## 4 Conclusions  
EIPAs can be prepared by reacting linear phosphazenes with alcohols of various structures, in particular, 2-ethylhexanol. The number of phosphorus-containing units in the chain does not  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**M**} & \multicolumn{4}{c}{**Extraction by EIPA-1**} & \multicolumn{4}{c}{**Extraction by EIPA-2**} \\ \cline{2-7}  & \(\mathbf{C_{aq}}\) & \(\mathbf{C_{org}}\) & \multicolumn{2}{c}{**Ratio**} & \multirow{2}{*}{\(\mathbf{C_{aq}}\)} & \multirow{2}{*}{\(\mathbf{C_{org}}\)} & \multicolumn{2}{c}{**Ratio**} \\  & & & & & & \(\mathbf{C_{EIPA-1}}\): \(\mathbf{C_{org}}\) & & \\ \hline Y & 0.20 & 0.10 & 3.02 & 0.15 & 0.10 & 3.16 \\ La & 0.22 & 0.06 & 5.03 & 0.16 & 0.09 & 3.53 \\ Ce & 0.24 & 0.06 & 5.08 & 0.16 & 0.09 & 3.33 \\ Nd & 0.21 & 0.07 & 4.25 & 0.16 & 0.09 & 3.43 \\ Sm & 0.21 & 0.07 & 4.28 & 0.15 & 0.10 & 3.06 \\ Gd & 0.21 & 0.07 & 4.33 & 0.15 & 0.10 & 3.09 \\ Dy & 0.19 & 0.08 & 3.76 & 0.16 & 0.09 & 3.24 \\ Ho & 0.20 & 0.09 & 3.30 & 0.17 & 0.08 & 4.00 \\ Yb & 0.16 & 0.10 & 2.97 & 0.15 & 0.10 & 2.93 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Extraction properties of EIPAs. The ratio of extractant concentrations to REE in extracts depending on the atomic number of element.  
Figure 7: Proposed structures of EIPA lanthanide [Ln(A)\({}_{3}\)] (**a**) and [Ln(A)\({}_{3}\)] 3 HA (**b**) complexes.  
change during the substitution of chlorine atoms in linear oligophosphazenes with 2-ethylhexyl groups. Therefore, the chain length of EIPAs can be set using the starting chlorophosphazene with the selected chain length. The chain length of the initial linear chlorophosphazenes can be easily adjusted due to the living cationic polymerization of Cl\({}_{3}\)P=NSiMe\({}_{3}\).  
Analysis of the chemical structure of compounds presented in the samples of polyalkylphosphonitrilic acid (PAPNA), EIPA-1 (Scheme 6) and EIPA-2 (Scheme 7) shows that the selectivity of PAPNA towards REE ions primarily depends on the presence of functional dihydroxymosphonylamines with structures **5** and **6** (Table 2).  
A significant content of non-functional fragments -P(O)(OR)-NR- along with functional -P(O)(OR)-OH or -P(O)(OR)-NH- groups in EIPAs contributes to a high collective extraction ability to REE. These fragments are observed mainly in EIPA-2 but also present in the PAPNA.  
Oligomeric 2-ethylhexyl esters based on linear phosphazenes, considered to be PAPNA analogues, show a clearer ability to collective REE extraction. An increase in the chain length of the ester leads to a better extraction capacity towards light lanthanides, but a decrease in their selectivity.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/4/1229/s1](http://www.mdpi.com/2076-3417/10/4/1229/s1)]([http://www.mdpi.com/2076-3417/10/4/1229/s1](http://www.mdpi.com/2076-3417/10/4/1229/s1)). Figure S1: The \({}^{31}\)P NMR spectrum of the products prepared by the reaction of TCDP and 2-ethylhexanol (20% excess) at 22 \({}^{\circ}\)C in dioxane. Figure S2: The \({}^{31}\)P NMR spectrum of the products prepared by the reaction of TCDP (1 mol) and 2-ethylhexanol (0.75 mol) at 70 \({}^{\circ}\)C in dioxane.  
Conceptualization, N.S.B., M.V.G., V.V.K.; Experiment design, synthesis and data analysis, N.S.B., A.S.E., A.A.B., and M.V.G.; Extraction investigation, O.A.S., M.D.R.; Resources and Materials, M.V.G., A.A.B., O.A.S.; Writing-original draft preparation, N.S.B.; Writing-review and editing, M.V.G., V.V.K., O.A.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation in the framework of the Federal Target Program \"Research and Developments in Priority Directions of the Scientific and Technological Complex of Russia for 2014-2020\" (unique project identifier RFMEFI57417X0171).  
The authors declare no conflict of interest.