*2.8. Methods*

1H NMR spectra were recorded on Bruker AC-400 (400.1 MHz), with the signals of the solvent (DMSO-d6, CDCl3) as the reference.

IR spectra were recorded using a Shimadzu IR Affinity-1S spectrophotometer in the ATR mode (multiple attenuated total internal reflection) with a 4 cm−<sup>1</sup> resolution and 30 scans.

Molecular weights and dispersities (Ð) of the samples were estimated by size exclusion chromatography (SEC) on Agilent-1260 Infinity fitted with differential refractive index (RID), light scattering (LS), and viscometry (VS) detectors equipped with two Agilent PLgel MIXED-C columns (7.5 × 300 mm, 5 μm), 1 × 43 PLgel 5 M guard column (50 × 7.5 mm), and autosampler. The analysis was carried out at 50 ◦C using 0.1 M LiBr in DMF as eluent at a flow rate of 1.0 mL min−1. The salt was added to suppress the aggregation of macromolecules. For triple detection analysis the system was calibrated using PMMA standard with Mp = 2.16 × 103 g mol−<sup>1</sup> with dn/dc=0.052. Before analysis, all samples were passed through a 0.45 μm Nylon filters. SEC data were analyzed using Agilent GPC/SEC Software version 1.2.

Thermogravimetric analysis (TGA) of obtained copolymers was performed using a TG 209 F1 Libra analyzer (NETZSCH, Germany) in the temperature range of 25–800 ◦C and heating rate of 10 deg min–1 under nitrogen. The weight of samples was 2–3 mg.

Melting temperatures and enthalpies of synthesized copolymers were determined by differential scanning calorimetry (DSC) using a DSC 204 F1 Phoenix device (NETZSCH, Germany) at a heating rate of 10 deg min–1 in the range from −30 to +120 ◦C under nitrogen. The weight of samples was 2.5–3.5 mg.

The wide-angle X-ray scattering (WAXS) was carried out on an upgraded DRON 2.0 diffractometer (Saint-Petersburg, Russia) with CuKα radiation with a 0.154 nm wavelength.

#### **3. Results and Discussion**

PCL blocks were introduced into copolyimides by two fundamentally different ways. In the first case, to obtain grafted copolyimides, a precursor molecular brush with poly(2 hydroxyethyl methacrylate) (PHEMA) side chains was synthesized. Hydroxyl groups located in each repeating unit of the side chains of such brushes were used as initiators for carrying out ROP of CL. As a result the "brush on brush" structures PI-*g*-(PHEMA-*g*-PCL) were obtained. The second approach was aimed at obtaining graft copolymers with PCL side chains directly attached to the PI backbone. For this, a combination of ROP and CuAAC was used in two different variations.

#### *3.1. Synthesis of Multifunctional Polyimide Initiators*

The synthesis of multifunctional polyimide initiators with initiating OH-groups PI1 and 2-bromoisobutyrate groups PI2 in each repeating unit (Figure 1) based on soluble polyimide obtained by polycondensation of 4,4'-(1,3-phenylene-dioxy)bisphthalic anhydride and 2,4-diaminophenol was carried out according to a previously developed method [29]. The synthesis of polyimide was carried out according to the standard two-stage scheme with the preparation of polyamido acid (PAA) at the first stage and its subsequent thermal imidization in solution. During the synthesis of PAA, the equimolar ratio of monomers was strictly observed. The presence of a signal at 10.3 ppm in the 1H NMR spectrum of the product of thermal imidization of PAA indicated the presence of phenolic groups in PI1 (Figure 2). To obtain initiator PI2, samples of initiator PI1 were used as precursors, and treated with 2-bromoisobutyryl bromide under conditions that ensure almost complete esterification of the hydroxyl groups of PI1 [29]. The degree of functionalization by initiating 2-bromo-isobutyrate groups of PI2 initiator intended to perform ATRP, was 97 ÷ 99 mol%

according to the 1H NMR spectroscopy data, i.e., virtually every repeating unit contained an initiating group.

**Figure 1.** Multifunctional polyimide macroinitiators.

In the IR spectrum of PI1 (Figure 3), in addition to the characteristic bands of symmetric and antisymmetric vibrations of C=O groups of imide rings at 1780 and 1720 cm<sup>−</sup>1, respectively, there was an absorption band in the region of 1660 cm<sup>−</sup>1, which belongs to the vibrations of OH-phenol groups. The absence of the band at 1680 cm−<sup>1</sup> indicates an almost complete conversion of *o*-carboxyamide groups of PAA to imide rings. It should also be noted that there are no absorption bands in the region of 1620 and 935 cm<sup>−</sup>1, the presence of which would indicate the development of benzoxazole rings during the high-temperature reaction with the participation of phenol and imide groups [31].

