Poly(lactic acid) (PLA) is one of the best candidates as a sustainable plastic material for a circular economy, being biodegradable, bio-based, recyclable, and displaying good thermal and mechanical properties. The industrial production of PLA is mainly based on the ring opening polymerization (ROP) of l-lactide (l-LA) promoted by tin(II) 2-ethylhexanoate [Sn(Oct)₂] in a continuous solvent-free process operating at temperatures between 180 and 200 °C, above the melting point of the resulting isotactic polymer. Despite the huge efforts in the research of alternative catalysts based on less toxic metals, resulting in a plethora of highly active catalysts under laboratory mild conditions, very few candidates can compete with Sn(Oct)₂ under industrially relevant conditions. We report a family of new Zn(II) complexes, bearing variously substituted monoanionic [N,O^–] (imidazole[1,5-a]pyrid-3-yl)phenolate ligands, as catalysts for the ROP of l-LA under both mild (20 °C, solvent) and industrially relevant (190 °C, in the melt, technical grade unpurified monomer, very low catalyst loading) conditions. Interestingly, the best performing catalyst under mild conditions is the worst performing under harsh conditions, and, on the contrary, the less active catalysts under mild conditions compete well with Sn(Oct)₂ under industrially relevant conditions. Kinetic and DFT mechanistic investigations shed light on the non-trivial role of the 2-pyridine substituent in the catalytic performances at different temperatures. Preliminary depolymerization tests on commercial PLLA samples suggested that the new catalysts can also be a suitable candidate for the chemical recycling of PLA under mild conditions.

中文翻译：

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在温和的实验室条件下，用于 l-丙交酯开环聚合的性能良好的催化剂是否适用于工业过程？新型高活性 Zn(II) 催化剂案例

聚乳酸 (PLA) 是循环经济可持续塑料材料的最佳候选材料之一，具有可生物降解、生物基、可回收以及良好的热学和机械性能。PLA 的工业化生产主要基于2-乙基己酸锡 (II) [Sn(Oct) _{2 ] 促进的}l-丙交酯 ( l -LA)在连续无溶剂工艺中的开环聚合 (ROP)温度在 180 至 200 °C 之间，高于所得全同立构聚合物的熔点。尽管在研究基于毒性较小的金属的替代催化剂方面付出了巨大努力，导致在实验室温和条件下产生了大量高活性催化剂，但很少有候选者可以与 Sn(Oct) _2竞争在工业相关条件下。我们报道了一系列新的 Zn(II) 配合物，它们带有各种取代的单阴离子 [N,O ^– ] (咪唑[1,5 - a ]pyrid-3-yl)苯酚配体，作为l -LA的 ROP 催化剂温和（20 °C，溶剂）和工业相关（190 °C，熔体，工业级未纯化单体，极低催化剂负载）条件。有趣的是，在温和条件下性能最好的催化剂在苛刻条件下性能最差，相反，在温和条件下活性较低的催化剂与 Sn(Oct) _{2竞争很好}在工业相关条件下。动力学和 DFT 机理研究揭示了 2-吡啶取代基在不同温度下催化性能中的重要作用。对商业 PLLA 样品的初步解聚测试表明，新催化剂也可以成为 PLA 在温和条件下化学回收的合适候选者。