Quantitative and Qualitative Counterion Exchange in Cationic

Ibrahim Isah Fagge , Khalisanni Khalid , Muhammad Azri Mohd Noh , Nor Saadah Muhammad Yusof , Sharifuddin Md Zain , Mohammad Niyaz Khan. Journal of ...
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Quantitative and Qualitative Counterion Exchange in Cationic Metallocene Polyelectrolytes Jiuyang Zhang, Perry J. Pellechia, Jeffery Hayat, Christopher G. Hardy, and Chuanbing Tang* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Metallocene-containing polyelectrolytes show many unique properties in a variety of electrochemical, optoelectronic, medicinal, and magnetic applications. The utilization of counterions plays crucial roles in many aspects of these polyelectrolytes. This paper reports the first quantitative analysis of counterion exchange in metallocene-containing polyelectrolytes. Particularly counterion exchange of cationic cobaltocenium-containing polyelectrolytes was analyzed by diffusion NMR, which determined molar fractions of dissociated and associated ions as well as ion-exchange constant. Qualitative impact of counterion exchange on macromolecular conformation was directly observed from cobaltocenium-containing molecular brushes. This study may provide valuable guidance on applications such as layer-by-layer assembly and ion-triggered drug delivery that are involved with ion exchange of polyelectrolytes.

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INTRODUCTION Charged polymers or polyelectrolytes have been widely used in many industrial applications, such as polyelectrolyte multilayers,1−4 water treatment,5,6 and high charge density batteries.7 Among these applications, the utilization of counterions plays crucial roles on many aspects of their polyelectrolytes, including chemical, electrical, and mechanical properties.8−10 Metallopolymers have attracted a lot of attention since they combine the synthetic efficiency and versatility of an organic polymer framework with redox, optoelectronic, magnetic, and catalytic properties of inorganic metals.11−21 Metallocenecontaining polymers have been widely utilized for applications ranging from electrochemical sensors to templates for advanced materials to biomedicines due to their unique physicochemical properties.22−25 Metallocene-containing polyelectrolytes,11,21 mostly based on ferrocene, show many unique properties in redox chemistry and assembly,26,27 therapeutics encapsulation and release,28 molecular electronics,29 sensing and medicinal chemistry (e.g., use as prodrugs).30−32 Mostly often this class of polyelectrolytes combines neutral metallocene as functional building block with external ionic groups, thus limiting the direct impact on tuning metallocene properties. Cationic metallocene such as cobaltocenium and rhodocenium possesses intrinsic cationic metal centers coupled with counterions.33−44 Among many applications, the utilization of counterion exchange for cationic metallocene-containing polymers receives special attention and allows for the preparation of amphiphilic diblock copolymers,40,44 ion shuttles,45 molecular capsules,46 DNA cooperation,47 and gene delivery.48 The control of counterions is expected to directly tune the properties such as the strength of association/dissociation, electrostatic Coulomb interaction, and hydrophobicity/hydrophilicity. However, in either scenario, a © 2013 American Chemical Society

quantitative analysis of counterion exchange has been rarely explored. To afford a rational foundation for these applications, herein we report the first quantitative analysis on counterion exchange of cationic metallocene polyelectrolytes (Scheme 1). Specifically, we target on cobaltocenium-containing polyelectrolytes using diffusion nuclear magnetic resonance (NMR) technique as a main tool to precisely determine the level of counterion association and dissociation with the metal center and their association constant. To qualitatively observe the counterion exchange effect on these polyelectrolytes, we prepared the first cobaltocenium-containing molecular brushes, whose counterion-dependent morphologies were directly imaged by atomic force microscopy (AFM).

