Waste-derived and recycled shape-memory polymers and composites: feedstocks, processing, performance, and circularity challenges.

Shape-memory polymers and polymer composites can recover a programmed shape in response to external stimuli and have been investigated for applications including actuators, sensors, adaptive structures, and 4D printing. The majority of well-established systems use virgin petrochemical feedstocks, which has prompted interest in waste-derived and recycled alternatives. This review examines waste-derived and recycled shape-memory polymers and polymer composites prepared from plastic waste, agricultural residues, industrial by-products, e-waste, and waste cooking oil. Feedstocks are distinguished as waste-derived, recycled, upcycled, virgin bio-based, or hybrid systems. Reported processing routes, shape-memory performance, thermal and mechanical properties, cyclic durability, and application status are evaluated where data are available. Selected systems show high shape-fixity and shape-recovery values; however, comparisons remain constrained by differences in testing protocols, feedstock quality, and incomplete reporting of recovery stress, long-term aging, and repeated-cycle performance. The review also discusses contamination control, recyclability, reprocessability, biodegradability, regulatory considerations, and the need for standardized testing, life-cycle assessment, and techno-economic analysis. Waste-derived and recycled shape-memory polymers may support circular-material development, but their environmental and practical advantages require case-specific validation.