Ionic Liquids as Green Solvents - American Chemical Society

Chapter 25

Downloaded by STANFORD UNIV GREEN LIBR on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch025

Polar, Non-Coordinating Ionic Liquids as Solvents for Coordination Polymerization of Olefins Kevin H. Shaughnessy*, Marc A. Klingshirn, Steven J. P'Pool, John D. Holbrey, and Robin D. Rogers* Department of Chemistry and the Center for Green Manufacturing, The University of Alabama, Tuscaloosa, AL 35487-0336

Increased polymerization yield is observed in the copolymerization of styrene and CO in ionic liquids (ILs) compared to commonly used molecular solvents using palladium catalysts. Conditions for the copolymerizations were optimized and the effect of changes in the cation and anion of the IL solvent were determined. These results suggest that polar, non-coordinating ILs accelerate olefin polymerization catalyzed by electrophilic, charge-separated catalyst species.

Introduction Ionic liquids (ILs), particularly those that are liquid at room temperature, are an exciting class of neoteric solvents. Although examples of ILs have been known for nearly a century, the past decade has seen a dramatic surge in interest in these materials (1). The development of air- and water-stable ILs based on imidazolium and pyridinium salts of anions, such as PF " and BF " (Figure 1) 6

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

301

/ C

H

3

/

^

\

Cl

BF4"

J

C„H [C mim] n

2n+1

"A.

PF " 6

9

O-S-CF3

F

° [TFA]

n

3

ο [OTf]

2 n +

C

3

R4N

"

C H i [Cnpyr] 0 ^ 0 0 ^ 0 » 3 -rN-S-CF C F -S-N-S-C F 11 n ' 11 11 R4P O O O O [Tf N] [BETI] F

+

j? C

^"Ν'

Ν

AICI4"

Cl"

2

5

2

5

+


2

Figure 1. Common ionic liquid cations and anions.

have provided materials that are more convenient to work with than the chloroaluminate ILs that received initial widespread interest. Since many ILs have negligible vapor pressure, there has been significant interest in their use as environmentallyfriendlyreplacements for volatile organic solvents. While the environmental friendliness of ILs remains to be determined, ILs possess a number of other unique properties that have led to their application in a variety of organic synthetic methodologies (2-4). ILs have tunable hydrophobicity that ranges from complete miscibility with water to highly hydrophobic materials. In addition, ILs are immiscible with a variety of nonpolar organic solvents. This tunable miscibility, in combination with the non-volatility of ILs, has lead to their wide application in bi- (5-13) and even tri phasic (14) catalytic processes. ILs are unique among potential reaction media in that they can be polar, yet can also be designed to be non-coordinating. Reported values for the solvent polarity of ILs have ranged from non-polar (hexane) (15) to polar (alcohols) (16,17) depending on the probes used. Despite the range of reported values, most authors agree that ILs have solvent polarities comparable to aprotic, dipolar solvents, such as acetonitrile and DMF. Although polar, ILs containing non-nucleophilic anions (BF ", PF ", Tf N") are weakly coordinating, with coordination abilities comparable to methylene chloride (13). Polar, noncoordinating IL solvents may be expected to accelerate certain catalytic processes by stabilizing charge separated catalytic intermediates or transition states. Acceleration of catalytic processes in IL solvents has been attributed to their polar, non-coordinating nature in some cases (5,18), but the true mechanism responsible for the observed rate increases has not been determined. Catalyst systems for the coordination polymerization of alkenes, such as metallocene-based Ziegler-Natta systems (19), late-transition metal olefin 4

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


302 polymerization catalysts (20), and alkene/CO copolymerization catalysts (21), share a common feature. In each case, the active species is a cationic metal complex with a weakly-coordinating anion. These highly electrophilic active species are necessary to allow alkene coordination and insertion. A highly polar, yet non-coordinating solvent may be expected to stabilize these active species and possibly accelerate the propagation steps. Chloroaluminate ILs have been used as solvents for the cationic (18,22-26) and metal-catalyzed (27,28) oligomerization or polymerization of olefins. The current generation of stable ILs have been successfully applied to radical polymerization of olefins (29-31). Wasserscheid (13) has shown that [C mim][PF ] ILs are effective solvents for ethylene oligomerization using nickel catalysts. Production of high molecular weight polyethylene catalyzed by both zirconocene and palladium catalysts has been claimed in a patent, although supporting data, including polymer yield or catalyst activity, were not reported (32). In this paper we present our initial efforts to apply IL solvents to palladium-catalyzed styrene/CO copolymerization and ethylene homopolymerization. n

