Theoretical Mechanistic Investigation into Metal-Free Alternating Copolymerization of CO₂ and Epoxides: The Key Role of Triethylborane

The copolymerization of carbon dioxide (CO₂) and epoxides has received much attention during the past decades for the production of aliphatic polycarbonates. Remarkably, the green synthesis of polycarbonates was recently demonstrated by copolymerization of CO₂ with epoxides under metal-free conditions. In this work, the reaction mechanism of this highly selective polymerization was further investigated using DFT calculations. Four steps were studied: step I describes the epoxide ring-opening by the chloride anion in the presence of the Lewis acid triethylborane (TEB); step II is related to the subsequent insertion of CO₂; step III corresponds to the alternating insertion of an epoxide facilitated by TEB; step IV is characterized by the leaving of TEB followed by a new round of polymerization. The high selectivity to form alternating polycarbonates and the suppression of backbiting and homopolymerization that respectively generate cyclic carbonates and polyethers were confirmed by the difference of energy barriers. The key role of TEB at every step was also elucidated. Our theoretical results support the proposed experimental outcomes and provide the fundamental mechanistic insights.