Influence of Salt on the Self-Organization in Solutions of Star-Shaped Poly-2-alkyl-2-oxazoline and Poly-2-alkyl-2-oxazine on Heating

The water–salt solutions of star-shaped six-arm poly-2-alkyl-2-oxazines and poly-2-alkyl-2-oxazolines were studied by light scattering and turbidimetry. The core was hexaaza[26]orthoparacyclophane and the arms were poly-2-ethyl-2-oxazine, poly-2-isopropyl-2-oxazine, poly-2-ethyl-2-oxazoline, and poly-2-isopropyl-2-oxazoline. NaCl and N-methylpyridinium p-toluenesulfonate were used as salts. Their concentration varied from 0–0.154 M. On heating, a phase transition was observed in all studied solutions. It was found that the effect of salt on the thermosensitivity of the investigated stars depends on the structure of the salt and polymer and on the salt content in the solution. The phase separation temperature decreased with an increase in the hydrophobicity of the polymers, which is caused by both a growth of the side radical size and an elongation of the monomer unit. For NaCl solutions, the phase separation temperature monotonically decreased with growth of salt concentration. In solutions with methylpyridinium p-toluenesulfonate, the dependence of the phase separation temperature on the salt concentration was non-monotonic with minimum at salt concentration corresponding to one salt molecule per one arm of a polymer star. Poly-2-alkyl-2-oxazine and poly-2-alkyl-2-oxazoline stars with a hexaaza[26]orthoparacyclophane core are more sensitive to the presence of salt in solution than the similar stars with a calix[n]arene branching center.


Introduction
The key property of "smart polymers", which determines a wide range of their practical use, is a nonlinear response to an external signal. In the case of thermoresponsive polymers, the phase transition in aqueous solutions is induced by temperature change. Accordingly, the temperature variation is a simple way to control the behavior of their solutions [1][2][3][4][5][6]. Thermoresponsive polymers are highly appealing for medical applications and biotechnology if the phase separation temperature is close to body temperature [7][8][9][10][11][12][13][14]. Polymers used in biomedical applications must be non-toxic, biocompatible, stable in biological media, biodegradable, and/or completely excreted from the body. These requirements are satisfied by poly-2-alkyl-2-oxazolines (PAlOx) and poly-2-alkyl-2-oxazines (PAlOz), many of which exhibit a thermosensitivity with a lower critical solution temperature.

Solution Investigation
The behavior of CPh6-PAlOz and CPh6-PAlOx in water-salt solutions was studied at polymer concentration c = 0.0050 g⋅cm −3 . For NaCl solutions, the salt concentrations csal were selected as one NaCl formula unit per one macromolecule, per one arm of the polymer star and per one monomer unit. Besides, physiological saline (0.154 M) and pure aqueous solutions were investigated. In the case N-PTS solutions, the csalt values were chosen in a similar way: one N-PTS molecule per one macromolecule, per one arm of the polymer star, and per one monomer unit. To expand the range of N-PTS content, solutions at csalt ≈ 0.10 and 0.15 M were prepared and studied. Thus, for both water-salt solvents the salt concentration varied from 0-1.54 M. . Figure 2. Structure of N-methylpyridinium p-toluenesulfonate.

Solution Investigation
The behavior of CPh6-PAlOz and CPh6-PAlOx in water-salt solutions was studied at polymer concentration c = 0.0050 g⋅cm −3 . For NaCl solutions, the salt concentrations csalt were selected as one NaCl formula unit per one macromolecule, per one arm of the polymer star and per one monomer unit. Besides, physiological saline (0.154 M) and pure aqueous solutions were investigated. In the case N-PTS solutions, the csalt values were chosen in a similar way: one N-PTS molecule per one macromolecule, per one arm of the polymer star, and per one monomer unit. To expand the range of N-PTS content, solutions at csalt ≈ 0.10 and 0.15 M were prepared and studied. Thus, for both water-salt solvents, the salt concentration varied from 0-1.54 M.

Solution Investigation
The behavior of CPh6-PAlOz and CPh6-PAlOx in water-salt solutions was studied at polymer concentration c = 0.0050 g·cm −3 . For NaCl solutions, the salt concentrations c salt were selected as one NaCl formula unit per one macromolecule, per one arm of the polymer star and per one monomer unit. Besides, physiological saline (0.154 M) and pure aqueous solutions were investigated. In the case N-PTS solutions, the c salt values were chosen in a similar way: one N-PTS molecule per one macromolecule, per one arm of the polymer star, and per one monomer unit. To expand the range of N-PTS content, solutions at c salt ≈ 0.10 and 0.15 M were prepared and studied. Thus, for both water-salt solvents, the salt concentration varied from 0-1.54 M.
