Epoxides

Nov 24, 2015 - Except for the development of efficient catalysts, the synthesis of ... catalyst 1 for COS/epoxides copolymerization to selectively pro...
6 downloads 0 Views 823KB Size
Download PDF
Article pubs.acs.org/Macromolecules

Single-Site Bifunctional Catalysts for COX (X = O or S)/Epoxides Copolymerization: Combining High Activity, Selectivity, and Durability Wei-Min Ren,* Ye Liu, An-Xiang Xin, Song Fu, and Xiao-Bing Lu State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China S Supporting Information *

ABSTRACT: Unprecedented activity (TOF > 270 000 h−1), high polymer selectivity (>99%), and excellent durability (TON > 600 000) were observed in the copolymerization of carbonyl sulfide (COS) and epoxides mediated by the single-site bifunctional chromium catalyst containing a Lewis acidic metal center and a sterically hindered organic base in a molecule, selectively producing the corresponding poly(thiocarbonate)s with completely alternating structure, high molecular weight, and narrow monodispersity. No oxygen−sulfur exchange reaction occurred even at an elevated temperature of 80 °C. Nevertheless, this catalyst was not efficient for CO2/epoxides copolymerization. The presence of CO2 completely inhibits the reactivity of COS. Contrarily, the corresponding Co(III) complex with the same ligand showed very low activity for COS/epoxides copolymerization but excellent activity (TOF > 13 000 h−1) for CO2/epoxides copolymerization at both ambient and high temperatures, affording the polycarbonates with >99% carbonate linkages and high molecular weight up to 467.0 kg/mol.

■

INTRODUCTION Since the seminal discovery of polycarbonate synthesis from the copolymerization of CO2 and epoxides reported by Inoue and co-workers in 1969,1 much attention was focused on developing efficient catalyst systems for enhancing catalytic activity and polymer selectivity.2 Among these studies, M(III)salen (M = Co or Cr) complex-based catalyst systems are currently subject to a resurgence of interest due to their facile synthesis and ease of handling. In 2002, Darensbourg and coworkers reported the use of the Cr(III)-salen catalysts for CO2/ cyclohexene oxide (CHO) copolymerization.3 These complexes have proven to be robust catalysts, producing completely alternating poly(cyclohexene carbonate) (PCHC) and showing high activity as approaching to that of the discrete β-diiminate zinc catalysts.4 However, these Cr(III)-based catalyst systems were inefficient in copolymerizing CO2 with propylene oxide (PO) to afford aliphatic poly(propylene carbonate) (PPC), usually suffering from the concomitant production of cyclic propylene carbonate.5 A breakthrough in this area came with the use of Co(III)salen-based catalyst systems for CO2/epoxides copolymerization,6−8 affording the corresponding polycarbonates with >99% carbonate linkages and nearly 100% copolymer selectivity even © XXXX American Chemical Society

under mild conditions. Notably, the highest activity with a turnover frequency (TOF) up to 26 000 h−1 and a turnover number (TON) up to 31 000 was realized by the bifunctional catalyst, which contains a Lewis acidic metal center and quaternary ammonium salt units in a molecule.8c This catalyst was also highly active even at very low catalyst loading of 0.001 mol %, which provided a copolymer with a molecular weight of up to 285.0 kg/mol. Except for the development of efficient catalysts, the synthesis of more diverse polycarbonates for improving material performance is another important and challenging theme in this area. Poly(thiocarbonate)s, as analogues of polycarbonates only with the substitution of sulfur atom(s) for oxygen atom(s) in the main chains, have attractive features, such as high optical property, remarkable chemical resistance, and excellent heavy-metal capture ability.9,10 These sulfurenriched polymers can be prepared by the condensation reaction of dithiols and phosgene11 or the ring-opening polymerization of five- or six-membered cyclic dithiocarbonates Received: September 24, 2015 Revised: November 9, 2015

