Reactive cycloalkane plasticizers covalently linked to energetic polyurethane binders via facile control of an in situ Cu-free azide–alkyne 1,3-dipolar cycloaddition reaction

Abstract

Alkyne-bearing reactive cycloalkane plasticizers (RCAPs) linked covalently to glycidyl azido polymer (GAP)-based polyurethane (PU) binders were developed to prevent the migration of plasticizers and decrease their sensitivities. Adamantane and norbornane used as a stable energetic source from ring strain energy were integrated into the RCAPs which were synthesized through direct esterification between cycloalkane carboxylic acids and alkynols. The inherent ring strain energy of the RCAPs was predicted by homodesmotic reactions at the B3LYP/6-31G* level of theory. The quantitative miscibility study of the RCAPs and GAP prepolymer showed that norbornane-based RCAPs could give the GAP prepolymer a more flexible chain. The experimental approach and theoretical prediction of the in situ Cu(I)-free azide–alkyne 1,3-dipolar cycloaddition (1,3-DPCA) reactivity associated with propargyl versus 3-butynyl species showed that the 1,3-DPCA reactivity could be manipulated by adjusting the methylene spacers between the electron-withdrawing groups and neighbouring alkynes. The addition of the RCAPs improved the mechanical properties and impact insensitivity of the GAP-based PU binders by forming a triazole linkage. The heat of formation was also determined from the heat of combustion of RCAP/GAP-based PU binders.