6. Synthesis of star-shaped polyimides with narrow molecular weight distribution

Star-shaped polymers (SSP) are of special interest among a variety of branching polymers. The state of the art in a field of SSP is considered in several excellent reviews [30]. In many papers, SSP with narrow MWR was synthesized using the methods of controlled chain polymerization. Yokozawa et al. [31]were the first to show that the "step growth" processes also can be applied to obtain SSP with narrow MWD. They synthesized SSP using polycondensation by the scheme (Bn + AB), where Bn is a multifunctional core initiator and AB is a low reactive monomer with two different functional groups A and B.

Narrow MWD can be achieved by using special AB' monomers, which are not able for auto-polycondensation, but able to react to active B groups of Bn core initiator. When AB' moiety attaches to Bn, terminal B<sup>0</sup> group becomes much more reactive due to the changing mesomeric effect in the course of condensation, and selective arm growth occurs.

To synthesize star-shaped oligoimides (SOI), we have used known general reaction scheme B3(B4) + AB, but another principle providing selective star arm growth not requiring special monomers AB [32, 33]. 1,3,5-(4-Aminophenoxy)toluene (TAPT) was used as B3, bis(3,5-diaminophenoxy)benzene (BAPB) as B4, and 3-APPA as latent AB monomer.

In the course of synthesis, APPA was introduced slowly to the reaction mixture containing TAPT and BAPB, providing that the current AB concentration in molten BA always being much less than that of TAPT or BAPB. The reaction scheme is given below (Figure 18). To check the possibility of obtaining stars with different average arm lengths, the overall 3-APPA/TAPT ratio was varied in a row: 10:1, 20:1, 40:1, and 100:1.

In IR spectra of SOI samples, characteristic absorption bands appear at imide cycle at 1720 and 1780 cm�<sup>1</sup> (symmetric and asymmetric C=O vibrations). With increase in 3-APPA/TAPT ratio, the intensity of N-H vibrations in NMR <sup>1</sup> H of the model compound obtained by treatment TAPT with excess of phthalic anhydride (TAPT-PhA) of terminal NH2 group at 3380–3480 cm�<sup>1</sup> decreases.

Comparison of the NMR <sup>1</sup> H spectra of TAPT (Figure 19) and the model compound TAPT-PhA, obtained by treatment of TAPT with phthalic anhydride (PhA) (Figure 19), shows that the replacement of the amino group on phthalimide group results in disappearance of the signal of the amino group protons (5.0 ppm) and the signal shift (from 5.8 to 6.6 ppm) of c, d, e, and f protons (Figure 20) belonging to central aromatic ring. There are new signals in the region of 8.0 ppm, referred to the protons "u, z" of the phthalimide fragment. Similar changes are observed in the spectra of SOI: c, d, e, and f signals are shifted download. From the comparison of NMR spectra of SOI 10 and SOI 40 (Figure 19), it is seen that the intensity of proton signals of terminal amino groups "p" at 5.4 ppm and methyl group of the central fragment (2.08 ppm) decreases with an increase in the APPA/TAPT ratio. Weak signals at 6.2–6.5 ppm are referred to aromatic protons "k, l, n" of the terminal fragment containing amino group. The structure of SOI is confirmed by the results of integration of NMR <sup>1</sup> H signals.

spectra of the starting B4 monomer (Figure 16a, spectrum 1), there are signals of protons of amino groups at 4.8 ppm, two signals of protons of aromatic fragments nearest to amino group at 5.58 and 5.42, and one signal of protons of central aromatic cycle at 6.9 ppm. In a spectrum of HB PI (Figure 16b, spectra 2), the signals of amino groups are shifted to 5.4 and 5.8 ppm due to appearance of imide fragments. Also, new signals of aromatic protons appear at 7–8 ppm belonging to

With the presence of reactive terminal amino groups in HB PI, we carried out its reaction with acetic or phthalic anhydrides (at 25°C in dimethylacetamide and in molten BA, respectively). In IR spectra of the product isolated after treatment with acetic anhydride (HB PIac) (Figure 16a, curve 3), an absorption band at 3300– 3500 cm<sup>1</sup> (NH2 group) disappears, and new bands at 1680 and 1550 cm<sup>1</sup> (defor-

5.8 ppm (NH2) disappear, and new signals at 10.0 and 10.2 ppm appear related to two isomeric "linear" fragments of HB PIac (Figure 17). From the fact of equal intensity of these signals, it should be concluded that reactivity of all amino groups

This observation is of importance because on its basis it can be concluded that all four amino groups of B4 monomer participate in chain growth with nearly the same probability, i.e., the scheme of reaction indeed is A2 + B4 and must result in HB

