Syntheses of Monosubstituted Rhodocenium Derivatives, Monomers

Article pubs.acs.org/Macromolecules

Syntheses of Monosubstituted Rhodocenium Derivatives, Monomers, and Polymers Yi Yan,† T. Maxwell Deaton,† Jiuyang Zhang,† Hongkun He,‡ Jeffery Hayat,† Parasmani Pageni,† Krzysztof Matyjaszewski,‡ and Chuanbing Tang*,† †

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

‡

S Supporting Information *

ABSTRACT: We report the first chemoselective, high yield synthesis of monosubstituted rhodocenium through a “η5 → η4 → η5” strategy detailing sequential nucleophilic addition and endohydride abstraction. Monosubstituted rhodocenium derivatives are then used as versatile synthons for the preparation of the first-ever vinyl monomers that allow controlled polymerizations including ROMP and RAFT, leading to rhodocenium-containing metallopolymers. Exploratory ion-exchange and self-assembly of this new class of polyelectrolytes cultivates the potential of side-chain rhodocenium-containing polymers.

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INTRODUCTION Metallopolymers combine inorganic metal centers with catalytic, magnetic, and electronic properties and organic polymeric frameworks with desirable mechanical and processing properties.1−4 Among them, metallocenes are the most important foci, as the field has been particularly inspired by the seminal work on the preparation of ferrocene-based polymers by the Manners group in the early 1990s.5,6 There are two major states of a metallocene: neutral metallocene and cationic metallocenium. Neutral metallocenes, predominantly 18-e ferrocene and ruthenocene, are widely explored.7−15 In comparison, cationic metalloceniums are disproportionally biased,16−22 mostly due to either their chemical instability (for 17-e metalloceniums) or synthetic challenges (for 18-e metalloceniums). As a matter of fact, cationic metalloceniums are fundamentally different from neutral metallocenes, as they are ionic, and can be hydrophilic or hydrophobic depending on their counterions.1 Thus, integration of metalloceniums into polymeric frameworks would constitute a class of polyelectrolytes that are ubiquitous for a variety of applications ranging from traditional electrolyte chemistry to biomedical applications to membranes, to name just a few.21,23−25 After the struggle in early years, cationic 18-e metalloceniumcontaining polymers have gained momentum over the past few years.18−20,22 Recent efforts have overcome many key hurdles to make 18-e cobaltocenium-containing materials not just a curiosity. Synthetic methodologies on cobaltocenium-containing main-chain19,20 and side-chain26,27 polymers as well as dendrimers28,29 have been established. Many of these materials have exhibited unprecedented properties such as peptide © 2015 American Chemical Society

nuclear delivery, DNA complexation, and antimicrobial bioconjugates.21,23,25 Compared with cobaltocenium, the isoelectronic 18-e rhodocenium is a nearly unexplored cationic metallocenium.30−35 To the best of our knowledge, there are almost no reports on either Rh- or rhodocenium-containing polymers,36,37 except that the Astruc group recently prepared rhodocenium-containing polymers and dendrimers via ionic interactions between parent substrates and unsubstituted rhodocenium.31 However, the major dilemma is the lack of a synthetic platform for the preparation of rhodocenium derivatives (both mono- and disubstituted), monomers, and polymers.30,33 Thus, potential electrolyte chemistry and utilities of rhodocenium materials are virtually unknown in the field of metallopolymers. Facile synthesis of rhodocenium derivatives and polymers would be critically needed and beneficial to the field of metallocenes and metallopolymers. Herein, we report the first successful synthesis of monosubstituted rhodocenium derivatives, which were used as key synthons for the preparation of vinyl rhodoceniumcontaining monomers. Controlled polymerizations38 were further carried out to prepare side-chain rhodoceniumcontaining polymers. Initial evaluations on the ion-exchange and self-assembly of these polymers were executed for exploration of their potential utilities. We demonstrate that Received: March 4, 2015 Revised: March 6, 2015 Published: March 11, 2015 1644

DOI: 10.1021/acs.macromol.5b00471 Macromolecules 2015, 48, 1644−1650

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Scheme 1. (a) η5 → η4 → η5 Strategy to Monosubstituted Metallocenium and (b) Synthetic Routes to η4/η5-Rh(I) and Rhodocenium Derivatives

Figure 1. 1H NMR spectra of monosubstituted η4/η5-Rh(I) and rhodocenium compounds: (a) IV, (b) M1, (c) M2, and (d) M3.

