Article pubs.acs.org/Macromolecules
Hyperbranched Polyethylenes Encapsulating Self-Supported Palladium(II) Species as Efficient and Recyclable Catalysts for Heck Reaction Pingwei Liu,†,‡ Zhibin Ye,†,§,* Wen-Jun Wang,‡,* and Bo-Geng Li‡ †
Bharti School of Engineering, Laurentian University, Sudbury, Ontario P3E 2C6, Canada State Key Lab of Chemical Engineering, Institute of Polymerization and Polymer Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China § Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario P3E 2C6, Canada ‡
S Supporting Information *
ABSTRACT: We demonstrate in this paper a unique selfsupporting strategy for the synthesis of hyperbranched polyethylenes (HBPEs) encapsulating Pd(II) species as efficient and recyclable catalysts for Heck reaction. This strategy combines remarkably the synthesis of HBPE polymer support and the immobilization of Pd(II) species in situ in one pot. It is achieved by chain walking copolymerization of ethylene with an acrylate comonomer (2) containing a disulfide functionality with a cationic Pd(II)−diimine catalyst (1). The copolymerization renders successfully HBPEs containing pendant disulfide groups at a controllable content, while with no poisoning effect from the disulfide functionality. Meanwhile, the Pd(II) catalyst in situ immobilizes itself onto the polymers by coordinative binding with the pendant disulfide groups, giving rise to the homogeneous self-supported Pd(II) catalysts. The resulting Pd-containing HBPEs having a low content of 2 (ca. < 1 mol %) have been found to be efficient and recyclable catalysts for the Heck reaction of iodobenzene and n-butyl acrylate while with a low/minimum Pd leaching (to form catalytically active species) during the reaction/recycling. In particular, the convenient recovery and reuse of the catalysts while at maintained high catalytic performance has been demonstrated with the use of a biphasic solvent system comprised of N,N-dimethylformamide and n-heptane in the Heck reaction.
■
INTRODUCTION Pd-based catalysts play a key role in modern organic chemistry. They have been extensively used in a large number of reactions for the synthesis of numerous organic small molecules and macromolecules.1 Some typical examples include the carbon− carbon bond forming reactions, like the notable Heck2 and Suzuki3 coupling reactions, and polymerization reactions such as olefin polymerization4 and olefin/CO alternating copolymerization.5 Given the high cost of Pd as one of the rarest elements, the recovery and reuse of Pd-based catalysts in organic coupling reactions is an important theme of great economic and environmental significances. To this end, a common solution is to immobilize the Pd-based complexes or Pd(0) nanoparticles onto a support, such as carbon, silica, polymers, metal oxides, etc.,6 to render recyclable supported catalysts with minimum/negligible catalyst leaching/loss in organic reactions. In the case of polymer support that is our primary interest, a great variety of insoluble cross-linked polymers and soluble polymers encapsulating Pd organometallic complexes or nanoparticles have been developed with the use of different catalyst encapsulation strategies. In particular, the use of soluble polymers as catalyst supporting materials has received increasing attention in recent years. In © 2012 American Chemical Society
addition to the advantage of homogeneity during synthesis and characterization, soluble-polymer-supported catalysts allow the catalytic reactions to be carried out under homogeneous conditions to achieve the maximized catalytic performance, meanwhile with the ease in the separation and recycling facilitated by the polymer supports.7 Various types of soluble polymers, including poly(ethylene glycol) (PEG),8 polystyrene (PS),9 polyisobutylene,10 poly(Nisopropylacrylamide) (PNIPAM),11 and dendrimers,12 have been extensively studied as the supports for Pd catalysts. Very often, these polymers contain specific functionalities (e.g., ligands) for Pd immobilization/entrapment and thus require special synthesis or postpolymerization modifications. Among them, dendrimers are particularly attractive because of their valuable characteristic structural features, such as well-defined three-dimensional symmetrical structure, controllable monodisperse size, tailored nanoenvironment, good solubility, and the presence of multiple functionalities.12 Extensive research has been undertaken on the synthesis of various dendrimers Received: October 17, 2012 Revised: November 30, 2012 Published: December 17, 2012 72
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
Scheme 1. Synthesis of Hyperbranched Polyethylenes (HBPEs) Containing Self-Supported Pd(II) Species by Copolymerization of Ethylene and 2-(2′-Bromoisobutyryloxy)ethyl 2″-Acryloyloxyethyl Disulfide (2) Facilitated with Pd−Diimine Catalyst 1
