Time-Resolved EPR as a Tool to Investigate ... - ACS Publications

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Time-Resolved EPR as a Tool to Investigate Oxygen Quenching in Photoinitiated Radical Polymerizations Daniela Hristova-Neeley, Dmytro Neshchadin, and Georg Gescheidt* Institute of Physical and Theoretical Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria

Downloaded by UNIV OF NEW SOUTH WALES on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 6, 2015 | doi: 10.1021/acs.jpcb.5b04263

S Supporting Information *

ABSTRACT: It is challenging to obtain absolute rate constants for the quenching of organic radicals by molecular oxygen because they often do not present absorbance in the UV−vis range. Here, it is shown that time-resolved EPR (chemically induced dynamic electron polarization, or CIDEP) spectroscopy is useful in establishing rate constants for the addition of benzoyl radicals to molecular oxygen. It was found that benzoyl radicals are particularly reactive toward O2 and can, therefore, act as oxygen scavengers in the initiating phase of radical polymerizations. Kinetic simulations underpin this reactivity.

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INTRODUCTION Photoinduced radical polymerizations have been successfully utilized in a broad range of applications.1 The particular advantages of this technique are rapid curing, time-based and spatial control, and the virtual absence of solvents. A drawback of radical polymerizations is, however, that molecular oxygen inhibits these efficient processes.2 This is shown in Scheme 1 for the class of acylphosphinoxide photoinitiators. In the presence of oxygen, the primary addition of a monomer competes with the oxygen addition to the initial radical pair (B· and P·). This leads to the peroxyl radicals B-OO· and P-OO·, which do not add to double bonds and, thus, do not initiate this type of polymerization. Technical applications of UV−vis irradiation curing are usually performed under atmospheric conditions, i.e., in the presence of oxygen. This generally causes a delay of the polymerization accompanied by a decreased rate of conversion.3 Moreover, the chain growth is hindered due to the formation of peroxyl radicals, which do not promote further polymerization. Beside these chemical aspects, triplet quenching by oxygen may occur; however, most commercial photoinitiators have very short-lived triplet states and, thus, bimolecular triplet quenching is often negligible. It has been shown that oxygen addition to carbon-centered α-amino and αhydroxyl radicals proceeds rather rapidly, with rate constants between 2.3 and 6.6 × 109 M−1 s−1.4 Benzoyl moieties are common structural features of many radical-based photoinitiating systems for radical reactions (e.g., those shown in Scheme 1). Their excitation at appropriate wavelengths leads to triplet n-π* excited states, undergoing αcleavage and yielding radical pairs, which initiate radical reactions.5 Nevertheless, it is challenging to assess their reactivity6 because they do not absorb in the visible region. Time-resolved IR spectroscopy has been a straightforward tool for assessing their reactivity.6c © XXXX American Chemical Society

Phosphinoyl radicals, in comparison, are much easier to detect.7 With typical absorptions around 450 nm and lifetimes in the ns or μs time range, they can be observed by laser-flash photolysis.7d EPR spectroscopy provides information on both components of a radical pair simultaneously. With the use of time-resolved EPR (TR-EPR) (chemically induced dynamic electron polarization, or CIDEP), the character of the radicals and their reactivity becomes accessible.8 It has been shown that kinetic rate constants for the addition of acrylates to these radicals can be determined using this technique.5c,7b,9 Here, we show that time-resolved EPR provides rate constants for the oxygen quenching of benzoyl and phosphinoyl radicals formed upon the photolysis of initiators 1 and 2 (Scheme 2). We will discuss the limits of this approach and relate the data to those available for carbon-centered radicals.4 Kinetic simulations will describe the early events of photopolymerizations.