(trimethylsilyl)acetylide lithium (prepared by reacting (trimethylsilyl)acetylene with n-BuLi) to carry out nucleophilic addition, resulting in exo(trimethylsilyl)acetylene-substituted η4/η5-Rh(I) (I) with a yield of >80%. However, subsequent endohydride abstraction of the η4/η5-Rh(I) by tritylium hexafluorophosphate ((C6H5)3C+PF6−) to convert η4-Cp to η5-Cp was unsuccessful, only leading to unsubstituted rhodocenium (Figure S9). This indicated that rhodocenium has different reactivities from cobaltocenium. One possible reason may be associated with the limited conjugation imparted from ethynyl group.32,40,42 An enhanced conjugation was attempted by the formation of a triazole group (III) via a copper-catalyzed azide−alkyne cycloaddition (CuAAC) between unprotected ethynyl η4/η5-Rh(I) and 2-azidoethanol. The endohydride abstraction was successfully carried out with either (C6H5)3C+PF6− or ferrocenium hexafluorophosphate (Fc+PF6−) as an oxidant, yielding monosubstituted rhodoce-

rhodocenium exhibits different chemical and physical properties from cobaltocenium.

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RESULTS AND DISCUSSION

Monosubstituted Rhodocenium and Monomers. Similar to cobaltocenium, direct electrophilic substitution on rhodocenium is yet to be accomplished, largely due to the reduced electron density on the cyclopentadiene rings in the presence of cationic metal centers.39 We then attempted to carry out the functionalization of rhodocenium through a new “η5 → η4 → η5” approach that was recently developed for cobaltocenium derivatization (Scheme 1a).40−42 Specifically the synthesis started with nucleophilic addition of unsubstituted rhodocenium (η5) to give cyclopentadienyl (Cp)-Rh(I)-1,3cyclopentadiene (η4), followed by endoselective hydride abstraction to produce monosubstituted rhodocenium (η5), as shown in Scheme 1b. The rhodocenium was first treated with 1645


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Figure 2. (a−c) Semilogarithmic plots for the ROMP and RAFT polymerization of monomers: (a) M1, (b) M2, and (c) M3. (d) GPC traces for polymers P1, P2, and P3. Insets are optical images for the corresponding polymers.

Rhodocenium-containing norbornene and methacrylate monomers were then derived by the direct oxidation of the corresponding η4/η5-Rh(I)-containing monomers or from monosubstituted rhodocenium IV (Scheme 2). Compound III was used directly to react with norbornene carboxylic acid, yielding a norbornene monomer M1 with η4/η5-Rh(I). The 1H NMR spectrum of M1 (Figure 1b) showed the characteristic triazole proton “e” at ∼7.1 ppm. The multiple peaks at 6.1 ppm were assigned as double bond protons “l, m” from the norbornene group. The η4/η5-Rh(I) structure was preserved after the click reaction, as shown by the proton signals from the Cp rings (protons “a, b, and c”) around 5.3, 5.1, and 3.6 ppm. Furthermore, 13C NMR (Figure S7) and organic elemental analysis also confirmed the structure of monomer M1.

nium salt (IV), although the use of the latter oxidant gave a much higher yield. The overall synthesis from unsubstituted rhodocenium to monosubstituted rhodocenium IV had a substantial yield of ∼55%. The identity of compound IV was confirmed by 1H NMR spectrum (Figure 1a), which showed (1) disappearance of the endo proton from the η4-Cp ring at 4.18 ppm and (2) downfield shift of the triazole proton from 7.1 to 8.4 ppm due to the deshielding effect of the charged rhodocenium. Peaks at 6.58, 6.10, and 5.93 ppm were assigned to protons from the Cp rings (protons “a, b, and c”). 13C and 19 F NMR spectra (Figure S10), high-resolution mass spectrum (Figure S11), and organic (C/H/N) elemental analysis results were consistent with the assigned structure. 1646