A

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules and trithiocarbonates.12 However, the former process is not a “green” synthetic route due to the usage of poisonous reactants, while the latter concerns the low reactive cyclic thiocarbonates. In 2007, Nozaki and co-workers reported an alternative route for the synthesis of poly(thiocarbonate)s by copolymerizing carbon disulfide and episulfides.13 Recently, the binary Cr(III)salen/ionic ammonium salt catalyst system was found to be active in synthesizing poly(propylene monothiocarbonate) from the coupling reaction of PO and carbonyl sulfide (COS, a greenhouse gas produced from the burning of fossil fuels, volcanic eruption, and animal decay).14 The resultant copolymer possessed a perfect alternating nature and high regioselectivity for epoxide ring-opening. However, an increase in reaction temperature from 25 to 60 °C caused significant decreases in both catalyst activity and copolymer selectivity. Additionally, the enhancement of reaction temperature also resulted in the occurrence of the oxygen−sulfur exchange (Scheme 1), observed in both the copolymer and cyclic

Scheme 2. Structure of catalyst

of small quantities of water was unavoidable. It has been well documented that for the reactions involving the ring-opening of terminal epoxides the active salen-Co(III) catalysts are easily reduced to the inactive Co(II) species in the presence of water. Therefore, in the present studies CO2 was purified by passing through 5 Å molecular sieve to remove the trace water. The simple pretreatment of CO2 led to the TOF from 410 increasing to 685 h−1 at the same conditions (entry 2). A rise in temperature from 25 to 100 °C resulted in a dramatic increase of TOF from 685 to 13 500 h−1, but the polycarbonate selectivity was up to 97% (entry 3). To achieve a high TON and therefore a higher molecular weight, we investigated the coupling reaction at a much higher molar ratio of PO to complex 2. Unfortunately, when the reaction was carried out at 100 °C, an increase in the molar ratio of epoxide to catalyst from 10 000 to 100 000 would significantly reduce the catalyst activity (entry 4).16 Indeed, salen-Co(III) is more easily transformed to the corresponding Co(II) species at a relatively high temperature in the presence of H2O.17 For comparison purposes, the copolymerization was performed at ambient temperature. The results show that a high TON of 11 500 was realized at a [PO]/[catalyst] ratio of 100 000; however, the molecular weight of the resultant polymer has not been improved greatly (entry 5). Alternately, in the presence of an organic solvent such as 1,2-dimethoxyethane, quantitative conversion of the epoxide was achieved with 99% polymer selectivity by a prolonged reaction time (entry 6). Notably, the isolated polymer has a high molecular weight up to 467.0 kg/ mol and a narrow molecular weight distributions less than 1.2. It seems that the coupling of epoxides with COS to form poly(monothiocarbonate)s has similar features with that of CO2 to afford polycarbonates, with one exception that the oxygen/sulfur atom exchange possibly occurred in the former reaction (Scheme 1). And two processes are often accompanied by the generation of undesired byproducts, such as polyether or ether linkages randomly dispersed within the copolymer chains and/or the more thermodynamically stable cyclic products. In view of the high efficiency of complex 2 in the copolymerization of CO2 and PO, it was also tested for COS/PO copolymerization. Unfortunately, a very low activity for copolymer formation was observed at ambient temperature (entry 7). A rise in the reaction temperature suppressed the formation of copolymer and concomitantly increased the selectivity for cyclic product formation (entry 8). These results showed that the Co(III)-based catalyst is unsuitable for the COS/PO copolymerization. Prior to the present study, we utilized a binary system composed of salen-type cobaltate complex and a ionic ammonium salt for mediating the coupling

Scheme 1. Copolymerization of Epoxides and COX (X = O or S) and the Possible Occurrence of Oxygen−Sulfur Exchange

products. Therefore, it would be highly desirable to develop an efficient catalyst for selective synthesis of completely alternating copolymers from COS and epoxides with high activities and molecular weights. Herein, we report a single-site bifunctional catalyst 1 for COS/epoxides copolymerization to selectively produce the corresponding copolymers with an unprecedented activity up to 271 000 h−1. The resultant poly(thiocarbonate)s possess completely alternating structure, high molecular weight, and narrow monodispersity. Another purpose of the present study is to aim at a comparison between the single-site Cr(III)-salen catalyst 1 and Co(III)-salen catalyst 2 in the copolymerization of epoxides with both CO2 and COS.

