[ACS Symposium Series] Controlled/Living Radical Polymerization Volume 944 (From Synthesis to Materials) || Loss in Activity and Catalyst Recyclability in Batch and Continuous Supported

Chapter 7

Loss in Activity and Catalyst Recyclability in Batch and Continuous Supported Atom Transfer Radical

Polymerization

Santiago Faucher, Shiping Zhu*

Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7, Canada

Cu1Br/HMTETA physically adsorbed to silica-gel is used in batch and continuous ATRP processes to study the loss in catalyst activity with catalyst reuse. The loss in catalyst activity cannot be attributed to an oxidation of the catalyst as the catalyst is found to be fully regenerated by an addition of fresh ligand. The primary location for catalytic activity is found to be in solution and it is the loss of these soluble species that account for the loss in activity. A partitioning equilibrium of the active catalyst species between the support's surface and the solution accounts for the recyclability of the catalyst and the differences in process performance.

Introduction

Atom transfer radical polymerization (ATRP) is one of the many recently discovered living free radical polymerization mechanisms that allows for the tailoring of macromolecules (1,2). The process is catalytic using a complexed metal salt, usually copper halides, to mediate the polymerization. Generally, high loadings of this catalyst are necessary to mediate the polymerization since the catalysts have low activities, as compared to olefin polymerizations. These high catalyst loadings contaminate the polymer product making purification necessary. While this is manageable in the laboratory, purification on a larger industrial scale makes this process less attractive.

Two approaches are taken to overcome this challenge. The first is to improve the catalyst's activity thereby allowing lower catalyst concentrations to

© 2006 American Chemical Society 85

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

86

be used; ideally to a level at which post-purification becomes unnecessary. While some advances have been made in this area, further developments in catalyst systems are required to achieve high conversions, fast rates and good control of the polymerizations at even lower catalyst loadings (3,4). The second approach is to support the catalyst making it easily recoverable and recyclable. Soluble/recoverable, by-phasic and solid supported catalysts have been successful towards these ends but further reductions of residual catalyst concentrations in polymer are sought (5-25).

In this group of supported systems the solid supported catalysts are found to lose a large fraction of their activity with recycling (5-72). Two hypotheses are generally presented to account for this loss in activity. The most common being the oxidation of the metal center to its deactivating state (Cu11), which causes a slowing of the polymerization (5-72). However where attempted, regeneration of the catalyst to its active form (Cu1) has not yielded complete recovery of the catalyst's activity (8-10). The second and seldom proposed hypothesis is that the catalyst complex is lost with recycling (5-7). This is on account of the low residual metal concentrations in polymer, which are in the order of 1 to 10% of the initial metal loading (7-9,11-14,23). These catalyst losses are lower than the observed losses in catalyst activity (7-9,11,12). Thus neither hypothesis has yielded a satisfactory explanation for the loss in catalyst activity with recycling and most importantly no method of avoiding or mitigating the loss in catalyst activity has been devised.

In this work we study the loss in catalyst activity in one of the physically adsorbed yet highly recyclable catalyst systems that has been developed by our group (CuBr/l,l,4,7,10,10-Hexamethyltriethylenetetramine physically adsorbed to silica gel) (6,15).

Experimental Section

Materials

M M A (Aldrich, 99.9%) is distilled under vacuum and stored at 4 °C before use. 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA, 99%), Cu*Br (98%), and methyl α-bromophenylacetate (MBP, 97%, initiator) are used as received from Aldrich. Toluene is distilled from CaH 2. Silica gel (100-200 mesh), chromatographic grade, Sargent-Welch Scientific Co. is boiled in deionized water for 5 hours, air-dried and then vacuum-dried.

Measurements

Conversions, number- and weight-average molecular weights (M„ and M w , respectively), and copper concentrations are measured by ! H NMR, gel permeation chromatography relative to narrow polystyrene standard and

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inductively coupled plasma atomic emission spectroscopy (ICP-AES), respectively, as described elsewhere (23).

