Nitrogen-Rich Polymers as Candidates for Energetic Applications

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.


Tetrazole-based polymers 2.1. Polymerization of vinyltetrazoles
During the last decade, two major papers have reviewed the synthesis and polymerization of vinyltetrazoles, and together they present an extensive literature coverage [ 1 , 2 ].

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 C-vinyltetrazole (5-vinyltetrazole) 2. Masking of the double bond of acrylonitrile 1 through addition of dimethylamine [ 3 ] or hydrogen chloride [ 4 ] circumvents this problem and renders the process more efficient (Figure 1).

Synthesis of N-vinyltetrazoles
Two main strategies have been used for the synthesis of N-vinyltetrazoles.The first one involves alkylation of the tetrazole N-H with a functionalized ethyl chain, followed by elimination.The second one is the catalyzed direct vinylation with vinylacetate or butylvinylether.Recently, a variant of the first approach, using dibromoethane as the alkylating agent, has been developed [ 5 ].This method appears now as the method of choice, since it is a one-step, high-yielding procedure that only needs readily available, non-toxic chemicals.In the majority of cases, a mixture of 1-and 2-vinyltetrazole are produced and selectivity depends on the 5-substituent (Figure 2).Nitrogen-Rich Polymers as Candidates for Energetic Applications 315

Polymerization of C-vinyltetrazoles
5-Vinyltetrazoles are readily involved in free-radical polymerization (Figure 4).A comparison between C-and N-vinyltetrazoles shows that the former are more active in such processes.A number of reaction media have been used for the polymerization of 5vinyltetrazoles, and it was shown that the activity was enhanced in more polar solvents, especially in water.Molecular weights also increased in aqueous systems.This trend prompted a polymerization study in ionic solvents.Of the various ionic liquids (ILs) tested, only 1-methyl-3-propylimidazolium bromide gave rise to decent yields.Other ILs gave low yields or even decomposition of the tetrazole ring [ 7 ].Poly-5-vinyltetrazole 6 contains 58.3% nitrogen, which clearly makes it a high-nitrogen compound.However, it seems that its N-vinyl congener has been favored for industrial applications.
New Polymers for Special Applications 316

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-1vinyltetrazole 8 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 N,Ndimethylformamide (DMF) than in water.Recently, emulsion polymerization gave porous monoliths of the corresponding so-called 'polyHIPEs' of 8, exhibiting remarkable mechanical properties [ 11 ].
The synthesis and polymerization of 5-(methyl)hydrazino-1-vinyltetrazole 9 have been described.It was necessary to add the initiator (AIBN) in several portions to enhance the conversion of the starting vinyl compound.However, while 9b (R = Me) yielded 85% of the corresponding polymer 10b, the yield of 10a from 9a (R = H) was limited to about 25-30% and the degree of polymerization was very low.The synthesis of the very nitrogen-rich azide-functionalized polymer 11 was attempted through diazotization of 10a.However, elemental analyses were unsatisfactory because of partial decomposition of the tetrazolylhydrazine moieties [ 12 ].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.

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 [ 13 ].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 [ 14 ].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 [ 15 ].A related patent subsequently extended this water-based synthesis [ 16 ].At 115°C tetrazolation was limited to 70%, but reached 95% at 170°C.Further reaction of the resulting poly(5vinyltetrazole) with ammonia yielded the corresponding ammonium salt 12 (Figure 7).The latter theoretically contains 61.9% of nitrogen and is therefore a promising material for gas generants.A related triaminoguanidinium salt 13 had been disclosed as early as in 1968 [ 17 ].Taking into account the 6% of PAN units remaining in the starting poly-5-vinyltetrazole, a theoretical value of 69.2% of nitrogen was achieved in the salt (Figure 7).All these conditions surpass earlier methods that yielded lower levels of tetrazole incorporation (see for example ref [ 18 ]).
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 [ 1 ].

