Synthesis of polyimides in supercritical carbon dioxide�

Ernest E. Said-Galiyev a,*, Yakov S. Vygodskii a, Lev N. Nikitin a,Rostislav A. Vinokur a, Marat O. Gallyamov b, Inna V. Pototskaya c,Vyacheslav V. Kireev c, Alexei R. Khokhlov a,b, Kjeld Schaumburg d

a A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Street, 28, Moscow 119991, Russiab Physics Department of Lomonosov State University, Vorobievy gory, Moscow 119992, Russia

c D.I. Mendeleev University of Chemical Technology of Russia, Moscow, Russiad Centre for Interdisciplinary Studies of Molecular Interactions, Department of Chemistry, University of Copenhagen, Universitetsparken

5, DK-2100 Copenhagen, Denmark

Received 15 September 2001; received in revised form 30 August 2002; accepted 5 September 2002

Abstract

One-step polycyclization reaction in supercritical carbon dioxide has been carried out for the first time and a thereby

set of polyimides has been synthesized. Diphenyl-2,2-hexafluoropropane-3,3?,4,4?-tetracarboxylic acid dianhydride

(dianhydride 6F), 4,4?-diaminodiphenyl-2,2-hexafluoropropane (diamine 6F) and 9,9-bis(4-aminophenyl)fluorene were

used as monomers. Maximum inherent viscosity of the synthesized polymers is 0.56 dl/g. It is supposed that carbon

dioxide act as an acidic catalyst for the reaction in the presence of water admixture.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Supercritical carbon dioxide; Polycyclization reaction; Synthesis; Polyimides

1. Introduction

Polymer synthesis in supercritical (sc) solvents is

a rapidly developing branch of the environment-

friendly chemistry [1]. Many types of radical

polymerization in sc solvents have already been

studied. The properties of the polymers obtained

by such reactions have not yet been studied

extensively. It was however shown that in most

cases without catalysts sc CO2 is an absolutely

neutral solvent. It was also shown that the sc

solvents can be an effective substitute for freons

and other undesirable halogen-containing organic

solvents [2].In contrast to the polymerization reactions, the

polycondensation reactions are by far not so well

studied [3�/9]. It can be due to a more complex

polycondensation technique in sc CO2.

Recently we reported the first synthesis of

polyimides (PI) in sc CO2 [10�/12]. Now some

new results and more detailed description of the

synthesis are presented. It is well known that

carboxylic acids, including acetic and benzoic

acid are effective catalyst for PI synthesis by the

�PII of original article: S 0 8 9 6 - 8 4 4 6 ( 0 2 ) 0 0 2 1 0 - 3

* Corresponding author. Tel.: �/7-095-135-05-22; fax: �/7-

095-135-50-85.

E-mail address: ernest@ineos.ac.ru (E.E. Said-Galiyev).

J. of Supercritical Fluids 27 (2003) 121�/130

www.elsevier.com/locate/supflu

0896-8446/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0896-8446(03)00146-3

Erratum122

reaction of tetracarboxylic acid dianhydrides anddiamines [13]. Since CO2 in the presence of water

admixture is acting as a weak acid, we suppose

that sc CO2 may act not only as a reaction

medium, but also as a catalyst for this reaction.

2. Experimental

2.1. Experimental materials

The choice of the monomers for a polyconden-

sation reaction was based on excellent solubility of

fluorinated PI in many organic solvents and a

good solubility of fluorinated polymers in sc CO2.

Diphenyl-2,2-hexafluoropropane-3,3?,4,4?-tetra-carboxylic acid dianhydride (dianhydride 6F) and

4,4?-Diaminodiphenyl-2,2-hexafluoropropane

(Diamine 6F) were received from ‘Aldrich’ and

have been purified by high vacuum sublimation.

Dianhydride A was purchased from ‘General

Electric’ (m.p., 186 8C). 9,9-bis(aminophe-

nyl)fluorene (Diamine AF) (m.p., 236�/237 8C)

was synthesized in our laboratory and purifiedby high vacuum sublimation [13]. Two kinds of

carbon dioxide was used: normal purity (�/99.0%)

and high purity (�/99.997%).

2.2. Synthesis technique

PI were synthesized by high temperature one-

step polycyclization reaction of the corresponding

diamines and dianhydrides.