Batch Supported ATRP with Catalyst Recycling and Regeneration

Polymerization: Cu*Br (64.5 mg, 0.45 mmol), and silica-gel (645 mg) are added to a Schlenk flask. The flask is degassed by five vacuum-nitrogen cycles. M M A (4.5 g, 45 mmol) and toluene (8.86 g) are added to the flask. The mixture is bubbled with nitrogen for 40 minutes with stirring. HMTETA (122.3 μ ι , 0.45 mmol) is added dropwise to the flask and bubbled with nitrogen for an additional 20 minutes. Degassed initiator, MBP (70.8 μ ι , 0.45 mmol), is then added dropwise to the flask. The flask is placed in an oil bath at 90°C and stirred by a magnetic bar. Kinetic samples, 0.3 ml, are withdrawn from the flask with a nitrogen-purged syringe. The samples are stored in hermetic vials and placed in a freezer for future assay.

Catalyst Recovery and Recycling: The catalyst is recovered for use in a second and later third polymerization using the following procedure. At the end of the polymerization the flask's contents are left to settle overnight. The supernatant is removed via syringe. The remaining catalyst solids are washed twice with 20 mL of toluene. Pre-purged toluene, MMA and MBP, in the quantities outlined above, are added to the flask containing the supported catalyst recovered. The flask is then placed in an oil bath preset at 90°C to run a subsequent polymerization. Al l described operations are completed under N 2

atmosphere. Catalyst Regeneration: To the used and recovered catalyst, HMTETA

(122.3 μ ί , 0.45 mmol) and toluene (8.86 g) are added. This mixture is left to stir for 15 minutes prior to the addition of MMA and initiator in the same quantities as outlined above for a subsequent polymerization.

Continuous Supported ATRP with Catalyst Regeneration

Process Description (see Scheme 1): A feed reservoir, blanketed by N 2 and cooled by dry ice, holds the monomer, initiator and solvent to be conveyed continuously by the pump to the column reactor via 1 mm stainless steel tubing. The column reactor is immersed in an oil bath preset at 90°C. The feed is activated by the supported catalyst in the column to undergo ATRP. The product polymer is collected at the column outlet. Catalyst Preparation: Silica gel (12 g) is weighted into a Schlenk flask and degassed by five vacuum-nitrogen cycles. Toluene (50 ml), Cu'Br (0.6 g) and HMTETA (0.958 g) are added to the flask under N 2 . The mixture is bubbled with N 2 for 10 min with stirring and then stirred for 3 hours. This catalyst is used to pack the column under N 2 ; approximately 7.1 g (dry) of the catalyst fills a stainless steel column 900 mm in length with an inner diameter of 4.5 mm.

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N2-

Feed Tank Pump

column

Oil Bath Product Tank

Scheme 1

Polymerization: A typical polymerization is as follows: 100 ml Degassed MMA/MBP/toluene solution (MMA/toluene = 1/3 (w/w), [MMA]:[MBP] = 100:1) is added to the feed reservoir. The pump flow rate is set to 1.2 ml/hr. The feed reservoir is refilled as required. Kinetic samples of the polymerized solution are collected from the column product stream.

Catalyst Regeneration: HMTETA (1 ml, 1.5 molar equivalent of the fresh ligand in the column) is loaded into a nitrogen purged syringe and injected into the column feed line via a built-in injection loop.

Results

Copper Losses with Catalyst Recycling in Batch ATRPs

A batch polymerization using the catalyst Cu'Br/HMTETA physically adsorbed to silica gel is run. At the end of the polymerization stirring is stopped and the catalyst settles out of solution. The batch reactor is then removed from the oil bath and the supernatant solution siphoned off by a N 2 purged syringe. The solution is split into three N 2 purged vials (4 ml). The first vial is submitted directly for copper assay. The supernatant solutions in the second and third vials are submitted for copper assay following 24 and 90 hours respectively. Thus in these delayed assays, the catalyst has more time to settle out of the solution and is not detected in the supernatant solution. The catalyst remaining in the reaction flask is washed twice with 20 ml of toluene at room temperature and the washes are assayed for copper.

