Loose Semirigid Aromatic Polyester Bottle Brushes at Poly(2-isopropyl-2-oxazoline) Side Chains of Various Lengths: Behavior in Solutions and Thermoresponsiveness

A polycondensation aromatic polyester with an oxygen spacer was synthesized and used as a macroinitiator for the grafting of linear poly(2-isopropyl-2-oxazoline) (PiPrOx) by the cationic polymerization method. The length of the thermosensitive side chains was varied by the initiator:monomer ratio. Using methods of molecular hydrodynamics, light scattering and turbidimetry, the copolymers were studied in organic solvents and in water. The molecular characteristics of the main chain and graft copolymers, the polymerization degree of side chains and their grafting density have been determined. The equilibrium rigidity of the macroinitiator and the conformations of grafted macromolecules were evaluated. In selective solvents, they take on a star-like conformation or aggregate depending on the degree of shielding of the main chain by side chains. The thermoresponsiveness of graft copolymers in aqueous solutions was studied, and their LCST were estimated. The results are compared with data for graft copolymers composed of PiPrOx side chains and flexible or rigid chain backbones of aromatic polyester type.


Introduction
Grafted polymers, the so-called cylindrical polymer brushes, are a class of polymers consisting of a long main chain and relatively short side chains grafted to it [1]. The physicochemical properties of molecular brushes in solution and in block are largely determined by their architectural parameters, such as the degree of polymerization of the main and side chains, the grafting density of side chains, etc. [1][2][3][4][5][6][7][8]. Due to the peculiarities of molecular architecture, cylindrical brushes are widely used to design new nanoobjects of various morphologies that cannot be obtained on the basis of linear polymers [9][10][11].
Research into molecular brushes began at the end of the 20th century due to the development of controlled synthesis of polymers of various architectures [2,3,12,13]. As a result, the main regularities of the behavior of graft copolymers in solutions, such as the effect of the length of the main and side chains and the grafting density of the latter, on the hydrodynamic characteristics and conformation of their macromolecules have been established [5][6][7][8][9][14][15][16][17][18][19][20].
The properties of grafted block-copolymers can depend on the interaction of chemically dissimilar side and main chains and on the solvent affinity to different blocks of the graft copolymer [4,13,[21][22][23][24][25][26]. Accordingly, one of the ways to control the self-organization of molecular brushes in selective solvents is the targeted synthesis of brushes with different chemical structures of the main and side chains and molecular architecture parameters. The self-organization of amphiphilic polymer brushes differs significantly from the behavior of linear copolymers in solutions [27][28][29][30]. For example, amphiphilic polymer brushes 50 • C, and that of 2-isopropyl-2-oxazoline at about 20 • C [63]. Cloud points observed for PiPrOx are generally higher than for the 2-isopropyl-2-oxazoline monomer; the thermal transition temperature of PiPrOx is affected by polymer concentration and molar mass, as well as terminal functional groups. When varying these parameters, T cp have been observed by various studies at temperatures from 36 to 63 • C. Molecular architecture can also influence T cp : for example, a cyclic PiPrOx exhibited T cp around 10 • C higher than a linear PiPrOx of the same molar mass. [40,41,64]. It has been shown that, in a selective solvent, APE 8 -g-PiPrOx polymerization-polycondensation molecular brush with short PiPrOx chains and a backbone containing sufficiently long alkylene fragments -(CH 2 ) 8in the monomer unit forms unimolecular micelles of a star-like conformation at a low z 0.5. On the contrary, molecular brushes with PAlOx side chains and the rigid main chain of aromatic APE (APE r.ch. -g-PAlOx) at a low z and short side chains aggregate into large supramolecular structures, and they are molecularly dispersed only at sufficiently long side chains [65]. In this case, the LCST is affected by the chemical structure of the side chains.
Thus, the chemical structure and equilibrium rigidity of the main chain essentially determine the mechanism of self-organization of a molecular brush in solution. It seems important to continue the series of copolymers by varying the structure of the main chain, passing to a main chain with an intermediate equilibrium rigidity. For this purpose, an aromatic polyester type macroinitiator with an oxygen atom as a spacer was synthesized (APE O ) and graft copolymers with PiPrOx side chains of various lengths were obtained ( Figure 1). Thus, the main objectives of this work were to determine the hydrodynamic and conformational characteristics of the APE O macroinitiator and its block copolymers with grafted PiPrOx chains, and to study the effect of the side chain length on the conformational characteristics of APE O -g-PiPrOx copolymers in selective solvents and on thermal responsiveness in aqueous solutions. to a decrease in phase separation temperatures [62]. Note that dehydration of 2-ethyl-2oxazoline units begins at 50 °C, and that of 2-isopropyl-2-oxazoline at about 20 °C [63]. Cloud points observed for PiPrOx are generally higher than for the 2-isopropyl-2-oxazoline monomer; the thermal transition temperature of PiPrOx is affected by polymer concentration and molar mass, as well as terminal functional groups. When varying these parameters, Tcp have been observed by various studies at temperatures from 36 to 63 °C. Molecular architecture can also influence Tcp: for example, a cyclic PiPrOx exhibited Tcp around 10 °C higher than a linear PiPrOx of the same molar mass. [40,41,64]. It has been shown that, in a selective solvent, APE8-g-PiPrOx polymerization-polycondensation molecular brush with short PiPrOx chains and a backbone containing sufficiently long alkylene fragments -(CH2)8in the monomer unit forms unimolecular micelles of a star-like conformation at a low z ~0.5. On the contrary, molecular brushes with PAlOx side chains and the rigid main chain of aromatic APE (APEr.ch.-g-PAlOx) at a low z and short side chains aggregate into large supramolecular structures, and they are molecularly dispersed only at sufficiently long side chains [65]. In this case, the LCST is affected by the chemical structure of the side chains. Thus, the chemical structure and equilibrium rigidity of the main chain essentially determine the mechanism of self-organization of a molecular brush in solution. It seems important to continue the series of copolymers by varying the structure of the main chain, passing to a main chain with an intermediate equilibrium rigidity. For this purpose, an aromatic polyester type macroinitiator with an oxygen atom as a spacer was synthesized (APEO) and graft copolymers with PiPrOx side chains of various lengths were obtained ( Figure 1). Thus, the main objectives of this work were to determine the hydrodynamic and conformational characteristics of the APEO macroinitiator and its block copolymers with grafted PiPrOx chains, and to study the effect of the side chain length on the conformational characteristics of APEO-g-PiPrOx copolymers in selective solvents and on thermal responsiveness in aqueous solutions.    [68] were obtained according to the known procedures. Diphenyl oxide, 1-chloronaphalene and 1,1,2,2-tetrachloroethane (Aldrich) were dried over calcium hydride and distilled. NMR spectra of solutions of samples in CDCl 3 were recorded using a Bruker AC 400 instrument (400 MHz). Dialysis was performed with the use of dialysis bags (CellaSep, Orange Scientific) with MWCO 3500 D. Chromatographic analysis was performed with the use of a Shimadzu LC-20AD chromatograph equipped with a TSKgel G5000HHR column (5 µm, 7.8 mm × 300 mm, Tosoh Bioscience) and a refractometric detector. Solution of LiBr in DMF (0.1 mol/L) at 60 • C was used as a mobile phase. Calibration was performed with the use of poly(ethylene glycol) standards (M w = 6 × 10 2 -4 × 10 4 ).

