news & views POLYMER SYNTHESIS
Chaining up carbon dioxide
The development of methods for efficiently using carbon dioxide in synthesis would enable chemists to tap into this abundant resource. Now, an indirect route to the copolymerization of alkenes with carbon dioxide shows how this greenhouse gas may prove useful in the search for new ‘green’ materials.
Andrew P. Dove
C
arbon dioxide is a focus of global attention because of the implication of its emission on the problem of climate change. Remediation strategies that convert CO2 into other materials are greatly preferable to storage options because they offer the potential to create new commercially viable products from a source of waste. The use of CO2 in chemical synthesis is largely limited by its low reactivity, so despite its abundance and low cost, few robust reactions are known that can effectively utilise it. Although some methods have been reported1 for the conversion of CO2 into more useful compounds, many of them rely on extremes of temperature and pressure to work. One notable exception, however, is the copolymerization of carbon dioxide with highly reactive epoxides to yield polycarbonates2,3. Some success has been achieved in producing new materials using this approach and there remains a broad scope of possible work to improve catalyst activity and selectivity. Unfortunately, these systems offer limited flexibility for the design of materials with diverse properties. The direct combination of CO2 with alkenes does, in theory, offer alternative routes to CO2-based materials, but such processes are thermodynamically unfavourable and so the notional reaction products have remained elusive. Now, as they report in Nature Chemistry, Kyoko Nozaki and co-workers have demonstrated4 a new method of incorporating CO2 into polymeric materials by copolymerizing it with butadiene in a process that involves a metastable lactone intermediate (Fig. 1). This approach uses chemistry that was first reported5 by Inoue and colleagues in 1976 and later optimized by Behr and others6–9 to predominantly produce a six-membered δ-lactone by direct reaction of CO2 and butadiene using a palladium catalyst. Nozaki and colleagues first investigated the problem computationally, and found that although the direct reaction of CO2 with either ethylene or butadiene has to compete 276
a O CO2
[Pd]
+
Tiglate
O
+ 1
O
Allylic
2
OR
O
3
OR
O
O
O 4
R=
5
6
H
V-40/ZnCl2 One-pot, two-steps
b
Random polymer of 1–4
O
O
V-40/ZnCl2
O
O
1 Purified
x
O
y
O
1–x–y
O
O
Figure 1 | The synthesis of alkene/CO2 copolymers via a lactone intermediate. a, CO2 reacts with butadiene in the presence of a palladium catalyst to form lactone 1 and other minor products 2–6. Treatment of this mixture with a radical initiator (V-40) and a Lewis acid such as ZnCl2 resulted in formation of a polymer butadiene/CO2 copolymer incorporating small amounts of alkenes 2–4. b, With a purification step included in the process, pure lactone 1 can be polymerized under the same conditions to give a copolymer with a higher glass-transition temperature and greater CO2 incorporation relative to the material obtained from the one-pot, two-step procedure shown in part a. V-40, 1,1’-azobis(cyclohexane-1-carbonitrile).
with the significantly more exothermic homopolymerization of the alkene, the formation of the lactone intermediate is isothermic and its subsequent polymerization is highly exothermic. Despite the potential of this approach, it seems that only one previous report exists in which polymerization of the lactone intermediate was attempted. Dinjus and co-workers reported10 that a photoinitiated polyaddition reaction between the lactone
and a range of multifunctional thiol compounds led to polymers with modest molar masses. They also noted that their attempts to induce direct polymerization of the lactone by anionic, cationic or radical mechanisms were unsuccessful. Importantly, Nozaki and colleagues observed that, on standing, the lactone underwent oligomerization, which led the team to further investigate radical conditions for the homopolymerization of the CO2-derived
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
news & views lactone monomer. It was found that by using emulsion conditions with a Lewis-acid additive, polymers with molar masses of up to 85 kg mol–1 could be produced. Analysis of the resulting polymer with NMR spectroscopy revealed that, in the absence of Lewis-acid additives, a camphor-like bicyclic structure was formed by the alternating insertion of tiglate and allylic moieties into the growing polymer chain — a process that overcomes the steric constraints that would be associated with the sequential addition of tiglates. Under the optimized conditions that used Lewis acids, however, other monocyclic rings were observed in the backbone of the polymeric product (Fig. 1b). Most notably, perhaps, is that unlike many polymers made from CO2, these materials were found to exhibit glass transitions up to 192 °C, which highlights their potential as engineering plastics. The team went on to show that the copolymers could also be made in a one-pot, two-step process in which — following lactone synthesis and venting of excess butadiene — the lactone intermediate could be polymerized without needing to be purified first. Under these conditions, the final CO2 content of the copolymers was slightly lower than for the stepwise procedure that involved purification (27 compared with 33 mol%). Moreover, the resulting polymers had reduced molar masses and glass-transition temperatures, most likely as a consequence of the