As seen in Table 1, the copper concentration decreases with increasing settling time. After 24 and 90 hours of settling, the supernatant solution contains 14% and 5%, respectively, of the total copper originally loaded. Washing the residual catalyst twice removes 2% of the total copper loaded. Thus under typical recycling conditions copper losses are in the range of 7 to 16%.

Loss and Regeneration of Catalyst Activity in Batch Supported ATRP

Three batch polymerizations are run. In the first polymerization fresh catalyst is used. At the end of the polymerization the catalyst is left to settle out

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Table 1: Copper Lost with the Supernatant Solution as a Function of Settling Time and Catalyst Washes

Description Copper in Solution Actual* Theoretical % of Total (ppm) (ppm) Loaded

Solution - end of pol.¥ 469 2137 21.9 Solution - 24 hours* 298 2137 13.9 Solution - 90 hourst 109 2137 5.1 1st Catalyst Wash 21 NA 1.2 2nd Catalyst Wash 16 NA 1.1 Total Cu Lost/Cycle 7 to 16

Notes; ¥Allowed solids to settle briefly prior to siphoning off solution. *,+ Left solution to settle 24 and 90 hours, respectively, under N 2 at room temperature prior to supernatant copper assay. ^Measured by ICP-AES. TAssuming all Cu reports to solution.

of the solution overnight. The clear supernatant is then siphoned off. The remaining catalyst is washed twice with toluene and then re-used in a second polymerization. MMA, toluene and initiator (as in the first polymerization) are loaded to the flask along with the used catalyst and the flask placed in the oil bath to polymerize. At the end of this second polymerization, the catalyst is recovered, washed and recycled again as described above. In this third polymerization, however, fresh ligand is added to the flask. Figure 1 shows the conversion and molecular weight development profiles for the three runs. A loss in catalyst activity occurs between the first and second polymerization (Figure 1). A comparison of the slopes of the first order rate plots (ln[M]o/[M] vs. time) shows that the apparent rate constant (propagation rate constant χ radical concentration) for the recycled catalyst is 30% lower than that for the fresh catalyst. Thus given the similar reaction conditions, 30% of the catalyst's activity has been lost following the first recycle. Polymer molecular weights and distributions are well controlled and narrow in both polymerizations. Polydispersities range between 1.1 and 1.3. Molecular weights increase linearly with conversion as expected for this living polymerization.

The catalyst in the third polymerization (recovered from the second run + fresh ligand) shows the same activity as the fresh catalyst used, see Figure 1. The conversion profiles of the first and third run are practically identical with conversions reaching 94% in both runs. The development of the molecular weight and polydispersity are also similar as evidenced in Figure 1.

Loss and Regeneration of Catalyst Activity in Continuous ATRP

A column packed with C^Br/HMTETA physically adsorbed to silica gel is placed in a bath at 90°C and continuously fed a solution containing MMA,

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50 100 150 200 250 300 350 400 Time (min)

• •

• •

• A

• • -P

• A

οσ 0 D

• • s* 0 on Δ

•

• ΐ>

1.8

1.6

1.4

1.2

1.0 0.0 0.2 0.4 0.6

Conversion

0.8 1.0

Figure 1: Monomer conversion, polymer molecular weights and distributions for three batch supportedATRPs: 90°C, [MMA]/[MBP]/[CuBr]/[HMTETA] =

100:1:1:1 (molar), toluene/MMA = 2 (w/w), and silica gel/CuBr = 10 fw/w). First run - fresh catalyst (Φ , OA second run - catalyst from run 1 (Α ,Δ ), third

run - catalyst from run 2 plus fresh ligand added (1 equiv to run 1) (Μ , Ώ).

