Oxime-Based and Catalyst-Free Dynamic Covalent Polyurethanes

Abstract

Polyurethanes (PUs) have many applications resulting from their preeminent properties, but being commonly used toxic catalysts, and the lack of processability for PU thermosets cause limitations. Herein, we report a new class of the PU-like dynamic covalent polymers, poly(oxime-urethanes) (POUs), which are prepared from the uncatalyzed polyaddition of multifunctional oximes and hexamethylene diisocyanate (HDI) at ambient temperature. Kinetics studies reveal that almost complete polymerization (∼99% conversion) can be achieved in 3 h at 30 °C in dichloromethane (DCM), the most effective among the solvents evaluated, producing linear POUs with comparable molecular weights to the catalyzed PUs. We find that the oxime-carbamate structures are reversible at about 100 °C through oxime-enabled transcarbamoylation via a thermally dissociative mechanism. The cross-linked POUs based on oxime-carbamate bonds show efficient catalyst-free healable/recyclable properties. Density functional theory (DFT) calculations suggest that the fast oxime-urethanation and the mild thermoreversible nature are mediated by the characteristic nitrone tautomer of the oxime. Given widespread urethane-containing materials, POUs are of promising potential in applications because of the excellent mechanical performances, facile preparation, and dynamic property without using catalysts.

Ultra‐Fast Microwave Assisted Self‐Healing of Covalent Adaptive Polyurethane Networks with Carbon Nanotubes

“…Most of the extensive research on vitrimers during the last few years has been focused on epoxy and hard network systems. There are only few papers reporting on PU vitrimers and between them, only very few works focused on PU elastomers using transcarbamoylation exchange reactions to carry out the self‐healing in the presence or absence of catalyst. Self‐healing is challenging for elastomers due to their high elongation, making it easy to reopen any cracks not completely healed.…”

A Transparent, High Refractive, Shape‐Memory, Healable, and Recyclable Phenolic Polyurethane Thermoset: Unexpected Roles of Bromine Substituents and Tertiary Amine Catalysts

“…Polyurethane (PU) thermosets, generally synthesized from diisocyanates and polyols, are a very versatile group of materials with outstanding properties 1. But as a result of permanent crosslinks, they cannot be reshaped, reprocessed, or rehealed after curing, leading to serious environment pollution and a huge waste of resources 2,3.…”

“…However, all these examples depend on the introduction of additional dynamic bonds, which would be limited by the complex synthetic routes, high cost of monomers, and availability of commercial raw materials 3. In fact, urethanes and ureas with certain substitutes like bulky tertiary butyl group10 and imine1 could also exhibit high reversible capability upon heating, showing great potential in scientific and industrial applications.…”

Imine‐Based Architectures at Surfaces and Interfaces: From Self‐Assembly to Dynamic Covalent Chemistry in 2D

“…[2][3][4][5][6] Both DCC and supramolecular chemistry are adaptable and modular because they allow for the selection and exchange of molecular components. [11] In particular,D CC includes the chemistry of disulfides, [12,13] acetals, [14,15] esters, [16,17] oxime, [18] boroxine, [19,20] alkynes, [21] and imines [22][23][24][25][26][27][28][29] to allow the generation of new covalents tructures. The equilibration processes are much faster in supramolecular systemsw hen compared to DCC systems, because in the latter the covalentb onds need to be broken and reformed.…”