incorporation of other alkene-containing by-products into the polymer chains. Nonetheless, this method was applied to the terpolymerization of CO2 and butadiene with isoprene and 1,3-pentanediene in an attempt to broaden the applicability of this approach to materials synthesis. Incorporating up to 33 mol% (29 wt%) of CO2 into a new polymeric material is an impressive feat, and the copolymerization with butadiene — which can itself be generated from renewable sources — makes this a particularly exciting advance in the area of ‘green’ materials. Although this methodology certainly overcomes the problem of copolymerizing CO2 with an alkene and presents a new method for the production of polymer materials that are derived from CO2, the general applicability of the methodology is unclear at this stage. In addition, more needs to be understood about the mechanical and degradative properties of these materials to discover if they can be applied as replacements for existing ones — and a question remains concerning how stable these materials are in the environment. Does this perhaps offer an opportunity to generate a commodity polymer that can overcome not only the problem of CO2 utilization, but also of persistence of materials in our environment? Time will tell. Some potential versatility in the feedstock has been shown, yet to access a wider range of materials the extension of this methodology to direct copolymerization of
CO2 with other dienes would be desirable. Post-polymerization functionalization of the lactone rings and residual alkenes also offers opportunities to expand the range of properties of these polymers. Finally, despite ATRP (atom-transfer radical polymerization) and nitroxidemediated polymerization methodologies being shown to be unsuccessful for the controlled polymerization of this system, the application of chain-transfer agents — including those capable of mediating RAFT (reversible activation-fragmentation chain transfer) polymerization — may provide opportunities to specifically tailor these polymers and firmly establish them at the forefront of a new generation of ‘green’ materials. ❐ Andrew Dove is in the Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. e-mail: [email protected] References 1. Aresta, M. (ed.) Carbon Dioxide as Chemical Feedstock (Wiley-VCH, 2010). 2. Darensbourg, D. J. & Wilson, S. J. Green Chem. 14, 2665–2671 (2012). 3. Coates, G. W. & Moore, D. R. Angew. Chem. Int. Ed. 43, 6618–6639 (2004). 4. Nakano, R., Ito, S. & Nozaki, K. Nature Chem. 6, 325–331 (2014). 5. Sasaki, Y., Inoue, Y. & Hashimoto, H. J. Chem. Soc. Chem. Commum. 605–606 (1976). 6. Behr, A. & Juszak, K.D. J. Organomet. Chem. 255, 263–268 (1983). 7. Braunstein, P., Matt, D. & Nobel, D. J. Am. Chem. Soc. 110, 3207–3212 (1988). 8. Dinjus, E. & Leitner, W. Appl. Organomet. Chem. 9, 43–50 (1995). 9. Behr, A. & Becker, M. Dalton Trans. 4607–4613 (2006). 10. Haack, V., Dinjus, E. & Pitter, S. Die Angew. Makromol. Chem. 257, 19–22 (1998).
METAL–ORGANIC FRAMEWORKS
Recognizing the unrecognizable
Separating carbon monoxide from chemically similar nitrogen gas is particularly challenging. Now, a flexible porous coordination polymer has been developed that recognizes carbon monoxide over nitrogen, with structural changes in the material leading to its accelerated adsorption.
Krista S. Walton
C
arbon monoxide is an important raw material in the synthesis of a variety of basic chemicals, including methanol and aldehydes, and can also be used in Fischer–Tropsch reactions for the production of liquid fuels. One of the largest sources of CO is the incomplete combustion of hydrocarbons, with more than 100 million tons of carbon monoxide produced annually in the United States alone1. However, most of these sources of CO are gas mixtures containing various amounts of nitrogen, carbon dioxide,
hydrogen, methane or water, complicating their use as a feedstock. The separation of CO from N2 in particular is a notoriously difficult problem due to the similarity of the molecules in terms of molecular weight and physical properties, with few CO-selective adsorbents available2. Materials that do provide high selectivity for CO over N2 typically contain transition metal sites to chemisorb the CO. However, because of the strong CO–metal interactions, the high temperatures needed to desorb the CO and reuse the material offsets
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
the advantage of this high selectivity. Therefore, designing a porous material that simultaneously recognizes CO with reasonable adsorption energies and rejects N2 would be a major advance. Writing in Science, Kitagawa and co-workers describe a novel porous coordination polymer (PCP) with open metal sites, that exhibits unique CO adsorption capabilities3. PCPs (or MOFs) are crystalline materials made from organic ligands and metal salts and exhibit an array of characteristics from high surface 277
Chaining up carbon dioxide
The development of methods for efficiently using carbon dioxide in synthesis would enable chemists to tap into this abundant resource. Now, an indirect route to the copolymerization of alkenes with carbon dioxide shows how this greenhouse gas may prove useful in the search for new ‘green’ materials.