Toluene and MBP as shown in Scheme 1. This continuous reactor is operated over 3 weeks (500 hours). At the exit to the column, samples of the polymer containing solution are collected to follow the systems performance (Figure 2). Upon depletion of the catalyst's activity, following 240 hours, a single injection of fresh ligand is made to the column's feed line. No new Cu*Br is added.

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Ο 40 80 120 160 200 240 280 320 360 400 440 480 520

Time (hours)

Figure 2: Monomer conversion, polymer molecular weight and molecular weight distribution of the continuous reactor product stream.

Monomer conversions reach 80% over the first 100 hours and then drop, as the catalyst loses activity, to plateau at 40%. Similarly, polymer molecular weights increase to 15,000 g/mol but later (140 hours) drop abruptly to 2500 g/mol. The trend in the polymer molecular weight distribution (M w /M n ) has similar inflection points. For the first 120 hours of operation polydispersities range between 1.42 and 1.55. However, at 140 hours of operation they begin to rise sharply, eventually reaching values of 15 at 230 hours. The polymer produced in the first 140 hours displays the characteristics of a controlled living polymerization (M n α conversion, low M w /M n ) . In contrast, the polymer produced after 140 hours appears poorly controlled. The catalyst in the column is therefore no longer sufficiently active to mediate the ATRP equilibrium and an uncontrolled free radical polymerization ensues.

Following 240 hours of operation, the feed line to the spent catalyst column is injected with a single shot of fresh ligand through a built-in injection loop. Subsequently, monomer conversions, polymer molecular weights and distributions return to levels comparable to those prior to the depletion of the catalyst in the column (t<140 hours). Conversions increase from 40% to 93%,

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molecular weights increase from 2500 to 9900 g/mol and polydispersities decrease from 15 to 1.63. The catalyst is rejuvenated for a period of approximately 50 hours as compared to the columns residence time of 8 hours. Following this, conversions, molecular weights and polydispersities return to levels observed under the spent catalyst conditions (40%, 1500 g/mol and 25 respectively). Thus an injection of fresh ligand reactivates the depleted catalyst in the column.

Performance Comparison of Catalyst in Continuous and Batch Reactors

Insight into the cause of the loss in catalyst activity can be gained by comparing the performance of the catalyst in the continuous and batch processes. Table 2 summarizes the key data. The catalyst in the continuous reactor is capable of producing 163 g of poly(methyl methacrylate) for every gram of copper used prior to becoming inactive. In contrast the catalyst used in the batch reactor produces (when recycled once) 284 g of poly(methyl methacrylate) for every gram of copper. In the batch case, the catalyst remains active and could be used to produce additional polymer.

Table 2: Catalyst Performance Comparison in Continuous and Batch Atom Transfer Radical Polymerizations

Parameters Reactor Type Continuous^ Batch

1st Use 2ndUse Total* Operating Time (hrs) 136 6 6 12 Polymer Produced (g) 25.6 4.2 3.9 8.1 Copper Used (g) 0.157T 0.0286 From first. 0.0286 Polymer/Cu (wgt/wgt/ 163 147 137 284 Used Catalyst Condition* Not Active Active Active Active

Notes: TData from Figures 1 and 2. TWhile polymerization is controlled. 'Polymer produced to copper used (weight ratio), *Used catalyst in continuous process no longer mediates ATRP, catalyst in batch process does and could be reused further.

Split Filter Test

Batch supported ATRPs are undertaken. Partway through the polymerization the solution is filtered away from the supported catalyst and the filtrate placed in a second reaction flask (case 1) to polymerize further in order to gauge its catalytic activity. Similarly, in a second experiment (case 2) the filtrate is placed in a flask loaded with Cu!Br. The filtered solutions' catalytic activities are compared to that of an unfiltered supported ATRP. Table 3 summarizes the results.

