Ring-Expansion Living Cationic Polymerization via Reversible Activation of a Hemiacetal Ester Bond

In this paper, we provide an effective route to cyclopolymers via the Lewis acid-assisted "ring-expansion" living cationic polymerization of vinyl ethers, directly from a simple "cyclic initiator" designed with a hemiacetal ester for dynamic and reversible initiation and propagation. The built-in hemiacetal ester, or a carboxylic acid-vinyl ether adduct, is a key to control the polymerization: as the leaving group, the activated carboxylate is well-suited for designing the ring structure, differing from monovalent halogens often employed in carbocationic initiation. The choice of a Lewis acid catalyst (SnBr₄) is equally crucial to retain the cyclic structure via the reversibly dissociable but relatively strong ester bond not only during propagation but also even after quenching. The formation of cyclic polymers was proved by irreversibly cleaving the hemiacetal ester linkage of the product via acidic hydrolysis into an open-chain structure, i.e., an increase in size exclusion chromatography (SEC) molecular weight (hydrodynamic radius), along with the clean transformation of the endocyclic hemiacetal ester into an α-carboxylic acid and ω-aldehyde terminals (by NMR). The polymerization was really "living" polymerization via ring-expansion, as demonstrated by successful monomer-addition experiments and a linear increase in molecular weight with conversion. This ring-expansion living polymerization would open a door to well-defined cyclic polymers free from terminus (end groups) and to hybrid macromolecules with combinations of cyclic and linear architectures.