Consecutive living polymerization from cationic to radical: a straightforward yet versatile methodology for the precision synthesis of “cleavable” block copolymers with a hemiacetal ester junction

Abstract

In this paper, we provide a straightforward yet versatile methodology via two consecutive living polymerizations, from cationic to radical, to synthesize “cleavable” block copolymers whose segment junction is a hemiacetal ester (HAE). The key compound is α-halophenyl acetic acid [HOOC–CH(Ph)–X: X = halogen] used as a hetero-bifunctional initiator: the carboxylic acid is for living cationic polymerization (LCP) and the carbon–halogen bond for metal-catalyzed living radical polymerization (LRP). The carboxylic acid efficiently reacted with a vinyl ether (e.g., isobutyl vinyl ether: IBVE) to give an adduct with a hemiacetal ester bond, which was reversibly activated with a suitable mild Lewis acid (SnBr₄) as a catalyst to initiate LCP of IBVE. Thus obtained poly(IBVE) quantitatively retained an α-end carbon–halogen bond from the initiator and thus worked as a macroinitiator for the LRP of methyl methacrylate (MMA) with a ruthenium catalyst, to form HAE-connected block copolymers (PIBVE-HAE-b-PMMA), whose junction can be cleaved under acidic conditions. Their solution-cast films exhibited mostly cylindrical microphase-separated structures, and both the junction cleavage and removal of the PIBVE segment were achieved in one shot, just by immersing the film into a trifluoroacetic acid solution in n-hexane without distorting film integrity with the cylinder morphology virtually intact, leading to nanoporous patterns.