A photocurable triple shape memory polymer (TSMP) resin based on acrylic monomers and an ion−pair comonomer (IPC) has been formulated and successfully 3D printed with a digital light processing (DLP) printer. The ion-rich and ion-poor domains produced by polymerizationinduced microphase separation (PIMS) generate two wellseparated glass transition temperatures and an excellent triple shape memory effect in the material, which is systematically studied by dynamic mechanical analysis (DMA) and atomic force microscopy (AFM). With the TSMP resin, an intermediate shape can be set to distinguish and program different shape evolution pathways (SEPs). To visualize the sequential shape shifting, several 3D models are printed and transform through distinct pathways. A potential application of shape memory microfluidics is also demonstrated as a proof-of-concept.

A polymer blend with high extensibility, exhibiting both shape memory and self-healing, was 4D printed using a lowcost fused filament fabrication (FFF, or fused deposition modeling, FDM) 3D printer. The material is composed of two commercially available commodity polymers, polycaprolactone (PCL), a semicrystalline thermoplastic, and polystyrene-block-poly(ethylene-cobutylene)-block-polystyrene (SEBS), a thermoplastic elastomer. The shape memory and self-healing properties of the blends were studied systematically through thermo-mechanical and morphological characterization, providing insight into the shape memory mechanism useful for tuning the material properties. In 3D-printed articles, the orientation of the semi-crystalline and micro-phaseseparated domains leads to improvement of the shape memory property and extensibility of this material compared to compression-molded samples. By controlling the orientation of the printed fibers, we achieved a high strain at break over 1200%, outperforming previously reported flexible 4D-printed materials. The selfhealing agent, PCL, enables the material to heal scratches and cracks and adhere two surfaces after annealing at 80 °C for 30 min. The high performance, multi-functionality, and potential scalability make it a promising candidate for a broad spectrum of applications, including flexible electronics, soft actuators, and deployable devices.

Four-dimensional (4D) printing is an emerging technology that integrates 3D printing and stimuli-responsive materials to fabricate reconfigurable 3D structures. Broadly speaking, the printed structures possess the ability to evolve their shape, properties, and/or function over time in response to an external stimulus. Compared to common 4D printing, sequential shapeshifting 4D printing not only defines the initial and final shapes, but also controls the shape evolution rate and pathway, serving as a powerful tool for reaching complex target geometries. After a brief introduction of the basic concepts in 4D printing and sequential shapeshifting, this review presents the current advances in sequential shapeshifting 4D printing from the viewpoint of their working approaches and is divided in five categories including multi-material assembly, multi-shape material, geometrical design, localized stimulus, and combinations of these approaches. A variety of 3D printing techniques and smart materials have been utilized to achieve sequential shapeshifting and its applications, which are reviewed in detail. Finally, the potentials and the future directions for improvement are discussed.

The substituents present upon five-membered bicyclic glucose carbonate monomers were found to greatly affect the reactivities and regioselectivities during ring-opening polymerization (ROP), which contrast in significant and interesting ways from previous studies on similar systems, while also leading to predictable effects on the thermal properties of the resulting polycarbonates. Polymerization behaviors were probed for a series of five fivemembered bicyclic 2,3-glucose-carbonate monomers having 4,6-ether, -carbonate, or -sulfonyl urethane protecting groups, under catalysis with three different organobase catalysts. Irrespective of the organobase catalyst employed, regioregular polycarbonates were obtained via ROP of monomers with ether substituents, while the backbone connectivities of polymers derived from monomers with carbonate protecting groups suffered transcarbonylation reactions, resulting in irregular backbone connectivities and broad molar mass distributions. The sulfonyl urethane-protected monomers were unable to undergo organobase-catalyzed ROP, possibly due to the acidity of the proton in urethane functionality. The thermal behaviors of polycarbonates with ether and carbonate pendant groups were investigated in terms of thermal stability and glass transition temperature (T g ). A two-stage thermal decomposition was observed when tert-butyloxycarbonyl (BOC) groups were employed as protecting side chains, while all other polycarbonates presented high thermal stabilities with a single-stage thermal degradation. T g was greatly affected by side-chain bulkiness, with values ranging from 39 to 139 °C. These fundamental findings of glucose-based polycarbonates may facilitate the development of nextgeneration sustainable highly functional materials.