A near-infrared-light (NIR)- and UV-light-responsive polymer nanocomposite is synthesized by doping polymer-grafted gold nanorods into azobenzene liquid-crystalline dynamic networks (AuNR-ALCNs). The effects of the two different photoresponsive mechanisms, i.e., the photochemical reaction of azobenzene and the photothermal effect from the surface plasmon resonance of the AuNRs, are investigated by monitoring both the NIR- and UV-light-induced contraction forces of the oriented AuNR-ALCNs. By taking advantage of the material's easy processability, bilayer-structured actuators can be fabricated to display photocontrollable bending/unbending directions, as well as localized actuations through programmed alignment of azobenzene mesogens in selected regions. Versatile and complex motions enabled by the enhanced photocontrol of actuation are demonstrated, including plastic "athletes" that can execute light-controlled push-ups or sit-ups, and a light-driven caterpillar-inspired walker that can crawl forward on a ratcheted substrate at a speed of about 13 mm min . Moreover, the photomechanical effects arising from the two types of light-triggered molecular motion, i.e., the trans-cis photoisomerization and a liquid-crystalline-isotropic phase transition of the azobenzene mesogens, are added up to design a polymer "crane" that is capable of performing light-controlled, robot-like, concerted macroscopic motions including grasping, lifting up, lowering down, and releasing an object.

A novel convergent route to 3-arm star polymers is described that takes advantage of RAFT-synthesized homopolymers serving as masked macromolecular terminal thiol-containing materials capable of undergoing thiol-ene click reactions.

Sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used as a facile and quantitative method for modifying end-groups on an N-isopropylacrylamide (NIPAm) homopolymer. A well-defined precursor of polyNIPAm (PNIPAm) was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization in DMF at 70 C using the 1-cyano-1-methylethyl dithiobenzoate/2,2 0 -azobis(2-methylpropionitrile) chain transfer agent/initiator combination yielding a homopolymer with an absolute molecular weight of 5880 and polydispersity index of 1.18. The dithiobenzoate end-groups were modified in a one-pot process via primary amine cleavage followed by phosphine-mediated nucleophilic thiol-ene click reactions with either allyl methacrylate or propargyl acrylate yielding ene and yne terminal PNIPAm homopolymers quantitatively. The ene and yne groups were then modified, quantitatively as determined by 1 H NMR spectroscopy, via radical thiol-ene and radical thiol-yne reactions with three representative commercially available thiols yielding the mono and bis end functional NIPAm homopolymers. This is the first time such sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used in polymer synthesis/end-group modification. The lower critical solution temperatures (LCST) were then determined for all PNIPAm homopolymers using a combination of optical measurements and dynamic light scattering. It is shown that the LCST varies depending on the chemical nature of the end-groups with measured values lying in the range 26-35 C.

N,N-Diethylacrylamide (DEAm) was homopolymerized by reversible addition-fragmentation chain transfer (RAFT) radical polymerization yielding a homopolymer with a calculated degree of polymerization of 30 (PDEAm 30 ), as determined by 1 H NMR spectroscopic end-group analysis, and a polydispersity index (M w /M n ) of 1.10. Aminolysis of the dithioester end groups followed by treatment with tris(carboxyethyl)phosphine hydrochloride yielded the corresponding macromolecular secondary thiol (PDEAm 30 -SH). Reaction of PDEAm 30 -SH with a range of commercially available isocyanates, catalyzed by NEt 3 , gave the corresponding thiocarbamate end-functional polymers in essentially quantitative yield as determined by a combination of 1 H NMR spectroscopy and UV-vis spectrophotometry. These reactions were shown to be rapid as evidenced by the real-time kinetics for the reaction between PDEAm 30 -SH and hexyl isocyanate in the presence of NEt 3 . Additionally, these facile reactions were shown to proceed without any apparent detrimental effect/side reactions to the basic polymer structure as evidenced by size exclusion chromatography with all modified polymers retaining their narrow molecular weight distributions. The lower critical solution temperatures (LCSTs) of the resulting ω-modified PDEAm homopolymers were evaluated by turbidimetry. For those samples that were soluble in aqueous media the measured LCSTs were between 3 and 11 °C lower than that determined for PDEAm 30 -SH (34 °C). Such differences were attributed to the hydrophobic nature of the newly introduced end groups;the effect being pronounced given the relatively low molecular weight of the precursor homopolymer. In two instances, and specifically the end-modified PDEAm homopolymers obtained from reaction with 9-isocyanato-9Hfluorene and 4-(2-isocyanatoethyl)biphenyl, the resulting materials were not soluble in water even at temperatures approaching 0 °C.

