Macromolecules 2008, 41, 9063-9066
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New Tetraalkylborate Initiators for Remote Polymerization† Andrey A. Ermoshkin,‡,| Ekaterina S. Nikolaeva,‡ Douglas C. Neckers,*,‡ and Andrei V. Fedorov*,§,⊥ Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403, and Wright Photoscience Laboratory, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed April 4, 2008; ReVised Manuscript ReceiVed September 3, 2008
ABSTRACT: The polymerization of acrylic esters without direct use of light and/or heat is an important goal. We have investigated a system where the activating component is delivered in a flow of a carrier gas which has unrestricted access to a surface covered with to-be-polymerized monomers. The active ingredient (H2O2 vapor) reacts with coating components (tetraalkylborates) to generate free radicals and initiate polymerization in the surface layer. Differences in reactivity are explained by differing oxidation potentials and alkyl substituents on the boron atom. Radical trapping experiments support the mechanism proposed.
Introduction Photopolymerization can be spatially directed and turned on or off.1,2 In free radical processes triggered by light, rates can be controlled by a combination of factors: source(s) of radicals, light intensity, and temperature. Most industrial photopolymerizations use solvent-free systems providing both economic and environmental benefits. Despite multiple advantages, photopolymerization has limitations that include low penetration of light through highly pigmented materials and the requirement of a direct line by which a light beam can reach a surface. These limitations prevent its use in a variety of applications. We were able to produce polymers without the direct use of light by delivering component(s) of an initiating system in a flow of a carrier gas. Upon contact with the surface to-bepolymerized, these component(s) generate free radicals and initiate the polymerization of the monomers that comprise the coating. We have called this “remote polymerization” and previously reported a system utilizing oxalate and glyoxylate esters.3a Another system utilizing trialkylborane/amine complexes and various vaporous release agents was also investigated.3b The focus of the present report is on tetraorganyl borate salts. Upon electron transfer, tetraorganyl borate anions produce alkyl radicals making this class of compounds useful radical initiators.4 Herein we describe novel systems in which tetraalkylborate salts dissolved in acrylic monomers are activated by an H2O2 vapor [hydrogen peroxide and other peroxides are strong oxidizing agents (Eox(H2O2) ) 0.9 V (SCE))5] to initiate radical polymerization. Experimental Section Materials. The following reagents were obtained from Aldrich and used as received unless indicated otherwise: γ-butyrolactone (98%), triethylborane (95%), tributylborane (98%), ethyl lithium reagent (0.5 M in C6H6/cyclohexanes 9/1), butyl lithium reagent (1.6 M in hexane), tetrabutylammonium bromide (99%), ethyl acrylate (99%), benzene (g99%, spectrophotometric grade), bromotrichloromethane (99%). The following HPLC grade solvents * Corresponding author. E-mail: (D.C.N.)
[email protected]; (A.V.F.)
[email protected]. † Contribution No. 607 from the Center for Photochemical Sciences, Bowling Green State University. ‡ Center for Photochemical Sciences, Bowling Green State University. § Wright Photoscience Laboratory, Bowling Green State University. | Present address: Spectra Group Limited, 27800 Lemoyne Rd., Millbury, OH 43447. ⊥ Present address: Pacific Biosciences, 1505 Adams Dr., Menlo Park, CA 94025.
