In 1973 Kobayashi, Inoue and Tsuruta reported that dialkylzinc–dicarboxylic acid and dialkylzinc–hydroxycarboxylic acid systems catalyze the copolymerization of carbon dioxide and propylene oxide to a polycarbonate; the detailed mechanism of the catalysis, the exact structure of the active species of the zinc carboxylate catalysts these systems produce, and the molecular origin of the acid R-group's effect on activity remained open. These systems evolved into the air-stable zinc dicarboxylates, above all zinc glutarate, the benchmark heterogeneous catalyst for poly(propylene carbonate) and the basis of its first commercial production. Later work established the chain-start stoichiometry in diethylzinc-based systems (one carboxylate residue per chain, with initiation considered to proceed by monomer insertion into a zinc–carboxylate bond), the bulk crystal structure of zinc glutarate, the full alternating-insertion mechanism for well-defined homogeneous dizinc complexes, and the empirical epoxide/organozinc scope. What remains unresolved, and is the surviving open core of the question, is the structure of the catalytically active species — above all the surface active site of the heterogeneous zinc dicarboxylate catalysts, with zinc glutarate as the benchmark — and the definitive set of elementary steps performed there: reviews as late as 2019 state that the elementary reaction steps of zinc glutarate 'are not known with certainty', competing active-site proposals coexist (bimetallic sites about 4–5 Å apart; surface or lattice hydroxyl initiation), and 2024–2026 studies continue to regulate, characterize and model the active sites. The open problem is to identify the active species and its active site, establish the catalytic cycle with discriminating evidence, and explain the R-group activity series at the molecular level.

Background

Carbon dioxide can be incorporated into polymers by the alternating copolymerization of CO2 with an epoxide — a three-membered cyclic ether such as propylene oxide (PO) — which yields an aliphatic polycarbonate; with PO the product is poly(propylene carbonate) (PPC), a degradable plastic. The reaction was discovered in 1969 by Inoue, Koinuma and Tsuruta, who used partially hydrolysed diethylzinc (Et2Zn, a pyrophoric dialkylzinc, that is, an organozinc compound bearing two alkyl groups on zinc) as the initiator. A family of two-component organozinc catalysts followed, in which the dialkylzinc is combined with a protic co-reactant: dihydric phenols (phenols bearing two OH groups; Kobayashi, Inoue and Tsuruta 1971, with a fuller study by Kobayashi, Tang, Tsuruta and Inoue in 1973), and — in the 1973 source work of Kobayashi, Inoue and Tsuruta — carboxylic acids. That communication reported that combinations of dialkylzinc with dicarboxylic acids (acids bearing two carboxyl groups, such as glutaric acid) and with hydroxycarboxylic acids (acids bearing both a hydroxyl and a carboxyl group, such as lactic acid) are effective catalysts for the CO2/PO copolymerization, observed structure–activity trends among the acids, and left the detailed mechanism, the exact structure of the active species, the role of the acid R group, and the scope of the reaction open.

The reaction product of a dialkylzinc with a dicarboxylic acid is a zinc dicarboxylate. In 1977 Soga and co-workers introduced air-stable zinc dicarboxylate catalysts that need no pyrophoric organozinc reagent, and for glutaric acid the product — zinc glutarate (ZnGA) — became the benchmark heterogeneous (that is, insoluble solid) catalyst for PPC synthesis and the catalyst of its first commercial production. Its bulk crystal structure is known (tetrahedral ZnO4 units linked by bridging glutarate ligands into a three-dimensional network), and empirical structure–activity rules are documented: activity rises with catalyst crystallinity, is enhanced by Zn(II)-ethylsulfinate initiating groups (ethylsulfinate being a sulfur-based ligand traceable to the organozinc precursor), and depends on the carboxylate R group and on the zinc source used in synthesis.

