2000 - one-pot polyimide synthesis in carboxylic acid medium

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DOI: 10.1088/0954-0083/12/3/307

2000 12: 445High Performance PolymersAlexander A Kuznetsov

One-Pot Polyimide Synthesis in Carboxylic Acid Medium

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High Perform. Polym. 12(2000) 445460. Printed in the UK PII: S0954-0083(00)14912-8

One-pot polyimide synthesis in carboxylic acid medium

Alexander A Kuznetsov

Institute of Synthetic Polymeric Materials, Russian Academy of Science, 70, Profsoyuznaya ul.,Moscow, 117393, Russia

Received 23 June 2000

Abstract. The melts of aromatic carboxylic acids such as benzoic acid (BA) at 130160 Cwere shown to be excellent media for the one-pot synthesis of high molecular weight, completelycyclicized polyetherimides (PEIs) from the corresponding diamines and tetracarboxylic aciddianhydrides. According to the data on the cyclodehydratation (CD) kinetics of oligomeric amic

acid model compounds, the rate of cyclicization of transient polyamic acids in BA at 140 C isveryhigh, and the polymer chain growth proceeds via the polyaddition of completely cyclicized PEIoligomers withaminoand anhydride endgroups. Tetracarboxylicacids in BAat 140 C were shownto react with aromatic diamines to obtain high molecular weight PEIs as well as the correspondingdianhydrides. The effect of the total concentration of comonomers and the presence of an inertdiluent on the kinetics of the growth of the inherent viscosity in the polycyclocondensation ofdiaminesand dianhydridesis discussed fromthe point of viewof the mechanism of the process. Theprospectof a novel synthetic approachin polyimide synthesis is discussed, including obtaining newhomopolyimides based on extremely-low-reactivity monomers; new statistical, regular, alternatingcopolyimides and block copolyimides; two-component blends with one glass transition point, andPEIs based on AB-type heteromonomers.

1. Introduction

Carboxylic acids are known to be catalysts in a large variety of organic reactions, including

the two reactions which are of great importance for polyimide chemistry, namely: (a) the

acylation of diamines with tetracarboxylic acid dianhydrides and (b) the cyclodehydratation

(CD) ofo-carboxyamide (amic acid) moieties. In this connection, the idea of using carboxylic

acids in polyimide synthesis as a catalytic agent and the reaction medium simultaneously seems

to be attractive.

The kinetics and thermodynamics of the acylation of aromatic diamines with carboxylic

acid dianhydrides has been investigated in detail [15]. Acid catalysis in the acylation of

aromatic diamines was probably first demonstrated by Litvinenko et al in the 1960s [6, 7]

and it was then studied in detail [810]. Thus, it was shown that addition of a small quantity

of chloroacetic acid (CAA) to a chloroform solution containing equimolar concentrations of

p-substituted anilines and phthalic anhydrides (PAs) results in the strong acceleration of thereaction (see scheme 1) due to appearance of a catalytic channel [9].

In the absence of CAA, a clear autocatalytic behaviour occurs in CHCl 3and acectonitrile

(AcCN) because of the catalysis of the reaction by phenylphthalamic acid (PPA), the reaction

product, the rate constant of the direct reaction via the catalytic channel being an order

of magnitude greater than that via ordinary reaction [10]. In more basic solvents such as

dimethylacetamide (DMAA), the autocatalytic regime does not occur because of the strong

interaction of the amic acid with the solvent, and the overall acylation rate is not too large.

0954-0083/00/030445+16$30.00 2000 IOP Publishing Ltd 445

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446 A A Kuznetsov

Scheme 1.

On the other hand, the strong interaction of the amic acids with the solvent leads to the

largest values of the acylation equilibrium constant (and connected with the latter, the degree

of polymerization of the polyamic acids) being achieved in just the amic solvents. A very

specific feature of the acid-catalysed acylation reaction in CHCl3 and AcCN is the extreme

dependence of the overall rate of the process versus the content of acid catalyst added, caused

by the protonation of amino groups with an excess of the catalyst. This fact probably explains

why high concentrations of carboxylic acid have not been used in one-pot polyimide synthesis.Acid catalysis in the CD reaction of polyamic acid (PAA) was demonstrated first in 1977

by Vinogradovaet al [11] and Lavrov et al [12, 13]. In the absence of acid additives, clear

second order was found for the CD reaction of PAA by Lavrov et al[13] and Kim et al [14].