■

RESULTS AND DISCUSSION Previously, we have developed a highly active cobalt-based catalyst 2 (Scheme 2) with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework for CO2 /epoxides copolymerization.8 Complex 2 and its analogues (only with different axial anion) have proven to be efficient catalysts for the copolymerization of CO2 and various epoxides, including epoxides with an electron-withdrawing group.15 As for the copolymerization of CO2/PO, a TOF of 410 h−1 was achieved at ambient temperature under a CO2 pressure of 1.5 MPa (entry 1).8d It is worth noting here that CO2 with industrial purity was used without any treatment, and thus the presence B

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Complex 2-Mediated Copolymerization of COX (X = O or S) and POa entry e

1 2 3 4 5 6f 7 8

[PO]/[catalyst]

temp (°C)

time (h)

TOFb (h−1)

selectivityc (% polymer)

Mnd (kg/mol)

PDId (Mw/Mn)

10000 10000 10000 100000 100000 10000 10000 10000

25 25 100 100 25 25 25 80

6.0 4.0 0.25 6.0 24.0 30.0 24.0 12.0

410 685 13500 33 480 330 34 103

>99 >99 97 99 99 99 60

101.0 133.0 65.2

1.05 1.05 1.25

137.0 464.0 11.5 4.3

1.18 1.15 1.21 1.33

a

The reaction was performed in neat PO (10 mL, 143 mmol) under a CO2 pressure of 1.5−2.5 MPa (entries 2−6) or COS to PO of 2/1 (molar ratio, entries 7 and 8) in a 50 mL autoclave. CO2 (99.995%) was purified by passing through 5 Å molecular sieve to remove the trace water molecule (entries 2−6). Carbonate or monothiocarbonate linkages of the resulted copolymer are all >99% based on 1H NMR spectroscopy. bTurnover frequency (TOF) = moles of product per mole of catalyst per hour. cDetermined by 1H NMR spectroscopy. dDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. eData from ref 8d. fThe reaction was carried out in 1,2-dimethoxyethane solution with PO/1,2-dimethoxyethane = 1/3 (volume ratio).

Table 2. Complex 1-Mediated Copolymerization of COX (X = O or S) and POa entry

COX

[PO]/[catalyst]

temp (°C)

time (h)

TOFb (h−1)

TON

Mnc (kg/mol)

PDIc

1 2d 3 4 5e 6f 7 8 9 10 11 12g

COS COS COS COS COS COS COS COS COS COS CO2 CO2

10000 10000 20000 50000 50000 50000 200000 500000 1000000 2000000 10000 10000

25 25 25 25 50 80 80 80 80 80 25 80

0.5 24.0 1.0 4.0 0.1 0.05 2.0 6.0 12.0 24.0 4.0 1.0

4300 110 4670 3880 112000 271000 31800 30600 29300 25700 99% based on 1H NMR spectroscopy. No oxygen−sulfur exchange reaction occurred as confirmed by 13C NMR spectroscopy. bTurnover frequency (TOF) = moles of product per mole of catalyst per hour. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. dThe molar ratio of N,N′-bis(3,5dibutylsalicylidene)-1,2-cyclohexenediamine−chromium nitrate complex to MTBD was 1/1. eReaction time is 360 s. fReaction time is 180 s. gOnly cyclic carbonate was produced.

reaction of carbon disulfide and epoxides, but unfortunately, no product was found.18 Indeed, only the chromium-based homogeneous catalyst system13,14 and zinc−cobalt double metal cyanide complexes19 could mediate COS/epoxides or CS2/episulfides copolymerization to selectively afford the poly(thiocarbonate)s so far. Therefore, we synthesized the single-site bifunctional Cr(III)salen catalyst 1 with the same ligand as complex 2 for the copolymerization of COS/epoxides. It was found that catalyst 1 exhibited an unprecedented activity of 4300 h−1 at ambient temperature and a high [epoxide]/[catalyst] ratio of 10 000 (Table 2, entry 1). This activity is about 40 times that of the binary system consisting of N,N′-bis(3,5-dibutylsalicylidene)1,2-cyclohexenediamine−chromium nitrate in conjunction with 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) under the same conditions (entry 2). The kinetic studies showed the first-order dependence on catalyst concentration. This is very different from the binary catalyst system, in which a reaction order of 1.58 of catalyst concentration was obtained (see Supporting Information, Figure S1a,b). The resultant poly(propylene monothoicarbonate) has a completely alternating structure. With further increase in the molar ratio of epoxide to catalyst, no obvious changes in copolymerization