Synthesis of Polyester-g-poly-2-isopropyl-2-oxazoline Copolymers
Solution of the initiator (30 wt.%) and the monomer in 1,1,2,2-tetrachloroethane (feed ratio of about 10% from the desired value) was heated in sealed tube at 100 • C for 3 h. Then, the rest of the monomer was added to reaction mixture and polymerization was allowed to proceed at 120 • C for 24 h. After completion of polymerization and removal of volatile compounds, the reaction mixture was diluted with ethanol, dialyzed against water for 48 hrs and lyophilized.

Determination of the Molecular Characteristics of Polymers
The weight-average molar masses M w of the macroinitiator and copolymers and the hydrodynamic radii R h of their macromolecules were determined in dilute solutions in organic solvents (chloroform, nitropropane) by static (LS) and dynamic (DLS) light scattering methods. Since there was practically no asymmetry in the scattered light intensity, M w was determined by the Debye method. The equation [69]: where I is the intensity of light scattering measured at 90 • , A 2 is the second virial coefficient, and H is the optical constant, N A being the Avogadro's number was applied. The refractive index increment dn/dc was measured on a Refractometer RA-620/600 (KEM, Kyoto, Japan), with LED Na-D Line light source. The LS and DLS measurements were carried out in a Photocor Complex set-up (Photocor Instrument, Moscow, Russia), which was equipped with a Photocor-PC2 correlator with 288 channels, and a Photocor-PD detector for measuring the transmitted light intensity. The light source was a Photocor-DL semiconductor laser with a wavelength of λ 0 = 659.1 nm. Calibration was carried out with toluene, the absolute scattering intensity being R v = 1.38 × 10 −5 cm −1 . Before measurements, the solutions were filtered into dust- The hydrodynamic radius R h was determined by the regularization method which is used as a part of the Photocor Complex software. The measurements were taken at an angle of 90 degrees.
Intrinsic viscosity [η] was measured in Ostwald type Cannon-Manning capillary viscometers (Cannon Instrument Company, State College, PA, USA) and an LOIP LT-100 temperature control unit (LOIP, St.-Petersburg, Russia). Viscosity data were analyzed using the Huggins equation where k is the Huggins constant characterizing the polymer-solvent hydrodynamic and thermodynamic interactions in solutions and η sp = (τ/τ 0 − 1), τ being the solution flow time [70,71]. The solvent efflux time was τ 0 = 116.0 s for chloroform, 105.4 s for nitropropane and 57.2 s for DMF.

