2. General features of the process

#### 2.1 Rate of the process and molecular weight

The process of synthesis of PIs in molten BA can be carried out under relatively mild conditions (140°C, 1–2 hour) at slow inert gas flow [11–13]. Within 1 hour after the start of synthesis, fully cyclized polyetherimides (PEIs) and PIs with logarithmic viscosity values ηlog = 0.4–1.2 dL\*g<sup>1</sup> (N-methyl-2-pyrrolidone (N-MP), 25°C), depending on the structure of the monomers, were isolated. In the IR spectrum of the products, there are typical absorption bands of the imide cycle in the regions of 720, 1370, 1720, and 1780 cm<sup>1</sup> ; no peaks of amic acid fragments (1660 cm<sup>1</sup> , 3600 cm<sup>1</sup> ) were observed. Imide peaks in IR spectrum did not change after additional heat treatment of polymers at 300°C for 0.5 hour; this allows us to conclude that during the synthesis, almost 100% conversion of imidization has been achieved. Molecular weight Mw = 5–15•10<sup>4</sup> (GPC) is sufficient for the subsequent processing of PIs into films, semifinished products, and bulk products by extrusion and injection molding or for use as binders for composite materials. The rate of PEI molecular weight increase, when synthesized in the BA melt at 140°C, is significantly higher than that in m-cresol at the same temperature.

The ability of carboxylic acids as additives to accelerate the process of one-pot synthesis of PIs was first demonstrated for synthesis of PIs with cardo fragments in nitrobenzene [14] at 160–210°C. However, it was noted that with an increase in the concentration of BA in a system above 2.5 mol BA per 1 mole of repeating unit (about 10%-weight solution in reaction mixture), the total rate of the process decreases—up to complete inhibition—probably due to the fact that the excess BA deactivates the amino group by the mechanism of acid–base interaction. In our works, we used concentration up to 95% in reaction mixture BA as a solvent. Under these conditions, the (BA/repeating unit) mole ratio is about 30–35. The absence of inhibition by excess BA can be explained by the fact that the melt of BA is a nonpolar liquid—by analogy with 100% acetic acid (AcOH) which has dielectric constant k6; therefore, the dissociation constant BA in its own melt can be significantly lower than that in nitrobenzene.

The general scheme of the process of obtaining PIs by high-temperature polycondensation of diamines and dianhydrides can be represented as follows (Figure 1). Figure 1 includes four conjugated reactions, including two main ones, acylation and

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

to be carried out in highly basic (amic) solvents at 20–40°C. In the case of one-stage process in high-boiling solvents, both reactions proceed simultaneously. In the process of one-stage high-temperature polycondensation (HTPC) of diamines and tetracarboxylic acids dianhydrides, the reactions of acylation and cyclization occur

PI synthesis in molten benzoic acid (BA) described in this review should be considered as a variant of the said PI synthesis by HTPC approach. The advantages of using molten BA compared to other high-boiling solvents used in this process (nitrobenzene, m-cresol, o-dichlorobenzene, etc.) are a strong catalytic effect, the lack of solvent toxicity, and easy isolation of polymer due to crystallization of solvent on cooling. In contrast to other novel ecologically improved ("green") solvents for polyimide synthesis such as ionic liquids [9] and supercritical CO2 [10],

The process of synthesis of PIs in molten BA can be carried out under relatively mild conditions (140°C, 1–2 hour) at slow inert gas flow [11–13]. Within 1 hour after the start of synthesis, fully cyclized polyetherimides (PEIs) and PIs with logarithmic viscosity values ηlog = 0.4–1.2 dL\*g<sup>1</sup> (N-methyl-2-pyrrolidone

(N-MP), 25°C), depending on the structure of the monomers, were isolated. In the IR spectrum of the products, there are typical absorption bands of the imide cycle in

after additional heat treatment of polymers at 300°C for 0.5 hour; this allows us to conclude that during the synthesis, almost 100% conversion of imidization has been achieved. Molecular weight Mw = 5–15•10<sup>4</sup> (GPC) is sufficient for the subsequent processing of PIs into films, semifinished products, and bulk products by extrusion and injection molding or for use as binders for composite materials. The rate of PEI molecular weight increase, when synthesized in the BA melt at 140°C, is signifi-

The ability of carboxylic acids as additives to accelerate the process of one-pot synthesis of PIs was first demonstrated for synthesis of PIs with cardo fragments in nitrobenzene [14] at 160–210°C. However, it was noted that with an increase in the concentration of BA in a system above 2.5 mol BA per 1 mole of repeating unit (about 10%-weight solution in reaction mixture), the total rate of the process decreases—up to complete inhibition—probably due to the fact that the excess BA deactivates the amino group by the mechanism of acid–base interaction. In our works, we used concentration up to 95% in reaction mixture BA as a solvent. Under these conditions, the (BA/repeating unit) mole ratio is about 30–35. The absence of inhibition by excess BA can be explained by the fact that the melt of BA is a nonpolar liquid—by analogy with 100% acetic acid (AcOH) which has dielectric constant k6; therefore, the dissociation constant BA in its own melt can be signif-