The self-catalysis of the CD reaction of model amic acids in concentrated solutions in amic

solvents was demonstrated by Kuznetsovet al [15], amic acid being the reagent and catalyst

simultaneously, and the solvent playing the role of co-catalyst.

Recently, we have discovered that the melts of aromatic carboxylic acids such as benzoic

acid (BA) at 130160 C can be used as the reaction media in catalytic one-pot syntheses

of high molecular weight completely cyclicized polyimides from corresponding diamines and

tetracarboxylic acid dianhydrides [1619]. This article is a development of the novel approach.

2. Experimental details

2.1. Starting materials

2,2-bis[(4-aminophenoxy)phenyl]propane (BAPP, technical grade product, Russia) was

purified by crystallization from isopropanol, Tm = 129C. m-phenylenediamine (PDA,

Tm = 6466C) and hexamethylenediamine (HMDA, Tm = 4445

C) were purified by

distillation in vacuo. 2,6-diaminopyridine (DAP, Aldrich) was purified by sublimation

in vacuo,Tm =122C.4,4-oxydianiline (ODA) (Russia) was purified by sublimation invacuo,

Tm = 190C. 4,4-diaminodihenyl sulfone (Russia) was crystallized from isopropanol,

Tm =175C.

3,3,4,4-biphenyltetracarboxylic acid (BPTA) was obtained by the dehydration of the

corresponding acid dihydrate (BPTA.2H2O) at 200C for 30 min. BPTA.2H2O was

synthesized by Professor E L Vulah of the Research and Technical Design Institute ofMonomers (Tula, Russia) and it was kindly presented for our disposal. The 3,34,4-

biphenyltetracarboxylic acid dianhydride (BPDA) was obtained by heating BPTA at 270 Cfor

2 h. BPDA displays only one peak in the differential scanning calorimetry (DSC) thermogram

at 300 C (literature data Tm = 286C [20]). 4,4-oxydiphthalic anhydride (technical

grade, Russia) was crystallized from acetic dianhydride and sublimated at 200 C in vacuo,

Tm = 221C. 2,2-bis[(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) and 4-

aminophenoxyphthalic acid (APPA) were synthesized by Professor M V Dorogov of the



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One-pot polyimide synthesis in carboxylic acid medium 447

Department of Organic Chemistry, State Yaroslavl Technical University (Russia) and used

as received;Tm =188189C (BPADA).

BA was purified by distillation under ambient pressure and an inert atmosphere,

Tm =122C; diphenylsulfone was crystallized from isopropanol, Tm = 126128

C; and

m-cresol was distilled under vacuum before use.

2.2. Synthesis of polyetherimide

The synthesis of polyetherimides (PEIs) from diamines and tetracarboxylic acid dianhydrides

in the melt of BA is described below by the example of PEI-2. 0.5825 g (1.42 mmol) of

BAPP, 0.4175 g (1.42 mmol) BPDA and 9.0 g of BA were loaded into a glass reaction vessel

equipped with a stirrer under a slow flow of argon. The reactor was placed into an oil bath at

140 C. After melting the BA a stirrer was turned on. The time to reach 140 C was less than

3 min. The reaction was carried out under an argon atmosphere for 1 h. At a definite time

after reaching 140 C, the probes were removed from the reaction medium. After cooling each

probe was treated with acetone to remove the BA; the polymer residue was dried and analysed

by Fourier-transform infrared (FTIR) spectroscopy and viscometry. The FTIR spectra wereobtained using a Nicollett FTIR, Impact 410 spectrometer; the samples for the FTIR were

made of CsI. The conversion of the CD reaction for PEI-2 was measured using the peaks at

1370 cm1 (CN vibration in the imide cycle) and 1500 cm1 (CC vibration in an aromatic

cycle; the internal standard). In the case of PEI-1 and other polymers based on BPADA, the

region of 1370 cm1 is not available because of its own absorption related to the isopropylidene

moiety. So, for the PEI of this group, the conversion of the CD reaction was estimated using

the intensity ratio of the peaks at 620 cm1 (imide cycle) and 690 cm1 (internal standard)

using a previously obtained calibration curve. The inherent viscosity () was measured for

solutions containing 0.5 g of polymer in 100 ml in N-methylpyrrolidone (N-MP) using an

Ubellode viscometer at 25 C. The PEIs on the basis of all other monomer pairs (diamine,

dianhydride) were obtained analogously.

