Heat-resistant resins derived from cyano-substituted Diels-Alder polymers

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Eur. Polym.J. Vol. 30, No. 4, pp. 465-472, 1994

Pergamon

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057/94 $6.00 + 0.00

HEAT-RESISTANT RESINS DERIVED FROM CYANO-SUBSTITUTED DIELS-ALDER POLYMERS CONSTANTINOS D. DIAKOUMAKOS a n d JOHN A. MIKROYANNIDIS* Chemical Technology Laboratory, Department of Chemistry, University of Patras, 261 I0 Patras, Greece

(Received 30 April 1993; accepted 9 June 1993) Abstract--Two structurally different monomers bearing furan terminal groups and cyano pendant groups were synthesized and polymerized with 4,4"-bismaleimidediphenylmethane (BMDM) through a Diels-Alder type reaction. In addition, furfurylamine reacted with half molar amount of terephthaloyl dichloride in the presence of triethylamine and the product obtained reacted with BMDM to yield a reference polymer. The starting materials as well as their intermediate compounds were characterized by FT-i.r., ~H-NMR spectroscopy and elemental analyses. The curing behaviour of monomers was investigated by DTA. It was shown that the Diels-Alder reaction between the maleimide and furan segments occurred at considerably lower temperature than that required for crosslinking of BMDM itself. Crosslinked resins were obtained upon curing the Diels-Alder polymers through their pendant cyano groups. They displayed higher thermal stability than that of reference polymer.

INTRODUCTION

M o r e recently, we synthesized fertain polyimides derived from Diels-Alder polymerization of furfurylsubstituted maleamic acids or from the reaction of bismaleamic with bisfurfurylpyromellitamic acids [4].

A r o m a t i c polyimides are well-known thermally stable polymers having m a n y technological applications [1]. They possess an unusual c o m b i n a t i o n o f o u t s t a n d i n g mechanical, electrical, thermal and chemical properties. On the other hand, bismaleimides are polymer precursors yielding widely used network polyimides with excellent thermal stability [1]. However, the cured bismaleimide resins are brittle due to their highly crosslinked structure. This disadvantage has been overcome by modification with r u b b e r or by chain-extended bismaleimide prepolymer molecules. The present investigation deals with the preparation of certain polyimides arising from A A a n d BB m o n o m e r s capable of Diels-Alder polymerization. M o r e particularly, three m o n o m e r s beating furan terminal groups were synthesized a n d they were polymerized with a bismaleimide. Some m o n o m e r s contained p e n d a n t cyano groups t h r o u g h them crosslinked polymers could be o b t a i n e d upon curing at relatively high temperatures. The synthesized polymers are expected to possess a n o u t s t a n d i n g thermal stability due to their network structure. In addition, the Diels--Alder polymerization of the maleimide group with the furan ring should take place at lower t e m p e r a t u r e than that required for crosslinking o f a c o m m o n bismaleimide itself. The present work is a c o n t i n u a t i o n o f o u r studies o n the p r e p a r a t i o n o f new thermally stable polymers arising from D i e l s - A l d e r polymerization. In earlier papers we reported the synthesis and polymerization o f certain furfurylidene[2]- or furoyl[3]-substituted maleamic acids. They were prepared by reacting the m o n o m a l e a m i c acid derived from a n aromatic diamine with furfural or 2-furoyl chloride, respectively.

EXPERIMENTAP LROCEDURES Characterization methods Melting temperatures were determined on an electrothermal melting point apparatus IA6304 and are uncorrected. FT-i.r. spectra were recorded on a Perkin-Elmer 16PC FT-i.r. spectrometer with KBr pellets. ~H-NMR spectra were obtained using a Varian T-60A spectrometer at 60 MHz. Chemical shifts (6 values) are given in parts per million with tetramethylsilane as an internal standard. DTA and TGA were performed on a DuPont 990 thermal analyser system. DTA measurements were made using a high temperature (1200 °) cell in N 2 atmosphere at a flow rate of 60 cm3/min. Dynamic TGA measurements were made at a heating rate of 20°/min in atmospheres of N 2 or air at a flow rate of 60 cm3/min. Elemental analyses were carried out with a Hewlett-Packard model 185 analyser.

Reagents and solvents 4,4'-Diaminodiphenylmethane, terephthaloyl dichloride and maleic anhydride were recrystallized from toluene, n-hexane and acetic anhydride, respectively. 2-Furoyl chloride was purified by distillation under N 2 atmosphere. Furfurylamine and triethylamine were also distilled. N,NDimethylformamide (DMF) was dried by refluxing and was fractionally distilled from CaH2. Cyanogen bromide, malononitrile, phosphorus oxychloride, chloroform as well as analytical grade acetone were used as supplied.

