Copolymerization of Epoxides and CO2: Polymer Chemistry for

Article pubs.acs.org/jchemeduc

Copolymerization of Epoxides and CO2: Polymer Chemistry for Incorporation in Undergraduate Inorganic Chemistry Donald J. Darensbourg* Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, United States ABSTRACT: Because fossil fuels are a nonrenewable resource, alternative sources of chemical carbon will be necessary as petroleum based chemicals decline during the 21st century. Carbon dioxide can serve as a source of carbon for the synthesis of useful chemicals, thereby contributing to a sustainable chemical industry. A promising technology for CO2 utilization is the alternating copolymerization of epoxides and CO2 to afford polycarbonates. This article describes the catalysis of this polymerization process employing well-defined transition metal catalysts. It is intended for use in advanced inorganic chemistry courses, which are required for an ACS-approved degree in Chemistry, and integrates concepts in inorganic chemistry and polymer science. Concomitantly, a goal of this material is to contribute to the recently established polymer requirement by the Committee on Professional Training (CPT) for an ACS-certified degree. The principles of inorganic chemistry are shown to play vital roles in the mechanistic aspects of this copolymerization process. KEYWORDS: Upper-Division Undergraduate, Polymer Chemistry, Epoxides, Inorganic Chemistry

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INTRODUCTION Polymeric materials are prominent when chemical educators examine areas where inadequate training is provided to undergraduates majoring in chemistry.1 Similarly there is a need for these students to be exposed to concepts in green and sustainable chemistry.2 This article is written as representative lectures which could be given in an advanced inorganic chemistry course required for an ACS-approved undergraduate degree in Chemistry. Such a lecture would integrate polymer chemistry into an inorganic course, thereby, contributing to the newly established polymer requirement by the Committee on Professional Training for an ACS-certified degree in Chemistry. It would also introduce undergraduate chemistry students to the use of a renewable resource and greenhouse gas, CO2, for the synthesis of useful polymers. The principles of inorganic chemistry are seen in play when understanding the mechanistic aspects of the copolymerization process involving the sequential coupling of two monomers, carbon dioxide and three-membered cyclic ethers (a.k.a., oxiranes or epoxides), in the formation of polycarbonates (eq 1)

epoxides and CO2 to polycarbonates, including very air-stable zinc derivatives of dicarboxylic acids.4 Disadvantages of these heterogeneous catalyst systems include requiring high catalyst loadings and, in general, providing extremely poor control of molecular weight (broad molecular weight distribution). The focus of this presentation will be to explore the preparation of polycarbonates from CO2 and epoxide employing discrete transition metal based catalysts. Transition-Metal Complexes as Catalysts for the Copolymerization of CO2 and Epoxides

A renewed interest in the coupling reaction of CO2 and epoxides was fueled by reports of effective discrete metal complexes capable of performing these transformations.5 Although there are numerous zinc complexes, both monoand dinuclear, that display excellent catalytic activity and regio-/stereoselectivity for this process, in the context of this account for pedagogical reasons, I will focus on the role of Cr(III) and Co(III) metal complexes. Namely, salen(salicylaldimine) derivatives of these first row transition metals, which represent the most widely utilized complexes for these copolymerization processes, will be considered. As illustrated in Figure 1, salen ligands substituted in the 3,5-positions by t-butyl groups are most commonly employed in these studies for both electronic and enhanced solubility reasons. First, it is necessary for students to have sufficient mechanistic background information for appreciating a discussion of why these particular metal complexes are effective at catalyzing this polymeric coupling process. The copolymerization of epoxides/CO2 can be summarized by the catalytic

Historical Perspective

Prior to addressing the developments in this field relevant to this lecture presentation, some historical facts about the subject matter are warranted. The first report of a metal-catalyzed sequential copolymerization of the two monomers CO2 and epoxides was reported by Inoue and co-workers in 1969.3 These researchers in Japan employed a heterogeneous catalyst system derived from a mixture of diethylzinc and water. Since these original studies, there have been numerous reports of other heterogeneous catalysts for the transformation of © XXXX American Chemical Society and Division of Chemical Education, Inc.

