The design of new energetic molecules is based on compounds exhibiting a high density and an elevated heat of formation. These fundamental properties, achieved through the presence of numerous nitrogen atoms and/or explosophoric groups, ensure high performance levels that can be useful in target applications such as explosives, propellants or gas generators.
The same basics also apply when considering the use of polymers, instead of single molecules, as energetic ingredients. However, the quest for high densities and heats of formation appears somewhat more challenging in the case of polymers, since the polymerization process often requires specific chemical functions that may be detrimental to the desired energetic properties. Currently, the most commonly used polymers in the field get their energetic content only from explosophoric groups: NO2 for polyNIMMO (polynitratomethyl methyloxetane) or polyGLYN (polyglycidyl nitrate), N3 for GAP (glycidyl azide polymer), to name a few. This survey gives an overview of research efforts that have been devoted to the synthesis of macromolecules with high nitrogen contents. Such a unique property is expected to produce materials with particularly elevated heats of formation, thus making them very valuable in the field of energetic compounds.
Azaheterocycles are obviously suitable scaffolds for achieving nitrogen-rich polymers. Tetrazole-, tetrazine-, triazole- and triazine-based materials are considered here. Three methodologies have been identified as the main routes for obtaining the target polymers: polymerization of vinylazaheterocycles, incorporation of the desired azaheterocycle in a preformed macromolecular architecture and polycondensation.
2. Tetrazole-based polymers
2.1. Polymerization of vinyltetrazoles
2.1.1. Synthesis of C-vinyltetrazoles
Polymerization of the starting and/or target vinyl compound is a major concern in the reaction of acrylonitrile with sodium azide leading to
2.1.2. Synthesis of N-vinyltetrazoles
Two main strategies have been used for the synthesis of
2.1.3. Synthesis of divinyltetrazoles
Following the above-mentioned strategies, divinyltetrazole compounds have been obtained. Vinylation of 5-vinyltetrazole gave a mixture of 1,5- and 2,5-divinyltetrazole
2.1.4. Polymerization of C-vinyltetrazoles
5-Vinyltetrazoles are readily involved in free-radical polymerization (Figure 4). A comparison between
Poly-5-vinyltetrazole 6 contains 58.3% nitrogen, which clearly makes it a high-nitrogen compound. However, it seems that its
2.1.5. Polymerization of N-vinyltetrazoles
Figure 5 presents the most interesting polymers obtained in terms of nitrogen-content or expected energetic output. To the best of our knowledge, polymerization of 5-nitro-1-vinyltetrazole8 is unknown. However, 1-vinyltetrazoles are generally readily polymerized in free-radical conditions, with water as the solvent of choice. The activity depends on the nature of the 5-substituent. For example, 5-amino-1-vinyltetrazole 7 is less active than the unsubstituted compound due to the electron-donating character of the amino group [9,10]. Unusually, yields of poly(5-amino-1-vinyltetrazole) 8 are slightly higher in
The synthesis and polymerization of 5-(methyl)hydrazino-1-vinyltetrazole
The different examples cited herein highlight the high nitrogen content of polyvinyltetrazoles and their prospects as energetic compounds. In 2003, a pilot-plant production of polyvinyltetrazoles was started in two companies in Russia.
2.2. Incorporation of tetrazole via polymer-analogous transformation
It is well-known that tetrazoles can be obtained by addition of an azide onto a cyano derivative. This method was applied to polyacrylonitrile (PAN) in order to synthesize poly(5-vinyltetrazole) (Figure 6).
When using NaN3/NH4Cl, it was shown that higher temperatures and molecular weights of the initial PAN resulted in a higher incorporation of tetrazole moieties in the polymer . Thus, at 105°C, a PAN with a Mw of 180,000 g/mol was almost completely transformed in the corresponding poly(5-vinyltetrazole) (tetrazole units: 97.5%). The tetrazole content was estimated by 2 independent methods: weight measurements and acid-base titration. According to the authors, this poly(5-vinyltetrazole) synthesis using polymer-analogous transformation is advantageous since there is no commercial source of 5-vinyltetrazole and its synthesis is difficult.
Zinc chloride has also been used as a catalyst to carry out the desired transformation . The best components ratio was NaN3/NH4Cl/RCN 4/4/1. Infrared and NMR spectroscopies demonstrated the total conversion of nitrile functions into tetrazoles. In addition to PAN, other nitrile-containing polymers were succesfully tetrazolated.
Poly(5-vinyltetrazole) for gas generants was also synthesized upon the action of a zinc salt (ZnBr2) but polymerization was carried out in emulsion in the presence of a surfactant . A related patent subsequently extended this water-based synthesis . At 115°C tetrazolation was limited to 70%, but reached 95% at 170°C. Further reaction of the resulting poly(5-vinyltetrazole) with ammonia yielded the corresponding ammonium salt
All these conditions surpass earlier methods that yielded lower levels of tetrazole incorporation (see for example ref ).