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high as 220 °C, while it is 150 °C for the η4/η5-Rh(I)containing polymer P1. Differential scanning calorimetry (DSC) measurement showed that the rhodocenium-containing polymer P2 displays a glass transition temperature at ∼65 °C (Figure S20b), which is similar to cobaltocenium-containing polymers with a similar backbone.11 UV−Vis and Electrochemistry Study of Rhodium/ Rhodocenium-Containing Polymers. Because of the presence of cationic charges in the rhodocenium-containing polymers P2 and P3, their UV−vis absorptions show a blueshift, compared with the neutral polymer P1. As shown in Figure 3a, the η4/η5-Rh(I)-containing monomer M1 and

Meanwhile, the isotope pattern in the high-resolution mass spectrum matched well with the rhodium element (Figure S8). Direct oxidation of η4/η5-Rh(I)-containing monomer M1 by Fc+PF6− gave rhodocenium-containing norbornene monomer M2 with a yield of ∼80%. As expected, a downfield shift of the triazole proton “d” was observed after oxidation (Figure 1c), suggesting the successful restoration of the η5-Cp ring. The methacrylate derivative M3 was synthesized through an esterification between methacryloyl chloride and compound IV. The 1H NMR spectrum of monomer M3 showed the vinyl protons (h and h′) are at ∼6.1 and ∼5.3 ppm (Figure 1d). Rhodium/Rhodocenium-Containing Polymers. Two controlled/“living” polymerization techniques were then carried out on the η4/η5-Rh(I)- and rhodocenium-containing monomers: reversible addition−fragmentation chain transfer (RAFT) polymerization,11 and ring-opening metathesis polymerization (ROMP)22,43 (Scheme 2). With the aid of Grubbs III catalyst, ROMP of monomer M1 at room temperature gave a η4/η5Rh(I)-containing neutral polymer P1 with dispersity (Đ = Mw/ Mn) as low as 1.09 (Figure 2d). As demonstrated by the linear relationship between ln([M]0/[M]) ([M]0 and [M] are the monomer concentrations at the beginning and at a given time, respectively) and reaction time (Figure 2a), the ROMP process followed a controlled/“living” characteristic. This η4/η5-Rh(I)containing polymer is stable under N2 and can be stored for at least 2 weeks, as indicated by 1H NMR spectra (Figure S17). Rhodocenium-containing norbornene monomer M2 was polymerized using Grubbs III catalyst. The upfield shift of norbornene double-bond protons from 6.2 ppm in monomer M2 to the broad peaks ∼5.2 ppm in the polymer P2 demonstrated the successful ROMP (Figure S18a). The polymerization achieved more than 90% conversion within 80 min (Figure 2b). The ROMP also followed a controlled/ “living” characteristic. The rhodocenium-containing polymer P2 is a brown powder. One of the major hurdles in the development of metallocenium polymers is the lack of direct approach to characterize molecular weight, especially dispersity, which has been puzzled for cobaltocenium-containing polymers.20,22 Using a method developed by the Matyjaszewski group,44 for the first time we successfully utilized gel permeation chromatography (GPC) to characterize molecular weight of rhodocenium polyelectrolytes. The rhodocenium-containing polymer P2 we prepared had Mn = 128 000 g/mol with dispersity as low as Đ = 1.20 (Figure 2d and Figure S19). In parallel, RAFT polymerization of monomer M3 produced a rhodocenium-containing polymer P3 as a brown powder with Mn = 38 000 g/mol and Đ = 1.26 (Figure 2d). The disappearance of vinyl peaks and appearance of broad peaks at ∼1−2 ppm suggested the successful polymerization (Figure S18b). The kinetic study (Figure 2c) showed that the RAFT process was in a controlled/“living” fashion, when the conversion was lower than 50%. The polymerization became slower after conversion >50%, indicating significant termination of propagating centers. Thermal Properties of Rhodium/Rhodocenium-Containing Polymers. The rhodocenium-containing polymers P2 and P3 are much more thermally and chemically (toward oxidation) stable than the η4/η5-Rh(I)-containing polymer P1 and show good solubility (>0.5 g/mL) in polar solvents, such as DMF and acetonitrile. As shown in the thermogravimetric analysis (TGA) curves (Figure S20a), the onset temperature of 5% weight loss for rhodocenium-containing polymer P2 is as

Figure 3. UV−vis spectra in acetonitrile: (a) η4/η5-Rh(I)-containing monomer M1 and polymer P1, inset is P1 solution; (b) rhodoceniumcontaining monomer M2 and polymer P2, inset is P2 solution.

