*2.2. Characterization of the NH4CN Polymers*

The Fourier transform infrared (FTIR) spectra were recorded as was reported in [1], but using a Nicolet Is50-FTIR spectrometer (Thermo Scientific, Waltham, MA, USA). The powder X-ray difracction (XRD) was performed as in [1], but by scanning the samples from 10◦ to 50◦ (2θ) and with a count time of 3 s. All of the parameters and the equipment for the measurements of the elemental analysis, thermal analysis and 13C solid-state cross polarization/magic angle spinning nuclear magnetic resonance (CP/MAS) NMR were described in [1]. The X-ray Photoelectron Spectroscopy (XPS) analysis of the samples (pure pellets of the polymers) was carried out in an ultra-high vacuum chamber equipped with a hemispherical electron analyser, using an Al Kα X-ray source (1486.6 eV) with an aperture of 4 mm × 7 mm (Specs company, Berlin, Germany). The base pressure in the chamber was 5 × <sup>10</sup>−<sup>9</sup> mbar, and the experiments were performed at room temperature. The following core level peaks were recorded under the same experimental conditions: C (1 s), N (1 s) and O (1 s). The pass energy applied to take the overview sample was 30 eV, while 20 eV pass energy was applied for the fine analysis of the core level spectra. The surface

morphologies of the polymers were determined by a ThermoScientific Apreo C-LV field emission electron microscope (FE-SEM) equipped with an Aztec Oxford energy-dispersive X-ray microanalysis system (EDX). The samples were coated with 4 nm of chromium using a sputtering Leica EM ACE 600. The images were obtained at 10 kV. This SEM study was analogous to those detailed in [13]. The values of the Z-average and polydispersity index (PdI) were registered using a Zetasizer Nano instrument (Malvern Instruments Ltd., Almelo, Netherlands), using ethanol as a solvent to scatter the samples.

**Table 1.** Experimental conditions used for the synthesis of NH4CN polymers assisted by MW radiation, as well as some of their properties. Note that polymer **2** is present in both series. <sup>1</sup> Maximum pressure reached along the syntheses. \* No reproducible measurements.


**Figure 1.** (**a**) Microwave reactor using in the present work; (**b**) equimolar water solution of NaCN and NH4Cl; (**c**) raw reaction. A representative suspension of the cyanide polymers; (**d**) filtration sys-tem. In the bottom part, dark solution or sol fraction which will be freeze-dried. In the upper part, black insoluble solid or gel fraction; (**e**) characteristic cyanide polymer over a glass fiber filter after drying by reduced pressure.