The general scheme of the process of obtaining PIs by high-temperature polycondensation of diamines and dianhydrides can be represented as follows (Figure 1). Figure 1 includes four conjugated reactions, including two main ones, acylation and

; no peaks of amic acid fragments

) were observed. Imide peaks in IR spectrum did not change

simultaneously in a high-boiling solvent at 180–210°C. The synthesis of PIs according to this scheme was first reported by Korshak, Vinogradova, and Vygodskii in 1967 [6–8]: the synthesis of so-called cardo-PIs. This approach has

this approach does not require any special chemicals and equipment.

found wide application.

Solvents, Ionic Liquids and Solvent Effects

2. General features of the process

2.1 Rate of the process and molecular weight

the regions of 720, 1370, 1720, and 1780 cm<sup>1</sup>

cantly higher than that in m-cresol at the same temperature.

, 3600 cm<sup>1</sup>

icantly lower than that in nitrobenzene.

(1660 cm<sup>1</sup>

44

Figure 1. Scheme of PIs obtaining by polycondensation of diamines and dianhydrides.

imidization, and two side reactions, binding of amino groups by acidic medium and hydrolysis of anhydride groups by water released during imidization. The degree of reversibility of each reaction depends on the temperature, acidity of solvent, and rate of water vapor removal. The replacement of dianhydride for corresponding tetracarboxylic acid does not influence on the overall rate and final molecular weight. This allows suggestion that dehyderatation of diphthalic acids is reversible and fast.

## 2.2 Leveling of monomer reactivity

In the course of experiments with different monomer pairs (Table 1), it was found that the chain growth rate in molten BA showed a rather weak dependence on the basicity of the diamines used [13, 15, 16]. This observation is in stark contrast to regularities of the low-temperature polycondensation in amic solvents in which the basicity of the diamines has a strong influence on the rate of polycondensation [1–3]. Such phenomenon of partial "reactivity leveling" in molten BA is of interest as a method of obtaining new high molecular weight copolyimides from low reactive diamines. In Table 2, the logarithmic viscosity (ηlog) values are given of polyimides synthesized in molten BA from dianhydrides and bridged aromatic diamines having different basicities expressed as pKb values.

It was observed that the change in basicity index of aromatic diamines in a range from pKb = 5.5 (ODA) to pKb = 2.5 (SDA) did not result in a significant difference in ηlog of final PIs (Table 2). It allows a conclusion that conversion of amino groups reached for 1.5 h was at least 90–95% in both cases. In other words, the difference in apparent reactivity of diamines of both the low and the high basicities is not so large. Such a behavior is quite different from the results reported for lowtemperature polycondensation of diamines and dianhydrides in amide solvents. For the latter reaction, changing the type of bridge substituent in diamines in a row ▬O▬; ▬CH2▬; ▬SO2▬ results in about three orders of magnitude decrease in the rate constant of polycondensation with pyromellitic dianhydride [1–3].

#### 2.3 Mechanism and kinetics

Initially, we suggested that low sensitivity of reaction rate to basicity of diamines is caused by interaction with acid medium, i.e., the higher the basicity of amino group, the higher its deactivation by acid medium. To check this supposition, we studied a character of interaction of different diamines with BA by the method of the phase diagrams.

In Figure 2a, b, the phase diagrams are shown of binary system BA-diamine constructed on the basis of DSC thermograms for BA-diamine mixtures of different compositions [15]. It is seen that highly basic 1,12-dodecamethylene diamine with


low melting point (Tm = 67°C) interacts with BA to form alkylene-bis-ammonium benzoate (Figure 2a) with Tm = 236°C, whereas less basic ODA does not form such a salt; the phase diagram of BA-ODA system has an appearance of ordinary physical mixtures of two substances with limited solubility in each other (Figure 2b). We also investigated the kinetics of model reaction—acylation of diamines ODA, MDA, SDA, and HMDA by phthalic anhydride (PhA) in glacial AcOH

Phase diagrams of binary system BA-diamines: DDA (a) and ODA (b); point (0.0) corresponds to 100%-Mol

Kinetic data were obtained by potentiometric titration of amino groups with solution of perchloric acid in AcOH after reaching an equilibrium state [16, 17]. Results are presented in Table 3 [16, 17]. It is seen that at the initial period of acylation, kinetics obeys the equation of second-order reaction (Figure 4);

ODA 490 8670 34.4 44.0 MDA 175 5800 33.1 41.5 SDA 6.9 1130 28.1 40.6 HMDA 0.054<sup>a</sup> 95.6<sup>a</sup> 77.1 13.0

) -ΔH (kJ\* mole<sup>1</sup>

)

considerable difference is observed in the rate constants of bridged

Diamine k1 (mole \* min)<sup>1</sup> (22°C) Kp (mole)<sup>1</sup> (22°C) Ea (kJ\*mole<sup>1</sup>

Parameters of the acylation reaction of different diamines in glacial acetic acid.

