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.

中文翻译：

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无金属交替发生的CO ₂和环氧化物的交替反应的理论机理研究：三乙基硼烷的关键作用

在过去的几十年中，二氧化碳（CO ₂）和环氧化物的共聚在脂族聚碳酸酯的生产中受到了广泛的关注。值得注意的是，最近在无金属条件下通过CO ₂与环氧化物的共聚证明了聚碳酸酯的绿色合成。在这项工作中，使用DFT计算进一步研究了这种高度选择性聚合的反应机理。研究了四个步骤：步骤I描述了在路易斯酸三乙基硼烷（TEB）的存在下氯离子对环氧化物的开环；步骤II与随后插入CO _2有关; 步骤III对应于TEB促进的环氧化物的交替插入；步骤IV的特征在于离开TEB，随后进行新的聚合。通过能垒的不同，证实了形成交替聚碳酸酯的高选择性以及分别生成环状碳酸酯和聚醚的抑制咬合和均聚的抑制作用。还阐明了TEB在每个步骤中的关键作用。我们的理论结果支持拟议的实验结果，并提供了基本的力学见解。