The one-pot high-temperature catalytic polycyclocondensation in molten BA was successfully used to obtain homo- and co-PEIs from 4-(3-aminophenoxy)-phthalic acid (3-APPA) which has a structure of latent "AB" monomer [29]. In solid state, 3- APPA exists in a zwitterionic form. In molten BA at 140°C, it transfers to the "open" form which is able to dehydrate to give monomer with amino and anhydride groups. It is able for auto-polycondensation with moderate rate. Homo-PEI prepared from 3- APPA in molten BA is high molecular weight amorphous thermoplastic PEI with a glass temperature of 210–220°C soluble in chloroform and DMSO. The yield and degree of imidization are close to 100%. In contrast, homo-PEI prepared at the same conditions from isomeric 4-(4-aminophenoxy) phthalic acid (4-APPA) is insoluble and intractable till 400°C. Properties of copolyimides 3-APPA/4-APPA depend on composition. When the 3-APPA/4-APPA ratio in staring monomer mixture is 30:70 or less, the polymer is amorphous and soluble in organic solvents. At 3-APPA/4-

H spectrum of HB PIac (Figure 16b, spectra 3), the signals at 5.4 and

protons of aromatic nuclei of dianhydride fragment as well as signals of

mation N-C vibrations, "amide 1" and "amide 2") appear.

5. Synthesis of polyimides from "AB" monomers

APPA ratios of 40:60 or higher, CPIs lose solubility and tractability.

isopropylidene fragment at 1.67 ppm.

Isomeric "linear" fragments in hyperbranched polyimide.

Solvents, Ionic Liquids and Solvent Effects

In NMR <sup>1</sup>

Figure 17.

is about the same.

polymer.

56

Figure 18. Synthesis of 3-arm star-shaped polyimide.

All SOI samples obtained are soluble in chloroform. For samples SOI40 and SOI 100, mechanically strong self-supporting films were obtained by casting from a solution in chloroform. According to DSC, increasing the APPA/TAPT ratio leads to

GPC chromatogram of SOI prepared at different APPA/TAPT mole ratios: 10/1 (1), 20/1 (2), 40/1 (3), and

Sample APPA/TAPT Mw Mn Mw/Mn Mw/Mn (without low MW fraction)

SOI 10 13,100 11,600 1.13 1.08 SOI 20 19,800 15,200 1.3 1.08 SOI 40 29,200 16,860 1.73 1.16 SOI 100 53,350 20,700 2.57 1.17 Poly APPA 24,400 9000 2.71 —

There is a small peak at low molecular weights in all chromatograms (Figure 21).

With increasing duration of synthesis, this peak does not change its position. Therefore, we refer it to low molecular cyclic oligomers. The Mw/Mn values calculated without this peak (Table 7, columns 5) do not differ much, in all cases Mw/Mn < 2. Formation of small quantity of cyclic oligomer can be the evidence of the low-rate by-reaction of AB auto-polycondensation to form linear oligomer with two complementary end groups A and B which can react to give cyclic oligomer. The effective reactivity of AB in auto-polycondensation and in the reaction with

terminal amino group of the growing star is thought to hardly differ from each other. The predominance of selective arm growth and narrow PDI is mainly due to the concentration of terminal arm's amino groups that is always greater than the

current concentration of AB monomer in reaction system.

In Figure 21, the GPC chromatograms of SOI samples are given. With an increase in the total ratio of 3-APPA/TAPT, the main peak on the GPC chromatogram shifts toward high molecular weights accomplished by small broadening of the peaks. All SOI samples have narrow MWD with Mw/Mn = 1.1–1.2 (Table 7), in which values are much less than the corresponding values for PI obtained by APPA auto-polycondensation in molten BA (Mw/Mn3) in the absence of additives. Small polydispersity is very unusual for polycondensation processes. It indicates that there is a selective growth of arms on the initiator molecules, similar to the process

an increase in the glass transition temperature.

Molecular weight characteristics of the oligoimides (GPC).

Synthesis of Polyimides in the Melt of Benzoic Acid DOI: http://dx.doi.org/10.5772/intechopen.87032

of "chain polycondensation" [30].

Figure 21.

100/1 (4).

Table 7.

59

#### Figure 19.

NMR <sup>1</sup> H spectra of TAPT (1) and TAPT-PA (2), SOI prepared at different 3-APPA/TAPT mole ratios: 10/1 (3) and 40/1 (4).

Figure 20.

Assignment of signals in NMR <sup>1</sup> H spectrum to corresponding hydrogen atoms in branching, linear and terminal.

Synthesis of Polyimides in the Melt of Benzoic Acid DOI: http://dx.doi.org/10.5772/intechopen.87032

#### Figure 21.

GPC chromatogram of SOI prepared at different APPA/TAPT mole ratios: 10/1 (1), 20/1 (2), 40/1 (3), and 100/1 (4).