were obtained from EMD: hexanes, THF, acetonitrile. THF was refluxed over Na/K alloy for 4 h and distilled in an Ar atmosphere to obtain oxygen-free solvent. Hexanes were dried by refluxing over sodium metal (99%, Aldrich) for 4 h followed by distillation. Trimethylolpropane triacrylate (TMPTA) was obtained from Sartomer and inhibitor-free monomer was prepared using commercial inhibitor remover (Aldrich). Aqueous H2O2 (30% - Fisher) was used as received. Nitrogen and argon were research grade. NMR Spectroscopy. 1H and 13C NMR spectra were recorded using a Bruker Avance 300 nuclear magnetic resonance spectrometer. Resonance frequencies of 1H and 13C were 300 MHz and 75 MHz, respectively. 11B NMR spectra were recorded using 400 MHz Varian Unity Plus system. The resonance frequency for 11B was 128 MHz. The samples were dissolved in chloroform-d or benzened6 (both from Aldrich). For quantitative measurements, signals from TMS of known concentration and residual amounts of C6D5H or CHCl3 in the deuterated solvents were used as internal references. Synthesis of Tetrabutylammonium Tetraalkylborates. Caution was exercised to keep, handle, and transfer all reagents and reaction mixtures under an atmosphere of dry argon since trialkylboranes and lithium tetraalkylborates are pyrophoric. Tetraalkylborate salts were prepared in two steps: synthesis of the lithium salt from corresponding trialkylborane and alkyl lithium reagent followed by Li+ exchange with Bu4N+. Synthesis of a Lithium Salt. In a 2-neck 250 mL flask 150 mL of absolute hexanes were placed. The flask was evacuated under stirring followed by an introduction of nitrogen to a pressure of ca. 800 mm Hg. This evacuation/filling cycle was repeated 5 times. Upon completion, the flask was equipped with an inlet and an outlet of an inert gas. Next, a designated amount (Table S1, Supporting Information) of liquid trialkylborane was taken from its storage container that had been equipped with septa, and argon inlet and outlet and transferred to the reaction flask. Alkyllithium reagent was added dropwise under stirring at 20 °C in an amount (Table S1, Supporting Information) to maintain 1:1 molar ratio with the corresponding trialkylborane. The reaction mixture was then stirred for 2 h at room temperature. The white precipitate of lithium tetraalkylborate formed was collected on a glass frit using an argon pressure gradient and washed twice with oxygen-free hexanes. The washed precipitate was dried in Vacuo. Cation Exchange. Lithium tetraalkylborate and tetrabutylammonium bromide (10% excess) were dissolved in degassed water (150-200 and 10-20 mL, respectively) and the resulting solutions combined under stirring. Instant formation of a white precipitate of tetrabutylammonium tetraalkylborate was observed. The precipitate was collected, washed with degassed water, and transferred into a 100 mL flask. The product was dried under vacuum and recrystallized from degassed ethanol. The recrystallized product was washed twice with cold degassed ethanol. The tetraalkylborates
10.1021/ma8007543 CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
9064 Ermoshkin et al.
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obtained were characterized by 1H NMR, 11B NMR, electrospray ionization LCMS and elemental analysis. Borate salts synthesized along with reagents, yields, and details of product characterizations are listed in Tables S1 and S2, Supporting Information. Tetrabutylammonium tetraisopropylborate and all asymmetrical tetraalkylborates were synthesized for the first time. Assignments of the 1H NMR spectra are also given in the Supporting Information. ATR FTIR Spectroscopy. FTIR spectra were recorded using a Shimadzu 8400S spectrometer. Double bond conversion was monitored in situ by a Pike Technology attenuated total reflectance (ATR) attachment. The samples tested contained tetrabutylammonium tetraalkylborate (0.12 M) and γ-butyrolactone (10% mass, to assist dissolution of the borate salt) dissolved in the TMPTA monomer. The borate (0.036 mmol) was weighed in an inert atmosphere bag followed by addition of γ-butyrolactone (0.333 g) and TMPTA (3.294 g). A typical formulation amount (V ) 0.3 mL) was placed on the horizontal surface of CdSe crystal of an ATR attachment (6 × 0.5 cm, 1 mm thickness of the uncured layer) mounted inside the instrument’s sample chamber. Nitrogen was bubbled through the tube filled with a 30% aqueous solution of hydrogen peroxide and the resulting vapors directed to the sample.3 Spectra were taken at designated intervals. The following acquisition parameters were used: 700 - 4000 cm-1 range; 30 scans; 2 cm-1 resolution. The percent of double bond conversion (%DB) was monitored by the decrease in the area of the acrylic wag peak at 809 cm-1 internally referenced to the area under either the carbonyl or CH mid-IR peaks. The ratios obtained were used to calculate the double bond conversion as follows:
Itime )
A809 A1700
Itime )
A809 A2900
% DB )
I0 - Itime × 100% I0
where I0 is the ratio measured before exposure; Itime is the ratio measured after exposure, A809 is the area under the 809 cm-1 peak, A1700 is the area under the carbonyl peak, and A2900 is the area under the CH peak. The results obtained for 1700 and 2900 cm-1 internal references were similar (see Supporting Information). Double bond conversion values averaged from both references are reported. Final double bond conversions obtained from ATR FTIR were verified by near IR using the methodology described previously.3 Control experiments without H2O2 resulted in no significant double bond conversion. Neither was conversion observed if formulations containing no borate salt were exposed to the stream of N2 saturated with H2O2 vapor. Electrochemistry. The oxidation potentials of the borates were measured by cyclic voltammetry on an Epsilon-2 BASi electrochemical workstation. Saturated calomel reference (SCE), platinum working (PTE), and platinum wire auxiliary electrodes were used with the following acquisition parameters: 0-1000 mV scanning range; 100mV/s scan rate; 100 µA scale. Oxidation of the borate anion resulted in irreversible formation of alkyl radical. Therefore, only half-potentials for borate salts could be measured. CH3CN solutions for electrochemical experiments contained 30 mM of analyzed borate and 200 mM of Bu4NPF6 as the supporting electrolyte. Solutions tested were placed into electrochemical cell equipped with a Teflon cover and deoxygenated prior to analysis by purging with argon for at least 15 min. The surface of the PTE electrode was freshly polished before each measurement. Laser Flash Photolysis. A detailed description of the experimental apparatus was given before.3,6 The samples consisted of 6.5 mM solution of benzophenone in benzene and tetrabutylammonium tetraalkylborate quencher of designated concentration. The quencher concentrations of 0.1, 0.25, 0.5, 0.75, 1, and 1.5 mM were obtained by dilution of a 2 mM solution. The samples were placed into a
Figure 1. Typical polymerization kinetics for tetrabutylammonium isopropyltriethylborate (0.036mmol)/trimethylolpropane triacrylate formulation (0.3 mL volume) exposed to a stream of nitrogen with (sample, dark squares) and without (control, hollow squares) hydrogen peroxide vapor. Inset: polymerization kinetics for tetrabutylammonium isopropyltriethylborate in TMPTA remotely initiated by hydrogen peroxide (dark squares), tert-butyl hydroperoxide (hollow circles) and di-tert-butyl peroxide (hollow squares).
standard 1 × 1 cm quartz cuvette. Sample solutions were deoxygenated by purging with argon for at least 15 min prior to the photolysis. GC Experiments. Tetrabutylammonium tetraalkylborate (0.025 mmol) was transferred to a vial under inert atmosphere followed by addition of solvent. The radical trapping agent (0.25 mmol) was added to the reaction followed by the addition of hydrogen peroxide (0.025 mmol). The reaction vessel was equipped with an inlet and outlet for argon, and the reaction mixture was ultrasonicated for 15 min. Solutions for NMR analyses were prepared in a similar fashion. Ethyl acrylate and bromotrichloromethane were used as radical trapping agents. A slow background reaction between CCl3Br and tetraalkylborates was observed, however, the contributions from this were separable from the outcome of the interaction between H2O2 and borate. Reaction products were analyzed using a GC-17A Shimadzu gas chromatograph. The parameters of GC analysis were as follows: 15 m capillary column; i.d., 0.250 mm; phase thickness, 0.250 µm; column gas flow, 1.1 mL/min; velocity, 32 cm/s; initial oven temperature, 70 °C; maximum oven temperature, 350 °C. The injection volume was 1 µL for all tested samples (Table S6, Supporting Information). Six controls (a solvent; borate salt-solvent; radical trapping agentsolvent; H2O2-solvent; trapping agent-H2O2-solvent; trapping agent-borate-solvent) were also analyzed for each experiment.