Synthesis of tetrazole-based polymers via polycondensation
5-Chloromethyltetrazole 14 is an interesting substrate as it has both a nucleophilic (tetrazole N-H) and a highly electrophilic (CH2-Cl) center.In the presence of triethylamine as a base, the tetrazole ring can be deprotonated which gives rise to poly(methylenetetrazole) 15 in good yields [ 19 , 20 ] (Figure 9).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 5chloroethyltetrazole can also participate in a similar process.However, polymerization is slower and yields do not exceed 20%.
The high reactivity of 5-chloromethyltetrazole 14 has been used to graft poly (5vinyltetrazole).Upon addition of an excess of 14 and triethylamine, a branched poly(5vinyltetrazole) bearing poly(methylenetetrazole) bridges was obtained in yields around 70%.Ten equivalents of 14 produced the best results.Characterization of the polymer revealed a 6/1 ratio of methylenetetrazole vs vinyltetrazole moieties.The number of vinyltetrazole units between branching junctions was around 2. Its energetic prospects were highlighted by a very elevated density of 1.70 and a nitrogen content of 67.9% [ 19,20 ]  (Figure 9).
A patent describes the polycondensation of various dinitriles with diazides to produce tetrazole containing polycondensates [ 21 ].Of these, the most valuable polymer for energetic applications is the one obtained from dicyanofuroxane 15.Indeed, despite the fact that the hexamethylene spacer lowers the N content, a high energetic output is expected from the presence of 2 tetrazole and 1 furoxane rings in the unit of polymer 16 (Figure 10).Isopropyl 2-(5-(1-methylhydrazinyl)tetrazol-1-yl)acetate 17 was polymerized at high temperature to give the corresponding polycondensate 18 [ 22 ].The yield was higher when working at 120°C rather than 140°C, however elemental analyses still showed a lower nitrogen content than expected (44.9 instead of 54.5%).The presence of low molecular weight polymers still bearing isopropoxy groups may account for this discrepancy.
The related monomer N-[1-(2-hydroxyethyl)tetrazol-5-yl]-N-methylhydrazine 19 was also involved in polycondensation processes.With succinyl chloride as the partner, only low molecular weights could be achieved, and results from elemental analysis were unsatisfactory.This was ascribed to the protonation of the hydrazino moieties during polycondensation.More positive results were obtained with hexamethylenediisocyanate but obviously the resulting polycondensate 20b is less attractive for energetic applications due to its lower nitrogen content [ 23 ].Another similar polymer was recently described by the same group [ 24 ] (Figure 10).

Polyvinyltetrazines
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 21 has been described (Figure 11).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 21 was obtained as a volatile, pink oil that solidified in the freezer.Preliminary studies showed that 21 exhibited a limited stability at high temperatures and in the presence of radicals, which obviously precludes its use in polymer chemistry.However, 3,6-divinyl-1,2,4,5tetrazine can be seen as a tetraaza analogue of the widely used 1,4-divinylbenzene and is therefore believed to be a useful building block in the synthesis of new tetrazine-based molecules and materials [ 25 ].Further work is needed to establish the properties and reactivity of 21.These studies also open the way towards the synthesis of unsymmetrical vinyl-1,2,4,5-tetrazine compounds that may exhibit improved stability and reactivity in polymerization conditions.

Synthesis of tetrazine-based polymers via polycondensation
One of the most convenient ways to construct the 1,2,4,5-tetrazine (s-tetrazine) ring is by condensation of hydrazine with a diimidate to produce the corresponding 1,2-dihydro-1,2,4,5-tetrazine, followed by oxidation.This strategy has been extended to the synthesis of diverse poly(phenylene-s-tetrazinylene)s by using diimidates.In order to increase the energetic prospects of such polymers, azahetaryl precursors were considered.However, the reaction of hydrazine monohydrate with pyridine-2,6-diimidate 22 did not yield the expected dihydrotetrazine polymer.Instead, the corresponding diamidrazone 23 was isolated in high yield.It is likely that the insolubility of 23 in THF is responsible for the uneffective polycondensation reaction.Diamidrazone 23 was used as the monomer in a microwave-assisted polycondensation to give the poly(2,6-pyridinediyl-dihydro-stetrazinylene) 24 which was further oxidized to the desired tetrazine polymer 25 using sodium nitrite/aq.AcOH [ 26 ] (Figure 12).Following the same strategy, the previously unknown 1,3,5-triazine-2,4,6-triamidrazone 26 was subjected to polycondensation to yield the hyperbranched polymer 27.In this case, microwave activation was not necessary.Polymer 27 stands out as the most nitrogenated tetrazine-containing polymer, even if the measured N content was lower than expected, presumably due to water incorporation.A density of 1.56 was determined for 27, which is significantly higher than the value reported for amorphous poly(p-phenylene) (1.11).This clearly shows how the introduction of nitrogen heterocycles in polyarylene structures is valuable in the field of energetic materials as it increases this crucial parameter.Unfortunately 27 was found unreactive to common oxidants, presumably because of its high insolubility.Only very strong oxidative conditions resulted in the formation of s-tetrazine rings, but oxidation remained uncomplete (Figure 12).
The structures of all polymers were confirmed using 13 C and 15 N 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).1-Vinyl-1,2,4-triazole 29 was readily polymerized in free-radical conditions, and the process was more efficient in the presence of water.However, it was less active than 1-vinyl-1,2,3triazole.This trend in activity was found to correlate with the corresponding Hammet constants [ 29 ].Poly(1-vinyl-1,2,4-triazole) 30, obtained in sufficiently aqueous media, was essentially insoluble in water or organic solvents.This polymer is a well-established compound that may be interesting in numerous applications.Recently, open-cell porous monoliths of poly(1-vinyl-1,2,4-triazole) cross-linked with a low amount of N,Nmethylenebisacrylamide were prepared (Figure 14).They exhibited interesting mechanical properties due to the presence of water in the polymer structure, as evidenced by NMR [ 32 ].For the synthesis of 3-amino-1-vinyl-1,2,4-triazole 31, the classical transvinylation using vinyl acetate in the presence of cuprous chloride was ineffective.The product was obtained through the reaction of acetylene in a basic medium at high temperature [ 33 ] (Figure 13).The presence of the amino group resulted in some changes compared to the unfunctionalized derivative: the polymerization activity was three times lower, and poly(3-amino-1-vinyl-1,2,4-triazole) 32 was soluble in water [ 34 ].The nitrogen content was also significantly improved to 50.9%, with a density of 1.30-1.36equalling that of polyglycidyl azide.

