Polynitroxides from Alkoxyamine Monomers: Structural and Kinetic

Article pubs.acs.org/Macromolecules

Polynitroxides from Alkoxyamine Monomers: Structural and Kinetic Investigations by Solid State NMR Frederik Behrends,‡,∥ Hendrik Wagner,†,∥ Armido Studer,*,† Oliver Niehaus,§ Rainer Pöttgen,§ and Hellmut Eckert*,‡ †

Organic Chemistry Institute, WWU Münster, Corrensstraße 40, 48149 Münster, Germany Institute of Physical Chemistry, WWU Münster, Corrensstraße 30, 48149 Münster, Germany § Institute of Inorganic and Analytical Chemistry, WWU Münster, Corrensstraße 30, 48149 Münster, Germany ‡

S Supporting Information *

ABSTRACT: A novel synthetic route toward poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTMA) is described. The polymerization of alkoxyamine-based monomers by atom transfer radical polymerization (ATRP) was investigated, as the polyalkoxyamine serves as the precursor for PTMA. The polydispersity indices (PDIs) and the kinetic data of the polymerization indicate a controlled reaction. The oxidative C−O bond cleavages of the polyalkoxyamine lead to PTMA. This transformation occurs with excellent yields, and it is possible to transfer the narrow PDIs of the prepolymer to PTMA. The material is characterized in detail using cyclic voltammetry in solution and magnetic susceptibility measurements as well as multinuclear solid state NMR and EPR spectroscopies. The conversion of the precursor polymer to the polynitroxide can be conveniently monitored by 1H and 19F magic-angle spinning (MAS) as well as 13C{1H} cross-polarization (CP)-MAS NMR. In addition, the intermolecular interaction of the nitroxide side chain units in the polymer at high conversion can be detected and monitored by the observation of pronounced low-frequency shifts.

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INTRODUCTION Within the past decade polynitroxide radicals have attracted great attention, and their application as redox reaction mediating material was in particular heavily investigated toward the development of organic radical batteries (ORBs).1−10 In addition, polynitroxide radicals have been applied as oxidation catalysts,11,12 and due to their radical character, they have been embedded into brush polymers or block copolymers as spinlabels for EPR studies.13,14 For all those reasons various nitroxides and radicals have been incorporated into monomer units of a polymeric backbone, such as 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO),5,15,16 nitronyl nitroxide,17 galvinoxyl,17−19 2,2,5,5-tetramethyl-1-pyrrolidineoxyl (PROXYL),20 and nitroxylphenyls.21 On the other hand, variations of the polymeric backbone were also reported, and therefore, different techniques for the preparation of polynitroxide radicals have been developed. For instance, ring-opening metathesis was applied to a norbornene functionalized nitroxide yielding a polynitroxide radical with an unsaturated polymer backbone. The double bonds of the backbone were then used for further cross-linking reactions.22 An allylated TEMPO derivative was incorporated into poly(methylhydrosiloxane) via hydrosilylation, and TEMPO radical substituted silicones were obtained.23 Radical polymerizations for the generation of polynitroxide radicals are usually performed using 2,2,6,6-tetramethylpiperidine (TMP) based precursor monomers, which are oxidized after polymerization.15,16,18,19,24,25 Especially the TEMPO substituent within a polymer frame is valuable, since TEMPO © 2013 American Chemical Society

moieties reduce the solubility of the corresponding polymers in a battery electrolyte.5 For this reason and because of its relatively high redox capacity2,4 poly(4-methacryloyloxy-2,2,6,6tetramethylpiperidine-N-oxyl) (PTMA) has become an important polynitroxide radical. The synthesis of PTMA is usually carried out by radical polymerization of 2,2,6,6-tetramethylpiperidin-4-yl methacrylate (TMPMA) monomers with 2,2′azobis(2-methylpropionitrile) (AIBN) as initiator.26 After polymerization of TMPMA subsequent oxidation of the polymer leads to the polynitroxide radical PTMA.5,15,16,18,25,26 The direct polymerization of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl was carried out under anionic conditions and by group-transfer polymerization, but in all these cases the polymerizations are not controlled and the resulting polymers are polydisperse.27−29 Herein, we present a new approach toward PTMA by ATRP30−35 of 4-methacryloyloxy-1-((1′-phenylethyl)oxy)2,2,6,6-tetramethylpiperidine (MPEOT, 1), resulting in polyMPEOT (PMPEOT) and subsequent high yielding aerobic oxidative cleavage of the C−ON bonds of the polyalkoxyamine to give PTMA (Scheme 1). In addition, we will introduce solid state nuclear magnetic resonance spectroscopy as a new powerful tool for the analysis of the kinetics of this reaction and the structure of the amorphous polyradical. The informaReceived: February 18, 2013 Revised: March 4, 2013 Published: March 19, 2013 2553

