Macromolecules 1999, 32, 2137-2140
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Copolymerization and Terpolymerization of CO2 and Epoxides Using a Soluble Zinc Crotonate Catalyst Precursor Donald J. Darensbourg* and Marc S. Zimmer Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842 Received November 9, 1998; Revised Manuscript Received January 20, 1999
ABSTRACT: A soluble catalyst precursor derived from the reaction of zinc bis(trimethylsilyl)amide, Zn[N(SiMe3)2]2, and crotonic acid has been found to be extremely active toward the copolymerization of cyclohexene oxide and carbon dioxide with turnover frequencies approaching 35 g/g of Zn/h at 80 °C. This catalyst precursor was also demonstrated to be an efficient terpolymerization catalyst when propylene oxide or styrene oxide was added to the cyclohexene oxide/CO2 feed. Extensive characterization of the metal complex proved difficult, but 31P NMR studies have shown that only 10% of the anticipated epoxide binding sites were available for catalysis. This suggests that the complex has several structures at its disposal, only one of which is conducive to copolymerization.
Nearly three decades ago, the first catalytic system for the copolymerization of epoxides and carbon dioxide was described by Inoue.1 The catalyst, prepared from diethylzinc and H2O, was reported to produce high molecular weight polypropylene carbonate in low yields. This initial discovery provided the stimulus for numerous investigations into zinc-catalyzed copolymerization systems which have been reviewed extensively in the literature.2 Among the list of ligands that have been appended to zinc include aromatic polyols3, aliphatic alcohols,4 and monocarboxylic acids.5 The most active first-generation catalysts were those synthesized from the zinc oxide and dicarboxylic acids.6 The most prominent example, zinc glutarate, which we demonstrated through a powder diffraction study to consist of parallel layers of zinc centers bridged by the dicarboxylate ligands,7 gave turnover numbers of nearly 70 g of polymer/g of Zn. Unfortunately, these systems suffer from the requirement of high catalyst loadings, broad molecular weight distributions, and poor reproducibility which can probably be attributed to their heterogeneous nature. More recent systems have enjoyed greater success with increased yields that are consistently reproduced. Beckman et al. have reported on a zinc acid-ester complex that, with its five-carbon perfluorinated chain, is soluble in supercritical carbon dioxide and gives turnovers approaching 450 g of polycyclohexene oxide carbonate per gram of zinc.8 In our laboratory, we have studied a group of soluble zinc(II) phenoxides in which sterically encumbering substituents at the 2- and 6-positions preclude the possibility of extended aggregation through oxygen-bridging interactions.9,10 These complexes are monomeric in zinc and possess a distorted tetrahedral geometry in which two of the sites are occupied by highly labile ethereal solvent molecules. Their activity toward the copolymerization of cyclohexene oxide and CO2 is quite good with turnover numbers exceeding 1000 g/g of Zn in some instances (eq 1).
Figure 1. Donation of an oxygen lone pair from the anionic chain end to another zinc center interrupts the backbiting reaction. Scheme 1
These zinc(II) phenoxides also serve as effective terpolymerization catalysts when propylene oxide is added to the copolymerization system. However, an increasingly greater proportion of the cyclic product, propylene carbonate, is produced at higher ratios of propylene oxide to cyclohexene oxide. It has been suggested by Kuran2d that a multicentered catalyst system (Figure 1) is necessary to circumvent the backbiting reaction (Scheme 1), a process that dominates in the monomeric zinc systems toward less bulky epoxides, which is responsible for cyclic carbonate formation. It was of interest to us to identify a soluble catalyst system with two or more zinc sites to test this idea. Straughan et al. have reported a zinc dimer complex11 in which the zinc centers are bridged by four crotonic acid carboxylates in a paddlewheel arrangement with quinoline ligated at the apical sites (Figure 2), whereas zinc crotonate, made from zinc oxide and crotonic acid in the absence of a nitrogen base, is polymeric.12 Concerned that quinoline, a strongly binding ligand, would inhibit the copolymerization reaction through competition for the remaining zinc binding site,13 we sought to make a soluble derivative with an available site for preassociation of the epoxide substrate. Herein
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2138 Darensbourg and Zimmer
Macromolecules, Vol. 32, No. 7, 1999 ring opening was established by characterization of the products of alkaline hydrolysis of the polymer.17 Phosphine Binding Study. Toluene (10 mL) was degassed using three freeze-thaw-degas cycles to remove adventitious air and was placed in the glovebox. Zinc crotonate (0.061 g, 0.259 mmol) was prepared inside the glovebox as previously described. The volatiles were removed in vacuo, and the complex was redissolved in deuterated toluene. Trimethylphosphine (0.026 mL, 0.259 mmol) was added to the NMR tube via a syringe. Phosphorus-31 NMR measurements were acquired incrementally between -80 and 20 °C. Similar methods were used when a binding study was carried out on zinc crotonate with 2 equiv of PBu3 at elevated temperature.