Synthesis of 1,2,4-triazole-based polymers via polycondensation
Among the few 1,2,4-triazole-based condensation polymers known [ 40 ], the most promising as energetic materials are poly(4-amino-1,2,4-triazole)s [ 41 ].The latter can be commonly obtained through isomerization of poly(dihydrotetrazine)s 45, following the general scheme depicted in Figure 18.This process is closely related to the one mentioned in the tetrazine section of this chapter.Polymer 46-C2 stands out as the most nitrogenated member of this family with a calculated N content of 50.9%.Even though the first synthesis of 46-C2 dates back to 1954 [ 42 ], interest for this polymer or similar derivatives has been maintained for a long time for diverse applications [ 43 , 44 ].
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-4amino-1,2,4-triazole 47 and diaminoguanidinium azide 48 with formaldehyde/glyoxal (4/1) [ 45 ].The exact structure of the resulting polymer 49 is not disclosed but has been claimed to provide a N/C ratio of 2.3 (Figure 19).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.
Copolymer 61a exhibited a measured N content of 47.2%.
Similarly, the salt 59-Na was used to quantitatively substitute the chlorine atoms on polyepichlorhydrin 42.As in the nitro-1,2,4-triazole series (see above), a mixture of isomers was obtained, but the selectivity was reversed: the N-2 isomer was the major one (60%) [ 56 ]  (Figure 22).

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 [ 57 ].The most interesting compound for energetic applications was polymer 64, obtained from 3-azido-1-butyne 63 (Figure 23).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 Nitrogen-Rich Polymers as Candidates for Energetic Applications 329 and formation of by-products [ 58 ].The hyperbranched product 65 offers an original combination of both triazine and triazole rings, although the nitrogen content was obviously lowered by the remaining propargyl ethers moieties .

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 [ 59 ], 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 (s-triazines), which have been studied for a long time.
Ref [ 60 ] is a comprehensive review of earlier work.
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 flameretardant compositions [ 68 ] (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 79 with formaldehyde followed by Nitrogen-Rich Polymers as Candidates for Energetic Applications 331 dehydrochlorination has been claimed to give a structure in which triazine rings are linked together through carbodiimide or dihydrocarbodiimide units [ 69 ] (Figure 27).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 80 was reacted with trichlorotriazine 81 (cyanuric chloride) in the presence of a base to produce a cross-linked polymer 82 with triazine units and nitro groups (Figure 28).The interfacial conditions readily employed for such polycondensations were naturally adapted for the synthesis of porous polyHIPEs of this material [ 70  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 83 in which triazine rings were connected by ethynylene moieties [ 72 ] (Figure 28).
Another well-known way to obtain triazine compounds is the cyclotrimerization of nitriles.However, only scarce examples deal with azaheterocyclic nitriles.2,6-dicyanopyridine 84 polycondensates was remarkably high, up to 77% [ 45 , 76 ].A related copolymer incorporating 4-amino-1,2,4-triazole units has already been described in the triazole section.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 [ 77 ] or photochemical [ 78 ] 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.

Conclusions
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.