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The properties of polyelectrolyte complexes and coacervates, both termed PECs, are influenced strongly by their ion and water content. Water plasticizes PECs, reducing their modulus and glass trans...

The spontaneous association of oppositely charged polyelectrolytes is an example of liquid–liquid phase separation. The resulting hydrated polyelectrolyte complexes or coacervates, both termed “PEC...

A one pot synthesis is applied to control the chain structure and architecture of multiply dynamic polymers, enabling fine tuning of materials properties by choice of polymer chain length or crosslink density. Macromolecules containing both non-covalent linkers based on quadruple hydrogen-bonded 2-(((6-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)hexyl)carbamoyl)oxy)ethyl methacrylate (UPyMA), and thermoresponsive dynamic covalent furan–maleimide based Diels–Alder linkers are explored. The primary polymer's architecture was controlled by reversible addition-fragmentation chain transfer (RAFT) polymerization, with the dynamic non-covalent (UPyMA) and dynamic covalent furfuryl methacrylate (FMA) units incorporated into the same backbone. The materials are crosslinked, taking advantage of the “click” chemistry properties of the furan–maleimide reaction. The polymer materials showed stimulus-responsive thermomechanical properties with a decrosslinking temperature increasing with the polymer's primary chain length and crosslink density. The polymers had good thermally promoted self-healing properties due to the dynamic covalent Diels–Alder bonds. Besides, the materials had excellent stress relaxation characteristics induced by the exchange of the hydrogen bonds in UPyMA units.

Oppositely charged polyelectrolytes in solution spontaneously associate into hydrated complexes or coacervates, PECs. The morphology, stability, and properties of PECs depend strongly on their ion ...

Dynamic nucleophilic exchange of quaternary anilinium salts has been incorporated into rehealable and malleable polymeric materials that can be activated under mild (60 °C) thermal stimulus. The mechanism of dynamic exchange between quaternary anilinium salt and free aniline was assessed in small-molecule model experiments. The dynamic exchange was found to be dissociative in nature, due to the indirect SN2 mechanism, where initially the bromide anion attacks the anilinium salt to generate an alkyl bromide which undergoes subsequent attack by a free aniline group. A quaternary anilinium-based cross-linker was synthesized to act as dynamic linkages in the polymer network. Cross-linked polymeric materials showed thermoresponsive rehealing and malleability properties at 60 °C along with being resistant to irreversible creep under ambient conditions. The use of anilinium salts enables dynamic exchange to occur with significantly milder thermal stimulus than other comparable materials, while maintaining mechan...

Recent progress on stretchable, tough dual-dynamic polymer single networks (SN) and interpenetrated networks (IPN) has broadened the potential applications of dynamic polymers. However, the impact of macromolecular structure on the material mechanics remains poorly understood. Here, rapidly exchanging hydrogen bonds and thermoresponsive Diels–Alder bonds were included into molecularly engineered interpenetrated network materials. RAFT polymerization was used to make well-defined polymers with control over macromolecular architecture. The IPN materials were assessed by gel permeation chromatography, differential scanning calorimetry, tensile testing and rheology. The mechanical properties of these IPN materials can be tuned by varying the crosslinker content and chain length. All materials are elastic and have dynamic behavior at both ambient temperature and elevated temperature (90 °C), owing to the presence of the dual dynamic noncovalent and covalent bonds. 100% self-healing recovery was achieved and a maximum stress level of up to 6 MPa was obtained. The data suggested the material's healing properties are inversely proportional to the content of the crosslinker or the degree of polymerization at both room and elevated temperature. The thermoresponsive crosslinker restricted deformation to some extent in an ambient environment but gave excellent malleability upon heating. The underlying mechanism was explored by the computational simulations. Furthermore, a single network material with the same crosslinker content and degree of polymerization as the IPN was made. The SN was substantially weaker than the comparable IPN material.

The kinetics and mechanism of the thermally activated dynamic covalent exchange of thiol-Michael adducts is investigated. A model system of thiol-Michael adducts between thiophenol and phenylvinylketone derivatives and adducts between 2-mercaptoethanol phenylvinylketone derivatives in N,N-dimethylformamide (DMF) at elevated temperatures is used to probe the underlying exchange mechanism. The kinetic data show negligible free Michael acceptor, which is consistent with the highly efficient thiol-Michael reaction being a "click"-like reaction that significantly favors the adduct form. At elevated temperatures of 90 °C in DMF the thiol-Michael adducts reach equilibrium after 24 h, although equilibration did not occur within 24 h at 60 °C or 75 °C, and negligible exchange occurs under ambient conditions. A kinetic model was developed to describe the dynamic covalent exchange and equilibration. The experimental and simulation kinetic data of dynamic covalent exchange are consistent with the thiol-Michael adducts undergoing a retro-Michael reaction, followed by subsequent addition of a free thiol to the liberated Michael acceptor. Kinetic analysis is consistent with the fragmentation, or retro-Michael reaction, being the rate-determining step in the dynamic covalent exchange. This suggests that the key step in dynamic covalent exchange is not enhanced by addition of free thiol or free Michael acceptor, since the addition reaction is much faster than the retro-Michael reaction. This fundamental study will guide the design of organic compounds, materials, and bioconjugates that utilize the thermally activated dynamic covalent thiol-Michael linkages.