situ in one pot (see Scheme 1). The HBPE support, which contains disulfide functionality as Pd(II) binding sites, is obtained by chain walking copolymerization of ethylene with an acrylate comonomer having a disulfide functionality, 2-(2′bromoisobutyryloxy)ethyl 2″-acryloyloxyethyl disulfide (2), using a chain walking Pd(II)−diimine catalyst (1).4,20,21 Following the polymerization, the Pd(II) species are autonomously immobilized onto the hyperbranched polymer by their coordination with the covalently tethered disulfide functionality. The catalytic performance and recyclability of these HBPE-supported catalysts in the Heck reaction, along with the effects from the composition of the HBPE support, have also been systematically studied. To the best of our knowledge, this is the first report on the synthesis of hyperbranched-polymersupported Pd(II) catalysts for coupling reaction via a selfsupporting approach, wherein the Pd(II) species play dual catalytic roles for both the synthesis of polymer support and subsequent coupling reaction.22 This strategy differs from conventional synthesis of polymer-supported Pd catalysts, which requires the preparation of polymer support followed with a separate step for catalyst immobilization. Meanwhile, it is also distinctively different from a previously reported selfsupporting strategy for the synthesis of heterogeneous chiral Pd(II) catalysts for enantioselective reactions, which involves the supramolecular interactions of Pd(II) ions with multifunctional chiral ligands.23
encapsulating Pd nanoparticles/complexes and their versatile applications in catalytic reactions. However, the preparation of dendrimers is inconvenient and costly, often requiring a multistep sophisticated synthesis. In this regard, hyperbranched polymers mimicking the architecture of dendrimers should be the ideal alternative to replace dendrimers. Though containing structural imperfection, hyperbranched polymers retain most desired properties of dendrimers. They can be easily produced in a large scale by convenient one-pot one-step synthesis and their supported catalysts are thus more promising for industrial applications.13 Thus far, there are, however, only limited examples on the use of hyperbranched polymers as the soluble support for Pd catalysts (complexes or nanoparticles/nanoclusters). Mecking et al. first reported the preparation of amphiphilic hyperbranched polyglycerols encapsulating palladium nanoclusters as the catalysts for hydrogenation of cyclohexene,14 in both batch15 and continuously operated membrane reactors.16 Therein, the weakly coordinating OH groups in the hyperbranched polyglycerols can stabilize the Pd(0) nanoclusters. Meanwhile, they further developed optically active hyperbranched polyglycerols encapsulating Pd(0) nanoparticles, which catalyzed the Heck reaction of 2,3-dihydrofuran and phenyl triflate successfully with a moderate yield.16 In another case, hyperbranched aramids have also been used to stabilize Pd(0) nanoclusters, rendering effective, selective, and recyclable catalysts for the hydrogenation of various unsaturated substrates.17 Further, a study of using hyperbranched polyamidoamines (an analogue of poly(amidoamine) dendrimers) to stabilize gold nanoparticles has proved that, the open architecture of hyperbranched polymers facilitates the interactions between metal ions and the functional groups of polymer.18 Despite the limited reports, hyperbranched polymers hold great promise as platforms for catalysts due to their distinctive advantages.19 In this article, we demonstrate the facile synthesis of a novel range of hyperbranched polyethylenes (HBPEs) encapsulating self-supported Pd(II) complexes as efficient and recyclable soluble supported Pd catalysts for the Heck reaction of iodobenzene (IB) and n-butyl acrylate (BA). The unique feature of this homogeneous supported catalyst system is that the synthesis of the HBPE support and the coordinative immobilization of the Pd(II) complexes are accomplished in
■
EXPERIMENTAL PART
Materials. All manipulations involving air- and/or moisturesensitive materials were conducted in an N2-filled glovebox or by using Schlenck techniques. The Pd−diimine catalyst, [(ArN C(Me)−(Me)CNAr)PdMe(NCMe)] + SbF 6 − (Ar = 2,6(iPr)2C6H3) (1), was synthesized by following a literature procedure.4a,b 2-(2-Bromoisobutyryloxy)ethyl acrylate (BIEA) was prepared by following a literature procedure.24 Ultrahigh-purity N2 (>99.97%) and polymer-grade ethylene (both obtained from Praxair) were purified by passing sequentially through a 3 Å molecular sieve column and an Oxiclear column before use. HPLC-grade dichloromethane (>99%), tetrahydrofuran (THF) (>99%) from Fisher Scientific were deoxygenated and dried by using a commercial solvent purification system (Innovative Technology) before use. Other chemicals, including 2hydroxyethyl acrylate (96%), 2-hydroxyethyl disulfide (technical grade, 90%), α-bromoisobutyryl bromide (98%), acryloyl chloride (≥97%), triethylamine (≥99%), n-heptane (anhydrous, 99%), iodobenzene (98%), n-butyl acrylate (≥99%), hydrogen peroxide (50 wt % in 73
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
Table 1. Synthesis of Hyperbranched Polyethylenes (HBPEs) Encapsulating Self-Supported Pd(II) Species by Chain Walking Copolymerizationa GPC resultse
polymer
amt of 1 (mmol)
HBPE1 HBPE2 HBPE3 HBPE4k HBPE5
0.15 0.30 0.30 0.30 0.15
comonomer
[M]0b (mol/L)
polymer productivityc (kg/ mol Pd)
comonomer contentd (mol %)
Mn (kDa)
2 2 2 2 BIEA
0.08 0.25 0.50 1.00 0.08
39 26 12 6 42
0.23 0.86 2.06 4.49 0.30
56 42 29 18 22
PDI
NS−Sf
Pd contentg (10−5 mol/g)
NPd
3.1 3.2 4.5 6.2 1.4
4.5 11.6 16.9 18.2 0
1.8 2.6 6.4 13.5 0.9
1.0 1.1 1.8 2.4 −
h
Pd encap, %i
branch densityj (per 1000 C)