Three bodies of knowledge frame the open question. First, for well-defined soluble dizinc complexes — most prominently zinc β-diiminate complexes (β-diiminate being a bidentate nitrogen ligand supporting two adjacent zinc centers) — the copolymerization mechanism is established: chain growth alternates between insertion of CO2 into a Zn–alkoxide bond and ring opening of the epoxide at a neighboring zinc center (the bimetallic mechanism), while intramolecular backbiting of the growing carbonate chain expels the five-membered cyclic carbonate (propylene carbonate) as by-product; quantum-chemical studies support this picture (Liu, Torrent and Morokuma 2002; Chisholm and Zhou 2004; reviewed by Coates and Moore 2004). Second, for the zinc carboxylate systems the chain-start stoichiometry is established: one carboxylate residue is incorporated per polymer chain in diethylzinc-based systems, and initiation is considered to proceed by insertion of a monomer into a zinc–carboxylate bond (Hino, Yoshida and Inoue 1984); which entity actually initiates on the heterogeneous zinc dicarboxylate surface remains among the open items. Third, for the heterogeneous zinc dicarboxylate surface itself the picture stops: the catalytically active site is an unknown, presumably small fraction of the solid's surface; synchrotron X-ray absorption spectroscopy (XAFS, which probes the local coordination of the zinc atoms) and NEXAFS (near-edge X-ray absorption fine structure, a surface-sensitive synchrotron probe) studies of zinc glutarates exist; theory has proposed bimetallic surface sites separated by about 4–5 Å; and reviews state that the elementary reaction steps of zinc glutarate 'are not known with certainty' and that the proposed surface hydroxyl initiating entities have 'remained difficult' to detect (Marbach, Höfer, Bornholdt and Luinstra 2019). This gap between the established homogeneous-model mechanisms and the unestablished heterogeneous active site is precisely the surviving part of the 1973 question.

Problem Statement

The 1973 work of Kobayashi, Inoue and Tsuruta raised the question the field has not yet completed: what is the detailed mechanism of catalysis, and what is the exact structure of the active species, by which dialkylzinc–carboxylic acid systems copolymerize carbon dioxide and epoxides? In its present, still-open form, the problem is to determine, for the zinc carboxylate catalysts that these systems produce — with the preformed heterogeneous zinc dicarboxylates (and zinc hydroxycarboxylates), above all zinc glutarate, ZnGA, as the technologically central benchmark members: (1) the structure of the catalytically active species/site(s), including the zinc coordination environment, the identity of the initiating group (zinc–carboxylate, zinc–hydroxyl, zinc–alkyl, or sulfinate), and the geometry and spacing of any cooperating zinc centers; (2) the elementary steps catalyzed at that site — initiation, propagation, chain transfer and cyclic-carbonate formation — and which of the competing published models the existing evidence best supports, together with the measurement that would discriminate among them; and (3) a molecular-level explanation of how the carboxylic acid's R group governs the observed catalytic activity and selectivity. The empirically documented scope of the catalyst family for other epoxides, other organozinc compounds, and CO2 with other comonomers (air-stable zinc dicarboxylates; the zinc glutarate benchmark; organozinc scope studies) is established background with which a solution must remain consistent.

