A Pseudo-Block Copolymerization Access to Cyclic Alternating Copolymers through Segment-Selective Transesterification

Efficient synthesis of cyclic polymers remains a frontier challenge. We report here that macromolecular transesterification during a pseudoblock copolymerization process can be utilized for such a purpose. Organobase-catalyzed ring-opening alternating copolymerization of 3,4-dihydrocoumarin and epoxide is conducted with four-armed poly(ethylene oxide) (PEO) as a macroinitiator. Intramolecular transesterification (backbiting) occurs selectively on the newly formed polyester segments. The disconnected cyclic alternating copolymers can be easily isolated by precipitation owing to their substantial solubility difference from the PEO-containing acyclic parts. The obtained cyclic alternating copolymers exhibit low dispersity (<1.2) and a molar mass of around 3 kg mol^–1, irrespective of the monomer-to-initiator feed ratio, indicating thermodynamic control over the ring size. The macrocyclic structure is confirmed by both mass spectroscopy and microscopic visualization and then utilized to prepare cyclic-brush terpolymer by thiol–ene modification, followed by graft polymerization of propylene oxide.