Results and Discussion Remote Polymerization Monitored In Situ by ATR. All tetraalkylborates are active in remote polymerization (Figures 1 and 2) but the phenyltributyl- and butyltriphenylborate salts showed no reactivity. Bu4N+(iPr)Et3B- is the most reactive with about 58% of the double bonds converted. Other borates also gave significant double bond conversion (Table 1). There is a reasonable agreement between the final double bond conversion obtained by ATR mid-IR (within 1 µm of the bottom) and near IR measurements (entire depth) that is indicative of a uniform cure throughout the entire polymerized layer. Although some control experiments where formulations were exposed only to the stream of nitrogen yielded noticeable double bond conversion (Table 1), in all cases the double bond conversion was significantly higher for formulations exposed to H2O2 vapor. These data demonstrate that each of the tetraalkylborate salts is capable of initiating radical polymerization when activated by H2O2 vapor. We have investigated the effect of tert-butyl hydroperoxide and di-tert-butyl peroxide (Figure 1 inset). Polymerization
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New Tetraalkylborate Initiators 9065 Scheme 1. Proposed Mechanism for Remote Polymerization
Table 2. Concentrations of EtBr in Different Samples
Determined from NMR Experiments solvent
Table 1. Final Percent of TMPTA Double Bond Conversion for
Remote Cure Experiments Measured by ATR FTIR and Near IR and Oxidation Potentials of Tetraalkylborates
borate salt
sample max double bond conversion (%)
control max double bond conversion (%)
sample max double bond conversion (%) by near IR
oxidation potential, mV
Bu4N+(iPr)Et3BBu4N+BuEt3BBu4N+Et4BBu4N+(iPr)4BBu4N+Bu4B-
58 52 50 46 40
7 18 13 6 22
58 57 54 51 37
328 406 435 223 437
occurred with neither. This is consisted with our previous observations for remote cure systems employing oxalate esters.3 Electrochemistry. The difference in the oxidation potentials is indicative of the relative ease of electron removal from each borate anion. The oxidation potentials (Eox) increase in the following order: Bu4N+(iPr)4B- < Bu4N+(iPr)Et3B- < Bu4N+BuEt3B- < Bu4N+Et4B- < Bu4N+Bu4B- , Bu4N+PhBu3B- , Bu4N+BuPh3B-. The latter two borates are remote cure inactive (Figure 3 and Tables 1 and S3 (Supporting Information)) and have much higher oxidation potentials of 547 and 998 mV. The observed differences in Eox between active and inactive borates provide additional support for the critical role of the electron transfer. Since the order of electron donating ability of the borates do not exactly correspond to the order of reactivity of borates in the cure process (Table 1 top to bottom), the activity of the tetraalkyl borates does not solely depend on the ease of an electron removal by an oxidizing agent.
Figure 3. Cyclic voltammograms and oxidation potentials for examined tetraalkylborates in acetonitrile. Solutions contained 30 mM of analyzed borate and 200 mM of Bu4NPF6 as the supporting electrolyte and were deoxygenated prior to analysis by purging with argon for 15 min.
[Solv.-H1], mM
Ca 12 670 Sb 12 670 CDCl3 Ca 5.2 25.5 Sb 5.2 25.5 a Control containing no H2O2 b Sample. C6D6
Figure 2. Polymerization kinetics for tetrabutylammonium tetraalkylborates (0.036mmol)/trimethylolpropane triacrylate formulations exposed to a hydrogen peroxide vapor.
[TMS], mM
[EtBr]TMS, mM
[EtBr]solv.-H1, mM
32.9 45.7 117.7 157.1
25.0 32.7 77.2 87.9
Laser Flash Photolysis. Laser flash photolysis was conducted in which the benzophenone triplet state was formed rapidly and used as an electron acceptor from borate anions instead of H2O2. The benzophenone triple state quenching rate constants (kQ) determined were similar for all the borates examined and approach the diffusion controlled limit of ∼109 M-1 s-1 (Table S5, Supporting Information). Unfortunately, we could not distinguish between the reactivities of different borates using LFP experiments. Proposed Mechanism for Remote Cure. We suggest that borate anion is oxidized by H2O2 producing boranyl radical (R4B•) and the radical anion (H2O2•-) (Scheme 1). The boranyl radical, in turn, irreversibly dissociates to trialkylborane (R3B) and an alkyl radical (R•). The radical anion (H2O2•)- may dissociate to hydroxyl radical (OH•) and hydroxyl anion (OH-). Furthermore, trialkylborane may react with oxygen dissolved in the formulation giving two oxygen-centered radicals (RO• and R2BO•) that also lead to the radical polymerization.7 According to the proposed mechanism, four different radicals are potentially capable of initiating chain polymerization (R•, OH•, RO•, and R2BO•). Mechanistic Studies. The alkyl radicals released upon borate oxidation by H2O2 were trapped using CCl3Br. The product of reaction of alkyl radical with CCl3Br is expected to be a corresponding alkyl bromide. NMR Experiments. 