Structural Effects in the Iridium Complex Series: Photoredox Catalysis and Photoinitiation of Polymerization Reactions under Visible Lights

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1 FULL PAPER Photopolymerizations Structural Effects in the Iridium Complex Series: Photoredox Catalysis and Photoinitiation of Polymerization Reactions under Visible Lights Mohamad-Ali Tehfe, Marc Lepeltier, Frédéric Dumur, Didier Gigmes, Jean-Pierre Fouassier, and Jacques Lalevée* A series of 18 iridium complexes (Ir) have been prepared and studied. Their abilities to initiate both a ring-opening cationic polymerization (CP) using an Ir/iodonium salt couple and a free radical polymerization (FRP) in the presence of an Ir/amine/alkyl halide system under very soft irradiation (i.e., halogen lamp) or upon exposure to laser nm nm are investigated. Interestingly, these photoinitiating systems operate through an oxidative or a reductive photoredox catalytic cycle, respectively. Excellent polymerization profiles are obtained (reactive function conversions >70% for the CP of an epoxide and >60% for the FRP of low viscosity triacrylate). The photochemical mechanisms are studied by fluorescence, steady state photolysis, cyclic voltammetry, and electron spin resonance spin trapping experiments. 1. Introduction Photoredox catalysis largely used in organic synthesis [1] can help to initiate a free radical polymerization (FRP), a cationic polymerization (CP), a free radical promoted cationic polymerization, or a controlled radical polymerization (CRP) under light exposure. [2] The use of organic or inorganic metal-based complexes (MC) as photoinitiators (PI) of FRP and CP is known for a long time. [3] Radical FRP reactions as well as CRP reactions in the presence of MCs have been already disclosed (see, e.g., in ref. [2]). In the recent years, ruthenium, iridium, zinc, copper, iron, titanium, or platinum organic metal complexes Dr. M.-A. Tehfe, Prof. J.-P. Fouassier, Prof. J. Lalevée Institut de Science des Matériaux de Mulhouse IS2M UMR CNRS 7361, UHA, 15, rue Jean Starcky Mulhouse Cedex 68057, France jacques.lalevee@uha.fr Dr. M.-A. Tehfe Carrefour of Innovative Technology and Ecodesign (CITE) Faculty of Engineering University of Sherbrooke blvd Université, Sherbrooke 2500, Canada Dr. M. Lepeltier Institut Lavoisier de Versailles, UMR CNRS 8180 Université de Versailles Saint Quentin en Yvelines 45 avenue des Etats Unis, Versailles 78035, France Dr. F. Dumur, Dr. D. Gigmes Aix Marseille Univ., CNRS, ICR, UMR 7273, Marseille F-13397, France DOI: /macp (OMC) have been proposed as efficient PIs and incorporated into photoinitiating systems (PISs) as presented in several reviews. [4] Copper or iron-based photoredox catalysts were particularly interesting. [4c e] The absorption properties as well as the generation of radicals, cations, or radical cations are strongly dependent on the kind of OMCs. The requirements for a high photosensitivity under soft irradiation conditions in the visible part of the spectrum or at defined irradiation wavelengths have led to the design of novel arrangements/chemical structures of the ligands in the various series of OMCs. Herein, we will consider the iridiumbased PIs series. Indeed, conventional Ir(III) complexes mainly absorb in the UV and the spectrum presents a rather weak visible tail that allows FRP and CP in the purple/blue range. [5] Incorporation of carefully selected ligands has already led to improved properties: for example, the presence of a phenylisoquinoline ligand in iridium complexes extended the absorption up to 532 nm (e.g., Ir_9 [6] or Ir_12 [7] in Scheme 1) and even 635 nm (e.g., Ir_7 