Figure 2. Ball-and-stick structure of [Zn(crot)2(quin)]2. Perpendicular crotonic acid carboxylate ligands bridge two zinc centers with quinoline ligated at the apical positions.
we describe a zinc crotonate catalyst precursor that displays notable activity toward the copolymerization of epoxides and carbon dioxide. Experimental Procedure General. Crotonic acid, propylene oxide, and styrene oxide were purchased from Aldrich and distilled before use. Cyclohexene oxide was purchased from Lancaster and was distilled from CaH2 prior to its use. ZnCl2 was purchased from Fischer and was dehydrated according to the published procedure.14 Zn[N(SiMe3)2]2 was synthesized and purified by distillation according to the literature procedure.15 1H NMR spectra were recorded on a Varian Unity+ 300 MHz superconducting instrument. 31P NMR spectra referenced to 85% H3PO4 were acquired on a Varian 200 MHz broad-band spectrometer operating at 81 MHz. Infrared spectra were recorded on a Mattson 6022 spectrometer with DTGS and MCT detectors. Mass spectral analysis (FAB) was acquired on a VG Analytical 70S (Manchester UK) high-resolution, double-focusing magnetic sector mass spectrometer. The VG 70S was equipped with a VG11/250J data system that allowed computer control of the instrument, data recording, and data processing. Catalyst Preparation. To a dilute toluene solution of Zn[N(SiMe3)2]2 (0.33 g, 0.86 mmol) was added crotonic acid (0.148 g, 1.71 mmol) to a total reaction volume of 15 mL, which was maintained under an argon atmosphere. The reaction mixture was stirred for 4 h during which the solution remained clear and colorless. All volatiles were then removed in vacuo, leaving behind a sticky, foamy, colorless residue. Attempts to purify the material by washing with hexanes produced a powder that was not very soluble in epoxides. Hence, the material obtained initially was employed in the catalytic run. 1H NMR (300 MHz, CDCl ) δ (ppm) 6.96 (m, 1H), 5.88 (d, 1H), 3 1.80 (d, 3H); FTIR (KBr pellet) 1664 (m), 1614 (m), 1545 (s), 1523 (s), 1425 (s); FTIR (toluene) 1660 (m), 1571 (s), 1418 (m). Typical Polymerization Reactions. The catalyst, made just prior to use, was dissolved in 20 mL of cyclohexene oxide. The mixture was then introduced into a 300 mL autoclave that had been dried overnight at 90 °C under vacuum. The reactor was charged with 800 psi of carbon dioxide and heated to 80 °C with stirring for the appropriate period of time. Upon completion of the reaction, the polymer mixture was taken up in a minimal amount of dichloromethane and precipitated from methanol. The polymer was then dried in the vacuum oven at 100 °C for several hours. Polymer Characterization. Once the high molecular weight polymer samples were isolated, they were probed using 1H NMR spectroscopy. The relative amounts of carbonate and ether diads were determined from the peak intensities of the broad resonances centered at 4.5 and 3.4 ppm, respectively. Molecular weights were obtained through GPC analysis on a polystyrene column. 13C NMR spectroscopy was used to assess the tacticity according to assignments that have been made in prior reports.16 The stereochemistry of cyclohexene oxide
Results and Discussion Our synthesis entailed the addition of Zn[NSiMe3]2, the same unconventional zinc source used in making the zinc phenoxide complexes, to 2 equiv of crotonic acid in toluene under argon. After stirring for 4 h, the solvent was removed in vacuo, leaving a colorless tacky solid. An IR spectrum of the reaction product in toluene displays absorbances at 1571 and 1418 cm-1 corresponding to the asymmetric and symmetric carboxylate stretching vibrations, respectively. The ν(CO)asym and ν(CO)sym separation is consistent with a bridging mode for the carboxylate ligand. However, when the material was washed with hexane to afford a fine white powder, two asymmetric stretching absorptions were observed at 1545 and 1523 cm-1. This observation is consistent with the polymeric structure noted for zinc crotonate in which zinc dimers bridged by three carboxylate ligands are joined together by a fourth, a common structural motif for zinc carboxylates.12,18 Another smaller asymmetric stretching absorption at 1614 cm-1 can be assigned to a monodentate-bound end group. In this form, the catalyst was only minimally soluble and, thus, was not catalytically active. For this reason, the catalyst was made just prior to use and was not subject to purification. In an attempt to elicit additional structural information, mass spectroscopy was performed on the complex. A spectrum of the tacky material dissolved in cyclohexene oxide revealed a group of peaks around the base peak at m/z ) 467 amu which can be attributed to the rearrangement product [Zn3O(CH3CHCHCO2)3]+.19 (A simulated isotopic pattern was in good agreement with this assignment.) Analogous peaks were observed in the mass spectrum of the crystallographically characterized quinoline derivative. Therefore, the