Special Issue: Polymer Concepts across the Curriculum Received: July 8, 2016 Revised: October 3, 2016

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DOI: 10.1021/acs.jchemed.6b00505 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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carbonates, the thermodynamic product of CO2/epoxide coupling (eq 2)

The characteristics of the transition metal complexes that are effective for the copolymerization process are inertness, that is, they undergo substitution reactions slowly (t1/2 > 1 min). For example, if we employ the Mn(III) analog of the chromium(III) complex in Figure 1, a d4 high spin labile complex, no binding is noted for weak ligands like epoxides.8 This in turn does not result in epoxide activation for the ring-opening process. Concomitantly, the metal catalyst cannot be too electrophilic, for the resulting metal alkoxide will not be sufficiently nucleophilic for carbon dioxide insertion. Relative to insertion of CO2 into the metal−alkoxide bond, this process does not require prior coordination of the CO2 molecule to the metal center.9,10 This is important because the metal center is coordinatively saturated. In general, this process is only a facile one in the presence of very nucleophilic metal−alkoxides because CO2 is a rather poor electrophile. The selectivity for copolymer formation vs cyclic organic carbonate can be controlled by the choice of catalyst or reaction conditions. In general, higher reaction temperatures enhance the quantity of cyclic carbonate formation. On the other hand, using bifunctional catalysts (Figure 2) can greatly enhance the

Figure 1. X-ray structure of an effective (salen)CrCl catalyst.

cycle shown in Scheme 1, where the catalyst is as defined in Figure 1 and X′ is an added nucleophile. Scheme 1. Catalytic Cycle for the Coupling of Epoxides and CO2, Where Following the First Cycle, ⊖X′ Represents the Anionic Growing Polymer Chain

The various six-coordinate anionic metal complexes resulting from the reaction of [M]−X and X′ have been identified in solution by infrared spectroscopy and in the solid state by X-ray crystallography.6 The epoxide metal bond involves donation of an electron pair from the epoxide monomer to the metal center. Because the lone electron pairs of the three-membered strained ether ring possess a great deal of s-character, that is, are tightly held by the oxygen atom, epoxides are weakly donating or binding ligands. Despite the fact that epoxides are poor ligands, because these polymerization processes are generally carried out in neat epoxide, they are able to effectively compete with X− for coordination to the metal center.7 Subsequent to epoxide coordination, the displaced X or X′ ligand serves as an initiator for ring-opening the bound (activated) epoxide monomer. The resulting alkoxide ligand is resistant to undergoing substitution by an epoxide (a process which would lead to ether linkages in the polymer chain)but instead rapidly inserts CO2 to afford a weakly bound carbonate species. The carbonate ligand is displaced by epoxide in a ratedetermining step and serves as the nucleophile to continue the polymer chain growing process. This latter process leading to a free organic carbonate is responsible for formation of cyclic carbonate byproduct, and hence needs to be controlled for copolymer production (vide infra). That is, when the polymer’s chain-end is not capped by the metal catalyst or other entities, it is prone to a facile backbiting process leading to cyclic organic

Figure 2. Bifunctional cobalt catalyst, where the onium salt which provides the initiating anion is attached to the salen ligand.

selectivity for copolymer production, even at high temperatures.11 This is ascribed to the anionic polymer chain remaining in close contact with the metal catalyst via an electrostatic interaction when it is displaced by the epoxide substrate in the rate-determining step.5e This ion-pairing interaction thereby reduces the probability of the f ree anionic polymer chain undergoing the backbiting reaction depicted in eq 2. Though it is not our intention to cover polymer properties to the extent discussed in numerous excellent textbooks on this subject, at this point, it is important to introduce two concepts from polymer chemistry relevant to polymerization mechanisms of the type shown in Scheme 1.12 That is, living polymerization or processes with no termination step manifest themselves in that the molecular weight of the resulting polymer correlates linearly with the percent conversion of monomers. In this instance, the observed molecular weight of the polymer can be theoretically predicted from the monomer/ initiator ratio. Hence, if the epoxide to metal catalyst ratio is 500, and the process is carried out in a large excess of CO2, the B



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rapid and reversible chain transfer process such as shown in Scheme 2. A bimodal molecular weight distribution (Figure 4) is observed which is ascribed to polymer chains initiated by the

polymer’s molecular weight at 100% conversion should be (500) (mole wt. epoxide + mole wt. CO2). In addition, living polymerization processes, where the initiation step is fast relative to the propagation step, exhibit narrow molecular weight distributions represented by polydispersity indices (PDIs) close to 1.0. PDI are defined as indicated below in eqs 3 through 5, where Ni = no. chains with mass Mi and Mi = mass of chain. Weight-average molecular weight (Mw) Mw =

2 ∑ NM i i ∑ NM i i

(3)

Number-average molecular weight (Mn)

Mn = PDI =

∑ NM i i ∑ Ni Mw , always ≥ 1 Mn

(4)

(5)

Although the copolymerization process of epoxides and CO2 appears to behave as a living polymerization, as illustrated in Figure 3 for a typical reaction, the observed molecular weight of

