Intrinsic chain-end stabilization enables chain-transfer-agent-free living cationic ring-opening polymerization of cyclic enol ethers

Cationic ring-opening polymerization of substituted six-membered cyclic enol ethers offers a distinct route to vinyl polymers with head-to-head alternating side chains. However, poor control over the propagating chain-end—conventionally assumed to be a ring-opened oxocarbenium ion—has historically limited its development. Here, we report a chain-transfer-agent-free living cationic ring-opening polymerization enabled by the discovery and utilization of intrinsic chain-end stabilization. Guided by computational analysis, we find that the ring-closed cationic form is thermodynamically preferred over the conventionally proposed ring-opened oxocarbenium form. By preserving this intrinsically stable cation with non-nucleophilic counteranions, we achieve living polymerization that yields polymers with number-average molecular weights up to 507 kg/mol and narrow dispersities. These high-molecular-weight materials exhibit tunable mechanical properties while retaining intrinsic photodegradability, providing a scalable route to degradable vinyl polymers.