The solutions and solvent were filtered into cells previously dedusted by benzene. Chromafil Xtra filters (Macherey-Nagel, Dueren, Germany) with a PTFE membrane with the pore size of 0.45 µm were used.
The self-organization in water-salt solutions of CPh6-PAlOz and CPh6-PAlOx was studied by light scattering and turbidimetry on a PhotoCor Complex setup (Photocor In- struments Inc., Moscow, Russia) with a sensor for measuring optical transmission. The light source was the Photocor-DL diode laser with wavelength λ = 659.1 nm and controllable power up to 30 mW. The correlation function of the scattered light intensity was obtained using the Photocor-PC2 correlator with 288 channels and processed using the DynalS software. The solution temperature T was changed discretely within the interval from 9-80 • C, with the steps ranging from 0.5 (near phase separation) to 6 • C (low temperatures). The temperature was regulated with the precision of 0.1 • C. The heating rate was 1.5 • C·min −1 .
The measurement procedure has been described in detail previously [55]. After the given temperature was achieved, all solution characteristics (light scattering intensity I, optical transmittance I*, and hydrodynamic radii R h of the scattering particles) began to change with time t. If intensity I changed at a high rate, the dependence of I (at the scattering angle 90 • ) and I* on time was measured only. The analysis of these dependencies makes it possible to determine the time of establishment of the "equilibrium" state of the system, in which I, I* and R h cease to change in time at given temperature. The hydrodynamic radii Rh of dissolved particles were determined when the intensity changed very weakly or independent of t. It should be noted that the values of R h can be obtained correctly if the light scattering intensity differs no more than 1% from its average value. Figure 3 as an example demonstrates the dependences of relative intensity I/I max of scattered light on the hydrodynamic radius R h of scattering species for CPh6-PEtOz water-salt solutions (I max is the maximum value of light scattering intensity I for a given solution). It is necessary to emphasize that the experiment time was equal to 1800 s at least at each temperature even if the measured characteristics did not depend on time. In "equilibrium" conditions, the angle dependences of I and R h were also analyzed within intervals from 45-135 • in order to justify the diffusion process ( Figure 4).
The solutions and solvent were filtered into cells previously dedusted by ben Chromafil Xtra filters (Macherey-Nagel, Dueren, Germany) with a PTFE membrane the pore size of 0.45 μm were used.
The self-organization in water-salt solutions of CPh6-PAlOz and CPh6-PAlOx studied by light scattering and turbidimetry on a PhotoCor Complex setup (Pho Instruments Inc., Moscow, Russia) with a sensor for measuring optical transmission light source was the Photocor-DL diode laser with wavelength λ = 659.1 nm controllable power up to 30 mW. The correlation function of the scattered light inte was obtained using the Photocor-PC2 correlator with 288 channels and processed the DynalS software. The solution temperature T was changed discretely withi interval from 9-80 °C, with the steps ranging from 0.5 (near phase separation) to 6 °C temperatures). The temperature was regulated with the precision of 0.1 °C. The he rate was 1.5 °C·min −1 .
The measurement procedure has been described in detail previously [55]. Afte given temperature was achieved, all solution characteristics (light scattering intens optical transmittance I*, and hydrodynamic radii Rh of the scattering particles) beg change with time t. If intensity I changed at a high rate, the dependence of I (a scattering angle 90°) and I* on time was measured only. The analysis of dependencies makes it possible to determine the time of establishment of "equilibrium" state of the system, in which I, I* and Rh cease to change in time at g temperature. The hydrodynamic radii Rh of dissolved particles were determined whe intensity changed very weakly or independent of t. It should be noted that the valu Rh can be obtained correctly if the light scattering intensity differs no more than 1% its average value. Figure 3 as an example demonstrates the dependences of re intensity I/Imax of scattered light on the hydrodynamic radius Rh of scattering specie CPh6-PEtOz water-salt solutions (Imax is the maximum value of light scattering inte I for a given solution). It is necessary to emphasize that the experiment time was equ 1800 s at least at each temperature even if the measured characteristics did not depen time. In "equilibrium" conditions, the angle dependences of I and Rh were also ana within intervals from 45-135° in order to justify the diffusion process ( Figure 4).

