Living Radical Polymerization Catalyzed with Hydrophilic and

Chapter 2

Living Radical Polymerization Catalyzed with Hydrophilic and Thermosensitive Ruthenium(II) Complexes in Aqueous Media 1

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Downloaded by UNIV OF GUELPH LIBRARY on August 10, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch002

Toshihide Yoshitani , Yasuhiro Watanabe , Tsuyoshi Ando , Masami Kamigaito , and Mitsuo Sawamoto 2

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Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

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Ruthenium(II) complexes with thermosensitive phosphine ligands carrying a poly(ethylene glycol) (PEG) chain, e.g., Ru(II)Cp*Cl[PPh C H (PEG)] actively catalyzed homo geneous and dispersion living radical polymerization of methyl methacrylate in toluene and in its mixture with water. Because of the PEG ligands, the catalysts are highly soluble in water at temperature below 80 °C (cloud point), while lipophilic at higher temperature, and thereby are reversibly transferred from an aqueous phase to an organic phase, allowing their efficient removal (> 97%) from the as-polymerized polymer solution by simply lowering the temperature below the cloud point. 2

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© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

15 Water has recently attracted attention as promising solvents and media for organic reactions and polymerizations, because it is abundant, nontoxic, and environmentally friendly, in addition to its high heat capacity for temperature regulation. In sharp contrast to ionic reactions, free radical polymerization is often performed in aqueous media mostly under heterogeneous conditions such as suspension, dispersion, or emulsion in industrial process to solve thermal and/or viscosity problems. Under most of such conditions, the polymers are usually obtained with uncontrolled molecular weights and architectures, because of complex heterogeneous reactions. Not very often but sometimes, in addition, water harms the active species or catalysts. Controlled or living polymerizations in aqueous media are thus challenging subjects and have been investigated in many systems: metathesis, cationic, ring-opening anionic, and radical polymerizations. Transition metal-catalyzed living radical polymerization is one of the hopeful candidates for controlled polymerization in water to which the growing radical is stable, while due care should be taken to ensure the stability of the catalysts that are, in general, potentially susceptible to deactivation and decomposition by water."* Recently, we have been investigating the metal-catalyzed living radical polymerization in organic and/or aqueous media. For example, finely controlled or living dispersion polymerizations of methacrylates, styrene, and related monomers are in fact feasible when coupled with selected ruthenium or iron catalysts. In these suspension aqueous media, the polymerizations proceed in the organic phase dispersed in water, and in some cases the reactions are faster than in homogeneous organic media. Despite such fine polymerization control and effectiveness in the aqueous-phase systems, however, the metal catalysts remain in the organic phase containing polymers, and thus the difficulty to separate the catalyst from the reaction mixture remains unsolved, as with the corresponding homogeneous systems in organic solvents. A solution for this problem may be to introduce thermally dictated phasetransfer catalysis into these living polymerizations, where the metal catalysts can change their solubility in water (and organic solvent) as a function of temperature. Most typical examples of such catalysis involve poly(ethylene oxide) [or polyethylene glycol (PEG)) ligands, which are soluble in water at temperature lower than a certain threshold (cloud point), while turning insoluble and thus soluble in organic solvents above this point (see Scheme 1). In this work, a thermosensitive (PEG) was introduced into the phosphine ligand (PEG-phosphine) of a ruthenium catalyst, which will change its hydrophilicity and solubility in water, reversibly in response to temperature (Scheme 1). Such a thermosensitive catalyst is supposed to be not only easy to removefromorganic solution containing polymers but also recyclable for further polymerization. Similar investigations have recently been reported by some groups but with fluorinated ligands/fluorined solvents or in ionic liquids. In this paper we report a living radical polymerization with the thermosensitive Ru(H) catalyst with a PEG ligand both in homogeneous organic solutions and in heterogeneous suspension systems in water, in the latter thermally regulated phasetransfer catalysis allowed ready separation of the catalysts residues from polymers and potential recycling of the catalyst. 1


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Thermosensitive Ru(il) Complex