Polyvinylchloride (PVC) can also be used as a polymer precursor. Upon reaction with a tetrazolate anion, the corresponding polyvinyltetrazole is formed (Figure 8). However, this method seems less practical since partial elimination of hydrogen chloride generates unsaturated fragments in the final product .
2.3. Synthesis of tetrazole-based polymers via polycondensation
This polyalkylation product is in fact an oligomer, with Mw below 2000 g/mol (ca 15-20 units). With a measured nitrogen content of up to 67.2%, it surpasses polyvinyltetrazole by almost 10%. In addition, its viscous state is attained at relatively low temperatures, thus making it a very promising candidate in gas-generating systems. It should be noted that 5-chloroethyltetrazole can also participate in a similar process. However, polymerization is slower and yields do not exceed 20%.
The high reactivity of 5-chloromethyltetrazole
A patent describes the polycondensation of various dinitriles with diazides to produce tetrazole containing polycondensates . Of these, the most valuable polymer for energetic applications is the one obtained from dicyanofuroxane
The related monomer
3. Tetrazine-based polymers
To the best of our knowledge, monovinyl-1,2,4,5-tetrazines are unknown, and the corresponding polyvinyltetrazines thus remain elusive compounds. However, the first member of the vinyltetrazine family, 3,6-divinyl-1,2,4,5-tetrazine
The synthesis was achieved by using the phenylsulfanylethyl group as a masked vinyl moiety, the latter being the result of oxidation to the sulfone and elimination. Compound
3.2. Synthesis of tetrazine-based polymers via polycondensation
One of the most convenient ways to construct the 1,2,4,5-tetrazine (
Following the same strategy, the previously unknown 1,3,5-triazine-2,4,6-triamidrazone
The structures of all polymers were confirmed using 13C and 15N solid state NMR [26,27]. High-resolution spectra also indicated low amounts of 4-amino-1,2,4-triazole moieties (coming from the known rearrangement of some 1,2-dihydro-1,2,4,5-tetrazine units).
4. Triazole-based polymers
Synthetic strategies for
4.1. 1,2,4-Triazole-based polymers
Figure 13 presents the most interesting polymers obtained in terms of nitrogen content or expected energetic output.
For the synthesis of 3-amino-1-vinyl-1,2,4-triazole
Poly(3-nitro-1-vinyl-1,2,4-triazole) 35 is also a useful target due to the expected output from the nitro group. Alkylation of 3-nitro-1,2,4-triazole 33 with dichloroethane followed by dehydrochlorination gave a mixture of 1- and 4-vinyltriazole. The former (34) was polymerized in DMF and gave a polymer that was soluble only in highly polar solvents  (Figure 13).
The strategy involving the construction of the vinyl fragment through dehydrohalogenation, dehydration or deamination is also applicable for the synthesis of
4.1.3. Energetic poly(vinyl-1,2,4-triazole)s salts
To further increase the potential of poly(1-vinyl-1,2,4-triazole), protonation with energetic inorganic or organic acids was investigated (Figure 15). Polymerization of protonated 1-vinyl-1,2,4-triazole
The equivalent work in the poly(
4.1.4. Incorporation of 1,2,4-triazole via polymer-analogous transformation
Energetic heterocyclic analogues of the well-known polyglycidyl azide (GAP) have been prepared via substitution of the chlorine atom of polyepichlorhydrin
4.1.5. Synthesis of 1,2,4-triazole-based polymers via polycondensation
Among the few 1,2,4-triazole-based condensation polymers known , the most promising as energetic materials are poly(4-amino-1,2,4-triazole)s . The latter can be commonly obtained through isomerization of poly(dihydrotetrazine)s
Another polycondensate incorporating 4-amino-1,2,4-triazole units has been mentioned in a patent. The synthesis is based on the simultaneous condensation of 3,5-dihydrazino-4-amino-1,2,4-triazole
It should also be noted that 3-amino-1,2,4-triazole can be subjected to electrooxidation to yield films of the corresponding polymer [46,47]. However, the use of these films seems to be limited to corrosion protection applications and they obviously cannot be implemented as energetic materials.
