2 , [PS-b-PVPP] = 2.6 × 10−3 M.

2.5. Characterization UV spectra were recorded on a Shimadzu UV-1601 spectrometer. The FTIR spectra were recorded at Perkin Elmer Spectrum One with an ATR Accessory (ZnSe, Pike Miracle Accessory) and cadmium telluride (MCT) detector. Resolution was 4 cm−1 and 24 scans with 0.2 cm/s scan speed. The conventional gel permeation chromatography (GPC) measurements were carried out with an Agilent instrument (Model 1100) consisting of a pump, refractive index (RI), and ultraviolet (UV) detectors and four Waters Styragel columns (guard, HR 5E, HR 4E, HR 3, HR 2), (4.6-mm internal diameter, 300-mm length, packed with 5-lm particles). The effective molecular weight ranges are 2000–4,000,000, 50–100,000, 500–30,000, and 500–20,000 g mol−1 , respectively. THF and toluene were used as eluent at a flow rate of 0.3 mL/min at 30 ◦ C and as an internal standard, respectively. The apparent molecular weights (Mn,GPC and Mw,GPC ) and polydispersities (Mw /Mn ) were determined with a calibration based on linear PS standards using PL Caliber Software from Polymer Laboratories. 2.6. Dynamic light scattering measurements of PS-b-PVPP PS-b-PVPP was dissolved in dichloromethane with a final concentration of 0.2 mg/mL. PS-b-PVPP solution was exposed to continuous irradiation at 365 nm using a UV lamp equipped with 80 led bulbs each with a power of 1 watt. Samples were taken every 15 min. Intensity-average hydrodynamic size measurements were performed using Zetasizer Nano-ZS equipment (Malvern Instruments Ltd.). Size values were obtained by cumulants analysis of the autocorrelation data. 2.7. Deposition of the PS-b-PVPP monolayer films for ellipsometric thickness measurements and atomic force microscopy (AFM) imaging

Fig. 2. UV–vis spectra of PS-b-PVP and its phenacyl pyridinium salt in dichloromethane. The concentration of all components was 2.6 × 10−3 M.

Silicon wafers were rinsed with DI water and dried under nitrogen flow prior to use. PS-b-PVPP was self-assembled at the surface by immersing the silicon wafers into either non-irradiated or irradiated solutions of PS-b-PVPP in dichloromethane for 30 min. PS-b-PVPP coated silicon wafers were dried under nitrogen flow and thickness of the films was measured using a Spectroscopic Ellipsometer of Optosense, USA (OPT-S6000). AFM imaging of the multilayers was performed using Nanomagnetics Instruments, Ambient AFM in dynamic mode.

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Table 1 Photoinitiated polymerizations of various monomers in the presence of PS-b-PVPP salt at room temperature.a Monomer (mol L−1 )

Polymerization mode

Time (min)

Conversion (%)

MMA (4.69) CHO (4.91) NVC (2.58)

Radical Cationic Radical and cationic

180 180 60

55 45 68

a b

Mn,GPC b (g mol−1 ) 38,400 33,000 48,800

Polymerization experiments were performed at 350 nm [PS-b-PVPP]: 5.56 × 10−3 mol L−1 . Molecular weight (Mn,GPC ) and distribution (Mw /Mn ) were determined by GPC with linear polystyrene standards.

Scheme 3. Radical and cationic mechanism of polymerizations via various monomers.

Fig. 4. Intensity (%) vs size of 0.2 mg/mL PS-b-PVPP before and after irradiation.

Mw /Mn b 1.26 1.38 3.76

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Hydrodynamic Size (nm)

140 before irradiation

A

120 100

continuous irradiation

80 60

Hydrodynamic Size (nm)

0

B

500 400 300 200 continuous irradiation

0

0

30

60

90

120 150 180 210 240

Time (min) Fig. 5. Evolution of intensity-average hydrodynamic size of PS-b-PVPP (Panel A: small aggregates; Panel B: large aggregates) as a function of time.

3. Results and discussion The precursor PS-b-PVP was synthesized by living anionic polymerization high vacuum techniques. The resulting diblock copolymer has narrow molecular weight distribution and the targeted molecular weight and chemical composition, as determined by GPC and 1 H NMR: Mw = 18,000, Mw /Mn = 1.08, 27 wt% P2VP. PS was chosen as the majority component in order to increase solubility of the resulting polymeric salt in organic solvents, as it is discussed in the following. Desired phenacyl salt of the block copolymer was synthesized via a two-step reaction procedure (Scheme 2). First, quaternization of nitrogen of poly(vinyl pyridine) segment of the block copolymer took place. The precipitation of insoluble PS-b-PVPP bromide salt in acetone strongly shifts the equilibrium reaction forward. Second, ion exchange was performed by the addition of KSbF6 . Addition of this non-nucleophilic ion makes the soluble bromide PV-b-PVPP salt insoluble in water. For this reason, the desired final product is isolated from the aqueous medium by a simple filtration. The obtained PS-b-PVPP salt was characterized by spectral analysis. The FT-IR spectra of PS-b-PVP and its phenacyl salt are shown in Fig. 1. As can be seen, the spectrum of the salt exhibits additional characteristic band of the carbonyl group at around 1686 cm−1 confirming successful incorporation of photosensitive phenacyl groups into the structure. PS-b-PVPP salt possesses n–␲* absorption with a maximum at about 300 nm, characteristic of acetophenone derivatives. Notably, compare to its precursor it has higher absorption at longer wavelengths which is well-matched with the absorption characteristics of low molar mass analog (Fig. 2).