The synthesis was performed in a high-pressure

cell with inner volume 10 cm3 described in detail

elsewhere [4]. A standard set-up was used to obtain

sc conditions in the cell. The reagents were loaded

into the cell (dianhydride 6F, 0.3 g, 6.75�/10�4

M, diamine AF 0.188 g, 5.4�/10�4 M and/or

diamine 6F 4.5�/10�2 g, 1.35�/10�4 M/0.2254 g,

6.75�/10�4 M with molar ratio 1:0.8:0.2/1:0:1.

The cell was then purged with CO2, sealed and

heated up to the temperature of 180 8C. We

intended to elucidate whether CO2 plays a specific

role in the polycyclization reaction. The role of sc

CO2 in reactions in a melt is very well known. Sc

CO2 reduces the melt viscosity that promotes a

better elimination of low molecular mass reaction

products and increases the polymer MM [2]. It was

not possible to elucidate the specific role of CO2 in

a polycondensation reaction at the temperature

higher than the m.p. of monomers.

It is well known that sc CO2 decreases Tg of

polymers owing to its plasticizing action [14].

Therefore the m.p. of the monomers could be

expected slightly to decrease.

When dianhydride A was used later as dianhy-

dride component the reaction temperature ap-

proached probably close to a melting

temperature of the reaction mass.

A syringe press provided by ‘High Pressure

Equipment’ was used to pump the liquid CO2

into the cell up to a pressure of 32.5 MPa where-

upon a magnetic stirrer was switched on. The

reaction time was varied from 3 to 6.5 h and it was

chosen based on known data about synthesis of

high MM PI under such conditions. Both batch

and flow modes of operation (persistent flow of sc

CO2) were used. CO2 flow rate was 15�/20 cm3/

min. The temperature was controlled with an

accuracy of 9/0.5 degrees, and the pressure within

9/1 bar.The cell was cooled after of run, the stirrer was

switched off and the cell was decompressed. The

Erratum 123

polymer was further reprecipitated from acetoneor 1-methyl-2-pyrrolidinone solution into excess

water and then the sample was dried overnight in

air and in vacuum at 50 8C for 1 h.

2.3. Methods of analysis

FT-IR spectral measurements were performed

using Nicolet ‘Magna 750’ spectrometer. The

specimen used for spectroscopy were KBr tablets

with added polymer powder.

Molecular weight analysis was performed with

GPC technique in THF solvent (flow rate is 1 ml/

min) using ‘Waters’ chromatograph consisting of

M600 delivery system, M 484 UV-VIS detector

(l�/260 nm) and U-Styrogel linear columns. The

chromatograph was calibrated on PS standards.

MAXIMA software was used.

Table 1

Run conditions

No. Dianhydride Diamine Molar ratio Temperature, 8C Pressure, bars Time Medium Regime

1 6F 6F�/AF 1:0.2:0.8 180 325 3 sc CO2 Batch

2 6F 6F�/AF 1:0.2:0.8 130 325 3 sc CO2 Batch

3 6F 6F�/AF 1:0.2:0.8 180 1 3 ?r Batch

3a 6F 6F�/AF 1:0.2:0.8 180 1 3 Ar Flow

3b 6F 6F�/AF 1:0.2:0.8 180 325 3 sc CO2 Flow

4 6F 6F�/AF 1:0.2:0.8 180 325 4 sc CO2 Flow

5 6F 6F�/AF 1:0.2:0.8 180 325 6 sc CO2 Batch

6 6F 6F�/AF 1:0.2:0.8 180 325 6 sc CO2 Batch

7 6F 6F�/AF 1:0.2:0.8 180 325 6.5 sc CO2 Flow

8 6F 6F 1:1 180 325 6.5 sc CO2 Flow

9 6F 6F�/AF 1:0.2:0.8 180 325 6.3 sc CO2a Flow

10 6F 6F 1:1 180 325 6.0 sc CO2b Flow

11 A 6F�/AF 1:0.2:0.8 180 325 6.0 sc CO2 Flow

12 A AF 1:1 180 325 4.0 sc CO2 Flow

a High-purity CO2 (�/99.997%).b High-purity CO2 (�/99.997%), special addition of water equal in quantity to that introduced into the cell with normal purity CO2

(99.0%) in batch mode of operation.

Table 2

Polymer characteristics

Appearance Yield, % Inherent viscosity, dl/g Imide cycle bands,

cm�1

Anhydride cycle bands,

cm�1

721 1380 1730 1780 1860

1 Light-yellow powder 93.7 0.14 m s s m w

2 Light-yellow powder 91.0 0.12 m s s w m

3 Yellow powder 94.6 0.19 w w m s s

4 Brown powder 81 0.26 s s s m m

5 Dark-yellow powder and brown crystal 100.0 0.12 m s �/ w

6 Dark-yellow powder and brown crystal 91.5 0.19 m s s w w

7 Dark-yellow powder and brown crystal 88.0 0.32 m s s w w

8 Light-yellow powder 80.0 0.43 m s s w �/

9 Light-yellow uniform powder 88.0 0.14 w w w s s

10 Grey powder 88.3 0.13 m s s w �/

11 Light-yellow powder 90.0 0.56 m s s w �/

12 Pale-yellow powder 93.0 0.40 m s s w �/

s, strong; m, medium; w, weak; �/, absent.

Erratum124

A solubility of the monomers was determind

visually by observation of a sample powder

particles behavior in a little glass located in the

spectroscopic cuvette filled with sc CO2. The

samples also were weighed before and after

exposition in sc CO2. A change of crystals form

without a weight loss was attributed to a slight

swelling.

Inherent viscosity (/h?inh) was measured at con-

centration of 0.5 g/dl in 1-methyl-2-pyrrolidinone

at 25 8C, using a 10 ml Ubbelohde viscometer.

These data permit a qualitative evaluation of the

Polyimide MM value because the intrinsic viscos-

ity and Polyimide MM are related via Mark�/

Kuhn Howink equation: [h ]tsolvent�/KMa , where

K and a are constants depending on the polymer

and the solvent. The experimental conditions are

shown in Table 1.

3. Results and discussion

Solubility of reagents is one of the crucial issues

for the polycondensation reaction. It is especially

important in the case of CO2 because only a few

classes of polymers have a considerable solubility

in sc CO2. The best known classes are fluorine-

containing polymers (polyfluoroalkyl acrylates,

fluorine-substituted polyethers, some of the poly-

perfluoroolefines and also their block and random

copolymers) and several silicon polymers (poly-

dimethylsiloxanes) [1].

Data on a new class of CO2-philic polymers

have been published recently. The polymers are

block copolymers of polyalkylene carbonate and

polyethylene oxide [15].

In our case the solubility tests showed that only

diamine 6F is soluble in sc CO2 among the

Fig. 1. FT-IR spectrum of polyimide (/h?inh 0.32 dl/g). Run conditions: dianhydride 6F and diamines mixture (6F�/AF) with molar ratio

(0.2:0.8), T�/180 8C, P�/325 bar, t�/6.5 h.

Erratum 125

reaction components under the run conditions.

Dianhydrides 6F and A exhibit a small swelling

degree, diamine AF and the polymers are insoluble

in sc CO2. Thus we suppose that the reaction

occurs in a heterogeneous medium. Fluorinated

polymers show a good solubility in many low-

temperature solvents: acetone, chloroform, THF,

etc. Such behavior is typical for aromatic PI

synthesized in high boiling solvents from dianhy-

drides and diamines containing bridging fluori-

nated groups [3]. The properties of the polymers

are shown in Table 2.

The polymer yield was practically quantitative

despite the heterogeneous reaction conditions.

Polymer viscosities in batch mode are rather low

(runs 1, 2, 5 and 6). Using flow mode and similar

other reaction parameters allows viscosity increase

approximately by a factor of 1.7 (up to 0.32 dl/g,

runs 6 and 7). Polymers synthesized in flow mode

show a high intensity of the imide cycle bands

(721, 1380 and 1730 cm�1). At the same time the

bands of anhydride cycle at 1860 cm�1, and those

of amide bond and 1654 and 3400 cm�1 are either

weak or absent at all (Fig. 1 and Tables 1 and 2;

runs 4, 7, 8, 11 and 12).

Polymers synthesized in batch mode show a low

intensity of anhydride bands, too. Therefore batch

mode (as well as flow mode) allows to reach a high

conversion monomers. However, in this case

oligomers are mostly produced. They should

have sufficiently great amount of end groups.

One of the reasons of why IR spectra of the

polymers synthesized in batch mode show a low

intensity of bands corresponding to anhydridegroups is both steps of polycondensation: forma-

tion of polyamide acid and its cyclization occur

almost simultaneously and then are followed by

chain growth. At the same time according to

Carothers: degree of polymerization DP�/1/1�/

p , where p is conversion. If p�/0.95 (it is close

to sensitivity limit of routine IR-spectroscopy

method) DP�/20 (only). On the other hand wehave the sample with h?inh 0.32 dl/g and

Mn(GPC)�/4700. MM of the elemental unit is

734. Simple calculations give p�/0.92. But never-

theless the anhydride groups in FT-IR spectrum of

the sample are practically absent (Fig. 1).

