Ruthenocene-Containing Homopolymers and Block Copolymers via

Nov 12, 2013 - Macromolecular Rapid Communications 2018 255, 1800372 ... and pyrolysis of ruthenium-containing polyferrocenylsilane block copolymers...
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Ruthenocene-Containing Homopolymers and Block Copolymers via ATRP and RAFT Polymerization Yi Yan, Jiuyang Zhang, Yali Qiao, Mitra Ganewatta, and Chuanbing Tang* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Ruthenocene-containing methacrylate homopolymer, poly(2-(methacrylolyoxy)ethyl ruthenocenecarboxylate) (PMAERu), was prepared by controlled polymerization methods such as reversible addition−fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). Kinetic studies showed that both the RAFT and ATRP process of monomer 2-(methacryloyloxy)ethyl ruthenocenecarboxylate (MAERu) followed a controlled/“living” polymerization behavior. The obtained polymer PMAERu was fully characterized by NMR, gel permeation chromatography, UV−vis spectroscopy, cyclic voltammetry, thermogravimetric analysis, and differential scanning calorimetry. By using the ruthenocene-containing homopolymer as a Macro-RAFT agent and 2-bromoisobutyryl end-capped poly(ethylene oxide) (PEO-Br) as a macroinitiator, different amphiphilic diblock copolymers were synthesized via successful chain extension. These amphiphilic diblock copolymers can self-assemble into micelles with different morphologies, including spherical and worm-like nanostructures.

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INTRODUCTION Among metallopolymers,1−5 metallocene-containing polymers6−9 have received broad attention due to their facile synthesis and diverse applications in advanced materials,10−14 sensing,15 catalysis,16 etc. Both main chain17−19 and side chain14,20−24 ferrocene-containing and charged cobaltoceniumcontaining25−31 polymers have been synthesized and widely explored for their unique functions. However, as an analogue to ferrocene, ruthenocene-containing polymers have been much less studied although they show potential applications in the areas including medicine32,33 and catalysis.16 Because of the lower electrophilic reactivity of ruthenocene than ferrocene,34 early methods to prepare ruthenocene-containing polymers by condensation between ruthenocene and different aldehydes did not yield high molecular weight polymers.35,36 A few ruthenocene-containing acrylate monomers,37,38 ruthenocene cyclophosphazene monomers,39,40 and ruthenocenophanes41,42 have been polymerized by conventional techniques such as free radical polymerization and ring-opening polymerization. However, most of these organometallic polymers lack control on molecular weight and molecular weight distribution and are incapable of producing advanced topology such as block copolymers. The only controlled/“living” polymerization of ruthenocene-containing monomers was reported by Schrock and co-workers using ethynylruthenocene initiated by Schrock catalysts.43 Over the past two decades, various controlled radical polymerization (CRP) techniques have been developed,44−47 which show advantages in providing polymers with controlled molecular weight, low polydispersity, high functionality, and diverse architectures. Major CRP techniques include nitroxide-mediated polymerization (NMP),48 reversible addi© 2013 American Chemical Society

tion−fragmentation chain transfer (RAFT) polymerization, 49−51 and atom transfer radical polymerization (ATRP).44,52−54 However, no CRP techniques have been used to prepare ruthenocene-containing polymers. Furthermore, ruthenocene-containing polymers could have potential for use as advanced protection coating materials to absorb highenergy ultraviolet radiation and in biomedical applications due to the unique optical (colorless and UV absorption) and electrochemical properties (higher oxidation potential than ferrocene) of ruthenocene. Herein, we report the first synthesis of well-defined ruthenocene-containing methacrylate polymers by ATRP and RAFT polymerization. Kinetic studies were used to characterize the controlled/“living” behavior of the polymerization process. Thermal, electrochemical, and spectral properties of these ruthenocene-containing polymers were studied. Furthermore, chain extension to hydrophilic poly(ethylene glycol) methacrylate (PEGMA) via RAFT and chain extension from hydrophilic poly(ethylene oxide) (PEO) by ATRP gave amphiphilic diblock copolymers. In addition, self-assembly behaviors of these block copolymers were preliminarily studied, with some interesting nanostructures observed.

