## 2 Results and Discussion  
A well-known method for preparing PNBI involves a high-temperature catalytic polyheterocondensation reaction in high-boiling phenolic solvents [15; 16] (Scheme 1). However, the lack of a developed monomer base holds back the progress in the synthesis and use of these representatives of PHA. In this work, we used 3,3\({}^{\prime}\),4\(A^{\prime}\)-tetraaminodiphenyl oxide [17] and 1,3-bis(3,4-diaminophenoxy)benzene [18] as tetraamines and dianhydride 1,4,5,8-naphthalenetetracarboxylic acid and 1,3-bis(1,8-dicarboxynaphthoyl-4)benzene dianhydride [19] as anhydride components. In earlier work [6], a polymer-analogous sulfonation reaction of preliminarily synthesized PNBIs with an oleum-sulfuric acid mixture was used for functional PNBIs isolation as shown in Scheme 2. Since the reaction of formation of the naphthoylenebenzimidazole ring proceeds in an acidic medium, it seemed appropriate to investigate a one-step method for the synthesis of functional SPNBI directly from the starting monomers in a sulfuric acid-oleum mixture, which plays the role of solvent, and sulfurizing as well as dehydrating agent (Scheme 3). This method allows to avoid environmentally hazardous chemicals such as phenol, cresol, p-chlorophenol, and amide solvents; lower the reaction temperature from 180 \({}^{\circ}\)C to 120 \({}^{\circ}\)C; eliminate the stages of precipitation and extraction of PNBI with large volumes of methanol; and finally to reduce energy costs. For the first time, we have developed a one-stage method for the synthesis of SPNBI in accordance with the following reaction scheme. Figure 1: General synthesis scheme of polynaphthoylenebenzimidazole. It is known that the parameters of the polycyclocondensation reaction significantly affect the process of polymer formation. Therefore, the effect of temperature, reaction time, and the ratio of the sulfurizing agents on the properties of the final polymer was studied. Under the most favorable conditions, a number of SPNBIs were synthesized. In the course of the reaction, we measured their characteristics such as sulfur content according to elemental analysis (S) and reduced viscosity (\(\eta_{\text{red}}\)), which are summarized in Table 1.  
The study of SPNBIs synthesized showed that their IR spectra exhibit absorption maxima at 1615 and 1547 cm\({}^{-1}\), which are characteristic for C = N absorption band and at 1700 cm\({}^{-1}\) assigned to C = O group of the benzimidazole cycle [20]. Additionally, a band at 1055 cm\({}^{-1}\) observed in the IR spectra is characteristic for the stretching vibrations of SO\({}_{3}\) groups in sulfonic acids. The synthesized SPNBIs were examined by solid-state \({}^{13}\)C NMR spectroscopy (Figure 1). The NMR spectra contain broad peaks in the range of 140-120 ppm, which correspond to the signals of carbon atoms of the benzimidazole ring, according to the data of [21]. For a more accurate determination of the structure of the synthesized polymers, a \({}^{13}\)C NMR spectrum of SPNBI-I,III in D\({}_{2}\)SO\({}_{4}\) was obtained (Figure 2). In the \({}^{13}\)C NMR spectrum of SPNBI-I,III signals of C = O and C = N groups (157.90 and 148.00, 147.6 ppm, respectively) are observed as well as signals assigned to carbon atoms of the benzimidazole ring at 136.59 and 127.88 ppm. The data obtained are in good agreement with the results of the work [21]. The structure of the monomers used affects the properties of the resulting functional polymers. SPNBIs based on dianhydride I are soluble in N-MP only when the sulfur content is more than 4.8 wt%. SPNBIs based on dianhydride II begin to dissolve in N-MP at a low (1.1 wt.%) sulfur content in their composition.