74 71 74 72 41
94 94 95 95 94
a
Other polymerization conditions: solvent, CH2Cl2; total volume, 10 mL for HBPE1, HBPE3, and HBPE5, 20 mL for HBPE2, 5 mL for HBPE4; ethylene pressure, 1 atm; temperature, 25 °C; time, 24 h. bComonomer feed concentration. cPolymer productivity in kg-polymer produced per molPd catalyst. dMolar percentage of comonomer in the polymer. eNumber-average molecular weight (Mn) and polydispersity index (PDI) determined through GPC characterization with universal column calibration. fAverage number of disulfide groups (−S−S−) per polymer chain estimated from the molar content of 2 and Mn. gMoles of Pd in 1 g of the polymer determined with atomic absorption spectroscopy. hAverage number of Pd(II) species per polymer chain estimated from the Pd content and Mn. iPercentage of Pd encapsulation in the polymer estimated from the amount of Pd catalyst 1 used and the actual amount of Pd encapsulated. jBranching density of polymers determined with 1H NMR spectroscopy. kHBPE4 contained a small fraction of insoluble components. water), as well as N,N-dimethylformamide (DMF) (HPLC, ≥ 99%), toluene (HPLC, > 99%), potassium carbonate (anhydrous, > 99%), hydrochloric acid (36.5 wt % in water), and methanol from Fisher Scientific, were all used as received. Synthesis of 2-(2′-Bromoisobutyryloxy)ethyl 2″-Acryloyloxyethyl Disulfide (2). A two-step procedure was employed for the synthesis of 2. 2-Hydroxyethyl 2-(2′-bromoisobutyryloxy)ethyl disulfide was first synthesized as follows. 2-Hydroxyethyl disulfide (16.8 g, 0.096 mol) and NEt3 (4.9 g, 0.048 mol) were dissolved in anhydrous THF (120 mL) in a Schlenk flask. The flask was placed in an ice bath. The mixture was stirred under the protection of nitrogen. αBromoisobutyryl bromide (7.6 g, 0.032 mol) dissolved in 40 mL of THF was added dropwise over a 2-h period under nitrogen protection. The mixture was further stirred in the ice bath for another 1 h, then warmed to room temperature, and stirred overnight. The resulting yellow-orange dispersion was filtered to remove the salt produced. The solvent in the filtrate was evaporated under vacuum. CH2Cl2 (60 mL) was then added and the solution was washed with deionized water for five times (ca. 20 mL each time). The product solution in CH2Cl2 was then dried with Na2SO4 overnight. 1H NMR (CDCl3, δ, ppm): 4.45 (m, 2H, C(O)OCH2), 3.89 (t, 2H, CH2OH), 2.97 (t, 2H, C(O)OCH2CH2S−S), 2.89 (t, 2H, S−SCH2CH2OH), and 1.94 (s, 6H, C(O)C(CH3)2Br). The dried product solution obtained above was placed in an ice bath and was added with NEt3 (6.6 g, 0.065 mol). Under stirring, acryloyl chloride (5.9 g, 0.065 mol) in 30 mL of CH2Cl2 was added dropwise into the solution over a period of 2 h in the presence of nitrogen protection. The mixture was stirred in the ice bath for another 1 h and then kept at room temperature overnight. After filtration, the resulting CH2Cl2 solution was washed with 1% NaOH aqueous solution (50 mL × 3) and deionized water (80 mL × 3), and then dried with Na2SO4 overnight. The removal of solvent by evaporation under vacuum yielded the brownish oil product 2 (9.0 g, 77% yield based on the amount of α-bromoisobutyryl bromide used in the first step). 1H NMR (CDCl3, δ, ppm): 6.43 (d, 1H, CH2CH), 6.14 (dd, 1H, CH2CHC(O)O), 5.87 (d, 1H, CH2CH), 4.42 (t, 4H, CH2O(O)C), 2.96 (t, 4H, CH2S), and 1.94 (s, 6H, C(O)C(CH3)2Br). Copolymerization of Ethylene with 2 for Synthesis of HBPEs Encapsulating Self-Supported Pd(II) Catalysts. All copolymerizations of ethylene and 2 (or BIEA as control) were conducted in a 50 mL Schlenk flask equipped with a magnetic stirrer at room temperature. The following is the typical polymerization procedure (taking the synthesis of HBPE1 in Table 1 as an example). The flask sealed with a rubber septum was first flame-dried under vacuum. After being cooled to room temperature, the reactor was purged with ethylene for at least three times, and then filled with ethylene to a pressure of 1 atm. A solution of Pd−diimine catalyst 1 (0.12 g, 0.15
mmol) in CH2Cl2 (5 mL) was injected into the reactor, followed with the immediate addition of the comonomer solution (0.29 g of 2 in 5 mL CH2Cl2). During the polymerization, ethylene pressure was kept constant by continuous feed from a cylinder. After 24 h, the polymer product was precipitated with a large amount of methanol. The precipitate was washed with methanol for 3 times, then redissolved in THF and precipitated in methanol for two cycles. Finally, it was dried overnight at 70 °C under vacuum, rendering HBPE1 (5.9 g). Heck Reaction of IB and BA in Toluene and Recycling of HBPE-Supported Pd Catalysts. Representatively, the following is a procedure employed for the Heck reaction of IB and BA in toluene with HBPE1 ([Pd]0/[IB]0 = 0.05%) as catalyst. A similar procedure was used for the reactions with other HBPE-supported Pd catalysts. HBPE1 (0.07 g) was dissolved in 3 mL of toluene in a 20-mL Schlenk tube equipped with a magnetic stirrer, followed with the addition of IB (0.5 g, 2.4 mmol), BA (0.47 g, 3.6 mmol), and NEt3 (0.37 g, 3.6 mmol). The tube was sealed with a rubber septum and was subject to three cycles of vacuum and nitrogen refill. The mixture was then placed in an oil bath set at 100 °C to start the reaction. Samples were taken at specific time intervals