polymer P1 are light yellow and show absorptions at 265, 291, and 389 nm. The solutions of rhodocenium-containing monomer M2 and polymer P2 are dark red, with absorptions at 200, 221, 253, and 329 nm (Figure 3b). We can assign the absorption around 200−300 nm to the π−π* transitions of rhodocenium, similar to those of cobaltocenium.22 While the shoulder at a longer wavelength around 329−389 nm is from the d−d transition of rhodocenium. As shown in Figure 4a, the η4/η5-Rh(I)-containing monomer M1 and polymer P1 display an irreversible redox peak ∼0.8−

Figure 4. Cyclic voltammetry curves in degassed DMF with TBAPF6 as supporting electrolyte, scan rate = 500 mV/s: (a) η4/η5-Rh(I)containing monomer M1 and polymer P1; (b) rhodoceniumcontaining monomer M2 and polymer P2. Arrows on the curves shows the initial scanning direction.

1.0 V vs the Ag/AgCl electrode due to the oxidation of η4/η5Rh(I).45 The rhodocenium-containing monomer M2 and polymer P2 show different redox behaviors (Figure 4b). Basically, two sets of irreversible redox peaks were observed around −1.4 and −2.0 V vs the Ag/AgCl electrode, which can be attributed to the reduction of [RhCp2]+ → [RhCp2] and [RhCp2] → [RhCp2]−, respectively.46,47 The irreversibility may be due to the short half-life of RhCp2, which is only 2 s at room temperature.47 1647


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Figure 5. (a) Reversible redox process between I−- and I3−-paired rhodocenium-containing polymers P2. (b) UV−vis spectra of I−-paired polymer P2 (black line), sequential oxidation by I2 (red line), reduction by sodium ascorbate (blue line). Inset shows corresponding plots for the absorption at 347 nm (I−-paired polymer) and 359 nm (I3−-paired polymer). (98%), ferrocenium hexafluorophosphate (97%) Cu(I)Br (99.999%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), tetrabutylammonium chloride (TBACl, 97%), Grubbs second-generation catalyst, and 4-(dimethylamino)pyridine (DMAP, 98%) were purchased from Sigma-Aldrich and used as received. N-(3(Dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDCHCl, 99%) was from ProteoChem. Rhodocenium hexafluorophosphate (RhCp2PF6) was synthesized in a modified way according to a previous report.33 (Triisopropylsilyl)acetylene (98%) was purchased from Oakwood Chemicals. Grubbs third-generation catalyst was synthesized according to the literature.48 Basic aluminum oxide was activated and adjusted to activity III by adding 6 wt % water prior to use. All solvents were dried before use. All other reagents were from commercial resources and used as received unless otherwise noted. Characterization. 1H (400 MHz), 13C (100 MHz), and 19F (376 MHz) NMR spectra were recorded on a Varian Mercury 400 NMR spectrometer with tetramethylsilane (TMS) as an internal reference. Mass spectrometry was conducted on a Waters Micromass Q-TOF mass spectrometer, and the ionization source was positive ion electrospray. UV−vis was carried out on a Shimadzu UV-2450 spectrophotometer with a 10.00 mm quartz cuvette using acetonitrile as solvent and monochromatic light of various wavelengths over a range of 190−900 nm. A Hitachi 8000 transmission electron microscope (TEM) was applied to take images at an operating voltage of 150 kV. TEM samples were prepared by dropping solution on carbon-supported copper grids and then dried before observation. Dynamic light scattering (DLS) was operated on a Nano-ZS instrument, model ZEN 3600 (Malvern Instruments). A BAS CV50W voltammetric analyzer was used to perform cyclic voltammetry (CV) characterization. Sample was dissolved in 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) solution in degassed DMF at a concentration of 0.5 mM. The samples were scanned at a rate of 500 mV/s with different potential ranges vs Ag/AgCl. Gel permeation chromatography (GPC) was performed at room temperature on a Varian system equipped with a Varian 356-LC refractive index detector and a Prostar 210 pump. The columns were STYRAGEL HR1, HR2 (300 mm × 7.5 mm) from Waters. HPLCgrade solvent (DMF or THF) was used as an eluent at a flow rate of 1.0 mL/min. Solvent and sample solution were filtered through microfilters with a pore size of 0.2 μm (Teflon, 17 mm Syringes Filters, National Scientific, USA). Polystyrene standards were used for calibration. For the cationic rhodocenium-containing polymers, to eliminate the strong electrostatic interaction between the cationic rhodocenium moieties and stationary phase of microstyragel columns, 10 mM KPF6 DMF solution was used as eluent.44 Organic elemental analysis (C, H, and N) was performed via combustion at 990 °C with the elemental analyzer by Midwest Microlab, LLC (Indianapolis, IN); each sample was duplicated. The synthesis of compounds I, II, III, and IV is given in the Supporting Information. Synthesis of Monomer M1. A mixture of compound III (0.95 g, 2.75 mmol) and exo-5-norbornene-2-carboxylic acid (0.46 g, 3.3 mmol) and DMAP (0.07 g, 0.55 mmol) were dissolved in 50 mL of