rate and polymer selectivity were observed in this reaction (entries 3 and 4). The reaction temperature has a strong influence on the rate (entries 5 and 6). For example, at a [epoxide]/[catalyst] ratio of 20 000, an increase in the temperature from 25 to 50 °C resulted in a dramatic enhancement in TOF from 4300 to 112 000 h−1. An activity of 271 000 h−1 was achieved when the reaction was performed at a higher temperature of 80 °C. Another advantage of the single-site chromium catalyst 1 is its high selectivity for copolymer formation over cyclic thiocarbonate (>99%). It is worth noting that no or negligible poly(propylene oxide) formation was observed in the system when the mixture of catalyst 1 and PO had been treated at 80 °C for 10 min. This result indicates that catalyst 1 is inactive in initiating homopolymerization of PO even at a high temperature and/ or in the absence of COS. Of importance, no oxygen−sulfur exchange reaction occurred, as confirmed by 13C NMR analysis of the resultant copolymer (see Figure S2). As shown in this spectrum, only one signal appears at δ 169.66 ppm, assigned to the carbon atom of monothiocarbonate linkage (−S(O)CO−). This result was far different from the previously reported binary catalyst system for PO/COS copolymerization, in which the C

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Stimulated by our success with this single-site Cr(III) catalyst 1 for the alternating copolymerization of COS and PO, we likewise applied this system to the copolymerization of COS and various epoxides at high [epoxide]/[catalyst] ratios. Pertinent to our study, Zhang and Darensbourg have very recently reported the copolymerization of COS and epoxides with electron-withdrawing group, such as styrene oxide22 and phenyl glycidyl ether,23 employing salen-Cr(III)Cl in the presence of an onium salt. The binary catalyst system provides for efficient coupling reactions even under mild conditions and in some cases allow for the regioselective polymerization. As expected, the catalyst 1 could also operate at high efficiency in catalyzing the copolymerization of COS with various monosubstituted terminal epoxides to selectively afford the corresponding poly(monothiocarbonate)s with more than 99% polymer selectivity at a high [epoxide]/[catalyst] ratio of 500 000 and 80 °C (Table 3, entries 1−4). All the resultant copolymers have perfect alternating structure, and their Mns exceed 100 kg/mol. In an effort to better assess the ability of this single-site bifunctional catalyst, the alicyclic epoxides were used as monomers to synthesize the corresponding polymers. Relevant to these investigations, Zhang and co-workers recently reported the copolymerization of COS and CHO via heterogeneous catalysis of a nanolamellar zinc−cobalt(III) double metal cyanide complex. An activity of 970 g polymer/g catalyst was achieved, although the formed polymer has a alternating degree of 93% and a slight oxygen−sulfur exchange reaction occurring.24 Unfortunately, catalyst 1 showed a low activity for the copolymerization of CHO and COS (entry 5). Interestingly, the addition of trace amounts of PO resulted in rapid conversion of CHO with a TOF of 15 800 h−1 to give the fully alternating poly(cyclohexene monothiocarbonate) (entry 6).25 Significantly, this catalyst system can operate efficiently in copolymerizing COS and cyclopentene oxide, a less reactive epoxide monomer (entry 7). Similarly, no oxygen−sulfur exchange reaction occurred with regard to all terminal and alicyclic epoxides, as confirmed by 13C NMR spectra (see Figures S8−S18). The thermal properties of various poly(monothiocarbonate)s were analyzed using differential scanning calorimetry (DSC) and thermogravimetry (TGA) methods. Compared with the corresponding polycarbonates, these sulfur-modified copolymers generally have reduced Tgs. The thermolysis curves indicate that all the resultant copolymers possess relatively high thermal stability, with the initial decomposition temperatures more than 200 °C and the temperatures at decomposition of 50% of >260 °C.