Behavior of Star-Shaped Six-Arm Pseudo-Polypeptoids in Water-Salt Solutions at Low Temperatures
The behavior of water-salt solutions of the studied polymer stars depends on the

Behavior of Star-Shaped Six-Arm Pseudo-Polypeptoids in Water-Salt Solutions at Low Temperatures
The behavior of water-salt solutions of the studied polymer stars depends on the chemical structure of both the arms and the salts. In CPh6-PAlOx solutions, the addition of NaCl and N-PTS does not change the set of scattering objects. Figure 5 shows the dependences of the hydrodynamic radii R h of the particles present in the solutions on the salt concentration c salt for CPh6-PAlOx. For all values of c salt , two types of particles with radii R m (small particles) and R s (large particles) were found in CPh6-PAlOx solutions. For both salts, R m did not depend on the salt content. In the case of CPh6-PEtOx, the average values <R m > = (7.4 ± 0.4) nm for NaCl solutions and (7.0 ± 0.4) nm for N-PTS solutions are approximately 2.5× larger than the hydrodynamic radius R h-D = 3.0 nm of CPh6-PEtOx molecules [59]. For the more hydrophobic CPh6-PiPrOx, the hydrodynamic radius <R m > is about 18 nm in both solvents, while the macromolecule radius R h-D was 2.6 nm [59]. Consequently, just as in pure water in water-salt solutions of CPh6-PAlOx, the species with radius R m are small aggregates, the reason for the formation of which is the interaction of hydrophobic CPh6 cores. These so-called micelle-like structures [75][76][77][78] were often observed in solutions of star-shaped PAlOx [79,80]. Taking into account that the form of star-shaped macromolecules with short arms and micelle-like structures [81] is close to spherical, the aggregation degree m agg can be estimated by comparing the volumes of macromolecules and aggregates using the formula: 3 (1) In the case of CPh6-PAlOz, the addition of salts leads to a change in the set of scattering objects ( Figure 6). At low temperatures, two types of species were also observed in aqueous solutions of these star-shaped polymers. However, unlike CPh6-PAlOx, the smaller particles were macromolecules. Indeed, the hydrodynamic radius Rf of these objects coincided within the experimental error with the radius Rh-D of macromolecules [59]. In NaCl solutions of CPh6-PiPrOz, macromolecules and large aggregates were present in the studied range of the salt concentration csalt. The Rf value did not depend on the NaCl content, while the Rs radius increased with the growth of NaCl concentration. A completely different behavior was observed for CPh6-PiPrOz solutions in the presence of N-PTS. Macromolecules were detected only at low N-PTS content. At csalt = 0.039 M, particles with a hydrodynamic radius Rm appeared in solutions, and at csalt > 0.01 M, species with radius Rf were not observed by dynamic light scattering ( Figure 6). The Rm values do not depend on the salt concentration. The average value <Rm> = (6.1 ± 0.4) nm, and therefore, taking into account that Rh-D = 3.0 nm [59], the aggregation degree is magg ≈ 8. This is half the magg value for N-PTS solutions of CPh6-PEtOx. At a low N-PTS content, the hydrodynamic size of large aggregates is close to 70 nm, and at csalt > 0.05 M, the Rs In the case of CPh6-PAlOz, the addition of salts leads to a change in the set of scattering objects (Figure 6). At low temperatures, two types of species were also observed in aqueous solutions of these star-shaped polymers. However, unlike CPh6-PAlOx, the smaller particles were macromolecules. Indeed, the hydrodynamic radius Rf of these objects coincided within the experimental error with the radius Rh-D of macromolecules [59]. In NaCl solutions of CPh6-PiPrOz, macromolecules and large aggregates were present in the studied range of the salt concentration csalt. The Rf value did not depend on the NaCl content, while the Rs radius increased with the growth of NaCl concentration. A completely different behavior was observed for CPh6-PiPrOz solutions in the presence of N-PTS. Macromolecules were detected only at low N-PTS content. At csalt = 0.039 M, particles with a hydrodynamic radius Rm appeared in solutions, and at csalt > 0.01 M, species with radius Rf were not observed by dynamic light scattering ( Figure 6). The Rm values do not depend on the salt concentration. The average value <Rm> = (6.1 ± 0.4) nm, and therefore, taking into account that Rh-D = 3.0 nm [59], the aggregation degree is magg ≈ 8. This is half the magg value for N-PTS solutions of CPh6-PEtOx. At a low N-PTS content, the hydrodynamic size of large aggregates is close to 70 nm, and at csalt > 0.05 M, the Rs value increased, reaching 110 nm. Thus, at csalt > 0.07 M in N-PTS solutions of CPh6-PiPrOz, micelle-like structures and large aggregates existed, which coincides with the set of scattering objects in water-salt solutions of CPh6-PAlOx.
.  Using Equation (1), it is also assumed that the densities of macromolecules and micellelike structures are the same. For CPh6-PEtOx, the aggregation degree is low (m agg ≈ 15), whereas for the more hydrophobic CPh6-PiPrOx, the m agg value approaches 300.
As for large scattering objects with a hydrodynamic radius R s , these are large loose aggregates. The addition of NaCl and N-PTS to the solution has a different effect on the size of these aggregates. The value R s is independent of the concentration of N-PTS and increases with growth of NaCl content in solution ( Figure 5).