This work reports synthesis of dynamic materials crosslinked with thiol-Michael linkages with distinct primary polymer architectures. RAFT polymerization allows control over degree of polymerization and macromolecular architecture. Well-defined branched and linear polythiol polymers were synthesized by RAFT and crosslinked using thiol-Michael chemistry. Branched and linear polymeric materials with different crosslink densities were evaluated by size exclusion chromatography, tensile testing, rheology, and differential scanning calorimetry. These materials are elastic and show dynamic behavior (e.g. healing ability and malleability) in response to thermal stimulus (90 °C) due to the presence of the stimulus responsive thiol-maleimide linkages as crosslinkers along the polymer backbone. The data suggest that materials synthesised by RAFT healed faster than materials of similar weight average chain length and crosslink density synthesized by conventional free radical polymerization. Healing ability and malleability properties of these dynamic materials are dramatically higher than materials crosslinked with static crosslinkers. Small molecular studies of thiol-maleimide adducts indicate the potential of using thiol-Michael linkages in dynamically crosslinked materials. In addition to significant re-healing and malleability properties, these materials showed mechanical stability in creep deformation, stress relaxation, and creep recovery experiments under ambient conditions due to their essentially static nature under these conditions.

Thiol-maleimide adducts have been used as dynamic crosslinkers to form soft, elastic, and stimuli responsive polymeric materials. Thiol-Michael adducts can undergo dynamic exchange at elevated temperature or elevated pH values. Due to the dynamic behaviour of thiol-Michael adducts, crosslinked polymeric materials display significant healing after cutting into half, and malleability upon exposure to solutions of elevated pH. These materials are also thermally responsive, showing self-healing properties and malleability at high temperatures (90 °C). The self-healing properties of these polymer materials are significantly higher than materials with non-dynamic crosslinkers. In addition, in mechanical stability experiments, these materials showed creep resistance and complete creep recovery at room temperature and pressure. These results indicate that the thiol-Michael reaction is dynamic and reversible in response to thermal and pH stimuli. These stimuli responsive self-healing, elastic, malleable, and mechanically stable polymeric materials open the door to have potential utilization in different applications such as coatings or elastomers with extended lifetimes.

The thiol–Michael adduct is used as a thermoresponsive dynamic cross-linker in polymeric materials. Recently, the thiol–Michael reaction between thiols and conjugated alkenes has been used as a ligation reaction for polymer synthesis and functionalization. Here, the thiol–Michael reaction is demonstrated to be thermally responsive and dynamic. Small molecule model experiments demonstrate the potential for the thiol–Michael adducts to be used in dynamic covalent chemistry. Thiol–acrylate adducts are incorporated into a cross-linker to form a soft polymeric material. These thiol–Michael cross-linked materials display healing after being cut and malleability characteristics at 90 °C. Additionally, the data suggest that there is limited creep and stress relaxation at room temperature with complete recovery of creep once the strain is removed. These thiol–Michael cross-linked polymers show dynamic properties upon thermal stimulus, with long-term stability against mechanical deformation in the absence of this s...

Recently, photochemical polymerizations have received interest since they can be performed under mild conditions and they offer temporal control over the reaction. In this study, the kinetics of photochemical dithiobenzoate-mediated RAFT polymerization in the presence of triethylamine is explored. This system uses mild light sources such as visible light and sunlight, and does not require the use of expensive or rare earth catalysts. Instead triethylamine is combined with reagents used in RAFT polymerization. This study investigates the effects of light source, RAFT chain transfer agent concentration, and amine concentration, to understand the kinetic contributions of each component and possible mechanism of this process. Data suggests that there is electron transfer from the amine to the excited RAFT end-group, which is the major radical generation pathway. Radicals are also generated directly from the excited RAFT end-group. This method yields living polymers as evidenced by the synthesis of well-defined block copolymers.

A self-healing material containing two reversible cross-linkers was made. Relatively rapidly exchanging hydrogen-bonded and slowly exchanging Diels–Alder based cross-linkers were incorporated. Two time scales allowed partial healing at room temperature, and near complete healing upon heating. Slow linkers limited creep at room temperature but allowed reshaping upon heating.