## 2 Materials and Methods
### Materials
A three-necked flask equipped with a magnetic stirrer, a reflux condenser with calcium chloride tube, and under a positive flow of argon was successively loaded with a solution of oligodichlorophosphazene (4.37 g) in dioxane (20 mL) and a solution of 2-ethylhexanol (11.41 g) in dioxane (25 mL). The further process was carried out in a similar way as for TCDP. The product obtained was a clear oily yellowish liquid (8.04 g). **Determination of EIPAs and PAPNA extraction characteristics**. The REE ions concentration in the initial and equilibrium aqueous phases was determined by the titrimetric method. A 250 mL conical flask was successively loaded with an aliquot of REE solution, 1% aqueous solution of ascorbic acid (2-3 drops), acetate buffer solution (20 mL, pH = 5.2-5.4), 1% aqueous solution of xylenol orange (3-5 drops) and diluted with water to 100 mL. The prepared solution was titrated with ethylenediaminetetraacetic acid disodium salt dihydrate solution (0.05 mol/L) until the color changed from violet to yellow. Acetate buffer solution was prepared according to the following procedure: acetic acid (32 mL) was mixed with 25% ammonia solution (34 mL). Then it was diluted with distilled water to 1000 mL in a volumetric flask. To calculate the concentration of metal ions in the aqueous phase the following Equation (1) was used:  
\[\text{C}_{1}=\text{C}_{2}\text{:}\text{V}_{2}/\text{V}_{1}, \tag{1}\]  
where C\({}_{1}\) is the concentration of metal ions in the aqueous phase, mol/L; C\({}_{2}\) is the concentration of ethylenediaminetetraacetic acid disodium salt dihydrate, mol/L; V\({}_{1}\) is the volume of an aliquot of the aqueous phase, ml; V\({}_{2}\) is the volume of titrant spent on the experiment, ml. The error in determining the concentration of REE ions in aqueous nitric acid solutions by complexometric titration does not exceed 3%. The equilibrium concentration of REE ions in the organic phase was calculated from the difference in equilibrium concentrations (Equation (2)):  
\[\text{C}_{\text{org}}=\text{C}_{\text{initial}}-\text{C}_{\text{aq}}, \tag{2}\]  
where C\({}_{\text{org}}\) is the equilibrium concentration of REE in the organic phase, mol/L; C\({}_{\text{initial}}\) is an initial concentration of REE in the aqueous phase, mol/L; C\({}_{\text{aq}}\) is the equilibrium REE concentration in the aqueous phase, mol/L. Extraction efficiency can be estimated as C\({}_{\text{org}}\) to C\({}_{\text{initial}}\) ratio. Conclusions about the formed complexes can be made from the ratios of initial extractant concentration to C\({}_{\text{org}}\)[28]. The concentration of the extractant was determined by potentiometric titration with a solution of NaOH. A 100 mL glass beaker was successively loaded with an aliquot of the extractant and 50% aqueous solution of isopropanol (50 mL). Electrodes were placed in a prepared solution and titrated using a magnetic stirrer. Based on the obtained titrimetric data, a graph was plotted in E/V coordinates. The calculation of the concentration of extractant was performed according to Equation (3):  
\[\text{C}_{\text{ext}}=\text{V}_{\text{NaOH}}\text{-C}_{\text{NaOH}}/\text{ V}_{\text{aliq}}, \tag{3}\]  
where C\({}_{\text{ext}}\) is the concentration of the extractant in the initial mixture, mol/l; V\({}_{\text{NaOH}}\) is the equivalent volume of a titrant solution corresponding to a jump in the titration curve, ml; C\({}_{\text{NaOH}}\)--titrant concentration, mol/L; V\({}_{\text{aliq}}\) is the volume of an aliquot of the extractant, ml. **The procedure of REE extraction with EIPA-1. 2 g (3.3 mmol) of EIPA-1 was dissolved in 10.5 mL of RED-3 solvent so that a final concentration of H\({}^{+}\) ions = 0.3 mol/L. The system was stirred until complete dissolution of extractant. The ratio of aqueous and organic phases has always been 1:1. The extraction was carried out in disposable plastic tubes for 15 min on a shaker in a horizontal position. All REE ions were taken in 0.25-0.3 mol/L concentration and were extracted from 0.1M nitric acid aqueous solutions.