Reactivity of polyamic acids in thermal cyclization

Eur. Polym. J. Vol. 29, No. 4, pp. 593-596, 1993

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REACTIVITY OF POLYAMIC ACIDS IN THERMAL CYCLIZATION ANDRZEJ ORZESZKOl and ANDRZEJ SIKORSKI2 tlnstitute of Chemistry, Agricultural University, ul. Rakowiecka 26/30, 02528 Warsaw, Poland 2Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02093 Warsaw, Poland

(Received 11 May 1992) Abstract--The reactions of pyromellitic anhydride, 3,3,'4,4'-benzophenonetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, o,L-bis(N-2-giutaric anhydride) pyromellitimide, O,Lbis(N-2-succinic anhydride) pyromellitimide and 1,2,3,4-cyclobutanetetracarboxylic dianhydride with aniline and benzidine lead to amic and polyamic acid. Thermal cyclization of these compounds gives the corresponding imides and the process was studied by DSC. It was shown that aromatic imides form at lower temperatures than aliphatic. Molecular modelling was used to determine geometrical parameters and space structures of five- and six-membered aromatic and aliphatic imide rings.

INTRODUCTION The reaction of tetracarboxylic dianhydrides and aromatic or aliphatic diamines is the most effective method for synthesis of polyimides. The mechanism and kinetics of this process have been examined and described in some detail [1-5]. It is well known that this reaction proceeds in two stages. First, addition of amine groups to cyclic dianhydrides leads to polyamic acids. Then, from these prepolymers, the corresponding polyimides are obtained by thermal or chemical dehydration. The influences of physical and chemical conditions on this reaction were considered in previous publications [3-5]. We found that polyamic acids obtained from diamines of high basicity (pKa) form imide rings at lower temperatures than prepolymers synthesized from amines of low basicity. Chemical cylization also depends on the pKa of the amine and in some cases not only imide but also isoimide rings are obtained

polyamic acids obtained from benzidine and the same anhydrides: P M D A (7), BPDA (g), NTCDA (9), G A P (10), SAP (11) and CBTDA (12). The above mentioned agents were selected to yield five- and six-membered aromatic or aliphatic imide cycles. The process of thermal cylization was observed by means of differential scanning calorimetry (DSC) and i.r. spectroscopy. For better understanding of the rate of cyclization and the stability of the polyamic acids and also to determine their geometrical parameters, we carried out molecular modelling. We studied simple molecules (N-phenyl substituted imides) only because of technical problems for polyamic acids. For this purpose, we applied the A L C H E M Y II program (by Tripes Associates, Inc.) which contains an energy minimizer. The potential energy and the geometry of the most probable conformations of model molecules were estimated.

[3--6].

Pyromellitic and trimellitic anhydrides are the most important agents used in the synthesis of polymers containing imide rings in the main chain but some results for other dianhydrides have been published [7-11]. From these publications as well as from our experience, it appears that the rate of cyclization of polyamic acids and properties of imides strongly depend on the kind of anhydride used [3, 4, 9]. Thus, in this paper we show how various anhydrides react with amines and form different types of imide rings. To this end, low molecular weight amic acids were synthesized from aniline and the following anhydrides; pyromellitic dianhydride (PMDA) (1), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPDA) (2), 1,4,5,8 naphthalenetetracarboxylic dianhydride (NTCDA) (3), D,L-bis(N-2glutaric anhydride)pyromellitimide (GAP) (4), D,L,bis(N-2-suceinic anhydride)pyromellitimide (SAP) (5) and 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBTDA) (6). For comparison, we also studied the

EXPERIMENTAL PROCEDURES

Materials All chemicals were of analytical grade, commercial products, and were used as received. Two anhydrides GAP (4) and SAP (5) were obtained and identified according to our previous description [9]. For most of the models and polymers, the methods of synthesis are known and so identification and purity control were provided by TLC and i.r. spectroscopy only. Synthesisof model compounds PMDA (1.09 g, 5 mmol) was dissolved in 10 ml of dry acetone; 0.95 g (10.5 retool) of aniline was added. The crude precipitate was crystallized from ethanol (95%). The structure of model 1 was confirmed by i.r. (--NH 3250cm -I, ---OH 2700-3500em -t, C--------O1635em-~). Yield 75%. BPDA (1.61 g, 5 retool) was dissolved in 10 ml of dry acetone; 0.95 g (10.5 retool) of aniline was added. Acetone was evaporated under vacuum and the crude product was crystallized from ethanol. The structure of compound 2 was confirmed by i.r. (--NH 3285 cm -~, -----OH2650-3500 crn -~, ~ 1690cra-~). Yield 60%. 593