and were analyzed by 1H NMR to monitor IB conversion. The following procedure was typically used to recycle the HBPEsupported catalyst after the reaction. The final reaction mixture was transferred to a centrifuge tube and was then centrifuged at 11 000 rpm to precipitate out the salt, which was further washed with petroleum ether (6 mL × 2) and centrifuged again. The supernatant solutions following centrifugation were combined, followed with the addition of methanol to precipitate out the HBPE-supported catalyst. The catalyst precipitate was further washed with methanol (6 mL × 3) and was then dried overnight under vacuum at room temperature, rendering the recycled catalyst ready for next cycle of reaction. Heck Reaction of IB and BA in Biphasic System and Recycling of HBPE-Supported Pd Catalysts. A typical procedure is as follows. In a 20 mL Schlenk tube was added a solution of HBPE1 (0.1 g) dissolved in n-heptane (4 mL). Subsequently, IB (0.74 g, 3.6 mmol), BA (0.69 g, 5.3 mmol), NEt3 (0.8 mL), and 4 mL DMF were added, followed with 3 cycles of vacuum and nitrogen refill. The mixture was then stirred in an oil bath at 100 °C to start the reaction. Samples were taken at specific time intervals and were analyzed with 1 H NMR in CDCl3 to monitor IB conversion. In the experiment involving the recycling of the HBPE-supported catalyst, the reaction mixture was cooled down to room temperature at the completion of the reaction to allow the occurrence of the phase separation. The bottom DMF phase was then removed with a syringe. Fresh substrate solution containing IB (0.74 g, 3.6 mmol), BA (0.69 g, 5.3 mmol), and NEt3 (0.8 mL) in 4 mL of DMF was added into the remaining heptane phase for the next cycle of the Heck reaction. 74
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
Characterizations and Measurements. Proton nuclear magnetic resonance (1H NMR) spectra of all samples were collected on a Varian Gemini 2000 spectrometer (200 MHz) with CDCl3 as the solvent. A Perkin-Elmer Precisely AAanlyst 400 atomic absorption spectrometer was used to determine the concentration of leached Pd in the coupling reaction solutions. Pd(II) atomic absorption standard solution (1061 μg/mL Pd in 5% HCl) from Sigma-Aldrich was used as the stock solution for the preparation of reference solutions with Pd concentration in the range of 0−10 ppm, which allowed the construction of a calibration curve. An aqueous solution with 11.1 vol % THF, 8.2 vol % H2O2 (30 wt % in H2O), and 6.7 vol % HCl (concentrated) was used as the blank solution. To determine the content of Pd encapsulated in HBPEs, the following procedure was adopted (with HBPE1 as an example). HBPE1 (0.081 g) was dissolved in 3 mL of THF in a 20 mL vial, followed with the addition of 0.5 mL of HCl (37.5%) and 0.25 mL of H2O2 (30 wt % in water). The mixture was then sonicated overnight. Subsequently, 0.8 mL of the solution was taken out and diluted with the blank solution for the analysis. To determine the leached Pd present in the salt produced or the DMF solution in the Heck reactions, a sample of the salt or DMF solution was added into 5 mL of blank solution, followed with sonication and then measurement. Gel permeation chromatography (GPC) characterization of all polymer samples was carried out on a Polymer Laboratories PLGPC220 system equipped with a dual detector array comprised of a differential refractive index (DRI) detector (from Polymer Laboratories) and a four-bridge capillary viscometer (from Polymer Laboratories). One guard column (PL# 1110−1120) and three 30 cm columns (PLgel 10 μm Mixed-B 300 × 7.5 mm) were used. HPLC-grade THF was used as the mobile phase at 1 mL/min. The complete GPC system, including columns and detectors, was maintained at 33 °C. A universal column calibration curve was generated with the use of narrow-distributed polystyrene standards (EasiVial PS-H from Polymer Laboratories) with molecular weight in the range of 580−6 000 000 g/mol. The average molecular weights and molecular weight distribution of all polymer samples were calculated with the universal column calibration curve. Melt rheological characterization of the polymers was conducted on a TA Instruments AR-G2 rheometer. A parallel plate measurement configuration with a diameter of 20 mm and a gap size of about 1.0 mm was used. The measurements were all carried out in the smallamplitude dynamic oscillation mode within a frequency range of 0.01− 100 Hz. To establish the linear viscoelastic region for each polymer, a strain sweep was performed at 10 Hz before frequency sweeps. The characterizations were performed in a temperature range from 15 to 65 °C with an interval of 10 °C. The temperature was maintained within ±0.1 °C with the peltier plate temperature control system. UV−vis spectra of the polymer samples were obtained on a Thermo Scientific Genesys 10S UV−vis spectrophotometer. HPLC-grade THF was used as the solvent. The measurements were conducted in the wavelength range of 190 to 800 nm with an optical path length of 1.0 cm. Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-1230 microscope operated at 80 kV. TEM specimens were prepared by placing a drop of THF solution of polymer samples onto copper grids coated with a holey carbon film, followed with drying at ambient temperature.