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EXPERIMENTAL SECTION

Materials. 2-Hydroxyethyl methacrylate (HEMA, 95%, VWR), ruthenocene (RuCp2, 99.9%, STREM), anhydrous aluminum chloride (AlCl3, reagent grade, Alfa Aesar), 2-chlorobenzoyl chloride (97%, Alfa Received: October 3, 2013 Revised: October 31, 2013 Published: November 12, 2013 8816

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Scheme 1. Synthesis of Ruthenocene-Containing Homopolymers and Diblock Copolymers by RAFT and ATRP

Aesar), potassium tert-butoxide (97%, Alfa Aesar), oxalyl chloride (98%, Alfa Aesar), 2,2′-bipyridine (bpy, 99%, Sigma-Aldrich), copper(I) bromide (Cu(I)Br, 99.999%, Sigma-Aldrich), ethyl 2bromoisobutyrate (EBiB, 99%, Sigma-Aldrich), and 4(dimethylamino)pyridine (DMAP, 99%, Alfa Aesar) were used as received. Ruthenocenecarboxylic acid,55 and cumyl dithiobenzoate (CDB)56 were synthesized according to reported procedures. 2,2Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. Poly(ethylene glycol) methacrylate (PEGMA, Mn = 360 g/mol, SigmaAldrich) was purified by passing a THF solution of PEGMA through a basic alumina column and removing solvent afterward under reduced pressure. 1,4-Dioxane was freshly distilled before use. ATRP macroinitiator PEO-Br was synthesized according to a reported procedure.57 All other reagents were from commercial resources and used as received unless otherwise noted. Characterization. 1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded on a Varian Mercury 400 NMR spectrometer with tetramethylsilane (TMS) as an internal reference. Mass spectroscopy 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 dichloromethane (DCM) as solvent and monochromatic light of various wavelengths over a range of 190−900 nm. 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. HPLC-grade DMF was used as an eluent at a flow rate of 1.0 mL/min. DMF and samples were filtered through microfilters with a pore size of 0.2 μm (Teflon, 17 mm Syringes Filters, National Scientific). Polystyrene standards were used for calibration. Thermogravimetric analysis (TGA) was conducted on a TA Instruments Q5000 with a heating rate of 10 °C/min from 40 to 1000 °C under constant nitrogen flow. 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 NanoZS instrument, model ZEN 3600 (Malvern Instruments). A BAS CV50W voltammetric analyzer was used to perform cyclic voltammetry (CV) characterization. Samples were dissolved in 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) solution in anhy-

drous degassed DCM at a concentration of 0.5 mM. The samples were scanned at different rate with different potential range vs Ag/AgCl under N2. Synthesis of Monomer 2-(Methacryloyloxy)ethyl Ruthenocenecarboxylate (MAERu, 1). Ruthenocenecarboxylic acid (1.00 g, 3.63 mmol) was dissolved in 20 mL of DCM; then, 10 mL of oxalyl chloride (7.38 g, 58 mmol) and two drops of dry DMF were added. The reaction was refluxed under N2 for 24 h. After the solvent was removed under reduced pressure, the prepared ruthenocenecarbonyl chloride was directly used in the next step. 2-Hydroxyethyl methacrylate (0.85 g, 6.53 mmol) and 4-(dimethylamino)pyridine (0.80 g, 6.53 mmol) were dissolved in 20 mL of DCM and stirred at room temperature for 30 min. Then, the as-prepared ruthenocenecarbonyl chloride solution in DCM was added dropwise to the above solution at 0 °C. The reaction was gently warmed to room temperature and stirred under N2 for 24 h. The reaction was quenched by 50 mL of ice water, extracted with DCM (3 × 50 mL), dried over anhydrous Na2SO4, concentrated, and purified by column chromatography (silica gel, eluent: dichloromethane) to yield 2(methacryloyloxy)ethyl ruthenocenecarboxylate (MAERu, 1) as light yellow powder. Yield: 1.1 g, 75%. 1H NMR (CDCl3, δ, ppm): 6.16 (s, 1H, CH2C), 5.61 (s, 1H, CH2C), 5.14 (t, J = 1.8 Hz, 2H, COOCH2CH2COO), 4.71 (t, J = 1.8 Hz, 2H, COOCH2CH2COO), 4.38−4.57 (m, 9H, Cp), 1.97 (s, 3H, COOC(CH3)CH2). 13C NMR (CDCl3, δ, ppm): 170.1 (CpCOOCH2), 167.1 (CH2COOCHCH2), 136.0 and 126.0 (CH2COOCHCH2), 72.9, 71.8, and 71.6 (CH of Cp), 62.5 and 61.8 (COOCH2CH2COO), 18.3 (COOC(CH3) CH2). MS (EI), m/z calcd for C17H18O4Ru 387.39; found 388.00 (M+). Synthesis of Poly(2-(methacrylolyoxy)ethyl ruthenocenecarboxylate) (PMAERu) via RAFT Polymerization. Monomer MAERu (1) (100 mg, 0.258 mmol), cumyl dithiobenzoate (CDB) (0.703 mg, 2.58 × 10−3 mmol), and AIBN (0.085 mg, 5.16 × 10−4 mmol) were dissolved in 0.22 mL of 1,4-dioxane in a 10 mL Schlenk flask and then degassed by three cycles of freeze−pump−thaw. The reaction was heated at 90 °C for 2.5 h and quenched by opening to air and cooling with ice−water. The reaction mixture was precipitated in cold diethyl ether. The obtained polymer was redissolved in DCM and reprecipitated in cold diethyl ether two times and vacuum-dried. Yield: 0.04 g, 40%. The polymer synthesized by RAFT process was denoted as PMAERu-RAFT (4). Mn (from 1H NMR end-group analysis): 14 200 g/mol, PDI = 1.15 (from GPC); Mn (from 1H NMR conversion): 8817

