Loss in Activity and Catalyst Recyclability in Batch and Continuous

Chapter 7

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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

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Cu Br/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 offreshligand. 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

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

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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 largefractionof 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 (Cu ), which causes a slowing of the polymerization (5-72). However where attempted, regeneration of the catalyst to its active form (Cu ) 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). 11

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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 . 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. 2

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

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87 inductively coupled plasma atomic emission spectroscopy respectively, as described elsewhere (23).

(ICP-AES),


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, M M A 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. All described operations are completed under N 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 M M A and initiator in the same quantities as outlined above for a subsequent polymerization. 2

Continuous Supported ATRP with Catalyst Regeneration Process Description (see Scheme 1): A feed reservoir, blanketed by N 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 . The mixture is bubbled with N for 10 min with stirring and then stirred for 3 hours. This catalyst is used to pack the column under N ; 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. 2

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88 N2

column Feed Tank

Pump

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 collectedfromthe 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 purged syringe. The solution is split into three N 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%. 2

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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


89 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. Solution - 24 hours* Solution - 90 hours 1st Catalyst Wash 2nd Catalyst Wash Total Cu Lost/Cycle

¥

469 298 109 21 16


t

¥

2137 2137 2137 NA NA

21.9 13.9 5.1 1.2 1.1 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 at room temperature prior to supernatant copper assay. ^Measured by ICP-AES. Assuming all Cu reports to solution. 2

T

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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Figure 1: Monomer conversion, polymer molecular weights and distribution 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 -freshcatalyst (Φ , OA second run - catalystfromrun 1 (Α ,Δ ), third run - catalystfromrun 2 plusfreshligand 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

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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 /M ) 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 α conversion, low M /M ). 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

Loss in Activity and Catalyst Recyclability in Batch and Continuous

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