Magnetic Nanoparticle Supported Catalyst for Atom Transfer Radical

Chapter 6

Downloaded by NORTH CAROLINA STATE UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch006

Magnetic Nanoparticle Supported Catalyst for Atom Transfer Radical Polymerization of Methyl Methacrylate *

Shijie Ding, Maciej Radosz, and Youqing Shen

Department of Chemical and Petroleum Engineering, University of Wyoming, 1000 East University Avenue, Laramie, WY 82071

Magnetic nanoparticles were used to support a CuBr/MNP -TEDETA catalyst used for an ATRP of methyl methacrylate (MMA). The supported catalyst mediated a living/controlled radical polymerization of M M A as effectively as unsupported catalysts. With the addition of 22 mol% of Cu(II)Br , the polymer molecular weights were well-controlled with initiator efficiency of 0.85 and polydispersity lower than 1.2. The supported catalysts could be easily separated/isolated using a magnetic field. The catalyst could be reused with slightly decreased activity but much improved control. The activity of the recycled catalyst could be regenerated by copper metal. A PEG-block-PMMA was synthesized with 92.2% conversion and low polydispersity by this supported catalyst. It was concluded that nanosized supports had reduced adverse effects on catalysis. 2

© 2006 American Chemical Society

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

71

72


Introduction Atom transfer radical polymerization (ATRP) has been successfully used in living polymerizations of various vinyl monomers (7-3) and functional monomers (4-8), producing polymers with well-controlled molecular weights. ATRP is also a versatile synthetic tool to prepare well-defined (co)polymer architectures including block (9-/0), star (77-/3), brush (14-16), comb (17), dendrimer-like (11,18-19) and hyperbranched (co)polymers (20-21). A remaining challenge for ATRP is the difficulty of separating the homogenous ATRP catalysts from their products. The residues of the transition metal complex catalysts color the products and may also make the products toxic. Therefore, the concentration of the catalyst residue in the products must be reduced to a low level for safe applications (22). Post-polymerization purification methods such as catalyst extraction with catalyst-soluble solvents (8), catalyst adsorption with ion-exchange resins (23) andfiltrationof polymer solutions through columns of alumina or silica gel, have been developed for removal of catalysts from polymers prepared by ATRP (22). The disadvantages of these methods include cost, loss of polymer, difficulty in scale-up, and difficulties in separating the catalyst from functional polymers that interact with the catalysts (22). Immiscible liquid-liquid biphasic polymerization, in which the transition metal complex catalysts were modified to preferentially locate in one phase and the polymer in the other, has also been extended from organic synthesis to ATRP. Fluorous solvent (24), supercritical C 0 (25) and ionic liquids (26-27) were explored for the removal of catalyst from polymers. Recently, we reported the development of an ionic liquid catalyst, which during the polymerization, this ionic liquid catalyst can be dispersed as small droplets in organic solvents to catalyze liquid-liquid biphasic ATRP; after polymerization, the phase separation of the catalyst from the polymerization solution leads to catalyst separation and recycling. In this way, the amount of ionic liquid used in the reaction is greatly reduced, only 5 wt% of the organic solvent (28). Immobilization of catalysts on solids also provides an efficient method for catalyst separation with a possibility to reuse the catalysts for cost saving. This concept was successfully used in batch (29-31) and continuous ATRP (32-33). Catalysts tethered on solid surfaces such as silica gel, polystyrene beads and Janda Jel resins can be easily recovered (34-40). However, the level of control over the polymerization by these covalently solid-supported catalysts was lower than that of unsupported catalysts. The deterioration of control over the polymerization was caused by the slower deactivation of the growing radicals. The deactivation rate constant for the solid-immobilized catalyst was reduced to the diffusion limit, ca. 10 Lmol^s" , which is much less than the diffusion limit of homogenous catalysts, ca.10 Lmol^s' . Because the activation rate constant 2

5

1

9

1



73 remains unchanged, the overall effect of catalyst immobilization on solids is slower deactivation of the generated radicals, resulting in a higher radical concentration and uncontrolled chain growth (22, 41). Another effect of the catalyst immobilization on solids is that the catalyst on the solid surface may not be able to reach inside the polymer coil to catalyze the reactions, especially when the solids are porous (22, 39-40). The addition of soluble deactivators or free ligand to the solid-supported catalysts (41-43), and use of a soluble supports (44-47) or "catalyst sponge" (48-50) that releases free catalyst for homogeneous catalysis, could significantly improve the level of control over the polymerization (22). Generally, the solid supports used for immobilization were in micrometer sizes, and some had porous structure. This causes a significant decrease in catalyst diffusivity. Inspired by the development of a supported catalysis for small molecular reactions (51), we used magnetic nanoparticles to support a catalyst for ATRP. It is envisioned that a smaller (nanometer sized) support with a regular shape may have reduced adverse effects on the diffusivity of the immobilized catalyst. The separation of nanosized magnetic supports can be easily achieved by applying a magnetic field.