References

  1. Makoto Kobayashi, Shohei Inoue, Teiji Tsuruta (1973). Copolymerization of carbon dioxide and epoxide by the dialkylzinc–carboxylic acid system. Journal of Polymer Science: Polymer Chemistry Edition 11(9), 2383–2385. DOI: 10.1002/pol.1973.170110929. https://doi.org/10.1002/pol.1973.170110929
  2. Shohei Inoue, Hideomi Koinuma, Teiji Tsuruta (1969). Copolymerization of carbon dioxide and epoxide with organometallic compounds. Die Makromolekulare Chemie 130(1), 210–220. DOI: 10.1002/macp.1969.021300112. https://doi.org/10.1002/macp.1969.021300112
  3. Makoto Kobayashi, Shohei Inoue, Teiji Tsuruta (1971). Diethylzinc-Dihydric Phenol System as Catalyst for the Copolymerization of Carbon Dioxide with Propylene Oxide. Macromolecules 4(5), 658–659. DOI: 10.1021/ma60023a033. https://doi.org/10.1021/ma60023a033
  4. Makoto Kobayashi, Yang-Lik Tang, Teiji Tsuruta, Shohei Inoue (1973). Copolymerization of carbon dioxide and epoxide using dialkylzinc/dihydric phenol system as catalyst. Die Makromolekulare Chemie 169(1), 69–81. DOI: 10.1002/macp.1973.021690107. https://doi.org/10.1002/macp.1973.021690107
  5. Kazuo Soga, Kazuya Uenishi, Satoru Hosoda, Sakuji Ikeda (1977). Copolymerization of carbon dioxide and propylene oxide with new catalysts. Die Makromolekulare Chemie 178(3), 893–897. DOI: 10.1002/macp.1977.021780325. https://doi.org/10.1002/macp.1977.021780325
  6. Yoshihiro Hino, Yasuhiko Yoshida, Shohei Inoue (1984). Initiation Mechanism of the Copolymerization of Carbon Dioxide and Propylene Oxide with Zinc Carboxylate Catalyst Systems. Polymer Journal 16(2), 159–163. DOI: 10.1295/polymj.16.159. https://doi.org/10.1295/polymj.16.159
  7. Shohei Inoue (1987). Copolymerization of Carbon Dioxide and Epoxide. In: Carbon Dioxide as a Source of Carbon (NATO ASI Series, Series C, vol 206), pp 331–337. DOI: 10.1007/978-94-009-3923-3_18. https://doi.org/10.1007/978-94-009-3923-3_18
  8. Moonhor Ree, Jin Yong Bae, Jae Hwan Jung, Tae Joo Shin, Yong-Taek Hwang, Taihyun Chang (2000). Copolymerization of carbon dioxide and propylene oxide using various zinc glutarate derivatives as catalysts. Polymer Engineering & Science 40(7), 1542–1552. DOI: 10.1002/pen.11284. https://doi.org/10.1002/pen.11284
  9. Yue-Qing Zheng, Jian-Li Lin, Hong-Liang Zhang (2000). Crystal structure of zinc glutarate, Zn(C5H6O4). Zeitschrift für Kristallographie – New Crystal Structures 215(4), 535–536. DOI: 10.1515/ncrs-2000-0435. https://doi.org/10.1515/ncrs-2000-0435
  10. Malcolm H. Chisholm, Diana Navarro-Llobet, Zhiping Zhou (2002). Poly(propylene carbonate). 1. More about Poly(propylene carbonate) Formed from the Copolymerization of Propylene Oxide and Carbon Dioxide Employing a Zinc Glutarate Catalyst. Macromolecules 35(17), 6494–6504. DOI: 10.1021/ma020348+. https://doi.org/10.1021/ma020348+
  11. Zhiwei Liu, Maricel Torrent, Keiji Morokuma (2002). Molecular Orbital Study of Zinc(II)-Catalyzed Alternating Copolymerization of Carbon Dioxide with Epoxide. Organometallics 21(6), 1056–1071. DOI: 10.1021/om0110843. https://doi.org/10.1021/om0110843
  12. Jong-Seong Kim, Moonhor Ree, Tae Joo Shin, Oc Hee Han, Sung June Cho, Yong-Taek Hwang, Joong Yeon Bae, Jae Min Lee, Ryong Ryoo, Heesoo Kim (2003). X-ray absorption and NMR spectroscopic investigations of zinc glutarates prepared from various zinc sources and their catalytic activities in the copolymerization of carbon dioxide and propylene oxide. Journal of Catalysis 218(1), 209–219. DOI: 10.1016/S0021-9517(03)00082-4. https://doi.org/10.1016/S0021-9517(03)00082-4
  13. Jong-Seong Kim, Moonhor Ree, Seung W. Lee, W. Oh, S. Baek, B. Lee, Tae Joo Shin, Ki J. Kim, B. Kim, J. Lüning (2003). NEXAFS spectroscopy study of the surface properties of zinc glutarate and its reactivity with carbon dioxide and propylene oxide. Journal of Catalysis 218(2), 386–395. DOI: 10.1016/S0021-9517(03)00122-2. https://doi.org/10.1016/S0021-9517(03)00122-2
