Phenylethynyl-Terminated lmide Oligomers and Polymers Therefrom

P. M. HERCENROTHER,’* R. C. BRYANT,’ B. J. JENSEN,’ and S. J. HAVENS*

’NASA Langley Research Center, Hampton, Virginia 23681 -0001, 2Lockheed Engineering and Sciences Company, Hampton, Virginia 23666

SYNOPSIS

Two new phenylethynyl endcapping compounds, 3- and 4-amino-4‘-phenylethynylbenzo- phenone, were synthesized and used to terminate imide oligomers from 3,4’-oxydianiline and 4,4’-oxydiphthalic anhydride at a calculated molecular weight of 9000 g/mol and from 3,4‘-oxydianiline (0.85 mol) , 1,3-bis (3-aminophenoxy) benzene (0.15 mol) , and 3,3’,4,4’- biphenyltetracarboxylic dianhydride at a calculated molecular weight of 5000 g/mol. Glass transition temperatures for the cured oligomers were - 249°C for the former and - 272°C for the latter. Films cured at 350°C for 1 h were tough and flexible and provided high tensile properties. The uncured oligomers were readily compression molded to provide tough, solvent-resistant moldings. 0 1994 John Wiley & Sons, Inc. Keywords: oligomers phenylethynyl termination thermally stable polymers polyimides

high-performance polymers films

INTRODUCTION

Recent research has demonstrated the advantages of using the phenylethynyl group as a reactive end group for polymers versus the ethynyl The phenylethynyl group also cures without the evolu- tion of volatiles, but does so at temperatures higher than that of the ethynyl group, thus providing a larger processing window. Cured polymers from phenylethynyl-terminated precursors also exhibit better thermooxidative stability than those from ethynyl-terminated precursors.

4- ( 3 and 4-Aminophenoxy ) -4‘-phenylethynyl- benzophenone have proven particularly useful as reactive end-capping compounds for imide oligo- m e r ~ . ~ As a continuation of this research, the syn- thesis and evaluation of two new phenylethynyl endcapping compounds, 3- and 4-amino-4’-phenyl- ethynylbenzophenone, are disclosed. These new compounds were used to terminate imide oligomers of two different compositions at calculated number- average molecular weight of 5000 and 9000 g/mol.

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 32,3061-3067 (1994) 0 1994 John Wiley & Sons, Inc. CCC 0%37-624X/94/163061-07

The chemistry and properties of these materials are discussed. This work was conducted to develop readily processable, high-performance structural resins for aerospace applications. These materials should have a favorable combination of processa- bility and performance.

EXPERIMENTAL

Starting Materials

4,4’-Oxydiphthalic anhydride (4,4’-ODPA) was ob- tained from Occidental Chemical Corporation and dried under vacuum at 180°C (mp 224-2255°C). 3,4’-Oxydianiline ( 3,4’-ODA) was obtained from Wakayama Seika Kogyo Co., Ltd., and recrystallized from ethanol-water (mp 82-84OC). 3,3’,4,4’-Bi- phenyltetracarboxylic dianhydride ( S-BPDA ) was obtained from Ube Industries, Ltd. and dried under vacuum at 250°C (mp 295-297°C). 1,3-Bis (3-ami- nophenoxy ) benzene ( APB ) was used as obtained from National Starch and Chemical Corporation ( mp 104-106°C). 3-Nitrobenzoyl chloride, 4-nitro- benzoyl chloride, and bis ( triphenylphosphine ) pal- ladium( 11) chloride were used as-received from Ald- rich Chemical Company. Triethylamine and pheny-

3061

3062 HERGENROTHER ET AL.

lacetylene were distilled and phthalic anhydride was sublimed before use. The various solvents were used as-received.

4-Bromo-3'-nitrobenzophenone

Anhydrous aluminum chloride (83.3 g, 0.65 mol) was added over a 10 min period to a mechanically stirred solution of 3-nitrobenzoyl chloride (92.79 g, 0.50 mol) in bromobenzene (527 mL, 5.0 mol) a t 5-10°C. The mixture was stirred a t room temperature for 1 h, heated a t 70-80°C for 3 h, then stirred overnight a t room temperature. The mixture was added to 3 L of ice water containing 200 mL of concentrated hydrochloric acid. The aqueous layer was decanted from the emulsion, which was washed with several portions of water. Bromobenzene was removed by co-distillation with water. The residue was washed with water, dried and recrystallized from ethanol to afford 4-bromo-3'-nitrobenzophenone ( 101.3 g, 66% yield) as pale yellow needles: mp 107-108.5"C (lit." mp 109.5"C).