ANDRZI~ ORZI~ZKO and ANDRZl~ SIKORSKI

594

NTDA (1.52 g, 5 mmol)was dissolved in 20ml of DMF and 0.95 g of aniline was added. A dark solid was filtered off and washed with acetone, i.r. showed that model (3) did not have characteristic bonds for pure amic acid. Yield 84%. Models 4 and 5 were synthesized and examined according to the published account [9]. CDTA (0.98 g, 5 retool) was dissolved in 10 ml of DMF; 0.95 g of (10.5 mmol) of aniline was added. White crystals precipitated in a few minutes. They were filtered off, washed with acetone and dried at 30°. The structure 6 was confirmed by i.r (--NH 3300 crn -I, - - O H 2800-3500cm -~, ~ 1640cm-~). Yield 80%.

Synthesis of polymers Polyamic acids were synthesized from benzidine and PMDA (7), BPDA (8) NDTA (9), GAP (10), SAP (11) and CBTDA (12) by conventional condensation in DMF at room temperature. Then, the solutions of polymers were poured into methanol. Polyamic acids were filtered off, dissolved and

reprecipitated, washed with acetone and dried at 30° in vacuum. Infrared spectra show characteristic amide and carboxylic bonds for all compounds under consideration with the exception of polymer 9 obtained from NTDA. Yields of all polymers were about 100%.

Measurements Calorimetric studies were carried out in a dynamic inert gas atmosphere using a Differential Scanning Calorimeter (Du Pont 1090). Model compounds and polyamic acids were heated to 350° at 10°/rain. The samples, after the calorimetric measurement, were examined by i.r. Spectra were recorded on a Carl Zeiss spectrophotometer UR-20 using KBr discs.

Molecularmodelling Model molecules were built using the ALCHEMY II program by the sequentional adding of appropriate atomic groups. In order to find the most probable conformations, we used the ALCHEMY minimizer. It uses a conjugate

Table 1. Thermal cyclization and calculated structures of the amic acids studied Geometrical parameters Distances (A) Angles (o) Model

I" 7" 2"

Tc(°C)

190 166 186 161

Infrared

1:

12

13

1780, 1760, 710 1780, 1720, 730 1770, 1730, 710 1775, 1780, 720

1.38

1.51

1.34

/4

ct

1.42 111

/3

7

¢o

107

105

125

Picture

~

N-_.../"

(A)

3b

Room

1700, 1650, 700

9b

Room

1720, 1670, 710

1.40

1.51

1.34 !.42

121

117

119

122

(B) II

4b lO b

241

--

240

--

!.54

1.51

1.34 1.42

118

113

109

~

123

(c) /

11' 6" 12"

255 252 250 250

1755, 1700, 715 1760, 1700, 720 1770, 1685, 740 1775, 1690, 740

1.54 !.50

! . 3 3 1.42

115

106

I00

/

123

(i)) (a)

(b) lx/~'Q C~ l,

!t

Ph

o

~ C ~PY_t~ !4 Ph

\

i

Reactivity of polyamic acids in thermal cyclization gradient procedure to find minima of the internal (potential) energy. The total energy of a model molecule consists of contributions arising from several types of interactions, viz. bond stretching, valence angle bending, torsion deformation, van der Waals interactions and out-of-plane bending. After every iteration, all atoms of a molecule are simultaneously moved. In order to find a global minimum of the potential energy, we performed the minimalization process many times starting from different initial conformations and by studying the energy of parts of these molecules. After the most probable conformations (the potential energy minimum) had been found, we measured geometrical parameters of these models using the same ALCHEMY II program. It should be pointed out that the ALCHEMY II program does not include charge distribution in the model molecule and so it does not calculate coulombic interactions. All results concern single molecules (not a solution) and do not include solvent effects.

595

~

o* /

~ o| :~

I 100

I 200

t 300

400

(oC) Fig. I. Endothermic effects of the cyclization of amic acids (a) 7 and (b) 12.