In the case of CPh6-PAlOz, the addition of salts leads to a change in the set of scattering objects (Figure 6). At low temperatures, two types of species were also observed in aqueous solutions of these star-shaped polymers. However, unlike CPh6-PAlOx, the smaller particles were macromolecules. Indeed, the hydrodynamic radius R f of these objects coincided within the experimental error with the radius R h-D of macromolecules [59]. In NaCl solutions of CPh6-PiPrOz, macromolecules and large aggregates were present in the studied range of the salt concentration c salt . The R f value did not depend on the NaCl content, while the R s radius increased with the growth of NaCl concentration. A completely different behavior was observed for CPh6-PiPrOz solutions in the presence of N-PTS. Macromolecules were detected only at low N-PTS content. At c salt = 0.039 M, particles with a hydrodynamic radius R m appeared in solutions, and at c salt > 0.01 M, species with radius R f were not observed by dynamic light scattering ( Figure 6). The R m values do not depend on the salt concentration. The average value <R m > = (6.1 ± 0.4) nm, and therefore, taking into account that R h-D = 3.0 nm [59], the aggregation degree is m agg ≈ 8. This is half the m agg value for N-PTS solutions of CPh6-PEtOx. At a low N-PTS content, the hydrodynamic size of large aggregates is close to 70 nm, and at c salt > 0.05 M, the R s value increased, reaching 110 nm. Thus, at c salt > 0.07 M in N-PTS solutions of CPh6-PiPrOz, micelle-like structures and large aggregates existed, which coincides with the set of scattering objects in water-salt solutions of CPh6-PAlOx.
In both water-salt solutions, the behavior of CPh6-PEtOz was similar to that observed for N-PTS solutions of CPh6-PiPrOz, namely, at a certain concentration c salt , micelle-like aggregates were formed in the solutions. Their hydrodynamic radius R m did not change with c salt . The average values <R m > were (6.6 ± 0.4) nm and (6.9 ± 0.4) nm for solutions with NaCl and N-PTS, respectively. Small radii of micelle-like aggregates indicate that they contain a small number of macromolecules, and the estimation of the aggregation degree according to Equation (1) leads to a value of m agg ≈ 7. Thus, the size of micelle-like aggregates formed in water-salt solutions of CPh6-PAlOz is smaller than the corresponding characteristics for CPh6-PAlOx. This can be explained by the fact that the arms of the CPh6-PAlOz molecules are longer [59] and better screen the nucleus. As regards the size of large aggregates, for CPh6-PEtOz, the R s value does not depend on the N-PTS content and increases with the NaCl concentration ( Figure 6).
Concluding the discussion of the behavior of water-salt solutions of CPh6-PAlOz at low temperatures, the following fact should be noted. The appearance of micelle-like aggregates in all cases occurs at a concentration c salt , which approximately corresponds to one salt molecule per one arm of a polymer star.

Temperature Dependences of Characteristics of Star-Shaped Pseudo-Dendrimers Water-Salt Solutions
All the results discussed below were obtained for a state of the investigated solutions when their characteristics (light scattering intensity I, optical transmission I*, hydrodynamic radii of scattering species R h , etc.) do not change with time. For the systems under study, the establishment time t eq of such "equilibrium" state after a discrete change in temperature is rather long. This is illustrated in Figure 7, which shows typical dependences of I and I* on time t. The moment when the solution temperature reached a given value was taken as t = 0. For each solution, the t eq value depended on temperature. As well as for other thermoresponsive polymers [82], at each c salt value, the establishment time t eq had a maximum value t eq (max) near the onset of phase separation. For the studied polymer stars, no systematic change in t eq (max) was found with a change in the salt content. Average values <t eq (max) > of maximum value of establishment time for each polymer are given in Table 1. They are noticeably less than <t eq (max) > for star-shaped eight-and four-arm PAlOx with a calix[n]arene core [82]. This is probably the effect of the structure of the branching center on the rate of self-organization of star-shaped polymers. The data in Table 1 show that the maximum establishment time for stars with side isopropyl groups is greater than <t eq (max) > for polymers with ethyl groups. Thus, an increase in the volume of the side radical slows down the aggregation processes in water-salt solutions of the star-shaped CPh6-PAlOz and CPh6-PAlOx. Note that earlier a decrease in the <t eq (max) > value with the passage from PiPrOx to PEtOx was found for PAlOx stars with a calix[n]arene core [81]. On the other hand, the influence of the salt structure on the times of establishing the equilibrium state has not been revealed. Indeed, for all studied polymers, the values of <t eq (max) > in solutions with NaCl and N-PTS coincide within the experimental error.   (Figure 8). The temperature of its onset T1 was determined as the beginning of the sharp decrease in optical transmittance I* and a rapid increase in the light scattering intensity I. At the temperature T2, which reflects the finishing of phase separation, the optical transmission becomes zero. At this temperature, for most of the studied solutions, the intensity I reaches maximum value. Note, that for the CPh6-PEtOx and CPh6-PEtOz solutions at low salt concentration (csalt < 0.005 M), the temperature T2 could not be determined because it was too high (>85 °C).  As seen in Figure 8, with heating, the light scattering intensity began to change at relatively low temperatures. For example, for CPh6-PEtOx in NaCl solutions, a reliably measurable increase in I was observed at 45 °C. At this temperature, the I value exceeds the value of light scattering intensity I at 21 °C (I21) by 10%, i.e., I/I21 = 1.1. A further increase in T leads to a slow increase in the I value up to a temperature of onset of phase separation magnitude. The dependence of I on T is caused by the increase in the size Rs of large aggregates on heating, while the values of Rf and Rm do not change with temperature. (Figure 9). The change in Rs is not high, but it is detected rather reliably. Therefore, at T < T1, the dominant process in the solutions of the studied star-shaped polymers was aggregation as a result of an increase in the dehydration degree of arms with temperature and the formation of intermolecular hydrogen bonds.   On heating, a structural-phase transition was observed in solutions of star-shaped CPh6-PAlOz and CPh6-PAlOx. Phase separation temperatures were measured by turbidimetry and light scattering methods (Figure 8). The temperature of its onset T 1 was determined as the beginning of the sharp decrease in optical transmittance I* and a rapid increase in the light scattering intensity I. At the temperature T 2 , which reflects the finishing of phase separation, the optical transmission becomes zero. At this temperature, for most of the studied solutions, the intensity I reaches maximum value. Note, that for the CPh6-PEtOx and CPh6-PEtOz solutions at low salt concentration (c salt < 0.005 M), the temperature T 2 could not be determined because it was too high (>85 • C).
As seen in Figure 8, with heating, the light scattering intensity began to change at relatively low temperatures. For example, for CPh6-PEtOx in NaCl solutions, a reliably measurable increase in I was observed at 45 • C. At this temperature, the I value exceeds the value of light scattering intensity I at 21 • C (I 21 ) by 10%, i.e., I/I 21 = 1.1. A further increase in T leads to a slow increase in the I value up to a temperature of onset of phase separation T 1 (for discussed solution, 70 • C according to turbidimetry data), at which I/I 21 = 3.3. Above T 1 , the rate of change in the light scattering intensity on heating increases by an order of magnitude. The dependence of I on T is caused by the increase in the size R s of large aggregates on heating, while the values of R f and R m do not change with temperature. (Figure 9). The change in R s is not high, but it is detected rather reliably. Therefore, at T < T 1 , the dominant process in the solutions of the studied star-shaped polymers was aggregation as a result of an increase in the dehydration degree of arms with temperature and the formation of intermolecular hydrogen bonds.
At T > T 1 , a sharp increase in the size of large aggregates was observed, and at the temperature of the phase separation finishing, the R s values reached hundreds of nanometers and even microns. Above T 2 , the radii of large aggregates slightly decreased, which reflects the macromolecule compaction. Note that, in the studied temperature range, the sizes of micelle-like structures did not depend on temperature, and in the phase transition (near T 1 ) these particles ceased to be detected by the dynamic light scattering. Therefore, they joined with large aggregates or formed new supramolecular structures.  Figure 10 shows the dependences of the phase separation temperature T 1 on the salt concentration c salt for water-salt solutions of CPh6-PAlOz and CPh6-PAlOx. It is clearly seen that the NaCl and N-PTS affect the behavior of the investigated star-shaped pseudopolypeptoids in different ways. For NaCl solutions, an increase in c salt leads to a monotonic decrease in T 1 , the rate of which decreases in the region of high values of c salt . Similar dependences were observed earlier for PAlOx of different architectures [69,[83][84][85][86][87], as well as linear and star-shaped PEtOz [60]. In the case of N-PTS solutions, for all polymers, the phase separation temperatures decline very quickly in the region of low salt content. At a concentration c salt corresponding to approximately one N-PTS molecule per one arm of polymer star, the decrease in T 1 slows down, the phase separation temperature reaches a minimum value T 1 (min) , and then the T 1 value begins to rise with increasing N-PTS content. Thus, for the studied polymer stars, at a low content in solution, N-PTS manifests itself as a kosmotropic agent, and at high c salt , N-PTS exerts chaotropic activity. What agent, chaotropic or kosmotropic, is a particular salt is a complex problem and its analysis, in particular the study of the interaction of thermoresponsive polymers with salt, has been devoted to a large number of works [66,67,71,72,[88][89][90]. Analyzing the effect of salt on the behavior of polymer solution, it is necessary to take into account not only the chemical structure of the polymer and salt, but also their concentration in solution, ionic strength, temperature, etc.