Fig. 6. AFM height images (Panel A: 0.5 ␮m × 0.5 ␮m scans and Panel B: 2 ␮m × 2 ␮m scans) of monolayer films of PS-b-PVPP before and after irradiation of PS-b-PVPP solution. Roughness values were determined from 2 ␮m × 2 ␮m scans.

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Scheme 4. Schematic representation of the interactions among PS-b-PVPP chains before and after irradiation at 365 nm.

Obviously, photodecomposition of the salt is important for its use in photopolymerization and photoswitching processes. For this purpose, UV spectral change of PS-b-PVPP salt on irradiation was investigated. As shown in Fig. 3, upon irradiation the absorption belonging to the salt structure rapidly diminishes. Provided that such decomposition would occur in monomer formulations, reactive species thus formed should be capable of initiating polymerizations (Scheme 3). Indeed, upon irradiation of the PS-b-PVPP salt solutions containing appropriate monomers in the UV range, polymerizations took place. MMA and CHO were selected as the free radical and cationically polymerizable monomers while NVC undergoes polymerization via both mechanisms. Table 1 lists the polymerization results obtained. As can be seen, in the case of NVC much higher conversions are attained in a short irradiation time due to its activity toward both radicalic and cationic species [25]. The molecular weight distribution is also broad as both polymerizations proceed concomitantly. We have also examined the photochemical transition of PS-b-PVPP in dichloromethane via monitoring the change in intensity-average hydrodynamic size as a function of irradiation time. PS-b-PVPP was dissolved in dichloromethane with a final concentration of 0.2 mg/mL and exposed to continuous irradiation at 365 nm. Upon photolysis, PVPP was converted to PVP as a result of the loss of ionic structure. Samples were taken from the solution every 15 min for size measurements. As seen in Fig. 4, there emerged a second peak with larger size after 15 min irradiation of the sample at 365 nm. We have also tracked the change in hydrodynamic size of PS-b-PVPP as a function of time. As seen in Fig. 5A, the hydrodynamic size of the smaller aggregates decreased by 30% within the first 15 min and no significant change in size was observed after 1 h irradiation. Larger aggregates which formed upon irradiation almost reached their size within the first 15 min. Formation of larger aggregates can be correlated with the enhanced association among PS-b-PVPP chains upon irradiation at 365 nm. Photochemical transition of the positively charged phenacyl benzoylpyridinium units of the PVPP blocks into electrically neutral pyridine groups upon irradiation decreases the electrostatic repulsion between the PVPP blocks resulting in enhanced association among the PS-b-PVPP chains and formation of larger aggregates. Scheme 4 shows schematic representation of the interactions among PS-b-PVPP chains before and after irradiation at 365 nm. To further understand the interaction among the PS-b-PVPP chains after irradiation, we prepared monolayer films of PS-bPVPP using non-irradiated and irradiated PS-b-PVPP solutions in dichloromethane. Table 2 and Fig. 6 show the ellipsometric film thickness and AFM images of the monolayer films, respectively. We found that the thickness of a monolayer of PS-b-PVP which was prepared using 30 min irradiated PS-b-PVPP solution was 20% lower than that of the monolayer film prepared using non-irradiated PS-b-PVPP solution. The difference in film thicknesses was also reflected to surface morphology of the films. As seen in Fig. 6,

Table 2 Thicknessa of PS-b-PVPP monolayer films dichloromethane solutions at different times.

prepared

Time (min)

Thickness (nm)

0 30

4.10 ± 0.17 3.28 ± 0.18

from

irradiated

a The ellipsometric film thickness was measured using a Spectroscopic Ellipsometer of Optosense, USA (OPT-S6000).

smoother films with lower rms roughness values were obtained when the monolayer films were prepared using 30 min irradiated PS-b-PVPP solution rather than non-irradiated PS-b-PVPP solution. The lower film thickness and surface roughness can be explained by formation of more intense film structures due to enhanced association among PS-b-PVPP after 30 min of irradiation [27]. 4. Conclusions In conclusion, we have demonstrated that anionically prepared PS-b-PVP can be readily converted to the corresponding photoactive phenacyl type salts. Depending on the type of monomer used in the formulation, the obtained salts are efficient photoinitiators for cationic, radical and their dual polymerizations. Their photochemical switch from cationic to neutral state induces molecular association resulting in the change of solid film surface morphology, as well as formation of aggregates in solution. Such photochemical behavior may lead to new pathways for various bioapplications and ways of controlling surface properties and thickness of polymeric films and multilayers. For example multilayered films formed from positively charged PVPP and negatively charged DNA through electrostatic interaction would dissemble DNA molecules by the photochemical loss of charges on the block copolymer. It is known that the encapsulation of DNA into polymeric nanocapsules can be realized by using cationic PS-QP2VP block copolymer micelles [31]. By using the described approach photochemical delivery of DNA molecules from block copolymer micelles can be accomplished. Further studies along these lines are now in progress. Acknowledgement The authors would like to thank Istanbul Technical University, Research Fund for the financial support. References [1] S.K. Dogruyol, Z. Dogruyol, N. Arsu, A thioxanthone-based visible photoinitiator, J. Polym. Sci. A: Polym. Chem. 49 (2011) 4037–4043. [2] D.K. Balta, N. Arsu, Thioxanthone-ethyl anthracene, J. Photochem. Photobiol. A: Chem. 257 (2013) 54–59. [3] O. Karahan, D.K. Balta, N. Arsu, D. Avci, Synthesis and evaluations of novel photoinitiators with side-chain benzophenone, derived from alkyl alphahydroxymethacrylates, J. Photochem. Photobiol. A: Chem. 274 (2014) 43–49.

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