It should be noted that in an attempt to obtain initiator similar to PI1 but with a hydroxymethyl group in each repeating unit, during the thermal imidization of PAA based on (3,5-diaminophenyl)methyl alcohol an irreversible gelation rapidly develops at 170–180 ◦C in the N-MP solution which leads to a product insoluble in amide solvents. Apparently, interlink and/or interchain interactions of a more mobile and reactive (than phenol group) hydroxymethyl group leads to the formation of insoluble supramolecular structures due to hydrogen bonds.

**Figure 3.** IR spectrum of macroinitiator PI1.

#### *3.2. Synthesis of Brush-on-Brush Type Copolyimides*

Previously, we reported [26], that polymerization of CL on PI macroinitiator PI2 (Figure 4) with phenol-type initiation groups practically does not proceed and the main polymerization product is homopolymer PCL. So, the introduction of PCL blocks into the side chains was carried out by performing ROP at the hydroxyethyl groups of the side chains of the polyimide brush with poly(2-hydroxyethyl methacrylate) side chains PI-*g*-PHEMA, which was used as a branched multifunctional macroinitiator. To obtain it, ATRP of 2-hydroxyethyl methacrylate was carried out on macroinitiator PI2. Table 1 shows the ATRP conditions and the monomer conversion values determined gravimetrically.

The formation of the copolymer PI-*g*-PHEMA was confirmed by the shift of the signals of the methyl protons of 2-bromo-isobutyrate groups from the 1.9 ppm to the 1.5 ppm in the 1H NMR spectrum of the ATRP products (Figure 5) and the appearance of signals of methylene protons of the –CH2–OH group at the 3.8 ppm while maintaining the signals of aromatic protons in the range of 6.0–8.5 ppm, related to the polyimide backbone. The intensities of the peaks from the PI backbone decreased due to its lower content in the final molecular brush.

In the IR spectrum (Figure 6) of the HEMA polymerization product, in comparison with the spectrum of the macroinitiator, absorption bands of OH groups at 3500 cm−1, of methylene groups at 2950 cm−<sup>1</sup> belonging to the PHEMA chains appeared, and the band at 1765 cm−<sup>1</sup> disappeared, which refers to the vibrations of an ester group with an electronegative substituent (2-Br-substituent) in the α-position.

The structure of the obtained copolymers was proved by the complex use of 1H NMR spectroscopy and SEC. In the 1H NMR spectrum (Figure 5) of the CL polymerization product, signals of methylene protons –OCH2– and –COCH2– groups of PCL chains were observed at 4.1 and 2.3 ppm, respectively.

**Figure 4.** Synthesis of PI-*g*-PHEMA and PI-*g*-(PHEMA-*g*-PCL).

**Table 1.** Synthesis of PI-*g*-PHEMA by ATRP on macroinitiator PI2 in DMF solution.


\*–molar ratio of the components in the reaction system, α–conversion of the monomer.

**Figure 5.** 1H NMR spectra of macroinitiator PI2 (**a**), PI-*g*-PHEMA (**b**) and PI-*g*-(PHEMA-*g*-PCL) (**c**).

**Figure 6.** IR spectra of macroinitiator PI2, PI-*g*-PHEMA and PI-*g*-(PHEMA-*g*-PCL).

Sequential offset of the MWD curves in the macroinitiator PI2, PI-*g*-PHEMA and PI-*g*-(PHEMA-*g*-PCL) series to the high MM values (Figure 7) and the presence of only one peak for PI-*g*-(PHEMA-*g*-PCL) indicates the grafting of PCL side chains to the PHEMA chains. According to the MW data corresponding to this series of polymers (Table 2), when passing from PI to the "brush on brush" structure, the MW increases by almost four times. A significant decrease in the value of the increment of the refractive index dn/dc in the series obviously reflects an increase in the ratio of aliphatic units in the copolymer.

**Figure 7.** MWD curves of macroinitiator PI2, PI-*g*-PHEMA, and PI-*g*-(PHEMA-*g*-PCL).

**Table 2.** MW characteristics of macroinitiator PI2, PI-*g*-PHEMA and PI-*g*-(PHEMA-*g*-PCL).


#### *3.3. Synthesis of Molecular Brushes PI-g-PCL with a Polyimide (PI) Backbone and Polycaprolactone (PCL) Side Chains Using a Combination of ROP and CuAAC*

To obtain the targeted copolymers PI-*g*-PCL, the following synthesis scheme was proposed, which includes two routes (Figure 8).

