The ring-opening polymerization of cyclic ester and carbonate catalyzed by trimethyl glycine and catalyst design: A DFT study

Organic catalytic ring-opening polymerization (ROP) of cyclic esters and carbonates is an effective strategy for obtaining biodegradable and biocompatible aliphatic polyesters and polycarbonates. The challenge is to develop highly active and green organocatalysts in this domain. Herein, the zwitterionic carboxybetaine catalyzed the ROP mechanism of trimethylene carbonate (TMC) cyclic ester and lactide (LA) was investigated by density functional theory (DFT). By comparing the ROP of TMC catalyzed by natural trimethyl glycine (TMG) and unnatural tetramethylammonium acetate (TMAA), we found the proton-accepting ability of the anion part in zwitterionic carboxybetaine is the main influencing factor of polymerization activity. Based on this finding, it is proposed to enhance the proton-accepting ability of the anion part in TMG by extending carbon spacer lengths between CH₃COO^‒ and N(CH₃)₄⁺ of TMG. Among them zwitterionic carboxybetaine molecules ((CH₃)₃N⁺(CH₂)_nCOO^‒ (n = 1–5)), the (CH₃)₃N⁺(CH₂)₄COO^‒ shows the highest catalytic activity in the ROP of TMC. Moreover, the ROP of LA catalyzed by (CH₃)₃N⁺(CH₂)₄COO^‒ showed very high catalytic activity compared to the TMG in both theoretical calculation and experiment. This work will inspire the future development of natural organocatalysts with high catalytic performance.