#### Table 7.

Figure 18.

Figure 19. NMR <sup>1</sup>

Figure 20.

58

Assignment of signals in NMR <sup>1</sup>

(3) and 40/1 (4).

Synthesis of 3-arm star-shaped polyimide.

Solvents, Ionic Liquids and Solvent Effects

H spectra of TAPT (1) and TAPT-PA (2), SOI prepared at different 3-APPA/TAPT mole ratios: 10/1

H spectrum to corresponding hydrogen atoms in branching, linear and terminal.

Molecular weight characteristics of the oligoimides (GPC).

All SOI samples obtained are soluble in chloroform. For samples SOI40 and SOI 100, mechanically strong self-supporting films were obtained by casting from a solution in chloroform. According to DSC, increasing the APPA/TAPT ratio leads to an increase in the glass transition temperature.

In Figure 21, the GPC chromatograms of SOI samples are given. With an increase in the total ratio of 3-APPA/TAPT, the main peak on the GPC chromatogram shifts toward high molecular weights accomplished by small broadening of the peaks. All SOI samples have narrow MWD with Mw/Mn = 1.1–1.2 (Table 7), in which values are much less than the corresponding values for PI obtained by APPA auto-polycondensation in molten BA (Mw/Mn3) in the absence of additives. Small polydispersity is very unusual for polycondensation processes. It indicates that there is a selective growth of arms on the initiator molecules, similar to the process of "chain polycondensation" [30].

There is a small peak at low molecular weights in all chromatograms (Figure 21). With increasing duration of synthesis, this peak does not change its position. Therefore, we refer it to low molecular cyclic oligomers. The Mw/Mn values calculated without this peak (Table 7, columns 5) do not differ much, in all cases Mw/Mn < 2. Formation of small quantity of cyclic oligomer can be the evidence of the low-rate by-reaction of AB auto-polycondensation to form linear oligomer with two complementary end groups A and B which can react to give cyclic oligomer.

The effective reactivity of AB in auto-polycondensation and in the reaction with terminal amino group of the growing star is thought to hardly differ from each other. The predominance of selective arm growth and narrow PDI is mainly due to the concentration of terminal arm's amino groups that is always greater than the current concentration of AB monomer in reaction system.

nonreversible reaction. Elimination of both reversible stages in polyimide synthesis makes it possible to control chain microstructure of copolyimides by

4.Mathematical modeling of copolyimide chain microstructure formation in molten BA has been developed. Results of prediction of chain microstructure

The work is supported by the Russian Foundation of Basic Research, Grant #19-03-00820, and the Ministry of High Education and Science of Russian

Kuznetsov Alexander Alexeevich\* and Tsegelskaya Anna Yurievna Enikolopov Institute of Synthetic Polymer Materials RAS, Moscow,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: kuznets24@yandex.ru

provided the original work is properly cited.

on the basis of independent kinetic data are in good consistence with experimental values obtained from NMR 13C data of copolyimides.

means of varying intermonomer loading.

Synthesis of Polyimides in the Melt of Benzoic Acid DOI: http://dx.doi.org/10.5772/intechopen.87032

Acknowledgements

Federation.

Author details

Russian Federation

61

Figure 22. IR spectra (a) and NMR <sup>1</sup> H spectra (b) of SOI 10 (1) and SOI-ac10 (2).

Thus, it is shown that star-shaped oligo- and polyimides with narrow MWD (Mw/Mn = 1.1–1.2) can be synthesized by polycondensation under these conditions.

To confirm the presence of reactive amino groups in SOI molecule, the following experiments were performed. In Figure 22a, the IR spectrum is shown of acetyl derivative SOI-ac10 obtained by the treatment of SOI 10 with acetic anhydride. The reaction of SOI with acetic anhydride leads to the disappearance absorption band of the amino group at 3480 cm<sup>1</sup> and the appearance of a new band at 1538 cm<sup>1</sup> (amide II). In the NMR <sup>1</sup> H spectrum of the SOI-ac10 (Figure 22b), the signals at 5.4 ppm (NH2) and 6.3–6.4 ppm (aromatic protons of terminal moiety) disappear, and new signals appear at 10.1 ppm (▬NH▬C=O)and 2.0 ppm (CH3▬C=O), in full correspondence with our expectations.

Presently, two novel tetrafunctional amines and corresponding tetra-arm starshaped oligomers with Mw/Mn < 1.6 were also synthesized via B4 + AB scheme, where AB is 2-APPA and B4 is the product of direct condensation of di-N-BOCprotected 3,5-diaminobenzoic acid with aromatic diamines (ODA, AFL) [34] in the presence of triphenyl phosphate-pyridine.
