Copper Removal in Atom Transfer Radical Polymerization - ACS

Chapter 18

Copper Removal in Atom Transfer Radical Polymerization 1

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Downloaded by COLUMBIA UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch018

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Mical E. Honigfort , Shingtza Liou , Jude Rademacher , Dennis Malaba , Todd Bosanac , Craig S. Wilcox , and William J. Brittain 2

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Department of Polymer Science, The University of Akron, Akron, O H 44325-3909 Department of Chemistry and The Combinatorial Chemistry Center, University of Pittsburgh, Pittsburgh, PA 15260

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We have developed three methods for simple copper removal in atom transfer radical polymerization (ATRP). The first involved use of a polyethylene-ligand which is homogeneous under polymerization conditions, but precipitates upon cooling. Less than 1% of the original copper was left after simple decantation; however, long (24 h) polymerization times were observed. The second method involved the use of JandaJel™ ligands. Normal polymerization times were observed with this heterogeneous system, but copper removal was less efficient with4-5%of the original copper remaining. The third method used precipiton ligands that are soluble in the cis-stilbene form, but precipitate when isomerized to the trans form. The precipitons were the most efficient at copper removal( Copper containing itandard solution; 0.9 wt% Qi

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\ PMMA/toluene solution after filtration Jo remove PE-iame/OiBr c©mpîex\

500 600 700 Wavelength (nm) Figure 1. UV-visible Spectrum of PMMA/toluene Solution After Filtration to Remove the Catalyst Complex

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Table IL Catalyst Reuse in A T R P of M M A Using PE-Imine Time(h) % Conv. M»(theo) M (exp) PDI te/mol) fe/mol) — — 800 2 8 1 Catalyst Use 3,100 2,500 1.32 31 4 1.38 4,600 4,700 47 7 1.38 7,500 6,300 63 10 1.44 9,100 8,000 80 15 1.52 9,400 8,700 87 21 1.44 2,000 2,400 24 8 2 Catalyst Use 5,700 6,300 1.50 57 16 1.54 7,900 7,500 75 21 n

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experimental Conditions:[PE-ligand]:[ethyl 2-bromoisobutyrate initiator]: [CuBr]:[MMA]==2.i:l:100; solvent = 2:1 v/v toluene/MMA; 100 °C; conversion measured by H-NMR relative to internal standard; M , M determined by GPC relative to P M M A standards. n

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To synthesize a diblock copolymer (PMMA-&-PDMAEMA), solvent and residual monomer from the first polymerization were removed in vacuo after the first ATRP reaction had reached 70-80% conversion and a second aliquot of monomer and additional solvent were added to the reaction mixture. The PMMA-Br product from the first reaction acts as a macroinitiator. Figure 2 shows the GPC trace for the P M M A block and for the diblock. Each block had a theoretical molecular weight of 10,000 g/mol at 100% conversion. There is

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

256 not good separation between the two peaks and the diblock peak has a small lower molecular weight tail. There is likely homopolymer still present in the mixture. The second block only reached 55% conversion and the molecular weight for the second block was 5,500 g/mol. This result is not surprising since the ATRP reactions using the PE-ligand were very long and thus, some termination is inevitable. The D M A E M A homopolymerization using the PEligand was also sluggish. PMMA- b - PDMAEMÂ M = 13,900gfaiol; 55% conversion


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Figure2. GPC Traces (RI Detector) for PMMA and PMMA-b-PD MAEMA Polymers Prepared via ATRP Using the PE-lmine Ligand

Understanding Long Reaction Times for PE-ligand System The preliminary work using M M A as the monomer demonstrated that the PE-imine ligand successfully mediates a controlled ATRP reaction and effectively removes the copper catalyst. The reactions proceeded as wellcontrolled ATRP reactions with good molecular weight control and with relatively narrow polydispersities. However, the reaction times were over twice as long to reach high conversion compared to a conventional ATRP reaction. Although the PE-ligand system is homogeneous at reaction temperature like the control reaction, the difference for the PE-ligand system is the long PE chains that surround this catalyst system (depicted in Scheme 3). Similar to branchedalkyl pyridylmethanimine ligands, there is a steric effect that retards Cu complexation by the PE-ligand. This slows the polymerization and leads to increased termination. The increased termination produces excess deactivator in accord with the "Persistent Radical Effect." Another effect that may be operating in this system is the immiscibility of the growing P M M A chains with the non-polar PE-ligand/Cu complex.