2.3. Synthesis of PEI-2 from diamine and tetracarboxylic acid (BAPPBPTA)

0.583 g (1.42 mmol) of BAPP and 0.463 g (1.42 mmol) BPDA and 9.0 g of BA were loaded

into a glass reactor. The process was carried out for 3 h at 140 C, analogously to that for the

dianhydride.

2.4. The kinetics of the CD of the model compound PAA-1 in the melt of BA

Oligomeric amic acid PAA-1 with = 0.13 dl g1 (Mw = 3000) was obtained as described

earlier [18]. For the goal of the measurement of the CD reaction rate, 30 mg of solid PAA-1

and 270 mg of BA were loaded into 10 identical thin-walled glass microtest-tubes which were

all placed into the oil bath simultaneously. In a control experiment, the time to reach 140 C inevery test-tube was estimated, by thermocouple, to be the same: 10 s. At a definite time after

reaching 140 C, the first, second, etc, test-tube was removed and cooled quickly to 20 C. The

BA was extracted with diethyl ether and the polymer residue was analysed by means of FTIR

spectroscopy (in a pellet of CsI). The conversion by CD was measured from the intensity ratio

of the peaks at 620 cm1 (imide cycle) and 690 cm1 (internal standard) using a previously

obtained calibration curve. The completely cyclicized oligomeric PEI was obtained by heating

the PEI-1 sample in a DSC cell at 300 C at a heating rate of 16 C min1.



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448 A A Kuznetsov

2.5. Preparation and analysis of two component polyimidepolyimide formulations

The formulations madeex situwere prepared by dissolution of solid PEI-1 and PEI-7 in the

melt of BA at 140 C. PEI-1 and PEI-7 samples were synthesized separately in the melt of BA

at 140

C for 1 h, each at a 10% load and a 1:1 molar ratio of diamine and dianhydride in thepresence of 1.0% PA as an endcapping agent. The PEI-7/PEI-1 ratio in the mixture was varied

from 0.1:0.9 to 0.9:0.1; the total concentration was equal to 10 wt%. The inherent viscosity

was equal to = 0.35 dl g1 (PEI-7) and = 0.42 dl g1 (PEI-1). After the loading of the

polymers, the mixture was stirred for 1 h, the reaction mixture was then poured into a crucible

and then treated as described above.

The in situ-made formulations were prepared by carrying out the catalytic synthesis of

PEI-7 from HMDA and BPADA (1:1 molar ratio) in the melt of BA at 140 C for 1 h, without

adding any PA. Then PDA and BPADA (1:1) were loaded into the hot reaction medium and

after 1 h PEI-1 was obtained in the presence of PEI-7. The PEI-7:PEI-1 ratio was varied. The

total concentration of PEI-7 and PEI-1 in the reaction mixture was held constant and equal to

10 wt%. The other details of the preparation the sample were the same.

Polymer formulations of both types were hot pressed at 280 C to give pellets. TheTg

values were determined by DSC (DTAP-4, Russia) at a heating rate of 16 C min1. The dataobtained in a second scan were used to determine theTg values to elucidate the possibility of

the influence of a thermal prehistory on the results.

3. Results and discussion

3.1. The melt of BA as a non-toxic catalytic medium for the one-pot synthesis of PEI

BA is known to possess a useful combination of properties: (1) it is a powerful catalyst of the

CD reaction of PAA; (2) as a melt at 140160 C, it is a good solvent for many aromatic PEIs.

