Stereoselective ring opening polymerization (ROP) of racemic lactide (rac-LA) is a challenging goal because a rationale connecting the catalyst structure and polymer microstructure (as has been established for α-olefin polymerization) is still missing. In this work, we reveal the origin of the stereoselective preference for D and L-lactide with two enantiopure salen–Al complexes, which have so far been claimed as the most efficient in enantiomorphic site control, using Density Functional Theory calculations. We introduce active site reorganization and monomer/chain switching throughout the reaction pathway, unconventional aspects necessitating careful consideration when confronting the intricacies associated with chiral catalyst recognition. We show how the catalytic pocket easily rearranges in the reaction path establishing a novel concept of the ligand coordination controlled by monomer binding. The resulting final picture of PLA stereoselectivity is much more complex than that of α-olefin polymerization catalysis, and a “complete” prediction by brute-force is (currently) hard, but the principles evolving should – even in their incomplete form – be useful in the design of new selective catalysts.

中文翻译：

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由单体结合控制的配体配位：DFT 对丙交酯立体选择性聚合的提示


外消旋丙交酯（ rac -LA）的立体选择性开环聚合（ROP）是一个具有挑战性的目标，因为仍然缺乏连接催化剂结构和聚合物微观结构（如α-烯烃聚合所建立的）的基本原理。在这项工作中，我们使用密度泛函理论计算揭示了具有两种对映体纯萨伦-铝配合物对D和L-丙交酯的立体选择性偏好的起源，迄今为止，这两种配合物被认为是对映体位点控制中最有效的。我们在整个反应途径中引入了活性位点重组和单体/链转换，在面对与手性催化剂识别相关的复杂性时需要仔细考虑非常规方面。我们展示了催化口袋如何在反应路径中轻松地重新排列，建立了由单体结合控制的配体配位的新概念。 PLA 立体选择性的最终结果比 α-烯烃聚合催化复杂得多，并且通过蛮力进行“完整”预测（目前）很困难，但不断发展的原理（即使是不完整的形式）应该是有用的新型选择性催化剂的设计。