**1. Introduction**

The development of new smart and multifunctional materials is currently encouraged to be constrained toward the design of low cost and free-solvent synthetic processes, or at least green-solvents, and easy, fast, effective and robust productions. The MW-driven polymerization of NH4CN may be considered as a good example of this kind of processes for the generation of polymeric macrostructures with potentially versatile properties. One of the advantages of this polymerization technique is to reduce the reaction times with respect to the conventional heating processes. However, the kinetic variations during the syntheses due to the effect of the MW radiation must be taken in consideration, as they may change the final properties of the products. In the case of the HCN-derived polymers, it is well established that their properties are strongly conditioned by the synthetic conditions and for extension, their potential applications are directly dependent on their chemical composition and structure, thermal stability, particle shape and size, among others.

Recently, it has been shown that the particle morphology of the known HCN-derived polymers can be tuned by the choice of the synthetic conditions [1–3]. Some polymers of this heterogeneous family [4] have been proposed as emergent materials with different applications, such as photocatalysts [5], semiconductors, nanowires, ferroelectric materials [6], capacitors [1], coatings with potential biomedical applications [2,7–9], protective

**Citation:** Pérez-Fernández, C.; Valles, P.; González-Toril, E.; Mateo-Martí, E.; de la Fuente, J.L.; Ruiz-Bermejo, M. Tuning the Morphology in the Nanoscale of NH4CN Polymers Synthesized by Microwave Radiation: A Comparative Study. *Polymers* **2022**, *14*, 57. https://doi.org/10.3390/ polym14010057

Academic Editor: Edina Rusen

Received: 2 November 2021 Accepted: 20 December 2021 Published: 24 December 2021

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films against corrosion [10,11], or for the development of antimicrobial media for passive filtration [12].

Different HCN-derived polymers present some structural characteristics resembling those of the extensively researched carbon nitrides [1,13,14], which are used as carbon materials in multiple fields [15–18]. Moreover, it has been proposed that the incorporation of HCN-derived polymeric nanoparticles as novel fillers in different polymeric matrixes could lead to the creation of new composite materials [1]. However, only one work has described the production of nanoparticles derived from HCN [13]. In that case, the aqueous polymerization assisted by the MW radiation of cyanide lead to the simultaneous generation of nanoparticles with different sizes and shapes, together with isolated long nanofibers. In this context, it is important to note that HCN polymer nanofibers have attracted considerable attention due to their attractive features, e.g., their high visible-light photocatalytic activity. These materials were prepared from formamide, the hydrolysis product of HCN, through conventional thermal heating at 200 ◦C and a prolonged reaction time of 48 h [5]. Therefore, these polymer materials, at the nanoscale, present an extraordinary potential in the different areas mentioned previously.

Taking into account the high potential of HCN-derived polymers for the development of multifunctional materials, as well as the key role of their polymeric morphology for these potential applications, several aspects related with the MW-driven synthesis of NH4CN polymers are explored here in detail, paying special attention to the relationships between the experimental conditions and the features and properties of the final polymeric products. In this way, two series of NH4CN polymers synthetized using MW radiation were compared between themselves and also against a control polymer produced under conventional heating using equivalent reaction times or by fixing this time, in order to determinate the influence of the MW radiation on the features of the insoluble polymers obtained.

Moreover, statistical methods were used for a better interpretation of the results reported in the present work. The obtained data were comparatively discussed in order to obtain useful synthetic conditions for the production of cyanide polymers using MW radiation, taking into special consideration the structural characteristic of the polymers and their morphological and textural properties for the further development of multifunctional materials.