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18 700 g/mol. 1H NMR (CDCl3, δ, ppm): 5.08 (broad, 2H, Cp), 4.66 (broad, 2H, Cp), 4.53 (broad, 5H, Cp), 4.24 (broad, 2H, OCH2CH2O), 4.10 (broad, 2H, OCH2CH2O), 1.78 (broad, 2H, CH2C), 0.87−1.03 (broad, 3H, CH3). Kinetic Study of RAFT Polymerization for MAERu. Monomer MAERu (1) (500 mg, 1.29 mmol), cumyl dithiobenzoate (CDB) (3.52 mg, 1.29 × 10−2 mmol), and AIBN (0.43 mg, 2.58 × 10−3 mmol) were dissolved in 1.1 mL of 1,4-dioxane in a 10 mL Schlenk flask and then degassed by three cycles of freeze−pump−thaw. The polymerization was performed at 90 °C. 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. The polymerization was quenched by placing the Schlenk flask in an ice bath and opening to air. Chain Extension To Prepare Amphiphilic Diblock Copolymers by RAFT. Amphiphilic diblock copolymers were synthesized by using PMAERu-RAFT (4) as a Macro-RAFT agent. A typical procedure was as follows: Macro-RAFT agent PMAERu-RAFT (4) (1 equiv), PEGMA (200 equiv, concentration: 0.5 mol/L), and AIBN (0.2 equiv) were dissolved in DMF in a 10 mL Schlenk flask and purged with N2 for 30 min. Polymerization was carried out at 60 °C. Samples were taken out during the polymerization to monitor the conversion of monomers. The final diblock copolymers were obtained by precipitation in cold diethyl ether, redissolved in DCM, precipitated in cold diethyl ether two times, and vacuum-dried. Synthesis of Poly(2-(methacrylolyoxy)ethyl ruthenocenecarboxylate) (PMAERu) via ATRP and Kinetic Studies. Monomer MAERu (1) (387 mg, 1 mmol), EBiB (1.95 mg, 0.01 mmol), and bpy (6.25 mg, 0.04 mmol) were dissolved in 1.5 mL of 1,4-dioxane in a 10 mL round-bottom flask and then purged N2 for 30 min. In a 10 mL Schlenk flask, CuBr (2.84 mg, 0.02 mmol) was added and purged N2 for 30 min. The monomer solution was transferred to the above Schlenk flask under protection of N2 and stirred at room temperature for 15 min. The reaction was heated at 90 °C. 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. The polymerization was quenched by placing the Schlenk flask in an ice bath. The mixtures were then diluted with THF and passed through a short neutral alumina plug, concentrated, and precipitated into cold diethyl ether. The obtained polymer was redissolved in DCM, precipitated in cold diethyl ether two times, and vacuum-dried. Yield: 0.3 g, 77%. The polymer synthesized by ATRP process was denoted by PMAERu-ATRP (2). Mn (from 1H NMR conversion): 31 000 g/mol, PDI = 1.13 (from GPC). 1H NMR (CDCl3, δ, ppm): 5.15 (broad, 2H, Cp), 4.73 (broad, 2H, Cp), 4.59 (broad, 5H, Cp), 4.30 (broad, 2H, OCH2CH2O), 4.16 (broad, 2H, OCH2CH2O), 1.84 (broad, 2H, CH2C), 0.95−1.09 (broad, 3H, CH3). Synthesis of Amphiphilic Diblock Polymer via ATRP Polymerization. Monomer MAERu (1) (0.77 g, 2.0 mmol), PEOBr macroinitiator (0.1 g, 0.02 mmol), and bpy (0.0125 g, 0.08 mmol) were dissolved in 3.0 mL of 1,4-dioxane in a 10 mL round-bottom flask and then purged N2 for 30 min. In a 10 mL Schlenk flask, CuBr (5.67 mg, 0.04 mmol) was added and purged N2 for 30 min. The monomer solution was transferred to the Schlenk flask under protection of N2 and stirred at room temperature for 15 min. The reaction was heated at 90 °C. 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. The polymerization was quenched by placing the Schlenk flask in an ice bath. The mixtures were then diluted with THF and passed through a short neutral alumina plug, concentrated, and precipitated into cold diethyl ether. The obtained polymer was redissolved in DCM, precipitated in cold diethyl ether two times, and vacuum-dried. Yield: 0.4 g, 46%. Mn (from 1H NMR conversion): 24 400 g/mol, PDI = 1.03 (from GPC). 1H NMR (CD2Cl2, δ, ppm): 5.08 (broad, 100H, Cp), 4.68 (broad, 100H, Cp), 4.54 (broad, 250H, Cp), 4.23−4.10 (broad, 200H, OCH2CH2O), 3.51 (broad, 452H, −OCHH2CH2-), 0.87−2.20 (broad, 150H, −CH2−C(CH3)).