**Figure 8.** Routes for the synthesis of grafted copolyimides with PCL side chains by ROP and CuAAC.

3.3.1. Synthesis of PI with Azide Groups

At the first stage, samples of prepolymer PI3 (Figure 8) with azide functional groups were obtained. For this, polyimide macroinitiator PI2 (Figure 8) with 2-bromo-isobutyrate groups was modified under the action of sodium azide NaN3. Azidating agent was taken in 4 ÷ 10-fold excess. The reaction proceeded at room temperature for two hours. According to the 1H NMR spectra (Figure 9), functionalization with sodium azide proceeded completely, as a result of which the signal at 1.9 ppm shifted to the 1.35 ppm. The degree of functionalization was determined by the content of 2-bromo-isobutyrate groups in the initial polyimide macroinitiator, which was 60%.

Table 3 presents the molecular weight characteristics of the initial PI2 macroinitiator with 2-bromo-isobutyrate groups and the obtained PI3 macroinitiators with azide groups.

During the isolation of the azidation product, the polymer is apparently fractionated, which leads to a decrease in the dispersity and MW of the obtained macroinitiators PI3.

**Table 3.** MW characteristics of PI macroinitiators.


**Figure 9.** 1H NMR spectra of macroinitiator PI2 (**a**) and macroinitiator PI3 (**b**).

3.3.2. Synthesis of PI with Hydroxyl Side Groups Attached to the PI Backbone through the 1,2,3-Triazole Linker

To implement the first route of the synthesis, the obtained prepolymer PI3 (Figure 8) was introduced into reaction with the propargyl alcohol. As a result of a click reaction, initiating ROP hydroxyl groups separated from the benzene ring of the backbone by a triazole ring (macroinitiator PI4, Figure 8) were introduced into the initiator. The reaction was carried out at a 50 ◦C for 2 hours in an N-MP using a 1.5-fold excess of propargyl alcohol and a CuCl/PMDETA catalytic system. Figure 10 shows the IR spectra of macroinitiators PI3 and PI4 (Figure 8). In the IR spectrum of the macroinitiator PI4, in addition to the characteristic absorption bands of imide rings (at 1370 cm–1 and a doublet at 1776 cm–1 and 1717 cm–1), there is a peak at 2300 cm–1, which refers to the stretching vibrations of the ethynyl group (–C≡CH) (Figure 10).

**Figure 10.** IR spectra of PI macroinitiators PI3 and PI4.

The resulting polymer PI4, according to the SEC data, had the following characteristics: *Mn* = 34 × <sup>10</sup><sup>−</sup>3, *<sup>Ð</sup>* = 1.2.

3.3.3. Synthesis of Molecular Brushes with a PI Backbone and PCL Side Chains by Polymerization of ε-Caprolactone on a Multicenter Macroinitiator PI4

The resulting macroinitiator PI4 (Figure 8) was used to carry out ROP of CL in bulk (initiator/monomer molar ratio = 1/1000) at 130 ◦C in an inert atmosphere in the presence of tin (II) octanoate as a catalyst. The 1H NMR spectrum of the obtained polymer (Figure 11) contains signals of methylene protons of the –OCH2– and –COCH2– groups at 4.0–4.1 ppm and 2.2–2.3 ppm and the middle methylene groups of the PCL side chains at 1.6–1.7 ppm and 1.35–1.45 ppm. In the spectrum of the polymerization product, signals of aromatic protons are not visible, due to the fact that in chloroform the polyimide is completely shielded by the PCL side chains.

**Figure 11.** 1H NMR spectrum of the polymerization product of CL on macroinitiator PI4.

Table 4 presents the polymerization conditions and MW characteristics of the obtained molecular brushes with PCL side chains.


**Table 4.** Polymerization conditions and MW characteristics of the PI-*g*-PCL.

Therefore, this approach allows to obtain graft copolymers with a PI backbone and relatively short PCL side chains, which is illustrated by the MWDs in Figure 12.

**Figure 12.** MWD curves of macroinitiators PI3 and PI4, and grafted copolyimide PI-*g*-PCL **5**.

3.3.4. Synthesis of Molecular Brushes with a PI Backbone and PCL Side Chains by CuAAC of Prepolymers

To implement the second route, a linear PCL (Figure 8) with a terminal triple bond was synthesized. Polymerization was carried out in bulk at 100 ◦C using propargyl alcohol as an initiator and tin (II) octanoate as a catalyst. The 1H NMR spectrum of the polymerization product (Figure 13) contains all characteristic signals of PCL: signals of methylene protons of the –OCH2– and –COCH2– groups at the 4.0–4.1 ppm and 2.2–2.3 ppm and the middle methylene groups of the PCL chains at 1.6–1.7 ppm and 1.35–1.45 ppm. Moreover, clear signal at the 4.3 ppm, related to the protons of the –C≡CH end groups is visible in the 1H NMR spectrum (Figure 13).

**Figure 13.** 1H NMR spectrum of linear PCL with pendant –C≡CH groups.

Table 5 presents the polymerization conditions and MW characteristics of the obtained linear PCL with the –C≡CH end groups.

**Table 5.** Polymerization conditions and MW characteristics of the linear PCL.


Azide-alkyne cycloaddition was carried out between linear PCL and macroinitiator PI3 (Figure 8), as a result of which the targeted copolyimides PI-*g*-PCL were also obtained. The click-reaction was carried out in an N–MP at 70 ◦C for 24 h, using CuCl/PMDETA as the catalytic complex. The molar ratio of the reagents was PI3/PCL/CuCl/PMDETA = 1/2/1/1.

Table 6 presents the MW characteristics of the products of click-reaction between linear PCL and macroinitiator with azide groups PI3. It can be seen that the implementation of the second approach leads to a more significant increase in molecular weight (Table 6), but unreacted linear PCL remains in the system. It is found in the form of a second peak in the chromatogram in the lower molecular weight region and its MW completely coincides with the mass of the previously analyzed linear PCL with alkyne end groups.

**Table 6.** MW characteristics of the click-reaction products.


#### *3.4. TGA, DSC and X-ray Study of the Synthesized Copolymers*

Table 7 and Figure 14 presents the data on the thermal stability of the PI macroinitiator, PCL macromonomer, and linear and grafted copolyimides based on it. All copolymers lost most of the weight before 400 ◦C. But they are mostly stable till 275 ◦C, while linear PCL start to decompose already at 210 ◦C. The residual mass depends on the content of PI block in the copolymer. While linear PCL retains less than 1% of mass after 800 ◦C, molecular brush with long PCL side chains (PI-*g*-PCL 1)–2%, and molecular brush with short PCL side chains (PI-*g*-PCL 2)–15%.

Melting temperatures (Tm) and enthalpy (ΔHm) of copolymers determined by the DSC method are presented in Table 7. The DSC curves of synthesized copolymers are presented with the endothermic melting peaks (Figure 15). All copolymers melt in the range from 52 to 58 ◦C and the melting enthalpy increases with the increase of PI content in the copolymer.


**Table 7.** Thermal properties of synthesized copolymers.

**Figure 14.** TGA curves for the synthesized macroinitiators, macromonomer, and copolymers.

**Figure 15.** DSC curves for the synthesized macromonomer and copolymers.

Comparison of diffractograms of linear PCL with films of its linear and grafted copolymers showed that they are practically symbate (Figure 16). Consequently, the introduction of a polyimide block into the polycaprolactone matrix practically did not lead to a change in the degree of crystallinity of PCL. Apparently, the length of the PI blocks between PCL blocks is not enough for the PI to form the amorphous areas in the copolymer, so they're ordered inside PCL crystalline structure.

**Figure 16.** Diffractograms of linear PCL, triblock-copolymer PCL-*b*-PI-*b*-PCL, and grafted copolyimide PI-*g*-PCL.

#### **4. Conclusions**

Multicomponent copolymers, in which PI blocks are combined with blocks of biocompatible aliphatic polymers, have great potential for the development of materials for tissue engineering. The introduction of PCL blocks into multiblock copolymers is of considerable interest from the point of view of further applications of these copolymers since PCL blocks are capable of undergoing alkaline and plasma etching and biodegradation. The synthesis of new initiators and the ROP of monomers and macromonomers containing functional groups on such initiators is a promising strategy for the preparation of macromolecules of complex architecture.

In this work we suggested an approach to the synthesis of the novel polymer molecular brushes with a polyimide (PI) backbone and poly(ε-caprolactone) (PCL) side chains. Using combination of polycondensation, ATRP, ROP, and CuAAC – targeted grafted copolyimides PI-*g*-PCL were synthesized as well as more complex "brush on brush" structures PI-*g*- (PHEMA-*g*-PCL). Comparison of different combinations of ROP and CuAAC showed that polymer-analogous transformations of a multicenter PI macroinitiator with an initiating hydroxyl group separated from the main chain by a triazole ring and carrying out ROP on it allows obtaining graft copolymers with a PI backbone and relatively shorter PCL side chains. While a separate synthesis of macromonomers with the desirable functional groups (polyimide with azide groups and PCL with terminal alkyne groups), followed by a click reaction leads to a more significant increase in molecular weight, but unreacted linear PCL remains in the system. X-ray scattering data showed that the introduction of a polyimide block into the polycaprolactone matrix practically did not change the degree of crystallinity of PCL.

**Author Contributions:** Conceptualization, T.K.M. and A.V.Y.; methodology, A.V.K. and T.K.M.; validation, A.V.K.; formal analysis, A.V.K.; investigation, A.V.K., N.N.B. and V.K.L.; writing—original draft preparation, A.V.K.; writing—review and editing, A.V.Y.; visualization, A.V.K.; supervision, T.K.M. and A.V.Y.; funding acquisition, A.V.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** We acknowledge funding from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2020-794).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

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