Andrew P. Dove
C
arbon dioxide is a focus of global attention because of the implication of its emission on the problem of climate change. Remediation strategies that convert CO2 into other materials are greatly preferable to storage options because they offer the potential to create new commercially viable products from a source of waste. The use of CO2 in chemical synthesis is largely limited by its low reactivity, so despite its abundance and low cost, few robust reactions are known that can effectively utilise it. Although some methods have been reported1 for the conversion of CO2 into more useful compounds, many of them rely on extremes of temperature and pressure to work. One notable exception, however, is the copolymerization of carbon dioxide with highly reactive epoxides to yield polycarbonates2,3. Some success has been achieved in producing new materials using this approach and there remains a broad scope of possible work to improve catalyst activity and selectivity. Unfortunately, these systems offer limited flexibility for the design of materials with diverse properties. The direct combination of CO2 with alkenes does, in theory, offer alternative routes to CO2-based materials, but such processes are thermodynamically unfavourable and so the notional reaction products have remained elusive. Now, as they report in Nature Chemistry, Kyoko Nozaki and co-workers have demonstrated4 a new method of incorporating CO2 into polymeric materials by copolymerizing it with butadiene in a process that involves a metastable lactone intermediate (Fig. 1). This approach uses chemistry that was first reported5 by Inoue and colleagues in 1976 and later optimized by Behr and others6–9 to predominantly produce a six-membered δ-lactone by direct reaction of CO2 and butadiene using a palladium catalyst. Nozaki and colleagues first investigated the problem computationally, and found that although the direct reaction of CO2 with either ethylene or butadiene has to compete 276
a O CO2
[Pd]
+
Tiglate
O
+ 1
O
Allylic
2
OR
O
3
OR
O
O
O 4
R=
5
6
H
V-40/ZnCl2 One-pot, two-steps
b
Random polymer of 1–4
O
O
V-40/ZnCl2
O
O
1 Purified
x
O
y
O
1–x–y
O
O
Figure 1 | The synthesis of alkene/CO2 copolymers via a lactone intermediate. a, CO2 reacts with butadiene in the presence of a palladium catalyst to form lactone 1 and other minor products 2–6. Treatment of this mixture with a radical initiator (V-40) and a Lewis acid such as ZnCl2 resulted in formation of a polymer butadiene/CO2 copolymer incorporating small amounts of alkenes 2–4. b, With a purification step included in the process, pure lactone 1 can be polymerized under the same conditions to give a copolymer with a higher glass-transition temperature and greater CO2 incorporation relative to the material obtained from the one-pot, two-step procedure shown in part a. V-40, 1,1’-azobis(cyclohexane-1-carbonitrile).
with the significantly more exothermic homopolymerization of the alkene, the formation of the lactone intermediate is isothermic and its subsequent polymerization is highly exothermic. Despite the potential of this approach, it seems that only one previous report exists in which polymerization of the lactone intermediate was attempted. Dinjus and co-workers reported10 that a photoinitiated polyaddition reaction between the lactone
and a range of multifunctional thiol compounds led to polymers with modest molar masses. They also noted that their attempts to induce direct polymerization of the lactone by anionic, cationic or radical mechanisms were unsuccessful. Importantly, Nozaki and colleagues observed that, on standing, the lactone underwent oligomerization, which led the team to further investigate radical conditions for the homopolymerization of the CO2-derived
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
news & views lactone monomer. It was found that by using emulsion conditions with a Lewis-acid additive, polymers with molar masses of up to 85 kg mol–1 could be produced. Analysis of the resulting polymer with NMR spectroscopy revealed that, in the absence of Lewis-acid additives, a camphor-like bicyclic structure was formed by the alternating insertion of tiglate and allylic moieties into the growing polymer chain — a process that overcomes the steric constraints that would be associated with the sequential addition of tiglates. Under the optimized conditions that used Lewis acids, however, other monocyclic rings were observed in the backbone of the polymeric product (Fig. 1b). Most notably, perhaps, is that unlike many polymers made from CO2, these materials were found to exhibit glass transitions up to 192 °C, which highlights their potential as engineering plastics. The team went on to show that the copolymers could also be made in a one-pot, two-step process in which — following lactone synthesis and venting of excess butadiene — the lactone intermediate could be polymerized without needing to be purified first. Under these conditions, the final CO2 content of the copolymers was slightly lower than for the stepwise procedure that involved purification (27 compared with 33 mol%). Moreover, the resulting polymers had reduced molar masses and glass-transition temperatures, most likely as a consequence of the