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RESULTS AND DISCUSSION Quantitative Analysis of Counterion Exchange by Diffusion NMR. The 2D DOSY NMR technique is a useful tool to measure diffusion coefficients of polymers. The diffusion experiments can be processed and displayed as a 2D matrix with chemical shifts and diffusion coefficients as each axis.49 Traditionally, the DOSY technique is based on the longitudinal eddy-current delay (LED) sequences, which allow the discrimination between diffusion coefficients even for similarsized molecules. Bipolar pulse longitudinal eddy current delay pulse (BPP-LED) sequence is a modification of the LED sequence, in which each gradient pulse is replaced by two pulses of different polarity separated by a 180° pulse.50−52 Compared to Received: January 1, 2013 Revised: January 19, 2013 Published: February 5, 2013 1618

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Scheme 1. Counterion Exchange of Cobaltocenium-Containing Polyelectrolytes

normal LED experiments, BPP-LED reduces eddy currents to a minimum and doubles the effective gradient output, thus making it very useful especially for measuring low diffusion coefficients that require large gradients. To perform a quantitative study, diffusion NMR based on BPP-LED was used to investigate the association/dissociation behaviors between cobaltocenium-containing polymers and their counterions by measuring their diffusion coefficients. Qualitative counterion exchanges on cobaltocenium-containing monomers and polymers have been reported.41−43,53 Because of the obvious difference in the hydrodynamic radii of polymers and free counterions, a significant increase of diffusion coefficient for counterions would be observed if the counterions dissociate from cobaltocenium cations on polymers. Thus, an experiment was designed by gradually increasing the amount of sodium tetraphenylborate (NaBPh4) in cobaltocenium-containing polymer solution (with hexafluorophosphate (PF6) as counterions) (Scheme 1). We chose poly(2-(methacrylolyoxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate) (PMAECoPF6, Mn = 22 000 g/mol, Mw/Mn = 1.24), which was prepared by reversible addition−fragmentation chain transfer polymerization (RAFT) according to our early work.54 The counterion exchange between PF6 and BPh4 was studied via a titration experiment. PMAECoPF6 was dissolved in acetonitrile. NaBPh4 solution in acetonitrile was then added to the polymer solution. The diffusion coefficients for polymer and BPh4 anion were recorded via 1H diffusion NMR in acetonitrile (Figure 1), while the PF6 anion diffusion coefficient was monitored via 19F diffusion NMR in acetonitrile (Figure 2). Four samples with different ratios of BPh4 to PF6 were measured by diffusion NMR. The diffusion coefficients are listed in Tables 1 and 2. 1H NMR spectra in Figure 1 showed peaks at 5.75−6.20, 4.40, 4.61, and 0.80−1.10 ppm from cobaltocenium polymers, and peaks at 6.90−7.30 ppm were from BPh4 anions. The correlation between diffusion coefficients and each characteristic chemical shift is clearly shown in the DOSY spectrum (Figure 1). The diffusion coefficient of BPh4 (through signals of aromatic protons) increased with the increase of the molar ratios of BPh4:PF6 , but in a lesser extent than the increase of concentration, indicating more complexation of BPh4 anions with cationic cobaltocenium moiety in the polymers due to counterion exchange. When the amount of BPh4 anions was gradually increased in polymer solution, an uptrend of diffusion coefficient for PF6 was also observed (Figure 2). Considering the increase of BPh4 anions in solution, PF6 anions were

Figure 1. 1H diffusion NMR spectra (DOSY processed and overlaid) for cobaltocenium-containing polymers PMAECoPF6 and BPh4 anions. The ratios in the spectra represented the molar ratios of BPh4:PF6 in each sample. (The amount of PF6 anions was 1.02 × 10−2 mmol in each sample, calculated from the weight of PMAECoPF6 polymers in samples.)