dx.doi.org/10.1021/ma400351q | Macromolecules 2013, 46, 2553−2561

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Scheme 1. Polymerization of 4-Methacryloyloxy-1-((1′-phenylethyl)oxy)-2,2,6,6-tetramethylpiperidine (MPEOT) and Subsequent Oxidative Cleavage toward the Poly(nitroxide radical) PTMAa

a

PMDETA: N,N,N′,N′,N″-pentamethyldiethylenetriamine; 2-EBiB: 2-bromoisobutyrate. of 0.5−2.5 s, and contact times of 100−5000 μs. Depending on the sample, 1440−98 304 scans were accumulated. 1H decoupling was achieved by applying the TPPM-15 decoupling scheme. 4096−13 312 scans were recorded using a 90° pulse length of 5 μs, a spinning frequency of 25 kHz, and a recycle delay of 5 s in the 19F-MAS NMR studies. For the quantification of fluorine the samples were mixed with BaF2 (Sigma-Aldrich, 99.999%) using a high precision balance (±1 μg). Spin−lattice relaxation times T1 for 1H and 19F were determined by the saturation recovery method using the parameters given above. Simulation of the NMR spectra was done using the DMFit program package.36 Solid state EPR spectra were obtained at 90 K on a Bruker Elexsys E580 spectrometer operating at around 9.5 GHz (X-band). The temperature was controlled by a continuous flow liquid helium Oxford cryogenic system. The deconvolution of the CW spectra was obtained using the program EasySpin 4.0.37

tional content of solid state 1H, 13C, and 19F NMR regarding the structural characterization is discussed in conjunction with solid state EPR studies.

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EXPERIMENTAL SECTION

ATRP of Alkoxyamines. MPEOT 1 (345 mg, 1.00 mmol, 1.00 equiv) was placed in a Schlenk tube under argon. To this, 1 mL of a solution of 2-EBiB (15 μL, 100 μmol) and PMDETA (21 μL, 100 μmol) in anisole (10 mL) were added. The tube was subjected to three freeze−thaw cycles, and CuBr (1.4 mg, 10 μmol, 1.0 mol %) was added. The mixture was stirred at 50 °C for various reaction times. The crude product was added to MeOH (25 mL); the precipitate was collected and washed with MeOH. It was dissolved in a small amount of CH2Cl2 (2−3 mL), and MeOH (ca. 30 mL) was added slowly. The polymer was collected and analyzed by GPC and NMR. The samples where filtered over SiO2 prior analysis. Conversions and theoretical Mn’s were gravimetrically determined based on the isolated polymer after drying in vacuo. Polymer Oxidation. Alkoxyamine-based polymer was dissolved in tert-butylbenzene (c = 0.01 g/mL), and the mixture was heated to 135 °C for 5 h with a reflux condenser. During the reaction oxygen was bubbled through the solution. The solvent was evaporated, and the crude mixture was dissolved in a small amount of CH2Cl2. A mixture of pentane/Et2O (1:1 v/v, ca. 25 mL) was added slowly, and the precipitate was collected. This procedure was repeated, and the nitroxide-based polymer was isolated as a red solid after drying in vacuo. The samples where filtered over SiO2 prior to analysis (EPR, CV, UV−vis, elemental analysis, and solid state NMR spectroscopy). Magnetic Susceptibility Measurements. Magnetic measurements were performed in the temperature range of 3−300 K using a Quantum Design Physical Property Measurement System (PPMS) with magnetic flux densities of up to 10 kOe. All measurements were carried out using the VSM option by packing the powdered samples in polypropylene capsules and attaching them to a sample holder. Of the PMPEOT samples after the reaction times of 0.5, 2, and 5 h, sample masses were 13.003, 10.817, and 9.930 mg, respectively. Solid State NMR and EPR Spectroscopy. The solid state NMR measurements were carried out on Bruker spectrometers equipped with 2.5 and 4 mm single and double resonance NMR probes. The resonance frequencies were 500.13 MHz for 1H at 11.74 T, 75.433 MHz for 13C at 7.04 T, and 188.154 MHz for 19F at 4.65 T. 1H and 13 C chemical shifts are reported relative to TMS, using adamantane (1.78 ppm for 1H and 38.56 ppm, methylene resonance for 13C) as secondary references. AlF3 (−172 ppm vs CFCl3) was used as a secondary reference for 19F MAS NMR. 1H-MAS NMR spectra were recorded using single pulse acquisition and rotor synchronized Hahn spin-echo experiments with an evolution time of one rotor period at a spinning frequency of 30.0 kHz. The 90° pulse lengths ranged from 2.1 to 3.3 μs. 16−960 FIDs were accumulated using recycle delays of 5−10 s, which were found to be sufficient for a quantitative analysis (>5T1) of all resonances. The 13C{1H} CPMAS-NMR experiments were measured at nutation frequencies corresponding to 90° pulse lengths of 4.3−4.9 μs, a spinning frequency of 10.0 kHz, recycle delays