Solution Self-Assembly of Amphiphilic Diblock Copolymers in Water. A certain amount of amphiphilic diblock copolymer was dissolved in THF, and then deionized water was added dropwise. The solution was sealed and stirred at room temperature for 24 h. THF was removed by opening the solution to air and the solution was stirred for another 24 h at room temperature. The concentration of the obtained solution was calculated without consideration of water evaporation. TEM samples were prepared by adding one drop of the above aqueous solution on a carbon-coated copper mesh grid and removing the excess solution by filtration paper. DLS was directly carried out using the above as-prepared solution.

Scheme 2. Synthesis of Monomer MAERu (1)

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RESULTS AND DISCUSSION 1. Synthesis and Polymerization of 2(Methacryloyloxy)ethyl Ruthenocenecarboxylate (MAERu (1)) via RAFT and ATRP. As shown in Scheme 2, ruthenocenecarboxylic acid (6) can be synthesized in two steps starting from ruthenocene with overall yield of 59%:58 (1) Friedel−Crafts acylation with 2-chlorobenzoyl chloride; (2) cleavage by potassium tertiary butoxide and acidify with HCl. The monomer MAERu (1) can be easily synthesized through an esterification reaction between ruthenocenecarbonyl chloride (7) and 2-hydroxyethyl methacrylate (HEMA). As shown in Figure 1A, the peaks around 6.16 and 5.61 ppm are attributed to vinyl protons, while the cyclopentadiene (Cp) protons can be found at 4.38−4.57 ppm. The two triplets at 5.14 and 4.71 ppm can be assigned to the methylene protons. Furthermore, the structure of monomer MAERu can also be confirmed by 13C NMR and mass spectrum. As shown in Figure 1B, all peaks of the 13C NMR were well assigned and matched well with the monomer MAERu structure. The peak at m/z = 388.00 in the mass spectrum (Figure S2) is in good agreement with the molecular weight of monomer MAERu. We then attempted to polymerize monomer MAERu by controlled radical polymerization methods, such as RAFT and ATRP. The RAFT polymerization was carried out at 90 °C in 1,4-dioxane with cumyl dithiobenzoate (CDB) as chain transfer agent. Figure 2 shows the 1H NMR spectrum of the synthesized homopolymer PMAERu-RAFT (4). The disappearance of the vinyl protons from the methacrylate around 6.2 and 5.6 ppm and appearance of broad peaks around 0.5−2.0 ppm suggested the successful polymerization. For the ATRP of MAERu, EBiB was used as initiator, Cu(I)Br/bpy as catalyst system and the polymerization was carried out at 90 °C in 1,4-dioxane. The synthesized PMAERu-ATRP (2) showed a similar 1H NMR spectrum to that of PMAERu-RAFT (Figure S3), suggesting the successful polymerization of MAERu. The synthesized polymer PMAERu is a white powder, which shows good 8818