Scheme 1. Living radical polymerization with thermosensitive Ru(II) catalyst


17 M M A Polymerization with Ru(Cp*)Cl(P£G-phosphine) in Toluene MMA polymerization was first examined with H-(MMA)2-C1 (an MMA dimer capped with a chlorine; initiator}, Ru(Cp*)Cl(PEG-phosphine) (1) (catalyst), which was prepared in situ by mixing [Ru(Cp*)Cl] [a cyclic tetramer of Ru(Cp*)Cl] and 8 eq. of PEG-phosphine ([Ru]/[phosphine] = 1/2), in toluene at 80 °C (Figure 1). By NMR analysis the catalyst 1 is considered to carry only one PEG-phosphine ligand, in contrast to RuCl (PPh ) and RuCl(Ind)(PPh ) (Ph: phenyl; Ind: indenyl), which have multiple phosphines to fulfill the 18-electron structures. The polymerization proceeded smoothly to reach 90% conversion in 170 h, and gave the polymers with controlled molecular weights (M = 11600) and very narrow molecular weight distributions (MWD) (MJM = 1.12), by size-exclusion chromatography (SEC). It is noteworthy that the PEG-phosphine system induced much faster polymerization than that of PPh counterpart (336 h, 60%). On the other hand, the addition of PEG to Ru(Cp*)Cl(PPh ) or [Ru(Cp*)Cl] resulted in much slower polymerizations. Thus, PEG-phosphine works not only as the ligand to induce living radical polymerization but also as an additive which enhances the catalytic activity. Equally important, the catalyst was readily removed by washing the reaction mixture with cold water after polymerization, thereby 93% of the catalyst was removed (determined by ICP-AES). 4


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Figure 1. Polymerization of MMA catalyzed with Ru complex 1 in toluene at 80 °C: [MMA] /[H-(MMA) -Cl] f[Ru]o = 2000/20/2.0 mM. (A) 1 (Φ);(Β) Ru(Cp*)Cl(PPh )2/PEG (A); (C) [Ru(Cp*)Cl]^PPh^/PEG (•) (D) [Ru(Cp*)Cl] . (

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Suspension Polymerization with 1 in Water: Thermally Regulated Phase Transfer Catalysis As mentioned above, Ru(II) catalyst 1 is thermosensitive to change in nature from hydrophilic to hydrophobic upon responding raising temperature across the cloud point (e.g., ca. 80 °C for 1 with a PEG chain of average DP = 45). The polymerization of M M A in aqueous media was then examined. The mixture of M M A and the initiator in toluene was added into a perfectly homogeneous solution of 1 in water (Figure 2A). At first, the mixture separated into two distinct phases at ambient temperature around 25 °C (Figure 2B), and stirring this mixture led to an emulsion, due to the amphiphilic nature of the PEG ligand (Figure 2C). When the emulsion was heated to 80 °C, during which process the catalyst turned hydrophobic, aqueous and organic phases separated again, but the catalyst was now transferred into the organic phase (Figure 2D). With the catalyst exposed to the monomer/initiator mixture in the organic phase, a polymerization proceeded in suspension under vigorous stirring at this temperature. The stirred suspension was then cooled down to 0 °C to terminate the polymerization, and the reaction media became an emulsion once again, with the catalyst now returning to the amphiphilic and more hydrophilic state at this low temperature (Figure 2E). A portion of toluene was added into the mixture to decrease the viscosity, and thereby the solution separated into two phases slowly (Figure 2F).

Figure 2. Procedure of the suspension polymerization ofMMA with 1.


19 Figure 3 shows the results of the suspension polymerizations catalyzed with 1. The polymerization turned out to proceed much faster than in the corresponding homogeneous system in toluene: Aqueous system; 33 h, 94% (Figure 3B); toluene system; 124 h, 92% (Figure 3C). Such acceleration effects are also observed in other aqueous systems. Addition of tributylamine, an additive, gave faster polymerization and polymers with controlled M and narrow MWD (Figure 3A). The molecular weights of the obtained polymer (by size-exclusion chromatography with PMMA standards) agreed well with the calculated values assuming that one initiator produced one polymer chain, and the molecular weight distribution remains very narrow (MJM =1.12). 93


