Macromolecular Research, Vol. 20, No. 6, pp 552-558 (2012) DOI 10.1007/s13233-012-0077-3

www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673

Atom Transfer Radical Polymerization (ATRP) of Methyl Methacrylate Mediated by Iron(II) Chloride in the Presence of Polyethers as Both Solvents and Ligands Yeap-Hung Ng1, Fabio di Lena*,†,1, and Christina L. L. Chai2,3 1

Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research, 1 Pesek Road, Jurong Island, 627833, Singapore 2 Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research, Chemistry@Neuros, 8 Biomedical Grove, Neuros #07-01, 138665, Singapore 3 Department of Pharmacy, National University of Singapore, 18 Science Drive 4, 117543, Singapore Received February 1, 2011; Revised October 10, 2011; Accepted October 22, 2011 Abstract: Iron(II) chloride in combination with various polyethers form the homogeneous, “green” catalytic mixtures that are capable of promoting the well-controlled atom transfer radical polymerization of methyl methacrylate in the absence of any additional ligands or solvents. With poly(ethylene glycol)s, linear semilogarithmic plots of conversion vs. time and of molecular weight vs. monomer conversion were obtained with polydispersities as low as 1.32. The effects of the polyether structure, molecular weight, and chain-end functionality on the polymerization kinetics were also investigated. Keywords: atom transfer radical polymerization, iron, ligand-free catalysis, poly(ethylene glycol).

Introduction

that the solvents coordinate to the metal centers predominantly with the “hard” oxygen atom.18,19 Recently, Matyjaszewski and coworkers showed that the solvated iron species resulting from the dissolution of FeBr2 in N-methyl pyrrolidone (NMP), DMF, or MeCN can act as effective ATRP catalysts for methyl methacrylate in the absence of additional ligands.20 The polymerization carried out in DMSO was not controlled. Solvents like DMF, however, are hazardous and toxic and thus nullify the advantage of using iron as a “green” catalytic center. In the present study, we demostrate that polyethers such as poly(ethylene glycol) (PEG), which has been approved by the US FDA for internal consumption,21 can act simultaneously as ligands and solvents for iron chlorides affording a well-controlled ATRP of methyl methacrylate (Scheme II).

Reversible-deactivation radical polymerizations (RDRPs)1 have made the preparation of complex and well-defined macromolecular architectures2,3 a facile and reliable process.4 Atom transfer radical polymerization (ATRP)5 is a particularly successful RDRP process in which the activation/deactivation cycles of the growing chains take place via (pesudo) halogen transfer reactions catalyzed by an appropriate, redox-active metal complex (Scheme I). Although a variety of transition-metal complexes has been used as ATRP catalysts,6 the iron-based species are gaining prominence in view of increasing awareness of environmental issues.7-10 Ligands that have been employed in the Fe-catalyzed ATRP6 include simple amines and phosphines,7,8 αdiimine,11 N-alkyl-2-pyridylmethanimine,12 imidazolylidine,13 triazacyclononane,14 salicycladiminato ligands,15 and phosphazenium halides.16 It is known that polar and coordinating solvents such as dimethylformamide (DMF) and dimethylacetamide (DMA) can readily solubilize iron halides.17 The resulting complexes have been characterized, both in solid state and in solution, by spectroscopic and electrochemical means, which revealed

Experimental Materials. Methyl methacrylate (MMA, 99%, Aldrich) was distilled under vacuum over CaH2, and was stored under Argon at 4 ºC. Tri(ethylene glycol) monoethyl ether (PEGME-178, technical grade, MW 178.23, Aldrich), poly (ethylene glycol) methyl ether (PEGME-550, Mn~550 g/ mol, Aldrich), poly(ethylene glycol) methyl ether (PEGME1000, Mn~1,000 g/mol, Fluka), poly(ethylene glycol) methyl ether (PEGME-2000, Mn~2,000 g/mol, Aldrich), poly(ethylene glycol) dimethyl ether (PEGDE-500, Mn~500 g/mol, Aldrich), poly(ethylene glycol) (PEG-400, Mn~400 g/mol,

*Corresponding Author. E-mail: [email protected] † Present address: Public Research Center “Henri Tudor”, Department of Advanced Materials and Structures, 66 rue de Luxembourg, L-4221 Esch-sur-Alzette. The Polymer Society of Korea

