Mechanism of cyclics formation in the cationic copolymerization of tetrahydrofuran with ethylene oxide in the presence of diols

Cationic copolymerization of ethylene oxide (EO) with tetrahydrofuran (THF) in the presence of diols leads to the formation of hydroxytelechelic copolymers. The process combines the features of an activated monomer mechanism (secondary oxonium ions as active species) with those of an active chain end mechanism (tertiary oxonium ions as active species). Due to the participation of tertiary oxonium ions, back-biting leads to the formation of cyclic oligomers. The effect of reaction conditions on the content of the cyclic fraction was studied, the individual cyclic oligomers were identified, and the results are interpreted in terms of a reaction mechanism.