Equilibrium concentration of water in batch

mode is rapidly reached and chain growth is

ended. Flow mode is specially used to removethe excess of water. This allows to shift the

equilibrium towards formation of polymers having

higher molecular mass (Table 2, runs 6 and 7).

Polycyclization reaction takes easily place under

a wide change of run conditions. Thus polymers

were synthesized even in argon atmosphere (runs 3

and 3a), but they had low h?inh:/Viscosity values equal 0.14 dl/g (run 1) and 0.19

dl/g (run 3) indicate a low molecular mass poly-

mer. However, viscosity of polymers synthesized in

argon was somewhat greater than of those synthe-

sized in CO2. This might lead to the conclusion

that CO2 does not play any role in batch mode

synthesis. One can suppose the humid CO2 in the

batch conditions brings more quantities of the

water in the system compared with Ar and therebyit promotes an achievement of an equilibrium in

condensation�/hydrolysis reaction for shorter time,

that influences the chain length. The imide bands

of the polymer in run 3 have medium intensity

(very intensive in run 1), anhydride bands in both

runs are very low and amide bands in run 3 have

more intensity higher than medium (medium one

in run 1). Thus the reaction in sc CO2 in batchmode promotes a higher conversion of polyamic

acid into polyimide.

The runs in argon and CO2 in flow mode were

performed (Table 1 and Table 3, runs 3a and 3b)

to determine the role of CO2 in polycyclization.

Since the viscosities of the polymers in both cases

are similar, we concluded that there is no con-

Table 3

The properties of polyimide synthesized in Ar (3a) and CO2

(3b) gas flow, 180 8C, 3 h

Run numbers 3a 3b

Physical properties Yellow powder Canary powder

Yield, % 60 74

Inherent viscosity, dl/g 0.17 0.20

Imide cycle bands, cm�1 721m, 1380m,

1730m

721m, 1380s 1730s

Anhydride cycle bands,

cm�1

1860w, 1780w 1860 is absent,

1780w

Amide group bands 1654s, 3400s 1654w, 3400w

s, strong; m, medium; w, weak.

Erratum126

siderable equilibrium shift in the polycyclization

reaction under these conditions (perhaps a rate of

water removal is not enough). Comparison of the

spectral data (Fig. 2 and Fig. 3, Table 3, runs 3a

and 3b) shows the bands corresponding to the

anhydride end groups are practically absent for the

polymer synthesized in sc CO2 (very low for

polymer synthesized in Ar) and those correspond-

ing to the amide bond (1654 and 3400 cm�1) have

a relatively low intensity, whereas they are very

intensive for polymer synthesized in argon. Imide

cycle bands (721, 1370 and 1730 cm�1) are more

intensive for the CO2-sample as compared with Ar

one. Polyimide is slowly formed in Ar atmosphere

as well (Table 3). Thus a moderate monomers

conversion into PI occurs also in Ar atmosphere

however an essential part of the polymer is present

as polyamic acid structure. Synthesis in sc CO2

leads to higher polyimide yield. These data give an

evidence of accelerating catalytic effect of CO2 in

both reaction steps.

If there is a catalytic effect in sc CO2 the same

one as we supposed must be observed in a CO2

flow as compared with Ar one at the pressure of 1

bar. We have carried out such runs. The results are

given in Table 4.

It is seen from the table CO2 provides for a

higher polymer viscosity and yield. These data also

point to the catalytic action of CO2 in PI synthesis.

It is well known that carboxylic acids catalyze

the polyimidation reaction [13]. We used high

purity (�/99.997%, H2O�/0.001%) to prove a

participation of water in catalysis (Table 2, run

9). All the other conditions were the same as in the

run 8. Polymer with low viscosity was obtained.

The result was reproduced several times and the

polymer inherent viscosities were in the range of

0.14�/0.16 dl/g. Therefore we can suggest that sc

CO2 can act as an acidic catalyst in the presence of

a water admixture in the system.On the other hand, an attempt to create

catalytic conditions by adding water to the reac-

tion cell while using high-purity CO2 did not lead

to synthesis of high MM polymer (Table 2, run

10). It may be due to excessive quantity of

added water. There are no exact data on the

minimum concentration of water necessary for

catalysis.