Study of Thermoresponsiveness of Copolymers in Solutions upon Heating
The thermoresponsiveness of copolymers was studied by LS, DLS and turbidimetry in aqueous solutions using the Photocor instrument described above. The temperature T was controlled with an accuracy of 0.1 • C, changing it discretely with a step of 1.0 • C near the phase separation point T 1 and with a larger interval up to 5.0-6.0 • C at temperatures far from T 1 .
Scattered light intensity I and transmitted light intensity I* were measured as functions of T upon heating. The temperatures T 1 and T* 1 of the onset of phase separation were determined from the dependences I(T) and I*(T), taking for T 1 and T* 1 the temperatures after which I began to increase or I* decreased, respectively. The hydrodynamic radii of scattering species R i and their contribution S i to the total scattering intensity S, i being associated with the type of scattering specie, were also measured at various T. The S i value was estimated as the area under the corresponding peak of I(R) distribution obtained by DLS. It should be noted that measurements of all parameters were carried out after the solution reached an equilibrium state, that is, when the I and I* reached constant values after an abrupt change in T.

Synthesis of Multicenter Oligoester Initiator
In the synthesis of the macroinitiator, high-temperature solution polycondensation without acceptor was used [72]; this method allows synthesize polyesters over a wide range of molecular masses with polydispersity index of 2.1-2.3. Synthesis of the APE O macroinitiator was conducted in accordance with that described in [59]. The conditions for the polycondensation were the following: 1-chloronaphthalene as the solvent; temperature of 200 • C, monomer concentration 25 wt.%, reaction time 2 h.

Synthesis of APE O -g-PiPrOx with the Use of APE O Macrointiator
Methods and approaches to the preparation of APE O -g-PiPrOx block copolymers are described in [59]. Graft copolymers were synthesized via CROP of 2-isopropyl-2-oxazoline according to the following scheme ( Figure 2): Grafting of PiPrOx chains was carried out in tetrachloroethane solution at 120 • C to improve solubility of the macroinitiator monomer, since thermodynamic quality of a solvent becomes higher with increasing temperature. The process was carried out in two stages. In the first stage, 10% from the calculated amount of the monomer was introduced into polymerization mixture, and the mixture was heated at 100 • C for 3 h. The addition of the remaining amount of oxazoline made it possible to post-polymerize the side chains due to the "living" character of the process. When loading the reaction mixture, the initiator/monomer ratio was 1/40 (sample 1), 1/100 (sample 2) and 1/120 (sample 3), which made it possible to obtain three samples of the graft copolymer with PiPrOx side chains of various lengths. GPC data confirm unimodality in their size distribution ( Figure 3). The amount of grafted PiPrOx was estimated using 1 H NMR data by the ratio of signal intensity of the terminal methyl group of 4-methylpiperidine (at about 0.9 ppm) and signal intensity of poly(ethylene imine) fragments (at about 3.4 ppm). The average degree of polymerization of monomer units N s in grafted oligo(oxazoline) chains was 30, when the initial initiator/monomer ratio was equal to 1/40; this value was 50 at a ratio of 1/100 and 60 at a ratio of 1/120. The fact that degree of polymerization of grafted chains is not proportional to initial initiator/monomer ratio is caused by chain transfer to monomer. This issue was analyzed by us earlier [59]. The observed decrease in initiation efficiency in the case of multicenter macroinitiators may be due to higher growth rate of side chains than the initiation rate; therefore, steric hindrances of side chains impede the process. Indeed, the asymmetric shape of the chromatogram obtained for graft copolymers ( Figure 3) indicates some inhomogeneity in distribution of grafted fragments alongside the backbone. Figure 2. Synthesis of APEO-g-PiPrOx graft copolymers.
Grafting of PiPrOx chains was carried out in tetrachloroethane solution at 120 °C to improve solubility of the macroinitiator monomer, since thermodynamic quality of a solvent becomes higher with increasing temperature. The process was carried out in two stages. In the first stage, 10% from the calculated amount of the monomer was introduced into polymerization mixture, and the mixture was heated at 100 °C for 3 h. The addition of the remaining amount of oxazoline made it possible to post-polymerize the side chains due to the "living" character of the process. When loading the reaction mixture, the initiator/monomer ratio was 1/40 (sample 1), 1/100 (sample 2) and 1/120 (sample 3), which made it possible to obtain three samples of the graft copolymer with PiPrOx side chains of various lengths. GPC data confirm unimodality in their size distribution ( Figure 3). The amount of grafted PiPrOx was estimated using 1 H NMR data by the ratio of signal intensity of the terminal methyl group of 4-methylpiperidine (at about 0.9 ppm) and signal intensity of poly(ethylene imine) fragments (at about 3.4 ppm). The average degree of