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ATRP of Methyl Methacrylate Mediated by Iron(II) Chloride in the Presence of Polyethers as Both Solvents and Ligands

Scheme I

Scheme II

Aldrich), poly(propylene glycol) (PPG-725, Mn~725 g/mol, Aldrich), poly(tetrahydrofuran) (PTHF-250, Mn~250 g/mol, Aldrich), ethyl α-bromoisobutyrate (EBiB, 98%, Aldrich) iron(II) chloride (FeCl2, 98%, Stream Chemicals), and iron(III) chloride (FeCl3, 98%, Aldrich) were used as received. Poly (ethylene glycol) methyl ether methacrylate (PEGMA, Mn= 300 g/mol, Aldrich) was passed through a short column of alumina to remove the inhibitors (100 ppm hydroquinone monomethyl ether (MEHQ) and 300 ppm butylhydroxytoluene (BHT)) before use. ATRP of MMA in PEGME-550. Table I entry 1-[MMA]0/ [EBiB]0/[FeCl2]0/[FeCl3]0=187/1/4/0.6. FeCl2 (50.7 mg, 0.40 mmol) and FeCl3 (9.7 mg, 0.06 mmol) were placed into a dry Schlenk flask in a glovebox. The flask was sealed with a rubber septum and subjected to three vacuum/argon cycles. PEGME-550 was deoxygenated by bubbling argon for ca. 1 h and 2 mL of it were added to the Schlenk flask using an argon-purged syringe. The mixture was stirred at 500 rpm and heated to 60 oC in an oil bath to form a homogeneous solution, followed by addition of distilled MMA (2 mL, 18.7 mmol). The Schlenk flask was heated to 90 oC and ethyl αbromoisobutyrate (EBiB, 14.7 µL, 0.1 mmol) was added to initiate the polymerization. The similar procedures were employed for the reactions carried out in PEGME-178, PEGDE500, PEG-400, PPG-725, and PTHF-250. ATRP of MMA in PEGME-1000. FeCl2 (50.7 mg, 0.40 mmol), FeCl3 (9.7 mg, 0.06 mmol), and solid PEGME-1000 (2 g) were placed into a dry Schlenk flask in a glovebox. The flask was sealed with a rubber septum and subjected to three vacuum/argon cycles. The mixture was heated to 60 oC in an oil bath to melt the PEGME-1000 and a homogeneous solution was formed after 10 min. 2 mL of distilled MMA were then added using and argon-purged syringe. The Schlenk flask was heated to 90 oC and EBiB (14.7 µL, 0.1 mmol) was added to initiate the polymerization. Bulk ATRP of PEGMA. For a typical polymerization procedure (entry 11-[PEGMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0= 50/1/2/0.3), FeCl2 (35.5 mg, 0.28 mmol), and FeCl3 (6.8 mg, 0.042 mmol) were placed into a dry Schlenk flask in a glovebox. The flask was sealed with a rubber septum and subjected to three vacuum/argon cycles. PEGMA was deoxyMacromol. Res., Vol. 20, No. 6, 2012

genated by bubbling argon for ca.1 h and then 2 mL of it were added to the Schlenk flask using an argon-purged syringe. The mixture was heated to the reaction temperature (50 or 70 oC) in an oil bath to form a homogeneous solution and EBiB (20.5 µL, 0.14 mmol) was added to initiate the polymerization. Charaterization. At timed intervals, samples were withdrawn with a purged syringe and diluted with CDCl3. The samples were passed through a thin column of silica to remove the iron catalyst. Monomer conversions were determined in CDCl3 by 1H NMR spectroscopy (Bruker Advance 400), by comparing the integrals of methyl ester protons of the unreacted monomer (3.73 ppm) and the polymer (centred on 3.58 ppm). The number- and weight-average molecular weights (Mn and Mw , respectively) and polydispersities (Mw/Mn, PDI) of the polymer samples were measured with a gel permeation chromatography (GPC) system equipped with Waters 515 HPLC pump, 717 plus autosampler, 2414 refractive-index detector, and the following Solvent-Efficient Styragel HR columns (4.6 mm ID×300 mm) arranged in series: guard, HR5E, HR3, and HR0.5, using THF as eluent operated at 0.3 mL/min and 40 oC. The instrument was calibrated with poly(methyl methacrylate) (PMMA) standards (Polymer Standards Service-USA Inc., Mp=904, 1,660, 2,580, 3,480, 4,680, 6,780, 10,100, 14,700, 18,700, 54,500, 93,300, and 158,000, Mw/Mn=1.03~1.10), using toluene as flow marker.

