The cycloaddition reaction of diynes with carbon dioxide is also applicable to the step-growth polymerization.18 The polymer thus produced has an all-carbon main-chain while the polymer can also be regarded as polyester (Scheme 2).

It should be noted that the polycarbonates produced in industry on a large scale are the aromatic ones, represented by the polycarbonates of bis-phenol A. This compound is widely used in daily life as Polycarbonate (PC) owing to its high impact resistance, stiffness, toughness, thermal stability, and transparency. In the Asahi process, now widely applied to industrial production, carbon originating from carbon dioxide is applied as the carbonyl group of polycarbonate.13 In this case, carbon dioxide is not directly used for the polycondensation with bisphenol and thus conversion of carbon dioxide to dialkyl and then diphenyl carbonates prior to polycondensation is required.

In addition to the direct incorporation of carbon dioxide into the polymer framework, preparation of a monomer from carbon dioxide and subsequent polymerization of the monomer is an alternative method for effective CO2 utilization. In the early 2010s, we focused our attention on lectone 17a,7b,8 which is generated from butadiene and CO2 using a Pd catalyst (Scheme 3).

KN received her doctorate from Kyoto University in 1991. Since 1991, she has been a faculty member at Kyoto University, moved to the University of Tokyo in 2002, and has been Professor at the University of Tokyo since 2003. Her research interests are focused on the development of homogeneous catalysts for polymer synthesis and organic synthesis.

Here in this article, polymer synthesis from CO2 was briefly reviewed. Whether it eventually contributes to mitigation of CO2 emission depends on the energy cost to pay for the synthesis and on the market size of the products. Recycling of the products also needs to be taken into consideration. In the case of propylene carbonate (PPC), a copolymer of propylene oxide and CO2, it took nearly half a century since its discovery11a to find industrial applications and the market is still very much smaller than that of the commodity polymers.

In addition to polycarbonates, carbon dioxide can be also incorporated into a polymer-chain as caboxylic esters, which will be the main topic of this article. The polymers discussed here can be classified into two types depending on the elements constituting the mainchain, namely polymers with all carbon mainchain and those containing oxygen. In both cases, the oxidation state of the carbon atom originating from carbon dioxide is in its reduced form. For the synthesis of other classes of polymers, such as poly urethanes and polyureas maintaining the same oxidation state as carbon dioxide, the readers are guided to refer the latest review articles.14

Carbon dioxide is the final product of the combustion of carbon containing materials and is in the most stable form of carbon in air. Owing to its easy availability, efficient chemical transformations of carbon dioxide to useful compounds have attracted much attention both in academia and in industry aiming at the realization of sustainable society. For conversion of carbon dioxide into other compounds, it is essential to combine it with additional chemical or physical energy. Here we focus on the use of chemical energy for the utilization of carbon dioxide.1

Polymer production from carbon dioxide is also an attractive research field. Polycarbonates, polymers having carbonate linkage (-OC(=O)O-) are one of the most representative classes of polymers made from carbon dioxide. As chain-growth polymerization, copolymerization of carbon dioxide with epoxides affording aliphatic polycarbonates has been studied for more than half a century since the first report by S. Inoue et al.5,11 Various homogeneous and heterogeneous catalysts have been developed by many groups including us.11g–11l Recently, an alternative approach for aliphatic polycarbonate synthesis was invented by Tamura, Tomishige, and their coauthors;12 a step-growth polymerization of carbon dioxide with aliphatic diols was accomplished using 2-cyanopyridine as a dehydrating reagent.

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b)J. H. Kamps, T. Hoeks, E. Kung, J. P. Lens, P. J. McCloskey, B. A. J. Noordover, J. P. A. Heuts, Polym. Chem. 2016, 7, 5294. 10.1039/C6PY00925E

j)K. Nakano, S. Hashimoto, M. Nakamura, T. Kamada, K. Nozaki, Angew. Chem., Int. Ed. 2011, 50, 4868. 10.1002/anie.201007958

Polyesters, polymers having carboxylic ester linkages (-C(=O)O-) are a major class of polymers, the fourth most produced polymer following polyethylene, polypropylene, and polyvinyl chloride. Especially, polyethylene terephthalate (PET) is one of the most common thermoplastic polymers used as fibers for cloths and containers for liquids. On the other hand, aliphatic polyesters capture increasing attention recently owing to their biodegradable and/or biocompatible nature. Ring-opening polymerization of lactones and lactides and dehydrative polycondensation of dicarboxylic acids and diols or the self-poly condensation of hydroxyacids are representative synthetic methods. Although copolymerization of olefins such as ethylene and propylene with carbon dioxide would be an ideal method for the synthesis of aliphatic polyesters, this still remains as a big challenge in polymer synthesis.15,16

Carbon dioxide (CO2) is an attractive raw material for chemical synthesis owing to its abundance and low toxicity. Among various utilizations of CO2 for the synthesis of useful compounds, this article focuses on its use for polymer synthesis. While synthesis of polyurea, polyurethane, and aliphatic and aromatic polycarbonates have been well-studied, this article focuses on another class of approach that uses a lactone derived from CO2 and 1,3-butadiene. Recent publications are summarized in relation to our synthesis of polylactone.