and Ir_8 in Scheme 1). [8] In the following, we prepare 14 novel derivatives (Ir_1 Ir_6, Ir_10, Ir_11, Ir_13 Ir_18) and explore the structural effects toward the initiation efficiency of the FRP of a triacrylate and the CP of a diepoxide under a halogen lamp or laser diodes at 457, 473, or 532 nm, in the presence of PISs consisting in an iridium complex (Scheme 1) and one or more additive chosen among N-methyldiethanolamine (MDEA), 2-bromoacetophenone (R-Br), or diphenyliodonium hexafluorophosphate (Iod, Ph 2 I + ). The photochemical properties as well as the associated chemical mechanisms are investigated by steady state photolysis, fluorescence measurements, theoretical calculations, and electron spin resonancespin trapping (ESR-ST), thereby allowing an access to their structure/reactivity/efficiency relationships. 2. Experimental Section 2.1. Materials The investigated iridium complexes (Ir_1 Ir_18) and other chemical compounds are shown in Schemes 1 and 2. They were prepared according to procedures presented in detail in the Supporting Information. MDEA, (1 of 7)

2 Scheme 1. Chemical structures of iridium complexes examined in this study. 2-bromoacetophenone (R-Br), and diphenyliodonium hexafluorophosphate (Iod, Ph 2 I + ) were obtained from Aldrich and used with the best purity available. (3,4-Epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX; Uvacure 1500; from Allnex) and trimethylol-propane triacrylate (TMPTA; from Allnex) have been selected as benchmarked monomers (Scheme 2) well known in the photopolymerization area Synthesis of the Different Iridium Complexes Mass spectroscopy was performed by the Spectropole of Aix-Marseille University. Electronspray ionization (ESI) mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The high resolution mass spectroscopy (HRMS) mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis apparatus driven by the Eager 300 software. 1 H and 13 C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1 H (400 MHz) and 13 C (100 MHz). The 1 H chemical shifts were referenced to the solvent peak dimethylsulfoxide (DMSO)-d 6 (2.49 ppm), CDCl 3 (7.26 ppm), and the 13 C chemical shifts were referenced to the solvent peak DMSO-d 6 (39.5 ppm), CDCl 3 (77 ppm). A detailed presentation of the synthesis is described or recalled in the Supporting Information Irradiation Sources Several lights were used for the irradiation of the samples: (i) a polychromatic light from a halogen lamp (Fiber-Lite, DC-950 incident light intensity: I 0 12 mw cm 2 ; in the nm range); (ii) monochromatic lights delivered by laser diodes at 457, 473, or 532 nm (MBL-III-BFIOPTILAS; I mw cm 2 ). The emission spectra of the light sources are given in ref. [9] Free Radical Photopolymerization (FRP) Experiments FRP experiments were carried out in laminate. The films (25 µm thick) deposited on a BaF 2 pellet were irradiated (see the irradiation sources). The evolution of the double bond content was continuously followed by real time Fourier Transform InfraRed (FTIR) spectroscopy (JASCO FTIR 4100) at 1630 cm 1 as in ref. [10]. Scheme 2. Chemical structures of monomers and additives (2 of 7)

