Supramolecular Control of Propagation in Cationic Polymerization of Room Temperature Curable Epoxy Compositions

The cationic ring-opening polymerization (ROP) of room temperature curable epoxy compositions was investigated in the presence of protic (alcohols), weakly chelating (linear polyethers), and strongly chelating (crown ethers) species. Epoxide conversion and gelation were monitored through infrared and rheological measurements. We demonstrate that the propensity of hydroxyl moieties to promote the activated monomer (AM) mechanism and the chelating ability of polyether groups toward the cationic species involved in this propagation mode can be combined to control two fundamental parameters of the gelation process of epoxy resins, the gel time (<i>t</i>_gel) and the critical conversion (conversion at the gel point, <i>x</i>_gel), by adequately adapting the amount of these additives. In the case of crown ether, a strong synergy between these two control tools was found and interpreted by the prolonged stabilization of protons involved in chain transfers, in the form of dormant supramolecular host–guest complexes. These results underline the potential of this new approach to control both the kinetic and architecture of epoxy growing network

Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy–acid and epoxy–anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics

Making Insoluble Polymer Networks Malleable via Olefin Metathesis

Covalently cross-linked polymers have many technological applications for their excellent properties, but they suffer from the lack of processability and adaptive properties. We report a simple, efficient method of generating adaptive cross-linked polymers via olefin metathesis. By introducing a very low level of the Grubbs’ second-generation Ru metathesis catalyst, a chemically cross-linked polybutadiene network becomes malleable at room temperature while retaining its insolubility. The stress relaxation capability increases with increasing level of catalyst loading. In sharp contrast, catalyst-free control samples with identical network topology and cross-linking density do not show any adaptive properties. This chemistry should offer a possibility to combine the dimensional stability and solvent resistance of cross-linked polymers and the processability/adaptibility of thermoplastics

Catalytic Control of the Vitrimer Glass Transition

Vitrimers, strong organic glass formers, are covalent networks that are able to change their topology through thermoactivated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. At low temperatures, exchange reactions are very long and vitrimers behave like classical thermosets. The transition from the liquid to the solid is reversible and is, in fact, a glass transition. By changing the content and nature of the catalyst, we can tune the transesterification reaction rate and show that the vitrimer glass transition temperature and the broadness of the transition can be controlled at will in epoxy-based vitrimers. This opens new possibilities in practical applications of thermosets such as healing or convenient processability in a wide temperature range

Supramolecular Thermoplastic with 0.5 Pa·s Melt Viscosity

Design of materials with polymer-like properties at service temperature but able to flow like simple liquids when heated remains one of the important challenges of supramolecular chemistry. Combining these antagonistic properties is highly desirable to provide durability, processability, and recyclability of materials. Here, we explore a new strategy based on polycondensation reactions to design supramolecular polymer materials with stress at break above 10 MPa and melt viscosity lower than 1 Pa·s. We report the synthesis and rheological and mechanical properties (uniaxial tensile tests) of supramolecular polymers based on a multiblock polyamide architecture. The flexibility of polycondensation reactions made it possible to control the molecular size distribution, the strength of hydrogen bonds, and the crystallization of middle and end groups and to achieve targeted properties