Microelectronics Technology - American Chemical Society

Chapter 34

Hybrid Polyimide-Polyphenylenes by the Diels—Alder Polymerization Between Biscyclopentadienones and Ethynyl-Terminated Imides

Downloaded by FUDAN UNIV on December 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch034

Uday Kumar and Thomas X. Neenan AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

The Diels-Alder reaction between biscyclopentadienones and pre -imidized ethynyl terminated diimides yields soluble, thermally stable polymers. The ethynyl terminated monomers were prepared by the reaction of commercially available 3-ethynylaniline with a series of dianhydrides. Polymerization of these monomers with substituted cyclopentadienones yielded soluble polymers with M and M /M in the range 8000-28000 and 1.7-4.4 respectively. The molecular weights of the polymers were strongly dependent on the reactant concentrations. The polymers have dielectric constants in the range 2.8-2.9. The materials retain many of the useful properties of polyimides, but require no curing step, suggesting their use as specialized organic dielectrics. n

n

w

The demand of the microelectronic industry for polymeric materials as organic dielectrics has fueled much novel work in polyimides (i,2). Polyimides have an outstanding combination of properties, including thermal and chemical stability, excellent processability and good adhesion to a variety of surfaces. Polyimides suffer from several disadvantages including the evolution of water upon thermal curing of the precursor polyamic acids at high temperature, and the tendency for the cured polyimides to absorb moisture when exposed to conditions of high humidity. Substantial progress has been made in addressing these problems, particularly by the introduction offluorinatedsubstituents into the polyimide structure (5-5). However efforts have continued to design new materials which combine low dielectric constants with mechanical and processing characteristics comparable to present commercial materials e.g Kapton. In our search for new materials as photodefinable organic dielectrics, we recently revisited the work of Stille and Harris on the preparation of polymers by the DielAlder polymerization of m- and p-diethynylbenzene with biscyclopentadienones. The original work (6-12) centered on the copolymerization of m- or p0097-6156/95/0614-0518$12.00/0 © 1995 American Chemical Society

Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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KUMAR & NEENAN

Hybrid Polyimide-Polyphenylenes

519


diethynylbenzene (1 and 2) with 3,3'-(oxy-p-phenylene)bis(2,4,5-triphenylcyclopentadienone) (3) (Scheme I), and yielded thermally stable, aromatic polymers (4) with high glass transition (Tg) temperatures. Of particular note was the unusually high solubility of these highly aromatic materials in common organic solvents such as toluene and chloroform. We extended the Stille-Harris reaction to the preparation of a broad class of new polymers through the use of a variety of diethynyl aromatic dienophiles (13). Specifically we showed that a range of both terminal and internal diethynyl aromatics could be used, but that terminal acetylenes yielded higher molecular weights. The polymerization reaction tolerated a variety of substituents on the aromatic ring of the dienophile, including fluorine, silicon and sulfur. Here we extend the synthetic approach to the preparation of copolymers from the Diels-Alder polymerization of biscyclopentadienones and ethynyl terminated, preimidized monomers. The latter materials are readily prepared in a two step, one pot synthesis from commercially available starting materials. We discuss the properties of the materials prepared, and show that this approach offers a new way to prepare soluble, thermally stable polymers. Our intent was to prepare materials which combined the advantages of polyphenylenes (low dielectric constant, hydrolytic stability) with those of polyimides (good mechanical and adhesive properties). In contrast to most conventional polyimides, our new approach yields materials in a pre-imidized form, which have several technological advantages. Poly(amic acid) solutions are notorious for molecular weight and polydispersity instabilities, imididization requires heating poly(amic acids) to temperatures of ~ 330°C., and the acids corrode copper and other metals. Experimental Synthesis of monomers 7-11. A typical procedure for the preparation of the ethynyl terminated imide 8 is given below. 2.57 g (22.0 mmol) of ethynylaniline (12) was added to a suspension of 10.4 mmoles of 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (14) in 80 mL of anhydrous THF under argon. The resulting mixture was stirred at 50°C for three hours. Upon cooling the reaction mixture, 20 mL acetic anhydride and 15 mmoles of anhydrous sodium acetate was added, and the resulting mixture stirred at 70°C for 5 h. Compound 8 slowly precipitated as an off-white solid. The reaction mixture was cooled, poured into ice cold water, filtered, washed with water, THF and methylene chloride and dried on a vacuum pump. Compounds 7,9-11 were prepared similarly. In the case of monomer 9, the reaction mixture after being poured into water was extracted with chloroform, the organic extract washed with brine and dried with anhydrous MgS04 and the solvent removed. Full experimental and characterization data will be reported elsewhere. Polymerization procedure. A representative procedure for the polymer preparations is as follows: A 25 mL Schlenk tube was charged with a suspension of 5 (0.86 mmol), 7 (0.86 mmol) and 5 gm of Ν,Ν-dimethylacetamide (DMAC), and the mixture was degassed by three freeze, pump, thaw cycles. The tube was sealed and placed in a oil bath at 200°C for 36 h. The solids dissolved within an hour and the deep magenta solution slowly became very viscous. At the end of the reaction period, the tube was cooled, about 60-100 mg of phenylacetylene added to the (light)


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MICROELECTRONICS TECHNOLOGY

5,R = H Fig. 1.

6,R = C H

3

Structures of biscyclopentadienones 5 and 6.


34.

KUMAR & NEENAN


521

magenta colored polymer solution, the tube wasreseated,and heated at 200°C for another 3 h. The reaction mixture was diluted with THF and precipitated into hexane/acetone (3:1). Polymer 20a was purified by dissolving in THF, reprecipitating into hexane/acetone,filteringand drying in a vacuum pump at 70°C for 24 h. The yields of polymers along with other polymer properties are listed in Table 1. Polymer Characterization.


Results and Discussion Monomers. Two biscyclopentadienones (5-6, Fig. 1) were prepared as described previously (13). A series offiveethynyl-terminated imide monomers (7-11, Scheme Π) were prepared via a two step procedure, involving first thereactionof the amino group of ethynylaniline on the bisanhydride rings of 13-17, followed by base catalysedringclosure of the intermediate bis-amic acids (14). With the exception of the hexafluoro-isopropylidene bridged monomer 9, the ethynyl terminated monomers were insoluble in all common organic solvents, making characterization by NMR impossible. However the IR C-H stretch at 3250-3270 cm ~ confirmed the presence of a carbon-carbon triple bond, and each monomer showed a carbonyl imide stretch at 1715-1725 cm" . The absence of a carbon-carbon triple bond absorption in the IR -2100 cm" is due to the presence of symmetry in the molecules. Litt has recently reported similar behavior in di-p-ethynylbenzoyl esters (75). The *H and C NMR spectrum of hexafluoroisopropylidene-3,3,4,4'-bis(phthalimide-N-3ethynyl-benzene) 9 is consistent with the structure and shows acetylenic proton and carbonresonancesat 3.13 and - 8 0 ppm respectively. In light of the extensive literature available on ethynyl terminated polyimide oligomers as melt processable thermosets (Id), we briefly examined the thermal behavior of monomers 7-11 by differential scanning calorimetry. The bis acetylenes 7-8 and 10-11 do not show a melting endotherm, but exhibit a strong exotherm in the DSC at 238, 250, 309 and 241°Crespectively,due to reaction of the ethynyl groups. Litt (15) has recently reported that di-p-ethynylbenzoyl esters behave similarly, undergoing exothermic cross-linking without melting in the solid state at 200 - 250°C. Diacetylene 9 does show a distinct melting endotherm at 212°C followed by a broad exotherm at 221°C corresponding toreactionof the acetylenes. 1

1

1

1 3

/

Polymerization Reactions. We chose to carry out the polymerizations between acetylenes 7-11 and the biscyclopentadienones 5-6 to form die polymers 18-27 in Schlenk tubes so that we could visually monitor the reaction (17). In a typical polymerization, a 1:1 molar mixture of 5 and 7 was prepared in n,ndimethylacetamide (DMAc) in a sealed tube. The mixture was degassed by a series of freeze pump thaw cycles, sealed, and immersed in a thermoregulated bath held at 200°C. (The diacetylenes, with the exception of 9 are initially insoluble in dimethylacetamide, but dissolve as the reaction proceeds). The polymerization reaction was monitored by the disappearance of the intense purple color of 5, appearance of a light brown coloration and an increase in viscosity of the reaction mixture. The polymerization reaction was cooled, 50 mg of phenylacetyene was added, the tube reseated and heated for a further period to terminate the polymerizationreaction.A color change from magenta to yellow (generally within


