Lactone Helps Push Carbon Dioxide To Couple With Olefins

Carbon dioxide has attracted attention as an inexpensive renewable carbon feedstock to incorporate into polymers. But the low reactivity of CO₂ has impeded progress in copolymerizing it with olefins. Kyoko Nozaki of the University of Tokyo described a strategy to overcome the co­polymerization barrier. It hinges on cobbling together CO₂ and a pair of butadiene molecules via a palladium-catalyzed reaction to form a lactone intermediate. The work builds on the Nozaki group’s palladium-catalyzed olefin copolymerization reactions and work by other groups to form lactones from CO₂ and epoxides to make polycarbonates. When mixing ethylene with CO₂, a thermodynamic barrier favors ethylene coupling and polyethylene formation, bypassing the less reactive CO₂ and failing to achieve copolymerization, Nozaki explained. But when using a 1,3-diene, such as butadiene, she said, the palladium catalyst tips the conditions in favor of coupling CO₂ and two diene molecules together. Free-radical polymerization of the resulting lactone leads to high-molecular-weight polylactones—a new class of polymers. The process also works when using two different 1,3-dienes, such as butadiene and isoprene, which leads to new terpolymers (Nat. Chem. 2014, DOI: 10.1038/nchem.1882).