Cationic ring-opening polymerization of 2-oxazolines in γ-butyrolactones using various initiators†

Solvent is often indispensable for polymerization reactions, especially to circumvent the negative effects of high viscosity. Cationic ring-opening polymerization (CROP) of 2-oxazolines has been studied for decades due to the bio-related characteristics and applications of the products. The commonly used solvent, acetonitrile (AN), raises sustainability concerns due to its volatility and toxicity. We report here that bio-sourced, high-boiling-point, high-polarity, and nontoxic (γ-alkyl) γ-butyrolactones are valid green solvents for the CROP of 2-ethyl-2-oxazoline. Polymerization efficiency and control are found to be the same as those in AN. The livingness of CROP is confirmed by block copolymerization with 2-phenyl-2-oxazoline. A series of organo-initiators, including phosphorus-based acids and carboxylic acids, are employed to gain insights into the structure–activity relationship. In particular, it is the first time to disclose the initiating activity of 2,6-dihydroxybenzoic acid. This work provides a solution for enhancing the environmental friendliness of synthesis of biomimetic poly(2-oxazoline).