Molecular and Conformational Properties of APEO and APEO-g-PiPrOx
The molecular and hydrodynamic characteristics of the macroinitiator were determined in chloroform. Graphs for their determination are presented in Figure 4; Table 1 presents the results. The values of the second virial coefficient A2 and the Huggins constant k' calculated from plots of Figure 4a,c (Equations (1)-(3)) testify to the good thermodynamic quality of CHCl3 as a solvent for APEO. The Rh was obtained by extrapolating the hydrodynamic radii determined at finite concentrations to c = 0 (Figure 4b). The viscometric hydrodynamic radius Rη = 5.7 nm for APEO, calculated from the formula significantly exceeds Rh, which may testify to an extended (asymmetric) conformation of the macroinitiator. The polymerization degree of the macroinitiator calculated from the Mw value (Table 1) is N0 = 38, since the molar mass of the APEO monomer unit is M0 = 580 g/mol. To estimate the length of the monomer unit of the macroinitiator L0, it was assumed that the length of valence bonds is 0.14 nm and the bond angles are tetrahedral; then, in accordance with the APEO structural formula, the sum of its bond lengths along the chain direction is L0 = 1.6 nm (without deformation of the bond angles) and the macromolecular chain length is L = 60.7 nm.

Molecular and Conformational Properties of APE O and APE O -g-PiPrOx
The molecular and hydrodynamic characteristics of the macroinitiator were determined in chloroform. Graphs for their determination are presented in Figure 4; Table 1 presents the results. The values of the second virial coefficient A 2 and the Huggins constant k calculated from plots of Figure 4a,c (Equations (1)-(3)) testify to the good thermodynamic quality of CHCl 3 as a solvent for APE O . The R h was obtained by extrapolating the hydrodynamic radii determined at finite concentrations to c = 0 (Figure 4b). The viscometric hydrodynamic radius R η = 5.7 nm for APE O , calculated from the formula significantly exceeds R h , which may testify to an extended (asymmetric) conformation of the macroinitiator. The polymerization degree of the macroinitiator calculated from the M w value (Table 1) is N 0 = 38, since the molar mass of the APE O monomer unit is M 0 = 580 g/mol. To estimate the length of the monomer unit of the macroinitiator L 0 , it was assumed that the length of valence bonds is 0.14 nm and the bond angles are tetrahedral; then, in accordance with the APE O structural formula, the sum of its bond lengths along the chain direction is L 0 = 1.6 nm (without deformation of the bond angles) and the macromolecular chain length is L = 60.7 nm. The equilibrium rigidity, which is measured by the Kuhn segment length A, for an APE O can be estimated by comparing its characteristics with the A value for a polymer of close chemical structure, for example, semi-rigid poly(m-phenylene isophthalamide) (PMPhIPhA), studied in [73]. For this polymer, the values A ≈ 4-4.7 nm were obtained, which are increased, compared to flexible-chain polymers, due to aromatic fragments in the meta position. Although the comparison of structures of APE O and PMPhIPhA is quite approximate, it can be assumed that the equilibrium rigidity of APE O should not be greater than that of PMPhIPhA, since the -O-spacer present in the structure of APE O allows rotation of the chain around the valence bond; thus, Kuhn segment length for APE O should be 2 nm < A < 4 nm, where A = 2 nm was obtained for APE macroinitiators with flexible alkylene spacers [74]. Then the APE O chain consists of more than 15 Kuhn segments, therefore it should be sufficiently coiled in solution. Thus, a worm-shaped cylinder with a contour length of 60.7 nm can serve as a model for the APE O macroinitiator chain in solution. The increased equilibrium rigidity of APE O and, consequently, the increased permeability of its macromolecules in NP probably cause their low R h (Table 1).   