The replacement of current petroleum-based plastics with sustainable alternatives is a crucial but formidable challenge for the modern society. Catalysis presents an enabling tool to facilitate the development of sustainable polymers. This review provides a system-level analysis of sustainable polymers and outlines key criteria with respect to the feedstocks the polymers are derived from, the manner in which the polymers are generated, and the end-of-use options. Specifically, we define sustainable polymers as a class of materials that are derived from renewable feedstocks and exhibit closed-loop life cycles. Among potential candidates, aliphatic polyesters and polycarbonates are promising materials due to their renewable resources and excellent biodegradability. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in catalytic transformations that lower the technological barriers for developing more sustainable replacements for petroleum-based plastics.

Ring-opening polymerization of lactones is a versatile approach to generate well-defined functional polyesters. Typical ring-opening catalysts are subject to a trade-off between rate and selectivity. Here we describe an effective catalytic system combining alkoxides with thioureas that catalyses rapid and selective ring-opening polymerizations. Deprotonation of thioureas by sodium, potassium or imidazolium alkoxides generates a hydrogen-bonded alcohol adduct of the thiourea anion (thioimidate). The ring-opening polymerization of L-lactide mediated by these alcohol-bonded thioimidates yields highly isotactic polylactide with fast kinetics and living polymerization behaviour, as evidenced by narrow molecular weight distributions (M/M < 1.1), chain extension experiments and minimal transesterifications. Computational studies indicate a bifunctional catalytic mechanism whereby the thioimidate activates the carbonyl of the monomer and the alcohol initiator/chain end to effect the selective ring-opening of lactones and carbonates. The high selectivity of the catalyst towards monomer propagation over transesterification is attributed to a selective activation of monomer over polymer chains.

The key challenge for few-shot semantic segmentation (FSS) is how to tailor a desirable interaction among support and query features and/or their prototypes, under the episodic training scenario. Most existing FSS methods implement such support/query interactions by solely leveraging plain operations -e.g., cosine similarity and feature concatenation -for segmenting the query objects. However, these interaction approaches usually cannot well capture the intrinsic object details in the query images that are widely encountered in FSS, e.g., if the query object to be segmented has holes and slots, inaccurate segmentation almost always happens. To this end, we propose a dynamic prototype convolution network (DPCN) to fully capture the aforementioned intrinsic details for accurate FSS. Specifically, in DPCN, a dynamic convolution module (DCM) is firstly proposed to generate dynamic kernels from support foreground, then information interaction is achieved by convolution operations over query features using these kernels. Moreover, we equip DPCN with a support activation module (SAM) and a feature filtering module (FFM) to generate pseudo mask and filter out background information for the query images, respectively. SAM and FFM together can mine enriched context information from the query features. Our DPCN is also flexible and efficient under the k-shot FSS setting. Extensive experiments on PASCAL-5 i and COCO-20 i show that DPCN yields superior performances under both 1-shot and 5-shot settings.

The use of dithiolane-containing polymers to construct responsive and dynamic networks is an attractive strategy in material design. Here, we provide a detailed mechanistic study on the self-assembly and gelation behavior of a class of ABA triblock copolymers containing a central poly(ethylene oxide) block and terminal polycarbonate blocks with pendant 1,2-dithiolane functionalities. In aqueous solution, these amphiphilic block copolymers self-assemble into bridged flower micelles at high concentrations. The addition of a thiol initiates the reversible ring-opening polymerizations of dithiolanes in the micellar cores to induce the cross-linking and gelation of the micellar network. The properties of the resulting hydrogels depend sensitively on the structures of 1,2-dithiolanes. While the methyl asparagusic acid-derived hydrogels are highly dynamic, adaptable, and self-healing, those derived from lipoic acid are rigid, resilient, and brittle. The thermodynamics and kinetics of ring-opening polymerization of the two dithiolanes were investigated to provide important insights on the dramatically different properties of the hydrogels derived from the two different dithiolanes. The incorporation of both dithiolane monomers into the block copolymers provides a facile way to tailor the properties of these hydrogels.

The design and generation of adaptable materials derived from structurally dynamic polymers provides a strategy for generating smart materials that can respond to environmental stimuli or exhibit self-healing behavior. Herein we report an expedient organocatalytic ring-opening polymerization of cyclic carbonates containing pendant dithiolanes (trimethylene carbonate/dithiolane, TMCDT) from poly(ethylene oxide) diols to generate water-soluble triblock (ABA) copolymers containing a central poly(ethylene oxide) block and terminal dithiolane blocks. Hydrogels generated from the triblock copolymers and a cross-linking dithiol exhibited dynamic behavior as a result of the reversible ring opening of the pendant 1,2-dithiolanes. These materials exhibit self-healing behavior, can be injected through a syringe, and rapidly recover their mechanical properties after a severe strain deformation. The dynamic properties of these gels can be modulated with the number of dithiolane units, pH, and temperature.