reported to facilitate the synthesis of functionalized HBPEs via the copolymerization of ethylene with various functionalized acrylate comonomers.21,25−29 The polar functionalities introduced onto HBPEs often have no or weak Pd binding capability to avoid the possible poisoning of the Pd catalysts. Herein, we have discovered that Pd-diimine catalyst 1 also tolerates disulfide functionality and facilitates effectively the copolymerization of ethylene with 2, which contains a disulfide group in addition to an α-bromoisobutyryl functionality. The copolymerization directly gives rise to HBPEs encapsulating self-supported Pd(II) catalysts. The disulfide-containing comonomer plays the key role in this one-pot strategy rendering the self-supported catalysts. Table 1 lists the copolymerization runs (HBPE1−HBPE4) undertaken at different feed concentration of 2 at an ethylene pressure of 1 atm and 25 °C, along with the results obtained from polymer characterization. A control run (HBPE5) with an analogue comonomer having no disulfide group, BIEA, was also carried out for the purpose of comparison. A drastic difference in the color of the polymerization solution was noted when the different comonomers were used. While the individual solutions of 1 and 2 in dichloromethane were between orange and yellow, the polymerization solution containing both 1 and 2 in the presence of ethylene had a deep red color throughout the polymerization, without the precipitation of Pd(0) black. On the contrary, the polymerization solution containing 1 and BIEA were initially orange, with an increasing amount of Pd(0) black precipitated out as a result of reductive catalyst decomposition during the polymerization. This difference indicates the presence of coordinative interactions between the sulfur atoms on the disulfide groups of monomeric/enchained 2 and the cationic Pd(II)−diimine centers. Such Pd−S coordination should be possible since homogeneous Pd(II) catalysts having various sulfur-coordinating ligands have been developed for various catalytic reactions.30 Clearly, its presence minimized/avoided the formation of Pd(0). This sulfur coordination, however, appears to be labile and replaceable by ethylene coordination since successful copolymerization took place. A very similar polymer productivity was found in the two runs for HBPE1 and HBPE5, respectively, which were carried out at identical conditions except the comonomer (2 vs BIEA). This demonstrates the absence of any apparent poisoning effect resulting from the coordinating disulfide group. After precipitation in methanol at the completion of each polymerization, the polymer product was simply washed and dried, rendering conveniently the various HBPEs containing self-supported Pd species (HBPE1− HBPE4). Polymer characterization with 1H NMR spectroscopy confirms the successful incorporation of 2 in the copolymers. Representatively, Figure 1 shows the 1H NMR spectrum of HBPE3, along with that of HBPE5 for comparison. In addition to the dominant signals corresponding to the ethylene sequences in the region of 0.7−1.5 ppm, characteristic signals (a−e in the Figure) attributed to the incorporated units of 2 are clearly seen. In particular, signal c at 2.96 ppm corresponds to the CH2 protons next to the disulfide group, whose chemical shift is identical to that in the comonomer. This confirms that the disulfide group is well preserved in the copolymers. Meanwhile, signals a and a′ at 2.30 ppm, assigned to the CH2 protons next to the incorporated acrylate ester group, confirm the characteristic 2,1-enchainment of both acrylate comonomers at branch ends. This enchainment pattern is typically
■
RESULTS AND DISCUSSION Synthesis and Characterization of HBPEs Encapsulating Self-Supported Pd(II) Complexes. Chain walking polymerization of ethylene with cationic Pd−diimine catalysts represents a novel strategy for the synthesis of HBPEs and their functionalized polymers.4c,20,21 The unique chain walking mechanism of the catalysts renders the hyperbranched polymer topology through nonlinear growth of the polyethylene chains. Featured with remarkably low oxophilicity and high tolerance of polar functionalities, the Pd−diimine catalysts have also been 75
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
complexes, which are immobilized via their coordinative binding to the tethered disulfide groups. The presence of Pd species in the polymers was quantitatively confirmed with atomic absorption (AA) spectroscopy after their complete extraction with H2O2 and HCl acid. The Pd content data determined with AA are listed in Table 1. In the copolymers of 2, the Pd content increases from 1.8 × 10−5 to 13.5 × 10−5 mol/g from HBPE1 to HBPE4 but with a nearly constant percentage of Pd encapsulation (in the narrow range of 71−74%). The increase of Pd content should result from the decreasing polymer yield from HBPE1 to HBPE4. The high percentages confirm the encapsulation of the majority of Pd species added following the polymerization. HBPE5 without disulfide groups, however, has a much lower Pd content (0.9 × 10−5 mol/g) and a lower Pd encapsulation percentage (41%) compared to HBPE1. This comparison clearly demonstrates that the higher Pd content and percentage of encapsulation in HBPE1−HBPE4 should result from their tethered disulfide groups. The solutions of HBPEs, catalyst 1, and comonomer 2 in THF were characterized with UV−vis spectroscopy. Figure 2a shows the UV−vis spectra of the solutions of HBPEs at the same Pd concentration of 0.05 mM. HBPE1−HBPE4 have similar absorbance in the wavelength range of 200−520 nm,
Figure 1. 1H NMR (200 MHz) spectra of HBPE3 and HBPE5 in CDCl3. The signals marked with an asterisk (∗) result from trace residual solvents.