continuously exchanged with BPh4, resulting in more PF6 to be released from polymer and therefore an increase of diffusion coefficient. Besides, a slight upfield shift of fluorine signal was also observed. It was probably led by the environmental change of PF6 anions due to their dissociation from cobaltocenium cations, further indicating the occurrence of ion exchange. Complete exchange from PF6 to BPh4 anions was achieved by flooding PF6 polymer solution with a large excess of BPh4 salts, as confirmed by 19F NMR that no fluorine signals were observed in BPh4 polymer (PMAECoBPh4, Figure S1). It is worthy to point out that such different states of PF6 anions did not lead to signal splitting of fluorine atoms, which may suggest that the dynamic nature of counterion exchange is fast on the NMR time scale. Addition of new ions will lead the increase of diffusion rate for “free ion”. However, the “free ion” also has the ability to exchange with bounded ions and becomes bounded ions again. This process is also fast. As a result, we can only observe the time average chemical shifts and diffusion constants from the NMR measurements. The time average diffusion 1619

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According to these diffusion coefficients in Table 1, molar fractions of dissociated ions and polymer-bound ions can be determined using eq 1 with the assumption that the diffusion coefficients of small counterions would be the same with polymers if they associate with large polymers:55−57 Dmean = xD bound + (1 − x)Dfree

(1)

where x is the molar fraction of counterions bounded with polymers. Dfree and Dbound are the diffusion coefficients for counterions dissociated and associated with polymers, respectively. The apparent Dmean is the weight-average diffusion coefficient of Dfree and Dbound. According to the above assumption, Dbound is the same as the diffusion coefficients of polymers, which can be measured by diffusion 1H NMR. According to Table 1 and eq 1, the molar fractions of associated and dissociated anions are summarized in Table 2. The counterion exchange process is described by eq 2, where [PMAECoPF6] and [PMAECoBPh4] are molar fractions of PF6 and BPh4 anions associated with polymers, while [PF6] and [BPh4] are those of dissociated anions. The anion exchange constant could be determined by eq 3. On the basis of the data in Table 2, [PMAECoPF6] × [BPh4] was plotted against [PMAECoBPh4] × [PF6]. As shown in Figure 3, there was a

Figure 2. 19F diffusion NMR spectra (DOSY processed and overlaid) for PF6 anions in PF6−BPh4 ion-exchange experiment. The doublet centered at about −72 ppm was from fluorine of PF6 anions. The ratios in graph represented the molar ratios of BPh4 to PF6 anions in each sample. (The amount of PF6 anions was 1.02 × 10−2 mmol in each sample, calculated from the weight of PMAECoPF6 polymers in samples.)

Table 1. Diffusion Constants Measured by 1H and 19F Diffusion NMR at Different Ratios of BPh4 to PF6 [BPh4]/[PF6]

Dbounda (m2/s)

Dmean for BPh4b (m2/s)

Dmean for PF6c (m2/s)

0.25:1 0.50:1 0.75:1 1.00:1

2.09 × 10−10 d 2.00 × 10−10 2.05 × 10−10 2.00 × 10−10

6.16 × 10−10 6.61 × 10−10 7.16 × 10−10 7.74 × 10−10

9.77 × 10−10 11.7 × 10−10 13.5 × 10−10 14.4 × 10−10

a Dbound is the diffusion coefficient for associated counterions, directly obtained from the diffusion coefficients of polymers. bDmean is the average of diffusion coefficients for all counterions (dissociated and associated). cIn the same polymer solution without NaBPh4, the diffusion coefficients for PF6 and polymers were also measured, which were 7.24 × 10−10 and 2.51 × 10−10 m2/s. dAll diffusion coefficients were normalized by using diffusion coefficients of H2O as internal standard.

Figure 3. A plot to determine counterion exchange constant. The slope shows the constant Ke = 0.898.

linear relationship observed. The slope of this plot, equal to 0.898, represented the anion exchange constant, Ke, as described in eq 3. This constant was close to 1.0, indicating that PF6 and BPh4 anions have very similar association strength to complex with cobaltocenium cations and should have nearly equal exchange with each other in acetonitrile solvent.