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EPR RATE CONSTANTS AND OXYGEN Kinetic data from CIDEP spectra is not obtainable in a straightforward manner from the decays of the signals because magnetic-field effects contaminate their intensities. However, it has been shown that the line widths in CIDEP or EPR spectra mirror the lifetime of the initiating radicals and can be translated into pseudo-second-order rate constants of radical reactions yielding, e.g., absolute kinetic data for the addition of radicals to double bonds.7b,10 The Stern−Volmer-type plot of line-widths versus the concentration of the radical quencher yields the corresponding rate constant (kadd). This method has been successfully utilized for the determination of absolute rate Special Issue: Wolfgang Lubitz Festschrift Received: May 4, 2015 Revised: July 13, 2015

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DOI: 10.1021/acs.jpcb.5b04263 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Scheme 1. General Scheme for the α-Cleavage of an Acylphosphinoxide Photoinitiator Followed by the Competition between Polymerization and Oxygen Addition to the Primarily Formed Radical Paira


a

Because our investigations target the reactivity of radicals B· and P·, the reactions of BM· and PM· are omitted.

Accordingly, the rate constants are not likely to be substantially deteriorated by Heisenberg exchange. It also has to be added that, if line broadening by the presence of oxygen would dominate the EPR line width, no differences between the values of the rate constants of the benzoyl and phosphinoyl radicals should emerge. Nevertheless, there are deviations between our data and those obtained by time-resolved IR and optical spectroscopy (Table1).6c,7d,14 This could be traced back to the different methods of keeping the O2 concentration constant and monitoring it. Moreover, the differences could also stem from solvent effects (the reference experiments were performed in CH2Cl2 and acetonitrile4,6c). With this, the contribution of line broadening by oxygen is only very small at the O2 concentrations employed in our experiments: At c(O2) between 10−5 and 10−4 mol·l−1, line broadening by O2 amounts to ca. 3.5 × 10−4 and 3.5 × 10−5 mT,15 whereas the line broadening detected in our experiments amounts to at least 3 × 10−2 mT.

Scheme 2. Photoinitiators 1 and 2 and Radicals B1·, P1·, and P2· Generated upon Their Photolysis

constants for the addition of acrylates to benzoyl11 and phosphinoyl radicals.4,5c Following the addition of radicals to oxygen requires special caution. It is well-established that the presence of oxygen, per se, leads to line broadening. This has been utilized for the determination of oxygen concentrations in several environments.12 Certainly, line broadening by the presence of oxygen has the potential effect of deteriorating the kinetic constants for the O2 addition to radicals. The contribution of O2 to the line width can be expressed by the following equation:13 8π −1 γ p·R·D·c ΔBpp = 3

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EXPERIMENTAL SECTION The compounds were dissolved in toluene (0.005−0.01 M, optical density ca. 0.5), and the solutions were saturated with argon or helium. Butyl acrylate (FLUKA purum) was used in concentrations between 0.3 and 2.8 M. A home-built flow system was used that consisted of a cylindrical quartz cell (diameter of 4 mm) and Teflon tubing with which to connect the pump system (Reglo digital pump, Ismatec, Glattbrugg, Switzerland). It provides a fast, turbulent flow. The different oxygen concentrations were adjusted by saturation of the toluene solution with oxygen−nitrogen mixtures at 297 K in the same way as described in references.7d,14 The Oxi Level 2 (inoLab) oxygen meter allowed us to perform rapid and reliable dissolved-oxygen measurements. The oxygen sensor CellOx 325 was placed in a chamber that was connected to the flow system of the TR-EPR spectrometer. This gave us the ability to measure the oxygen concentration in situ during TR-EPR measurement (see Figure S3). TR-EPR experiments were performed without modulation of the external magnetic field on a Bruker ESP 300E spectrometer equipped with a microwave amplifier, a TE102 microwave cavity (microwave power of 9 mW and response time of 40 ns). The response of the EPR spectrometer after a laser pulse at a fixed value of the static magnetic field was stored in a LeCroy 9400

where ΔBpp is the increase in line width caused by the presence of oxygen (in T), γ is the magnetogyric ratio of the electron, p is the reaction probability (equal to 1), R is the interaction distance (which we estimate as 5 Å, certainly representing an upper limit), D the diffusion coefficient (ca. 1.5 × 10−5 cm2 s−1), and c is the concentration of oxygen. In our experiments, the oxygen concentration was varied by the saturation of the toluene solution with different oxygen− nitrogen mixtures at a 1 atm partial pressure of oxygen at 297 K. Using the typical values in the above equation, ΔBpp yields 3.35 mT/(mol of oxygen). Accordingly, in the O2 concentration range utilized in our experiments, the contribution of line broadening by oxygen is between 3.35 × 10−4 and 3.35 × 10−5 mT, two orders of magnitude smaller than the line broadening observed in our experiments. B