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Figure 1. (A) 1H and (B) 13C NMR spectra of monomer MAERu (1) in CDCl3.

solubility in most organic solvents, such as DCM, acetone, acetonitrile, DMF, etc. To investigate whether RAFT and ATRP processes of MAERu are controlled/“living”, kinetic studies were carried out. The monomer conversion was determined by comparing the integration of vinyl protons at ∼6.2 ppm and protons from the Cp rings at 4.4−4.6 ppm. As shown in Figures 3A and 3C, the semilogarithmic plot of RAFT polymerization of MAERu displays a linear relationship between ln([M]0/[M]) and reaction time, while the molecular weight increased linearly with conversion. These studies demonstrated that the RAFT polymerization of MAERu followed a controlled/“living” characteristic. Meanwhile, the PDI was below 1.16 during the polymerization, which also indicated the well-controlled nature of the polymerization. The conversion achieved 72% after 140 min. However, when the conversion was above 72%, the kinetic curve started to level off, indicating that there were some termination reactions. For the ATRP of MAERu (as shown in

Figure 2. 1H NMR spectrum of homopolymer PMAERu-RAFT (4) in CDCl3.

Figure 3. Semilogarithmic plots: (A) RAFT polymerization of MAERu and (B) ATRP of MAERu. Plots of molecular weight (Mn, GPC), PDI (GPC) vs monomer conversion (1H NMR): (C) RAFT polymerization of MAERu and (D) ATRP of MAERu. 8819

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Figure 4. Normalized GPC traces of (A) PMAERu-RAFT (4), PMAERu-b-PEGMA (5), and (B) PEO-Br macroinitiator and PEO-b-PMAERu.

Figure 5. (A) UV−vis spectra of ruthenocene, monomer MAERu, and homopolymer PMAERu (2); inset is a picture of homopolymer PMAERuRAFT (4). (B) Cyclic voltammetry curve of the homopolymer PMAERu-ATRP (2) (Mn = 31 000 g/mol) in CH2Cl2 with 0.1 M TBAPF6 as supporting electrolyte. Scan rate = 100 mV/s.

Figures 3B and 3D), one can find that the semilogarithmic plot also displays a linear relationship between ln([M]0/[M]) and reaction time, even when conversion was as high as 85%, which displayed slightly better control on the polymerization than the RAFT process. Furthermore, the molecular weight increased linearly with the conversion, with PDI about 1.15, which was similar to the RAFT polymerization. 2. Chain Extension To Prepare Amphiphilic Diblock Copolymers. Side-chain ruthenocene-containing amphiphilic diblock copolymers can be easily prepared by using PMAERuRAFT (4) as a Macro-RAFT agent or PEO-Br as an ATRP macroinitiator (Scheme 1). First, PMAERu-RAFT (4) was used for chain extension to poly(ethylene glycol) methacrylate (PEGMA) to prepare amphiphilic diblock copolymer PMAERu-b-PEGMA (5). The chain extension was carried out in DMF at 60 °C with a molar ratio of [macroinitiator]: [AIBN]:[PEGMA] = 1:0.2:100. As shown in the 1H NMR spectrum (Figures S4), the disappearance of vinyl protons at 5.6 and 6.1 ppm and the appearance of peaks at 3.6 ppm suggested that PEGMA was successfully polymerized. As shown in Figure 4A, GPC traces shifted to higher molecular weight, confirming the successful chain extension. However, when conversion was higher than 40%, the chain extension efficiency was low as no further polymerization was observed (Figure S5). As shown in Figure 3B, ATRP seemed to be more efficient for homopolymerization of monomer MAERu than the RAFT process. Therefore, we attempted to use PEO-Br as macroinitiator to prepare PEO-b-PMAERu diblock copolymer. Two amphiphilic diblock copolymers with different degree of polymerization (DP = 35 and 50) of MAERu were prepared

with polydispersity index