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Figure 3. Suspension polymerization of MMA catalyzed with 1 in the presence (Α, Φ) or absence (B, A) of tributylamine in toluene/water (organic/water = 1/1 v/v), and solution polymerization in toluene (C, M) at 80 °C. [MMA]q/[H(MMA)r-ClJo/f[Ru(Cp*)ClJ ]o/fPEG-LigandJ = 4000/40/1.0/8.0 mM. 4

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Catalyst Removal by Phase Transfer Catalysis and Catalyst Recycling Catalyst RemovalfromProducts Catalyst removal and recycle was then examined. Taking advantage of the thermosensitivity of the PEG-phosphine ligand, a simple temperature cycle enabled an efficient removal of the catalyst (and possibly its residues) from the product polymers. In the suspension polymerization describe above, for



20 example, the product polymers were isolated from the final two-phase mixture (Figure 2F). The visually colorless and white solid indicated an efficient removal of the Ru residue, which should be dark brown. According to the metalresidue analysis by inductively coupled plasma atomic emission spectroecopy (ICP-EMS), the Ru content in the isolated polymers was 28 ppm, indicating ca. 97-% removal of the catalyst relative to the original content (1000 ppm) calculatedfromthe initial catalyst concentration. In the solution polymerization in toluene, also described above, the aspolymerized polymer solution in toluene, intensely colored dark brown due the catalyst, was washed three times with cold water, giving a nealy colorless transparent liquid from which the polymers were isolated by evaporation. ICPEMS analysis of the isolated polymer showed a 93-% catalyst removal (from 1000 ppm to 72 ppm). A similar attempt for the polymers obtained with Ru(Cp*)Cl(PPh ) (a lipophilic catalyst), in contrast, resulted in just a 50-% removal (to 503 ppm). 3

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Catalyst Recycle After the first-phase polymerization in aqueous suspension at 80 °C had been completed (Figure 2F), the reaction mixture was cooled to below room temperature to induce phase separation and the solubility change in catalyst 1 into a hydrophilic state. The organic phase was removed, and afreshmixture of M M A and the initiator [H-(MMA)2-C1] in toluene was then injected into the reaction tube followed by heating to 80 °C for radical polymerization; no amine addtive was employed. 100

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1st M Conv. Λ (/W^MnXTime)

Conv. (Time) 36% (2 h)

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/ \ 4100 25% // V(1.10) (24 h)

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7200 60% (1.09) (54 h)^

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11600 84% (1.12) (170h>

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Figure 4. Catalyst recycle for living radical suspension polymerization of MMA with H-(MMA)2-Cl/1 in toluene/water at 80 °C (organic/water = 1/1 v/v): [Mmjo/fH^Mmjr^lJ^ffRufCpVClJJo/fPEG-LigandJo = 4000/40/1.0/8.0 mM.


21 Though the second polymerization was slower than the first, it gave polymers with controlled M and relatively narrow MWDs (MJM ~ 1.3) (Figure 4), indicating that the recovered catalyst, though handled in air upon phase separation, remains active for living radical polymerization. The slower polymerization with the recovered and recycled catalyst, however, suggests a partial loss of the complex 1 and/or its decomposition, which was not clarified further and should be a subject of future study. For the same reason, a repeated recovery and recycling of the catalyst was not attempted. Nevertheless, the results indicate that the thermosensitive catalyst with the PEGphosphine ligand is pormissing for efficient removal of the Ru reside from the products as well as possible recycling of the precious catalyst by simple phase transfer catalysis.


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Conclusions In conclusion, the new ruthenium complex 1 bearing PEG chain exhibited the higher catalytic activity as well as thermosensitivity that enables catalyst removal and possibly catalyst recycle.