4-Bromo-4'-nitrobenzophenone

This compound was similarly prepared using 4-ni- trobenzoyl chloride. Recrystallization from 2-pro- panol afforded 4-bromo-4'-nitrobenzophenone (80% yield) as pale yellow crystals: mp 122-123°C (lit.'' mp 122°C).

3-Nitro-4'-phenylethynylbenzophenone

A mechanically stirred mixture of 4-bromo-3'-nitro- benzophenone (30.61 g, 0.10 mol), phenylacetylene (11.24 g, 0.11 mol), bis (tripheny1phosphine)pal- ladium( 11) chloride (0.10 g), triphenylphosphine (0.20 g) , and triethylamine (260 mL) was heated to - 60°C. Cuprous iodide (0.10 g) was added and the mixture stirred a t reflux for 2 h. The mixture was allowed to cool and the precipitated solid was collected by filtration and washed with triethyl- amine. The solid was stirred with water containing 100 mL of concentrated hydrochloric acid, collected by filtration, washed with water, and dried. Recrys- tallization from 1-butanol provided 3-nitro-4'- phenylethynylbenzophenone (28.14 g, 86% yield) as white crystals: mp 185-186°C. IR (KBr): 3086,2209 (CEC), 1654 and 1612 (C=O) , 1601 cm-'. MS (re1 intensity), m / e : 327.3 (38%, M+) , 205.2 (10076, P h C=CPhCO+ ) .

ANAL. Calcd for C2,H13N03: C, 77.05%; H, 4.00%; N, 4.28%. Found C, 76.54%; H, 4.26%; N, 4.02%.

4-Nitro-4'-phenylethynyl benzophenone

This compound was similarly prepared using 4- bromo-4'-nitrobenzophenone. Recrystallization from toluene provided 4-nitro-4'-phenylethynyl- benzophenone (82% yield) as pale yellow crystals: mp 192.5-193.5"C. IR (KBr) : 3086, 2212 (CEC), 1656 ( C = O ) , 1602 cm-'. MS (re1 intensity), m / e : 327.1 (53% M+) , 205.1 (loo%, Ph C=CPhCO+ ) .

ANAL. Calcd for C21H13N03: C, 77.05%; H, 4.00%; N, 4.28%. Found: C, 76.84%; H, 3.94%; N, 3.81%.

3-Amino-4'-phenylethynyl benzophenone (3A4'PEB)

A solution of sodium sulfide nonahydrate (48.04 g, 0.20 mol) in water (120 mL) was added over 1 h to a mechanically stirred suspension of 3-nitro-4'- phenylethynylbenzophenone ( 16.37 g, 0.050 mol) and ammonium chloride (10.70 g, 0.20 mol) in ethanol (120 mL) at reflux temperature. The mix- ture was stirred at approximately 75°C for 2 h. Wa- ter (240 mL) was added and the mixture was allowed to cool. The precipitated solid was collected by fil- tration, washed with water, and dried. Recrystalli- zation from ethanol provided 3A4'PEB ( 11.26 g , 76% yield) as yellow crystals: mp 139-141°C. IR ( KBr): 3463, 3356, and 3208 (NH,) , 2217 (C=C), 1646 cm-' (C=O) . 'H-NMR (CDC13), 6: 3.63 (s, 2H, NH,), 6.7-8.0 (m, 13H, aromatic). MS (re1 inten- sity), m / e : 297 (7496, M + ) , 205 (loo%, Ph C=CPhCO+ ) .

ANAL. Calcd for C21H15NO: C, 84.82%; H, 5.08%; N, 4.71%. Found C, 84.99%; H, 5.12%; N, 4.52%.

4-Amino-4'-phenylethynyl benzophenone (4A4'PEB)

A mechanically stirred mixture of 4-nitro-4I-phe- nylethynylbenzophenone (32.73 g, 0.10 rnol), am- monium chloride ( 12.52 g, 0.234 mol) , sodium sulfide nonahydrate (56.04 g 0.234 mol) , ethanol (500 mL) , and water (50 mL) was heated a t reflux for 4 h. Solvents were evaporated and the resulting solid washed with water, collected by filtration, and dried a t 100°C. Recrystallization from 1-butanol provided 4A4'PEB (22.92 g, 77% yield) as yellow crystals: mp 195-196.5"C. IR (KBr): 3422, 3340, and 3224 ( NH2), 2216 ( C e C ) , 1637 and 1628 cm-' ( C = 0). 'H-NMR (DMF-d7),6:6.28 (s,2H,NH2),6.65-7.9 (m, 13H aromatic). MS (re1 intensity), m/e: 297.2 (47%, M S ) , 120 ( loo%, H,NPhCO+) .

PHENYLETHYNL-TERMINATED IMIDE OLIGOMERS 3063

ANAL. Calcd for C2>H15NO: C, 84.82%; H, 5.08%; N, 4.71%. Found: C, 84.60%; H, 5.26%; N, 4.60%.

N- (4-Phenylethynylbenzoyl-3’- pheny1)phthalimide

A mixture of 3-amino-4’-phenylethynylbenzophen- one (2.97 g, 0.010 rnol), phthalic anhydride ( 1.48 g, 0.010 mol), and glacial acetic acid (50 mL) was heated at reflux for 16 h and then cooled. The pre- cipitated solid was collected by filtration, washed with water, and dried at 100°C. Recrystallization from 1-butanol provided a white solid (3.50 g, 82% yield): mp 195-195.5”C. IR(KBr): 2214 (CFC), 1727 (imide C=O) , 1654 (C=O). MS (re1 inten- sity), m / e : 427.3 (6%, M+), 69.1 (100%).

ANAL. Calcd for C29H17N03: C, 81.49%; H, 4.01%; N, 3.28%. Found C, 81.38%; H, 3.93%; N, 3.10%.

N- (4-Phenylethynylbenzoyl-4’- phenyl ) phthalimide

This compound was similarly prepared using 4- amino-4’-phenylethynylbenzophenone, followed by recrystallization from 1-butanol to provide white crystals (71% yield) : mp 204-205°C. IR ( KBr ) : 2216 (C-C), 1774 and 1718 (imide C = O ) , 1654 cm-’ (C=O).MS(relintensity),m/e:427.4 (6%,M+) 69.1 (100%).

ANAL. Calcd for CZ9Hl7NO3: C, 81.49%; H, 4.01%; N, 3.28%. Found: C, 81.38%; H, 3.82%; N, 2.89%.

Synthesis of Phenylethynyl-Terminated Oligomer (9000 g/mol)

4,4’-ODPA (13.9599 g, 0.0450 mol) was added to a mechanically stirred solution of 3,4’-ODA ( 8.5482 g, 0.04269 mol) and either 3- or 4-amino-4’-pheny- lethynylbenzophenone ( 1.3738 g, 0.00462 mol) in N-methylpyrrolidinone (NMP, 54 mL) . The solu- tion was stirred at room temperature for 48 h. A portion of the polyamide acid solution was diluted to a concentration of 20% (w/w) with NMP and used to cast a film (doctor blade setting, 23 mils) onto plate glass. Toluene (50 mL) was added to the remainder of the solution and a toluene / water mix- ture was removed by azeotropic distillation using a Dean-Stark trap. The reaction mixture was diluted to 15% (w/w) by the addition of additional NMP and the mixture maintained at 180-190°C for 2 h. The precipitated imide oligomer/NMP mixture was added to methanol in a blender and the solid was

collected by filtration, washed with boiling methanol, and dried under vacuum at 220°C to give a white powder. The powder had a volatile content of - 0.6% as determined by heating at 350°C for 0.5 h. The film on glass was allowed to dry to a tack-free form in a dust-free chamber, then thermally converted to the polyimide by heating in air at 100, 225, and 350°C for 1 h at each temperature. Mechanical properties of the 0.051-0.076 mm (2-3 mil) thick film were determined according to ASTM D882 us- ing four to five specimens per test condition.

Synthesis of Phenylethynyl-Terminated Oligomer (5000, g/mol)

This material was similarly prepared by the addition of S-BPDA (13.2399 g, 0.0450 mol) to a stirred so- lution of 3,4’-ODA (6.9684 g, 0.03480 mol), APB ( 1.7950 g, 0.00614 mol) , and either 3- or 4-amino- 4’-phenylethynylbenzophenone (2.4146 g, 0.00812 mol) in NMP (95 mL). At a temperature of 180- 190°C the imide oligomer remained in solution, but precipitated upon cooling. The imide oligomer pow- der was isolated by filtration, washed with boiling methanol, and dried under vacuum at 240°C. Vol- atile content of the powder was - 0.6%.

Moldings

Neat resin moldings of the phenylethynyl-termi- nated imide oligomers were fabricated by compres- sion molding of the powder at 350°C under 1.38 MPa (200 psi) for 1 h in a 3.2 cm (1.25 in) square stainless steel mold. Compact tension specimens [1.57 X 1.57 X 0.64 cm thick (0.62 X 0.62 X 0.25 in. thick) ] were cut from the 8-9 g moldings and subsequently tested to determine fracture toughness ( K1, , critical stress intensity factor) according to ASTM E399 using four specimens per test.

Characterization

Visual melting points were determined using a Thomas-Hoover capillary melting point apparatus and are uncorrected. Proton nuclear magnetic res- onance ( ‘H-NMR) spectra were taken on a Varian EM 360 A spectrometer with tetramethylsilane as internal standard. Infrared spectra were obtained using a Perkin-Elmer 1600 FT-IR. Mass spectra were obtained using a Finnigan Model 4500 mass spectrometer operating in a solid probe mode. Ele- mental analysis was performed by Galbraith Lab- oratories, Inc., Knoxville, TN. Inherent viscosities (ainh) of the uncured imide oligomers were deter-

3064 HERGENROTHER ET AL.

mined in rn-cresol at 0.5% concentration (w/v) at 25°C. Differential scanning calorimetry (DSC ) was performed using a Shimadzu DSC-50 at a heating rate of 10"C/min for monomers and model com- pounds and at a heating rate of 20"C/min for oligo- mers and polymers. Polymers were heated - 50°C above the glass transition temperature ( T,) , quenched and rerun. The T, was taken at the in- flection point of the AT versus temperature curve. Thermogravimetric analysis ( TGA ) was performed on uncured imide oligomers using a Seiko TG/DTA at a heating rate of 2.5"C/min under an atmosphere of flowing air or nitrogen at 40 mL/min.

RESULTS AND DISCUSSION

3- and 4-Nitro-4'-phenylethynylbenzophenone were prepared by the palladium-catalyzed coupling of 4- bromo-3 and 4-nitrobenzophenone with phenyl- acetylene [ eq. ( 1 ) ] . Reduction of these compounds with sodium sulfide afforded the reactive endcapping compounds, 3A4'PEB and 4A4'PEB [ eq. ( 1 ) 1. Low- pressure catalytic hydrogenation of the nitro com- pounds using palladium or platinum( IV) oxide re- sulted in partial reduction of the carbonyl and ethy- nyl groups.

Model compounds were prepared by the reaction of 3A4'PEB and 4A4'PEB with phthalic anhydride to form phenylethynyl phthalimides. Differential scanning calorimetric analysis showed sharp melting endotherms with peaks at 198 and 207"C, respec- tively. Intense exotherms due to reaction of the phenylethynyl groups were observed with maxima at 400 and 391°C for the 3A4'PEB and 4A4'PEB derived phthalimides, respectively. The DSC curve of one model compound is shown in Figure 1. After heating these compounds at 350°C for 1 h, no melt-

i Pd

,- 391

0

Heating Rate: 10°C/min. Atmosphere: Static Air

2 0 7 1 F I I I I I

1w 200 300 4M) 500 Temperature, "C

Figure 1. N - ( 4-phenylethynylbenzoyl-4'-phenyl) phthalimide.

Differential scanning calorimetric curve of

ing endotherms or reaction exotherms were observed when the samples were rerun.

The phenylethynyl-terminated imide oligomers as depicted in eq. ( 2 ) were prepared by initially forming a phenylethynyl-terminated amide acid by reaction of the appropriate diamine(s) and end- capper with the appropriate dianhydride in NMP. Toluene was added to the NMP solution and heated to convert the amide acid to imide. Water was re- moved by azeotropic distillation. The stoichiometric offset ratio was calculated for the indicated molec- ular weights using the modified Carother's equation with the stoichiometry offset in favor of the dian- hydride. 3A4'PEB or 4A4'PEB was used to end-cap

W R + n H,N-Ar'NH, + 2 H Z N a a C @

mete or para isomer

Amide Acid Oligomer

(2)

cured polymer

A r = or

Ar' I or

0.85 m and 0.15 m

Scheme 1. Scheme 2.

PHENYLETHYNL-TERMINATED IMIDE OLIGOMERS 3065

Table I. Properties of Phenylethynyl-Terminated Imide Oligomers

Calcd M,, End-Capping Imide Backbone (g/mol) Compound

3,4‘-ODA/4,4‘-ODPA 9000 3A4‘PEB 3,4‘-ODA/4,4’-ODPA 9000 4A4‘PEB 3,4‘-ODA (0.85) APB (0.15)/S-BPDA 5000 3A4’PEB 3,4’-ODA (0.85) APB (0.15)/S-BPDA 5000 4A4‘PEB

Temperature of 5% Weight

Loss (“C) Vinh

(dL/g) Uncured Cured Film

0.32 225 249 0.31 230 248 0.32 235 272 0.35 235 271

Air Nitrogen

500 489 493 487 515 522 499 521

the anhydride-terminated oligomers. Previous work2 has shown that the measured number-average mo- lecular weights of phenylethynyl-terminated imide oligomers are less than the calculated values.

Cured films of the 9000 g/mol ODPA/3,4’-ODA imide oligomers using 3A4‘PEB and 4A4’PEB were tough and creasible with virtually identical Tgs (Ta- ble 1 ) . Cure exotherms of the imide oligomers were not readily detectable. Tgs of the 350°C air-cured films were 10-20°C higher than the Tgs of the mold- ings or powder samples cured in a DSC pan for 1 h at 350°C. The linear polymer from ODPA/3,4’-ODA is designated LARCTM-IA and has displayed excel- lent properties as an adhesive, film, and composite rnatrix.l2J3

The mechanical properties of unoriented thin films from 3A4’PEB- and 4A4’PEB-terminated im- ide oligomers are compared in Table I1 with those of phthalic anhydride end-capped LARC TM-IA

where the stoichiometry was upset by 3 mol 9%. The most striking differences in film properties are the lower moduli of the 4A4’PEB/3,4’-ODA/4,4’-ODPA polymer at 23 and 177°C as compared to the two other 4,4’-ODPA-based materials and the lower elongation of the 3,4-ODA/4,4’-ODPA polymer as compared to the two other materials. Part of this difference may be attributed to film quality (e.g., presence of gel or foreign particles) and/or the con- trolled molecular weight of the linear polymer. However, more work needs to be done to resolve these differences.

The uncured imide oligomer powders (volatile contents - 0.6% ) were readily compression molded under 1.38 MPa (200 psi) at 350°C for 1 h. Extensive resin flow was observed. The critical stress intensity factors (K,,, fracture toughness), were 3.40 MPa m112 (3097 psi in.”2) and 2.16 MPa m112 ( 1964 psi in.’12), respectively, for moldings from the 3A4’PEB-

Table 11. Mechanical Properties of Cured Unoriented Thin Films

Tensile Test Tensile Strength Modulus, Elongation

Calcd M,, End-Capping Temperature at Break GPa at Break Imide Backbone (g/mol) Compound (“C) [MPa (Ksi)] (Ksi) (%o)

3,4‘-ODA/4,4‘-ODPA

3,4’-ODA/4,4’-ODPA

3,4’-ODA/4,4’-ODPA (Stoichiometry offset

by 3 mol %) 3,4’-ODA (0.85), APB (0.15)/S-BPDA 3,4‘-ODA (0.85) APB (0.15)/S-PBDA 3,4’-ODA (0.85) APB (0.15)/S-BPDA

9000 3A4’PEB 25 177

9000 4A4’PEB 25 177 - 16,000 None 25 177

5000 3A4’PEB 25 177

5000 4A4’PEB 25 177

High None 25 177

121.3 (17.6) 51.0 (7.4)

113.1 (16.4) 58.6 (8.5)

125.5 (18.2) 61.4 (8.9)

137.9 (20.0) 91.7 (13.3)

128.9 (18.7) 80.7 (11.7)

139.3 (20.2) 80.0 (11.6)

2.93 (425) 1.96 (284) 2.75 (399) 1.46 (212) 3.22 (467) 2.29 (332)

3.46 (502) 2.70 (391) 3.33 (483) 2.45 (355) 3.32 (482) 2.47 (359)

42 76 27 50 6.2

12.6

68 95 46 90 44 96

3066 HERGENROTHER ET AL.

f c w

and 4A4'PEB-terminated imide oligomers. The dif- ference in fracture toughness was unexpected and the reasons are unknown. Critical strain energy re- lease rates ( GI,, fracture energy), calculated from these values and the moduli reported in Table I1 were 3.95 kJm-' (22.6 in. lb in.-') and 1.69 kJm-' (9.7 in. lb in.-'), respectively. Both the moldings and the cured films were not noticeably affected by immer- sion in methylene chloride or N,N-dimethylacetamide ( DMAc ) for 48 h.

Recently, another polyimide composition was found to have an excellent combination of proper- ties, including good processability. This composition is a polyimide from 85 mol % 3,4'-ODA and 15 mol % 1,3-bis( 3-aminophenoxy) benzene (APB) with 3,3',4,4'-biphenyltetracarboxylic dianhydride (S-BPDA) and is designated LARCTM-8515.14 Cured films of the 3A4'PEB- and 4A4'PEB-termi- nated 5000 g/mol 3,4'-ODA, APB/S-BPDA imide oligomers were also tough and creasible. During the curing, the films cracked and then rehealed as the temperature was increased. Tg's of the cured films were approximately 272"C, regardless of which end- capping compound was used. DSC exothermic max- ima for the 3A4'PEB- and 4A4I-PEB-terminated imide oligomers of this composition were 451 and 444"C, respectively. The latter material has a crys- talline melt transition at 349°C by DSC (Fig. 2). When this uncured imide oligomer was heated in a DSC pan for 1 h at 350°C and the sample was rerun, the polymer melt endotherm was not detected. Cur- ing for l h at 350°C leaves an amorphous material with a Tg of 263°C. The DSC curve of the film is also shown in Figure 2.

Cured 350°C 1 h, + - --y DSC part

R m : cured 350% 1 h. air

I 7 - 4 4 4

Uncured 4A4'PEB Term. hide Oligomer

Heating Rate: 20'CImin.

I

I W m 3M) 4M) 500 Temperature, "C

Figure 2. Differential scanning calorimetric curves of 4A4'PEB terminated 3,4'-ODA (0.85), APB (0.15)/S- BPDA imide oligomers (calc. mol. wt. 5000 g/mol) and cured polymers.

Films from oligomers terminated with both end- capping compounds were amorphous by x-ray anal- ysis. Again, the Tgs of the air-cured films were ap- proximately 10°C higher than the Tgs of the mold- ings or powder samples heated in a DSC pan for 1 h at 350°C. Oxygen is apparently involved in the cure mechanism of the films and other phenylethy- nyl-terminated imide forms although the chemistry is unknown. These films exhibited significantly higher tensile strengths and moduli than the ODPA/ 3,4'-ODA-based polymeric films (Table 11). The enhanced mechanical properties at 25 and 177°C probably result from increased rigidity of the imide backbone due to incorporation of S-BPDA. The un- end-capped high molecular weight polyimide from 3,4'-ODA, APB/S-BPDA exhibited a Tg of 263°C and about the same thin film tensile strength as films from 3A4'PEB- and 4A4'PEB-terminated oligomers as presented in Table 11. The unoriented film prop- erties were excellent with high strengths, moduli, and elongations at 23 and 177°C.

The imide oligomers (volatile contents - 0.6% ) were readily compression-molded under 1.38 MPa (200 psi) at 350°C for 1 h. Moldings were void-free and exhibited extensive resin flow. Molding flash was tough and creasible. The moldings and also the films were unaffected by immersion in methylene chloride and DMAc. Fracture toughness values from moldings of 3A4'PEB- and 4A4'PEB-terminated materials were 4.16 MPa m1/2 (3780 psi in.'/') and 3.91 MPa m'/' (3550 psi in."'), respectively. Frac- ture surfaces were somewhat irregular, indicative of a tough material. Fracture energies were calculated to be 4.99 kJm-2 (28.5 in. lb in.-') and 4.58 kJm-' (26.2 in. lb in.-2), respectively.

All the imide oligomers exhibited excellent ther- mal stability. Temperature of 5% weight loss as de- termined by dynamic TGA ranged between 487- 522°C in atmospheres of both air and nitrogen. Long-term thermal stability with retention of me- chanical properties is unknown.

CONCLUSIONS

Two new phenylethynyl end-capping compounds, 3- and 4-amino-4'-phenylethynylbenzophenone, were synthesized in good overall yield and used to ter- minate imide oligomers. Properties of the imide oligomers and the cured polymers were nearly iden- tical regardless of the end-capping compound. The phenylethynyl-terminated oligomer prepared by re- action of 3,4'-ODA and APB with S-BPDA was

PHENYLETHYNL-TERMINATED IMIDE OLIGOMERS 3067

readily processed into tough, solvent-resistant ma- terials with high mechanical properties.

REFERENCES AND N O T E S

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Received February 23, 1994 Accepted June 7, 1994