Article


In the Table1, the rate constants determined in our work are summarized together with published data. We have determined the highest rate constant toward oxygen (kadd(O2)) for benzoyl radical B1· (8.9 ± 1.7 · 1010 M−1 s−1); those for phosphinoyl radicals P1· and P2· are only slightly lower (Table1). These values, however, are considerably higher than those for the carbon centered radicals C1·−C3· being formed from photoinitiators of the α-hydroxyl- or α-aminoacetophenone type (Scheme 3).17 In contrast, radicals B1·, P1·,


dual 125 MHz digital oscilloscope. The central control unit consists of a PC computer with a Debian Linux operating system and a GPIB interface board. The experiment is controlled by the program fsc2 (J. T. Toerring, Institut fuer Experimentalphysik, Freie Universitaet Berlin). A Continuum Surelite II Nd:YAG laser with a fixed repetition frequency of 20 Hz (4−6 ns pulse width, frequency-tripled, 355 nm) was used as the light source. For each photoinitiator and each concentration of the butyl acrylate, the experiment was performed four times with freshly prepared reaction solutions. Within the period of the constant EPR line width, the signal attributed to the benzoyl radical was simulated for 10 slices along the magnetic field. Thus, a set of 40 values led to the average line width represented in Figure 1; the error bars show

Scheme 3. Radicals C1·−C3· Formed from Photoinitiators of the α-Hydroxyl- or α-Amino-acetophenone Type

P2·, and C1·−C3· add to the double bond of butyl acrylate (BA) with rates, being ca. 2−3 orders of magnitude lower than those for oxygen addition.4,7d Using the experimental data (Table1), we have performed kinetic simulations using concentrations typical for radical polymerization: initiator radicals (10 mM), butyl acrylate (2 M), and oxygen (200 μM).3 In Figure 2, we present the oxygen

Figure 1. Left side: line-width variation dependence on O 2 concentration (270 ns after the laser flash). Right side: determination of kadd(O2) for B1·.

individual errors for each data point, determined from the standard deviation of the fit procedure. The error margins of the addition constants given in the Table1 contain the Table 1. Comparison of the Rate Constants for the Addition of Selected Benzoyl, Phosphinoyl, and Carbon-Centered Radicals toward O2 and Butyl Acrylate Determined in This Work with Published Onesa radical B1· P1· P2· C1· C2 · C3·

kadd(O2), our worka; M−1 s−1 × 1010

kadd(O2), literature; −1 −1 M s × 1010

kadd(BA)b, our work; M−1 s−1 × 106

8.9 ± 1.7 5.7 ± 0.9 4.3 ± 0.5

0.3d6c 0.42d7d 0.277d 0.664 0.634 0.234

1.6 ± 0.1 17.9 ± 0.7 7.6 ± 0.2

kadd(BA), literature; M s−1 × 106 −1

0.18d6c d7d

28 11d7d 13c4 29c4 < 0.1c4

a

Solvent, toluene, T = 297 K. bButyl acrylate. cSolvent, MeCN, no error margins available. dSolvent, CH2Cl2, no error margins available

Figure 2. Concentration of oxygen, peroxyl radicals (B1−OO· and P1−OO·), and primary polymer radicals B1−M· and P1−M· in the initial phase of a photoinitiated polymerization. The inset shows B1− M· and P1−M· at a longer time scale (up to 1 μs).

