Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Design of Betaine Functional Catalyst for Efficient Copolymerization of Oxirane and CO2 Kaihong Chen,†,‡ Guiling Shi,† Haoran Li,† Congmin Wang,*,† and Donald J. Darensbourg*,‡ †
Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, Hangzhou 310027, China Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
‡
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S Supporting Information *
ABSTRACT: A new Co(III) salen betaine functional catalyst is reported which displays high activity and high selectivity for the copolymerization reaction of oxirane and carbon dioxide. Importantly, this catalyst system is not only effective for propylene oxide and CO2 but also efficient for more difficult epoxides to selectively provide copolymers. For example, styrene oxide and 2vinyloxirane are readily coupled with CO2 to provide the corresponding completely alternating copolymers. This catalyst system is thereby able in several instances to avoid the use of tediously prepared bifunctional Co(III) salen catalysts. Through a combination of control experiments, MALDI-TOF measurements, and DFT computations, the addition of a cocatalyst is shown to play a significant role in this process.
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INTRODUCTION The transformation of CO2 into important industrial products has received much recent attention, since CO2 is an abundant, nontoxic, and renewable C1 resource.1 The synthesis of polycarbonates from CO2 and epoxides represents an important contribution for the utilization of carbon dioxide in the production of polymeric materials of much commercial value.2−6 Since the early disclosure of Inoue on the first example of the copolymerization of CO2 and epoxides using a poorly defined heterogeneous zinc catalyst, significant advances in catalyst development have been achieved.7,8 These include a large variety of metal systems.9−19 Presently, the most widely studied among these are binary systems consisting of Cr(III) or Co(III) salen derivatives in the presence of an onium salt initiator such as quaternary ammonium or phosphonium salts. These catalyst systems are in several instances very effective at affording perfectly alternating copolymers, often with high levels of regio- and stereoselectivity. Nevertheless, these catalyst systems often exhibit a prolonged induction period at low catalyst loading. The most detrimental feature of these binary metal salen catalysts is their tendency to promote the coupling of CO2 and epoxides to yield the cyclic carbonate product. This becomes a particular issue for reactions performed at elevated temperatures and processes involving epoxides with electron-withdrawing substituents. To compensate for these shortcomings, bifunctional catalysts have been developed which contain the onium salt appended to the salen ligand.20−31 These catalysts resist cyclic carbonate production involving a wide variety of epoxides, including reactions performed at elevated temperatures. Notwithstanding, because of the very tedious procedures for the synthesis of these bifunctional catalysts, their applications have been significantly restricted.23,28 © XXXX American Chemical Society
Thus far, limited efforts have been expended at modifying the nature of the fifth ligand (X) in the five-coordinate (salen)MX (M = Cr(III) or Co(III)) complexes and the effect of that modification on the CO2/epoxide coupling process. Related to this subject, task-specific anion- and cationfunctionalized ionic liquids (ILs) have exhibited outstanding performance in carbon capture and utilization (CCU) processes.32−50 In this connection, we have employed a betaine functionalized ILs as the precursor in the synthesis of the catalyst (Figure 1). This modification was inspired by the cation on the appended arm of the salen ligand, ensuring the high activity and selectivity noted for bifunctional metal salen catalysts (Figure 2). Herein, we report on the copolymerization of propylene oxide and CO2 using these axially modified catalysts 2a and 2b (Figure 1). In addition, the effectiveness of these catalysts on the more difficult to copolymerize with CO2 epoxides, styrene oxide, and 2-vinyloxirane is reported. These latter epoxides were chosen because, along with epichlorohydrin, these represent the most challenging monomers to provide copolymers with CO2 thus far investigated with (salen)CoX catalyst systems.
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RESULTS AND DISCUSSION As indicated in Figure 1, catalysts 2a and 2b were synthesized from betaine hydrochloride and betaine 2,4-dinitrophenolate (DNP) precursors and used in these copolymerization reactions. Table 1 contains data for the copolymerization of propylene oxide and CO2 under various reaction conditions as well as the nature of the catalyst. These new catalysts exhibit Received: May 25, 2018 Revised: July 12, 2018
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DOI: 10.1021/acs.macromol.8b01103 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Catalysts used in this work.
Figure 2. Traditional bifunctional catalysts.
Table 1. Coupling Reaction of CO2 and POa
entry
cat.
