## 3 Results and Discussion
According to the results of MALDI-TOF mass spectrometry, EIPA-2 product represents a mixture of oligomeric imidophosphoric esters with 4, 5 and 6 phosphorus atoms in the main chain (Table 1). Structures in Table 1 are given in accordance with the NMR spectroscopy data. Fragments of the phosphazene form -P(OR)\({}_{2}\)=N- are not excluded. MALDI-TOF analysis of the industrial extractant PAPNA shows the presence of shorter-chain oligomers (Table 2). The extraction capacity of EIPA-1, EIPA-2, and PAPNA as a reference sample, was estimated by change in REE ions concentration in the aqueous phase during extraction. The decrease in the concentration of REE ions in the aqueous phase is the result of their transition to the organic phase (RED-3 diluent was used). REE were extracted from nitric acid aqueous solutions (C\({}_{\rm initial}\) = 0.25-0.3 mol/L, C\({}_{\rm HNO3}\) = 0.1 mol/L) using an organic solution of the corresponding extractant (C\({}_{\rm ext}\) = 0.3 mol/L). Figure 4: Tri- and tetra-substituted derivatives of TCDP (R = 2-ethylhexyl radical). Figure 5: MALDI-TOF spectrum of the product from the reaction of TCDP with an excess of 2-ethylhexanol. Figure 3: \({}^{31}\)P NMR spectra of EIPA-1 (**a**), EIPA-2 (**b**) and PAPNA (**c**). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number of Phosphorus** & **Chemical Structure** & **m/z (Ions)** & **Intensity** \\ \hline
4 & RO & OR & 1047 (K\({}^{*}\)) & 4182 \\  & RO & OR & OR & 1121 (H\({}^{+}\)) & 709 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 1143 (Na\({}^{*}\)) & 779 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 1159 (K\({}^{*}\)) & 8311 \\ \hline  & OR & OR & OR & 1233 (H\({}^{+}\)) & 8711 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 1271 (K\({}^{*}\)) & 1600 \\ \hline
5 & RO & OR & OR & 1335 (Na\({}^{+}\)) & 1375 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 1350 (K\({}^{*}\)) & 9332 \\ \hline  & OR & OR & OR & 1424 (H\({}^{+}\)) & 6764 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 1462 (K\({}^{*}\)) & 1665 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Proposed chemical structures of 2-ethylhexyl esters of oligoimidophosphoric acids in EIPA-2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number of Phosphorus** & **Chemical Structure** & **m/z (Ions)** & **Intensity** \\ \hline
2 & HO & OR & 513 (H+) & 19072 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)OH (**5**) & 536 (Na\({}^{*}\)) & 14952 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)N & 552 (K\({}^{*}\)) & 7122 \\ \hline
3 & HO & OR & OR & 593 (H\({}^{*}\)) & 2063 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)OH (**6**) & 615 (Na\({}^{*}\)) & 3077 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)N & 631 (K\({}^{*}\)) & 1647 \\ \hline  & OR & OR & OR & 705 (H\({}^{*}\)) & 6390 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)N & 727 (Na\({}^{*}\)) & 7078 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)N & 3415 \\ \hline  & OR & OR & 817 (H\({}^{+}\)) & 3982 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 840 (Na\({}^{*}\)) & 3360 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)N & 856 (K\({}^{*}\)) & 1623 \\ \hline  & OR & OR & & \\  & RO & OR & & \\  & RO & OR & & \\  & RO & P \(-\)N & P \(-\)N & P \(-\)OR & 969 (K\({}^{*}\)) & 1569 \\  & HO & P \(-\)N & P \(-\)N & P \(-\)OR & 1081 (H\({}^{*}\)) & 1981 \\  & RO & P \(-\)N & P \(-\)N & P \(-\)N & P \(-\)N \\  & HO & P \(-\)N & P \(-\)N & P \(-\)N & 1081 (H\({}^{*}\)) & 1981 \\  & HO & P \(-\)NThe extractant concentration in the organic phase corresponds to the concentration of H\({}^{+}\) ions, which can be replaced by REE ions. A volume ratio of organic and aqueous phases was 1:1. It is clearly shown (Figure 6) that EIPA-1 and EIPA-2 compounds are characterized by higher extraction ability and lower selectivity than PAPNA during REE extraction. To confirm the participation of P-OH and NH groups in the complex formation and determine the role of P=O groups, the extraction ability of model triphenoxyphosphazodiphenoxyphosphonyl (PhO)\({}_{3}\)P=N-(PhO)\({}_{2}\)P=O containing only one P=O group was studied.