A convergent approach to ring polymers with narrow molecular weight distributions through post dilution in ring expansion cationic polymerization

Abstract

In this paper, we demonstrate a convergent approach to convert “fused” ring chains obtained via ring expansion cationic polymerization of vinyl ether with a hemiacetal ester (HAE)-based ring initiator (1) into “sing” ring ones of narrow MWDs. In the ring-expansion propagation process, the propagation can be controlled without any side reactions but an intermolecular counteranion exchange reaction between HAE bonds between propagating ring polymers ineluctably occurs resulting in broad molecular weight distributions (MWDs) composed of “fused” ring chains with multiple HAE bonds as well as a “sing” ring chain with one HAE bond. Hence, after monomer conversion reached over 95%, the polymerization solution was diluted (i.e., post dilution) without deactivation of an employed Lewis acid activator (i.e., SnBr₄) to induce intramolecular counteranion exchange in the fused ring chain. Size exclusion chromatography (SEC) curves of the product eventually became almost unimodal, though a small shoulder peak from the fused ring remained. Importantly, the HAE bond in ring chains still survived even after the post dilution process, which was confirmed by ¹H NMR, and the retention of the ring structure was also supported by an acidolysis experiment where the apparent peak top molecular weight (M_p) was increased in SEC due to topology conversion from a ring to linear. The approach was proved to be effective even for higher molecular weight ring polymers that were prepared with a higher [monomer]/[1] ratio as well as ring block copolymers.