Are Well Performing Catalysts for the Ring Opening Polymerization of l‑Lactide under Mild Laboratory Conditions Suitable for the Industrial Process? The Case of New Highly Active Zn(II) Catalysts

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.

聚乳酸（Poly(lactic acid), PLA）是循环经济背景下可持续塑料材料的最佳候选之一，兼具可生物降解性、生物基来源特性与可回收性，同时具备优良的热学与力学性能。当前聚乳酸的工业化生产主要采用辛酸亚锡[Sn(Oct)₂]作为催化剂，通过l-丙交酯（l-lactide, l-LA）的开环聚合（Ring Opening Polymerization, ROP）实现，工艺为连续无溶剂聚合，反应温度控制在180~200℃，该温度高于所得等规聚合物的熔点。尽管学界围绕低毒金属基替代催化剂开展了大量研究，已在实验室温和条件下开发出众多高活性催化体系，但在工业相关的严苛工况下，鲜有催化剂能与辛酸亚锡相匹敌。本工作报道了一类新型锌(II)配合物，其搭载有不同取代模式的单阴离子[N,O-]型咪唑并[1,5-a]吡啶-3-基苯酚盐配体，可作为l-丙交酯开环聚合的催化剂，适配两种反应条件：温和条件（20℃、溶剂体系）与工业相关工况（190℃、本体聚合、使用工业级未纯化单体、极低催化剂负载量）。值得注意的是，在温和条件下催化性能最优的催化剂，在严苛工业工况下表现最差；反之，在温和条件下活性较低的催化剂，在工业相关工况下可与辛酸亚锡实现相当的催化效果。动力学与密度泛函理论（DFT）机理研究阐明了2-吡啶取代基在不同温度下对催化性能的非平凡调控作用。对商用聚左旋乳酸（Poly(L-lactic acid), PLLA）样品开展的初步解聚测试表明，这类新型催化剂同样可作为温和条件下聚乳酸化学回收的潜在候选催化体系。