1 2 3 4 5d 6 7 8d,e 9f 10g 11h
1 2a 2b 2a 2a 2b 2b 2b 1 3a 4
cocat. betaine
PPNDNP PPNDNP PPNDNP PPNBF4 PPNBF4 betaine + PPNBF4
convb (%)
TOFb (h−1)
selectb (polymer %)
Mnc (kg/mol)
PDIc
22.1 28.7 29.1 76.3 26.5 84.0 66.2 78.1 56.2 24.5 38
111 144 146 382 2650 420 331 1562 281 612 253
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 99
4.8 3.7 9.7 23.0 5.6 18.5 38.3 26.9 17.9 134.6 12.6
1.02 1.47 1.06 1.52 1.10 1.45 1.03 1.16 1.17 1.09 1.13
a Reaction conditions: PO:catalyst:cocatalyst = 1000:1:1, CO2 (1.5 MPa), temperature = 25 °C, reaction time = 2 h. bDetermined by 1H NMR spectroscopy. cBased on GPC in THF. dReaction time = 0.1 h. eTemperature = 60 °C. fPO:catalyst:betaine:PPNBF4 = 1000:1:1:1. gReference 23; PO:catalyst = 10000:1, reaction time = 4 h. hReference 20; PO:catalyst = 2000:1, reaction time = 3 h; CO2 (1.4 MPa).
slower rates of polymerization than that of the traditional bifunctional Co(III) salen catalysts (entries 10 and 11). However, these new catalysts show excellent selectivity for copolymer formation even at 60 °C. More importantly, these catalysts are much easier to synthesize than their state of the
art bifunctional counterparts and hence are more readily available. The activity of catalysts 2a and 2b was greatly enhanced in the presence of an added cocatalyst (entries 4−6). It is wellestablished that the anion of the cocatalyst can initiate the B
DOI: 10.1021/acs.macromol.8b01103 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 3. MALDI-TOF mass spectra of polypropylene carbonate (PO:2a:PPNDNP = 1000:1:1, CO2 (1.5 MPa), 25 °C, 0.1 h).
Figure 4. DFT optimized structure of 2b in the presence and absence of the BF4 anion.
activity and molecular weight are decreased and PDI is increased slightly. This is consistent with a greater quantity of adventitious water which serves as a chain transfer agent being present in entry 9 because of the mode of catalyst addition. In an attempt to explain the augment in catalytic activity noted upon addition of PPNBF4 to catalyst 2b, DFT calculations were performed to establish the site of binding of the dinitrophenolate ion in this instance (DFT of cation added in SI). That is DNP to the metal center in 2b is approximately 21.6 kcal/mol more stable than to betaine. It should be noted parenthetically that we have not been able to obtain suitable crystals of 2b for X-ray structural analysis. However, we have determined an X-ray structure of complex 2b in the presence of an added equivalent of betaine which clearly shows two betaine ligands bound to the cobalt center (see the Supporting Information). This structure strongly suggests betaine is a better ligand, binding via carboxylate group, than dinitrophenolate. The significantly greater copolymer chain ends with DNP as opposed to betaine are consistent with this observation.
copolymerization process. In this regard, the main chain sequence of the copolymer product obtained in the presence of the catalyst system 2a and PPNDNP was analyzed by MALDITOF mass spectroscopy. As illustrated in Figure 3, three series of signals are found, all of them having the same regular interval of 102 m/z but with a different end group, i.e., DNP, Cl, or betaine. Of the three, betaine was the least active, and DNP was the most active at initiating the copolymerization process. When 2b is utilized in the presence of the non-nucleophilic anion from the cocatalyst PPNBF4, the activity for the copolymerization of propylene oxide and CO2 is greatly enhanced (entries 3 vs 7), i.e., TOF 146 h−1 vs 331 h−1. In Table 1, the significant decrease in TOF in entries 4 and 5 is not due to catalyst degradation; instead, it is due to the extended reaction time of entry 4 (2 h) relative to entry 5 (0.1 h). Upon raising the temperature to 60 °C, catalytic activity was further increased (TOF of 1562 h−1 for a 0.1 h run, entry 8) while maintaining high selectivity for copolymer formation. In Table 1, entry 9, where the catalyst system composition is very similar to that of entry 7, relative to the latter, catalytic C
DOI: 10.1021/acs.macromol.8b01103 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 2. Coupling Reaction of Styrene Oxide and CO2a
entry
cat.
cocat.
convb (%)
TOFb (h−1)
selectb (polymer %)
Mnc (kg/mol)
PDIc
1 2 3 4 5d
2a 2a 2b 2b 3b
PPNDNP PPNDNP PPNBF4
4.7 89.3 84.0 32.1 24.0
3 60 56 21 60
57 97 >99 >99 >99
27.6 29.7 17.6 76.4
1.17 1.17 1.21 1.03
Reaction conditions: SO:catalyst:cocatalyst = 1000:1:1, CO2 (3 MPa), temperature = 25 °C, reaction time = 15 h. bDetermined by 1H NMR spectroscopy. cBased on GPC in THF. dReference 25. Reaction conditions: SO:catalyst = 3000:1, CO2 (2 MPa), temperature = 25 °C, reaction time = 12 h. a
Table 3. Coupling Reaction of CO2 and 2-Vinyloxiranea
entry
cat.
cocat.
convb (%)
TOFb (h−1)
selectb (polymer %)
Mnc (kg/mol)
PDIc
1 2 3 4 5d 6e
2a 2a 2b 2b 1 5
PPNDNP PPNDNP PPNBF4 PPNDNP