Experimental Materials Methyl methacrylate (Tokyo Kasei, >99%) was dried overnight over calcium chloride, and purified by double distillationfromcalcium hydride before use. H-(MMA) -C1 and PEG-phosphine and [Ru(Cp*)Cl] were prepared according to the literatures. The ligand and Ru complex were handled in a glovebox (M. Braun Labmaster 130) under a moisture- and oxygen-free argon atmosphere (H 0 < 1 ppm, 0 < 1 ppm). Toluene and THF were dried overnight over calcium chloride and distilled from sodium/benzophenone ketyl. «-Octane (internal standard for gas chromatography for MMA) was dried overnight over calcium chloride and was distilled twice from calcium hydride. Polyethylene glycol) methyl ether (Aldrich; M ~ 2,000; DP - 45), thionyl chloride (Wako, >95%), 4-iodophenol (Aldrich, 99%), sodium hydride (Wako, ca. 65% dispersion in mineral oil), potassium acetate (Aldrich, 99.98%), transdi(μ-acetato)bis[o-(di-o-tolyl-phosphino)benzyl]dipalladium(II) (STREM, 98%), diphenylphosphine (Tokyo Kasei, 90%), and potassium carbonate (Wako, 99.5%) were used as received. 13

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22 Synthesis of End-Chlorinated PEG Poly(ethylene glycol) methyl ether (8.0 g, 4.0 mmol) was refluxed in 10.0 mL of thionyl chloride at 90 °C for 17 h. Volatile substances were removed by evaporation to obtain end-chlorined PEG [CHCH CH 0) -CH ] (8.07 g). H NMR (500.16 MHz, CDC1 ): δ 3.76 (t, 2H,C1C# ), 3.48-3.80 (br, 180H, OC// ), 3.38 (s, 3H, OC// ). l

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Synthesis of Sodium 4-Iodophenoxide A solution of 4-iodophenol (7.68 g, 34.9 mmol) in 10 mL of THF was added into slurry of sodium hydride (2.60 g, 71.5 mmol) in 10 mL of THF at room temperature and stirred for I h. The reaction solution was filtered and evaporated to obtain sodium 4-iodophenoxide (8.45 g).

Synthesis of PoIy(ethylene Glycol) Methyl Ether 4-Iodophenyl Ether In a two-necked round bottom flask, end-chlorined PEG (6.12 g, 3.06 mmol) and sodium 4-iodophenoxide (1.20 g, 4.62 mmol) were reacted in 25 mL of DMA at 90 °C for 5 h. The solvent was removed by evaporation and the product was extracted with toluene (6.29 g, yield = 95.5%). *H NMR (500.16 MHz, CDCI3): δ 7.54 (d, 2H, ortho to I), 6.69 (d, 2H, meta to I), 3.48-3.86 (br, 180H, OC# ), 3.38 (s, 3H, OC# ). 2

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Synthesis of PEG-Phosphine

In a two-necked round bottom flask, poly(ethylene glycol) methyl ether 4iodophenyl ether (6.29 g, 2.89 mmol), potassium acetate (0.670 g, 6.83 mmol), and trans-di(μ-acetato)bisto-(di-o-tolyl-phosphino)benzyl]dipalladium(II) (10.7 mg, 0.0114 mmol) were combined under an inert atmosphere. A 30 mL aliquot of dimethylacetoamide was added, and the mixture was degassed. After addition of 1.00 mL of diphenylphosphine (1.08 g, 5.80 mmol), the reaction mixture was heated at 110 °C for 17 h. After removal of the solvent the residue was dissolved in 40.0 mL of chloroform, 1.30 g of potassium carbonate was added, and the mixture was stirred overnight. After filtration, the phosphine-ligand was obtained by evaporation (3.95 g, yield = 61%). H NMR (500.16 MHz, CDC1 ): δ 6.85-7.40 (br, 14H, aromatic), 3.48-3.90 (br, 180H, OC# ), 3.38 (s, 3H, l

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23 Solution Polymerization The solution polymerization of M M A with catalyst 1 was carried out by the syringe technique under argon in baked glass tubes equipped with a three-way stopcock. Typical procedures are as follows (see Figure 1): In a 50 mL roundbottomed flask was placed [Ru(Cp*)Cl] (0.0035 mmol, 3.8 mg). To this, toluene (4.74 mL), PEG-phosphine (0.028 mmol, 0.28 mL, 100 mM in toluene), «-octane (0.32 mL; internal standard for gas chromatography), MMA (14 mmol, 1.50 mL), and a solution of initiator H-(MMA) -C1 (0.14 mmol, 0.156 mL, 900 mM in toluene) were added sequentially in this order at 25 °C under argon. The total volume of the reaction mixture was thus 7.0 mL. Five aliquots (1.0 mL each) of the solution were distributed into the baked glass tubes with a three-way stopcock and placed in an oil bath at 80 °C. In predetermined intervals, the reaction was terminated by cooling the mixture to -78 °C. Conversion of M M A was determined by gas chromatography with «-octane as an internal standard. The quenched reaction solutions were diluted with toluene (ca. 15 mL), washed with cold water, and evaporated to dryness to give the polymers. 4


