## 3 Results and Discussion  
One of the most recognized 2-ethylhexyl derivatives of imidophosphoric acid used as extractant is polyalkylphosphonitrile acid (PAPNA), but despite its widespread industrial use, generally in Eastern Europe countries, its structure has not been fully established and analytical data is almost absent. Scanty literary data about PAPNA chemical structure varies a lot (Figure 2) [33; 34; 41]. Due to these differences in proposed structures, the relationship between the chemical structure of EIPAs and their effectiveness in REE extractive separation remains unclear. Within our investigation, we carried out the syntheses of EIPAs in order to provide an exhaustive description of their molecular structures and determine their ability to complex formation with REE ions, as well as the possibilities for these ions separation. Next, starting materials were used for synthesis of the desired EIPAs: TCDP as a model compound and linear oligodichlorophosphatezene [Cl-(PCI2=N)n-PCI3]+[PCI6]-, where n = 4-7, as a substrate for the preparation of PAPNA analogue, with the same chain length according to the literature data [41]. The proposed reagents were considered to be prospective candidates for EIPAs preparation due to the possibility of well-known phosphazene-phosphazane rearrangement (PPR) in their units. The driving force of such an arrangement is the supposed high susceptibility of hydrogen or \(\alpha\)-carbon atoms in phosphate-like groups, which are attacked by the most basic nitrogen atom of the main chain [42]. The occurrence of PPR and molecular structures of resulting isomers was determined by NMR analysis. MALDI-TOF mass spectrometry was additionally used for confirmation. PPR is observed already at 25 \({}^{\circ}\)C for various phosphazenes with hydroxyl groups at the phosphorus atoms. For mono-substituted hydroxy or alkoxy derivatives of TCDP, PPR leads to a significant change in the character of the signals in their \({}^{31}\)P NMR spectra. Magnetically nonequivalent phosphorus atoms of hydroxychosphazo (alkoxyphosphazo) and phosphonyl groups, initially characterized by two doublet signals, become magnetically equivalent with only one singlet in spectra after the rearrangement due to the formation of the second phosphonyl group. In the products, containing 2-ethylhexyl substituents, the rearrangement proceeds very slowly at 25 \({}^{\circ}\)C. Moreover, as it can be seen from the \({}^{31}\)P NMR spectrum (Figure S1) of the product obtained via TCDP and an excess of 2-ethylhexanol reaction at room temperature, the substitution of chlorine atoms itself is insignificant. In accordance with NMR data the main product is a mono-substituted non-rearranged derivative of TCDP (**1**), characterized with two doublet signals: at \(-0.2\) ppm (phosphazenyl atom) and \(-11.2\) ppm (phosphonyl atom). The temperature increase leads to the formation of products with a higher degree of chlorine atoms substitution as well as to PPR. At elevated temperatures, PPR probability (70 \({}^{\circ}\)C and higher) approaches 100% even for the mono-substituted phosphazene and decreases slightly when all chlorine atoms in chlorophosphazene have been replaced by 2-ethylhexyl substituents. Thus, the resulting product of the reaction between  
Figure 2: Various polyalkylphosphonitrile acid chemical structures according to different literary sources, n = 2–4; here and below, R is a 2-ethylhexyl radical. 1 mol of TCDP and 0.75 mol of 2-ethylhexanol at 70 \({}^{\circ}\)C in dioxane is actually a mixture of the original TCDP and its mono-substituted derivative, tetrachlorodiphosphonyl-N-(2-ethylhexyl)-amine (**2**), which has a singlet signal at -5.2 ppm in the \({}^{31}\)P NMR spectrum (Figure S2) due to the magnetic identity of the phosphorus nuclei. The formation of **2**, in this case, occurs according to Scheme 3.