The equilibrium rigidity, which is measured by the Kuhn segment length A, for an APEO can be estimated by comparing its characteristics with the A value for a polymer of   Figure S1. The A 2 values are positive and decrease with increasing iPrOx monomer content indicating that the solvent is worsening for the copolymers upon the initiator/monomer ratio. The hydrodynamic radii obtained by the DLS method did not depend on the concentration, and Table 1 presents the concentration-averaged R h values. The hydrodynamic radii of APE O -g-PiPrOx exceed the R h obtained for the macroinitiator (Table 1). This increase is conditioned by significant contribution of side chains to the friction coefficient of the macromolecule in the solvent. However, the values of both R h and intrinsic viscosities [η] are very small at sufficiently large M w , which can be seen from the Figure 5. Here, the points corresponding to the data for samples 1 and 2 in NP lie well below the logarithmic dependence [η](M) carried out for linear PEtOx according to the equation [η] = 5.0 × 10 −2 M 0.63 from the work [75], and also lower than for APE 8 -g-PiPrOx in NP [62]. This testifies to a high intramolecular density of APE O -g-PiPrOx macromolecules. As was shown in [32,62], the grafted block-copolymer can adopt a star-like conformation, in case of low grafting degree of side chains and short grafted blocks; here the arms are the PiPrOx side chains and the core is the backbone macromolecule collapsed in a nonsolvent. The [η] and R h values of sample 3 were measured in DMF (Table 1). They noticeably exceed [η] and R h obtained for APE O -g-PiPrOx in NP ( Figure 5), which, at close M w of all copolymers, could be caused by swelling of PiPrOx side chains in this solvent. However, the increase in R h by more than 2 times in DMF rather attributes to intermolecular aggregation, what is indicated by a negative k' value. This issue will be discussed below. Based on the experimentally measured values of Mw for APEO (MAPE) and the copolymers, as well as the mass of the side chain Ms, the architecture parameters of the APEOg-PiPrOx graft copolymer were calculated, which are given in Table 2   Based on the experimentally measured values of M w for APE O (M APE ) and the copolymers, as well as the mass of the side chain M s , the architecture parameters of the APE O -g-PiPrOx graft copolymer were calculated, which are given in Table 2. The side chain length in the extended conformation without deformation of bond angles was calculated using the formula L s = λ × N s , since the length of the projection of the alkylene unit onto the direction of the PiPrOx chain is λ = 0.252 nm. Accordingly, molar masses M s of side chains are equal to M s = N s × M 0s , where the mass of the PiPrOx monomer unit, M 0s = 113 g/mol. Then the degree of functionalization of the macroinitiator (or grafting density of side chains) is z = m/(n + m) = m/N 0 , where m is the number of the backbone monomer units containing side chains (i.e., the number of side chains) and n is the number of the monomer units of the macroinitiator which do not contain side chains [59,60]; m was calculated using the formula m = (M w − M APE )/M s . It can be seen from Table 2 that in all cases the value of z is small, i.e., APE O -g-PiPrOx macromolecules are brushes with rather loosely attached side chains. Less than half of the monomer units in the main chain carry the side chains. The value of z decreases upon an increase in L s , and for sample 1 this is about two times greater than for samples 2 and 3. At the same time, the side chains are 2.2-2.7 times longer than the distance between the grafting points, calculated as ∆L = L 0 /z ( Table 2). The parameter L s /∆L makes it possible to compare the distance between the grafting points of the side chains and their length, which is maximum for sample 1.
The degree of shielding of the molecular brush backbone by side chains and its accessibility for contacts can be quantitatively characterized by the parameter δ that takes into account the size of the side chain folded into a molecular coil [24]. This parameter is conveniently calculated in terms of the number of bonds in the side chains and in the fragments of the main chain between their grafting points, δ = [(N A × N s )] 0.5 /(N 0 × z), where the number of bonds in the Kuhn segment of the side chain N A = 3 for PiPrOx [74], and N 0 × z characterizes the number of bonds in the backbone of the brush between adjacent side chains. Characteristically, the degree of shielding calculated for the studied brushes is always low (Table 2). However, the parameter δ obtained for brush 1 significantly exceeds this parameter for brushes 2 and 3 with longer side chains, but with a lower side chain grafting density.