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Results Alternating Styrene/CO Copolymerization with a Drent-Type Catalyst To initially test the ability of ILs to be used as solvents in the copolymerization of styrene and CO, we carried out the copolymerizations using a Drent-type catalyst system derived from LPd(OAc) (L = 2,2'-bipyridine (bipy)), excess ligand, benzoquinone, and p-toluenesulfonic acid (Figure 2) (33). The [Tf N] anion was chosen for initial screening due to its stability and low coordinating ability. Initial attempts to copolymerize styrene and CO in [C pyr][Tf N] gave only styrene homopolymers. Since methanol is known to react with Pd(II) complexes in the presence of CO to give catalytically active [LPdC(0)OCH ] species, we added methanol to the reaction mixture. When the copolymerization was repeated using a 10:1 ratio of [C pyr][Tf N]:MeOH perfectly alternating copolymer was formed with a productivity of 1.11 kgCP/gPd (Trial 2, Table 1) (34). Under these conditions little or no polystyrene is formed as determined by *H NMR and FTIR. The copolymerization conditions were optimized for the [C pyr][Tf N]/methanol solvent system. Doubling the volume of the IL, while holding the methanol volume constant ([C pyr][Tf N]:MeOH = 20:1) resulted in an approximately 50% increase in copolymer yield (Trial 6). If both the IL and methanol volumes were doubled, the productivity of the system increased to 2.7 kgCP/gPd (Trial 7). In addition, the copolymer produced under these conditions had a much higher molecular weight than with other MeOH:IL ratios. Further 2

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χ

+CO

p h

Catalyst 2.2-Bpyridine />-TsOH Ο Benzoquinone xf^N^S"^ [C ][Tf N],MeOH Τ » 6PF

2

Catalyst


Figure 2. Styrene/CO Copolymerization with a Drent-type Catalyst System.

Table 1. Styrene/CO Copolymerization in IÙ MeOH T ('C) ΡCO TON (mL) (mL) (bar) 0 2 70 40 PS" 2 0.2 70 40 1.11 2 0.2 40 0.70 50 2 0.2 90 40 PS" 1 0.2 70 40 1.23 4 0.2 70 40 1.66 4 0.4 70 40 2.73 0.4 2.54 4 70 20 4 0.4 70 60 1.78 0

Trial 1 2 3 4 5 6 7 8 9 a

[C pyrl[Tf Nl M 6

g

2

a

w

PDf

9,700 20,800

4,700 4,700

2.1 2.6

6,000 10,000 34,000 12,200 8,400

3,100 6,200 25,300 7,200 5,000

1.9 1.6 1.3 1.7 1.7

b

c

NOTES: See Reference 34 for general procedure. lL = [C pyr][Tf N] TON = kgCP/gPd ^Determined by GPC Polymer produced was primarily polystyrene. 6

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SOURCE: (Chem. Commun., 2002,1394-1395) Reproduced by permission of the Royal Society of Chemistry

optimization of the reaction temperature and CO pressure resulted in no improvement in the productivity of this system (Table 1). The copolymers produced in these studies were analyzed by GPC, NMR, and IR to determined their molecular weight and composition. Copolymer molecular weights were generally modest ( M = 5,000 - 10,000), although the copolymer produced under the optimized conditions had a molecular weight (M ) of 25,000 (Trial 7, Table 1). The polydispersity values were generally narrow suggesting a single-site catalyst system. The polymer produced under the optimal conditions showed a very narrow PDI value of 1.30. IR and NMR spectroscopic characterization of the copolymer showed it to be perfectly alternating copolymer with a predominately syndiotactic microstructure (21). n