contributions of the line fit and linear-regression errors. The simulations of the EPR signals were performed with a homebuilt routine using Matlab (Version 6, Mathworks Inc., Natick, USA). T2*−1 values were directly determined from the leastsquare fit of the Gauss-type EPR signals. Kinetic simulations were performed with COPASI.16

concentration versus time as well as that of the primarily formed peroxyl radicals, B1−OO·and P1−OO·, together with the primary adduct between BA and the initiating radicals (B1− M· and P1−M·). In the initial phase of the reaction, O2 is consumed (within ca. 5 ns) predominately by benzoyl radical B1·. Accordingly, B1· acts as an efficient oxygen scavenger for the initial steps of the reaction. Because the second-order rate constant for the initial addition to BA is substantially higher for P1 than B1, P1−M· is formed substantially more rapidly than is B1−M· (cf. the inset in Figure 2). A slightly retarded but

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RESULTS AND DISCUSSION Figure 1a indicates the line-width variation of the signal attributed to radical B1·, and the Stern−Volmer-type plot in Figure 1b shows the line-width variation of the EPR signal of B1· versus the oxygen concentration yielding the rate constant (kadd) for the addition of O2 to radical B1·. C


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Recent developments in photoinitiated radical polymerization. Macromol. Symp. 1999, 143, 45−63. (2) (a) Biswal, D.; Hilt, J. Z. Analysis of Oxygen Inhibition in Photopolymerizations of Hydrogel Micropatterns Using FTIR Imaging. Macromolecules 2009, 42 (4), 973−979. (b) Ligon, S. C.; Husar, B.; Wutzel, H.; Holman, R.; Liska, R. Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization. Chem. Rev. 2014, 114 (1), 557−589. (c) Taki, K.; Watanabe, Y.; Ito, H.; Ohshima, M. Effect of Oxygen Inhibition on the Kinetic Constants of the UVRadical Photopolymerization of Diurethane Dimethacrylate/Photoinitiator Systems. Macromolecules 2014, 47 (6), 1906−1913. (3) Scherzer, T.; Langguth, H. Temperature dependence of the oxygen solubility in acrylates and its effect on the induction period in UV photopolymerization. Macromol. Chem. Phys. 2005, 206 (2), 240− 245. (4) Jockusch, S.; Turro, N. J. Radical Addition Rate Constants to Acrylates and Oxygen: a-Hydroxy and a-Amino Radicals Produced by Photolysis of Photoinitiators. J. Am. Chem. Soc. 1999, 121 (16), 3921− 3925. (5) (a) Dietliker, K.; Broillet, S.; Hellrung, B.; Rzadek, P.; Rist, G.; Wirz, J.; Neshchadin, D.; Gescheidt, G. Photophysical Investigations on Photoinitiators with Covalently Linked Thioxanthone Sensitizer Moieties. Helv. Chim. Acta 2006, 89 (10), 2211−2225. (b) Liska, R. Photoinitiators with Functional Groups. VI. Chemically Bound Sensitizers. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (9), 2285− 2301. (c) Weber, M.; Turro, N. J. A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes. J. Phys. Chem. A 2003, 107 (18), 3326−3334. (6) (a) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J.; Ingold, K. U. First Direct Detection of Transient Organic Free Radicals in Solution by Time-Resolved Infrared Spectroscopy. Kinetic Studies on Some Acyl Radicals. J. Am. Chem. Soc. 1991, 113 (5), 1869−70. (b) Brown, C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J. Kinetic and Spectroscopic Studies on Acyl Radicals in Solution by Time-Resolved Infrared-Spectroscopy. Aust. J. Chem. 1995, 48 (2), 363−379. (c) Colley, C. S.; Grills, D. C.; Besley, N. A.; Jockusch, S.; Matousek, P.; Parker, A. W.; Towrie, M.; Turro, N. J.; Gill, P. M. W.; George, M. W. Probing the Reactivity of Photoinitiators for Free Radical Polymerization: Time-Resolved Infrared Spectroscopic Study of Benzoyl Radicals. J. Am. Chem. Soc. 2002, 124 (50), 14952−14958. (7) (a) Weber, M.; Khudyakov, I. V.; Turro, N. J. Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes. J. Phys. Chem. A 2002, 106 (10), 1938−1945. (b) Gatlik, I.; Rzadek, P.; Gescheidt, G.; Rist, G.; Hellrung, B.; Wirz, J.; Dietliker, K.; Hug, G.; Kunz, M.; Wolf, J.-P. Structure-Reactivity Relationships in Radical Reactions: A Novel Method for the Simultaneous Determination of Absolute Rate Constants and Structural Features. J. Am. Chem. Soc. 1999, 121 (36), 8332−8336. (c) Baxter, J. E.; Davidson, R. S.; Hageman, H. J.; Overeem, T. Photoinitiators and Photoinitiation. 8. The Photoinduced.alpha.-Cleavage of Acylphosphine Oxides: Identification of the Initiating Radicals Using a Model Substrate. Makromol. Chem. 1988, 189 (12), 2769−80. (d) Jockusch, S.; Turro, N. J. Phosphinoyl Radicals: Structure and Reactivity. A Laser Flash Photolysis and TimeResolved ESR Investigation. J. Am. Chem. Soc. 1998, 120 (45), 11773− 11777. (e) Savitsky, A. N.; Galander, M.; Mobius, K. W-Band TimeResolved Electron Paramagnetic Resonance Spectroscopy on Transient Organic Radicals in Solution. Chem. Phys. Lett. 2001, 340 (5−6), 458−466. (f) Sumiyoshi, T.; Schnabel, W.; Henne, A. The Photolysis of Acylphosphine Oxides. III: Laser Flash Photolysis Studies with Pivaloyl Compounds. J. Photochem. 1986, 32 (2), 191−201. (8) (a) Salikhov, K. M.; Molin, Y. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Effects in Radical Reactions. Elsevier: Amsterdam, The Netherlands, 1984; Vol. 22. (b) Nagakura, S.; Hayashi, H.; Azumi, T. Dynamic Spin Chemistry: Magnetic Controls and Spin Dynamics of Chemical Reactions. Wiley-Kodansha: Tokyo, 1999.