Counterion-Exchange for Rhodocenium-Containing Polymers. We next explored ion-exchange study on rhodocenium-containing polyelectrolytes. We employed a phase-transfer ion-exchange17 using homopolymer P2 with PF6− as a model polymer. By treating an acetonitrile solution of P2 with tetrabutylammonium chloride (TBACl), a new homopolymer P2 paired with Cl− immediately precipitated out with a quantitative yield (Figure S21). The Cl−-paired P2 is highly water-soluble. Similarly, the counterion can be exchanged to iodide (I−) (Figure 5a). Interestingly, I−-paired P2 exhibited reversible redox chemistry. In a mixture of DCM and methanol (1:1 v/v), I− and triiodide (I3−) can be reversibly switched in the presence of iodine or sodium ascorbate (Figure 5b), which could tune the amphiphilicity of the polymer. After oxidization by I2, the solution became darker and opaque due to the reduced solubility of I3−-paired polymers. We also demonstrated that ROMP can be used to prepare amphiphilic rhodocenium-containing block copolymers. Hydrophobic and nonpolar N-cetyl-cis-5-norbornene-exo-2,3-dicarboximide was chosen as the second monomer. As shown in Figure S25, the phase transfer process allowed rhodoceniumcontaining diblock copolymers underwent exchange from PF6− to Cl− with the aid of TBACl. Initial evaluation indicated that a block copolymer with the block ratio of 50:50 could selfassemble into micelles in water (Figure S26).

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CONCLUSIONS In summary, we conceptualized an efficient “η5 → η4 → η5” strategy to the synthesis of monosubstituted rhodocenium derivatives, which were served as substrates for the preparation of rhodocenium-containing methacrylate and norbornene monomers. RAFT and ROMP were respectively carried out on these monomers, both exhibiting controlled/“living” characteristics. These rhodocenium derivatives and polymers display characteristic UV−vis absorptions and electrochemistry. Facile counterion exchange and self-assembly of amphiphilic rhodocenium-containing copolymers were successfully carried out. Establishment of the synthetic platforms may pave the way to the exploration of broader chemistries and applications of rhodocenium-containing metallopolymers.

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EXPERIMENTAL SECTION

Materials. Sodium azide, ethyl vinyl ether (EVE, 99%), and 2bromoethanol (97%) were from Acros Organics. Dicyclopentadiene (95%, Acros Organics) was freshly cracked before use. Trimethylsilylacetylene (98%), 2.5 M n-butyllithium solution in hexane, exo-5norbornene-2-carboxylic acid (97%), methacryloyl chloride (98%), rhodium(III) chloride (RhCl3, 98%), tritylium hexafluorophosphate 1648