observed in olefin-acrylate copolymers synthesized with Pd− diimine catalysts.4b,c,21,25−29 The comonomer content has been calculated from the 1H NMR spectra (see Table 1). With the increase of feed concentration of 2 from 0.08 to 1 M, its molar percentage increases consistently from 0.23% to 4.49% but at decreasing polymer productivity (from 39 to 6 kg/mol Pd). The reduced polymer productivity should be attributed to the lower reactivity of the acrylate comonomer relative to ethylene. Between HBPE1 and HBPE5, a slightly higher comonomer percentage is noticed in HBPE5, indicative of the slightly higher reactivity of BIEA relative to 2. On the basis of the methyl, methylene, and methine signals of the ethylene sequences in the 1H NMR spectra, the branching density data in the polymers were calculated (see the data listed in Table 1). Characteristic of Pd−diimine polyethylenes, all the polymers are highly branched with a similar branching density of ca. 95 branches per 1000 carbons, regardless of the different comonomer and its concentration. Such highly branched structures result from the characteristic chain walking mechanism of the Pd−diimine catalyst.20,21,31,32 All the polymer products are oil-like liquids but with colors (see Figure S1 in Supporting Information) despite extensive wash applied, indicating their encapsulation of Pd species. The copolymers of 2, HBPE1−HBPE4, show a color between red and brown while HBPE5 is black. When dissolved in THF at a concentration of 5.3 mg/mL, solutions of the copolymers of 2 have increasingly darkened yellow-orange color from HBPE1 to HBPE4. In particular, the solution of HBPE4 is turbid, containing a small fraction of insoluble components. After filtration with a 20 nm syringe filter, the solution color of HBPE1−HBPE4 does not change. The solution of HBPE5 is slightly blackish and it becomes nearly colorless after filtration. The different color of the copolymers indicates that the encapsulated Pd species are at different valence states. It has been found in our prior studies that HBPE homopolymers and copolymers synthesized with Pd−diimine often contain black Pd(0) particles resulting from reductive catalyst decomposition in the polymerization.33 Given the black color of HBPE5 in both melt and solution states, the Pd species in HBPE5 likely exist predominantly as physically entrapped Pd(0) particles, some of which can be filtered. On the contrary, the Pd species in HBPE1−HBPE4 are hypothesized to be supported Pd(II)
Figure 2. UV−vis spectra of (a) HBPE1−HBPE5 at [Pd] = 0.05 mM in THF (the extracted HBPE2 was included for comparison) and (b) Pd−diimine catalyst 1, comonomer 2, and their mixture at [Pd] = 0.05 mM and/or [−S−S−] = 0.45 mM in THF (spectrum of HBPE3 is included for comparison). 76
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
Figure 3. TEM image of HBPE4 (a) and HBPE5 (b).