analogous behavior follows also for the (smaller) rate constants for the reaction of B-type radicals with oxygen reported in the literature (see Figure S1). Benzoyl radicals (B·), are inherent active components of most (radical) photoinitiating systems based on α-cleavage. They are not the most efficient initiating radicals because their reactivity toward addition to CC double bonds is lower than that for most phosphinoyl and α-amino- or α-hydroxyalkyl radicals.4,5c Rather, benzoyl radicals rapidly react with oxygen, acting as “internal antioxidants”. Consequently, phosphoruscentered radicals such as P1· and P2·, and carbon-centered ones as C1·−C3· can preferentially react with the acrylate. Due to the higher addition rate constant of the corresponding α-hydroxyl- and α-aminoalkyl radicals such as C1· or C2· to BA, the scavenging effect is more pronounced for α-hydroxyl- or α-aminoacetophenone-type photoinitiators than for acylphosphine oxides, which show a considerable reactivity toward oxygen (Table1). This is in good agreement with the experimental observation that oxygen inhibition is more pronounced for acylphosphine oxide-type photoinitiators than, for example, for α-hydroxy ketones. The rate constants kadd(O2) determined in this investigation are somewhat higher than those determined previously by timeresolved IR spectroscopy and (dye-assisted) laser-flash photolysis.6b,c,7f These differences could be traced back to the different solvents. The solubility of oxygen in toluene (used in this investigation) is higher than in acetonitrile and CH2Cl2.18 Moreover, we have used in situ monitoring of the oxygen content using a Clark electrode. Importantly, the oxygenscavenging effect of the benzoyl radicals also follows from the formerly published rate constants (see Table S1). This oxygen-protecting effect in the initiating phase of photoinitiated radical polymerization may be also responsible for the high efficiency of, for example, bisacylphosphinoxides compared to that of monoacylphosphine oxides, in which a prolonged photolysis (several seconds) leads to the formation of two benzoyl radicals.19 Accordingly, these effects should also be addressed in further development of photoinitiators.20

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b04263. Details of the kinetic simulations, including all radicals presented in Table1 and literature data, and the experimental setup, as well as references. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NAWI Graz. G.G. is indebted to Wolfgang Lubitz for his encouragement over many years. REFERENCES

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