Table 2. Normalized Molar Fractions for Associated or Dissociated BPh4, PF6 Anions under Different Ratios of BPh4 to PF6 [BPh4]/[PF6]

0.25:1

0.50:1

0.75:1

1:1

[PMAECo+BPh4−]a free [BPh4−]b [PMAECo+PF6−]a free [PF6−]b

0.146c 0.104 0.670 0.330

0.270 0.230 0.585 0.415

0.361 0.389 0.510 0.490

0.426 0.574 0.470 0.530

[PMAECoPF6] + [BPh4] ⇌ [PMAECoBPh4] + [PF6] (2)

Ke =

[PMAECo+BPh4−] and [PMAECo+PF6−] were the fractions of BPh4, PF6 anions associated with polymers. b[BPh4−] and [PF6−] were the molar fraction of free anion. Diffusion coefficients for free BPh4 and PF6 anions in acetonitrile were measured by 1H and 19F diffusion NMR, which were 12.0 × 10−10 and 25.64 × 10−10 m2/s, respectively, similar to those in previous reports after correction.58,59 cAll the fractions were normalized by setting the total amount of PF6 anions as 1.00. a

[PMAECoBPh4][PF6] [PMAECoPF6][BPh4]

(3)

Qualitative Counterion Exchange Impact on Conformation of Molecular Brush. The above quantitative analysis indicated that the exchange from small PF6 and bulky BPh4 anions might impart significant conformational change to cobaltocenium-containing polymers. Therefore, we designed a qualitative experimental system aiming to directly observe such effect. Considering the high grafting density of cobaltocenium cations and unique nanoscale unimolecular morphology with high persistence length, such cobaltocenium-containing molecular brushes may show a sensitive and drastic response toward

provides information regarding the fraction of the ions in both bounded and free states. 1620

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Scheme 2. Synthesis of Cationic Cobaltocenium-Containing Molecular Brushes and Their Proposed Conformational Response to Counterion Exchange

transition from wormlike morphology to spherical nanoparticles was observed when the amount of BPh4 anion increased to 25 mol % (Figure 4C). However, large aggregates with an average of height at 17.1 nm (Figure S7) were formed when BPh4 was increased to 100 mol %, compared to PF6 anions (Figure 4D). The gradual change of height profiles of these molecular brushes at different stages further confirmed that these brushes contracted, condensed, and collapsed with the addition of bulky BPh4 anions. According to a previous report,41 acetonitrile is a good solvent for cobaltocenium-containing polymers with PF6 anion but a relatively poor solvent for polymers with BPh4 anion. The exchange from PF6 to BPh4 would change solubility of these molecular brushes and thus enable them to respond by changing their conformation. It should be noted that it should be different between solution measurements by diffusion NMR and dried nanostructure images by AFM. Nevertheless, the trend by direct observation is very suggestive of conformational change due to ion exchange. Such change in morphology of molecular brushes in response to counterions is proposed in Scheme 2. The observation of by AFM was also confirmed by dynamic light scattering (DLS) experiments. Four samples used for DLS measurement were prepared by the same procedure as those for AFM imaging. While fitting DLS data to a cylinder model (molecular brushes with only PF6 anions) was not straightforward, the DLS plots showed that the “diameter” of molecular brushes increased with the increase of molar ratios of BPh4 and PF6 anions (Figure S8). This was consistent with the trend observed in AFM imaging.

