The current atmospheric concentration of carbon dioxide is around 414 ppm, which means more than three thousand gigatons of CO2 gas is floating in the atmosphere. The abundance and inexpensiveness of CO2 make it an attractive feedstock for chemical reactions. However, the direct incorporation of CO2 into polymerization is challenging due to the thermodynamic instability of carbon dioxide as well as the fact that aliphatic alkenes do not possess enough energy to effective fixation. An alternative approach for this challenge is to convert CO2 into polymerizable intermediates; one ideal candidate would be the formation of 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP) by telomerization of butadiene with CO2. Polyesters can thus be obtained from the ring-opening of EVP-derived molecules.
Figure 1: The overall scheme of the paper, both CO2 and butadiene were used as chemical feedstock in the production of the recyclable polyesters.
The direct use of EVP as a chemical feedstock in polymerization is still quite limited. Various attempts with ring-opening polymerizing (ROP) EVP derivatives to their respective polyesters have been unsuccessful. These attempts have led to some researchers question whether the ring-opening polymerization is thermodynamically favourable because of the lack of ring-strain and the deleterious impact of substitution. In 2022, a group of researchers from the US believed they found the answer to this question. They discovered novel organocatalyzed ROP of the semi-hydrogenated 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one (EtVP) and fully hydrogenated 3,6-diethyl-tetrahydro-2H-pyran-2-one (DEP), both with rich CO2 weight contents (28.5% and 28.2%, respectively).
In the start, the research team tried a well-established Bronsted acid catalyst, diphenyl phosphate or (DPP). While DPP is known for the well-controlled ROP of ε-caprolactone and δ-valerolacton, bulk polymerization of EtVP with DPP was found to be slow. For this reason, the researchers turned their attention to organobase catalysts. Two catalysts, namely 1,8-diazabicyclo [5.4.0] undec7-ene (DBU) and 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) met researchers’ expectations. TBD worked particularly well as it led to a 78% conversion of bulk EtVP and a number-average molecular weight (Mn) of 10,700 gmol−1 for the derived polymer. For DEP, it was found that Waymouth’s NaOMe/1-cyclohexyl3-phenylurea catalyst system led to 74% conversion after one day at room temperature and a number-average molecular weight (Mn) of 9,700 gmol−1 for the derived polymer. Interestingly, poor yields (<50%) were observed when these two catalyst systems were used interchangeably. This indicates even a small change in the lactone monomer can impact the kinetics of polymerization dramatically.
Figure 2: The overview of the catalyst systems used for both EtVP and DEP, the percent conversion, Mn of the polymer were also included.
Such polyesters made from EtVP and DEP have two attractive features: chemical recyclability and its pendent alkene side chain. Chemical recyclability is an important measure for the overall sustainability of the material. The research group discovered an 84% of depolymerization ratio of poly (Et)VP when exposed to 3% tin (II) 2-ethylhexanoate (Sn (Oct)2) at 165 °C. The subsequent biodegradability test for the polymer also met the expectation by reaching 67.4% of the theoretical CO2 removal within 60 days. The alkene side chain allows for facile post-polymerization modification via high-yielding and well-established alkene reaction chemistry. Some explored modifications include Michael-type addition of thiols to the acrylates or photoinitiated thiol-ene click reactions. These post-modifications can alter the utility of the polymers beyond their high CO2 content.
In this work, the authors present a novel organocatalyzed ring-opening polymerization of carbon dioxide and butadiene derivatives. Since both starting materials are readily available and the polymerization is high yielding, this ROP is an ideal reaction for the industrial synthesis of polyesters. This CO2-derived homopolymer also demonstrated promising recyclability and biodegradability for sustainable closed-loop recycling. In addition, the pendent alkene side chain broadens the applications of the polymers through facile post-polymerization functionalization.
The finding of this research has been published on Nature Chemistry: Rapagnani, R.M., Dunscomb, R.J., Fresh, A.A. et al. Tunable and recyclable polyesters from CO2 and butadiene. Nat. Chem.14, 877–883 (2022). https://doi.org/10.1038/s41557-022-00969-2
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