Results and Discussion Due to its non-toxic nature, biodegradability, biocompatibility and relatively low price, PEG has been widely employed in several research fields including organic synthesis, where it has been used as a replacement for volatile organic compounds (VOCs) as reaction media.21 Another intriguing property of PEG is its ability to chelate metals, the structure of the resulting species being markedly dependent on the PEG’s average degree of polymerization.21-23 Ethylene glycol dimers and trimers typically arrange themselves into a pseudocyclic, crown ether-like coordination motif.22 Higher oligomers can occupy all the coordination sites available on the metal center and also displace inner-sphere ligands such as solvent molecules and halides.22,23 Moreover, it has been proposed that a single molecule of PEG may host several catalytic centers when its chain is sufficiently long.24 These intriguiging properties prompted us to investigate the possibility of using PEGs and other polyethers simultaneously as solvents and ligands for iron-mediated atom transfer radical polymerization. The effects of the polyether’s type, molecular weight and chain-end functionality on the polymerization kinetics were also investigated under comparable experimental conditions. Effect of the PEG Molecular Weight. In a first set of experiments, a blend of iron(II) and iron(III) chlorides was dissolved in PEGME-550 at 90 oC and the resulting mixture 553

Y.-H. Ng et al.

Figure 1. (a) Kinetic plot and (b) number-average molecular weight (Mn) and polydispersity indices (Mw/Mn) of PMMA vs. percent monomer conversion for the ATRP of MMA in PEGs methyl ethers with different molecular weights.

used to polymerize MMA with EBiB as the initiator according to the molar ratios [MMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0 = 187/1/4/0.6. The plot ln([M]0/[M]) vs. time (where [M]0 is the initial concentration of the monomer and [M] is the monomer concentration at given reaction time) showed a deviation from linearity at the highest reaction times (Figure 1(a)), which indicates a loss of propagating centers. Although slightly higher than the theoretical values, the molecular weights increased linearly with monomer conversion, which reached 82% after 8 h (Figure 1(b)). This is suggestive of an initiation efficiency lower than 100%. Yet, the polydispersity index decresead monotonically with monomer conversion down to 1.37. Figure 2 shows a representative 1H NMR spectrum of PMMA relative to entry 1 in Table I. The signals in the ranges 1.25-0.54, 2.12-1.25, and 3.88-3.18 ppm are assigned to the protons of the methyl groups in -C(CH3) (COOCH3), the methylene groups in the backbone, and the

Figure 2. Representative 1H NMR spectrum of a PMMA prepared in this work (Table I entry 1). 554

methoxy groups in -C(CH3)(COOCH3), respectively. The signals in the range 4.08-3.97 ppm are assigned to the CH3CH2O- of the α chain-end, whereas those between 2.502.34 ppm to the methylene protons of the ω chain-end.25 The omission from the reaction mixture of iron(III) chloride as a deactivator afforded a more curved kinetic plot without compromising the polymerization control significantly (Figure 1). In contrast, no polymerization occurred when only FeCl3 was used as the metal center with the molar ratios [MMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0=187/1/0/0.6 and 187/1/ 0/4.6 at 90 oC. This indicates that neither the monomer nor PEGME-550 are able to reduce FeCl3 to iron(II) chloride under the experimantal conditions employed. When PEGME-178 was used under comparable reaction conditions, the control over the molecular weights improved whereas the polymerization rate lowered significantly, only 63% of monomer being converted after 8 h (Figure 1(a)). Furthermore, the downward curvature of the plot ln([M]0/ [M]) vs. time was more accentuated and polydispersities higher than 1.4 were obtained (Figure 1(b)). Interestingly, the use of PEGME-1000 had opposite effects on the kinetics yielding a faster polymerization (86% monomer conversion after 8 h), a narrower PDI (Mw/Mn as low as 1.33) and a more marked deviation of molecular weights from the theoretical line. Unsurprisingly,5 reducing the total amount of iron by a factor of two or four while keeping the ratio Fe(II)/ Fe(III) constant did not yield any appreciable change in the evolution of molecular weights with conversion (Figure 3(b)). In contrast, due to ubiquitus and irreversible radical-radical termination events,5 slower reaction rates and higher polydispersities were obtained (Figure 3). The direct proportionality between the polymerization rate and the molecular weight of PEGME may be traced to the different reducing power of the complexes that FeCl2 forms with the three PEGs. It is known, in fact, that the staMacromol. Res., Vol. 20, No. 6, 2012

