An effective route to cyclopolymers has been developed via the Lewis acid-assisted �ring-expansion� living cationic polymerization of vinyl ethers, directly from a simple �cyclic initiator� with a hemiacetal ester for dynamic and reversible initiation and propagation (ref 1). The built-in hemiacetal ester, or a carboxylic acid-vinyl ether adduct, is a key to control the ring-expansion polymerization: as the leaving group, the activated carboxylate is well-suited for retaining the ring structure, differing from monovalent halogens often employed in carbocationic initiation. The choice of a Lewis acid catalyst (SnBr4) is equally crucial, to form a 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, as shown by an increase in SEC molecular weight (hydrodynamic radius), along with the clean transformation of the endocyclic hemiacetal ester into an a-carboxylic acid and ?-aldehyde terminals (by NMR). Importantly, the propagation is well-controlled to prepare ring block copolymers by sequential addition of another monomer.Reference:  1) ACS Macro Lett. 2013,  2, 531.

Ring-expansion living cationic polymerization: Precision syntheses of ring-based architectures

248th National Meeting (2014)

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