Controlled cationic ring-opening polymerization of L-lactide by organic ion pair: novel approach to isotactic-rich and crystalline polylactide

Living/controlled cationic ring-opening polymerization (ROP) of L-lactide is a promising approach to isotactic-rich and crystalline poly(L-lactide). In contrast with the unsubstituted lactones, L-LA can not be polymerized by organic Lewis acids or carbenium ions, and the state of the art in this field is the cationic ROP of L-LA catalyzed by Brønsted acid/alcohol system via activated monomer mechanism. Herein, we reported the first example of controlled cationic ROP of L-LA by using Meerweintype ion pair [Me₃O]⁺[B(C₆F₅)₄]⁻ as the catalyst. [Me₃O]⁺[B(C₆F₅)₄]⁻ promoted rapid L-LA cationic polymerization in the absence of alcohol, producing isotactic-rich and crystalline PLLA without transesterification and epimerization side reactions. An activated chain end mechanism, involving twice S_N2 substitution and configuration-inversion (S→R→S) with the assistance of released Me₂O, was proposed and further verified by density functional theory and control experiments. This work expands the catalytic toolbox of isotactic-rich and crystalline polylactide synthesis. It represents a unique example of cationic-controlled polymerization of L-LA catalyzed by an organic ion pair.