Nettles Jared A, Alfarhan Saleh, Pascoe Cameron A, Westover Clarissa, Madsen Margaret D, Sintas Jose I, Subbiah Aadhi, Long Timothy E, Jin Kailong
Chemical Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe 85287, Arizona, United States.
Biodesign Center for Sustainable Macromolecular Materials and Manufacturing, Arizona State University, Tempe 85287, Arizona, United States.
JACS Au. 2024 Jul 12;4(8):3058-3069. doi: 10.1021/jacsau.4c00403. eCollection 2024 Aug 26.
The cross-linked structures of most commodity polyurethanes (PUs) hinder their recycling by common mechanical/chemical approaches. Catalyzed dynamic carbamate exchange emerges as a promising PU recycling strategy, which converts traditional static PU thermosets into reprocessable covalent adaptable networks (CANs). However, this approach has been limited to thermoset-to-thermoset reprocessing of PU CANs, accompanied by their well-preserved network structures and extremely high viscosities, which pose challenges to processing and certain applications. This study reports a catalytic decross-linking extrusion process aided by small-molecule carbamates, which can upcycle PU thermosets into easily processable and functional PU thermoplastics in a solvent-free and high-throughput manner. Key to this process is the employment of small-molecule carbamates as decross-linkers to simultaneously deconstruct cross-linked PUs and functionalize the decross-linked PU chains, through catalyzed carbamate exchange reactions in a twin-screw extruder. This strategy applies to both aromatic and aliphatic cross-linked PU films and foams, and the amount of small-molecule carbamates required to decross-link PU networks is determined through thermal, chemical, and structural analyses of the resulting extrudates. This approach is generalizable to small-molecule carbamates with various steric/electronic structures and chemical functionalities including methacrylate, anthracene, and stilbene groups. The chain-end functionalization is confirmed by analyzing the purified decross-linked extrudates after dialysis. This thermoset-to-thermoplastic extrusion process represents a powerful approach for upcycling postconsumer PU thermosets into a library of thermoplastic PUs with controlled molecular weights and chain-end functionalities for diverse applications, including adhesives, photoresins, and stimuli-responsive materials, as demonstrated herein. In the future, this strategy could be extended to upcycle many other step-growth networks capable of undergoing catalytic bond exchange reactions, such as cross-linked polyureas and polyesters, contributing to plastic waste management in general.
大多数商用聚氨酯（PU）的交联结构阻碍了它们通过常见的机械/化学方法进行回收利用。催化动态氨基甲酸酯交换作为一种有前景的PU回收策略应运而生，该策略将传统的静态PU热固性材料转化为可再加工的共价自适应网络（CAN）。然而，这种方法仅限于PU CAN的热固性材料到热固性材料的再加工，伴随着其保存完好的网络结构和极高的粘度，这对加工和某些应用构成了挑战。本研究报道了一种由小分子氨基甲酸酯辅助的催化脱交联挤出工艺，该工艺可以将PU热固性材料以无溶剂和高通量的方式升级回收为易于加工的功能性PU热塑性材料。该工艺的关键是使用小分子氨基甲酸酯作为脱交联剂，通过在双螺杆挤出机中进行催化氨基甲酸酯交换反应，同时解构交联的PU并使脱交联的PU链功能化。该策略适用于芳香族和脂肪族交联的PU薄膜和泡沫，通过对所得挤出物进行热、化学和结构分析来确定解构PU网络所需的小分子氨基甲酸酯的量。这种方法可推广到具有各种空间/电子结构和化学功能的小分子氨基甲酸酯，包括甲基丙烯酸酯、蒽和芪基。通过分析透析后纯化的脱交联挤出物来确认链端功能化。这种热固性材料到热塑性材料的挤出工艺代表了一种强大的方法，可将消费后的PU热固性材料升级回收为一系列具有可控分子量和链端功能的热塑性PU，用于多种应用，包括粘合剂、光致抗蚀剂和刺激响应材料，如本文所示。未来，这种策略可以扩展到升级回收许多其他能够进行催化键交换反应的逐步增长网络，如交联聚脲和聚酯，总体上有助于塑料废物管理。
