Chemically Circular Poly(Orthoester) Elastomers Synthesized by Halogen‐Bond Donor Catalyzed Cationic Ring‐Opening Polymerization

ABSTRACT Synthetic polymers are essential to modern life, yet their large‐scale production and environmental accumulation necessitate the development of more sustainable alternatives. Existing synthetic approaches to next‐generation polymers largely focus on well‐established monomer classes, yet developing methodologies for the polymerization of underexplored functionalities would unlock challenging‐to‐access polymer classes and exploration of uncharted materials space. Poly(orthoesters) are an underexplored class of biodegradable and chemically recyclable polymers, and here we overcome long‐standing synthetic challenges by developing a molecular weight‐controlled synthesis via cationic ring‐opening polymerization of spiro orthoesters. Polymerization control is imparted by a tridentate halogen‐bond donor catalyst that selectively promotes reversible‐deactivation via anion‐binding, while suppressing undesired monomer activation. Mechanistic studies reveal that precise molecular weight control is maintained despite termination events, constituting a rare example of a polymerization exhibiting immortal‐like behavior. Combined with controlled degradation, chemical recycling to monomer, and tunable material properties, this work establishes poly(orthoesters) as a promising platform for sustainable and circular materials.