Experimental

Materials Acryloyl chloride (Lancaster, 96%), methyl a-bromophenylacetate (Aldrich, 97%, MBP), 3-aminopropyltrimthoxysilane (Aldrich, 97%), Ν,Ν,Ν' ',Ν' -tetraethyldiethylenetriamine (Aldrich, tech.90%, TEDETA), ^AT^^ jV''-pentamethyldiethylenetriamine (Aldrich, 99%, PMDETA), nitric acid (EM, A.C.S.), Fe 0 powder (Nanostructured & Amorphous Materials Inc., APS 20-30 nm) were used directly without further purification. Toluene (Baker) and methyl methacrylate (Aldrich, 99%, MMA) were distilled before use. Copper(I) bromide (Aldrich, 98%) was stirred with glacial acetic acid, filtered, washed with ethanol and dried. ,

l

3

4

Grafting TEDETA ligand onto Fe 0 magnetic nanoparticles (MNPTEDETA) 3

4

As shown in Scheme 1, Fe 0 magnetic nanoparticles (5.0 g, 20-30 nm) were refluxed with 3-aminopropyltrimthoxysilane (20 ml, 0.113 mol) in dry 3

4


74 toluene (30.0 ml) for 4 days. The solids were separated by applying an external magnetic field, and washed with 4 χ 100 mL dry toluene and then 2 χ 100 ml dry acetone. The resulted product was dried under vacuum for 12 h. Elemental analysis: N , 0.61 %. The resulting solids were dispersed in 50 ml of dry dichloromethane and the flask was cooled in an ice-water bath. Triethylamine (16.0 ml, 0.113 mol) was added and stirred for 30 min. Then acryloyl chloride (4.0 ml, 49.2 mmol) was added dropwise over 30 min. The mixture was stirred at room temperature for 48 h. It was successively washed with acetone (3 χ 100 ml), DI H 0 (3 χ 100 ml), and again acetone (2 χ 100 ml). The resulting product was dried under vacuum for 5 h, then dispersed in methanol (30.0 ml) and stirred with TEDETA (5.0 ml, 19.4 mmol) at room temperature for one week. The solids were separated by magnetic field and washed with acetone 5 χ 100 mL. The resulting product was dried under vacuum for 12 h. Elemental analysis: Ν, 1.87 %.


2

Preparation of po!y(ethylene glycol) macroinitiator(Ini-PEG) P E G 4 5 - O H (20.0 g, 0.010 mol) was dissolved in 300 mL of toluene in a 500 ml three-neck flask then triethylamine (2.78mL, 0.020 mol) was added and the solution was cooled to 0 °C. 2-Bromoisobutyryl bromide (2.47 mL, 0.020 mol) was added dropwise via a syringe over 1 h. The reaction mixture was stirred overnight at room temperature. The solution was filtered and the solvent was removed in vacuo. The crude polymer was dried under vacuum overnight, then dissolved in water at pH 8.5, and extracted with dichloromethane. The organic layer was collected, dried over CaCl , and purified under vacuum to remove the residual solvent. A 10-fold excess of diethyl ether was added to precipitate the PEG initiator (Ini-PEG). The product was dried under high vacuum overnight. Yield = 15.8 g. 2

Polymerization by CuBr/MNP-TEDETA catalyst Reaction conditions for a typical polymerization are as follows: CuBr (0.0135g, 0.0941 mmol), MNP-TEDETA (0.2818g, TEDETA 0.0941 mmol) and a stirring bar were put into a reaction tube. The tube was tightly sealed and degassed by ten vacuum/nitrogen cycles. Degassed toluene (3.0 ml) was added via a degassed syringe. The mixture was stirred and bubbled with nitrogen for 5 minutes. The sealed tube was sonicated at room temperature for 10 min. Degassed M M A (0.943 g, 9.41 mmol) was added and then the mixture was degassed again for 2 minutes and stirred at 25 °C for 1 h. The tube was