Influence of Salt Content on Phase Separation Temperatures
nanometers and even microns. Above T2, the radii of large aggregates slightly decreased, which reflects the macromolecule compaction. Note that, in the studied temperature range, the sizes of micelle-like structures did not depend on temperature, and in the phase transition (near T1) these particles ceased to be detected by the dynamic light scattering. Therefore, they joined with large aggregates or formed new supramolecular structures. Figure 10 shows the dependences of the phase separation temperature T1 on the salt concentration csalt for water-salt solutions of CPh6-PAlOz and CPh6-PAlOx. It is clearly seen that the NaCl and N-PTS affect the behavior of the investigated star-shaped pseudopolypeptoids in different ways. For NaCl solutions, an increase in csalt leads to a monotonic decrease in T1, the rate of which decreases in the region of high values of csalt. Similar dependences were observed earlier for PAlOx of different architectures [69,[83][84][85][86][87], as well as linear and star-shaped PEtOz [60]. In the case of N-PTS solutions, for all polymers, the phase separation temperatures decline very quickly in the region of low salt content. At a concentration csalt corresponding to approximately one N-PTS molecule per one arm of polymer star, the decrease in T1 slows down, the phase separation temperature reaches a minimum value T1 (min) , and then the T1 value begins to rise with increasing N-PTS content. Thus, for the studied polymer stars, at a low content in solution, N-PTS manifests itself as a kosmotropic agent, and at high csalt, N-PTS exerts chaotropic activity. What agent, chaotropic or kosmotropic, is a particular salt is a complex problem and its analysis, in particular the study of the interaction of thermoresponsive polymers with salt, has been devoted to a large number of works [66,67,71,72,[88][89][90]. Analyzing the effect of salt on the behavior of polymer solution, it is necessary to take into account not only the chemical structure of the polymer and salt, but also their concentration in solution, ionic strength, temperature, etc. The effect of NaCl and N-PTS on the behavior of CPh6-PAlOz and CPh6-PAlOx solutions depends on the arm structure. It is convenient to analyze the effect of the chemical structure of arms on the characteristics of water-salt solutions of the studied stars, comparing not only the dependences of the phase separation temperatures on the salt content ( Figure 10), but also the dependences ΔT1 = T1 (0) -T1 (c) on csalt (Figure 11), where T1 (0) is the temperature of onset of phase separation at csalt = 0 and T1 (c) is this temperature at a given csalt. The ΔT1 value determines the change in the phase separation temperature upon salt addition. As can be seen in Figure 10, for both salts in the studied range of csalt, the phase separation temperatures decrease in the series CPh6-PEtOx-CPh6-PEtOz-CPh6-PiPrOx-CPh6-PiPrOz. Therefore, in water-salt solutions, a regularity, which is valid for solutions of CPh6-PAlOz and CPh6-PAlOx in water, is preserved. The ΔT1 values in NaCl solutions change in the same way ( Figure 11). In solutions containing N-PTS, this The effect of NaCl and N-PTS on the behavior of CPh6-PAlOz and CPh6-PAlOx solutions depends on the arm structure. It is convenient to analyze the effect of the chemical structure of arms on the characteristics of water-salt solutions of the studied stars, comparing not only the dependences of the phase separation temperatures on the salt content ( Figure 10), but also the dependences ∆T 1 = T 1 (0) − T 1 (c) on c salt (Figure 11), where T 1 (0) is the temperature of onset of phase separation at c salt = 0 and T 1 (c) is this temperature at a given c salt . The ∆T 1 value determines the change in the phase separation temperature upon salt addition. As can be seen in Figure 10, for both salts in the studied range of c salt , the phase separation temperatures decrease in the series CPh6-PEtOx-CPh6-PEtOz-CPh6-PiPrOx-CPh6-PiPrOz. Therefore, in water-salt solutions, a regularity, which is valid for solutions of CPh6-PAlOz and CPh6-PAlOx in water, is preserved. The ∆T 1 values in NaCl solutions change in the same way ( Figure 11). In solutions containing N-PTS, this sequence occurs only at low c salt concentrations (Figure 11). At c salt > 0.02 M, the ∆T 1 value for CPh6-PEtOz becomes lower than the corresponding characteristic for CPh6-PiPrOx. This is due to the fact that after reaching the minimum value T 1 (min) , the phase separation temperature for stars containing side ethyl and isopropyl groups in the arms increases with different rates. For CPh6-PEtOz and CPh6-PEtOx, the temperature T 1 at high c salt exceeded the value T 1 (min) by 29 and 18 • C, respectively. For CPh6-PiPrOx and CPh6-PiPrOz, the increase in T 1 in the region of high N-PTS content was smoother, and the change in T 1 did not exceed 4 • C ( Figure 11).