Figure 4. Gel permeation chromatography (GPC) trace of polycarbonate produced via copolymerization reaction of CO2 and an epoxide.14 The GPC instrument is equipped with a column made from a styrene divinyl-benzene material using tetrahydrofuran as eluent. Polystyrene standards are used to calibrate the system. The molecular weight and distribution were determined by size-exclusion chromatography coupled with refractive index and light scattering detectors.

nucleophile X and those initiated by the chain transfer reagent. A similar situation arises upon deliberately adding protic reagent such as alcohols. Hence, upon addition of large excesses of chain transfer reagents, small molecular weight polyols can be produced.13 This phenomenon is illustrated in the table inserted in Scheme 2 which shows the molecular weight of the polyol to decrease with an increase in the quantity of chain transfer reagent. As indicated in Scheme 2, the polymer can be removed from the metal center by acid hydrolysis in methanol, resulting in an insoluble white polymeric material with an −X and an −OH end group. Hence, following the chain transfer process, a second polymer is afforded, resulting in two overlapping molecular weight distributions as observed in the GPC traces

Figure 3. Molecular weight of polycarbonate and polydispersity vs % of monomer conversion, where • represents Mn and ★ represents PDI.

the copolymer generally differs significantly from that predicted from the monomer/initiator ratio. This is the consequence of an inability to eliminate adventitious water, which leads to a

Scheme 2. Process Illustrating the Chain Transfer Reaction That Occurs in the Presence of Adventitious Trace or Added Water to Afford Macropolyols

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ACKNOWLEDGMENTS The author acknowledges funding from the National Science Foundation (CHE-1057743) and the Robert A. Welch Foundation (A-0923) for his groups’ contribution to this area of chemistry.

shown in Figure 4. Furthermore, if the process is carried out in excessive amounts of chain transfer reagents (see table insert in Scheme 2), the resulting polymer mostly contains two −OH end groups, that is, a macrodiol or polyol is produced. These polyols are currently being used as drop-in replacements for poly(propylene oxide)-derived polyols for polyurethane production.15 Thus far, the most important copolymers with regard to industrial applications are derived from ethylene oxide and propylene oxide. In general, poly(alkylene carbonate) copolymers are amorphous, clear, readily processable, and possess good mechanical properties. Some of the applications where these copolymers are used include food packaging materials, lost foam casting, electronic passive components, and ceramic binders (see the Web sites of Novomer, Inc. and Empower Materials). The subject matter in this area can easily be extended with discussions of regioselective and stereoselective processes. For example, employing the propylene oxide monomer, both site of ring-opening (Figure 5) and kinetic resolution of racemic

