Overcoming entropic constraints to selectively produce high-molecular-weight cyclic poly(lactide) (c-PLA) is a challenge. In this work, we use an air-stable cationic alkyl indium complex featuring a hemilabile pyridine pedant arm to reproducibly synthesize c-PLA exclusively with low dispersity (Đ ∼ 1.30) and high molecular weights (up to 416,000 g mol^–1). The complex remains active after extended periods of exposure to high-humidity air. The process allows the formation of high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex with no epimerization. We propose that polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism where the monomer coordination to the cationic indium center and ring-opening of lactide by the hemilabile pyridine donor group are essential steps.

中文翻译：

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用于形成高分子量环状聚乳酸的空气稳定阳离子铟催化剂

克服熵限制以选择性地生产高分子量环状聚丙交酯 (c-PLA) 是一项挑战。在这项工作中，我们使用具有半可溶性吡啶前臂的空气稳定的阳离子烷基铟络合物，可重复地合成具有低分散性 ( Đ ∼ 1.30) 和高分子量（高达 416,000 g mol ^{-1 ）的 c-PLA。}）。该复合体在长时间暴露于高湿度空气后仍保持活跃。该过程允许形成高分子量 c-PLLA、c-PDLA 及其立体复合物，而无需差向异构化。我们提出聚合是通过基于路易斯对的协同配位插入机制发生的，其中单体与阳离子铟中心的配位和半可比吡啶供体基团对丙交酯的开环是必不可少的步骤。