Composition of Scattering Species in Solution and Their Hydrodynamic Size at Room Temperature
At room temperatures (T < 27 • C), two modes associated with scattering species with hydrodynamic radii R f and R s , were registered in DLS experiments in water solutions. For all samples, species of large hydrodynamic size, which are aggregates, were observed. Their averaged hydrodynamic radii R s were 300 nm for brush 1, 40 nm for brush 2 and 170 nm for brush 3 (Table 1). It can be noted that R s tends to increase with concentration despite a general significant scatter of points ( Figure 6). The proportion of light scattered by aggregates, S s , in the total light scattering always exceeds 98.5%. However, it should be understood that a high contribution from large species to scattering intensity does not mean their high concentration in solution. Indeed, for a particle of type i, the relation I i~ci M i is true, where M i and c i are the molar mass of the specie and its content in the solution. In this case, M i~Ri x , where the parameter x depends on the particle shape. For example, x = 1, 2 and 3 for the long rod, the macromolecular coil, and for the sphere, respectively. Then the partial concentration for spheres is c i~Ii /R i 3 . It can be estimated that for spherical particles, whose sizes differ by 1-2 orders, even at a high contribution of the scattering intensity, the concentration c i can be very small. However, since the shape and density of aggregates are a priori unknown, we cannot obtain their content in solution. Though measurements at a large scattering angle could be advantageous when using DLS to determine the relative concentration of nanoparticles in the samples with bimodal size distributions [76], the question of the physical and topological properties of supramolecular structures does not allow us to carry out a quantitative analysis of their content in solution. Nevertheless, we can trace the change in the contribution of various species to light scattering with a change in the external parameters of the medium, such as concentration, temperature, etc. measurements at a large scattering angle could be advantageous when using DLS to determine the relative concentration of nanoparticles in the samples with bimodal size distributions [76], the question of the physical and topological properties of supramolecular structures does not allow us to carry out a quantitative analysis of their content in solution. Nevertheless, we can trace the change in the contribution of various species to light scattering with a change in the external parameters of the medium, such as concentration, temperature, etc. From Table 1 it follows that the size of the fast mode R f , obtained for aqueous solutions, exceed the radii of macromolecules R h , determined in organic solvents. On the one hand, this may indicate increased size of macromolecules caused by swelling of PiPrOx chains in water, which apparently occurs in the case of brush 1. On the other hand, with a loose attachment of side chains, it is more likely that even far from the phase separation temperatures, the macromolecules form small supramolecular structures. As was shown above, the degree of shielding of the main chain by side chains is low for the studied brushes, especially in copolymers 2 and 3, which gives preconditions for hydrophobic interactions of their main chains and hence their intermolecular aggregation.

Thermoresponsiveness of APE O -g-PiPrOx in Water While Heated
Upon heating APE O -g-PiPrOx aqueous solutions, a significant increase in I and, respectively, a decrease in optical transmission I* are observed, indicating a phase separation (Figure 7). For copolymer 1, the increase in I was quite sharp. However, a smooth appearance in I/I 0 (T) and I*/I* 0 (T) dependencies, where I 0 and I* 0 are initial scattering intensity and optical transmission at T = 15 • C, respectively, for brush solutions 2 and 3 were obtained. Similarly, a smooth augmentation in I was noted in the study of the PEtOx graft copolymer synthesized on a flexible-chain aromatic macroinitiator of a similar structure APE 6 -g-PEtOx containing an alkylene spacer [60,61]. The temperature T 1 was determined from the beginning of the drop in the optical transmission I* (Figure 7).
The hydrodynamic dimensions of dissolved species of both types, R f and R s , in solutions of all samples increased upon heating (Figures 8a and 9a), augmentation beginning in the vicinity of T 1 . The increase in size takes place due to attaching of macromolecules to aggregates and to coupling of existing supramolecular structures. Thus, the fast mode in the sample 1 aqueous solutions transforms from macromolecules to small aggregates. The aggregate formation upon heating is conditioned by the decrease of PiPrOx solubility due to dehydration of 2-isopropyloxazoline units, which begins already at T ≈ 20 • C [63]. Note that the relative rate of change in the size of small aggregates R f is noticeably higher than the rate of increase in R s . Thus, R f increases almost 20 times when heated to T ≈ 80 • C, while R s changes by approximately 30%. Accordingly, the contribution of large aggregates to light scattering S s at temperatures above T 1 decreases, while S f increases (Figures 8b and 9b). tion (Figure 7). For copolymer 1, the increase in I was quite sharp. However, a smooth appearance in I/I0(T) and I*/I*0(T) dependencies, where I0 and I*0 are initial scattering intensity and optical transmission at T = 15 °C, respectively, for brush solutions 2 and 3 were obtained. Similarly, a smooth augmentation in I was noted in the study of the PEtOx graft copolymer synthesized on a flexible-chain aromatic macroinitiator of a similar structure APE6-g-PEtOx containing an alkylene spacer [60,61]. The temperature T1 was determined from the beginning of the drop in the optical transmission I* (Figure 7). The hydrodynamic dimensions of dissolved species of both types, Rf and Rs, in solutions of all samples increased upon heating (Figures 8a and 9a), augmentation beginning in the vicinity of T1. The increase in size takes place due to attaching of macromolecules to aggregates and to coupling of existing supramolecular structures. Thus, the fast mode in the sample 1 aqueous solutions transforms from macromolecules to small aggregates. The aggregate formation upon heating is conditioned by the decrease of PiPrOx solubility due to dehydration of 2-isopropyloxazoline units, which begins already at T ≈ 20 °C [63].
Note that the relative rate of change in the size of small aggregates Rf is noticeably higher than the rate of increase in Rs. Thus, Rf increases almost 20 times when heated to T ≈ 80 °C, while Rs changes by approximately 30%. Accordingly, the contribution of large aggregates to light scattering Ss at temperatures above T1 decreases, while Sf increases (Figures  8b and 9b).