(Figure 3).

a

47

Table 3.

Extrapolated value.

Figure 3.

Figure 2.

BA and point (1.0) to 100%-Mol diamine [15].

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

Model reactions of amic acid formation and imidization.

## Table 1.

Monomers used for polycondensation.


#### Table 2.

Characterization of polyimides obtained in molten BA, at 140° C (1.5 hour) [12].

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

#### Figure 2.

ODPA Oxydiphthalic dianhydride

BPADA Dianhydride of 2,2-bis-[(3,4-

RDA 1,3-Phenylenedioxy-bis-4-phthalic

6F 2,2-Hexafluorpropylidene-diphthalic

ODA 4,4'-Oxydianiline

SDA 4,4'-Sulfonyldianiline

ADA 1,3-Bis-(2-aminoethyl)-adamantane

m-PDA m-Phenylene diamine

TFPDA Tetrafluoro-p-phenylene diamine

)a

MDA 4,4<sup>0</sup>

H2N–(CH2)6–NH2 HMDA 1,6-Hexamethylenediamine H2N–(CH2)12–NH2 DDA 1,12-Dodecametylene diamine

Diamine Dianhydride ηlog(dL g�<sup>1</sup>

ODA (pKb = 5.5) ODPA 0.57 (H2SO4) SDA (pKb = 2.5) ODPA 0.44 (N-MP) ODA BPADA 1.0 (N-MP) SDA BPADA 0.45 (N-MP)

Characterization of polyimides obtained in molten BA, at 140° C (1.5 hour) [12].

Table 1.

a

46

Table 2.

N-MP, 0.5 g/dL, 25° C

Monomers used for polycondensation.

Solvents, Ionic Liquids and Solvent Effects

dicarboxyphenoxy)phenyl]propane

anhydride

anhydride


Phase diagrams of binary system BA-diamines: DDA (a) and ODA (b); point (0.0) corresponds to 100%-Mol BA and point (1.0) to 100%-Mol diamine [15].

#### Figure 3.

Model reactions of amic acid formation and imidization.

low melting point (Tm = 67°C) interacts with BA to form alkylene-bis-ammonium benzoate (Figure 2a) with Tm = 236°C, whereas less basic ODA does not form such a salt; the phase diagram of BA-ODA system has an appearance of ordinary physical mixtures of two substances with limited solubility in each other (Figure 2b).

We also investigated the kinetics of model reaction—acylation of diamines ODA, MDA, SDA, and HMDA by phthalic anhydride (PhA) in glacial AcOH (Figure 3).

Kinetic data were obtained by potentiometric titration of amino groups with solution of perchloric acid in AcOH after reaching an equilibrium state [16, 17]. Results are presented in Table 3 [16, 17]. It is seen that at the initial period of acylation, kinetics obeys the equation of second-order reaction (Figure 4); considerable difference is observed in the rate constants of bridged


#### Table 3.

Parameters of the acylation reaction of different diamines in glacial acetic acid.

Figure 4.

Acylation kinetics of diamines ODA (I), MDA (II), and SDA (III) with phthalic anhydride in AcOH at 22°C (second-order reaction plot). Starting concentration of amino groups: ODA, 0.005; MDA, 0.01; and SDA, 0.03 Mol L<sup>1</sup> [16].

It should be noted that a very low acylation rate was observed in the case of aliphatic diamines at 22°C (Table 3). The reason is occurrence of the concurrent reaction of salt formation with AcOH. This conclusion is confirmed by the appearance of adsorption peaks of benzoate anion and alkylene-bis-ammonium cation in

Parameters of acylation and imidization in carboxylic acid media (140°C).