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RESULTS AND DISCUSSION Synthesis and Kinetic Studies. To develop a new synthetic approach toward PTMA using alkoxyamines as protected precursor groups for nitroxides, we first studied the homopolymerization of MPEOT by ATRP. Alkoxyamines are thermally not stable and undergo C−O bond homolysis at higher temperatures accompanied by the generation of radicals.38,39 Therefore, we chose ATRP which can be conducted at lower temperature. We performed polymerization of the MPEOT monomer 1 in anisole in the presence of CuBr and N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) as a ligand and 2-bromoisobutyrate (2-EBiB) as an initiator in sealed tubes at 50 °C. The conversion was determined gravimetrically after precipitation with methanol. Mn and PDIs were determined by gel permeation chromatography (GPC) by using PMMA standards. Reaction time and the stoichiometry of 2-EBiB, CuBr and PMDETA were systematically varied (Table 1). As expected, polymerization of 1 proceeded straightforwardly and the alkoxyamine unit did not inhibit ATRP at 50 °C. 80% conversion was achieved after 4 h, and PMPEOT with narrow PDI was isolated (Table 1, entries 1−6). Nitroxide exchange reaction is not occurring under the applied conditions since such an exchange would lead to formation of an alkoxyamineterminated PMMA type polymer (exchange of Br by a nitroxide). It is well-known that PMMA cannot be prepared by NMP because tertiary alkoxyamine terminated esters occurring as dormant species in such processes are not stable.8 They eliminate the hydroxylamine moiety, and this in turn will lead to termination of the polymerization process. Preparation of high molecular weight polymer would not be possible. Polymerization using 0.75 mol % of CuBr/EBiB/PMDETA resulted in polymers with higher molecular weights (Table 1, 2554


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by simply removing the residual monomer through evaporation. Furthermore, we used PMPEOT (Table 1, entry 7) as a macroinitiator in the presence of PMDETA/CuBr in anisole for polymerization of MMA and obtained poly(MPEOT-b-MMA), supporting the living character of the polymerization of MPEOT (Table 1, entry 9). For the preparation of PTMA, we then studied the oxidative cleavage of the C−O bonds of the alkoxyamine units in PMPEOT. Oxidation of the alkoxyamine unit in PMPEOT was first investigated using 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-yl (IB-TEMPO-PhEt) isobutyrate as a model compound. We found that the corresponding nitroxide (4isobutyryloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) was formed in 93% yield in tert-butylbenzene by bubbling O2 for 5 h at 135 °C through a solution of the alkoxyamine (see Supporting Information). Encouraged by this result, we reacted PMPEOT with O2 under the same conditions and isolated PTMA as a red solid. The analysis (GPC) showed a significant diminution of the molecular weight (Figure 2). Furthermore, no change of the

Table 1. ATRP of 1 in Anisole entry

time (h)

conv (%)

Mn,th (g/mol)

Mn,exp (g/mol)

PDI

1a 2a 3a 4a 5a 6a 7a 8b 9c 10d

0.5 1 1.5 2 3 4 9 9 6 5

14 39 43 50 79 80 90 95

4 800 13 000 14 700 17 100 27 400 27 700 31 000 43 600

76

24 100

13 700 19 500 23 000 24 300 31 800 33 500 34 600 55 200 44 600 34 200

1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.2 1.1 1.2

a

[1]:[CuBr]:[EBiB]:[PMDETA] = 100:1:1:1 in anisole (1 mL). b[1]: [CuBr]:[EBiB]:[PMDETA] = 100:0.75:0.75:0.75 in anisole (1 mL). c Preparation of a block copolymer with MMA and PMPEOT (entry 7). 1H NMR indicates 20% MMA incorporation within poly(MPEOTb-MMA). dpF-MPEOT was used as monomer.

entry 8). We also investigated the controlled character of the polymerization by analyzing the conversion as a function of time and by analyzing the molecular weight of the polymer as a function of conversion. Results are presented in Figure 1. The

Figure 1. (a) Kinetic plot (black squares) and corresponding PDIs (open rhombi) for the ATRP of MPEOT 1 at 50 °C in anisole ([MPEOT]0 = 1 M); [2-EBiB]0 = [CuBr]0 = [PMDETA] = 10 mM. (b) Dependence of the molecular weight on the conversion of MPEOT 1 as monomer.

Figure 2. (a) GPC traces of PMPEOT (i) and PTMA (ii). (b) Cyclic voltammogram of PTMA (5 mg/mL) in 0.1 M CH2Cl2 solution. Scan rate from inside outward: 0.01, 0.02, 0.05, 0.07. Inset: EPR spectrum of PTMA; total sweep range: 1.95 ≤ g ≤ 2.03.

molecular weight increased in an approximately linear manner with the conversion of the monomer as well as the conversion increased with time. Small deviations from the linearity in these plots are likely due to the fact that analysis occurred after precipitation of the polymers which leads to errors. As the monomer is not volatile, conversion could not be determined

PDI was noted showing that the narrow PDI of the prepolymer was preserved (PDI: 1.1 in both cases). In order to determine the conversion, we analyzed the polynitroxide by elemental analysis. The conversion based on the calculated and found ΔC values of PMPEOT and PTMA indicated a conversion of 93% (PMPEOT: anal. calcd for [C21H31NO3]n: C 73.01 H 9.04 N 2555


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situation of the PTMA synthesis. In the absence of trapping reagents recombination of released C-radicals with the nitroxide might influence the kinetics. A quantitative solid state NMR analysis was done by comparing the 19F signal areas in mixtures of the samples with a known amount of an internal standard (BaF2; −14.2 ppm) whose 19F resonance is sufficiently separated from the pF-PMPEOT (−114.3 ppm) signal. To obtain the amount of residual fluorine per mol and therefore the degree of conversion, the molar weight of the samples had to be calculated using the value of 19F/g as a firstorder approximation. This was sufficient because the absolute changes in the second-order approximation were smaller than the additional error introduced. Figure 3 shows the conversion

4.05; found: C 72.74 H 9.17 N 4.01; PTMA: anal. calcd for [C13H22NO3]n: C 64.97 H 9.13 N 5.48; found: C 65.23 H 9.13 N 5.48). We also estimated conversion using UV−vis spectroscopy. Correlation of the absorption coefficient by UV−vis (wavelength: 461 nm) of PTMA resulting from PMPEOT and 4-isobutyryloxy-2,2,6,6-tetramethylpiperidine-Noxyl as a model monomer unit for PTMA revealed a 99% conversion. Moreover, cyclovoltammetry, and EPR measurements in solution confirmed the presence of nitroxide moieties in the polymer (Figure 2). We also determined the rate constant kd and the activation energy EA for the C−O bond cleavage of the model compound IB-TEMPO-PhEt as well as of PMPEOT by using an established method.40 To this end, we treated each compound with thiophenol as a radical scavenger in deuterated xylene in a sealed NMR tube at 115 °C. The decrease of the benzylic proton in the alkoxyamine unit was readily monitored by 1H NMR.40 C−O bond homolysis rate constants kd of 1.9 × 10−4 s−1 for the model compound IB-TEMPO-PhEt and of 1.5 × 10−4 s−1 for PMPEOT were measured. The corresponding EA values were calculated with the Arrhenius equation, assuming a prefactor A of 2.4 × 1014 s−1, as this is a typical A value for C− O homolysis of benzylic TEMPO-based alkoxyamines.41,42 For IB-TEMPO-PhEt the EA was 32.1 kcal/mol and for PMPEOT 32.3 kcal/mol. These values agree very well with the EA of the C−ON bond cleavage of styryl-TEMPO (31.8 kcal/mol).41,42 These data show that the polymeric nature of PMPEOT does not influence the kinetics of the C−O bond homolysis. We further studied reaction kinetics by solid state NMR and prepared 1-(1-(4-fluorophenyl)ethoxy)-2,2,6,6-tetramethylpiperidin-4-yl methacrylate (pF-MPEOT), a fluorine-tagged alkoxyamine-based monomer. Controlled polymerization by ATRP at 50 °C yielded pF-PMPEOT (Table 1, entry 10). Oxidation of pF-PMPEOT gave PTMA (Scheme 2), and the

Figure 3. Degree of conversion as a function of reaction time, monitored for polymerization of 1-(1-(4-fluorophenyl)ethoxy)-2,2,6,6tetramethylpiperidin-4-yl methacrylate (pF-MPEOT). Conversion was measured by quantification of the 19F-MAS NMR signal of the monomer’s leaving group.

per gram and per mole. The conversion after 5 h was 94 ± 2%, which is in excellent agreement with the values obtained for IBTEMPO-PhEt and pF-MPEOT by elemental analysis. 19F-MAS NMR enabled us to accurately analyze the reaction in the high conversion regime due to the fact that the procedure is applicable even for highly paramagnetic samples. Assuming first-order kinetics, kd was determined as 5.6 × 10−4 s−1 for conversions lower than 85%. This corresponds to an EA of 32.7 kcal/mol, which is in very good agreement with the value obtained in the NMR study discussed above. At higher conversion strong deviations from the first-order kinetics can be observed. The decrease in kd suggests that a second reaction starts to contribute, thereby influencing the kinetics. This is most likely the back-reaction (trapping of the released benzylic radical with the polynitroxide) that becomes more and more important at higher conversion as the local concentration of radicals steadily increases. Magnetic Susceptibility Studies. The magnetic properties of the nitroxide polymer and the degree of conversion were additionally characterized by magnetic susceptibility measurements. Figure 4 shows the result obtained on the sample after a reaction time of 5 h. The temperature dependences of the magnetic susceptibility (χ and χ−1) measured at an applied magnetic field strength of 10 kOe as well as the kink-point measurement at a low field of 100 Oe illustrate perfect Curietype paramagnetic behavior and the absence of cooperative ordering phenomena between the spins down to 2.5 K. Applying a diamagnetic correction χdia = −196.0 × 10−6 emu/ monomer (sum of Pascal’s constants obtained from ref 43), an effective magnetic moment of μeff = 1.57(1) μB/fu can be deduced. This value can be compared to an effective moment of μeff = 1.69(1) μB/fu measured on the radical momomer 4-

Scheme 2. Oxidative Cleavage of Fluorine-Containing Polyalkoxyamine (pF-PMPEOT) Yielding in PTMA

conversion (97%) was determined by elemental analysis (pFPMPEOT: anal. calcd for [C21H30FNO3]n: C 69.39 H 8.32 N 3.85; found: C 69.24 H 8.17 N 3.65. PTMA: anal. calcd. for [C13H22NO3]n: C 64.97 H 9.13 N 5.48; found C 64.96 H 8.93 N 5.29). The F-substituent of the leaving group allowed monitoring the cleavage-reaction conveniently by solid state 19F-MAS NMR. To this end, C−O homolysis was conducted in solution, and reactions were stopped at defined times. The solid material was washed with CH2Cl2, the solvent was evaporated, and the resulting material analyzed for fluorine content by solid state NMR spectroscopy. In contrast to the thiophenol trapping experiments discussed above, a reducing reagent is not present during homolysis. Hence, this setup exactly represents the 2556


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Figure 5. Temperature dependence of the magnetic susceptibility of PTMA (χ−1 data, uncorrected for diamagnetic contributions, measured at an applied magnetic field strength of 10 kOe). MOTMP denotes the radical monomer.

Figure 4. (a) Temperature dependence of the magnetic susceptibility χ and of its inverse χ−1 of PMPEOT after a reaction time of 5 h. Data are measured at an applied magnetic field strength of 10 kOe and have been corrected for diamagnetic contributions. (b) Low-temperature susceptibility zero-field cooling (ZFC) and field cooling (FC) modes of PTMA at 100 Oe.

methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl radical (MOTMP, see Supporting Information). On the basis of this ratio, it can be concluded that the yield of the aerobic oxidation cleavage amounts to 93 ± 3%, in excellent agreement with the result deduced from elemental analysis. Determination of the yields for the aerobic oxidation cleavage at shorter reaction times is not possible from the magnetic measurements because of the non-negligible mass and the variable diamagnetic contributions arising from the leaving group in these cases. Nevertheless, the susceptibility measurements can be used to monitor this reaction in a qualitative fashion (see also ref 5). The susceptibility measurements give no hint of magnetic ordering. Structural Characterization. The polynitroxide was characterized by multinuclear solid state NMR and EPR spectroscopies. Only few NMR applications to solid radicals have been published,44−48 and to the best of our knowledge, no solid state NMR studies of nitroxide polymers are available. As a solid reference sample, we grafted mesoporous silica nanoparticles (N-MSN) with a nitroxide radical (details of the synthesis are given in the Supporting Information). Because of the very dilute radical content of this sample, the paramagnetic species are assumed to be spatially isolated and noninteracting. The 13C{1H}CPMAS-NMR spectra of N-MSN, PMPEOT, and PTMA are shown in Figure 6. The spectra of N-MSN (top trace) and PMPEOT (bottom trace) give clear evidence that

Figure 6. 13C{1H}CPMAS-NMR spectra (top) and molecular structures (bottom) of the compounds under study, including proposed peak assignments. The asterisks mark spinning sidebands. “ct” denotes the contact time used for achieving magnetization transfer under Hartmann−Hahn matching conditions.

the targeted compounds were formed, as all of the peaks could be assigned to the various types of chemically inequivalent carbon atoms. In the case of MSN, we assume that the resonances of the ring C-atoms of the spin-carrying moiety are not observable owing to slow spin exchange. Based on the comparison with the spectrum of the corresponding alkoxyamine (see Supporting Information) we conclude that a diamagnetic reduction product, presumably present as an impurity, contributes to the signals a, b, c, and d. Although the spectrum of PTMA is significantly broadened due to the highly 2557


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chemical shift values, as probed by spinning-speed-dependent measurements (see Supporting Information). Both signals exhibit extremely short spin−lattice relaxation times (T1 ≈ 8 × 10−4 s (−16.0 ppm) and 6 × 10−4 s (−27.3 ppm)) which are close to the experimentally accessible limit and about 1 order of magnitude shorter than the other values measured. Solid state 1 H−1H-COSY results (see Supporting Information) also confirm an interaction of these species with 1H nuclei contributing to the main peak at 1.4 ppm. Furthermore, the 1:3.2 area ratio between the two signals is close to that expected (1:3) for an assignment of the −16 and −27 ppm signals to the CH2 groups of the six-membered ring of the nitroxide moiety and the β-methyl protons, respectively. Similar signals have been observed previously in solid state NMR spectra of other TEMPO-derived radical iodide salts as well as for the monomer reference compound 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl radical in this work (see Supporting Information) but are found to be conspicuously absent from the spectra if these radicals were incorporated into layered host structures at low concentrations.51−53 Further quantitative considerations based on signal areas are not possible due to the strongly different relaxation characteristics of the different types of protons involved, the wide spinning sideband patterns observed for the paramagnetically shifted peaks, and the unknown amounts of remaining precursor material still present in the polymeric product. Overall, Figure 7 indicates that the local paramagnetic spin densities of the TEMPO protons are strongly influenced by intermolecular electronic interactions between several TEMPO side chain residues, while leaving the electronic environment of the backbone almost unaltered. The hypothesis of strong intermolecular radical−radical interactions in the nitroxide polymer is strongly supported by Figure 8, which shows the X-band CW-EPR spectra observed

paramagnetic character of this material, the nearly complete disappearance of the resonances of the aromatic carbon atoms (128 and 146 ppm) belonging to the protecting group is clearly evident. As expected, the spectrum still features the pronounced signals of the polymer backbone, indicating that the oxidative bond cleavage was successful. In addition, a new signal near −10 ppm (y) is observed, the origin of which will be discussed below. It has to be noted that in the amorphous materials under study no 13C resonances were detected at very high (δ > 200 ppm) or low (δ < −200 ppm) frequencies that have been found for TEMPO in concentrated solutions,49,50 and the 13C-MAS NMR spectra feature the same resonances as the 13C{1H}CPMAS-NMR spectra. Figure 7 shows the 1H-MAS NMR spectra of N-MSN, PMPEOT, and PTMA. The spectrum of N-MSN (top trace)

Figure 7. 1H-MAS NMR spectra of N-MSN, PTMA, and PMPEOT. The asterisks mark spinning sidebands. Chemical shifts of significant resonances are indicated.

shows a resonance at 1.5 ppm that can be assigned to the methylene protons of the linker unit. In addition a signal at 4.8 ppm is observed, which is attributable to the protons of type a, overlapping with the signal of surface-adsorbed water. The signals of the protons b and the β-methyl groups appear to be unobservable, presumably because of the slow electron relaxation characteristics also evident in the EPR spectra (see below). In addition, the spectrum also contains diamagnetic impurities arising from partial reduction of the nitroxyl group (see below). The spin−lattice relaxation times T1 are found to be 0.12 s (4.8 ppm) and 0.16 s (1.5 ppm) which are typical values for a paramagnetic material. The spectrum of PMPEOT (bottom trace) exhibits three signals at 7.2, 5.1, and 1.2 ppm in a 4.9:2.2:23.8 ratio with a T1 of 0.68 s for all the resonances. They correspond to the aromatic protons of the phenyl ring (7.2 ppm), the benzylic protons as well as to the protons of the nitroxide-CHO moiety (5.1 ppm), and the aliphatic protons (1.2 ppm), respectively. The 1H-MAS NMR spectrum of PTMA after 5 h of reaction still shows a weak signal at 7.5 ppm arising from the residual aromatic group. T1 is 0.59 s, which is close to the value obtained for the PMPEOT, indicating that the residual phenyl protons do not experience a strong interaction with the paramagnetic centers as expected. In addition, a pronounced signal from the backbone is observed at 1.4 ppm. Saturation recovery experiments show a strongly biexponential relaxation behavior for this signal, with two contributions (92% with 0.0015 s and 8% with 0.14 s). The most interesting feature of this spectrum is the appearance of two distinct new resonances at −16.0 and −27.3 ppm, revealing strongly negative paramagnetic shift effects. Paramagnetic shift interactions are also suggested by the strong spinning sideband patterns and a pronounced temperature sensitivity of the

Figure 8. X-band CW-EPR spectra of PTMA and N-MSN and their simulations (dashed curves) based on anisotropic g- and hyperfine tensor parameters (see text).

for N-MSN and PTMA. The isolated nitroxide moieties of NMSN show the expected anisotropic hyperfine interaction with the g-tensor components gxx = 2.007 892, gyy = 2.006 906, gzz = 2.001 284 and the 14N (I = 1) hyperfine tensor components Axx = 5.7 G, Ayy = 6.9 G, Azz = 36.8 G. These parameters are in good agreement with hyperfine tensor data reported for various nitroxide radicals in the literature.54−59 In contrast, the PTMA shows only one broad signal envelope (gxx = 2.009 288, gyy = 2.005 778, gzz = 2.004 811), indicating that the distinct hyperfine interaction is not detected owing to fast spin relaxation caused by strong electron−electron interactions. This finding is consistent with a close spatial proximity between the paramagnetic units in the polymer. Further insights are 2558


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available on the basis of the 13C−CPMAS-NMR spectra, recorded as a function of contact time, ct (Figure 9). Those

again mainly reflects resonances from the polymer backbone and the variety of the local chemical environments decreases again. The evolution of the low-frequnency shifted resonances can be further used to monitor the partial oxidation of the PMPEOT precursor to the radical species. Figures 10 and 11

Figure 9. 13C{1H}CPMAS-NMR spectra of PTMA as a function of the contact time used for realizing Hartmann−Hahn matching conditions for magnetization transfer. Figure 10. 1H-MAS NMR spectra of partially oxidized PMPEOT as a function of reaction time at 110 °C. The asterisk marks a spinning sideband.