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h followed by addition of 0.2 mL of EVE. The polymer was purified by precipitation in cold methanol and redissolved in DMAc and precipitation in methanol for another two times and vacuum-dried. Yield: 100 mg, 80%. Mn (from 1H NMR conversion ∼100%): 60 900 g/mol. 1H NMR (CD3CN, 400 MHz, δ, ppm): 8.19 (s, triazolyl H), 6.46 (s, Cp), 5.96 (s, Cp), 5.79 (s, Cp), 5.22 (broad, backbone double bond H), 4.6−4.5 (broad, CH2CH2), 2.95 (broad), 2.54 (broad), 1.12−1.89 (broad). GPC (10 mM KPF6 DMF): Mn = 128 000 g/mol, Đ = 1.20. Synthesis of Polymer P3. Monomer M3 (150 mg, 0.269 mmol), CDB (0.92 mg, 3.38 × 10−3 mmol), and AIBN (0.22 mg, 1.34 × 10−4 mmol) were dissolved in 0.22 mL of anhydrous DMAc in a 10 mL Schlenk flask and then degassed by three cycles freeze−pump−thaw. The reaction was heated under 70 °C for 6 h and quenched by opening to air and cooling with ice−water. The reaction mixture was precipitated in methanol three times and vacuum-dried. Yield: 45 mg, 30%. Mn (1H NMR conversion ≈45%): 20 000 g/mol. 1H NMR (CD3CN, 400 MHz, δ, ppm): 8.25 (s, triazolyl H), 6.13 (s, Cp), 5.50 (s, Cp), 4.56 and 4.00 (broad, CH2CH2), 2.30−0.88 (m). GPC (10 mM KPF6 DMF): Mn = 38 000 g/mol, Đ = 1.26 Synthesis of Rhodocenium-Containing Copolymers. Grubbs third-generation catalyst (1.2 mg, 1.65 × 10−3 mmol) was dissolved in 0.2 mL of degassed anhydrous dimethylacetamide (DMAc), and then monomer M2 (50 mg, 0.08 mmol) in 0.5 mL of degassed anhydrous DMAc was added quickly. The reaction was stirred at room temperature until 100% conversion was achieved (∼1.5 h, conversion was monitored by 1H NMR), and then 15 mg of N-cetyl-cis-5norbornene-exo-2,3-dicarboximide monomer in 0.2 mL of DMAc was added and stirred for another 1 h, followed by addition of 0.2 mL of EVE. The polymer was purified by precipitation in cold diethyl ether and redissolved in DMAc and precipitation in diethyl ether for another two times and vacuum-dried. Yield: 80 mg, 80%. Mn (from 1H NMR conversion ∼100%): 51 500 g/mol. Following the same procedure, another two block copolymers with block ratios of 50:25 and 50:100 were prepared.