with a peak maximum at 212 nm. The extracted HBPE2, whose encapsulated Pd species have been removed by extraction with H2O2 and HCl acid, shows much reduced absorbance intensity. This indicates that the absorbance in the polymers stems mainly from the encapsulated Pd species. Unlike HBPE1− HBPE4, HBPE5 has absorbance in the whole wavelength range (200−800 nm) though the absorbance is weak above 500 nm. Similar UV−vis spectra have been observed previously with dendrimers encapsulating Pd nanoparticles, with the absorbance in the high wavelength range (>500 nm) resulting from the encapsulated Pd(0) nanoparticles.34 This spectrum thus confirms the presence of Pd(0) nanoparticles in HBPE5. Meanwhile, the absence of absorbance above 520 nm in HBPE1−HBPE4 illustrates that their encapsulated Pd species should be Pd(II) species rather than Pd(0) nanoparticles. Figure 2b shows UV−vis spectra of 1, 2, and their mixtures in THF, along with that of HBPE3. The measurement solutions have the same Pd and/or disulfide concentrations. Both 1 and 2 shows the absorbance in the region from 200−500 nm, with the peak maximum at 213 and 212 nm, respectively. Upon their mixing, the peak shows a red shift to 217 nm and broadens, indicating the occurrence of coordinative complexation between 1 and 2. Comparing the spectra of HBPE3 and the mixture of 1 and 2, a similar absorbance is noted in the range of 240−500 nm, indicative of the similar complexation of Pd(II) species with the disulfide groups in HBPE3. TEM characterization was undertaken on HBPE4 and HBPE5. Figure 3 shows representative TEM images of both polymer samples. Pd(0) nanoparticles/nanoclusters with a size around 3−10 nm are extensively dispersed in HBPE5 while in contrast they are absent in HBPE4 (except possibly impurity particles). In compliance with the above evidence, the TEM results further confirm that the Pd species in HBPE5 are trapped Pd(0) particles while they are Pd(II) species complexed with the tethered disulfide groups in HBPE1− HBPE4. Figure 4 shows the molecular weight distribution curves of the polymers determined with GPC, wherein the molecular weight is calculated on the basis of universal column calibration with polystyrene narrow standards. The number-average molecular weight (Mn) and polydispersity index (PDI) data are summarized in Table 1. While HBPE5 shows a clean monomodal distribution with a PDI of 1.4, HBPE1−HBPE4 have broadened distribution patterns with the presence of a significant shoulder peak at the high molecular weight end (in the range of 2 × 104−2 × 106 g/mol) and a additional weak but
Figure 4. Molecular weight distribution curves of HBPE1−HBPE5. The molecular weight is determined on the basis of universal column calibration.
persistent peak characteristic of ultrahigh molecular weight (>2 × 106 g/mol). From HBPE1 to HBPE4, the peak-maximum molecular weight of the polymers is gradually reduced due to the enhanced chain transfer rates and/or reduced chain propagation rate upon the increase of the feed concentration of 2. The high-molecular-weight shoulder peak and ultrahighmolecular-weight peak of the polymers suggest the presence of intermolecular coupling and cross-linking among the polymers via the coordinative binding of one Pd(II) species with two or more disulfide groups. From HBPE1 to HBPE4, the Mn value is reduced from 56 to 18 kg/mol. However, the PDI value increases from 3.1 to 6.2 due to the enhanced coupling and/or cross-linking among the polymers as a result of the increased Pd content. The Mn and PDI data reported were calculated with the ultrahigh molecular weight peak (its fraction generally 99% is reached after 4 h of reaction in the n-heptane/DMF mixture while it is 91% and 18% in toluene and n-heptane, respectively, after 24 h. This indicates the use of polar DMF is also beneficial to the reaction rate,39 in addition to facilitating the catalyst recovery and reuse through phase separation.
Figure 7. Fractional conversion of IB as a function of reaction time in the Heck reactions of IB and BA with HBPE1−HBPE4 at the [Pd]0/ [IB]0 ratio of 0.02%. Other reaction conditions: [IB]0:[BA]0:[NEt3]0 = 1:1.5:1.5; [IB]0 = 0.56 M; solvent, 3 mL of toluene for the reactions with HBPE1−HBPE3 and 4.2 mL for that with HBPE4; T = 100 °C.
are in consistency with the literature reports that the leached Pd species catalyze the reaction.6c The increase of NS−S from HBPE1 to HBPE4 reduces the leaching of the Pd(II) species necessary for catalyzing the reaction, thus deteriorating the catalytic performance. Appropriate Pd leaching is thus necessary with these supported catalysts, but excessive leaching should be avoided to maintain their recyclability. The catalysts without containing disulfide functionality, Pd−diimine catalyst 1 and Pd(0)-containing HBPE5, were also used for the Heck reaction (see Figure S3, Supporting Information). Both of them are also effective in catalyzing the reaction, but with the lower activity compared to HBPE1. This is attributed to the fast deactivation/loss of active Pd(II) species in the absence of necessary stabilization in the reaction. Visible Pd(0) black colloids were noticed in both reactions. Recycling studies on HBPE1 and HBPE2 were conducted by recovering the supported catalysts after each reaction by precipitation and reusing them in the next reaction for 5 cycles. For the purpose of comparison, the recycling of HBPE5 was also attempted. In each reaction, IB conversion was determined to evaluate the catalytic performance of the recycled catalysts and the amount of leached Pd species was measured with AA. Table 2 summarizes the results from recycling experiments. With both HBPE1 and HBPE2, the catalytic performance of the supported catalysts is well maintained within five cycles of experiments, while at the low Pd leaching in all five cycles. In
Table 2. Recycling Performance of HBPE1, HBPE2, and HBPE5 in the Heck Reactiona cycle I
cycle II b
cycle III
cycle IV
cycle V
HBPE Catalyst
convn (%)
leached Pd (%)
convn (%)
leached Pd (%)
convn (%)
leachedPd (%)
convn (%)
leached Pd (%)
convn (%)
leached Pd (%)
total leached Pd (%)
HBPE1 HBPE2 HBPE5
87 90 78
2.5 2.7 86
90 90 68
2.3 5.0 n.d.c
95 92 ∼0
2.0 1.2 −
92 88 −
0.5 0.9 −
64 96 −
0.2 0.8 −
7.5 10.5 86
a
Reaction conditions: in those with HBPE1, 0.10 g of HBPE1 was used with 0.74 g of IB in 4.4 mL of toluene for a reaction time of 16 h; in those with HBPE2, 0.10 g of HBPE2 was used with 1.08 g of IB in 6.5 mL of toluene for a reaction time of 40 h; in those with HBPE5, 0.175 g of HBPE5 was used with 0.68 g of IB in 4.0 mL of toluene for a reaction time of 24 h. In all runs, [IB]0 = 0.56 M, [Pd]0/[IB]0 = 0.05%, [IB]0:[BA]0:[NEt3]0 = 1:1.5:1.5, and T = 100 °C. bLeached Pd % = the amount of Pd leached/the total amount of Pd added ×100. cNot detected. The leached Pd was too low to be detected with AA. 79
dx.doi.org/10.1021/ma3021739 | Macromolecules 2013, 46, 72−82
Macromolecules
Article
Table 3. Ten-Cycle Recycling Experiment with HBPE1 in a Biphasic Systema cycle IB convn% [Pd] leached (ppm)e leached Pd %f
Ib >99 3.5 11.6
II >99 0.22 0.7
III >99 0.16 0.6
IV >99 n.d.g n.d.