)<sup>a</sup> <sup>k</sup><sup>1</sup> (min<sup>1</sup>

ODA 24,200 960 25.2 0.8 MDA 8100 370 22.0 — SDA 150 15.6 9.6 0.4 HMDA 430 20 21.4 0.6

)<sup>a</sup> Kp (l\*mole<sup>1</sup>

)<sup>a</sup> k2 (min<sup>1</sup>

)b

Formation results in increase in effective activation energy of acylation. Due to this, the acylation rate increases sharply with increasing temperature (Figure 5a,

We also estimated the value of effective rate constants for the imidization step [16, 17]. Low molecular weight model amic acids were synthesized from ODA, SDA, and HMDA and PhA. Kinetics of their imidization in the melt of BA at 140°C was followed by FTIR. First-order reaction rate constants were determined and corrected taking into account the conjugated reactions of decay and resynthesis of amic acid. Corresponding set of kinetic equations was written and solved numerically to give best fitting with experimental data on kinetics of imide cycle accumulation. From the analysis of the kinetic data, the following conclusions are apparent:

1. Imidization of amic acid moieties at 140°C in molten BA acid medium is a first-

2. Due to catalysis of the acylation step in molten BA in combination with low equilibrium constant, this stage becomes kinetically insignificant, and

Multiblock (MB) copolymers attract big attention due to their ability to selforganize and form continuous two-phase morphology with controlled characteristic size of the phase particle in the micro- or nanoscale range. MB copolyimides (MB CPIs) are also known as promising materials for design of gas separation and

Conventionally, polycondensation-type MB copolymers are synthesized by polycondensation of two preliminary synthesized oligomers containing terminal reactive groups. MB CPIs can also be obtained by transimidization reaction of oligoimides with pyrimidine end groups. The limitation of this approach is the fact

imidization becomes the rate-determining step of PI formation. In comparison with acylation reaction, imidization is less sensitive for chemical structure of reagents, so in one-pot PI synthesis, partial effective reactivity leveling of the

order reaction with a very fast pre-equilibrium stage (Figure 3).

low and high reactive diamines is observed.

proton-conductive membranes for fuel cells.

49

3. Synthesis of random and multiblock copolymers

that only few oligoimides are soluble in organic solvents.

IR spectrum.

a

b

Table 4.

Extrapolated.

Experiment in closed system.

line 4, and Tables 3 and 4).

Diamine k1 (l\*mole<sup>1</sup> min<sup>1</sup>

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

#### Figure 5.

Arrhenius plots (a) and logarithm of the equilibrium constant vs. the reciprocal temperature (b) for the acylation of diamines ODA (1), MDA (2), SDA (3), and HMDA (4) with phthalic anhydride [16, 17].

▬O▬; ▬CH2▬; ▬SO2▬ diamines. So, partial effective reactivity leveling in a row of aromatic diamines (not able to form salt) observed experimentally in one-pot PI synthesis in molten BA at 140°C hardly can be explained by difference in acid–basetype interaction of amino groups with BA.

Alternative explanation of this phenomenon is the change of limiting stage from amic acid moiety formation for amic acid moiety imidization; the latter is much less sensitive to chemical structure of starting reagent.

From Arrhenius plots (Figure 5a), the values of activation energy (Ea) of acylation in glacial acetic acid (AcOH) were determined (Table 3). At elevated temperatures (50°C and higher), the reversible character of the acylation was established. On the basis of experimentally measured equilibrium amino group concentration, the equilibrium constants (Kp) were determined at different temperatures. In Figure 5b, temperature dependences of equilibrium constants for acylation reaction different diamines with PhA are shown.

From these data, using the vant' Hoff equation, the enthalpy change (ΔH) values for the acylation reaction of diamines in AcOH were determined (Table 2). On the basis of the values of the equilibrium constants and the rate constants for direct reaction (k1), the first-order rate constants for the back-reaction (k<sup>1</sup>) were calculated for each temperature. In Table 4, the values of rate constants at 140°C obtained by extrapolation are given.


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

#### Table 4.

Parameters of acylation and imidization in carboxylic acid media (140°C).

It should be noted that a very low acylation rate was observed in the case of aliphatic diamines at 22°C (Table 3). The reason is occurrence of the concurrent reaction of salt formation with AcOH. This conclusion is confirmed by the appearance of adsorption peaks of benzoate anion and alkylene-bis-ammonium cation in IR spectrum.

Formation results in increase in effective activation energy of acylation. Due to this, the acylation rate increases sharply with increasing temperature (Figure 5a, line 4, and Tables 3 and 4).

We also estimated the value of effective rate constants for the imidization step [16, 17]. Low molecular weight model amic acids were synthesized from ODA, SDA, and HMDA and PhA. Kinetics of their imidization in the melt of BA at 140°C was followed by FTIR. First-order reaction rate constants were determined and corrected taking into account the conjugated reactions of decay and resynthesis of amic acid. Corresponding set of kinetic equations was written and solved numerically to give best fitting with experimental data on kinetics of imide cycle accumulation. From the analysis of the kinetic data, the following conclusions are apparent:

