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

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.