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Scheme 3. General Schematic for ATRP Using the PE-Ligand

JandaJel™ Supported Ligands for ATRP JandaJel™ resins were developed by Janda and coworkers (12) as an alternative to divinylbenzene (DVB) crosslinked polystyrene (PS) resins. These new resins are insoluble but swellable due to a more flexible crosslinker and are said to h ave increased homogeneity and site accessibility as compared with D V B crosslinked resins. The synthesis and structures of the two JandaJel™ ligands that were used are shown in Scheme 4. The precursors to these ligands are commercially available resins functionalized with - O H or - N H 2 (1.0 mmol functionality per gram). The JandaJel™-imine ligand, as characterized by H-NMR, had a functionality of 0.9 mmol/g. The JandaJel™-TEDETA ligand, characterized by elemental analysis for nitrogen had a functionality of 0.7 mmol/g. l

A T R P Using JandaJel™ Ligands The ATRP reactions for M M A , styrene, and D M A E M A were much faster for this system compared to the PE-iigand/CuBr catalyst system. For M M A , high conversions (up to 100% for some of the reactions) were reached in 6-8 h. One contributor to the fast rate could be the polar molecule used to crosslink the PS in the JandaJeis™ since polar media are known to increase reaction rates in ATR?.(25,26) The polar nature of JandaJel™ could help increase solubility of the activating species (CuBr) and therefore increase reaction rate. The polydispersity narrowed over the course of the ATRP reaction. Even though these PDIs are significantly higher than for a conventional ATRP of M M A , they are much narrower than those for ATRP of M M A using ligands supported by D V B crosslinked PS resins (PDI values up to 10 were reported).(2) With the JandaJel™ system, we would expect a higher PDI compared to a conventional reaction since there will be a diffusion effect on the deactivation reaction, which would not be present with a typical homogeneous



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ligand system. Also, some of the previously cited heterogeneous ATRP reactions have been reported as not very reproducible due to factors such as stirring rate (2) but these reactions with the JandaJel™ ligands were very reproducible. Despite the broader polydispersity, these reactions proceeded with a linear increase of molecular weight with monomer conversion and good control of molecular weight for both catalyst uses (Table III, Figure 3). At high conversions the molecular weights were higher than predicted; this has been observed in the other reported heterogeneous ATRP systems.(2,5) There was -20% decreased activity for the second catalyst usage, which can be attributed to excess CuBr2 (deactivator) due to the"Persistent Radical Effect.". The first order kinetic plot was linear for both the first use and for the recycled catalyst (Figure 3). The JandaJel™-TEDETA ligand exhibited the same behavior as the JandaJel™-imine ligand in ATRP of M M A .


259 Table III. A T R P of M M A Using the JandaJel™-Imine Ligand* PDI Time (h) % Conv. M (theo) M (exp) (%/mol) (%/mol) 1.40 5,200 5,800 2 1 Catalyst Use 58 7,500 1.36 72 7,200 3 1.32 9,400 80 8,000 5 1.29 10,800 9,600 8 96 1.81 5,000 37 2 3,700 2 Catalyst Use 1.76 7,500 64 6,400 3 1.52 9,000 7,100 5 71 1.50 8,500 9,900 6 85 experimental Conditions:[JandaJel™-ligand]:[ethyl 2-bromoisobutyrate initiator]:[CuBr]: [MMA] = 2:1:1:100; solvent = 3:1 v/v toiuene/MMA; 100 °C. Conversion measured by H-NMR relative to an internal standard; M M determined by GPC using universal calibration. n

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Figure 3. f Order Kinetic Plot for A TRP of MMA for f and 2 Catalyst Uses for JandaJel™-Imine Ligand

The ATRP of styrene and D M A E M A both showed controlled behavior in the polymerization. The experimental molecular weights were in agreement with the predicted values and the molecular weights increased linearly throughout the reaction (Table IV, Figure 4). The styrene reaction was sluggish and only reached 63% conversion while the D M A E M A polymerization was very fast, reaching 92% conversion in less than 3 h. The D M A E M A result is


260 consistent with what is seen in conventional ATRP of this monomer.(27) Both reactions followed first order kinetics (Figure 5). The P D M A E M A had broader polydispersities than the other polymers (the PDI progressed from 1.8 at lower conversions to 1.6 at the end of the reaction) probably due to its polar nature, which helps to increase solubility of the activating species, shifting the atom transfer equilibrium and increasing the concentration of the active radicals in solution.

Table IV. A T R P of Styrene and D M A E M A Using the JandaJel™-Imine Ligand PDI Time (h) % Conv. M (theo) M„(exp) (z/mol) (z/mol) 1.70 1,500 1,500 Styrene 2 15 2,400 1.62 2,900 6 29 1.54 5,800 6,300 10 63 1.85 6,000 5,800 DMAEMA 0.5 37 9,000 1.77 10,800 2 69 1.54 12,500 13,200 84 3 1.50 14,100 14,400 6 92 Conversion measured by iH-NMR relative to internal standard; M , M determined by GPC using universal calibration or reference to P M M A or PS standards. [JandaJel -ligand]: [ethyl 2-bromoisobutyrate initiator]: [CuBr]:[styrenel = 2:1:1:100; solvent = 3:1 v/v toluene/styrene; 100 ° C . [ J a n d a J e l -ligand]:[ethyl 2-bromo-isobutyrate initiator]:[CuBr]: [DMAEMA]=2:1:1:100; solvent = 3:1 v/v t o l u e n e / D M A E M A ; 60 °C 8


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A diblock copolymer was synthesized using the JandaJel™ ligand system. For the diblock experiment, solvent and residual monomer from the first polymerization were removed in vacuo after the first ATRP reaction has reached 70-80% conversion and a second aliquot of monomer and additional solvent are added to the reaction mixture. A PMMA-6-PDMAEMA diblock was synthesized; the GPC trace for the first block and the diblock is shown in Figure 6. Copper removal from the JandaJel™ ligand ATRP systems was determined in the same manner as described for the PE-ligand system. Elemental analysis for copper indicated 4-5% of the original copper remained in the unpurified polymers.