4.2. 1,2,3-Triazole-based polymers
4.2.3. Incorporation of 1,2,3-triazole via polymer-analogous transformation
Reaction of the sodium salt of 4(5)-nitro-1,2,3-triazole
This is the only method rendering it possible to obtain this nitrated polymer since it cannot be synthesized through polymerization of 1-vinyl-4-nitro-1,2,3-triazole
The same chemistry was applied to copolymers of vinylchloride and 2-methyl-5-vinyl-tetrazole to yield the corresponding nitro-1,2,3-triazole containing copolymers
Similarly, the salt
4.2.4. Synthesis of 1,2,3-triazole based polymers via polycondensation
The well-established reaction of an azide with an alkyne, that has been extensively used for monomeric 1,2,3-triazole compounds, can be further extended to the synthesis of polymers.
Thus, (azidoalkyl)- or (azidoaryl) acetylenes were polymerized without catalyst to afford materials that were presumed to contain 1,4- and 1,5-substituted 1,2,3-triazole rings in a random distribution . The most interesting compound for energetic applications was polymer
Similarly, an azido-1,3,5-triazine containing propargyl ether was used as the monomer. Of the different conditions (bulk, solution, with or without catalyst or heating), polymerization in bulk without catalysts was found to be the best way to minimize decomposition reactions and formation of by-products . The hyperbranched product
5. Triazine-based polymers
To the best of our knowledge, vinyl-1,2,4-triazines are unknown. Some poly-1,2,4-triazines have been obtained by polycondensation , however their nitrogen content is much too low for them to be considered as energetic materials. This section will thus focus on polymers containing 1,3,5-triazines (
2,4,6-Trivinyl-1,3,5-triazine would also be an interesting compound (as a nitrogen-containing cross-linker) but the reported yield for its synthesis is poor . In fact, the most valuable vinyltriazine is 2,4-diamino-6-vinyl-1,3,5-triazine
5.2. Synthesis of 1,3,5-triazine-based polymers via polycondensation
Amino-substituted triazines bearing a leaving group have been heated to obtain the corresponding homopolycondensates. A linear polytriazinylamine
In a similar fashion, 2,4-bis(methylamino)-1,3,5-triazine
Aminoalkyl units can also serve as linkages between triazine rings, as exemplified by the commercially available polymer 78, which has been used as a charring agent in flame-retardant compositions  (Figure 26).
Melamine-formaldehyde resins constitute a well-known class of compounds. Not surprisingly, related polymers can also be considered as derivatives with high nitrogen contents. For example, the reaction of trichloromelamine
The chemistry shown above in the case of polytriazinylamines has also been widely explored for the synthesis of polytriazinylethers. The latter are obviously less attractive in terms of nitrogen content. However, commercially available nitro-containing diols are valuable compounds when one wishes to incorporate explosophoric functions in the final material. Thus, 2-nitroresorcinol
It should also be noted that carbon dinucleophiles are effective in polycondensation reactions with cyanuric chloride. Thus, sodium carbide enabled the construction of cross-linked architectures
Another well-known way to obtain triazine compounds is the cyclotrimerization of nitriles. However, only scarce examples deal with azaheterocyclic nitriles. 2,6-dicyanopyridine
The quest for graphitic forms of carbon nitride (g-C3N4) has stimulated a vast amount of research in order to find suitable molecular precursors. With a calculated nitrogen content of 60.9%, C3N4 surely has its place in this overview, although only a few precursors lead to materials approaching the theoretical N/C ratio due to the presence of hydrogen or oxygen. The structure of C3N4 is still under consideration, but surely involves 1,3,5-triazine moieties. Many useful references are provided to the reader in . Theoretical as well as characterization results have indicated that tri-
Azide salts of bis(aminoguanidinium) compounds can be condensed with formaldehyde to produce the corresponding polymers
Gaseous cyanogen (NC-CN) is a promising monomer as it shows a C/N ratio of 1/1. Studies have been devoted to its polymerization and given rise to so-called paracyanogen, either through chemical  or photochemical  methods. The determination of the exact structure of the polymer is a difficult task and a number of different hypotheses have been postulated (Figure 32). This work is made complicated by the incorporation of solvent or water/oxygen during the polymerization process, which leads to a significant presence of oxygen in the elemental analyses. Therefore, the theoretical figure of 50% of nitrogen can hardly be obtained. However, certain reaction conditions have enabled the synthesis of materials exceeding 40% of N.
This review highlights a number of azaheterocycles-based polymer structures containing a high content of nitrogen, frequently around 50%. In some cases, over 60% and 70% N can be achieved, mainly in tetrazole-based materials. The presence of additional explosophoric groups such as nitro may also be encountered in such polymers which strengthens the interest in using them for energetic applications. The diversity of the described structures renders it possible to foresee a wide range of properties for these energetic materials. Consequently, a great opportunity is offered to select the appropriate material for a specific application in this field. Certain patents cited in this survey show that some of these polymers have already been exploited, and many other applied high-nitrogen polymers will surely see the light in the future.