precursor’s structure is likely that of a dimer with four bridging carboxylate ligands. In any instance, the complex does not appear to exist as a highly aggregated structure given its solubility in many common organic solvents including THF, benzene, and chloroform. Table 1 displays the results for a representative sampling of the co- and terpolymerization attempts. Where cyclohexene oxide was the comonomer, high yields of polycyclohexene oxide carbonate were reliably obtained and at relatively shorter reaction times. To our knowledge, the turnover frequency shown in entry 2 is among the higher values documented for an epoxide/ carbon dioxide copolymerization.20 The percentage of polyether linkages, as determined by 1H NMR, was ordinarily between 12 and 16%, which is higher than that observed from the polymers made with our zinc(II) phenoxides. This is especially surprising in light of the fact that zinc crotonate is a poor homopolymerization catalyst for cyclohexene oxide.21 When supercritical
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Co- and Terpolymerization of CO2 and Epoxides 2139
Table 1. Summary of Results for the Zinc Crotonate (0.2 g)-Catalyzed Copolymerization of Cyclohexene Oxide and/or Propylene Oxide/Styrene Oxide with Carbon Dioxide at 80 °C and 800 psi Initial Pressure of CO2 mol fractiona of cyclohexene oxide
mol fractiona of propylene oxide
mol fractiona of styrene oxide
1 1 0.5 0.3 0.67 0.5
0 0 0.5b 0.7b 0 0
0 0 0 0 0.33 0.5
% propylene/styrene oxide carbonate
% polyether linkage
time (h)
turnover freq (g/g of Zn/h)
18 28 19 22
16 16 12 15 NAc 12
15 7.5 30 24 27 24
19.44 33.71 3.98 2.18 4.05 1.01
a Mole fraction as a percentage of the total number of moles of epoxide. b Under these conditions very small quantities (1-2%) of cyclic carbonate were produced. c An unassigned peak due to an impurity overlapped the proton signal corresponding to the ether unit.
CO2 conditions were employed to increase the percentage of carbonate linkages, the catalyst was inactive, which is attributed to its insolubility in this medium.22 On the other hand, reducing the CO2 pressure to 350 psi did not significantly change the overall conversion or the percent carbonate content. Lowering the temperature to 60 °C rendered the catalyst almost completely inactive as only trace amounts of copolymer were detected by infrared spectroscopy. Weight-average molecular weights (Mw) ranged from 84 000 to 150 000, and the corresponding polydispersities varied between 6.51 and 15.97. The high polydispersities recorded for these polymers are characteristic of a coordinative anionic polymerization system.23 Inequivalent catalyst sites are normally invoked to explain this observation. As stated earlier, a condensed catalyst species involving two or more zinc sites is believed necessary to block the backbiting pathway that produces cyclic carbonates when propylene oxide is the comonomer. Although zinc crotonate seems to meet this minimum requirement, no polypropylene carbonate was produced at 100 °C and 800 psi of CO2. (Propylene oxide appeared to be completely unreactive at 80 °C.) Instead, the five-membered cyclic product propylene carbonate was obtained as indicated by the intense carbonyl stretch at 1803 cm-1 in the IR spectrum of the product. However, when propylene oxide was combined with cyclohexene oxide and carbon dioxide, a terpolymer was obtained in moderate (32-44%) yield. The polyether content and polydispersities recorded for the terpolymer were comparable to those observed in the copolymer. The molecular weights and total conversion, markedly lower by comparison, appear to be inversely proportional to the amount of propylene oxide in the feed. Predictably, the backbiting reaction becomes more prevalent as the mole fraction of propylene oxide, Xpropylene oxide, is increased. The percentage of propylene oxide incorporation, approximately 18% at Xpropylene oxide ) 0.5, undergoes an expected increase as the mole fraction of the monomer is raised. Notwithstanding, a point of diminishing returns is quickly reached as a 30% increase in Xpropylene oxide brings only a 10% increase in propylene oxide incorporation at the expense of lowering the polymer yield. To further explore the utility of zinc crotonate, its activity toward the polymerization of styrene oxide was examined. Preliminary experiments demonstrate that, like propylene oxide, the monomer was inert to copolymerization with CO2 at 80 °C, yet when cyclohexene oxide was added, a terpolymer was produced in low yield and with nearly 20% styrene oxide incorporation (entry 5). Increasing Xstyrene oxide to 0.5 afforded only a small change in the amount of styrene oxide in the terpolymer while the overall conversion was greatly compromised. Having met the solubility requirement, it was of
Figure 3. 31P NMR spectra of PMe3 (1 equiv) binding study with zinc crotonate in CD2Cl2 as a function of temperature. Only 10% of PMe3 is bound at -80 °C.