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REFERENCES

(1) Daus, K.; Rigsby, R., Eds. The Promise of Chemical Education: Addressing our Students’ Needs; ACS Symposium Series Books; American Chemical Society: Washington, DC, 2015; Vol. 1193. (2) Kovacs, D. G. ConfChem Conference on Educating the Next Generation: Green and Sustainable ChemistryTeaching Green Chemistry: The Driving Force behind the Numbers! J. Chem. Educ. 2013, 90, 517−518. (3) Inoue, S.; Koinuma, H.; Tsuruta, T. Copolymerization of Carbon Dioxide and Epoxide. J. Polym. Sci., Part B: Polym. Lett. 1969, 7, 287− 292. (4) Soga, K.; Imai, E.; Hattori, I. Alternating Copolymerization of CO2 and Propylene Oxide with the Catalysts Prepared from Zn(OH)2 and Various Dicarboxylic Acids. Polym. J. 1981, 13, 407−410. (5) (a) Coates, G. W.; Moore, D. R. Discrete Metal-Based Catalysts for the Copolymerization of CO2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism. Angew. Chem., Int. Ed. 2004, 43, 6618− 6639. (b) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux, D. R. Copolymerization of CO2 and Epoxides Catalyzed by Metal Salen Complexes. Acc. Chem. Res. 2004, 37, 836−844. (c) Darensbourg, D. J. Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2. Chem. Rev. 2007, 107, 2388−2410. (d) Kember, M. R.; Buchard, A.; Williams, C. K. Catalysts for CO2/ epoxide Copolymerisation. Chem. Commun. 2011, 47, 141−163. (e) Darensbourg, D. J.; Wilson, S. J. What’s New with CO2? Recent Advances in Its Copolymerization with Oxiranes. Green Chem. 2012, 14, 2665−2671. (f) Lu, X.-B.; Darensbourg, D. J. Cobalt Catalysts for the Coupling of CO2 and Epoxides to Provide Polycarbonates and Cyclic Carbonates. Chem. Soc. Rev. 2012, 41, 1462−1484. (g) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.; Saini, P. K.; Williams, C. K. Ringopening Copolymerization (ROCOP): Synthesis and Properties of Polyesters and Polycarbonates. Chem. Commun. 2015, 51, 6459−6479. (6) Darensbourg, D. J.; Moncada, A. I. Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO2 Catalyzed by (salen)CrX Complexes. Inorg. Chem. 2008, 47, 10000−10008. (7) This is the reason coordinating solvents cannot be used in these reactions, because these would serve as better ligands than epoxides for the metal center and thereby inhibit the copolymerization process. (8) Darensbourg, D. J.; Frantz, E. B. X-Ray Crystal Structures of Fivecoordinate (Salen)MnN3 Derivatives and Their Binding Abilities Towards Epoxides: Chemistry Relevant to the Epoxide−CO 2 Copolymerization Process. Dalton Trans. 2008, 5031−5036. (9) Darensbourg, D. J.; Sanchez, K. M.; Reibenspies, J. H.; Rheingold, A. L. Synthesis, Structure, and Reactivity of Zerovalent Group 6 Metal Pentacarbonyl Aryl Oxide Complexes. Reactions with Carbon Dioxide. J. Am. Chem. Soc. 1989, 111, 7094−7103. (10) Darensbourg, D. J.; Lee, W.-Z.; Phelps, A. L.; Guidry, E. Kinetic Study of the Insertion and Deinsertion of Carbon Dioxide into fac(CO)3(dppe)MnOR Derivatives. Organometallics 2003, 22, 5585− 5588. (11) Ren, W.-M.; Liu, Z.-W.; Wen, Y.-Q.; Zhang, R.; Lu, X.-B. Mechanistic Aspects of the Copolymerization of CO2 with Epoxides Using a Thermally Stable Single-Site Cobalt(III) Catalyst. J. Am. Chem. Soc. 2009, 131, 11509−11518. (12) Cowie, J. M. G. Polymers: Chemistry and Physics of Modern Materials, 2nd ed.; Blackie Academic & Professional: London, 1991. (13) Kember, M. R.; Copley, J.; Buchard, A.; Williams, C. K. Triblock Copolymers from Lactide and Telechelic Poly(cyclohexene carbonate). Polym. Chem. 2012, 3, 1196−1201.

Figure 5. Regiochemistry of ring-opening. Note ring-opening occurring at the methylene site does not alter chirality at the methine carbon center.

epoxides by a R,R-cyclohexene or S,S-cyclohexylene chiral metal catalyst as shown in Figure 1 could be considered.16 These topics can also lead into a consideration of polymer tacticity. This topic can be further expanded to include epoxides derived from renewable resources, such as limonene oxide, where limonene is a cyclic terpene obtained from the rind of citrus fruit.17

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CONCLUSION This article covers concepts in inorganic/organometallic chemistry at the undergraduate level pertinent to understanding a polymerization reaction utilizing CO2 as a renewable C1 feedstock. The coupling of epoxides and CO2 catalyzed by metal complexes can be described mechanistically via a coordination−insertion pathway. Specifically, topics of importance to inorganic chemistry include: 1. ligand substitution in terms of labile and inert metal complexes based on metal’s electron count and ligand field strength; 2. mechanism of CO2 insertion into metal−alkoxide bonds; 3. design of metal complex architecture to improve product selectivity; 4. living polymerization processes and molecular weight distributions.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 979.845.2983. Fax: 979.845.0158. E-mail: djdarens@ chem.tamu.edu. Notes

The author declares no competing financial interest. D



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(14) GPC separates macromolecules in solution by their effective size. That is, molecules of various sizes elute the column at different rates, with the low molecular weight polymers retained longer (retention time) than high molecular weight polymers. The broader the GPC trace, the broader the molecular weight distribution. (15) Langanke, J.; Wolf, A.; Hofmann, J.; Böhm, K.; Subhani, M. A.; Müller, T. E.; Leitner, W.; Gürtler, C. Carbon Dioxide (CO2) as Sustainable Feedstock for Polyurethane Production. Green Chem. 2014, 16, 1865−1870. (16) Darensbourg, D. J. Salen Metal Complexes as Catalysts for the Synthesis of Polycarbonates from Cyclic Ethers and Carbon Dioxide. Adv. Polym. Sci. 2011, 245, 1−28. (17) Byrne, C. M.; Allen, S. D.; Lobkovsky, E. B.; Coates, G. W. Alternating Copolymerization of Limonene Oxide and Carbon Dioxide. J. Am. Chem. Soc. 2004, 126, 11404−11405.

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Copolymerization of Epoxides and CO2: Polymer Chemistry for

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