n


304 Specifically, IR spectra of the copolymers showed a strong absorption at 1709 cm" for the C=0 stretch, C NMR showed 8 major resonances, and *H NMR showed only the expected copolymer resonances with no resonances attributable to polystyrene (35). Analysis of the ipso carbon region of the C N M R spectrum showed the copolymer to be approximately 80% syndiotactic. The effect of variation of the IL cation and anion was determined using the optimized conditions (Table 2). Choice of the IL anion was expected to be important, since the activity of these catalyst systems are known to be strongly anion dependent (36). Addition of small amounts (> 0.1%) of [C pyr][Br] significantly inhibits the catalyst system (Trial 2). Thus, it is critical that all residual halide be removed from ILs used in these reactions. Polymerization activity in imidazolium ILs decreased along the series [Tf N] « [BETI] > [PF ] > [TfO] » [TFA] (Trials 5-8), which seems to correspond to the coordinating ability of the anion. In the [Tf N] series, the imidazolium salt gave a slightly lower yield than the pyridinium IL (Trials 1 and 6), although the difference is only about twice the standard deviation (0.112 kg CP/g Pd) obtained in repeated runs under optimal conditions. Copolymer molecular weight and polydispersity were not affected by choice of IL solvent. Concurrent with our own work, Seddon (37) reported the copolymerization of styrene and CO with a similar catalyst system and found that [C pyr][Tf N] gave optimal results. 1

13

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Styrene/CO Copolymerization with a Cationic Palladium Complex The Drent-type catalyst system produces a palladium complex with moderately weakly coordinating ligands under the reactions conditions. Since

Table 2. IL Solvent Effects in the Styrene/CO Copolymerization* Trial IL TON M M PDf 1 [C pyr][Tf N] 1.3 34,000 25,300 2.73 2 [QpyrHTfiN]'' 0.01 3 [C pyr][BETI] 11,900 1.5 2.51 18,200 4 [C pyr][TFA] 0.00 5 [C mim][BETI] 12,300 1.5 2.85 18,600 6 [C mim][Tf N] 20,500 12,600 1.6 2.53 7 [C mim][PF ] 20,100 11,600 1.7 2.35 8 [C mim][TfO] 1.5 1.74 20,300 13,200 9 [C mimlfTFA] 0.03 0

c

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NOTES: See reference 34 for general procedure. *TON = kgCP/gPd determined by GPC *[C pyr][Br] 0.5% (w/w) in [C pyr][Tf N] SOURCE: (Chem. Commun., 2002,1394-1395) Reproduced in part by permission of the Royal Society of Chemistry 6

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305 /7-toluenesulfonic acid is used in large excess relative to palladium (ca. 20:1 /?TsOH:Pd), there is a significant concentration of weakly coordinating anions (TsO" and AcO") in the reaction mixture. We were interested to see what effect IL solvents would have on a catalyst system where there were no coordinating anions present. Brookhart (38) has shown that preformed cationic complexes, which are analogous to the presumed active species in the Drent-type system, are active for the copolymerization of styrene and CO. Copolymerizations were carried out with (bipy)Pd(CH )Cl in combination with NaB(Arp)4 or AgSbF (Figure 3). Initial optimization was carried out in [C6pyr][Tf N]. Our initial results showed that catalysts derived from AgSbF gave higher yields of copolymer than those made from NaB(Ar )4 (Table 3, Trials 1 and 2 vs. 3 and 4) (39). Further optimization was performed with the AgSbF derived catalyst. This catalyst system showed little pressure dependence on catalytic activity. There was a dramatic increase in copolymer yield as the temperature was raised to 50 C, but the yield decreased at higher temperature (Trials 5-8). An increase in copolymer molecular weight was also observed at 50 °C (M = 20,000) compared to lower temperatures (M « 12,000) Copolymers formed at 50 °C had larger polydispersities than those made at room temperature. IR and NMR spectra of these copolymers were identical to those obtained in the Drent system described previously/55^ 3

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2

6

F

6

e

n

N

N

\

n

/CH3 M Y

/^\

Solvent

Cl

M Y = NaB(Ar ) ,AgSbF . T3(Ar ) = B~Kx F

4

6

F

4

a

Figure 3. Brookhart-type catalyst system

At 50 °C, copolymerization in [C pyr][Tf N] gave approximately 3 times more copolymer than was produced in methylene chloride (Trial 9). Molecular weight values and polydispersities were nearly identical in the two solvents. In contrast to the Drent system, higher copolymer yields were obtained in [C mim][OTf] than in the [PF ] IL (Trials 10-11, Table 3). A 1,106