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Conversion in the filtered solution (case 1) increased from 22 to 78% as compared to 26 to 94% for the control experiment (unfiltered). The apparent rate constants (kpx[R*]) for the filtered solution is 50% of its unfiltered counterpart. Thus a large fraction of the catalytic activity is attributable to unsupported catalytic species. When the filtered solution is placed in contact with fresh Cu*Br (case 2), the catalyst activity of the filtrate increases and is comparable to that of the control experiment. Thus the presence of C^Br has an accelerating effect on the polymerization. Since Cu!Br does not promote polymerization of M M A in the absence of ligand, it must accelerate the system via a reduction of the soluble Cu nBr 2/L to QnBr/L (the activating catalyst).

Table 3: Kinetic Data for Solutions Filtered Away from the Supported Catalyst 30 minutes into the Polymerization and an Unfiltered

Polymerization

Rxn. Control Case 1 Case 2 Fil Time Not Filtered Filteredft¥ + CuBr"

(') Conv. Mn MJMn Conv. Mn MJMn Conv. 30 26 3180 1.08 22 3250 1.10 19 60 43 4800 1.07 27 4110 1.07 49 90 59 6400 1.08 35 5020 1.06 57 135 73 8170 1.11 47 6440 1.07 64 195 85 9890 1.16 61 7960 1.08 71 255 90 10850 1.22 70 9200 1.09 71 315 94 11220 1.27 74 10200 1.11 375 93 11530 1.29 78 11030 1.11

Notes:. fData from reference 24. Reaction conditions: toluene/methyl methacrylate = 2 (w/w), [MMA] : [Initiator] : [Cu'Br] : [HMTETA] = 100:1:1:1 (molar), silica gel/Cu'Br = 5 (w/w), 90°C. filtered at 30 minutes of polymerization time through a 0.2 μιτι PTFE filter into another schlenk flask under N 2 atmosphere immersed in a 90°C oil bath. TIn the new reaction flask 0.45 mmol of Cu !Br was added (1 equivalent to Cu !Br prior to filtration).

Effect of Silica Gel Loading

ATRPs of MMA are run with varying silica gel concentrations while all other reaction parameters are kept constant (Figure 3). At or below silica gel concentrations of 60 g/L the polymerization kinetics are comparable. At higher loadings the polymerization rates decrease. At concentrations of 130 and 170 g/L, polymerization rates are 50% and 35%, respectively, of those at the lower

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concentrations. Molecular weight developments as a function of conversion for the varying silica gel loadings (not shown) are comparable indicating similar initiator efficiencies. Molecular weight distributions are generally below 1.4.

Discussion

In the literature, the main causes to which the loss in surface supported catalyst activity are attributed are the catalyst's oxidation and a loss of catalyst (5-12). For the catalyst studied here, however, the data does not support the oxidation hypothesis since the catalyst can be regenerated with an addition of fresh ligand (Figures 1 and 2). Further evidence of this lack of oxidation is found through visual observation of the spent catalyst in the continuous process which retains its blue un-oxidized colour. We therefore focus on the second hypothesis, a loss of catalyst, to explain the loss in activity.

The measured copper losses (7 to 16%, Table 1) are significantly lower than the activity lost with recycling (30%, Figure 1). This is a first indication that the lost catalyst species contribute disproportionately more to the system's activity than the catalyst retained by the silica gel. This is demonstrated more directly by the split-filter tests and the deteriorated performance of the continuous

0 100 200 300 400

Time (min)

Figure 3: Effect of silica-gel loading on the kinetics of batch supported ATRPs of MMA using Si-gel/CuBr/HMTETA as catalyst. Conversion (filled in symbols)

and first order rate plot (clear symbols) are shown. Silica gel concentrations (g/L): 0 (W ,V;, 22 ( · ,Ο), 40 (Φ ,Ολ 60 (+ ,Ο), 130 (Α ,Δ), and 170 (M

All other reaction conditions as per Figure 1.