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Suspension Polymerization Ru complex 1 was prepared by mixing [Ru(Cp*)Cl] (0.069 mmol, 74.3 mg) and PEG-phosphine (0.55 mmol, 1.20 g) under dry nitrogen at 80 °C in toluene for 1 h, and the solvent was removed by evaporation; no further purification was The isolated complex 1 (0.024 mmol, 112 mg) was dissolved in 6.0 mL of water, and the solution was distributed into glass tubes equipped with a threeway stopcock (1.0 mL each). A mixture of toluene (2.54 mL), «-octane (0.56 mL), M M A (24 mmol, 2.58 mL), and H - i M M A ) - C l (0.24 mmol, 0.32 mL, 734 mM in toluene) was added into the glass tubes; thus the total volume of the solution was 6.0 mL. The polymerization was initiated by placing the tubes in an oil bath at 80 °C. During the reaction period, the reaction media was stirred by a magnetic stirrer. 4

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Polymer Characterization The M , MJM , and MWD curves of the polymers were determined by sizeexclusion chromatography in chloroform at 40 °C on the three polystyrene gel columns (Shodex K-805L χ 3) that were connected to Jasco PU-980 precision pump and a Jasco RI-930 refractive index detector. The columns were calibrated against eleven standard poly(MMA) samples (Polymer Laboratories; N

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24 M„ = 630-1,200,000; MJM = 1.04-1.22) as well as the monomer. The metal residue content in polymers was analyzed by inductively coupled plasma atomic emission spectroecopy (ICP-EMS) on a Jarrell-Ash IRIS AP instrument. n


Acknowledgments This work was supported by a Grant-in-aid for Scientific Research on Priority Area (No. 13128201) from Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank Mr. H. Sasaki and Mr. T. Ono (Kuraray Co. Ltd.) for ICP-AES analysis.

References 1. 2. 3.

4. 5. 6. 7. 8. 9.

10. 11.

Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous Media; Jown Wiley & Sons: New York, 1997. Moad, G.; Solomon, D. H. The Chemistry of Free Radical Polymerization; Elsevier Science: Oxford, U. K., 1995. (a) Kamigaito, M.; Ando, T.; Sawamoto, M . Chem. Rev. 2001, 101, 3689. (b) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (c) Hawker, C. J.; Bosma, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661. (d) Matyjaszewski, K., Ed.; ACS Symposium Series 768; American Chemical Society: Washington, DC, 2000. (e) Sawamoto, M . ; Kamigaito, M . In Synthesis of Polymers; Schlüter, A.-D., Ed.; Materials Science and Technology Series; Wiley-VCH: Weinheim, Germany, 1999, Chapter 6; pp 163-194. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614. Percec, V.; Barboiu, B. Macromolecules 1995, 28, 7970. Granel, C.; Dubois, Ph.; Jérôme, R.; Teyssié, Ph. Macromolecules 1996, 29, 8576. Haddleton, D. M . ; Jasieczek, C. B.; Hannon, M . J.; Shooter, A . J. Macromolecules 1997, 30, 2190. (a) Nishikawa, T.; Kamigaito, M . ; Sawamoto, M . Macromolecules 1999, 32, 2204. (b) Fuji, Y . ; Ando, T.; Kamigaito, M . ; Sawamoto, M . Macromolecules 2002, 35, 2949. (c) Sawamoto, M . ; Kamigaito, M . Macromol. Symp. 2002, 177, 17. Haddleton, D. M.; Jackson, S. G.; Bon, S. A. F. J. Am. Chem. Soc. 2000, 122, 1542. Carmichael, A. J.; Haddleton, D. M.; Bon, S. A. F.; Seddon, K. R. Chem. Commun. 2000, 1237.


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Living Radical Polymerization Catalyzed with Hydrophilic and

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