oxygen−sulfur exchange reaction occurred at an enhanced temperature of 60 °C.14 A catalyst’s power is estimated not only by its activity (TOF) but also by the durability (TON). The latter might be more important for industrial application. To achieve a high TON, we investigated the coupling reaction at high [PO]/[catalyst] ratios of 200 000−2 000 000 in a prolonged time (entries 7− 10). A high TON of 352 000 was obtained in catalyst 1mediated COS/PO copolymerization at a [PO]/[catalyst] ratio of 1 000 000 and 80 °C within 12 h (entry 9). When the reaction was prolonged to 24 h, a higher TON of 616 000 was achieved (entry 10). In this case, the level of catalyst-derived metal residue in the resultant copolymer decreased to 0.7 ppm. The accurate control of polymer molecular weight (Mn) remains an important goal in polymerization catalysis.20 In the present study, the isolated polymers have narrow molecular weight distributions and exhibit monomodal distributions. In contrast, the bimodal distributions were frequently found in the copolymerization of CO2 and PO catalyzed by the corresponding cobalt complex with the same ligand. Moreover, there is a good linear relationship between polymer molecular weight and percentage conversion (see Figure S3). These features are consistent with a controlled polymerization. Notably, an elevated glass-transition temperature (Tg) of 30.0 °C was observed in the copolymer with a high Mn of 220.0 kg/mol (see Figure S4).21 The single-site catalyst 1 was also applied to catalyze the copolymerization of CO2 and PO. At a [epoxide]/[catalyst] ratio of 10 000, almost no activity was found at 25 °C, and only cyclic product with a TOF of 25 h−1 was observed at 80 °C. To better compare the discrepancy in reactivity between CO2 and COS during the coupling with PO, a control experiment was performed by in situ FTIR. Figure 1 displays a typical reaction

■

Figure 1. Changes to the intensity of IR resonances during formation. The plot shows the copolymerization of COS and PO by the use of complex 1 as catalyst (the reaction conditions are in accord with Table 2, entry 4), the addition of CO2, and the copolymerization of CO2 and PO.

CONCLUSION In summary, we have developed a highly active bifunctional Cr(III)-salen catalyst 1 (TOF > 270 000 h−1, TON > 600 000) for COS/epoxides copolymerization to selectively produce the corresponding poly(thiocarbonate)s with completely alternating structure, high molecular weight, and narrow monodispersity. No oxygen−sulfur exchange reaction occurred even at an elevated temperature of 80 °C. However, this catalyst was not efficient for CO2/epoxides copolymerization. The presence of CO2 completely inhibit the reactivity of COS. On the contrary, the Co(III)-salen catalyst 2 showed very low activity for COS/ PO copolymerization, but excellent activity (TOF > 13 000 h−1) for CO2/PO copolymerization at both ambient and high temperatures, affording the poly(propylene carbonate) with

profile of infrared absorbance at 1715 cm−1 due to poly(monothiocarbonate) production with time. When the reaction was performed for 40 min, CO2 was added into the reaction. After this time, the intensity of the (monothio)carbonate linkage resonance stopped growing, and there was no change in the polycarbonate signal at 1750 cm−1. This result indicates that the presence of CO2 completely inhibits the reactivity of COS. This observation is consistent with the copolymerization results. D

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 3. COS/Various Epoxides Copolymerization Resultsa

a The reaction was mediated by complex 1 in neat epoxides (71.5 mmol) at a [COS]/[PO] of 2/1 under 80 °C in a 50 mL autoclave. No cyclic thiocarbonates and ether linkages were found by 1H NMR spectroscopy. No oxygen−sulfur exchange reaction occurred as confirmed by 13C NMR spectroscopy. bTurnover frequency (TOF) = moles of product per mole of catalyst per hour. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards. dGlass transition temperature determined by DSC. eThe temperature at polymer decomposition of 50% measured by TGA. fAddition of 0.7 mmol of PO.

>99% carbonate linkages and high molecular weight up to 467.0 kg/mol.

