Acidity Reversal Enables Site-Specific Ring-Opening Polymerization of Epoxides from Biprotonic Compounds

Abstract

Polyethers are versatile materials extensively used in advanced as well as everyday applications. The incorporation of primary amine functionality into polyethers is particularly attractive due to its well-established coupling chemistries. However, the inherent nucleophilicity of amine group poses a challenge in the anionic ring-opening polymerization (ROP) of epoxides and requires the use of robust protecting groups that can withstand the harsh conditions of ROP without triggering undesirable side reactions. In this work, we present streamlined synthesis of amino-functionalized polyethers using classic N-carbamate-protected aminoalcohols as initiators for the ROP of epoxides. A Lewis acid-excess two-component organocatalytic system is found to trigger efficient anionic ROP of epoxides while preserving the integrity of the carbamate protection. Despite the higher intrinsic acidity of the carbamate group compared to the hydroxyl group, it is noncompetitive in both the deprotonation and ring-opening steps. This is due to an intriguing acidity-reversing effect of the catalyst, which allows site-specific ethoxylation to proceed exclusively from the hydroxyl group. The resulting poly(propylene oxide) and poly(ethylene oxide) exhibit the targeted molar mass, low dispersity, and well-defined end groups. The fidelity of the amino functionalities is further corroborated and utilized in construction of polypeptoide-based hybrid block copolymers using the synthesized polyethers as macroinitiators.