1H NMR spectra of reaction mixtures containing Et4B- in C6D6/TMS in the presence and absence of H2O2 (Figure S12, Supporting Information) display new signals in both the reaction and control mixtures in addition to intense signals from the starting borate. These signals match the 1H NMR signals of ethyl bromide in C6D6/TMS (Figure S11, Supporting Information). Since the amount of TMS and C6D6 and degree of isotopic substitution in C6D6 (99% atom D) are known, the concentrations of TMS and C6D5H in the reaction mixture could be estimated. The concentration of EtBr referenced to TMS (C6D5H) was 1.39 (1.30) times higher in the reaction mixture than in the control (Table 2). This indicates that addition of H2O2 produces additional EtBr, thus, supporting the mechanism for remote polymerization (Scheme 1). Similar experiments were conducted in CDCl3 (Figures S13-S14 (Supporting Information), Table 2). Again, the concentration of EtBr from the reaction mixture was higher than that from the control mixture (1.34 and 1.14 times relative to [TMS] and [CHCl3], respectively). The amount of EtBr formed, however, was higher than the theoretical yield calculated based on the initial borate salt concentration. The possible formation of free
9066 Ermoshkin et al. Scheme 2. Steps of the Remote Cure Process
triethylborane (Et3B) during the borate oxidation that is capable of producing ethyl radicals7 may explain the additional amount of EtBr observed. Gas Chromatography Experiments. GC analyses of the reaction mixtures in CDCl3 are consistent with the results of NMR experiments. The signal having a retention time of 0.89 min corresponds to the expected EtBr product (Figure S15). The signal identity was confirmed by an injection of the standard solution of ethyl bromide. The intensity of the EtBr signal formed in the presence of H2O2 is 1.7 times higher compared to the control. These data confirm the formation of ethyl radical from tetraethylborate upon oxidation by H2O2 and, hence, further supports the mechanism proposed (Scheme 1). Factors Determining Borate Reactivity. Remote cure involves two steps (Scheme 2). First, electron transfer between borate salt and hydrogen peroxide results in the formation of alkyl radical and trialkylborane. Second, an addition of the alkyl radical(s) formed to an acrylic monomer, initiates a chain process leading to polymer formation. Additional pathways of radical initiation may be available owing to the initiating capabilities of trialkylborane,7 a second active component produced during the first step of the remote cure (Scheme 1). The first step occurs at diffusion controlled rates4 (ca. 109 M-1 -1 s ) while the second step is slower. The following rates on addition to methyl acrylate were reported: 3.8 × 105 M-1 s-1 for methyl and 6.2 × 105 M-1 s-1 for pentyl radicals.8 Therefore, the overall reactivity of the borate salt in the remote cure process should be limited by the second step assuming that reaction conditions allow for the first step to proceed. This reactivity depends directly on the ability of the free radical formed from borate anion and H2O2 to initiate acrylic polymerization. In the symmetrically substituted borates, only one radical can be produced. If the boron atom has two different substituents, however two different radicals may be produced. In the latter case, the predominant formation of the most stable radical takes place resulted from the cleavage of the weakest bond. From borates examined, three different radical species may be formed having the following stability order: isopropyl . n-butyl > ethyl. Borates producing the more stable isopropyl radical are more reactive than others. This applies to Bu4N+(iPr)4B- which produces only isopropyl radicals and Bu4N+(iPr)Et3B- which predominantly yields iPr radical. Bu4N+Bu4B- and Bu4N+BuEt3B- yield n-butyl radical, which is second in stability. Bu4N+Et4B- gives the least stable ethyl radical. The reactivity order based on the stability of the radicals produced correlates reasonably with the oxidation potentials of borates studied (Bu4N+(iPr)4B- < Bu4N+(iPr)Et3B< Bu4N+BuEt3B- < Bu4N+Et4B- < Bu4N+Bu4B-), except for a reduced reactivity of tetrabutylborate. However, the conditions of the electrochemical experiments where potential is applied to a bulk solution differ from those of the remote cure experiments where electron transfer between two molecules occurs. Therefore, steric factors also need to be taken into consideration. Bu4N+Et4B-, Bu4N+(iPr)Et3B- and Bu4N+BuEt3B- are more sterically accessible than Bu4N+Bu4B- and Bu4N+(iPr)4B-. This influences both the overall efficiency of remote cure and final degree of a double bond conversion. The rate of the second step is determined by the ability of the radical formed during the first step to attack the acrylic double