ATRP of Methyl Methacrylate Mediated by Iron(II) Chloride in the Presence of Polyethers as Both Solvents and Ligands

Table I. ATRP of MMA Catalyzed by FeCl2 in Different Polyethersa Entry

Solvent

Time (h)

Conv. (%)

Mn,theob

Mn,GPC

Mw/Mn

1

PEGME-550

8 20

82.3 93.6

15,600 17,700

17,300 19,600

1.38 1.37

2c

PEGME-550

8

84.8

16,100

19,600

1.34

3

PEGME-178

8 20

63.1 84.8

12,000 16,100

11,100 14,400

1.48 1.42

4

PEGME-1000

5 8

77.2 86.0

14,700 16,300

18,200 20,500

1.37 1.33

5d

PEGME-1000

8 20

80.3 91.2

15,200 17,300

17,600 20,500

1.52 1.45

6e

PEGME-1000

8 20

65.6 80.1

12,500 15,200

15,000 17,100

1.74 1.65

7

PEGDE-500

5 8

79.8 90.9

15,100 17,200

17,500 20,500

1.37 1.33

8

PEG-400

5 8

77.2 88.7

14,600 16,800

19,000 21,500

1.38 1.37

9

PPG-725

8 20

54.2 67.1

10,300 12,800

12,200 14,800

1.32 1.32

10

PTHF-250

8 20

9.8 17.1

3,400

5,600

1.24

a

Conditions: [MMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0=187/1/4/0.6, temperature of 90 oC, unless specified otherwise. Monomer conversion was determined by 1H NMR spectroscopy (Entries 1-10, Supporting Information). bMn,theo=(Conv.% × [MMA]0/[I]0 × MWMMA)+MWInitiator . c[MMA]0/[EBiB]0/ [FeCl2]0/[FeCl3]0=187/1/4/0. d[MMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0=187/1/2/0.3. e[MMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0=187/1/1/0.15.

Figure 3. (a) Kinetic plot and (b) number-average molecular weight (Mn) and polydispersity indices (Mw/Mn) of PMMA vs. percent monomer conversion for the ATRP of MMA in PEGMA-1000 with different amount of iron catalysts.

bility constant of PEG/metal complexes increases, up to a plateau, with increasing molecular weight of PEG.21,26 Ligands forming very stable complexes lead to strongly reducing metal complexes, high KATRP (=kact/kdeact) and, thus, high catalytic activity.6,27,28 The validation of this hypothesis is currently under investigation. The molecular weight of PEG also influences the viscosity of the reaction mixture, which may become so high that, as observed with PEGME-1000, the stirrer-bar is unable to agitate the solution efficiently. This may explain the increasing deviation of PMMA molecular Macromol. Res., Vol. 20, No. 6, 2012

weights from the expected values when higher molecular weight PEGs are used. Effect of the PEG End-Groups. The stability constant of a PEG/metal complex depends also on the PEG’s end-group substituents.21,26 Generally, hydroxy-terminated PEGs afford metal complexes that are slightly more stable than those with ether end-groups. This effect, however, was not reflected on the polymerization kinetics of MMA. Indeed, when PEGDE-500 and PEG-400 were used with EBiB as the initiator, both the polymerization rate and the linearity of the 555

Y.-H. Ng et al.