75 immersed in an oil bath preset at the polymerization temperature. Finally, the initiator (MBP, 15.0μ1, 0.0955 mmol) was added to the mixture with stirring. Samples (0.050 ml) were withdrawn at timed intervals from the tube using nitrogen-purged syringes, and dissolved in CDC1 . The conversions were measured by *H-NMR spectroscopy and the molecular weight and polydispersity of the polymers were measured by GPC. 3


Recycle of CuBr/MNP-TEDETA catalyst After the polymerization was complete, the reactor tube was cooled to room temerpature and placed on a magnetic bar. The catalyst was attracted to the bottom of the reactor. The polymer solution was decanted and the residual solid was washed by 3 χ 3.0 ml degassed toluene under nitrogen. Degassed M M A , toluene and initiator were charged to the tube and the second polymerization run was conducted following the same procedure as the first polymerization run. Regeneration of CuBr/MNP-TEDETA catalyst The recovered solid was washed with 3 χ 3.0 ml degassed toluene under nitrogen. About 1 g of copper metal beads (~1 mm diameter) and 3.0 ml of toluene were added and stirred with the catalyst at 40°C overnight. The resulting reactivated catalyst was separated from the copper beads and transferred into a degassed, tightly sealed tube. Then degassed MMA, toluene and initiator were added, and the polymerization was performed following the same procedure as the first polymerization run. Characterization. Gel permeation chromatography (GPC) was used to determine polymer molecular weights and molecular weight distributions (PDI) using polystyrene standards (Polysciences Corporation) to generate a universal calibration curve. The measurements were operated on a Waters SEC equipped with a Waters 2414 refractive index detector and two 300 mm Solvent-Saving GPC Columns (molecular weight ranges: 1*10 -5χ10 , 5*10-5>) 2

2


81

2.8

12000

h 2.6 ioooo H

S

2.4

Ί

8000

• •I

A

.•. /

6000

2.2 •

1.6


4000

0 *

1.4

r

2000 I

I

20

40

1.2

—r—

ι

60

80

1 100

Conversion (%)

Figure 6. PMMA molecular weight and polydispersity as a function of conversion in the polymerizations catalyzed by fresh 0) first re-used (• A), second-reused (Μ, D), and regenerated (Φ, O) catalyst of CuBr/MNPTEDETA. See Figure 5 for experimental conditions. t

Figure 7. GPC traces of (a) PEG-b-PMMA block copolymer and (b) PEG macroinitiator. 70 °C, [MMA] = 2.35 mol/L, [Cu(I)Br] =0.0183 mol/L, [Cu(II)BrJ = 0.00520 mol/L, [Ligand] = [Ini-PEG] = 0.0235 mol/L.


82 Block copolymerization

The ability of the magnetic supported CuBr/MNP-TEDETA catalyst to mediate a block copolymerization was examined by conducting a block copolymerization of M M A using a PEG macroinitiator (Ini-PEG) under similar conditions to the homopolymerization reactions. A block copolymer of PEG-bPMMA was isolated with 92.2% M M A conversion after 18 h polymerization. The resulting polymer had M of 16000 and polydispersity of 1.44 (Figure 7), This demonstrates that the magnetic nanoparticle supported catalyst is also good for block copolymerization.


n

Conclusions We have demonstrated that a magnetic nanoparticle supported CuBr/MNPTEDETA catalyst can effectively mediate a living polymerization of M M A forming polymers with a polydispersity less than 1.3. In the presence of 22 mol% of Cu(II)Br , the initiator efficiency could be increased to 0.85 and the polydispersity could be decreased to less than 1.2. The supported catalyst could be recycled and reused to conduct a second-run and a third-run with improved control over the polymerization but decreased activity. The activity of recycled catalyst could be regained by regenerating the supported catalyst with copper metal. The supported catalyst was also good for block copolymerization forming a PEG-b-PMMA. This work demonstrates that nanometer sized supports have minor effects on catalysis in terms of activity and control of polymerization. 2

Acknowledgement We thank the University of Wyoming and the Government of State of Wyoming (EORI) for financial support. We also appreciate Dr. Steven Boese of University of Wyoming, Geology and Geophysics Department for ICP-MS analysis.

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Magnetic Nanoparticle Supported Catalyst for Atom Transfer Radical

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