  14. Malcolm H. Chisholm, Zhiping Zhou (2004). Concerning the Mechanism of the Ring Opening of Propylene Oxide in the Copolymerization of Propylene Oxide and Carbon Dioxide To Give Poly(propylene carbonate). Journal of the American Chemical Society 126(35), 11030–11039. DOI: 10.1021/ja0394232. https://doi.org/10.1021/ja0394232
  15. Robert Eberhardt, Markus Allmendinger, Manuela Zintl, Carsten Troll, Gerrit A. Luinstra, Bernhard Rieger (2004). New Zinc Dicarboxylate Catalysts for the CO2/Propylene Oxide Copolymerization Reaction: Activity Enhancement Through Zn(II)-Ethylsulfinate Initiating Groups. Macromolecular Chemistry and Physics 205(1), 42–47. DOI: 10.1002/macp.200350081. https://doi.org/10.1002/macp.200350081
  16. Geoffrey W. Coates, David R. Moore (2004). Discrete Metal-Based Catalysts for the Copolymerization of CO2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism. Angewandte Chemie International Edition 43(48), 6618–6639. DOI: 10.1002/anie.200460442. https://doi.org/10.1002/anie.200460442
  17. Gerrit A. Luinstra (2008). Poly(Propylene Carbonate), Old Copolymers of Propylene Oxide and Carbon Dioxide with New Interests: Catalysis and Material Properties. Polymer Reviews 48(1), 192–219. DOI: 10.1080/15583720701834240. https://doi.org/10.1080/15583720701834240
  18. Stephan Klaus, Maximilian W. Lehenmeier, Eberhardt Herdtweck, Peter Deglmann, Anna K. Ott, Bernhard Rieger (2011). Mechanistic Insights into Heterogeneous Zinc Dicarboxylates and Theoretical Considerations for CO2–Epoxide Copolymerization. Journal of the American Chemical Society 133(33), 13151–13161. DOI: 10.1021/ja204481w. https://doi.org/10.1021/ja204481w
  19. Gemma Trott, Prasad K. Saini, Charlotte K. Williams (2016). Catalysts for CO2/epoxide ring-opening copolymerization. Philosophical Transactions of the Royal Society A 374(2061), 20150085. DOI: 10.1098/rsta.2015.0085. https://doi.org/10.1098/rsta.2015.0085
  20. Christoph Wulf, Ulrike Doering, Thomas Werner (2018). Copolymerization of CO2 and epoxides mediated by zinc organyls. RSC Advances 8(7), 3673–3679. DOI: 10.1039/c7ra12535f. https://doi.org/10.1039/c7ra12535f
  21. Jakob Marbach, Theresa Höfer, Nick Bornholdt, Gerrit A. Luinstra (2019). Catalytic Chain Transfer Copolymerization of Propylene Oxide and CO2 using Zinc Glutarate Catalyst. ChemistryOpen 8(7), 828–839. DOI: 10.1002/open.201900135. https://doi.org/10.1002/open.201900135
  22. Yulong Wu, Yingfang Nie, Li Sun, Zhenhao Xi, Zhen Liu (2024). ZnGA/DMC catalysts for the synthesis of high molecular weight poly(propylene carbonate): The crucial role of water treatment. Molecular Catalysis 564, 114267. DOI: 10.1016/j.mcat.2024.114267. https://doi.org/10.1016/j.mcat.2024.114267
  23. Norio Saito, Masayoshi Honda, Hiroshi Sugimoto, Takahiro Takei, Nobuhiro Kumada (2024). Solvent-free mechanochemical synthesis of zinc glutarate for the heterogeneous catalysis of propylene oxide and CO2 copolymerization. Bulletin of the Chemical Society of Japan 97(12), uoae132. DOI: 10.1093/bulcsj/uoae132. https://doi.org/10.1093/bulcsj/uoae132
  24. Xinkai Fu, Hongjun Yin, Jing Huang, Yang Wang, Xuhui Zhang, Bihua Xia, Ting Li, Shibo Wang, Weifu Dong (2025). Regulating Active Sites of ZnGA Catalyst for Highly Efficient CO2 Copolymerization. Journal of Applied Polymer Science 143(10), e70172. DOI: 10.1002/app.70172. https://doi.org/10.1002/app.70172
  25. Kihyuk Sung, Hye Jeong Joe, Changsu Ha, Hee-Seong Yang, In-Hwan Lee, Seok Ki Kim, Hye-Young Jang (2025). Understanding heterogeneous Zn-gallate catalyzed copolymerization of propylene oxide and CO2. Journal of CO2 Utilization 102, 103241. DOI: 10.1016/j.jcou.2025.103241. https://doi.org/10.1016/j.jcou.2025.103241
  26. Masayoshi Honda, Hiroshi Sugimoto (2026). Heterogeneous cerium phosphate catalyst for the alternating copolymerization of carbon dioxide and epoxide. Journal of CO2 Utilization 108, 103461. DOI: 10.1016/j.jcou.2026.103461. https://doi.org/10.1016/j.jcou.2026.103461