The ester groups in the polymer of 1 are reactive for further derivatization.22 Hydrolysis of the lactone units took place to afford a polymer possessing trans-cyclopentan-1,2-diyl moiety in the chain (Scheme 8, top). Since the carboxy group and the hydroxy group are located cis to each other on the cyclopentane group, they are located close to each other in this open form. As such, the dehydrative lactonization took place upon simple heating. Notably, the molecular weight and polydispersity hardly changed during the ring-opening hydrolysis–ring-closing dehydration process. On the other hand, while the lactone ring was partially opened by addition of benzylamine, the polymer was unchanged upon heating (Scheme 8, bottom). When the hydroxy group was converted to a leaving group by treatment with mesityl chloride under basic conditions, the nitrogen not on the same cyclopentane ring but on the neighboring group replaced the oxygen to form a seven-membered lactone. This can be explained in that the nucleophilic attack by the amide nitrogen needs to approach from the backside of the leaving mesylate.

The polymers here introduced still need further optimization of physical properties to find their commercial application. Although copolymerization with other monomers or kneading with other polymers would improve their material properties, then we face the dilemma of decreasing the total CO2 consumption. Continuous efforts are needed to make the polymers truly “green”.

The examples mentioned above are rather straightforward in a sense that the carbon dioxide is used as a comonomer in the same way as its copolymerization with epoxides. Unfortunately, however, the large-scale availability of the diynes is rather limited.

In 2014, we found lactone 1 undergoes radical polymerization under emulsion polymerization conditions.16 When lactone 1 was allowed to react under neat conditions in the presence of AIBN as an initiator at 100 °C, a polymeric material was obtained in 18% yield. The molecular weight of the product was estimated by SEC to be Mn 2,200 (Mw/Mn 1.4) (Scheme 5, top). Based on 13C NMR analysis, the sole structure of the repeating unit was assigned to be unit a. The radical addition to lactone 1 may take place to provide either A (path i) or B (path ii) as shown in Scheme 6. In path i, the cyclized radical A′ is less stable than A but should be more reactive to add to the next monomer owing to less steric demand around the radical center. In path ii, the generated radical B can also cyclize to B′ which also possibly attacks the next monomer. In both cases, cyclized structure unit a is provided.

For reviews and selected publications from our group; b)H. Sugimoto, S. Inoue, J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5561. 10.1002/pola.20319

Aiming at the improvement of molecular weight, addition of Lewis acid was examined since they are known to interact with the α,β-unsaturated ester moiety under radical polymerization conditions.20 When ZnCl2 was added in an ethylene carbonate solution, the molecular weight was increased to Mn 85,000 (Mw/Mn 1.5) by the expense of structural regularity; namely, in addition to unit a, two other repeating units b and c were detected in the polymer chain in a ratio of a:b:c = 3:5:2 (Scheme 5, middle). Based on the mechanism drawn in Scheme 6, the formation of units b and c can be explained as follows: If radical B generated by path ii attacks the next monomer without cyclization, unit b forms (path iii). Meanwhile, if radical A produced by path i isomerizes to radical C via 1,5-hydrogen transfer (path iv), C and C′ are resonance structures to each other and C′ gives unit c.

V. H. Duparc, R. M. Shakaroun, M. Slawinski, J.-F. Carpentiner, S. M. Guillaume, Eur. Polym. J. 2020, 134, 109858. 10.1016/j.eurpolymj.2020.109858

Since the advantage of using carbon dioxide as a carbon source is its abundance and low price, the transformations should be oriented to a large-scale synthesis of bulk chemicals. The compound to be brought into the process as chemical energy must be easily available in a large quantity, and hopefully is from renewable resources.2 Representative examples of such transformations are summarized in Figure 1. In addition to the industrialized processes such as dialkyl carbonate, urea, and salicylic acid synthesis, recent publications are targeted to hydrogenation of carbon dioxide to formic acid or methanol,3 reaction with ethylene to form acrylates,4 coupling with epoxides for cyclic carbonate synthesis,5,6 etc. Cycloaddition reactions of carbon dioxide with unsaturated carbon–carbon bonds providing lactones has been known since the pioneering work by Sasaki, Y. Inoue, and Hashimoto in the 1970s.7 Mediated by palladium or nickel catalyst, six-membered lactones are obtained from 1,3-dienes,7a,7b,8 acetylenes,7c,9 and 1,2-dienes.10

b)For the synthesis of small complex molecules, see A. Tortajada, F. Juliá-Hernández, M. Börjesson, T. Marogas, R. Martin, Angew. Chem., Int. Ed. 2018, 57, 15948. 10.1002/anie.201803186

Since lectone 1 has reactive functional groups such as carbon–carbon double bonds and an ester group, various chemical transformations have been reported, such as hydrogenation, hydroformylation, hydroamination, alkoholysis, etc.8a As for polymer synthesis, step-growth polymerization was reported by the addition of dithiols to both of the double bonds in lactone 1 by Dinjus et al. in 1998 (Scheme 4a).19 In this paper, the authors reported their attempt at the homopolymerization of 1 at 80 °C resulting in no polymerization reaction (Scheme 4b). They explained that the low reactivity of lactone 1 toward radical polymerization came from the stability of radicals A and B (Scheme 4c).

