Ion-Exchange Catalyst Marries Cationic Ring-Opening Polymerization with Functional Carboxylic Acid Initiators

Abstract

Compared with the conventionally used strong acid/electrophile initiators, carboxylic acids are much more compatible with other functional groups but are incapable of initiating cationic polymerization for the one-step synthesis of end-functionalized polymers. Using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a catalyst, we realized carboxylic acid-initiated cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx). The dynamic exchange between carboxylate and TFSI^– anions, driven by Li⁺–carboxylate interaction, significantly enhances the activity of oxazolinium propagating species and ensures uniform chain growth, as shown by both experiments and calculations. Interestingly, a proper solvent, γ-valerolactone, also plays a vital role, likely by promoting ion-pair separation at elevated temperatures. A wide scope of functional groups is readily introduced by commercialized carboxylic acids, especially benzoic acid derivatives and analogues, with controlled molar mass, low dispersity, and high/quantitative end-group fidelity achieved, and polymerization rate varying with the initiator structure. The end functionalities of polyEtOx are exemplified by surface adsorption–protein resistance and aggregation-induced emission properties. This catalytic paradigm breaks through long-standing limitations in weak-acid-initiated CROP, indispensably upgrading the toolkit for macromolecular engineering.