Energetic polymers derived from oxetanes

Abstract. Within energetic formulations, polymeric binders are frequently used to both disperse and homogenise the components into a polymer matrix, and to provide stability by improving structural and mechanical integrity. Energetic polymeric binders play a key role in enhancing the stability of these formulations, while also contributing to enhanced outputs. One of the most widely used energetic binders is nitrocellulose, but this poses several challenges, namely: it is derived from cellulose and therefore has natural variation in both structure and properties; it has a high glass transition temperature that means formulations must be plasticised to ensure acceptable mechanical properties; and stabilisers are required to prevent autocatalytic degradation. Because of this inherent variability and difficulty in sourcing reliable NC, attention has turned to the development of synthetic energetic polymer alternatives. For this study, polyoxetanes have been selected due to their tunability and relatively simple synthesis. The synthetic approach for the preparation of nitrate ester functionalised polyoxetanes can follow two pathways. The first involves the direct nitration of the oxetane-based heterocycle using a highly selective nitration method of dinitrogen pentoxide (N2O5). Utilising this nitrating agent removes the need for strong acids as the reaction medium, thereby allowing the ring to remain intact. Following this, the energetic monomer can be polymerised using cationic ring opening polymerisation (CROP). The second approach to achieving nitrate ester functionalised polyoxetanes involves polymerising the oxetane species before nitrating the inert polymer. The steps taken to date to prepare energetic binders by these two approaches are reported here, along with comparisons of their structure property relationships, and sensitivity to initiation.