Therefore, we propose the melt of BA as alternative solvent for one-pot polyimide synthesis

[18, 19]. An additional advantage of BA appears from consideration of the toxicity data. It is

well known thatm-cresol ando-dichlorobenzene (DCB), which are commonly used as high-

boiling-point organic solvents in high-temperatureone-pot polyimide synthesesat 130180 C,

are known to be very toxic substances: the minimal toxic concentrations ofm-cresol and DCB

are 5 and 20 mg m1, respectively. In contrast, BA at ambient temperature is a non-toxic

solid; moreover, in small quantities it is used as a preserving additive for tinned food. Besides

the superiority over m-cresol in toxicity, BA can be easily separated from the polymer after

completing the process. On cooling, the reaction mixture solidifies and microphase separation

takes place, BAforming its own crystalline phase. The latter can be easily removed by repeated

washing with acetone.

In figure 1 and figure 2 (curve 1) the kinetics of the inherent viscosity growth of PEI-1

and PEI-2 from a one-pot synthesis in the melt of BA is shown; the starting monomer pairs

were PDABPADA and BAPPBPDA, respectively. The macromolecules PEI-1 and PEI-2

contain the same two hinge fragments of bis-phenol A (BPA) in the elementary chain unit,

but differ from each other by its location: PEI-1 contains a BPA fragment in the anhydride

moiety, whereas PEI-2 contains it in the diamine moiety. In both cases PEI synthesis proceedsas a homogeneous process throughout all the time of synthesis. In a typical experiment, the

reaction of equimolar quantities of PDA and BPADA at a total of 10% comonomer mixture

load takes about 1 h to give a completely cyclicized product (according to FTIR data) with

= 0.78 dl g1. The structure and characteristics of several PEIs obtained by us in one-pot

syntheses in the melt of BA are collected together in table 1.

In all cases, values of the inherent viscosity () of about 0.40.5 dl g1 and more were

achieved in 1 h. It should be noted that the level of = 0.5 dl g1 seems to be optimal for a



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Figure 1. Kinetics of the growth of the inherent viscosity of PEI-1 from PDA and BPADA in theone-pot synthesis at 140 C in the melt of BA (total monomer concentration 10 wt%).

Figure 2.Kineticsof thegrowth of theinherent viscosityof PEI-2 from BAPP andBPDA at 140 Cin different media: (1), the melt of BA; and (2), the melt of BA:DPS, 1:1 mixture (total monomerconcentration 10 wt%).

PEI intended for further melt processing of the PEI. If necessary, the molecular weight of the

PEI can be controlled by the choice of the stoichiometric ratio or by monofunctional additives

such asp-anisidine and PA (figure 3).



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450 A A Kuznetsov

O

O

C

CO

CH3

CH3

CO

O

O

C

COO

OO

OO

O

C

C

O

C

C

C

C

O

O

O

C

C

O

O

O

Structure 1.

Table 1. Characteristicsof PEIs obtained from dianhydrides (structure 1) and diamines of common

formulae H2NXNH2 in the melt of BA, (140C for 1 h). Note that the glass transition

temperatures and the melting pointsare given for the amorphous and crystallinephase, respectively.

Viscosity () in Tg Tm

X Diamine Dianhydride PEI N-MP (dl g1) (C) (C)

PDA BPADA PEI-1 0.78 220

BAPP BPDA PEI-2 0.50 240

BAPP ODPA PEI-3 0.40 220

BAPP BPADA PEI-4 0.48 200

ODA ODPA PEI-5 0.42 265 340

ODA BPADA PEI-6 0.52 220

(CH2)6 HMDA BPADA PEI-7 0.40 120 165

SO2 DDS BPADA PEI-8 0.5 230

DAP BPADA PEI-9 0.56 220

+

NPDA: BPDA PEI-10 0.50 220

DAP

(1:1)



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Figure 3. The influence of the addition of (1) PA and (2) anisidine as endcapping agents on theinherent viscosity of PEI-1 (140 C for 1 h).