Photothermal-induced self-healable and shape memory materials have drawn much attention due to the rapidly growing technical applications and environmental requirements. As epoxy natural rubber (ENR) is a kind of bio-based elastomer with good mechanical properties, weather resistance, and air impermeability, it is of great significance to incorporate ENR with recyclable, photothermal-induced self-healable and shape memory properties. In this study, we report a simple method to cross-link ENR with dodecanedioic acids (DAs) through esterification reaction, and during the cross-linking process, a little aniline trimer (ACAT, a kind of oligoaniline) was added at the same time. Then, the ENR-DA-ACAT vitrimers that were covalently cross-linked with recyclable, self-healable, and multiple responsive properties were obtained, which also possessed various functions. As a result of the transesterification reactions at elevated temperatures, the ENR-based vitrimers possess the ability to be reprocessed and self-healed, and the mechanical properties could be maintained even after three consecutive breaking/mold pressing cycles. Besides, the vitrimer is also responsive to near-infrared (NIR) light and pH with the introduction of ACAT, and we also find that ACAT can be used as a catalyst to accelerate the transesterification reaction. Moreover, it is demonstrated that the ENR-DA-ACAT vitrimer could also be used to construct the reconfigurable shape memory polymer; the shape fixing ratio and shape recovery ratio are both above 95% in the reconfiguration process, and the multistage shape memory performance can also be achieved by NIR irradiation, which will potentially lead to a wide application for ENR in the field of actuators.

No abstract

Vitrimer is a new class of polymeric materials which can be reprocessed to any shape while being permanently cross-linked. We designed and synthesized a catalyst-free network with poly-(dimethylsiloxane)etherimide (PDMS-NH 2 ), terephthalaldehyde (TA), and tri(2-aminoethyl)amine (TREA) through the condensation reaction between amino groups and aldehyde groups. As a result of the exchange reaction of the dynamic imine bond obtained, this PDMS network exhibits the nature of vitrimer-like material, which is examined by solubility and stress-relaxation experiments, and the relaxation time is as short as 64 s at 130 °C. In addition, the vitrimer-like PDMS is malleable and capable of self-healing, and the mechanical properties can be maintained even after three consecutive breaking/mold pressing cycles. Especially, besides heating, this vitrimer-like PDMS can also be recycled and reshaped at ambient temperature due to the exchange reaction of dynamic imine bond when immersed in water, which will potentially lead to green processing of the elastomers.

Reversible addition-fragmentation chain transfer (RAFT) radical polymerization, mediated by 4-cyanopentanoic acid dithiobenzoate and 4,4'-azobis(4-cyanovaleric acid) (V-501) in water at 70 degrees C, of biocompatible 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) yields a macro-chain transfer agent (CTA) that was employed in the synthesis of a range of stimulus-responsive AB diblock copolymers in protic media. Well-defined block copolymers of varying molar composition, with narrow molecular weight distributions (M(w)/M(n) = 1.10-1.24) were prepared with N,N-diethylacrylamide (DEAm), 4-vinylbenzoic acid (VBZ), N-(3-sulfopropyl)-N-methacryloyloxyethyl-N,N-dimethylammonium betaine (DMAPS), and the newly synthesized N,N-di-n-propylbenzylvinylamine (DnPBVA) in either methanol, 2,2,2-trifluoroethanol, or aqueous media. When a combination of (1)H NMR spectroscopy and dynamic light scattering is used, it is shown that all block copolymers are capable of existing as molecularly dissolved chains in aqueous media with average hydrodynamic diameters of approximately 6-7 nm provided the aqueous environment is appropriately tuned. Similarly, these unimers can be induced to undergo self-assembly in the same aqueous environment provided the correct external stimulus (change in temperature, pH, or electrolyte concentration) is applied. In such instances, aggregates with average sizes in the range of approximately 22-180 nm are formed and are most likely due to the formation of polymeric micelles and vesicles. Such self-assembly is also completely reversible. Removal, or reversal, of the applied stimulus results in the reorganization to the unimeric state.