Influence of Salt Content on Phase Separation Temperatures
sequence occurs only at low csalt concentrations (Figure 11). At csalt > 0.02 M, the ΔT1 value for CPh6-PEtOz becomes lower than the corresponding characteristic for CPh6-PiPrOx. This is due to the fact that after reaching the minimum value T1 (min) , the phase separation temperature for stars containing side ethyl and isopropyl groups in the arms increases with different rates. For CPh6-PEtOz and CPh6-PEtOx, the temperature T1 at high csalt exceeded the value T1 (min) by 29 and 18 °C, respectively. For CPh6-PiPrOx and CPh6-PiPrOz, the increase in T1 in the region of high N-PTS content was smoother, and the change in T1 did not exceed 4 °C ( Figure 11). As is known, with the same structure of side groups, PAlOz are more hydrophobic than PAlOx. Accordingly, at the given concentration and molar mass of the polymer, the phase separation temperatures for aqueous solutions of PAlOz are lower than for the corresponding PAlOx [14,43,91]. This regularity is observed for water-salt solutions of the studied star-shaped polymers, namely, at all salt concentrations csalt; in N-PTS and NaCl solutions, the temperature T1 decreased with passage from CPh6-PAlOx to CPh6-PAlOz. Note that the molar masses of the CPh6-PAlOz samples are higher than the MM of CPh6-PAlOx. An increase in the MM usually leads to a growth in the phase separation temperatures [85,[92][93][94]. Consequently, some contribution to the observed difference in the T1 values for water-salt solutions star-shaped of CPh6-PAlOz and CPh6-PAlOx can be made by changing MM.
As seen in Figure 11, for the star-shaped CPh6-PEtOz, the maximum change in the phase separation temperature ΔT1 is approximately the same in both water-salt solvents: The maximum ΔT1 value is around 45 °C. For CPh6-PiPrOz, the maximum ΔT1 values are noticeably lower (ΔT1 ∼ 10 °C), but they also coincide in different solvents. In the case of CPh6-PAlOx, a similar behavior was detected for CPh6-PiPrOx, while for CPh6-PEtOx, the maximum ΔT1 values in NaCl and N-PTS solutions differed by 10 °C.
Comparison of the obtained results with the literature data for other star-shaped pseudo-polypeptoids shows that their behavior in water-salt solutions depends on the core structure. For example, for water-salt solutions of eight-arm star-shaped poly-2isopropyl-2-oxazoline with calix [8] arene core (at polymer concentration c = 0.0050 g⋅cm −3 ), the dependence of the phase separation temperature on the N-PTS content was monotonic [70], and the decrease in T1 in the csalt range from 0-0.06 M was about 4 °C. Note that for the CPh6-PiPrOx studied in this work, the ΔT1 value exceeded 20 °C. Figure 12 compares the dependences of ΔT1 on csalt for the six-arm CPh6-PEtOz and CPh6-PiPrOx studied in this work, four-arm PEtOz with a calix [4] arene core (C4A-PEtOz) [60], and eight-arm PiPrOx with calix [8] arene core (C8A-PiPrOx) [69]. For sixand four-arm PEtOz, the considered dependences differ insignificantly, and only in the region of high NaCl content, the ΔT1 value for CPh6-PEtOz is noticeably higher than ΔT1 for C4A-PEtOz. For star-shaped PiPrOx at all concentrations, the ΔT1 value for the As is known, with the same structure of side groups, PAlOz are more hydrophobic than PAlOx. Accordingly, at the given concentration and molar mass of the polymer, the phase separation temperatures for aqueous solutions of PAlOz are lower than for the corresponding PAlOx [14,43,91]. This regularity is observed for water-salt solutions of the studied star-shaped polymers, namely, at all salt concentrations c salt ; in N-PTS and NaCl solutions, the temperature T 1 decreased with passage from CPh6-PAlOx to CPh6-PAlOz. Note that the molar masses of the CPh6-PAlOz samples are higher than the MM of CPh6-PAlOx. An increase in the MM usually leads to a growth in the phase separation temperatures [85,[92][93][94]. Consequently, some contribution to the observed difference in the T 1 values for water-salt solutions star-shaped of CPh6-PAlOz and CPh6-PAlOx can be made by changing MM.
As seen in Figure 11, for the star-shaped CPh6-PEtOz, the maximum change in the phase separation temperature ∆T 1 is approximately the same in both water-salt solvents: The maximum ∆T 1 value is around 45 • C. For CPh6-PiPrOz, the maximum ∆T 1 values are noticeably lower (∆T 1~1 0 • C), but they also coincide in different solvents. In the case of CPh6-PAlOx, a similar behavior was detected for CPh6-PiPrOx, while for CPh6-PEtOx, the maximum ∆T 1 values in NaCl and N-PTS solutions differed by 10 • C.