carbon atoms whose magnetization transfer originates from protons with very short T1 (i.e., those that are highly influenced by the free electron) will have their intensity maximum at very short contact time values, but their CP signal decays rapidly toward longer times. This is most obvious for the −10 ppm resonance (y) in the 13C{1H}CPMAS-NMR spectrum of PTMA. Also, the low-frequency shift of this signal suggests that the intermolecular electron−electron interaction effect described for the protons can also be found in the 13C{1H}CPMAS-NMR spectra. Because of the inherently nonquantitative character of 13C{1H}CPMAS-NMR spectra, it is not possible to interpret the peak areas in terms of site populations. However, we can draw conclusions from the extent of line-broadening effects. Those signals in Figure 6 that experience the strongest broadening are (e), (g), and (h), which are those in the direct vicinity of the N−O radical moiety, which is in excellent agreement with the insights gained from 1H-MAS NMR. One can assume that it is one of these groups that contributes to the “y”-resonance at −10 ppm. The line width of signal (a) shows that those resonances arising from C atoms distant from the radical exhibit almost no broadening. In addition, we suspect that the broad resonances g and h comprise nitroxide-methyl resonances from TEMPO moieties that are not affected by intermolecular electron− electron interactions, in contrast to those methyl species contributing to the −10 ppm peak. If the ideas developed above are valid and assuming a statistical distribution of MPEOT and nitroxide monomer units along the polymer chain at any time of the reaction, it is possible to predict the development of the 1 H-MAS-NMR spectra as a function of conversion. Starting from the PMPEOT spectrum, the signals arising from the aromatic carbon atoms will decrease in intensity. At the same time the overall line widths are expected to increase, as a result of both dipolar broadening with the unpaired electrons (∼r−3) as well as chemical shift distribution effects, reflecting the variety of local chemical environments due to the status (protection or not) of neighboring monomer units. At higher conversion, the low-frequency signals will be observed, once the concentration of free radical units exceeds a certain threshold necessary to produce strong intermolecular radical−radical interactions. The appearance of this new signal should then be accompanied by a successive sharpening of the main proton signal, as the paramagnetically broadened 1H resonances from the nitroxide units no longer contribute to it and the signal

Figure 11. 1H-MAS NMR spectra of partially oxidized pF-PMPEOT obtained as a function of reaction time at 135 °C. The asterisks mark spinning sidebands.

show the 1H-MAS NMR spectra recorded for this substrate as well as for the substrate pF-PMPEOT. The spectra of samples obtained from the partial oxidation of PMPEOT at 110 °C (Figure 10) emphasize the low conversion regime, resulting in lower nitroxide radical concentrations. In contrast, the spectra of samples obtained from the partial oxidation of pF-MPEOT at 135 °C (Figure 11) emphasize the high conversion regime, with a higher density of nitroxides, as also seen in the 19F-MAS NMR spectra (Figure 3 and Supporting Information). The predicted evolution of the 1H-MAS NMR line shapes is supported very well by these data. While the line shapes are strongly broadened at low conversions, the widths of both the resonances centered at 7.2 and 1.4 ppm decrease again, as the low-frequency signals appear. The final line widths observed for these signals at high conversion reflect the combined influence of weak paramagnetic interactions and chemical shift distribution effects of the polymeric backbone and the residual aromatic protons, i.e., the same effects influencing the residual 19 F-MAS NMR signals in the oxidized pF-PMPEOT polymer.

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CONCLUSIONS In summary, we report a novel synthetic strategy for the synthesis of the polynitroxide radical PTMA in a controlled way and with narrow PDIs. In this regard, we investigated the polymerization of alkoxyamine monomer MPEOT (1) to give 2559


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PMPEOT as a precursor polymer for PTMA. The polyalkoxyamine PMPEOT can be prepared with a narrow PDI applying ATRP as a controlled radical polymerization technique. The key step of our new approach involves generation of the polynitroxide by oxidative aerobic C−O bond homolysis. The present contribution also applies, for the first time, solid state NMR spectroscopy to analyze a polynitroxide providing detailed structural information about this versatile material. Quantitative 19F-MAS NMR spectra can serve to monitor the conversion, and systematic 1H-MAS- and 13C{1H}CPMASNMR spectra allow for an unambiguous identification of the products. Furthermore, the 1H and 13C chemical shifts and line narrowing effects can serve as sensitive indicators of intermolecular radical−radical interactions, possibly reflecting initial stages of cooperative magnetic ordering effects, even though magnetic susceptibility measurements give no indication of such ordering in the bulk. Altogether, our results indicate the unique power and potential of solid state NMR methods for the structural analysis of polyradical systems.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, characterization of all compounds, and further experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.E.); [email protected] (A.S.). Author Contributions ∥

F.B. and H.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Fernando José Lima (Physics Institute, University of Sao Paulo) for recording the EPR spectra. The authors thank the SFB858 for funding. F.B. acknowledges support by the Fonds der Chemischen Industrie for a doctoral fellowship. O.N. thanks the NRW Forschungsschule “Molecules and Materials − A Common Design Principle” for a doctoral fellowship.

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Polynitroxides from Alkoxyamine Monomers: Structural and Kinetic

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