■

EXPERIMENTAL SECTION

Synthesis of Complex 1. The ligand (1.34 g, 2.0 mmol) and CrCl2 (0.26 g, 2.11 mmol) were dissolved in THF (30 mL). The mixture was stirred under nitrogen at ambient temperature for 12 h. Then the reaction mixture was exposed to air and stirred for an additional 12 h. After the reaction mixture was poured into diethyl ether (60 mL), the organic layer was washed with aqueous saturated NH4Cl (3 × 60 mL) and brine (3 × 60 mL) followed by drying with MgSO4. After filtration to remove solid impurities and drying agent, the solvent was removed in vacuo. The resulting brown powder and excess AgNO3 were dissolved in CH3CN, and the precipitation of AgCl was observed immediately. The mixture solution was further stirred overnight to ensure the complete reaction. Then, the reaction mixture was filtered, and the precipitate was washed with CH2Cl2. The filtrate was further dried and concentrated to yield a dark green powder. Yield: 85%. HRMS: calcd for [C42H61N5O2Cr]+ ([1 − NO3]): 719.4230; found 719.4216. Representative Procedures for COS/Epoxide Copolymerization. A tailor-made autoclave (75 mL) equipped with a magnetic stirrer was heated to 120 °C under vacuum for 8 h and cooled under vacuum to room temperature. COS (8 g) was pressurized into the autoclave by a mass flow controller and heated by oil bath. Meanwhile, complex 1 (5.4 mg, 0.0715 mmol) and propylene oxide (5.0 mL, 71.5 mmol, 20 000 equiv) were placed in the vessel E-1 equipped with an electric heat tape under an argon atmosphere. When the temperature of COS in vessel E-2 and the catalyst/epoxide mixture in vessel E-1 reached to 80 °C, the valves V-3 and V-4 were opened to let the mixture pouring into E-2. After the allotted reaction time, a small amount of the resultant polymerization mixture was removed from the autoclave for 1H NMR analysis to quantitatively give the information regarding copolymer selectivity and monothiocarbonate linkages. This sample was also used for GPC analysis. The residual propylene oxide and the excessive COS were removed in vacuo. The crude polymer was dissolved in CHCl3/MeOH (5/1, v/v) mixture and precipitated from 1 N HCl/methanol. This process was repeated 3−5 times to completely remove the catalyst, and white polymer was obtained by vacuum-drying.

Figure 2. Equipment used for the reaction.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02108. General experimental procedures and characterizations of copolymers and kinetic studies (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-M.R.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Gratitude is expressed to the National Natural Science Foundation of China (NSFC, Grant 21134002) and Program for Changjiang Scholars and Innovative Research Team in E

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

M.; Wang, Y.-M.; Zhang, R.; Jiang, J.-Y.; Lu, X.-B. J. Org. Chem. 2013, 78, 4801−4810. (18) Wang, Y.-M.; Li, B.; Wang, H.; Zhang, Z.-C.; Lu, X.-B. Appl. Organomet. Chem. 2012, 26, 614−618. (19) (a) Zhang, X.-H.; Liu, F.; Sun, X.-K.; Chen, S.; Du, B.-Y.; Qi, G.R.; Wan, K.-M. Macromolecules 2008, 41, 1587−1590. (b) Darensbourg, D. J.; Andreatta, J. R.; Jungman, M. J.; Reibenspies, J. H. Dalton Trans. 2009, 41, 8891−8899. (c) Darensbourg, D. J.; Wilson, S. J.; Yeung, A. D. Macromolecules 2013, 46, 8102−8110. (20) Gόmez, F. J.; Waymouth, R. M. Science 2002, 295, 635−636. (21) The Tg of COS/PO copolymer with a Mn of 24.4 kg/mol was 22.4 °C (see ref 14). (22) Luo, M.; Zhang, X.-H.; Darensbourg, D. J. Macromolecules 2015, 48, 6057−6062. (23) Luo, M.; Zhang, X.-H.; Darensbourg, D. J. Polym. Chem. 2015, 6, 6955−6958. (24) Luo, M.; Zhang, X.-H.; Du, B.-Y.; Wang, Q.; Fan, Z.-Q. Polymer 2014, 55, 3688−3695. (25) Very recently, Rieger and co-workers developed a variety of new dinuclear zinc catalysts for CHO/CO2 copolymerization. The highest activities by far with a TOF up to 155 000 h−1 were achieved by using the catalyst with electron-withdrawing groups. See: Kissling, S.; Lehenmeier, M. W.; Altenbuchner, P. T.; Kronast, A.; Reiter, M.; Deglmann, P.; Seemann, U. B.; Rieger, B. Chem. Commun. 2015, 51, 4579−4982.