It is believed that the one-pot polyimide synthesis in BA includes two chemical reactions,

thefirst being the acylation of the diamine with the tetracarboxylic acid dianhydrides to give

PAAs, the second being the CD of transient o-carboxyamide moieties to give imide cycles

(scheme 2). The peculiarity of the process in BA is that the rate of CD of transient PAAs

is very high under the conditions chosen. To demonstrate this, we tried to estimate the

CD rate of a model oligomeric amic acid PAA-1 in the melt of BA at 140 C. PAA-1 was

previously synthesized by us for this experiment. The data obtained are presented infigure 4

(curve 1). It is seen that the CD kinetics of PAA-1 in the melt of BA at 140 C is very

fast, too fast for us to calculate accurately the CD rate. However, these data are enough to

conclude that the chain growth in the one-pot synthesis proceeds basically via a polyaddition

reaction of thefully cyclicized oligomerswith amino andanhydrideendgroups. Dueto thefast

disappearance of theo-carboxyamide moieties in thecourse of theCD reaction, theequilibrium

of the polycondensation is shifted continuously to the side of polymer chain growth, and high

molecular weight completely cyclicized PEIs can be obtained in the melt of BA; contrast this

to that in amic solvents at comparable temperatures.

In contrast to the high rate of the CD, the growth rate of the inherent viscosity of PDA

BPADA in BA at 140 C is about the same order of magnitude as that in polycondensation

at 20 C that results in PAA. Taking into account the large difference in temperature (120 C)between the conditions of polycondensation and the one-pot synthesis in BA, it should be

concluded that diamines in the melt of BA are strongly deactivated, probably due to hydrogen

bonding with BA. As a result, the efficient acylation rate in the melt of BA at 140 C is about

the same as that in the polycondensation reaction in an amic solvent at 20 C. The acylation

rate in the melt of BA is probably much lower than that of the CD reaction; the acylation rate

only is thought to determine the time necessary to obtain completely cyclicized polyimide of

a definite molecular weight.



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452 A A Kuznetsov

Scheme 2.

0 2 4 6 8 1 0

0

2 0

4 0

6 0

8 0

1 0 0

T i me, mi n

P ,% 2

1

Figure 4.Kinetics of the CD of PAA-I at 140 C: (1) in the solid state; and (2) in the melt of BA.

To confirm the correctness of the proposed mechanism for the process, we investigated

the one-pot synthesis in the melt of BA in the presence of a co-solvent. In figure 2 (curve 2)

the kinetic curve of the inherent viscosity growth in mixtures of BA/diphenylsulfone (DPS)

(50:50 by weight) is shown. The appearance of the viscosity maximum at the start of the



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Figure 5. Dependence of the 10 min inherent viscosity value of PEI-2 against the total weight

concentrationCof mononers in BA, at 140 C.

process can be explained by the presence of a considerable quantity ofo-carboxyamide (amic

acid) moieties in the growing polymer chain, which afford higher hydrodynamic radius to

macromolecules in comparison with that for PEI for an equal degree of polymerization. In

turn, the increased concentration of the transient product in a case of BA/DPS can only be a

result of changing the ratio of the cyclicization/acylation rates in comparison with that in pure

BA. Indeed, the addition of any inert diluent to BA is expected to retard the CD because of the

decrease of the concentration of the cyclicization agent (BA). At the same time, the dilution

will lead to a decrease of the hydrogen bonding of the diamine and BA and, as intended, to

the acceleration of the acylation stage. Moreover, in BA/DPS mixtures, the balance of the

acylation/CD rates is shifted considerably in favour of acylation in comparison with that in

pure BA.

In figure 5, the experimental dependence of the rate of the initial inherent viscosity growth

of PEI-2 against the total monomer concentration is shown. The true reason for the decrease

of the rate at high monomer concentration is not clear. The preliminary supposition is that

this effect is connected with the change of any of the medium characteristics influencing the

balance of the acylation/CD rates.

3.2. New polyimides and statistic copolyimides from low-reactivity diamines

It can be seen from table 1 that high molecular weight PEIs were obtained successfully

even from low-reactivity diamines such as 4,4-diaminodiphenylsulfone (DDS) and 2,6-

diaminopyridine (DAP). The low reactivityof the DDS in polycondensation is connected

with its low basicity. DAP is known to possess extremelylow reactivityas a nucleophylic

reagent due to occurrence of the aminoimine tautomerism. Furthermore, DAP is known to

give no polyimides under conditions of high-temperature one-pot synthesis in cresol. To check

this, in a control experiment, the reaction between DAP with BPADA (1:1 molar ratio, 10%



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454 A A Kuznetsov

total monomer load) was carried out by us in m-cresol: after 1 h at 170 C no polymer was

precipitated after adding an excess of methanol. In contrast to this, the high molecular weight

polymer PEI-9 was isolated in quantitative yield after synthesis in BA at 140 C for 1 h.

According to the inherent viscosity data (table 1), the difference in efficient reactivity of

low- and high-reactivity diamines is not observed in the melt of BA, in contrast to that in amicsolvents at ambient temperature. The equal reactivity of highly basic ODA and low basicity

DDS in the melt of BA can be explained by the phenomenon of theequal basicityof strong

and weak bases in acidic media. As concerns the high reactivity of DAP in the BA medium,

the effect observed maybe connectedwith the ability of BA to coordinate thepyridine nitrogen

atom through hydrogen bonding and prevent the aminoimine tautomerism and, hence, lead

to an increase in the reactivity of the amino groups in DAP. Besides, the reactivity equalization

phenomenon in the melt of BA can be useful for obtaining new statistical copolyimides.

To demonstrate this, we made experiments on copolycyclocondensation of BPADA with the

mixture of PDA and DAP (1:1 molar ratio). As can be seen from table 1, a high molecular

weight co-PEI was successfully obtained with =0.50 dl g1 for 1 h at 140 C. The large

value of the inherent viscosity is evidence for the comparable reactivity of the two diamines

in BA, otherwise the more reactive of these two must be in a large excess of the dianhydride,resulting in an oligomeric product.

3.3. Regular and alternating copolyimides and block copolyimides

The new approach makes it possible to obtain new regular, alternating and block copolyimides

in the one-pot process. In this work, by means of two-step chain growth in the course of an

in situconsequent catalytic polycyclicization, a series of novel regular PEIs was synthesized.

They consist of two types of polyimide blocks: A and B. Block A being a fragment of PEI-

1 (an amorphous, thermoplastic and soluble polymer) and block B being a product of the

cyclopolycondensation of PDA and BPDA (a non-thermoplastic and non-soluble polymer, see

structure 2).

As a first step, the oligomer of the PEI-1 structure with amino end groups was obtained

by cyclopolycondensation of PDA and BPADA taken at molar ratios of 1:0.9, 1:0.85, 1:0.7

and 1:0.6. As a second step, BPDA was added in a quantity corresponding to the overall 1:1

stoichiometric ratio of all amino and anhydride groups, and reaction was continued for 1 h

more (table 2). Theoretically, the synthesis strategy chosen must lead to a so-called regular

copolymer, i.e. to the polymer containing individual fragments of the first type (in our case

PDABPDAPDA)separatedfrom each otherby longersequences of theothertype (inourcase

(PDABPADA)n). Characteristics of the polymeric products obtained are given in table 2.

The values of the glass transition temperature Tg increase smoothly with increasing

numbers of BPDA fragments, from 220 C (PEI-1, sample 1) to 245 C (sample 5). The

samples 24 are soluble in N-MP, sample 5 is soluble only in H2SO4. Theoretically, further

increases of the BPDA content in a regular copolymer upto 50:50 BPADA:BPDA ratio mustlead to strongly alternating structures. In this series we met a solubility limitation. However,

it is believed that by the use of appropriate monomers, alternating copolymers are achievable

through thesynthesis strategyoffered. It shouldbe noted that thefragment(PDABPDAPDA)

which is present in the copolyimides (samples 25) is a long enough rigid block consisting

of six aromatic and heteroaromatic moieties connected without hinge atoms. Therefore, the

regular copolyimides obtained can also be considered as block copolymers. In the future we

intend to apply this strategy to obtain other types of block copolyimides.



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Structure 2.(a) Block A, PEI-1 and (b) block B, poly-(PDABPDA).

Table 2.Characteristics of regular copolyimides based on PDA(BPDA + BPADA) obtained by atwo-step one-pot synthesis in the melt of BA at 140 C (1 + 1 h)

BPDA/BPADA ratio Tgby DSC Solubility in Inherent viscosity in

SampleN (mol:%) (C) N-MP H2SO4(dl g1)

1 0:100 (PEI-1a, control) 220 + 0.40

2 10:90 225 + 0.35

3 15:85 230 + 0.33

4 30:70 235 + 0.34

5 40:60 245 + 0.34

6 0:100 (control)b 0.28

a In the presence of 0.5% PA.b The polymer precipitates from the reaction medium.

3.4. Two-component polyimidepolyimide systems with one glass transition temperature

An interesting application of the new synthesis method is the preparation of polyimide

polyimide blends with one glass transition temperature via consequent one-pot polyimide

synthesis in situ of twopartially compatible polyimides [21]. Two types of the two-component

formulations (PEI-7/PEI-1) were prepared: thein situ-madeformulations and the ex situ-

made formulations. The in situ-made formulations were prepared by carrying out therespective catalytic synthesesof PEI-7 andPEI-1 in BA. The mechanically-made formulations

were prepared by the dissolution of separately synthesized PEI-7 and PEI-1 (each in the

presence of PA as an endcapping agent) in the melt of BA. All other details of the preparation

of the samples were kept the same. The phase diagrams were designed for all the formulations

on the basis of the DSC data. In the case of an ex situ-made formulation (figure 6), a clear

microphase separation occurs in a wide interval of PEI-7:PEI-1 ratios from 80:20 to 10:90,

the Tg values for each phase being equal to 140 and 210C, respectively. In contrast, the



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456 A A Kuznetsov

Figure 6. Phase diagram of binary polyimidepolyimide formulations (PEI-1/PEI-7):mechanically-made formulations (a and b branches); in situ-made formulations (points AE);andin situ-made formulations with complete endcapping of PEI-7 (points G and F). See text for

further details.

experimentalTgvalues of the in situformulations depend on the content; the corresponding

points AE in figure 6 for the in situ-made formulations are located inside the region of the

phase separation of the ex situ-made blends. Thus, thein situformulations behave as systems

with a molecular level of compatibilization. At the same time, by means of turbidimetry

analysis, the in situ-made formulations were identified as a mixture of homopolymers (or

a long-chain diblock copolymer), but not as a statistical copolymer. In a course of special

experiments using completely end-capped PEI-7, it was shown that the quasi-one-component

morphology behaviour observed in the in situformulation was caused by the formation of

small quantity of a diblock copolymer playing the role of stabilizer of the polymerpolymer

microemulsion particles and retarding the phase separation. More details will be given in a

separate paper [22].

3.5. Direct polycondensation of diamines and tetra-acids

It is well known that a high quality of comonomers is a very important factor in fluencingthe molecular weight of thefinal polyimides. In the long-term storage of dianhydrides, the

hydrolysis productsthe corresponding di- and tetra-acidscan accumulate as impurities.

Tetra-acids are not reactive as acylating agents. Therefore, the purification of dianhydrides

and the current control of the quality are strong requirements in usual polyimide chemistry. As

was shown above, the melt of BA can act as a powerful dehydrating agent in relation to amic

acid moieties. Hence, it was suggested that it could serve as an in situdehydrating agent for

tetracarboxylic acids. In this work we investigated the possibility of using the tetracarboxylic



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Figure 7.DSC thermograms of (1) BPTA.2H2O, (2) BPTA and (3) BPDA.

acid BPTA instead of the dianhydride BPDA in the catalytic synthesis of PEI-2 in the melt

of BA. As can be seen from DCS thermograms of BTPA (figure 7 (curve 2)), anhydridization

of solid BPTA on heating occurs in a region of 250270 C (wide endo peak); the second

endo peak at 300 C corresponds to the melting of the BPDA formed in situ. The second

scan obtained after cooling sample 2 (figure 7 (curve 3)) contains only a peak at 300 C which

coincides with the melting peak of pure BPDA. The temperature of the beginning of the BPTA

anhydridization is much higher than that of the loss of the coordinated water molecules by

BPTA.2H2O (140170C) (figure 7 (curve 1)) and higher than the optimal temperature of the

one-pot synthesis in the melt of BA (140 C).To check thesuggestion offered above,a seriesof

experiments were carried out on polyimide synthesis in BA using the tetra-acid BPTA instead

of the corresponding dianhydride (BPDA).

Infigure 8 (curve 2) the kinetics of the growth of the inherent viscosity of PEI-2 obtained

from BAPPBPTA at 140 C is shown. It is seen that a high molecular weight PEI-2 with an

inherent viscosity of 0.5 d l g1 wasobtained after 3 h, although thechain growth is slower than

that in the case of BPDA (figure 7 (curve 1)). Obviously, the cyclopolycondensation process

of BTDA with BAPP includes the stage of formation of BPDAin situ(scheme 3), the latter

reaction being the rate-determining step in the process as a whole.

3.6. Polycondensation of heteromonomers (AB-type monomers)

The opportunity demonstrated above for the direct polycyclocondensation of diamines and

tetra-acids has reinitiated our interest in AB-type heterocomonomers for polyimides, such as

APPA. According to the data of IR spectroscopy [23], APPA exists in the form of a zwitter ion

(the equilibrium structures in scheme 4).Intramolecular proton transfer makes AB-type heterocomonomers extremely low in

reactivity in polycondensation reactions. According to [23], they can be converted to

polyimides only by a two-stage synthesis, thefirst stage beingthedirect homopolycondensation

in the presence of (Ph)3P=O, and the second being chemical imidization. Infigure 9 FTIR

spectra areshown of theinitial APPA (spectrum1) andof theoligomeric product (0.20dlg1 in

H2SO4solution)obtained byheating APPA in themelt of BAat 140C for 45min (spectrum 2).

The absorption near 16001660 cm1 related to the zwitter-ion structure of APPA disappears



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458 A A Kuznetsov

Figure 8. Kinetics of the growth of the inherent viscosity of PEI-2 obtained in the melt of BA

starting from pairs of (1) diaminedianhydride (BAPPBPDA) and (2) diaminetetracarboxylicacid (BAPPBPTA) (140 C, total monomer concentration 10 wt%).

Scheme 3.

Scheme 4.

and the imide peaks at 1720 and 1790 cm 1 appear instead. The spectrum of the oligomeric

product obtained in BA is similar to that obtained by polymerization on heating of solid APPA

up to 250 C (figure 9 (spectrum 3)). So, it can be concluded that treatment of AB compounds



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Figure 9. FTIR spectra of (1) APPA, (2) the oligomeric product ( =0.20 dl g1 in H2SO4)

obtained by heating the APPA in the melt of BA at 140 for 45 min; and (3) the product (insoluble

in H2SO4) obtained after heating solid APPA at 250C for 5 min.

in the presence of an acid catalyst at 140 C results in the corresponding PEI (poly-APPA,

figure 9 (scheme 4)), although the molecular weight is not too large. Even more interesting

opportunities appear for the synthesis of linear and star-shaped block copolymers. This work

is also in progress.

4. Conclusion

The main conclusions are as follows.

(1) The meltof BA at140160 C is showntobeanadvancedreactionmediumfor thecatalytic

one-pot process for obtaining high molecular weight, completely cyclicized polyimide

products, including:

(a) thermoplastic PEIs known from the literature;

(b) new polyimides based on low-reactivity diamines;

(c) new statistic copolyimides, containing both high- and low-reactivity diamine

moieties;(d) new regular, alternating and, probably, block copolyimides;

(e) two-component polyimidepolyimide blends with one glass transition temperature;

(f) PEIs based on AB-type heteromonomers.

(2) The direct polycyclocondensation of diamines with tetracarboxylic acids is demonstrated

to occur in the melt of BA at 140160 C to give high molecular weight completely

cyclicized polyetherimides.



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460 A A Kuznetsov

Acknowledgments

The work was supported by the General Electric Plastics Co. and also by RFBR, grant

97-03-32740. The author also express his thanks to Dr S V Lavrov, Dr V I Berendyaev,

A Yu Tsegelskaya, Dr N V Kozlova, I D Egorov (all from the Karpov Institute of PhysicalChemistry, Moscow), Dr G K Semenova, M Yu Yablokova, L V Ryabova (ISPM RAN), Dr

Belov M Yu (Institute of Problem of Chemical Physics, Chernogolovka) for their participation

in the experimental work; to Professor E L Vulah and Professor M V Dorogov for the synthesis

of the starting materials; and Professor B V Kotov for fruitful discussions of the results.

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