anion exchange. In addition, molecular brushes could be imaged directly by microscopy techniques due to their nanoscale sizes. We prepared the first cationic cobaltocenium-containing molecular brushes and studied their morphological response to the exchange of counterions. A cobaltocenium-containing brush polymer was synthesized via a “graft from” technique by a combination of ring-opening metathesis polymerization (ROMP) and RAFT, as shown in Scheme 2. A norbornenefunctionalized chain transfer agent was first synthesized to be used as monomer 1 (Scheme S1), which was polymerized by ROMP to produce the brush polymer backbone 2. Then, a cobaltocenium-containing methacrylate monomer with PF6 counterion, MAECoPF6, was polymerized from the backbone via RAFT, yielding the final molecular brush 3. NMR spectra (Figures S2, S3, S4, and S6) coupled with many other characterizations of 1, 2, and 3 demonstrated the successful execution for each synthetic step, which are detailed in the Experimental Section and Supporting Information. These brushes were then dissolved in acetonitrile. A series of diluted solutions were drop-cast onto mica substrates. Tapping-mode AFM was subsequently used to image the morphologies of cobaltocenium-containing molecular brushes (Figure 4). The molecular brush 3 deposited from pure acetonitrile exhibited an extended wormlike morphology with an average of height at 2.7 nm as shown in Figure 4A, very similar to many other molecular brushes, primarily due to long persistent length imparted from densely grafted side chains. With the addition of BPh4 anions into solution of these cobaltocenium-containing brushes, a conformational transition and change was clearly observed. As shown in Figure 4B, a more condensed, though still wormlike, conformation was formed when only 10 mol % BPh4 anion (compared to overall PF6 anion) was added. A sharp

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CONCLUSIONS In conclusion, the exchange of counterions (PF6 and BPh4) in cobaltocenium-containing polyelectrolytes was quantitatively 1621

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Figure 4. AFM height images of cobaltocenium-containing molecular brushes with different molar ratios of BPh4 and PF6 anions: (A) only PF6 anions; (B) 0.1:1.0; (C) 0.25:1.0; (D) 1.0:1.0. Image size: 800 nm × 800 nm. (DMF) was dried and freshly distilled. Sodium tetraphenylborate (99%, NaBPh4, Alfa Aesar) was used as received. All other reagents were from commercial sources and used as received. Characterization. 1H NMR (400 MHz) spectra were recorded on a Varian Mercury 400 MHz spectrometer with tetramethylsilane (TMS) as an internal reference. Diffusion NMR (bipolar pulse longitudinal eddy current delay, BPP-LED) experiments were carried out without spinning and under ambient temperature (298 K)52 on a 500 MHz Bruker Avance III-HD spectrometer with a microprocessor-controlled gradient unit and a multinuclear probe with an actively shielded Zgradient coil. With duration of 1.00 ms, sine-shaped gradient pulses were used in the experiments. In both the 1H and 19F BBP-LED experiments, the diffusion delay was set to 100 ms. The number of scans was set at 8 per increment with a recovery delay of 2 s. All the spectra were acquired using 16K points with a line broadening of 1 Hz. The 1H spectral width was −11.00 ppm and centered at 5.00 ppm, while the 19F window was 25 ppm with a center at −70 ppm. All spectra were processed with Bruker Topspin (version 3.2) using the standard DOSY routine to determine the diffusion coefficients. Sixteen spectra were collected with a range of 95% to 5% of the maximum gradient strength for each regression analysis. All diffusion coefficient measurements used H2O as internal reference. Gel permeation chromatography (GPC) was performed in 1% LiBr DMF solution at a flow rate of 0.8 mL/min on a Varian system equipped with a ProStar 210 pump and a Varian 356-LC RI detector and three Phenogel 5 μm columns with narrow dispersed polystyrene as

studied by diffusion NMR, which determined the level of ions both in the dissociated state and at the complexation and therefore the ion exchange constant. We have designed and prepared cationic cobaltocenium-containing molecular brushes to qualitatively confirm counterion exchange. The imaging of individual brushes by AFM showed counterion exchange transformed macromolecular conformations from extended wormlike cylinders to contracted, condensed, and finally collapsed nanoparticles. This quantitative approach could provide a foundation to study many other metallocene-based polyelectrolytes and to guide a variety of applications such as layer-by-layer assembly and ion-triggered drug delivery that are involved with ion exchange.