interest to confirm the availability of an open coordination site on the catalyst. Because phosphines offer an additional spectroscopic handle, we elected to probe zinc’s coordination sphere using trimethylphosphine.24 At ambient temperature, no phosphine binding is observed as only a single peak is observed at -60 ppm representative of free PMe3. When the temperature is lowered to -20 °C, however, a signal emerges at -38 ppm (Figure 3). This peak grows larger and better resolved as the temperature is decreased whereas the peak at -60 ppm broadens at lower temperatures. Such behavior is characteristic of a process where there is rapid phosphine ligand exchange (eq 2). Concomitantly, the line shape of the free PMe3 ligand is dependent on the total concentration of PMe3.25
Zn-(-O2CCHdCHCH3)2 + PMe3 h Zn-(-O2CCHdCHCH3)2‚PMe3 (2) Inasmuch as the bound peak is comparatively smaller than that for the free phosphine over a broad temperature range, the “initial” complex does not appear to easily accommodate additional ligands. Integration of the bound and free phosphine peaks demonstrated that only ≈10% of the zinc sites are occupied at -80 °C. A comparable observation was made when 2 equiv of PBu3 was added to zinc crotonate at 80 °C. Furthermore, when a 0.5 equiv of PBu3 was added to the catalyst to examine its effects on CO2 incorporation into the polymer product, copolymerization was inhibited presumably because the phosphine blocked any available coordination sites.26 Regardless, the results in Table 1 suggest that the term catalyst precursor might be more appropriate for the following reasons. (1) At temperatures approaching 60 °C, the catalyst exhibited no
2140 Darensbourg and Zimmer
activity whatsoever. By comparison, the zinc phenoxide derivatives displayed moderate efficiency even at 40 °C. (2) Attempted polymerization of CO2 and propylene oxide or styrene oxide in the absence of cyclohexene oxide yielded only the cyclic product. Under Kuran’s model, formation of the polymer material might have been expected. To summarize, we have developed a new inexpensive soluble catalyst, synthesized from Zn(II) bis(trimethylsilylamide) and crotonic acid, that exhibits much improved activity toward the copolymerization of CO2 and cyclohexene oxide. The complex is also capable of producing terpolymers when either propylene oxide or styrene oxide is added to the feed. Despite the fact that the catalyst appeared to possess two or more zinc centers as evidenced by the absence of any terminal carboxylate stretching modes in the infrared spectrum, polymers were not obtained from propylene oxide and CO2 alone. It seems likely that the complex lacks the proper geometry for the growing chain end(s) to interact with another zinc site. Alternatively, the aggregation is broken prior to initiation of the polymerization which seems to justify the inefficiency of the catalyst at temperatures below 80 °C. The methodology used to produce soluble zinc carboxylates is quite general; thus, future investigations will target related catalysts for copolymerization catalysis. Acknowledgment. The financial support of this research by the National Science Foundation (Grant CHE96-15866) and the Robert A. Welch Foundation is greatly appreciated. We also thank the National Science Foundation (Grant CHE-8705697) for instrumentation funding to purchase the VG sector instrument and the Vestec 201 and PAC Polymers for molecular weight determinations on the polymer samples. References and Notes (1) Inoue, S.; Koinuma, H.; Tsuruta, T. Polym. Lett. 1969, 7, 287. (2) (a) Kuran, W.; Rokicki, A. J. Macromol. Sci., Rev. Macromol. Chem. 1981, C21, 135. (b) Beckman, E.; Super, M. Trends Polym. Sci. 1997, 5, 236. (c) Beckman, E. J. Science 1999, 283, 946. (3) (a) Inoue, S.; Kobyashi, M.; Tang, Y.-L.; Tsuruta, T. Makromol. Chem. 1973, 169, 69. (b) Kuran, W.; Pasynkiewicz, S.; Skupinska, J. J. Makromol. Chem. 1976, 177, 1283. (c) Kuran, W.; Listos, T. Makromol Chem. Phys. 1994, 195, 401. (d) Kuran, W.; Pasynkiewicz, S.; Skupinska, J.; Rokicki, A. Makromol. Chem. 1976, 177, 11. (4) Kuran, W.; Listos, T. Makromol. Chem. Phys. 1994, 195, 401. (5) Inoue, S.; Kobayashi, M.; Tsuruta, T. J. Polym. Sci. 1973, 11, 2383.