University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (T2011056) from the Ministry of Education of the People’s Republic of China.

■

REFERENCES

(1) Inoue, S.; Koinuma, H.; Tsuruta, T. Makromol. Chem. 1969, 130, 210−220. (2) For recent reviews of CO2/epoxide copolymerizations, see: (a) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388−2410. (b) Luinstra, G. A. Polym. Rev. 2008, 48, 192−219. (c) Kember, M. R.; Buchard, A.; Williams, C. K. Chem. Commun. 2011, 47, 141−163. (d) Klaus, S.; Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Coord. Chem. Rev. 2011, 255, 1460−1479. (e) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. Acc. Chem. Res. 2012, 45, 1721−1735. (3) Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124, 6335−6342. (4) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120, 11018−11019. (b) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew. Chem., Int. Ed. 2002, 41, 2599−2602. (5) (a) Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C. J. Am. Chem. Soc. 2003, 125, 7586−7591. (b) Darensbourg, D. J.; Phepls, A. L. Inorg. Chem. 2004, 43, 6024−6034. (c) Eberhardt, R.; Allmendinger, M.; Rieger, B. Macromol. Rapid Commun. 2003, 24, 194−196. (6) Qin, Z.-Q.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem., Int. Ed. 2003, 42, 5484−5487. (7) (a) Lu, X.-B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574− 3577. (b) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc. 2005, 127, 10869−10878. (c) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng, X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664−1674. (8) (a) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed. 2006, 45, 7274−7277. (b) Noh, E. K.; Na, S. J.; Sujith, S.; Kim, S. W.; Lee, B. Y. J. Am. Chem. Soc. 2007, 129, 8082−8083. (c) Sujith, S.; Min, K. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int. Ed. 2008, 47, 7306−7309. (d) Ren, W.-M.; Liu, Z.-W.; Wen, Y.-Q.; Zhang, R.; Lu, X.-B. J. Am. Chem. Soc. 2009, 131, 11509−11518. (9) (a) Ochiai, B.; Endo, T. Prog. Polym. Sci. 2005, 30, 183−215. (b) Kuran, L. W. Prog. Polym. Sci. 1998, 23, 919−992. (10) (a) Montaudo, G.; Puglisi, C.; Berti, C.; Manaresi, P.; Pilati, F. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2277−2290. (b) Marianucci, E.; Berti, C.; Pilati, F.; Manaresi, P. Polymer 1994, 35, 1564−1566. (11) (a) Berti, C.; Marianucci, E.; Pilati, F. Makromol. Chem. 1988, 189, 1323−1330. (b) Choi, W.; Sanda, F.; Endo, T. Macromolecules 1998, 31, 2454−2460. (c) Berti, C.; Marianucci, E. Eur. Polym. J. 2002, 38, 1281−1288. (12) Soga, K.; Imamura, H.; Ikeda, S. Makromol. Chem. 1975, 176, 807−811. (13) Kokano, K.; Tatsumi, G.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 15116−15117. (14) Luo, M.; Zhang, X.-H.; Du, B.-Y.; Wang, Q.; Fan, Z.-Q. Macromolecules 2013, 46, 5899−5904. (15) (a) Wu, G.-P.; Wei, S.-H.; Lu, X.-B.; Ren, W.-M.; Darensbourg, D. J. Macromolecules 2010, 43, 9202−9204. (b) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Xu, T.-Q.; Darensbourg, D. J. J. Am. Chem. Soc. 2011, 133, 15191−15199. (c) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Li, B.; Zu, Y.-P.; Darensbourg, D. J. Energy Environ. Sci. 2011, 4, 5084−5092. (d) Zhang, H.; Grinstaff, M. W. J. Am. Chem. Soc. 2013, 135, 6806−6809. (16) Although PO was refluxed over a mixture of KOH/CaH2 for more than 10 h, the water content in the epoxide was in the range of 20−25 ppm, which was determined by means of Karl Fischer coulometry. In this case, the molar ratio of water molecule to complex 1 is more than 7/1. (17) (a) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277, 936−938. (b) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen, K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124, 1307−1315. (c) Ren, W.F

DOI: 10.1021/acs.macromol.5b02108 Macromolecules XXXX, XXX, XXX−XXX