Fig. 2. FT-IR spectrum of polyimide (/h?inh 0.17 dl/g). Monomers: dianhydride 6F and diamine mixture (6F�/AF) with molar ratio

(0.2:0.8) T�/180 8C, P�/1 bar (Ar flow), t�/3 h.

Erratum 127

Polymer viscosity values in runs 6 and 7 lead to

the conclusion that the synthesis mode plays

nevertheless a certain role in sc conditions namely

the flow mode of operation gives rise to higher

MM.

As it can be seen from Table 2, decrease of the

temperature in the run by 50 8C in batch mode

does not influence the MM (runs 1 and 2).

The increase of the reaction time from 3 to 6 h in

batch mode leads to a certain increase of the

inherent viscosity (runs 1 and 6). Therefore an

equilibrium is reached under these conditions in

more than 3 h. Increase of the reaction time in flow

mode leads to an increase of inherent viscosity too

(runs 4 and 7). If the reaction is carried out in flow

mode from reaction start, then monomers dis-

solved in CO2 can be carried away by the gas flow

and stoichiometry can be changed.

On the other hand as it was noted in [2], in case

of polycarbonate synthesis from 4,4-bis(hydroxy

Fig. 3. FT-IR spectrum of polyimide with h?inh 0.20 dl/g. Monomers: dianhydride 6F and diamine mixture (6F�/AF) with molar ratio

(0.2:0.8), 180 8C, 325 bar (in CO2 flow), 3 h.

Table 4

Dependence of polymer inherent viscosity and yield on gas

nature

Gas Inherent viscosity, dl/g Yield, %

Argon 0.43 84

Carbon dioxide 0.51 89

The conditions: 180 8C, 1 bar, m -cresol, 3 h.

Erratum128

phenyl)-2,2-propane and diphenylcarbonate, theoligomer precipitates from the solution in sc CO2

already after 1�/2 condensation acts. Therefore the

initial phase (1.5 h) of all the flow mode runs with

monomers soluble in sc CO2 was performed in

batch mode. Molecular mass of a sample with

inherent viscosity 0.32 dl/g was determined with

gel-permeation chromatographic technique: Mw�/

12 400, Mn�/4700, Mw/Mn�/2.6. Polymers withsimilar MM were synthesized with heterogeneous

polymerization: Polycarbonate (Mw�/4500�/

27 000, Mn�/2200�/11 000) and PET (M�/3000�/

6300) in sc CO2 [2].

Such moderate MM are most likely due to the

heterophase reactions conditions. It is known that

the reactions in solid phase develop slower than in

liquid phase. Heterophase conditions prevent theremoval of low molecular weight reaction products

and shift the equilibrium towards lower monomers

conversion, thus preventing the formation of high

MM polymers.

Nevertheless we succeeded in subsequent runs in

the synthesis of PI with h?inh up to 0.56 dl/g under

heterogeneous conditions. Therefore heteroge-

neous conditions in given system are not theprincipal limiting factor for reaching high MM.

On the other hand a higher degree of homogeniza-

tion facilitates the increase of polyimide MM.

(Table 1 and Table 2, runs 8, 12 and 11. In these

runs soluble diamine 6F was used. Homogeniza-

tion can also increase due to dianhydride A

melting because of depression of its melting point

in sc CO2.)Run 11 extends the set of potential monomers.

Run 12 shows that polymers with relatively high

molecular mass can be synthesized using mono-

mers insoluble in sc CO2. This opens wide

perspectives for carbon dioxide substitution of

toxic solvents in polyimide production with envir-

onment-friendly.

4. Conclusions

1) PI have been synthesized by one-step poly-

cyclization in sc carbon dioxide for the first

time.

2) A solubility of monomers is not a limitingcriterion for achievement of high molecular

weight of PI.

3) The flow mode of synthesis leads to an

increase of polyimide molecular weight.

4) The hypothesis about catalytic action of CO2

in synthesis of PI in the presence of water

admixture is suggested. Some confirming ex-

perimental data have been obtained.

Acknowledgements

This work was supported by Russian Founda-

tion for Basic Research, projects 02-03-32089 and

No. 01-03-32766a, by the Danish Research Coun-

cil THOR program FUCOMA. and by NATO

Programme ‘Science for Peace’, SfP-977998.

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