V >99 n.d. n.d.
VI >99 − −
VII >99 − −
VIII >99 − −
IX >99 − −
Xc >99 n.d. n.d.
XId >99 n.d. n.d.
a
Other conditions: 0.10 g of HBPE1, [IB]0 = 0.35 M, [Pd]0/[IB]0 = 0.05%, [IB]0: [BA]0: [NEt3]0 = 1: 1.5: 1.5, a mixture of DMF (4 mL) and nheptane (4 mL) as solvent, T = 100 °C, 16 h. bA conversion of 42% at 2 h and >99% at 4 h. cA conversion of 46% at 2 h and >99% at 4 h. dThe filtered heptane-phase solution of cycle X was used in this run; the conversion is 17% at 4 h, 33% at 6 h, and >0.99 at 24 h. eThe concentration of leached Pd in the DMF phase determined with AA. fLeached Pd % = the amount of leached Pd in DMF phase/the total amount of Pd added ×100. g Not detected. The concentration of leached Pd was too low to be detected with AA.
recycling experiment) in the Heck reaction of electron-deficient 4-iodoacetophenone and BA with the same n-heptane/DMF biphasic solvent.10 Therein, a modest amount of Pd leaching, ca. 0.2−1% of the Pd loading, was also found in each cycle during the recycling.
A ten-cycle recycling experiment was carried out for the Heck reaction of IB and BA with HBPE1 in the n-heptane/ DMF biphasic system. In each cycle, the reaction time was set at 16 h. At the end of each cycle, the reaction solution was cooled to allow the phase separation, followed with the removal of the DMF phase and the addition of fresh substrate solution in DMF for next reaction cycle. In each cycle, no removal of the product in the n-heptane phase was undertaken since its concentration should be quickly saturated after several cycles of reaction and newly generated product should stay in the DMF phase.10,40 Table 3 summarizes the results (IB conversion, the concentration and percentage of leached Pd in the DMF phase in each cycle) obtained in the recycling experiment. From Table 3, a conversion of >99% was achieved in all ten cycles. By comparing the conversion data in cycles I and X, no obvious deterioration in the catalyst performance is observed after ten cycles. The conversion is 42% and 46% in cycles I and X, respectively, at 2 h, and is >99% in both cycles at 4 h. About 11.6% of the total amount of Pd added leached into the DMF phase in the first cycle, followed with negligible leaching in the following nine cycles. However, an obvious change in the color of the heptane phase was noted from cycle I to X. As shown in Figure S5 in Supporting Information, the color of the heptane phase darkened within the first several cycles due to the formation of small Pd(0) clusters. These small Pd(0) clusters may float in the heptane phase and may also be bound to the disulfide functionalities on the HBPE. Afterward, the color of the heptane phase turned to light yellow as the small Pd(0) clusters underwent aggregation in subsequent cycles to form large precipitated Pd(0) colloids (see the picture for cycle X in Figure S5, Supporting Information). The Pd(II) species should be still present after each recycling since the recycled heptane phase always has a yellow/orange color though darkened in the initial cycles. To investigate the effect of these Pd(0) clusters or colloids, the heptane-phase obtained in cycle X was filtered, and the filtrate was then used the catalyst solution for cycle XI. Compared to the conversion of >99% at 4 h in cycle X (TOF = 500), a slower reaction was found in XI, with a conversion of 17% at 4 h (TOF = 85), 33% at 6 h (TOF = 110), and >99% at 24 h (TOF = 83). This indicates that the floating Pd(0) particles have a significant catalytic effect on the reaction. However, in view of the long reaction time (16 h in each cycle) and the high number of cycles (10), as well as the low leached Pd concentration (