incorporation of other alkene-containing by-products into the polymer chains. Nonetheless, this method was applied to the terpolymerization of CO2 and butadiene with isoprene and 1,3-pentanediene in an attempt to broaden the applicability of this approach to materials synthesis. Incorporating up to 33 mol% (29 wt%) of CO2 into a new polymeric material is an impressive feat, and the copolymerization with butadiene — which can itself be generated from renewable sources — makes this a particularly exciting advance in the area of ‘green’ materials. Although this methodology certainly overcomes the problem of copolymerizing CO2 with an alkene and presents a new method for the production of polymer materials that are derived from CO2, the general applicability of the methodology is unclear at this stage. In addition, more needs to be understood about the mechanical and degradative properties of these materials to discover if they can be applied as replacements for existing ones — and a question remains concerning how stable these materials are in the environment. Does this perhaps offer an opportunity to generate a commodity polymer that can overcome not only the problem of CO2 utilization, but also of persistence of materials in our environment? Time will tell. Some potential versatility in the feedstock has been shown, yet to access a wider range of materials the extension of this methodology to direct copolymerization of
CO2 with other dienes would be desirable. Post-polymerization functionalization of the lactone rings and residual alkenes also offers opportunities to expand the range of properties of these polymers. Finally, despite ATRP (atom-transfer radical polymerization) and nitroxidemediated polymerization methodologies being shown to be unsuccessful for the controlled polymerization of this system, the application of chain-transfer agents — including those capable of mediating RAFT (reversible activation-fragmentation chain transfer) polymerization — may provide opportunities to specifically tailor these polymers and firmly establish them at the forefront of a new generation of ‘green’ materials. ❐ Andrew Dove is in the Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. e-mail: [email protected] References 1. Aresta, M. (ed.) Carbon Dioxide as Chemical Feedstock (Wiley-VCH, 2010). 2. Darensbourg, D. J. & Wilson, S. J. Green Chem. 14, 2665–2671 (2012). 3. Coates, G. W. & Moore, D. R. Angew. Chem. Int. Ed. 43, 6618–6639 (2004). 4. Nakano, R., Ito, S. & Nozaki, K. Nature Chem. 6, 325–331 (2014). 5. Sasaki, Y., Inoue, Y. & Hashimoto, H. J. Chem. Soc. Chem. Commum. 605–606 (1976). 6. Behr, A. & Juszak, K.D. J. Organomet. Chem. 255, 263–268 (1983). 7. Braunstein, P., Matt, D. & Nobel, D. J. Am. Chem. Soc. 110, 3207–3212 (1988). 8. Dinjus, E. & Leitner, W. Appl. Organomet. Chem. 9, 43–50 (1995). 9. Behr, A. & Becker, M. Dalton Trans. 4607–4613 (2006). 10. Haack, V., Dinjus, E. & Pitter, S. Die Angew. Makromol. Chem. 257, 19–22 (1998).
METAL–ORGANIC FRAMEWORKS
Recognizing the unrecognizable
Separating carbon monoxide from chemically similar nitrogen gas is particularly challenging. Now, a flexible porous coordination polymer has been developed that recognizes carbon monoxide over nitrogen, with structural changes in the material leading to its accelerated adsorption.
Krista S. Walton
C
arbon monoxide is an important raw material in the synthesis of a variety of basic chemicals, including methanol and aldehydes, and can also be used in Fischer–Tropsch reactions for the production of liquid fuels. One of the largest sources of CO is the incomplete combustion of hydrocarbons, with more than 100 million tons of carbon monoxide produced annually in the United States alone1. However, most of these sources of CO are gas mixtures containing various amounts of nitrogen, carbon dioxide,
hydrogen, methane or water, complicating their use as a feedstock. The separation of CO from N2 in particular is a notoriously difficult problem due to the similarity of the molecules in terms of molecular weight and physical properties, with few CO-selective adsorbents available2. Materials that do provide high selectivity for CO over N2 typically contain transition metal sites to chemisorb the CO. However, because of the strong CO–metal interactions, the high temperatures needed to desorb the CO and reuse the material offsets
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
the advantage of this high selectivity. Therefore, designing a porous material that simultaneously recognizes CO with reasonable adsorption energies and rejects N2 would be a major advance. Writing in Science, Kitagawa and co-workers describe a novel porous coordination polymer (PCP) with open metal sites, that exhibits unique CO adsorption capabilities3. PCPs (or MOFs) are crystalline materials made from organic ligands and metal salts and exhibit an array of characteristics from high surface 277
