A novel and versatile method for the synthesis of 2H-imidazoles via iron-catalyzed [3 + 2] annulation from readily available oxime acetates with vinyl azides has been developed. This denitrogenative process involved N–O/N–N bond cleavages and two C–N bond formations to furnish 2,4-substituted 2H-imidazoles. This protocol was performed under mild reaction conditions and needed no additives or ligands. Furthermore, this is a green reaction involving oxime acetate as internal oxidant, acetic acid, and nitrogen as byproducts.

Excellent mechanical properties and remarkable self-healing ability are difficult to unify in one hydrogel. We integrated acylhydrazone bonds and Pluronic F127 (PF127) micelle cross-linking, as two kinds of dynamic cross-links, in one system and developed hydrogels with superior stretchability, high toughness, and good self-healing ability. The hydrogel could stretch up to 117 times its initial length and self-heal approximately 85% of its initial strength within 24 h. The toughness of the hydrogel, indexed by the work of extension, W, reached 14.1 MJ m–3. Energy dissipation occurred from the simultaneous decomposition of the PF127 micelles and chain sliding facilitated by the reconfiguration of the acylhydrazone bonds. This unique combination and dynamics led to pronounced hysteresis in the loading–unloading cycles, as well as good recovery and self-healing of the hydrogel. Dynamic cross-linking of the covalent acylhydrazone bonds was comparable to those of physical interactions, such as coordination and ionic bonding.

A novel copper-catalyzed C(sp3)–H oxidative functionalization of aromatic oxime acetates with α-oxocarboxylic acids was reported. This process involved N–O/C–C bond cleavages and C–C bond formations to furnish substituted enaminones under redox-neutral conditions. The oxime acetates served as both reactants and internal oxidants. Furthermore, this transformation also features good functional group tolerance and needs no ligands or additional bases.

•Self-healing PDMS elastomers based on acylhydrazone groups were prepared.•A reversible transition was observed around 80 °C.•Role of H-bonds on self-healing properties was elucidated.A PDMS elastomer based on acylhydrazone groups with both acid- and heat-assisted self-healing properties was successfully prepared from tetra-acylhydrazine-terminated PDMS and terephthalaldehyde through solution casting. The good healing performance was obtained with catalytic acetic acid for 24 h at 25 °C or by annealing at 120 °C for 2 h. The elastomer exhibited a reversible transition near 80 °C observed by rheological measurements and variable-temperature FTIR, which corresponded to the dissociation and reconstruction of hydrogen bonds between acylhydrazone groups. Since the non-equimolar sample presented similar behaviors with the equimolar sample, it verifies that the reversible dissociation/reformation of hydrogen bonds dominates the heat-assisted self-healing process. This finding will enable better understanding of the contribution of hydrogen bonding interactions in acylhydrazone self-healing systems, thus promoting the development of self-healing bulk materials based on acylhydrazone groups.Download high-res image (321KB)Download full-size image

An efficient strategy for synthesis of isoquinolines via Pd(II)-catalyzed cyclization reaction of oximes with vinyl azides or homocoupling of oximes is reported. Oximes could serve as a directing group and an internal oxidant in the transformation. This reaction features good functional group tolerance and provides a useful protocol for the synthesis of different kinds of isoquinolines under mild conditions. Some control experiments and 15N isotope labeling experiments were conducted for the mechanistic research.

A macroscopic organohydrogel hybrid was prepared by fast adhesion between the hydrogel and organogel which often repel each other. The two original gels were prepared by condensation of two poly(ethylene glycol) (PEG) gelators in anisole and water, respectively. Reversible acylhydrazone bonds formed in the condensation act as linking points of the polymer networks in the gels. When the two gels were brought into contact, a robust hybridized gel was obtained in 10 min. An emulsion layer formed at the interface between the two gels and dynamic chemistry of acylhydrazone bonding are key factors in rapid adhesion of the two inherently different gels. We hope this finding will enable the development of intelligent soft objects whose macroscopic water and oil phases contain different functional components.