Concentration Dependence of the Phase Separation Temperatures
The T1 values obtained for APEO-g-PiPrOx at various concentrations are generally in the same region (Figure 10a,b). The T1 for brush 1 with shorter side chains and higher z is practically independent of concentration, and the concentration-averaged temperature of the phase transition is <T1> = (30 ± 1) °C. As for brushes 2 and 3, despite the rather large scatter of experimental points, they fit into the general dependence T1(c), demonstrating a tendency for T1 to decrease from 35 to 24 °C with an increase in c within a more than 10fold concentration range. This trend is consistent with the previously obtained dependence for PiPrOx molecular brushes with flexible and rigid main chains [62,65]. In this case, in general, the region of the onset of phase separation T1 for the studied APEO-g-PiPrOx lies above the entire corresponding temperature range 20-25 °C for the phase separation of graft copolymer with a flexible backbone APE8-g-PiPrOx [62].

Concentration Dependence of the Phase Separation Temperatures
The T 1 values obtained for APE O -g-PiPrOx at various concentrations are generally in the same region (Figure 10a,b). The T 1 for brush 1 with shorter side chains and higher z is practically independent of concentration, and the concentration-averaged temperature of the phase transition is <T 1 > = (30 ± 1) • C. As for brushes 2 and 3, despite the rather large scatter of experimental points, they fit into the general dependence T 1 (c), demonstrating a tendency for T 1 to decrease from 35 to 24 • C with an increase in c within a more than 10-fold concentration range. This trend is consistent with the previously obtained dependence for PiPrOx molecular brushes with flexible and rigid main chains [62,65]. In this case, in general, the region of the onset of phase separation T 1 for the studied APE O -g-PiPrOx lies above the entire corresponding temperature range 20-25 • C for the phase separation of graft copolymer with a flexible backbone APE 8 -g-PiPrOx [62].
The above results show similar behavior of copolymers with longer side chains but lower z in solutions at room temperature and upon heating: the I/I 0 (c) profile, the composition and size of the scattering particles, and the phase separation temperatures as functions of concentration are similar for samples 2 and 3. This can undoubtedly be due to their similar molecular properties, which can be described using architectural parameters, such as z, L s , δ and so on. The above results show similar behavior of copolymers with longer side chains but lower z in solutions at room temperature and upon heating: the I/I0(c) profile, the composition and size of the scattering particles, and the phase separation temperatures as functions of concentration are similar for samples 2 and 3. This can undoubtedly be due to their similar molecular properties, which can be described using architectural parameters, such as z, Ls, δ and so on.

Discussion and Conclusions
The studied APEO-g-PiPrOx copolymers with blocks that differ in affinity to the solvents, at low z exist in the regime of strong intra-and intermolecular interactions. For side chains of different length and similar backbone, their behavior can be controlled by the parameter δ, which is a parameter to compare the distance between the grafting points on the macromolecule backbone (and hence, z) and the model dimensions of the side chains, taking into account the size and rigidity of their structural units. The studied copolymers exhibiting similar molecular and thermoresponsive properties, namely samples 2 and 3, are characterized by a close value of δ, which differs by about 1.5 times from the δ value for sample 1 ( Table 2). Due to the ability of the semirigid APEO to change the conformation, the main chain is always folded to avoid the solvent and is surrounded by swollen side chains. Schematically, the considered macromolecules with the same length of the main chain but different side chain length and different low grafting degree can be represented as star-shaped structures, with a core of collapsed APEO chain, and loosely grafted PiPrOx arms which are in a coiled conformation. In the case of sample 1, where the degree