3 2.5. Ring Opening Cationic Photopolymerization (ROP) Experiments The photosensitive formulations were also deposited on a BaF 2 pellet (25 µm thick) and irradiated under air. The evolution of the epoxy group content is continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100), as described in ref. [11], at 790 cm Fluorescence Experiments The luminescence properties of the iridium complexes (Ir) were studied in acetonitrile using a JASCO FP-750 spectrometer. The interaction rate constants k q between Ir and Ph 2 I + (or MDEA) were extracted from classical Stern Volmer treatments (I 0 /I = 1 + k q τ 0 [Ph 2 I + ]; where I 0 and I stand for the luminescence intensity of the Ir complexes in the absence and in the presence of Ph 2 I + or MDEA, respectively; τ 0 stands for the lifetime of the excited iridium complex in the absence of Ph 2 I + or MDEA). The fluorescence lifetimes were determined from time-resolved experiments (Jobin-Yvon Fluoromax 4) Redox Potentials The redox potentials of the iridium complexes were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 m) as a supporting electrolyte (Voltalab 6 Radiometer). The free energy change ΔG et for an electron transfer reaction is calculated from the equation (ΔG et = E ox E red E T + C) [12] where E ox, E red, E T, and C are the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the excited state energy, and the coulombic term for the initially formed ion pair, respectively. C is neglected as usually done in polar solvents ESR-ST Experiments The ESR-ST experiments were carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were produced at room temperature (RT) upon a halogen lamp exposure under N 2 and trapped by phenyl-ntert-butylnitrone (PBN) according to a procedure described in detail in ref. [13]. The ESR spectra simulations were carried out with the WINSIM software. 3. Results and Discussion 3.1. Absorption Properties of the Studied Iridium Complexes (Ir) The ground state absorption spectra of the proposed Ir (Figure 1) allow a large and efficient matching with the Figure 1. UV vis absorption spectra of the different Ir complexes in acetonitrile (3 of 7)

4 Table 1. Parameters characterizing the photochemical properties of the iridium complexes (Ir): redox properties (in acetonitrile); excited state energy levels (E T1 ), and excited state lifetimes under air (τ), interaction rate constants with Ph 2 I + and MDEA in acetonitrile. The free energy changes for 3 Ir/Ph 2 I + (ΔG ox ) and 3 Ir/MDEA (ΔG red ) were calculated with the Rehm Weller equation. Iridium complexes E ox [V/SCE] E T1 [ev] τ a) [ns] k ox (Ph 2 I + ) [m 1 s 1 ] [ΔG ox [ev]] Ir_ [ 1.28] Ir_ [ 1.30] Ir_ [ 1.15] k red (MDEA) [m 1 s 1 ] [ΔG red [ev]] Ir_ [ 1.23] Ir_ [ 1.41] [ 0.89] Ir_ [ 1.33] Ir_ [ 1.32] a) Under air. emission spectra of the various visible light sources (halogen lamp and laser diodes). As expected, interesting absorption possibilities up to 635 nm are found. All the Irs exhibit an intense absorption in the nm wavelength range. Most of them have an interesting absorption, e.g., in the nm region (all compounds), in the nm range (e.g., Ir_1, Ir_3, Ir_4, Ir_5, Ir_6, Ir_8, Ir_10, Ir_11, Ir_12), above 500 nm (e.g., Ir_5, Ir_7, Ir_15) or even up to 650 nm (e.g., Ir_9, Ir_16, Ir_18) Redox Properties of the Studied Iridium Complexes (Ir) The redox properties of typical compounds are gathered in the Table 1. The cyclic voltammetry investigation reveals that the oxidation process is nearly reversible: e.g., the ratio of the anodic/cathodic peak currents i a /i c is <1 (i.e., the reversibility for Ir_3 and Ir_16 is rather good: i a /i c 0.94 and 0.8, respectively; Figure 2A,B). For all complexes, the Ir/Iod interaction reactions are highly favorable according to the highly negative free energy change (Table 1; e.g., ΔG = 1.28 and 1.41 ev for Ir_3 and Ir_12, respectively; using E red (Ph 2 I + ) = 0.2 V). [14] The same holds true for the Ir/MDEA reactions (e.g., ΔG = 0.89 ev for Ir_12; using E ox (MDEA) = 1 V). [15] 3.3. Excited State Reactivity of the Studied Iridium Complexes (Ir) All the Irs are luminescent (see, e.g., the case of Ir_3 in Figure 3A). The excited state lifetimes under air are