Comparison of the obtained results with the literature data for other star-shaped pseudo-polypeptoids shows that their behavior in water-salt solutions depends on the core structure. For example, for water-salt solutions of eight-arm star-shaped poly-2-isopropyl-2-oxazoline with calix [8] arene core (at polymer concentration c = 0.0050 g·cm −3 ), the dependence of the phase separation temperature on the N-PTS content was monotonic [70], and the decrease in T 1 in the c salt range from 0-0.06 M was about 4 • C. Note that for the CPh6-PiPrOx studied in this work, the ∆T 1 value exceeded 20 • C. Figure 12 compares the dependences of ∆T 1 on c salt for the six-arm CPh6-PEtOz and CPh6-PiPrOx studied in this work, four-arm PEtOz with a calix [4] arene core (C4A-PEtOz) [60], and eight-arm PiPrOx with calix [8] arene core (C8A-PiPrOx) [69]. For six-and four-arm PEtOz, the considered dependences differ insignificantly, and only in the region of high NaCl content, the ∆T 1 value for CPh6-PEtOz is noticeably higher than ∆T 1 for C4A-PEtOz. For star-shaped PiPrOx at all concentrations, the ∆T 1 value for the polymer with CPh6 core is higher than for the star with C8A. These facts suggest that star-shaped pseudo-polypeptoids with a hexaaza [2 6 ]orthoparacyclophane core are more sensitive to the presence of NaCl than similar stars with a calix[n]arene core. However, it should be remembered that the compared polymers differed not only in the structure of the branching center, but also in the number and length of arms. The values of the latter characteristics determine the intramolecular density of the macromolecule, and, accordingly, the accessibility of the core for solvent molecules and molecules of low molecular weight salts.

Conclusions
The effect of NaCl and N-PTS on the self-organization in aqueous solutions of sixarm star-shaped CPh6-PAlOz and CPh6-PAlOx and on phase separation temperatures were investigated. It was shown that in the case of CPh6-PAlOx at low temperatures, the addition of salts does not lead to significant changes in the solution characteristics. A different situation took place for CPh6-PAlOz, in solutions of which, with a salt content corresponding to approximately one salt molecule per arm of star, the set of scattering objects changed. At this concentration, the micelle-like structures appeared in solutions, and isolated molecules ceased to be detected by dynamic light scattering. The observed effect depended on the arm structure. In CPh6-PEtOz solutions, micelle-like aggregates appeared with the addition of both salts, while in CPh6-PiPrOz solutions they formed only with N-PTS addition. In NaCl solutions of CPh6-PiPrOz, macromolecules and large aggregates were present in solutions at all studied salt concentrations. The effect of the salt structure was traced in the fact that in most N-PTS solutions the sizes of the aggregates were constant, while in NaCl solutions they increased with growth of salt concentration.
On heating, a phase transition with the formation of supramolecular micron-sized structures was observed in all the studied water-salt solutions of the star-shaped CPh6-PAlOz and CPh6-PAlOx. As well as in aqueous solutions, in both used solvents, at the same salt concentration, the phase separation temperature decreased in the series CPh6-PEtOx-CPh6-PEtOz-CPh6-PiPrOx-CPh6-PiPrOz. This is caused by an increase in the hydrophobicity of the polymers both with growth of the size of the side radical in the arms and with an elongation of the monomer unit by one -CH 2 -group.
The effect of the structure of salt and polymer on the phase separation temperature T 1 was found. For all the stars studied, the temperature T 1 monotonically decreased with increase in NaCl content in solution from c salt = 0 to 0.154 M. This reduction for CPh6-PEtOz and CPh6-PEtOx polymers reached 49 and 37 • C, respectively. For more hydrophobic stars with isopropyl side groups, the discussed change was much smaller, 23 • C for CPh6-PiPrOx and 11 • C for CPh6-PiPrOz. In N-PTS solutions for all polymers, the dependence of the phase separation temperature on the salt concentration was non-monotonic. In the region of low salt content, T 1 decreased sharply, reaching a minimum value at concentration c salt corresponding to approximately one N-PTS molecule per one arm of a polymer star. Above this concentration, an increase in the phase separation temperature was observed. As well as in NaCl solutions, in solutions with the addition of N-PTS, the maximum change in T 1 was greater for polymers with ethyl side radicals. Comparison of the obtained results with the literature data for star-shaped pseudo-polypeptoids with a calix[n]arene branching center showed that PAlOz and PAlOx stars with a hexaaza [2 6 ]orthoparacyclophane core are more sensitive to the presence of salt in solution.

Conflicts of Interest:
The authors declare no conflict of interest.