Discussion and Conclusions
The studied APE O -g-PiPrOx copolymers with blocks that differ in affinity to the solvents, at low z exist in the regime of strong intra-and intermolecular interactions. For side chains of different length and similar backbone, their behavior can be controlled by the parameter δ, which is a parameter to compare the distance between the grafting points on the macromolecule backbone (and hence, z) and the model dimensions of the side chains, taking into account the size and rigidity of their structural units. The studied copolymers exhibiting similar molecular and thermoresponsive properties, namely samples 2 and 3, are characterized by a close value of δ, which differs by about 1.5 times from the δ value for sample 1 ( Table 2). Due to the ability of the semirigid APE O to change the conformation, the main chain is always folded to avoid the solvent and is surrounded by swollen side chains. Schematically, the considered macromolecules with the same length of the main chain but different side chain length and different low grafting degree can be represented as star-shaped structures, with a core of collapsed APE O chain, and loosely grafted PiPrOx arms which are in a coiled conformation. In the case of sample 1, where the degree of shielding is δ = 0.21, the macromolecules are soluble, similarly to that for APE 8 -g-PiPrOx with flexible backbone [62] (Figure 11a). At δ ≤ 0.14 (samples 2 and 3), molecular coils of a soluble PiPrOx do not shield the core sufficiently, which creates the preconditions for the aggregation of copolymers due to hydrophobic interactions of the main chains (Figure 11b). For comparison, for the brush APE 8 -g-PiPrOx with long flexible -(CH 2 ) 8 -spacer in the main chain and rather short PiPrOx arms (N s = 37), the calculated value of δ = 0.18 is close to that of sample 1 (Table 3).
of shielding is δ = 0.21, the macromolecules are soluble, similarly to that for APE8-g-PiPrOx with flexible backbone [62] (Figure 11a). At δ ≤ 0.14 (samples 2 and 3), molecular coils of a soluble PiPrOx do not shield the core sufficiently, which creates the preconditions for the aggregation of copolymers due to hydrophobic interactions of the main chains ( Figure  11b). For comparison, for the brush APE8-g-PiPrOx with long flexible -(CH2)8-spacer in the main chain and rather short PiPrOx arms (Ns = 37), the calculated value of δ = 0.18 is close to that of sample 1 (Table 3).  Since we determined the transition temperatures T1 over a wide, more than 10-fold concentration range from 0.0012 to 0.016 g/cm 3 (see Figure 10), we attempted to estimate the LCST for the APEO-g-PiPrOx samples. For sample 1, whose solutions did not demonstrate a noticeable dependence T1(c), we can apparently assume LCST ≈ <T1> ≈ 30 °C. For sufficiently hydrophobic samples 2 and 3, the value of T1 decreases monotonically with increasing concentration. Judging by our observations of the dissolution of the samples, it is unlikely that, with a further increase in c above the measurement interval, the solubility of the polymer will improve, and hence T1 will increase. Therefore, LCST < 24 °C for samples 2 and 3 can be estimated. It should be understood that the LCST estimation performed for APEO-g-PiPrOx is rather rough, but it is helpful for comparing the thermosensitivity of polyoxazoline brushes with aromatic polyester backbones of different rigidities (Table  3). Table 3 represents parameters for PiPrOx grafted copolymers with APE main chain of different rigidity. Due to the high proportion of the hydrophobic component ω = M/Mw in the composition of APEO-g-PiPrOx, where M = Mw − MAPE (Taблe 3), its hydrophilichydrophobic balance in water solutions is shifted to the temperature below the LCST detected for linear PiPrOx. As discussed in the Introduction, the cloud points for PiPrOx were observed at 36 < T < 63 °C depending on concentration, molar mass and architecture [40,41,64]. For block copolymers that are identical in chemical structure, but with a difference in architectural parameters, at close values of δ, a similar behavior of the APEO copolymers upon heating is observed; at higher δ, the LCST of the polymer is higher. However, upon transition to a rigid aromatic polyester main chain, the thermoresponsiveness range is significantly lower, than it is for APEO-g-PiPrOx at close structural parameters. Thus, the ability of the main chain of the grafted block copolymer to undergo intramolecular transformations plays a significant role in the formation of its properties in solutions at room temperature and upon heating.  Since we determined the transition temperatures T 1 over a wide, more than 10-fold concentration range from 0.0012 to 0.016 g/cm 3 (see Figure 10), we attempted to estimate the LCST for the APE O -g-PiPrOx samples. For sample 1, whose solutions did not demonstrate a noticeable dependence T 1 (c), we can apparently assume LCST ≈ <T 1 > ≈ 30 • C. For sufficiently hydrophobic samples 2 and 3, the value of T 1 decreases monotonically with increasing concentration. Judging by our observations of the dissolution of the samples, it is unlikely that, with a further increase in c above the measurement interval, the solubility of the polymer will improve, and hence T 1 will increase. Therefore, LCST < 24 • C for samples 2 and 3 can be estimated. It should be understood that the LCST estimation performed for APE O -g-PiPrOx is rather rough, but it is helpful for comparing the thermosensitivity of polyoxazoline brushes with aromatic polyester backbones of different rigidities (Table 3). Table 3 represents parameters for PiPrOx grafted copolymers with APE main chain of different rigidity. Due to the high proportion of the hydrophobic component ω = M/M w in the composition of APE O -g-PiPrOx, where M = M w − M APE (Taблe 3), its hydrophilichydrophobic balance in water solutions is shifted to the temperature below the LCST detected for linear PiPrOx. As discussed in the Introduction, the cloud points for PiPrOx were observed at 36 < T < 63 • C depending on concentration, molar mass and architecture [40,41,64]. For block copolymers that are identical in chemical structure, but with a difference in architectural parameters, at close values of δ, a similar behavior of the APE O copolymers upon heating is observed; at higher δ, the LCST of the polymer is higher. However, upon transition to a rigid aromatic polyester main chain, the thermoresponsiveness range is significantly lower, than it is for APE O -g-PiPrOx at close structural parameters. Thus, the ability of the main chain of the grafted block copolymer to undergo intramolecular transformations plays a significant role in the formation of its properties in solutions at room temperature and upon heating.