rather long ( 20 ns, Table 1) and strongly quenched by O 2. The *Ir/Iod interactions occur with very high rate constants (diffusion controlled processes are found, e.g., k = and m 1 s 1 for Ir_3 and Ir_12, respectively). This quenching process promotes the decomposition of the iodonium salt through an electron transfer (1) Ir * Ir(h ν) + + * Ir + PhI Ir + Ph I and Ph I Ph + Ph I Aryl radicals (Ar ) are produced as supported by ESR-ST experiments (e.g., in an irradiated Ir_3/Iod solution, the PBN/ phenyl radical adduct (characterized by a N = 14.2 and a H = 2.2 G; reference values in ref. [16]; Figure 4A) is easily observed. A very fast bleaching is also observed during the photolysis of the Ir/Iod solution (e.g., in Figure 5; halogen lamp irradiation) in full agreement with the chemical mechanisms presented above (reaction (1)) and the highly favorable ΔG and k values for the Ir/Iod interaction (Table 1). The Ir complex is not regenerated here (to do that, a third compound such as a silane must be added as seen, e.g., in ref. [17]). Luminescence quenching experiments (Figure 3B) of the Irs by MDEA also lead to high interaction rate constants k (but approximately two orders of magnitude lower than those determined for the Ir/Iod couples; e.g., for Ir_12, k = m 1 s 1 ). The electron transfer reaction (2) is also favorable as revealed by the calculated negative free energy change ΔG (Table 1). In addition to (2), two other electron transfer reactions can occur in the Ir/R-Br/amine systems. Indeed, in the presence of the phenacylbromide, PhC( O)CH 2 (R ) are clearly generated (1) Figure 2. Examples of cyclic voltammograms: A) Ir_3 and B) Ir_ (4 of 7)

5 Figure 3. Luminescence quenching of Ir_3 by A) Iod and B) MDEA. In acetonitrile. Figure 4. Examples of ESR-ST spectra for the irradiation of different solution: A) Ir_3/Iod and B) Ir_12/ethyldimethylaminobenzoate (EDB)/R-Br. Experimental a) and simulated b) spectra. [Iod] = 0.01 m. [EDB] = m. [R-Br] = m. In tert-butylbenzene. 457 nm laser diode irradiation phenyl-n-t-butylnitrone (PBN) as a spin-trap agent. EDB is used here to avoid the known problem encountered with MDEA in ESR. according to (3) and (4): the characteristic PBN adduct of the phenacyl radical is clearly detected by ESR-ST (a N = 14.4 G; a H = 4.1 G; reference values in ref. [16]; Figure 4B). In such a system, the Ir complex is regenerated in (4) 3 + MDEA Ir Ir + MDEA + (2) 3 + R Br + + R Br followed byr Br Ir Ir R + Br (3) Ir + R Br Ir + R + Br (4) 3.4. The Iridium Complex/Iodonium Salt: A Photoinitiating System for Cationic Polymerization Figure 5. Steady state photolysis of Ir_10/Iod in acetonitrile upon the halo gen lamp exposure. UV vis spectra recorded at different irradiation times. The ring-opening cationic photopolymerization of EPOX (Scheme 2) under air in the presence of the Ir/Iod systems is quite excellent both under the halogen lamp and the laser (5 of 7)

6 Table 2. Compared polymerization profiles (epoxy function conversion vs irradiation time) for EPOX upon a laser diode exposure at 457 nm under air in the presence of iridium complex/iodonium salt couples (1%/2% w/w): rate of polymerization (R p ) and final epoxy function conversions. Ir R p /[M 0 ]*100 a) [s 1 ] Conversion [%] for t = 800 s Ir_1, Ir_2, Ir_4, Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ a) [M 0 ]: the initial monomer concentration. Table 3. Polymerization rates (Rp) and final acrylate function conversions for TMPTA in laminate. Upon a laser diode irradiation at 457 nm. Initiating system: Ir/MDEA/R-Br (1%/4%/3% w/w). Ir R p /[M 0 ]*100 a) (s 1 ) Conversion [%] for t = 200 s Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ Ir_ a) [M 0 ]: the initial monomer concentration. diodes at 457 or 532 nm (Table 2 and Figure 6). Rather high final conversions are reached after 6 min of irradiation (e.g., >70% with Ir_13, Ir_14, or Ir_16). Tack free coatings are always obtained. This highlights the ability of Ir + generated in (1) to open the epoxy ring, the kind of Ir + as well as the absorption properties of the Irs (Figure 1) driving the CP results + Ir + EPOX Polymer (5) 3.5. The Ir/Amine/Alkyl Halide Reductive Cycle: A Photoinitiating System for FRP The free radical polymerization of TMPTA in laminate in the presence of Ir/MDEA/phenacylbromide (RBr) is rather efficient under visible lights (Table 3 and Figure 7; 57% of acrylate function conversion within 200 s under a laser diode at 457 nm exposure). In the absence of phenacylbromide, no polymerization is observed (curve 1 vs curve 2). The phenacyl radicals R formed in (3) and (4) initiate the FRP (6). The relative efficiency of the different iridium complexes can be ascribed to (i) their different light absorption properties (Figure 1), (ii) the relative 1 Ir/ MDEA and 1 Ir/RBr interaction rate constants (reactions (2) (4)), and (iii) the efficiency of the Ir /R-Br electron transfer reactions R + TMPTA Polymer (6) 4. Conclusion Figure 6. Typical polymerization profiles of EPOX (epoxy function conversion vs irradiation time), under air, in the presence of Ir_16/Iod (1%/2% w/w) upon a (1) laser diode at 457 nm, (2) laser diode at 532 nm, and (3) a halogen lamp irradiation. Insert: IR spectra recorded during the photopolymerization reaction using (1): before and after polymerization. In the present paper, new iridium complexes that can work according to oxidative (in the presence of an iodonium salt) or reductive (in the presence of an amine and alkyl halide) cycles are presented. In the first case, the Ir oxidized form (Ir + ) is easily generated and starts a ring opening polymerization under visi ble light irradiation conditions (halogen lamp and laser (6 of 7)

7 Figure 7. Typical polymerization profiles of TMPTA (acrylate function conversion vs irradiation time) upon a laser diode irradiation at 457 nm in laminate in the presence of (1) Ir_12/MDEA (1%/4% w/w) and (2) Ir_12/MDEA/R-Br (1%/4%/3% w/w). diodes) and under air. In the second case, phenacyl radicals are formed and initiate the FRP. The performance of these Ir-based PISs have been mainly checked under a laser diode at 457 nm or a halogen lamp. Many of them are obviously able to work under light emitting diode (LEDs) at 365, 385, or 395 nm. The present Ir series allows a tunable character (from 300 to 635 nm) for the excitation and could extend their applications in other areas such as in the synthetic organic chemistry as photoredox catalysts. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Conflict of Interest The authors declare no conflict of interest. Keywords free radical polymerization, iridium complexes, LED, photopolymerization, ring-opening cationic polymerization (ROP) Received: April 10, 2017 Revised: June 2, 2017 Published online: July 10, 2017 [1] a) H. W. Shih, M. N. Vander Wal, R. L. Grange, D. W. C. MacMillan, J. Am. Chem. Soc. 2010, 132, 13600; b) J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011, 40, 102; c) K. Zeitler, Angew. Chem., Int. Ed. 2009, 48, 9785; d) M. H. Larraufie, R. Pellet, L. Fensterbank, J.