Preparation of starting materials 1-4 (Scheme 1) 4,4"-Bis(cyanamidefuroyl)diphenylmethane (1). A flask was charged with a solution of cyanogen bromide (10.3811 g, 98 mmol) in water (50 ml) which was cooled at 0 °. DMF (50ml) and sodium bicarbonate (6.4015g, 76.2 mmol) were added to the solution. 4,4'-Diaminodiphenylmethane (6.4636 g, 32.6 mmol) dissolved in DMF (50 ml) was subsequently added dropwise at 0-10 ° for 2 hr. The mixture was subsequently stirred at this temperature

*To whom all correspondence should be addressed. 465

466

CONSTANTINOS D. DIAKOUMAKOSand JOHN A. MIKROYANNIDIS

for additional 2 hr. The insoluble material was removed by filtration and the filtrate was added to cold water (500 ml). The white solid precipitated was filtered off, washed with water and dried in a vacuum oven at about 30° to afford la (7.68g, 95%). It was recrystallized from a mixture of 2-methoxy-ethanol/water (vol. ratio I: l) and it did not show a melting temperature upon gradual heating up to 300 °. i.r, (KBr)cm-I: 3500-3000 (N--H stretching); 2240 (C~N); 1618 (N--H deformation); 1518 (aromatic); 1258 (C--N stretching). ~H-NMR (DMSO-d6) 8:9.68 (s, 2H, NHCN); 7.26-6.80 (m, 8H, aromatic); 3.71 (s, 2H, CH2). Anal: Calcd for CI~HI:N4: C, 72.56%; H, 4.87%; N, 22.67%. Found: C, 71.98%; H, 4.91%; N, 22.52%. A flask was charged with a solution of la (I.2660g, 5.1retool) in DMF (20ml). Triethylamine (1.0322 g, I0.2 mmol) was added to the solution. 2-Furoyl chloride (I.3314 g, 10.2 mmol) diluted with DMF (10 ml) was dropwise added to the stirred solution under N z at 0 °. Stirring of the mixture was continued at ambient temperature in a stream of N 2 for 2 hr. It was subsequently poured into ice-water and the white solid was filtered off, washed with water and dried to afford l in 94% yield (I.25g). A purified sample obtained by recrystallization from a mixture of DMF/water (vol. ratio I:I) had m.p. 130-135 o. i.r. (KBr) cm - ~: 2230 ( ~ N ) ; 1701 (C----O); 1607 (C-----C); 1513 (aromatic); 1465 (CH2); 1020, 895 (furan ring). ~H-NMR (DMSO-d6) 8:7.83 (m, 2H, furan proton of 5 position); 7.564/.70 (m, 8H, aromatic and 4H, other protons of furan ring); 3.94 (m, 2H, CH2). Anal: Calcd for C25H~6N404: C, 68.79%; H, 3.69%; N, 12.84%. Found: C, 68.32%; H, 3.72%; N, 12.76%.

N,N "-Bis ( l -furyl- 2,2-dic yanov inyl)-4, 4"-diaminodiphenylmethane (2). 2-Furoyl chloride (5.0000 g, 38.3 mmol) and malononitrile (2.5507 g, 38.3 mmol) were dissolved in chloroform (50ml). To the vigorously stirred mixture an aqueous solution 6 N KOH (60 ml) containing a catalytic amount of tetramethylammonium bromide was added portionwise at 0 °. Stirring of the mixture at this temperature was continued for 2 hr. The precipitated white solid was filtered off, washed with isopropanol, and dried to afford 2a (4.50 g, 60%). It was recrystallized from ethanol 95% (m.p. 327-331 °). i.r. (KBr)cm-~: 3463 (OK); 2209 ( ~ N ) ; 1581, 1544 (C-----C); 1371,1361 (OK deformation and C - 4 ) stretching); 1020, 867 (furan ring). ~H-NMR (CDCI3) 8:7.97 (m, IH, furan proton of 5 position); 7.62-7.54 (m, 1H, furan proton of 3 position); 6.94-6.82 (m, IH, furan proton of 4 position). Anal: Calcd for CsH3N202K: C, 48.47%; H, 1.52%; N, 14.13%. Found: C, 48.01%; H, 1.53%; N, 14.10%. A flask was charged with a mixture of 2a (4.0000 g, 20.0 mmol) and phosphorus oxychloride (40 ml). It was refluxed for 2hr. Phosphorus oxychloride and volatile components were stripped off by distillation under reduced pressure. The residue was stirred at ambient temperature for about 30min with a mixture of chloroform/water. The organic layer was washed with water, dried with MgSO4 and concentrated by means of a rotary evaporator to afford 2b as a yellowish solid in 90% yield (2.01 g, m.p. 70-74°). i.r. (KBr)cm-J: 2209 ( ~ N ) ; 1575 (C----C); 1031, 884 (furan ring); 968, 926 (C--C1). ~H-NMR (CDCI3) 8:7.96 (m, IH, furan proton of 5 position); 7.63-7.57 (m, lH, furan proton of 3 position); 6.93-6.83 (m, 1H, furan proton of 4 position).

Anal: Calcd for CsH~N2OCI: C, 53.80%; H, 1.69%; N, 15.68%. Found: C, 53.36%; H, 1.71%; N, 15.60%. A flask was charged with a solution of 2b (l.3000g, 7.2mmol) and 4,4'-diaminodiphenylmethane (0.7194g, 3.6mmol) in acetone (20ml). Triethylamine (0.7286g, 7.2 mmol) was added to the solution and it was refluxed overnight. It was subsequently poured into ice-water and the purple solid precipitated was filtered off, washed with water and dried to afford 2 in 96% yield (1.70 g). A purified sample having m.p. 110-115 ° was obtained by recrystallization from a mixture of acetone/water (vol. ratio 1:2). i.r. (KBr)cm-~: 3235 (N--H stretching); 2209 ( ~ N ) ; 1586 (C---~-); 1560 (N--H deformation); 1523 (aromatic); 1465 (CH2); 1031, 879 (furan ring). IH-NMR (DMSO-d6) 8:8.16 (m, 2H, NH); 7.83 (m, 2H, furan proton of 5 position); 7.5(b6.66 (m, 8H, aromatic and 4H, other protons of furan ring); 3.96 (m, 2H, CH2). Anal: Calcd for C29H~sN602: C, 72.18%; H, 3.76%; N, 17.42%. Found: C, 71.53%; H, 3.80%; N, 17.31%.

N,N-Bisfurfurylterephthalamide (3). A flask was charged with a solution of furfurylamine (2.9699 g, 30.5 mmol) and triethylamine (3.0811g, 30.5mmol) in acetone (25ml). Terephthaloyl dichloride (3.0963 g, 15.2 mmol) dissolved in acetone was stepwise added to the stirrdd solution at 0 ° under N2. An exothermic reaction was observed and the mixture was stirred at ambient temperature in a stream of N z for 2 hr. It was subsequently poured into ice-water and the white solid obtained was filtered off, washed with water and dried to afford 3 in 71% yield (3.50g). A purified sample obtained by recrystallization from a mixture of DMF/water (vol. ratio 1:2) had m.p. 165-170 °. i.r. (KBr)cm-J: 3318 (N--H stretching); 1676 (amide C---10); 1540 (N--H deformation); 1500 (aromatic); 1425 (CH2); 1010, 870 (furan ring). IH-NMR (DMSO-d6) 8:9.18 (m, 2H, NHCO); 8.01 (m, 4H, aromatic and 2H, furan proton of 5 position); 7.61 (m, 2H, furan proton of 3 position); 6.38-6.32 (m, 2H, furan proton of 4 position); 4.50-4.38 (m, 4H, CH2). Anal: Calcd for CjsH~6N204: C, 66.65%; H, 4.97%; N, 8.64%. Found: C, 66.02%; H, 5.00%; N, 8.51%.

4,4"-Bismaleimidediphenylmethane (4). Maleic anhydride (3.9224 g, 40 mmol) dissolved in DMF (20 ml) was stepwise added to a solution of 4,4'-diaminodiphenylmethane (3.9654 g, 20 retool) in DMF (20 ml) at 0 ° under N 2. The mixture was stirred at ambient temperature in a stream of N2 for 2 hr. Acetic anhydride (5 ml) and a catalytic amount of fused sodium acetate were added to the solution and it was heated at 90 ° overnight. It was poured into ice-water and the brown solid was filtered off, washed with water and dried to afford 4 in 80% yield (5.73 g). A purified sample having m.p. 120-130 ° was obtained by recrystallization from a mixture of CH3CN/MeOH (vol. ratio 1:2). i.r. (KBr)cm-~: 1780, 1720 (imide C-----O); 1523 (aromatic); 1380, 730 (imide structure). ~H-NMR (DMSO-dD 8:7.67-7.15 (m, 8H, aromatic); 6.45 (m, 4H, olefinic); 3.88 (s, 2H, CH2). Anal: Calcd for C2~H~N204: C, 70.38%; H, 3.94%; N, 7.81%. Found: C, 69.74%; H, 4.00%; N, 7.80%.

Preparation of polymers (Scheme 2) A mixture of equimolar amounts of 4 and 1, 2 or 3 was heated in a shallow dish by means of a heating plate. The melt was stirred to obtain homogeneous mixture. It was subsequently cured by heating in an oven at 265 ° for 20 hr to yield a Diels-Alder polymer.

Heat-resistant resins derived from Diels-Alder polymers RESULTS AND DISCUSSION

Scheme l outlines the preparation of starting materials 1--4. More particularly, 4,4'-diaminodiphenylmethane reacted with cyanogen bromide to afford 4,4'-biscyanamidediphenylmethane (la). This reaction was carried out according to a published method [5-8]. 2-Furoyl chloride reacted with la utilizing triethylamine as acid acceptor to yield I. In addition, 2-furoyl chloride reacted with malononitrile in the presence of potassium hydroxide in a two phase system to afford the potassium enolate 2a. Tetramethylammonium bromide was used as phase transfer catalyst in this reaction. Compound 2a reacted with phosphorus oxychloride to yield 2b. Purification of 2b was accomplished by extracting the crude product with a mixture of water/chloroform. Thus the impurities remained in the water phase whereas the pure product was isolated by evaporation of the organic layer. Compound 2b was condensed with a half molar amount of 4,4'-diaminodiphenylmethane in the presence of triethylamine to yield 2. On the other hand, a reference compound 3 was synthesized from the reaction of furfurylamine with

467

terephthaloyl dichloride utilizing triethylamine as acid acceptor. Finally, 4,4"-bismaleimidediphenylmethane (4) was prepared from the reaction of 4,4'-diaminodiphenylmethane with maleic anhydride and subsequent cyclodehydration of the intermediate bismaleamic acid 4a using acetic anhydride and a catalytic amount of sodium acetate. The starting materials and their intermediate compounds were characterized by FT-i.r., JH-NMR spectroscopy and elemental analyses (see Experimental Procedures). Figure l presents the FT-i.r. spectra of compounds l, 2 and 3. The presence of cyano groups in 1 and 2 was confirmed from their sharp absorptions at 2230 and 2209 cm-~, respectively. Compound 2 displayed stronger absorption band of the C ~ N groups than that of 1 due to the higher concentration of these segments. The absorption around 1700 cm- ~of I and 3 was associated with their carbonyl groups whereas compound 2 lacked this absorption. Figure 2 presents typical ~H-NMR spectrum of compound 2 in DMSO-d6 solution. It showed peaks at 8.16 (2H, NH); 7.83 (2H, protons next to oxygen

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468

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Fig. 1. FT-i.r. spectra of 1 (top) and 2 (middle) as well as of 3 (bottom). of furan ring); 7.50-6.66 (8H aromatic and 4H, other protons of furan ring) and 3.96 6 (2H, CH2). Homogeneous mixtures of bismaleimide 4 with an equimolar amount of compound 1, 2 or 3 were prepared by melting and stirring of these compounds. Prolonged heating of these mixtures was avoided to

preclude or minimize Dieis-Alder reactions between the maleimide and furan moieties. The curing behaviour of the mixtures as well as of the starting materials themselves was investigated by DTA. Figure 3 presents the DTA traces of the mixture (2 + 4) as well as of its ingredients in N2. Compounds

Heat-resistant resins derived from Diels-Alder polymers

469

NCr"bN

NC~'CN

(2)

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maleimide and furan segments occurred at significantly lower temperature than the temperature required for crosslinking of bismaleimide 4 itself. This curing feature conformed with our previous data [4]. An analogous behavior was observed in the curing temperature of the mixture (1 + 4) whose the DTA trace showed exotherm at 227 °. Scheme 2 outlines the polymerization reactions of monomers. Upon curing at relatively high

2 and 4 themselves showed an exotherm at 313 and 279 °, respectively. The mixture (2 + 4) displayed a well distinguished exotherm at 200 ° attributable to Diels--Alder reactions since the corresponding cured (at 265 ° for 20 hr) sample lacked an exotherm at this temperature range (Fig. 3). Note that the T G A thermogram of the mixture (2 + 4) in N2 displayed a weight loss of about 20/, at 200 ° . Thus the Diels--Alder polymerization reactions between the

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Fig. 3. D T A traces of 2, 4 and the mixture (2 + 4) as well as of cured polymer (2 + 4)' in N~. Conditions: N 2 flow 60 cm3/min heating rate 20°/min.

CONSTANTINOSD. DIAKOUMAKOSand JOHN A. MIKROYANNIDIS

470

O

O

0

0

(Fig. 4). In the case of monomer 2, a rearrangement at least to a measure, of the enamino nitrile segment to 4-aminoquinoline could take place during curing at high temperatures [12-18] (>250°). The crosslinked polymers obtained upon curing at 265 ° for 20 hr the mixtures (1 + 4), (2 + 4) and (3 + 4) are referred to by the designations (1 + 4)', (2 + 4)' and (3 + 4)' respectively. Bismaleimide 4 was cured itself under same conditions to afford a crosslinked polymer 4' for comparative purposes. The thermal stabilities of polymers were ascertained by dynamic TGA and isothermal gravimetric analysis (IGA). Figure 5 presents their TGA traces in N 2 and air atmospheres. The initial decomposition temperature (IDT), the polymer decomposition temperature (PDT) and the maximum polymer decomposition temperature (PDTm,,) both in N2 and air as well as the char yield (Yc) at 800 ° in N: for all polymers are summarized in Table 1. PDT was determined for a temperature at which 10% weight loss was observed. PDT,,a, corresponds to the temperature at which the maximum rate of weight loss occurred. Taking the IDT as creterion of thermal stability, the relative order of thermal stability was as follows:

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(2+4)'>/4'>(1+4)'>(3+4)' Thus the cyano-substituted monomers I and 2 afforded more thermally stable polymers than did the reference monomer 3. The thermal stability of 4' was equal or slightly lower than that of polymer (2 + 4)'. Polymers (1 + 4)' and (2 + 4)' afforded higher char yield than did the reference polymers (3 + 4)' and 4'. Since the trimerization reaction of the cyano groups is a very slow reaction occurring at high temperatures, it is reasonable to accept that this reaction was not completed under the utilized curing conditions in the present study. Evidence for this feature was obtained from the FT-i.r. spectra of (1 + 4 ) ' and

Scheme 2 temperature, the intermediate endoxide ring was dehydrated to a more stable aromatic system [2--4, 9]. The curing process of monomers was complex and afforded crosslinked polymers through the trimerization reaction of the cyano groups [10, 11]. The cured resins were completely insoluble even in polar aprotic solvents as well as in H.,SO4 98%. Their FT-i.r. spectra displayed a considerable reduction of the absorption band assigned to the ~-~N groups

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800

Fig. 4. FT-i.r. spectra of cured polymers (1 + 4)' (top) and (2 + 4)' (bottom).

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Heat-resistant resins derived from Diels-Alder polymers

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Fig. 5. TGA thermograms of cured polymers (1 + 4)', (2 + 4)" and (3 + 4)' as well as of 4' in N 2 and air. Conditions: gas flow 60 cm3/min; heating rate 20/min.

known [19] that curing of cyano-substituted compounds in the presence of a catalytic amount of an aromatic diamine enhances their thermal stability. Figure 6 presents the I G A traces of polymer (2 + 4)' at 280, 300 and 320 ° in static air. After 20 hr isothermal aging at these temperatures, the polymer showed a weight loss of 14.0, 25.4 and 40.7%, respectively.

(2 + 4 ) ' that exhibited residual absorption of the ~N groups (Fig. 4). To improve thermal stability of polymers (1 + 4)' and (2+4)' they were mixed with 4,4'diaminodiphenylmethane (ca 5 0 by weight) and additional curing was accomplished at 300 ° for 20 hr. After this treatment, they showed I D T values 422 and 426 ° respectively and Yc at 800 ° 7 5 0 in N2. It is well

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(C)

(C)

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58

355

426

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*Initial decomposition temperature. ?Polymer decomposition temperature. SMaximum polymer decomposition temperature. {}Char yield at 800 .

472

CONSTANTINOSD. DIAKOUMAKOSand JOHN A. MIKROYANNIDIS

Acknowledgement--A grant from the Greek Ministry of Industry, Energy and Technology (General Secretariat of Research and Technology) in partial support of this work is gratefully acknowledged.

REFERENCES

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