Wallenberg Wood Science Center, WWSC, Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56-58, 100 44, Stockholm, Sweden.
Angew Chem Int Ed Engl. 2022 Aug 15;61(33):e202204531. doi: 10.1002/anie.202204531. Epub 2022 Jun 9.
Chemical recycling of poly(L-lactic acid) to the cyclic monomer L-lactide is hampered by low selectivity and by epimerization and elimination reactions, impeding its use on a large scale. The high number of side reactions originates from the high ceiling temperature (T ) of L-lactide, which necessitates high temperatures or multistep reactions to achieve recycling to L-lactide. To circumvent this issue, we utilized the impact of solvent interactions on the monomer-polymer equilibrium to decrease the T of L-lactide. Analyzing the observed T in different solvents in relation to their Hildebrand solubility parameter revealed a "like recycles like" relationship. The decreased T , obtained by selecting solvents that interact strongly with the monomer (dimethyl formamide or the green solvent γ-valerolactone), allowed chemical recycling of high-molecular-weight poly(L-lactic acid) directly to L-lactide, within 1-4 h at 140 °C, with >95 % conversion and 98-99 % selectivity. Recycled L-lactide was isolated and repolymerized with high control over molecular weight and dispersity, closing the polymer loop.
聚（L-乳酸）的化学循环回收为环状单体 L-丙交酯受到低选择性以及差向异构化和消除反应的阻碍，这限制了其大规模应用。大量的副反应源于 L-丙交酯的高上限温度（T），这需要高温或多步反应才能实现 L-丙交酯的回收。为了规避这个问题，我们利用溶剂相互作用对单体-聚合物平衡的影响来降低 L-丙交酯的 T。分析不同溶剂中观察到的 T 与它们的 Hildebrand 溶解度参数之间的关系表明存在“相似的循环”关系。通过选择与单体相互作用强的溶剂（二甲基甲酰胺或绿色溶剂γ-戊内酯），降低 T，在 140°C 下 1-4 小时内可直接将高分子量聚（L-乳酸）化学循环回收为 L-丙交酯，转化率>95%，选择性>98-99%。回收的 L-丙交酯被分离出来，并与高控制分子量和分散度重新聚合，从而封闭了聚合物循环。