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bond. The isopropyl radical is the most efficient because its lifetime is comparable with the time scale of the propagation step of radical polymerization.9 The more reactive ethyl and butyl radicals are too “hot” to propagate the chain owing to their enhanced ability to abstract hydrogen atoms from acrylates. Therefore, borates producing isopropyl radicals are expected to display better reactivity in remote cure. This, combined with the radical stability and steric considerations, may explain the reactivity order observed from the ATR FTIR measurements. Bu4N+(iPr)Et3B- is the most efficient because it produces isopropyl radical and has good steric accessibility. Then follow Bu4N+BuEt3B- and Bu4N+Et4B- where steric factors favor the radical formation; however, these radicals are less efficient toward conversion of a double bond. Finally, sterically hindered Bu4N+(iPr)4B- and Bu4N+Bu4B- display the smallest degree of a double bond conversion. Several other factors may critically affect the overall efficiency of the remote cure. Further studies are underway to evaluate the effect of the initial viscosity of the formulations on the remote cure efficiency as well as the cure depth dependence on various factors. Acknowledgment. The authors acknowledge greatly the Office of Naval Research for funding this project (Grant No. N00014-04-10406). The Ohio Department of Development and Bowling Green State University are acknowledged for funding this project through the Wright Photoscience Laboratory. Supporting Information Available: Additional details of tetraalkylborate synthesis and characterization, including tables of synthesis and charcaterization results, elemental analysis results, figures showing assignments of 1H NMR and 11B NMR spectra, text giving additional details of ATR measurements, with a figure showing the ATR FTIR curves, tables containing electrochemical data, results of laser flash photolysis, and mechanistic and GC studies, and a figure showing the GC plots. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Oster, G.; Yang, N.-L. Chem. ReV. 1968, 68, 125. (2) Neckers, D. C., Jager, W. Photoinitiation for Photopolymerization: UV & EB at the Millenium, Chemistry & Technology of UV & EB Formulations for Coatings, Inks and Paints; Wiley-Interscience: New York, 1998; Vol. VII, p 1. (3) (a) Ermoshkin, A. A.; Neckers, D. C.; Fedorov, A. V. Macromolecules 2006, 39, 5669. (b) Fedorov, A. V.; Ermoshkin, A. A.; Neckers, D. C. J. Appl. Polym. Sci. 2008, 107, 147. (4) For example see: (a) Schuster, G. B. Pure Appl. Chem. 1990, 62, 1565. (b) Sarker, A. M.; Polykarpov, A. Y.; de Raaff, A. M.; Marino, T. L.; Neckers, D. C. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2817. (c) Lan, J. Y.; Schuster, G. B. Tetrahedron Lett. 1986, 27, 4261. (d) Valdes-Aguilera, O. M.; Pathak, C. P.; Shi, J.; Watson, D.; Neckers, D. C. Macromolecules 1992, 25, 541. (e) Hassoon, S.; Neckers, D. C. J. Phys. Chem. 1995, 99, 9416. (f) Chatterjee, S.; Davis, P. D.; Gottschalk, P.; Kurtz, M. E.; Sauerwien, B.; Yang, X.; Schuster, G. B. J. Am. Chem. Soc. 1990, 112, 6329. (g) Hassoon, S.; Sarker, A.; Rodgers, M. A. J.; Neckers, D. C. J. Am. Chem. Soc. 1995, 117, 11369. (h) Polykarpov, A. Y.; Hassoon, S.; Neckers, D. C. Macromolecules 1996, 29, 8274. (i) Hassoon, S.; Sarker, A.; Polykarpov, A. Y.; Rodgers, M. A. J.; Neckers, D. C. J. Phys. Chem. 1996, 100, 12386. (j) Mejiritski; A.; Polykarpov, A. Y.; Sarker, A. M.; Neckers, D. C. J. Photochem Photobiol. A: Chem. 1997, 108, 289, and references therein. (5) Santhanam, K. S. V. Pure Appl. Chem. 1998, 70, 1259. (6) Merzlikine, A. G.; Voskresensky, S. V.; Fedorov, A. V.; Neckers, D. C. Photochem. Photobiol. Sci. 2004, 3, 892. (7) (a) Sonnenschein, M. F.; Webb, S. P.; Kastl, P. E.; Arriola, D. J.; Wendt, B. L.; Harrington, D. R. Macromolecules 2004, 37, 7974. (b) Fedorov, A. V.; Ermoshkin, A. A.; Mejiritski, A.; Neckers, D. C. Macromolecules 2007, 40, 3554. (8) Beckwith, A. L. J.; Poole, J. S. J. Am. Chem. Soc. 2002, 124, 9489. (9) Moad, G., Solomon, D. H. The Chemistry of Radical Polymerization; Elsevier: Oxford, U.K., 2006; Chapter 1, pp. 8-27.
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