Direct Synthesis of Alternating Polycarbonates from CO₂ and Diols by Using a Catalyst System of CeO₂ and 2-Furonitrile

The control technique of polymer molecular weight is required for the synthesis of versatile polymers with various properties. In our previous work, we found that CeO₂ + 2-cyanopyridine catalyst system was effective for the direct synthesis of alternating polycarbonates from CO₂ and diols, however, the maximum average molecular weight was ∼1000 g mol^-1 (degree of polymerization = 7-8). In this study, we succeeded in the synthesis of alternating polycarbonates with higher molecular weight from CO₂ and diols by using a catalyst system of CeO₂ + 2-furonitrile. The average molecular weight reached up to 5000 g mol^-1 and could be controlled by adjusting the amount of diols and 2-furonitrile. Moreover, polycarbonate diols, polycarbonates without capping of OH groups at the ends, were obtained with the average molecular weight of ∼2000 g mol^-1. The catalyst system was applicable to the direct polymerization of CO₂ and various α,ω-diols, providing the corresponding alternating polymers. Comparison of CeO₂ + 2-cyanopyridine and CeO₂ + 2-furonitrile catalyst systems based on the kinetics and DFT calculations showed two main causes for the formation of polycarbonates with higher molecular weight in the CeO₂ + 2-furonitrile catalyst system: First, the reactivity of 2-furamide, which was formed from 2-furonitrile, with produced polycarbonate diols was lower than that of 2-picolinamide, which was formed from 2-cyanopyridine, leading to decrease of formation of ester-capped polycarbonates. Second, the adsorption of 2-furonitrile on CeO₂ was weaker than that of 2-cyanopyridine, leading to low steric hindrance at the active sites of CeO₂ and enabling the reaction of longer diols, such as polycarbonate diols with CO₂.