Water-Degradable Oxygen-Rich Polymers with AB/ABB Units from Fast and Selective Copolymerization

Researchers have been chasing plastics that can automatically and fully degrade into valuable products under natural conditions. Here, we develop a series of water-degradable polymers from the first reported fast and selective cationic copolymerization of formaldehyde (B) with cyclic anhydrides (A). In addition to readily accessible monomers, the method is performed at industrially relevant temperatures (~100 °C), takes tens or even minutes, and uses common acid as the catalyst. Interestingly, such polymers possess tunable AB/ABB-type repeating units, which are considered to be thermodynamic and kinetic products, respectively, resulting in low carbon content ([O] : [C] up to 1 : 1). Notably, the polymers can completely degrade to valuable diacids within 150 days in water at ambient temperature owing to the incorporation of carboxyl terminals and acid-responsive acetal units. By washing with aqueous sodium carbonate, the polymers are relatively stable over several months.

Elucidation of the Alternating Copolymerization Mechanism of Epoxides or Aziridines with Cyclic Anhydrides in the Presence of Halide Salts

Organic halide salts in combination with metal or organic compound are the most common and essential catalysts in ring-opening copolymerizations (ROCOP). However, the role of organic halide salts was neglected. Here, we have uncovered the complex behavior of organic halides in ROCOP of epoxides or aziridine with cyclic anhydride. Coordination of the chain-ends to cations, electron-withdrawing effect, leaving ability of halide atoms, chain-end basicity/nucleophilicity, and terminal steric hindrance cause three types of side reactions: single-site transesterification, substitution, and elimination. Understanding the complex functions of organic halide salts in ROCOP led us to develop highly active and selective aminocyclopropenium chlorides as catalysts/initiators. Adjustable H-bonding interactions of aminocyclopropenium with propagating anions and epoxides create chain-end coordination process that generate highly reactive carboxylate and highly selective alkoxide chain-ends.