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MMA DMAEMA Styrene Theo (DMAEMA) Theo (MMA, Styrene)

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Figure 5. 1 Order Kinetic Plot ComparingATRP of MMA, DMAEMA, and Styrene Using the JandaJel -Imine Ligand


262 PMMÀ-i -PDMÂEMA M =18,700 gfaol; 73% conversion


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Figure 6. GPC traces (RI Detector) for PMMA and PMMA-b-PD MAEMA Polymers Prepared via ATRP Using JandaJel™Ligands

Précipitons for ATRP Catalyst Removal Precipiton ligands-1 and -2, shown in Scheme 5, were successfully used for ATRP of M M A mediated by CuBr, using toluene as the polymerization solvent and ethyl 2-bromoisobutyrate as the initiator (Table V).(14) Upon completion of the polymerization, the solution was cooled to room temperature and exposed t o U V radiation for 2 h. The precipiton-ligand precipitated and remained complexed with the Cu catalyst. The precipitated product can be isolated by décantation, filtration, or eentrifugation. Copper content of the polymer solution was determined by U V spectroscopy and ICP analysis. Less than 1% of the original copper was observed in the P M M A produced using either ligand1 or -2. The P M M A from this reaction required no purification other than simple décantation. Compared to the various heterogeneous copper removal techniques, this precipiton-ligand system offered somewhat better control of molecular weight and molecular weight distribution in reasonable reaction times. The data in Table V indicate that M (exp) is in good agreement with M (theo) until high conversion. At high conversions, M (exp) is greater than expected, indicating some termination. The reactions achieved 90-93% conversion in 12 h while polydispersity narrowed slightly over the course of the reaction. Better control over polydispersity was obtained using monofunctional ligand-1. n

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Ligand-2 Scheme 5. Precipiton Ligands for ATRP

The precipiton-bound ligands successfully mediated the ATRP of M M A and allowed for easy and fast removal of the copper catalyst by exposure of the product solution to a U V light source. The present inability to reuse or recycle the ligand in this catalyst system is an undesirable feature. In other supported catalyst systems, the ligand can be recovered and reused, but the précipitons used in our experiments cannot be recycled. The results demonstrate that the method can be useful in specialty applications. If ongoing efforts to develop recyclable précipitons are successful, then this system will provide a general and economically attractive way to remove metals from ATRP systems.

Summary Overall, the PE-ligands worked well in ATRP to produce homopolymers although reaction times were long and polydispersity increased over the course of the reaction. The system was able to produce a diblock copolymer although


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all the chains did not initiate polymerization of the second block. The catalyst complex retained its activity for a second catalyst reuse. The PE-ligands were very effective for copper removal, with only 1% of the original copper remaining in the unpurified polymer. The JandaJel -ligands are applicable to various monomers and the ATRP reactions generally proceeded much faster as compared to the PE-ligands. Catalyst reuse is possible using these ligands. However, copper removal with the JandaJel™ system was less effective than for the PE-ligand systems or other reported ATRP catalyst removal systems.(2,3,8,28) Table V I compares our results for copper removal with representative literature examples. Inspection of this table reveals that the precipiton ligand produced the lowest amount of residual copper. However, the inability to recycle these ligands mitigates this performance.

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Table V. ATRP of MMA Using Precipiton Ligands 1 and 2 Mnfexp) PDI Ligand Time % Conv. M (theo) (g/mol) (g/mol) (min) 1.45 4,600 4,000 1 120 40 1.45 5,800 300 54 5,400 1.44 7,700 480 74 7,400 1.42 8,100 7,800 600 78 1.40 10,400 9,100 660 91 1.40 13,700 9,300 800 93 1.22 3,200 2,700 2 240 27 1.20 6,100 5,100 360 51 1.20 7,500 6,800 510 68 1.19 8,200 7,800 600 78 1.19 10,100 9,000 720 90 [Precipiton ligand]:[ethyl 2-bromoisobutyrate inititator]:[MMA] = 1.5:1:100; 50% (v/v) toluene; 90°C; conversion was measured by Ή N M R relative to an internal standard; A/ , M determined by GPC by comparison to P M M A standards. n

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Table VI. Comparison of Copper Removal Techniques Method Percent Original Copper Remaining 1% PE-ligand (M = 2000 g/mol) Precipiton Ligand

Copper Removal in Atom Transfer Radical Polymerization - ACS

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