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones with dramatic (> 100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. The heterostructured materials field explores heterostructures to control defect distributions, long-range internal stresses, and nonlinear inter-zone interactions for unprecedented performances. This paper is aimed to provide perspectives on this novel field, describe the state-of-the-art of heterostructured materials, and identify and discuss key issues that deserve additional studies. IMPACT STATEMENT This paper delineates heterostructured materials, which are emerging as a new class of materials with unprecedented properties, new materials science and economic industrial production.

Bicyclic isothioureas 1 and 2 mediate controlled ring opening polymerizations (ROP) of lactides in the absence of protic initiators to afford high molecular weight polylactides (PLA) with narrow polydispersities. The cyclic structure of the resulting PLA was determined by dilute solution viscosity measurement and MALDI-TOF mass spectrometry. Compared to DBU initiator, isothioureas are more selective for producing cyclic PLA without appreciable linear contaminants. Mechanistic studies involving acyl amidinium support our hypothesis that DBU-initiated ZROP generates linear chains from a ketene aminal intermediate. Z witterionic ring opening polymerization (ZROP) mediated by organic nucleophiles has been developed as an effective strategy for the synthesis of cyclic polymers. 1−7 In the absence of protic initiators, nucleophiles such as N-heterocyclic carbenes, 1,6 pyridines, 8,9 imidazoles, 10 amidines, 11 tertiary amines, 8,12,13 or phosphines 9 can mediate the ring opening of strained cyclic monomers to form zwitterionic intermediates 14,15 that propagate and cyclize to release macrocycles (Scheme 1). Lactones, 1,2,16 thiolactones, 8 N-carboxyanhydrides, 6,17,18 or carbosiloxanes 19 are among the types of monomers that can be polymerized with this approach, but the factors that control the rate of initiation, propagation, and cyclization and competitive side reactions remain incompletely understood and depend sensitively on the reactivities of nucleophilic initiator, the monomer, the nature of the reactive intermediates, and the polymerization conditions. 1,4,20,21 The ZROP of lactide with N-heterocyclic carbenes is rapid (minutes), but molecular weights >30 kDa are difficult to obtain. 16,22,23 We recently reported that the amidine 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) was sufficiently nucleophilic to mediate the ZROP of lactide to generate cyclic polylactides but was accompanied by the cogeneration of linear polylactides. 11 We had proposed that the linear polylactides might arise from competitive deprotonation of the amidininium zwitterion to generate the neutral ketene aminal (KA, Scheme 2). 11 To test this hypothesis and to provide a strategy for eliminating this competitive pathway, we investigated the ZROP of lactide with the isothioureas 24−26 1 and 2. While the isothioureas are less nucleophilic than the amidine DBU, 27,28 we reasoned that the acylated isothiourea zwitterions would be unable to deprotonate to the ketene aminal and, thus, be less likely to generate linear chains. Herein, we employ ITU 1 and 2 as nucleophilic initiators for the ZROP of LA and compare the results with the DBU initiator.The ROP of L-lactide in the absence of added alcohol initiators was carried out in dichloromethane at room temperature with ITU 1 or 2 as catalysts (Table 1). High molecular weight PLAs with M n ranging from 38000 to 66000 Da and polydispersities of M w /M n = 1.3−1.7 were produced at high conversion (∼90%). The polymerization rates with isothioureas are significantly slower than those with DBU or NHCs. K...

Hybrid nanostructures that comprise two or more nanoscale functional components are fascinating for applications in electronics, energy conversion devices, and biotechnologies. Their performances are strongly dependent on the characteristics of the individual components including the size, morphology, orientation, and distribution. However, it remains challenging to simultaneously control these structural properties in a three-dimensional (3D) hybrid nanostructure. Here, we introduce a robust strategy for concurrently manipulating these characteristics in a bulk SmCo/Fe(Co) nanocomposite. This method can tune nanocrystals in size (down to sub-10 nm), morphology (sphere, rod, or disc), and crystallographic orientation (isotropic or anisotropic). We have therefore achieved the desired nanostructures: oriented hard magnetic SmCo grains and homogeneously distributed soft magnetic Fe(Co) grains with high fractions (∼26 wt %) and small sizes (∼12.5 nm). The resulting anisotropic nanocomposite exhibits an energy product that is approximately 50% greater than that of its corresponding pure SmCo magnet and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Our findings pave a new way to manipulating 3D hybrid nanostructures in a controllable manner.