-P. Goddard, E. Lacôte, M. Malacria, C. Ollivier, Angew. Chem., Int. Ed. 2011, 50, 4463; e) M. A. Ischay, Z. Lu, T. P. Yoon, J. Am. Chem. Soc. 2010, 132, 8572; f) G. Zhang, I. Y. Song, K. H. Ahn, T. Park, W. Choi, Macromolecules 2011, 44, [2] a) B. P. Fors, C. J. Hawker, Angew. Chem., Int. Ed. 2012, 51, 8850; b) D. Konkolewicz, K. Schröder, J. Buback, S. Bernhard, K. Matyjaszewski, ACS Macro Lett. 2012, 1, 1219; c) D. M. Haddleton, Rooftop Reactions Nat. Chem. 2013, 5, 366; d) X. Zhang, C. Zhao, Y. Ma, H. Chen, W. Yang, Macromol. Chem. Phys. 2013, 214, 2624; e) O. S. Taskin, G. Yilmaz, M. A. Tasdelen, Y. Yagci, Polym. Int. 2014, 63, 902; f) M. A. Tasdelen, M. Ciftci, Y. Yagci, Macromol. Chem. Phys. 2012, 213, 1391; g) M. Ciftci, M. A. Tasdelen, W. Li, K. Matyjaszewski, Y. Yagci, Macromolecules 2013, 46, 9537; h) A. Aguirre-Soto, C.-H. Lim, A. T. Hwang, C. B. Musgrave, J. W. Stansbury, J. Am. Chem. Soc. 2014, 136, 7418; i) J. Lalevée, N. Blanchard, M. A. Tehfe, F. Morlet-Savary, J. P. Fouassier, Macromolecules 2010, 43, 10191; j) M. Bouzrati, J.-P. Fouassier, J. Lalevée, N. Zivic, F. Dumur, A. Kermagoret, D. Gigmes, ChemCatChem 2016, 8, 1617; k) P. Xiao, J. Zhang, F. Dumur, M.-A. Tehfe, F. Morlet-Savary, B. Graff, D. Gigmes, J.-P. Fouassier, J. Lalevée, Prog. Polym. Sci. 2015, 41, 32. [3] A. F. Cunningham, V. Desobry, in Radiation Curing in Polymer Science and Technology, Vol. 2 (Eds: J. P. Fouassier, J. F. Rabek), Elsevier, Barking, UK, 1993, pp [4] a) J. P. Fouassier, J. Lalevée, Photoinitiators for Polymer Synthesis-Scope, Reactivity, and Efficiency, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2012; b) J. Lalevée, J. P. Fouassier, Dye Photosensitized Polymerization Reactions: Novel Perspectives, RSC Photochemi stry Reports (Eds: A. Albini, E. Fasani), London 2014; c) P. Xiao, F. Dumur, J. Zhang, J. P. Fouassier, D. Gigmes, J. Lalevée, Macromolecules 2014, 47, 3837; d) J. Lalevée, P. Xiao, D. Gigmes, F. Dumur, Patent No. WO , 2015; e) J. Zhang, D. Campolo, F. Dumur, P. Xiao, J. P. Fouassier, D. Gigmes, J. Lalevee, J. Polym. Sci., Part A 2015, 53, 42. [5] J. Lalevée, M. A. Tehfe, F. Dumur, D. Gigmes, N. Blanchard, F. Morlet-Savary, J. P. Fouassier, ACS Macro Lett. 2012, 1, 286. [6] J. Lalevée, P. Xiao, S. Telitel, M. Lepeltier, F. Dumur, F. M. Savary, D. Gigmes, J. P. Fouassier, J. Beilstein, Org. Chem. 2014, 10, 863. [7] S. Telitel, F. Dumur, S. Telitel, O. Soppera, M. Lepeltier, Y. Guillaneuf, J. Poly, F. Morlet-Savary, P. Fioux, J.-P. Fouassier, D. Gigmes, J. Lalevée, Polym. Chem. 2015, 6, 613. [8] S. Telitel, F. Dumur, M. Lepeltier, D. Gigmes, J.-P. Fouassier, J. Lalevée, Comptes rendus chimie 2016, 19, 71. [9] M. A. Tehfe, F. Dumur, A. Zein-Fakih, S. Telitel, D. Gigmes, E. Contal, D. Bertin, F. Morlet-Savary, B. Graff, J. P. Fouassier, J. Lalevée, Eur. Polym. J. 2013, 49, [10] M. A. Tehfe, J. Lalevée, S. Telitel, E. Contal, F. Dumur, D. Gigmes, D. Bertin, M. Nechab, B. Graff, F. Morlet-Savary, J. P. Fouassier, Macromolecules 2012, 45, [11] M. A. Tehfe, J. Lalevée, S. Telitel, E. Contal, F. Dumur, D. Gigmes, D. Bertin, M. Nechab, B. Graff, F. Morlet-Savary, J. P. Fouassier, Macromolecules 2012, 45, [12] D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259. [13] J. Lalevée, N. Blanchard, M. A. Tehfe, F. Morlet-Savary, J. P. Fouassier, Macromolecules 2011, 43, [14] J. P. Fouassier, D. Burr, J. V. Crivello, J. Photochem. Photobiol., A 1989, 49, 317. [15] a) D. Kim, A. B. Scranton, J. W. Stansbury, J. Appl. Polym. Sci. 2009, 114, 1535; b) D. Kim, J. W. Stansbury, J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 887. [16] Landolt Bornstein: Magnetic Properties of Free Radicals, Vol. 26d (Ed: H. Fischer), Springer Verlag, Berlin [17] a) J. Lalevée, N. Blanchard, M.-A. Tehfe, M. Peter, F. Morlet-Savary, J.-P. Fouassier, Polym. Bull. 2012, 68, 341; b) M.-A. Tehfe, D. Gigmes, F. Dumur, D. Bertin, F. Morlet-Savary, B. Graff, J. Lalevée, J.-P. Fouassier, Polym. Chem. 2012, 3, (7 of 7)