c)J. Artz, T. E. Müller, K. Thenert, J. Kleinekorte, R. Meys, A. Sternberg, A. Bardow, W. Leitner, Chem. Rev. 2018, 118, 434. 10.1021/acs.chemrev.7b00435

d)F. M. Baena-Moreno, M. Rodríguez-Galán, F. Vega, B. Alonso-Fariñas, L. F. V. Arenas, B. Navarrete, Energy Sources, Part A 2019, 41, 1403. 10.1080/15567036.2018.1548518

V. Haack, E. Dinjus, S. Pitter, Angew. Makromol. Chem. 1998, 257, 19. 10.1002/(SICI)1522-9505(19980601)257:1%3C19::AID-APMC19%3E3.0.CO%3B2-T

b)Y. Gu, K. Matsuda, A. Nakayama, M. Tamura, Y. Nakagawa, K. Tomishige, ACS Sustainable Chem. Eng. 2019, 7, 6304. 10.1021/acssuschemeng.8b06870

Although intensive efforts devoted for ring-opening polymerization of 1 or its hydrogenated derivatives, no successful report appeared yet, so far.25 As another class of monomer derived from CO2, Lu reported a multistep synthesis of α-methylene β-lactone 3 from 2-butyne and CO2. Subsequent ROP of the lactone 3 afforded polyester (Scheme 10).26

c)K. Sordakis, C. Tang, L. K. Vogt, H. Junge, P. J. Dyson, M. Beller, G. Laurenczy, Chem. Rev. 2018, 118, 372. 10.1021/acs.chemrev.7b00182

In 2017, Lin reported that the polymerization of 1 takes place at 180 °C under air in the absence of any additives or solvents (Scheme 5, bottom).21 The authors characterized the four unit-structures in the obtained polymer; namely, in addition to units a, b, and c, the authors detected the formation of unit d. Seemingly, at high temperature under neat condition, even the very hindered and stabilized radical A can add to another monomer generating unit d (path v in Scheme 6).

d)S. Fukuoka, I. Fukawa, M. Tojo, K. Oonishi, H. Hachiya, M. Aminaka, K. Hasegawa, K. Komiya, Catal. Surv. Asia 2010, 14, 146. 10.1007/s10563-010-9093-5

Kyoko Nozaki, New Polymers Made from Carbon Dioxide and Alkenes, Bulletin of the Chemical Society of Japan, Volume 94, Issue 3, March 2021, Pages 984–988, https://doi.org/10.1246/bcsj.20200402

c)S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya, K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa, S. Konno, Green Chem. 2003, 5, 497. 10.1039/B304963A

a)W.-H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck, E. Fujita, Chem. Rev. 2015, 115, 12936. 10.1021/acs.chemrev.5b00197

The use of bifunctional monomers for step-growth polymerization is an effective approach. Namely, when bis(carbon nucleophile) and bis (carbon electrophile) are allowed to react with carbon dioxide, polyesters are possibly produced. For example, dianions derived from diynes react with carbon dioxide to form dicarboxylic acid anions and then undergo dialkylation by α,ω-dihaloalkanes on both ends providing polymeric materials (Scheme 1).17

The polylactone can be also synthesized by a one-pot two-step process. After the palladium-catalyzed reaction of butadiene and CO2 in ethylene carbonate, all volatiles were removed, and then to the residue, addition of ZnCl2 and V-40 provided poly-1 (Scheme 7). Similarly, a mixed lactone was obtained starting from butadiene and isoprene or 1,3-pentadiene and the crude lactone was successfully subjected to further polymerization.

c)N. Huguet, I. Jevtovikj, A. Gordillo, M. L. Lejkowski, R. Lindner, M. Bru, A. Y. Khalimon, F. Rominger, S. A. Schunk, P. Hofmann, M. Limbach, Chem.—Eur. J. 2014, 20, 16858. 10.1002/chem.201405528

a)M. L. Lejkowski, R. Lindner, T. Kageyama, G. É. Bódizs, P. N. Plessow, I. B. Müller, A. Schäfer, F. Rominger, P. Hofmann, C. Futter, S. A. Schunk, M. Limbach, Chem.—Eur. J. 2012, 18, 14017. 10.1002/chem.201201757

a)S. Fukuoka, I. Fukawa, T. Adachi, H. Fujita, N. Sugiyama, T. Sawa, Org. Process Res. Dev. 2019, 23, 145. 10.1021/acs.oprd.8b00391

Lactone 1 was further converted to another multifunctional monomer 2 by methanolysis of 1 followed by conversion to a methacrylate ester of the resulting alcohol moiety (Scheme 9). Two α.β-unsaturated ester moieties coexist in compound 2. Only methacrylate moiety underwent radical polymerization under mild conditions while the crotonate also contributed to the polymerization under harsh conditions.23 The monomer 2 is also applicable to palladium-catalyzed copolymerization with ethylene.24