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EXPERIMENTAL SECTION

Materials. 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid (97%, CPPA) and ethyl vinyl ether (99%) were purchased from Aldrich and used directly. Monomer (2-(methacrylolyoxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate) (MAECoPF6), side-chain cobaltocenium-containing methacrylate polymers (PMAECoPF6, Mn = 22 000 g/ mol, Mw/Mn = 1.24), and N-[3-hydroxylpropyl]-cis-5-norbornene-exo2,3-dicarboximide (NPH) were synthesized according to our earlier reports.42,54 Grubbs catalyst, third generation, was synthesized following a procedure reported in the literature.60 N,N-Dimethylformamide 1622

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2.00 (broad, CH2C, 2H), 0.80−1.10 (broad, CCH3, 3H). Mn(1H NMR) = 7 708 400. Response of Molecular Brushes toward Counterions. Cobaltocenium-containing molecular brush (3, 1.0 mg) with PF6 anions was dissolved in 1.0 mL of acetonitrile. The solution was then diluted 20 times to make the polymer brush concentration to 0.05 mg/ mL. Four samples were prepared. Each sample had 2 mL of diluted polymer brush solution, which contained 2.05 × 10−4 mmol of PF6 anions. Then, four samples were added in 10 μL of NaBPh4 solution with different concentrations (0.00, 0.70, 1.75, and 7.00 mg/mL), which made the molar ratio of BPh4:PF6 in each sample at 0:1, 0.10:1, 0.25:1, and 1:1, respectively. All samples were then characterized by AFM to investigate the response toward counterions (Figure S7).