RESULTS AND DISCUSSION The syntheses of the anhydrides (described above) with aniline and benzidine lead to the corresponding amic acids. However, i.r spectra showed that, when using NTDA, six-membered aromatic imide tings were formed instead of amide and carboxylic bonds almost immediately at room temperature. This observation is in agreement with Hodgkin's data [7]. The model compounds (1-6) and polymers (7-12) were studied by DSC. During calorimetric measuremerits, heated samples lost water and were converted into imides according to the scheme: CO /CONH-_H20 / ~ R ~ --R N-~COOH ~ CO / where R is one of the following substituents:

1, 2, 7, 8

i

Y

l

--

C~ T

C~ i

C -T

--

C D,--C--

i

!

T

l

for

3, 9

for

4, 10

for

5,6,11,12

DSC measurements show that the cyclization processes are strongly endothermic. An interesting fact is that aromatic amic acids 1, 2, 3, 7, 8 and 9 lose

water at much lower temperatures than aliphatic 4, 5, 6, 10, 11 and 12. Temperature (To) of the maximum rate of imidization are listed in Table 1. DSC curves for amic acids (12) and (7) are shown in Fig. I as examples. Curve 'a' shows the cyclization of prepolymer (7). Curve 'b' (prepolymer 12) shows that there is a clear difference between the temperatures of the thermal cyclization for aliphatic and aromatic amic acids. As shown in Table 1 and Fig. 1, aromatic imide tings were formed at lower temperatures than aliphatic. For better recognition of geometrical properties of four kinds of imide rings, simple molecules of Nphenyl substituted imides containing such types of rings were studied by molecular modelling. Valence angles and bond lengths for these rings were found by means of A L C H E M Y II program and are included in Table I. These parameters are similar to X-ray spectroscopic data obtained for pyromellitimide tings many years ago [12]. There are also computer pictures of these molecules in Table 1. The shown conformations correspond to the minima of the internal energy of imides under consideration. One can see that the bonds lengths are almost identical for all rings but bond angles of five-membered cycles are different from those of six-membered imides. For that reason, i.r. characteristic frequencies of most of the model molecules and polymers are similar. For aromatic six-membered imides, absorption bonds are slightly shifted towards longer wavelenghts. The same behaviour was described by Hodgkin [7]. There are no spectroscopic data for compounds 4 and 10 in Table 1 because cyclization of these amic acids proceeds with simultaneous gradual decomposition [9].

CONCLUSIONS DSC measurements showed that the thermal cyclization of amic acids occurs with a strong endothermic effect. It was confirmed that aromatic imide rings were formed more easily than aliphatic. Computer

596

ANDRZFJ ORZ~SZKO and Abrt)RZ.~JSIzoltSrd

calculations provide detailed information about imide cycles. Interatomic distances are almost identical for all the kinds of imides under consideration but bond angles are different for aliphatic and aromatic rings. Unfortunately, there is no correlation between geometrical parameters and the ease of cyclization because of many complex kinetic and thermodynamic factors. For example, the influence of the acidity of amic acids is apparently stronger than the shape and dimensions of the rings being formed. As known, most aromatic amic acids have higher values of pK, than aliphatic acids. According to our previous observation, higher acidity of amic acids should make the cychzation process earlier [5, 6]. Acknowledgements--The authors thank Professor Andrzej Kolinski for helpful discussion and also assistance in using ALCHEMY II program. We thank Dr Boleslaw Kowalski for DSC measurements.

nEF~nr.NCV.s 1. S. D. Bruck. Polymer 4, 435 (1964). 2. C. E. Sroog. J. Polym. $ci. Part .4 3, 1373 (1965). 3. G. M. Bower and L. W. Frost. J. Polym. $ci. A-l, 3135 (1962). 4. A. L. Endrey. Can. Pat. 645, 073 E.I. Do Point and Co. (1962). 5. J. Zurakowska-Orszagh, A. Orzeszko and T. Chreptowicz. Fur. Polym. J. 16, 289 (1980). 6. A. Orzeszko and W. Kosinska-Banbula. Fur. Polym. J. 27, 1107 (1991). 7. J. H. Hodgkin. J. Polym. Sci.; Polym. Chem. Edn 14, 409 (1976). 8. J. S. Vygodski. Vysokomolek. Soed. A19, 1516 (1977). 9. A. Orzeszko. J. appl. Polym. Sci. 42, 2349 (1991). 10. H. R. Kricheldorf and P. Jahnke. Fur. Polym. J. 26, 1009 (1990). I 1. H. R. Kricheldoff, R. Pakull and S. Buchner. Macromolecules21, 1929 (1988). 12. J. G. Baklagina. Vysokomolek. Soed. A18, 1235 (1976).