anhydrous dichloromethane. Then EDC-HCl (0.79 g, 4.13 mmol) and TEA (0.42 g, 4.13 mmol) were added to the above solution. The solution was allowed to stir overnight at room temperature. After reaction, wash with water, dried, and purified on basic Al2O3 column (activity III) with dichloromethane to give monomer M1 (1.25 g, 98%) as light yellow solid. 1H NMR (CDCl3, 400 MHz, δ, ppm): 7.08 (s, 1H, triazole H), 6.10−6.15 (m, 2H), 5.25 (s, 5H), 5.16 (s, 2H), 4.40−4.50 (m, 4H, CH2CH2O), 4.19 (t, J = 0.6 Hz, 1H), 3.63 (s, 2H), 2.96 (m, 2H), 2.20 (br, 1H), 1.86 (br, 1H), 1.40 (m, 3H). 13C NMR (CDCl3, 100 MHz, δ, ppm): 175.66, 138.20, 136.02, 120.06, 82.60, 73.59, 62.35, 48.90, 46.53, 43.14, 41.76, 30.46. MS (EI), m/z calcd for C22H24RhN3O2 465; found 465 (M+). Elemental analysis (%) calcd for C22H24RhN3O2: C 56.78, H 5.20, N 9.03; found: C 57.17, H 5.58, N 9.52. Synthesis of Monomer M2. Monomer M1 (0.2 g, 0.43 mmol) was dissolved in 20 mL of dry THF, and then ferrocenium hexafluorophosphate (0.144 g, 0.43 mmol) was added. The mixture was stirred in the dark for 2 h. After removing solvent, the mixture was purified on basic Al2O3 column (activity III) with hexane and then dichloromethane to dichloromethane:methanol = 10:1 to give monomer M2 (0.2 g, 77%) as a yellow powder. 1H NMR (CD3CN, 300 MHz, δ, ppm): 8.16 (s, 1H, triazole H), 6.45 (t, J = 1.8 Hz, 2H, Cp), 6.18 (m, 2H, CHCH), 5.95 (t, J = 1.8 Hz, 2H, Cp), 5.79 (s, 5H, Cp), 4.51−4.67 (m, 4H, CH2CH2O), 2.91 (m, 3H), 1.81 (m, 1H), 0.88−1.33 (m, 3H). 13C NMR (CD3CN, 75 MHz, δ, ppm): 138.61, 135.87, 124.61, 88.46, 88.37, 87.32, 87.22, 84.33, 84.24, 62.62, 50.09, 46.86, 46.24, 43.31, 42.02, 30.56. 19F NMR (CD3CN, δ, ppm): −71.59, −74.06. MS (ESI), m/z calcd for C22 H23RhN3O2PF6 609.3067; found: 464.0844 (M+). Elemental analysis (%) calcd for C22H23RhN3O2PF6: C 43.37, H 3.80, N 6.90; found: C 43.95, H 4.16, N 7.24. Synthesis of Monomer M3. Compound IV (0.64 g, 1.31 mmol) was stirred with 0.27 mL of triethylamine (0.20 g, 1.96 mmol) at room temperature for 30 min. Then cool to 0 °C by an ice bath followed by the addition of 0.19 mL of methacryloyl chloride (0.20 g, 1.96 mmol). Slowly warm to room temperature and stir overnight. Remove solvent under vacuum, redissolve in dichloromethane, wash with NaPF6 aqueous solution, and separate to collect DCM phase. Dry over anhydrous Na2SO4, concentrate, and precipitate in cold ether to give monomer M3 (0.68 g, 93%) as a dark orange solid. 1H NMR (CDCl3 and CD3OD, 300 MHz, δ, ppm): 8.25 (s, 1H, triazole H), 6.47 (pseudo t, 2H, Cp), 6.03 (s, 1H, CH2C), 5.89 (pseudo t, 2H, Cp), 5.76 (s, 5H, Cp), 5.55 (s, 1H, CH2C), 4.68 (t, J = 5.1 Hz, 2H, CH2CH2O), 4.53 (t, J = 5.1 Hz, 2H, CH2CH2O), 1.84 (s, 3H, CH3). 13 C NMR (CDCl3 and CD3OD, 75 MHz, δ, ppm): 126.84, 124.43, 88.11, 87.90, 86.49, 85.80, 84.83, 84.05, 83.80, 80.73, 65.81, 63.39, 61.00, 53.53, 46.53, 28.94, 18.16, 14.93, 8.65, 7.22. 19F NMR (CDCl3 and CD3OD, δ, ppm): −71.27, −73.79. MS (EI), m/z calcd for C18H19RhN3O2PF6 557.2322; found: 412.0522 (M+). Elemental analysis (%) calcd for C18H19RhN3O2PF6: C 38.80, H 3.44, N 7.54; found: C 38.49, H 3.86, N 7.43. Synthesis of Polymer P1. Grubbs third-generation catalyst (3.75 mg, 5.16 × 10−3 mmol) was dissolved in 0.5 mL of dichloromethane, and then monomer M1 (120 mg, 0.258 mmol) in 1.5 mL of dichloromethane was added quickly. The reaction was stirred at room temperature for 30 min followed by quenching with EVE. The reaction mixture was precipitated in cold ether three times and vacuum-dried. Yield: 40 mg, 32%. Mn (from 1H NMR conversion ∼50%): 10 600 g/ mol. 1H NMR (CDCl3, 400 MHz, δ, ppm): 7.11 (s, triazolyl H), 5.14− 5.34 (m, 8H, Cp), 4.34−4.46 (br, 4H, CH2CH2), 4.17 (s, 1H), 3.61 (s, 2H), 2.63−3.04 (m, 4H), 1.0−2.0 (m, 5H). GPC (DMF): Mn = 8100 g/mol, Đ = 1.09. An initial sample was taken to accurately determine the reaction conversion by 1H NMR. Samples were periodically taken over the course of the polymerization for 1H NMR and GPC analysis for kinetic study. Synthesis of Polymer P2. Grubbs third-generation catalyst (1.5 mg, 2.06 × 10−3 mmol) was dissolved in 0.5 mL of degassed anhydrous dimethylacetamide (DMAc), and then monomer M2 (125 mg, 0.206 mmol) in 2 mL of degassed anhydrous DMAc was added quickly. The reaction was stirred at room temperature under N2 for 1.5

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental data including details of synthesis of compounds I, II, III, and IV, 1H and 13C and 19F NMR spectra, mass spectra, GPC traces, TGA and DSC curves, and TEM images. 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] (C.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support from the National Science Foundation (CHE1151479) is acknowledged. We thank Dr. John Sheats for very helpful discussion.

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REFERENCES

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