Efficient metal-free crosslinking of common propellant binders using nitrile oxide–alkene click ligation

The use of isocyanate-free reactions for crosslinking binders is highly desired in solid propellants. The nitrile oxide–alkene click cycloaddition reaction is a promising isocyanate-free reaction due to its metal-free and byproduct-free ligation. In general, ditopic nitrile oxides are synthesized and used as crosslinkers of unsaturated bond-containing polymers, but they are unsuitable for crosslinking common binders containing two reactive end groups due to undesirable chain-extension. Herein, a tritopic nitrile oxide with high stability and high reactivity was synthesized via three simple steps. Thereafter, several matching alkenyl-terminated poly(ethylene oxide-co-tetrahydrofuran) binders were prepared, and the corresponding crosslinking with nitrile oxide was successfully achieved at room temperature based on an efficient nitrile oxide–alkene cycloaddition reaction. All resulting elastomers presented excellent mechanical properties, particularly an obviously improved elongation at break. It is worth mentioning that such metal-free crosslinking can also be well used for the energetic binder poly(3-nitratomethyl-3-methyloxetane) matrix, revealing a versatile method. This work offers a powerful and general isocyanate-free curing strategy, promoting the application of the nitrile oxide–alkene click ligation technique in green crosslinking of propellant binders.