Anionic ring-opening polymerization of 2-oxabicyclo[2.1.1]hexan-3-one: manipulating topology and conformation for circular polymer design.

Polyesters featuring a linear topology and in-chain 1,3-cyclobutane rings, synthesized via ring-opening polymerization (ROP) of 2-oxabicyclo[2.1.1]hexan-3-one (4R-BL, R = Bu, Ph) through a coordination-insertion mechanism, display excellent thermal and hydrolytic stability, making them promising candidates for sustainable circular materials. However, achieving diverse topological and stereochemical structures remains challenging. Herein, we demonstrate precise control over linear and cyclic topologies of these polyesters and the conformation of in-chain cyclobutane rings through anionic  ROP of 4R-BL with appropriate catalysts or initiators. Using tert-butoxide (tBuOK) as the catalyst, low loading (0.05-0.1 mol%) produces high-molar-mass cyclic polyester P(4R-BL) (up to 571 kg/mol), whereas high loading (2 mol%) promotes transesterification and isomerization, ultimately yielding cyclic oligomers. Remarkably, the tetramer (4Ph-BL)4undergoes conformational turnover of the puckered cyclobutane rings and can be repolymerized into polymer P(4Ph-BL). This establishes a "monomer ⇄ polymer ⇄ tetramer" dual closed-loop life cycle, enhancing the potential for a circular material economy.