Figure 4. (a) Kinetic plot and (b) number-average molecular weight (Mn) and polydispersity indices (Mw/Mn) of PMMA vs. percent monomer conversion for the ATRP of MMA in PEGs with different end-groups, poly(propylene glycol) (PPG-725) and poly(tetrahydrofuran) (PTHF-250).

graphs ln([M]0/[M]) vs. time decreased in the order PEGDE500>PEG-400>PEGME-550 (Figure 4(a)). On the other hand, the deviation of Mn from the theoretical values increased in the order PEG-400>PEGDE-500>PEGME550, with PEGDE-500 affording the smallest PDI (1.33, Figure 4(b)). The reasons for these contradictory findings are presently under investigation. Notably, the use of one of these polymers (Table I entry 4) as a macroinitiator (PMMA-MI) in the ATRP of MMA in toluene at 85 oC with CuBr/bpy as the catalytic system resulted in effective chain extension (Figure 5) indicating a high degree of end-functionality retention of the PMMA produced with the PEG/Fe(II) system. ATRP in PPG and PTHF. In another set of experiments, the iron-mediated ATRP of MMA in PEG-400 was compared with those in poly(propylene glycol) (PPG-775) and poly(tetrahydrofuran) (PTHF-250) (Figure 4). In PPG-775, while the polymerization turned out to be slower (54% monomer conversion vs. 89% in PEG-400 after 8 h) and the kinetics less linear, the molecular weights were closer to the theoretical line and their distribution narrower (PDI as low as 1.32). On the other hand, with only 9.8% of monomer polymerized after 8 h, PTHF-250 was the least effective among all the ATRP reactions carried out in this study. ATRP of PEGMA in Bulk. In order to further simplify the system, we investigated the possibility of employing

Figure 5. SEC traces relative to the chain extension of a poly (methyl methacrylate) macroinitiator (PMMA-MI, Table I entry 4, black trace) with MMA in toluene at 85 oC ([MMA]/[PMMAMI]0/[CuBr]0/[bpy]0=200/1/4/8, reaction time=18 h, grey trace).

PEG methyl ether methacrylate (PEGMA, Mn~300) simultaneously as monomer, solvent and ligand in bulk ATRP (Figure 6). At 50 ºC, a linear kinetics was obtained, monomer conversion reached 48% after 8 h and the corresponding polyPEGMA had a Mw/Mn=1.55. Molecular weights higher than the expected ones and a downward curvature

Table II. Bulk ATRP of PEGMA Catalyzed by FeCl2a Entry

Temperature (oC)

Time (h)

Conv. (%)

Mn,theo

Mn,GPC

Mw/Mn

11

70

5.5

95.6

14,500

13,000

1.73

12

50

8 20

47.9 76.3

7,400 11,600

9,600 10,900

1.55 1.64

a

Conditions: [PEGMA]0/[EBiB]0/[FeCl2]0/[FeCl3]0=50/1/2/0.3 (Entries 11 and 12, Supporting Information).

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Macromol. Res., Vol. 20, No. 6, 2012

ATRP of Methyl Methacrylate Mediated by Iron(II) Chloride in the Presence of Polyethers as Both Solvents and Ligands

Figure 6. (a) Kinetic plot and (b) Number-average molecular weight (Mn) and polydispersity indices (Mw/Mn) of polyPEGMA vs. percent monomer conversion for the ATRP of PEGMA in bulk at different temperatures.

in the Mn vs. conversion plot are indicative of an initiation efficiency lower than 100% and of non-neglectable chain transfer reactions, respectively.29 Not surprisingly, increasing the temperature to 70 ºC produced a much faster polymerization reaction (>90% conversion after 8 h) but also broadened the molecular weight distributions (Mw/Mn≥2).

Conclusions In summary, iron(II) chloride dissolved in a variety of polyethers afforded the well-controlled ATRP of MMA without the need for additional ligands and solvents. The best results were obtained with PEGs and in the presence of a small amount of FeCl3 as a deactivator. In contrast, the bulk polymerization of a PEGylated methacrylate was not well-controlled. This convenient catalytic system represents a significant step toward the development of ‘greener’ ATRP processes. Our future work will focus on the characterization of the catalytic species, the rationalization of the effects of PEG molecular weight and chain end-groups on the reaction kinetics, the possibility of using more advanced initiation strategies (e.g., ICAR and ARGET)30 in order to improve further the sustainability of the process, and the scope of applications to different monomers and initiators. Acknowledgments. The Agency for Science, Technology and Research, Singapore, is gratefully acknowledged for financial support. Supporting Information: Information is available regarding the kinetic data for MMA polymerizations. The materials are available via the Internet at http://www.springer.com/ 13233. Macromol. Res., Vol. 20, No. 6, 2012

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