3-Nitromethyleneoxetane: A very versatile and promising building block for energetic oxetane-based monomers  Max Borna‡, Thomas C. Fessardb, Lucas Göttemannb, Konstantin Karaghiosoffa, Jakob Planka and Thomas M. Klapötke*aIn the field...

Today's ammunition still consists of about 70 wt% structure‐providing materials such as metals providing no energetic contribution. Therefore, reactive structure materials (RSMs) offer tremendous room for improvement. While current research focuses on rather heavy, metal‐based materials (e. g., alloys, thermites), energetic polymers appear as an under‐recognized opportunity for very lightweight RSMs. Unfortunately, suitable polymers are unavailable as energetic polymer research has almost exclusively focused on elastic binders with the least possible glass transition temperature. An application as RSM, however, requires rigid polymers with a glass transition above operational temperatures. Accordingly, monomers with fundamentally different structures are required. The first step in this particular direction is 3‐(2,4,6‐trinitrophenoxy)oxetane (TNPO). Herein, we report the synthesis of its homopolymer and investigate its polymerization behavior by copolymerization with prior art energetic oxetanes. All polymers were intensively studied by vibrational and multinuclear (1H, 13C, 14N) NMR spectroscopy, elemental analysis, gel permeation chromatography, and differential scanning calorimetry (DSC). Hereby, DSC revealed the high effect of the TNPO repeating unit on the glass transition temperature. The performance of all polymers was calculated using the EXPLO5 code to evaluate the potential performance range of polymeric RSMs. Further, their shock and friction sensitivity was determined by BAM standard procedures.

Despite the variety of energetic polyoxetane binders, the oxirane-based glycidyl azide polymer (GAP) has largely succeeded in the market due to its advantageous properties. Nevertheless, it suffers from various drawbacks such as non-uniform chain termination, possible chlorine content (flame retardant), and toxic epichlorohydrin required for its synthesis. These problems can be bypassed using the structurally related poly­(3-azidooxetane). Unfortunately, it is only accessible in moderate yield by polymerization of 3-azidooxetane. Herein, we describe its synthesis by polymer-analogous transformation using the new polymers poly­(3-tosyloxyoxetane) and poly­(3-mesyloxyoxetane) as precursors. This results in a significantly increased yield and improved safety as handling of the very sensitive 3-azidooxetane is avoided. The aforementioned prepolymers were prepared using boron trifluoride etherate as well as triisobutylaluminum as catalysts. The latter provides polymers of particularly high molecular weight, and the corresponding poly­(3-azidooxetane) species was obtained and studied for the first time. In order to shed light on the applicability of poly­(3-azidooxetane) as a GAP substitute, it was thoroughly studied with regard to thermal behavior, energetic performance (EXPLO5), plasticizer compatibility, and curing. Moreover, the aquatic toxicity of all involved monomers was analyzed and compared to epichlorohydrin. Here, poly­(3-azidooxetane) turned out as a fully adequate, if not more environmentally benign, substitute.

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