Macromolecules, Vol. 32, No. 7, 1999 (6) Soga, K.; Imai, E.; Hattori, I. Polymer 1981, 13 (4), 407. (7) Darensbourg, D.; Holtcamp, M.; Reibenspies, J. Polyhedron 1996, 15, 2341. (8) Beckman, E.; Super, M.; Berluchi, E.; Costello, C. Macromolecules 1997, 30, 368. (9) Darensbourg, D.; Holtcamp, M. Macromolecules 1995, 28, 7577. (10) Darensbourg, D.; Holtcamp, M.; Struck, G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.; Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107. (11) Straughan, B. P.; Little, I. R.; Clegg, W. J. Chem. Soc., Dalton Trans. 1986, 1283. (12) Straughan, B. P.; Clegg, W.; Little, I. A. Acta Crystallogr. 1986, C42, 919. (13) Neutral nitrogen bases are considerably stronger bases than their oxygen counterparts and, as an illustration, have been used to disrupt aggregation in zinc complexes that were insoluble in THF or ether. (14) Burger, H.; Sowadny, W.; Wannget, U. J. Organomet. Chem. 1965, 3, 113. (15) Pray, A. Inorg. Synth. 1957, 5, 153. (16) (a) Kuran, W.; Listos, T. Macromol. Chem. Phys. 1994, 195, 977. (b) Koinuma, H.; Harai, H. Makromol. Chem. 1977, 178, 1283. (c) Hasebe, Y.; Tsuruta, T. Makromol. Chem. 1987, 188, 1403. (17) Inoue, S.; Yokoo, Y.; Tsuruta, T. Makromol. Chem. 1971, 143, 97. (18) Clegg, W.; Harbron, D.; Hunt, P.; Littler, I.; Straughan, B. Acta Crystallogr. 1990 C46, 750. (19) Clusters larger than the parent compound are often seen in the mass spectra of metal carboxylates. (20) Coates et al. have recently described a monomeric singlesite catalyst that exhibits excellent catalyst activity producing polymers with polydispersities near 1. Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120, 11018. (21) A cyclohexene oxide homopolymerization attempt using 0.2 g of the catalyst at 85 °C yielded only trace amounts of polymer. (22) The final product mixture contained an insoluble white powder which was determined to be the carboxylate catalyst by IR spectroscopy. (23) Ivin, K. J.; Saegusa, T. Ring Opening Polymerization; Elsevier Applied Science Publishers: Amsterdam, 1984; Vol. 1, p 241. (24) Recently, we demonstrated through binding study on zinc phenoxides that phosphines with significant σ-donor capabilities are bound to zinc at room temperature in solution in: Darensbourg, D. J.; Zimmer, M. S.; Rainey, P.; Larkins, D. Inorg. Chem. 1998, 37, 2852. (25) Tolman proposes that, for equilibria such as these, only the line shape of the free phosphine actually depends on the overall phosphine concentration. Tolman, C.; Seidel, W.; Gosser, L. J. Am. Chem. Soc. 1974, 96, 53. (26) We recently determined that the addition of phosphine in our zinc phenoxide systems had a marked effect on the amount of CO2 incorporated into the polymer.
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