In the end: Alkyl end groups can induce a thermoreversible sol–gel transition in an otherwise sol‐like suspension by affecting the macroscopic self‐assembly behavior. The picture shows PLGA‐PEG‐PLGA triblock copolymer/water mixtures in the sol (polymer a), gel (polymers b and c), and precipitation states (polymer d). PLGA=poly(lactic acid‐co‐glycolic acid), PEG=poly(ethylene glycol).

The spontaneous hydrogel formation of a sort of biocompatible and biodegradable amphiphilic block copolymer in water was observed, and the underlying gelling mechanism was assumed. A series of ABA-type triblock copolymers [poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(D,L-lactic acid-co-glycolic acid)] and different derivatives end-capped by small alkyl groups were synthesized, and the aqueous phase behaviors of these samples were studied. The virgin triblock copolymers and most of the derivatives exhibited a temperature-dependent reversible sol-gel transition in water. Both the poly(D,L-lactic acid-co-glycolic acid) length and end group were found to significantly tune the gel windows in the phase diagrams, but with different behaviors. The critical micelle concentrations were much lower than the associated critical gel concentrations, and an intact micellar structure remained after gelation. A combination of various measurement techniques confirmed that the sol-gel transition with an increase in the temperature was induced not simply via the self-assembly of amphiphilic polymer chains but also via the further hydrophobic aggregation of micelles resulting in a micelle network due to a large-scale self-assembly. The coarsening of the micelle network was further suggested to account for the transition from a transparent gel to an opaque gel.

Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.

Large-scale synthesis of monodisperse ultrasmall metal ferrite nanoparticles as well as understanding the correlations between chemical composition and MR signal enhancement is critical for developing next-generation, ultrasensitive T magnetic resonance imaging (MRI) nanoprobes. Herein, taking ultrasmall MnFeO nanoparticles (UMFNPs) as a model system, we report a general dynamic simultaneous thermal decomposition (DSTD) strategy for controllable synthesis of monodisperse ultrasmall metal ferrite nanoparticles with sizes smaller than 4 nm. The comparison study revealed that the DSTD using the iron-eruciate paired with a metal-oleate precursor enabled a nucleation-doping process, which is crucial for particle size and distribution control of ultrasmall metal ferrite nanoparticles. The principle of DSTD synthesis has been further confirmed by synthesizing NiFeO and CoFeO nanoparticles with well-controlled sizes of ∼3 nm. More significantly, the success in DSTD synthesis allows us to tune both MR and biochemical properties of magnetic iron oxide nanoprobes by adjusting their chemical composition. Beneficial from the Mn dopant, the synthesized UMFNPs exhibited the highest r relaxivity (up to 8.43 mM s) among the ferrite nanoparticles with similar sizes reported so far and demonstrated a multifunctional T MR nanoprobe for in vivo high-resolution blood pool and liver-specific MRI simultaneously. Our study provides a general strategy to synthesize ultrasmall multicomponent magnetic nanoparticles, which offers possibilities for the chemical design of a highly sensitive ultrasmall magnetic nanoparticle based T MRI probe for various clinical diagnosis applications.