standards. Tapping mode AFM experiments were carried out using a Multimode Nanoscope V system (Veeco (now Bruker), Santa Barbara, CA). The measurements were performed using commercial Si cantilevers with a nominal spring constant and resonance frequency at 20−80 N/m and 230−410 kHz, respectively (TESP, Bruker AFM Probes, Santa Barbara, CA). The height and phase images were acquired simultaneously at the set-point ratio A/A0 = 0.9−0.95, where A and A0 refer to the “tapping” and “free” cantilever amplitudes, respectively. The samples were visualized by AFM after dryness. Dynamic light scattering (DLS) was performed on a Zetasizer Nano ZS (Malvern Instrument) at a scattering angle of 90° under room temperature. The polymer brush concentration was 0.05 mg/mL. Synthesis of N-[4-Cyano-4-(phenylcarbonothioylthio)pentanoate]-cis-5-norbornene-exo-2,3-dicarboximide (1). The norbornene-functionalized chain transfer agent 1 was synthesized via an esterification reaction between NPH and CPPA (Scheme S1). CPPA (0.600 g, 2.10 mmol), NPH (0.620 g, 2.80 mmol), and 4dimethylaminopyridine (0.042 g, 0.34 mmol) were dissolved in dry dichloromethane (DCM) and cooled to 0 °C. Dicyclohexylcarbodiimide (0.440 g, 2.14 mmol) was first dissolved in 5 mL of DCM and then added dropwise into reaction over 15 min. The reaction was stirred under room temperature for 24 h. The solution was concentrated and then separated by column chromatograph (silica gel, eluent: ethyl acetate/hexane = 4/6). The product was collected, concentrated, and vacuum-dried to give a viscous red compound (yield: 65%, 0.65 g). 1H NMR (Figure S2) (CDCl3, δ, ppm): 7.90 (d, Ph, 2H), 7.55 (t, Ph, 2H), 7.41 (t, Ph, 2H), 6.24 (s, CHCH, 2H), 4.11 (t, COOCH2, 2H), 3.57 (t, NCH2CH2, 2H), 3.23 (t, CHCON, 2H), 2.72 (m, CH2CHCH, CH2COO, 4H), 2.49 and 2.52 (m, CH2CH2C, 2H), 1.96 (m, CH2CH2CH2O, CNCCH3, 5H), 1.50 and 1.23 (m, CHCH2CH, 2H). 13 C NMR (Figure S3) (CDCl3, δ, ppm): 221 (CS), 177.9 (CHCON), 171.4 (OCOCH2), 144.5 (Ph), 137.8 (CH2CHCH), 125−135 (Ph), 118.5 (CN), 62.2 (CH2CH2O), 47.8 (CHCH2CH), 45.7 (CH2CHCH), 45.2 (CHCHCO), 42.8 (NCH2CH2), 35.5 (CCH3), 33.3 (CCH2), 29.2 (CH2CH2CH2), 26.8 (COCH2), 24.1 (CCH3). Synthesis of Poly(N-[4-cyano-4-(phenylcarbonothioylthio)pentanoate]-cis-5-norbornene-exo-2,3-dicarboximide) (2) via ROMP. 0.10 mL of Grubbs III catalyst solution (5.10 × 10−4 mmol in DCM) was charged into 0.5 mL of dry DCM. Compound 1 (0.101 g, 0.210 mmol) was dissolved in 0.5 mL of dry DCM and then added into reaction. The conversion of monomers was monitored by 1H NMR by comparing the integration of peaks at 6.24 ppm with peaks at 7.40−8.00 ppm. The reaction was stopped after 11 min with a conversion of 63% The mixture was precipitated in diethyl ether three times, vacuum-dried to yield red polymers (38 mg, 60%). 1H NMR (Figure S4) (CDCl3, δ, ppm): 7.94 (d, Ph, 2H), 7.53 (t, Ph, 2H), 7.46 (t, Ph, 2H), 5.50−5.50 (broad, CHCH, 2H), 4.12 (t, COOCH2, 2H), 3.57 (broad, NCH2CH2, 2H), 3.00−3.50 (broad, CH2CHCH, CHCON, 4H), 2.70 (broad, CH2COO, CH2CH2C, 4H), 2.46 and 1.52 (broad, CHCH2CH, 2H), 1.95−2.05 (broad, CH2CH2CH2O, CNCCH3, 5H). Mn (NMR) = 124 800. Mn (GPC) (Figure S6) = 153 000 g/mol, Mw/Mn = 1.28. Synthesis of Cationic Cobaltocenium-Containing Molecular Brush: Poly(N-[4-cyano-4-(phenylcarbonothioylthio)pentanoate]-cis-5-norbornene-exo-2,3-dicarboximide)-g-poly(2-(methacrylolyoxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate)) (3) via RAFT. The cobaltocenium-containing molecular brush was synthesized by a “graft from” approach using macroinitiator 2. Polymer 2 (1.65 mg, 1.37 × 10−5 mmol), AIBN (0.17 mg, 1.04 × 10−3 mmol), and cobaltocenium-containing methyl acrylate monomer (MAECoPF6, 0.25 g, 0.512 mmol) were dissolved by 0.35 mL of dry DMF in a Schlenk flask. The solution was purged by nitrogen gas for 30 min and then placed in 90 °C oil bath for 150 min. The conversion was determined from 1H NMR by comparing the integrations of peaks from 6.10 ppm and peaks at 5.75−6.00 ppm. The polymerization was stopped with monomer conversion at 40%. The reaction mixture was precipitated in DCM three times, vacuum-dried to yield yellow polymers (52 mg, 52%). 1H NMR (Figure S6) (acetonitrile-d3, δ, ppm): 6.24 (broad, Cp, 2H), 5.95 (broad, Cp, 2H), 5.81 (broad, Cp, 5H), 4.61 (broad, CH2CH2, 2H), 4.42 (broad, CH2CH2, 2H), 1.90−

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ASSOCIATED CONTENT

S Supporting Information *

Scheme of norbornene-functionalized chain transfer agent synthesis, NMR spectra and GPC trace for synthesis of cobaltocenium-containing molecular brushes, and ion-response of cobaltocenium-containing molecular brushes. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. John E. Sheats for some helpful discussions. The support from National Science Foundation (CHE-1151479) is acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ma4000013 | Macromolecules 2013, 46, 1618−1624