Dynamic polymer hydrogels with an environmental adaptive self-healing ability and dual responsive sol–gel transitions were prepared by combining acylhydrazone and disulfide bonds together in the same system. The hydrogel can automatically repair damage to it under both acidic (pH 3 and 6) and basic (pH 9) conditions through acylhydrazone exchange or disulfide exchange reactions. However, the hydrogel is not self-healable at pH 7 because both bonds are kinetically locked, whereas the hydrogel gains self-healing ability by accelerating acylhydrazone exchange with the help of catalytic aniline. All of the self-healing processes are demonstrated to be effective without an external stimulus at room temperature in air. The hydrogel also displays unique reversible sol–gel transitions in response to both pH (HCl/triethylamine) and redox (DTT/H2O2) triggers.

In our previous work [Macromolecules2010, 43, 1191–1194], we synthesized dynamic covalent cross-linked polymer gels through condensation of acylhydrazines at the chain ends of poly(ethylene oxide) (A2) and aldehyde groups in tris[(4-formylphenoxy)methy]ethane (B3) and reported reversible sol–gel transition and self-healing properties of the gels. For those dynamic gels, this paper examines the gelation kinetics and rheological behavior in pre- and postgelation stages and discusses the molecular mechanism underlying the mechanical and self-healing properties. The results showed that the condensation reaction before the critical gelation point can be treated as the pseudo-second-order reaction. The scaling exponent n (=0.75) for the frequency dependence of the complex moduli at the critical gel point, the exponent γ (=1.5) for the concentration dependence of the viscosity in the pregel regime, and the exponent z (=2.5) for the concentration dependence of the equilibrium modulus in the postgel regime were found to not exactly obey the relationship for covalent gels, n = z/(z + γ), possibly because of the dynamic nature of the gels. The terminal relaxation of the dynamic gels at high temperature (125 °C) accorded with the Maxwellian model, as often observed for transient associating networks. In contrast, at low temperature (25 °C) where this transient network reorganization was essentially quenched in a time scale of experiments (∼50 s), the uniaxial stress–strain behavior of the gel was well described by the classical model of rubber elasticity σeng = G(λ – 1/λ2) up to 300% stretch (as similar to the behavior of usual gels chemically cross-linked in a swollen state). Ultimately, the gel cut into two pieces was found to exhibit self-healing under ambient conditions in 8 and 24 h, respectively, when the edges of those pieces were coated and not coated with acid (catalyst for dynamic covalent bond formation).

ABC-type miktoarm star polymers, poly(ethylene oxide)-block-polystyrene-block-poly (ε-caprolactone)s (PEO-b-PS-b-PCL) were synthesized via combination of “click” chemistry, atom-transfer radical polymerization (ATRP) and ring opening polymerization (ROP). Azide ended PEO arms, PEO–N3, and a trifunctional molecule, propargyl 2-hydroxylmethyl-2-(α-bromoisobutyraloxymethyl)-propionate (PHBP), were prepared first, respectively. A “click” reaction of PEO–N3 and PHBP generated a PEO macroinitiator, PEO–(Br)(OH) with two functionalities, one is hydroxyl group and the other is α-bromoisobutyraloxyl group. Consecutive ATRP of styrene (St) and ROP of ε-caprolactone (ε-CL) from the PEO macroinitiator produced the PEO-b-PS-b-PCL miktoarm stars. All the structures of the polymers were determined.

A novel copper-catalyzed C(sp3)–H oxidative functionalization of aromatic oxime acetates with α-oxocarboxylic acids was reported. This process involved N–O/C–C bond cleavages and C–C bond formations to furnish substituted enaminones under redox-neutral conditions. The oxime acetates served as both reactants and internal oxidants. Furthermore, this transformation also features good functional group tolerance and needs no ligands or additional bases.