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reactor following 140 hours. The split-filter tests show that a small amount of soluble (in constrast to adsorbed) catalyst species account for more than 50% of the total catalytic activity (Table 3) (24). Similarly, in the continuous process the higher conversions achieved prior to 140 hours show that the catalyst species lost have high activity (Figure 2). The simultaneous loss in the control of polydispersity at 140 hours also demonstrates that the lost catalyst species are necessary to mediate the ATRP equilibrium and that the retained catalyst is incapable of accomplishing this. The ability of the supported catalyst species to mediate the faster deactivation mechanism is questionable given diffusion constraints (9). Thus the loss of catalyst activity and control of polydispersity results from a loss of the soluble catalyst species.

The regeneration of the system's activity with ligand addition (Figures 1 and 2) shows that Cu!Br is not the limiting reagent (lost or oxidized) but rather that the ligand is. It is not clear why this occurs. An adsorption of uncomplexed ligand and/or ill defined metal-ligand complexes to the support's surface (25) may account for this. Once fresh ligand is added, however, new soluble complexes form from non or partly complexed Cu!Br on the silica gel surface thereby completely regenerating the catalytic activity (Figures 1 and 2).

The recyclability of the catalyst is explained by a reversible partitioning equilibrium between the silica gel and solution that is temperature dependent. Thus in batch reactions when the system is heated the catalyst desorbs into solution where it mediates the process. At the end of the polymerization a fraction of this catalyst readsorbs to the support's surface, the rest is lost with the product accounting for the loss in catalyst activity upon recycling.

This explains the lower catalyst activity retention in the continuous process. Here, the desorbed catalyst remains in solution and exits the reactor with the product as a result of the constant reaction temperatures (90°C). In the batch reactor, the cooler (25°C) catalyst recovery temperatures allow a larger fraction of the desorbed catalyst to be readsorbed by the support. More direct evidence of a partitioning equilibrium is presented in Figure 3. Here reaction rates are found to decrease with increasing silica gel concentration. This indicates that the surface area has an impact on the concentration of soluble catalyst species, which is consistent with the partitioning theory.

Scheme 2 summarizes the demonstrated ATRP mechanisms for the system studied here. This mechanism is inspired by the work of Matyjaszewski who developed a shuttle bus system to overcome diffusion limited deactivation in supported ATRPs (P). There are some key differences, however, between the shuttle bus system and that which we present here for C^Br/HMTETA/Si-gel. First, the soluble catalyst species result from desorption of catalyst from the surface. Second, the soluble catalyst accounts for the majority of the catalysis and activation by the surface is a minor component. Third, an acceleration mechanism via reduction of soluble Cu nBr 2/L to Cu!Br/L by solid C^Br is demonstrated.

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Conclusions

The loss in catalyst activity with recycling using Cu !Br/HMTETA physically adsorbed to silica gel does not result from oxidation of the catalyst. The catalyst is found to retain its rich blue unoxidized colour in the continuous process and the catalyst's activity can be regenerated by an addition of fresh ligand. Rather, the loss in catalyst activity results from the loss of a small quantity of soluble catalyst species that partition between the support's surface and the solution where they are active. The soluble catalyst accounts for a large fraction of the system's catalytic activity (+50%) and is essential for mediating the ATRP equilibrium. The catalyt adsorbed to the surface is incapable of mediating this equilibrium. A mechanism, depicted in Scheme 2, is presented to summarize the observed phenomena.

Scheme 2: Demonstrated Mechanism of ATRP using Ct/Br/HMTETA Adsorbed to Silica-gel as Catalyst. Notes: Abbreviations are ligand (L), temperature

sensitive (T), dormant polymer chain (P-Br), activation and deactivation rate constants (ka, ty; fopent catalyst activates but does not mediate ATRP;

fdeactivation via soluble catalyst; *uncomplexed Cu!Br does not on its own promote ATRP; propagation not shown for clarity.

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[ACS Symposium Series] Controlled/Living Radical Polymerization Volume 944 (From Synthesis to Materials) || Loss in Activity and Catalyst Recyclability in Batch and Continuous Supported