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306 Table 3. Styrene/CO Copolymerization Catalyzed by (bipy)Pd(Me)CI/MY" PDf Trial Solvent M Τ TON M ΡCO CO (bar) 2.45 1 24,200 9,900 [C pyr][Tf N] 23 3.4 0.07 2 [C pyr][Tf N] 16,200 10,300 1.57 23 0.11 6.9 3 [C pyr][Tf N] 23 26,000 14,600 1.78 3.4 0.18 4 [C pyr][Tf N] 28,500 15,300 1.87 23 6.9 0.14 5 15,500 12,500 1.24 [C pyr][Tf N] 23 13.8 0.13 6 70,900 24,700 2.87 [C pyr][Tf N] 40 0.27 6.9 7' 90,000 20,500 4.39 [C pyr][Tf N] 50 1.14 6.9 8 11,600 2.24 [C pyr][Tf N] 25,900 60 0.59 6.9 9 57,000 18,700 3.05 CH C1 50 0.40 6.9 1.47 10 10,800 [C mim][OTf] 16,000 50 0.97 6.9 11 [C mim][PF ] 25,400 9,700 2.63 50 0.56 6.9 12 [C pyr][Tf N] 50 1.22 6.9 0

c

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NOTES: See reference 39 for procedure. MY = AgSbF unless noted. Reaction time = 12 h. T O N = kgCP/gPd 'Determined by GPC *MY = Na[B(Ar ) ] (phen)Pd(Me)Cl 6

è

e

F

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phenanthroline (phen) based catalyst gave about the same yield as was obtained with the bipyridine catalyst (Trial 12). The crude polymer produced in this system is gray, suggesting that it is contaminated with palladium metal. In contrast, the polymer produced by the Drent-type system was pale yellow in color. Copolymerizations were carried out with different reaction times to probe for catalyst deactivation (Figure 4). A plot of TON vs. time shows that the catalyst was most productive in thefirst2 hours, after which it continued to produce polymer at a slower rate for up to 12 hours. After the first two hours, the catalyst produces polymer at a fairly constant rate of approximately 0.08 kgCP/gPd»h. The cumulative TOF decreases slowly throughout the reaction period reflecting the lower activity after the first 2 hours. The apparent increase in Pd precipitation compared with the Drent-type catalyst system may reflect catalyst decomposition upon workup rather than during the reaction period.

Homopolymerization of Ethylene in IL Solvents Based on the accelerating effect of ILs on the copolymerization of styrene and CO using a cationic palladium complex, we were encouraged to try a class of related ethylene polymerization catalysts (Figure 5). Palladium and nickel (40-44), as well as cobalt and iron (45-47), catalysts supported by bulky diimine


307


ligands have been shown to be active for the polymerization of ethylene and aolefins. In each system, the active species is thought to be a cationic, monoalkyl metal complex with a non-coordinating anion to provide an open site for alkene coordination. Based on our hypothesis and results with the styrene/CO copolymerization, we would expect ILs to accelerate the polymerization.

Figure 4. Copolymerization productivity as a function of time. f B TON; · = TOFfrom start of reaction; Δ = incremental TOF.

:

Ethylene polymerization using catalyst 1 in combination with NaB(Ar )4 and AgSbF was carried out in IL solvents and compared to results obtained in methylene chloride (Figure 5, Table 4). Ethylene polymerization in IL solvents gave much lower yields than were obtained in methylene chloride. Of the systems tested, the combination of l/AgSbF in [C6mim][PF ] gave the highest yield of polyethylene (0.19 kgPE/gPd). The best activity obtained in IL solvents was an order of magnitude lower than that observed in methylene chloride. GPC analysis showed the polymer was moderately low molecular weight material (M„ ~ 10,000). The molecular weight of polyethylene made in ILs was an order of magnitude lower than for polyethylene made in methylene chloride under identical conditions. The branching ratio ranged from 55-89 branches per 1000 C's as determined by *H NMR for polyethylene prepared in ILs, which is comparable to that obtained in methylene chloride. F

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308 Branched Polyethylene Ar H C^N 3

N

Ionic Liquids as Green Solvents - American Chemical Society

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