522


Table 1. Molecular weight dependence on monomer concentration tolymer mmol mmol Solvent Yield M grams % (χ 10 ) w


3

18a 18b 19 20a 20b 21 22a 22b 23 24a 24b 25 26a 26b 27

Monomer 8 0.86 1.24 Monomer 8 1.30 Monomer 7 0.86 1.42 Monomer 7 1.30 Monomer 9 0.86 1.24 Monomer 9 1.24 0.86 1.40 Monomer 10 1.42 Monomer 11 0.78 1.42 Monomer 11 1.42

Monomer 5 0.86 1.24 Monomer 6 1.30 Monomer 5 0.86 1.42 Monomer 6 1.30 Monomer 5 0.86 1.24 Monomer 6 1.24 0.86 1.40 Monomer 6 1.42 Monomer 5 0.78 1.42 Monomer 6 1.42

M (χ 10 )

D

n

3

5.0 4.0

95 92

38 18

16 11

2.5 1.7

4.3

98

28

16

1.7

5.0 4.5

96 51

74 19

25 8

3.0 2.4

4.3

99

37

17

2.2

5.0 4.0

96 94

42 35

22 20

1.9 1.7

4.0 5.0 4.5

88 100 87

50 22 33

28 10 10

1.8 2.2 3.3

5.0

94

86

20

4.4

4.5 4.5

99 96

19 24

10 11

1.8 2.2

4.5

98

41

21

2.0

Solvent was dimethyl acetamide; M = Weight average molecular weight; M„ = Number average molecular weight; D = Polydispersity. w


KUMAR & NEENAN



: ° Ο 13, X = nil 14, X = S 0 15, X = C(CF ) 16, x = co

+

0 ~

N

H

2

12

Η

2

3

H

e

a) THF, 60 C 2

b) Ac 0, sodium acetate, 80 °C 2

7, X - n f l ;

8, X = S 0

9, X = C(CF ) ; 3

Q

Scheme II. monomers.

2

2

10, X = CO 0

Preparation of ethynyl-terminated imide


524


#

Ό

Η

Η


DMAc, 200 °C., -CO

Ο ForR=H, 18(a-b),X=S0 ; 20(a«b),X = nfl;22(a-b),X = C(CT )2; 2

3

24(a-b), Χ = CO;. For R = CH ,19, Χ = SO2; 21, Χ = nil; 23, Χ = QCF^; 25, X = CO. 3

Polymers 26a-b (R = H) and 27 (R = CH3) have monomer 11 as the imide component.

Scheme III.

Preparation of polymers 18-27.

Table 2. Physical of polyimides 18-27 Tg(°C)properties Polymer UV *TGA(°C) TGA(°C) (in air) (in argon) Im« (a) b

a

18a 18b 19 20a 20b 21 22a 22b 23 24a 24b 25 26a 26b 27

270 260 245 275 250 245 265 265 245 245 245 240 280 280 260

440 420 420 450 470 450 470 440 330 430 400 490 470 450 380

440 420 420 470 470 450 470 450 380 440 400 460 420 430 450

C

UV(AU; (cutoff)

252(113.0) 252 (104.5)

344 334

252 (119.6) 256 (113.4)

370 366

248 (103.0)

320

252 (120.9) 254(116.2)

336 348

252 (107.6) 254(112.7)

344

a. Onset temperature for weight change in thermogravimetric analysis (TGA) for polymers. b. Absorbance measured in THF solution. The value in parenthesis is the absorptivity. c. The absorbance value for an absorptivity of 1.0 measured in THF solution.


34.

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525


fifteen minutes) served as an indication of reaction termination. Dilution of the solution with THF, followed by precipitation into hexanes/acetone gave polymer 20a as afibrous,off-white material. The preparation of the other polymers followed the same procedure. See Scheme ΙΠ. Characterization of the polymers. The polymers 18-27 are all white/tan fibrous materials which are readily soluble in a variety of solvents including THF, pdioxane, cyclohexylbenzene, N-methylpyrrolidinone, toluene, xylene, chloroform, chlorobenzene or Ν,Ν-dimethylformamide. Solutions of up to 15 weight % can be easily prepared, and the solutions are indefinitely stable to precipitation. The solubility is limited by solution viscosity and is not inherent to the polymer. Table 1 shows die strong dependence of the molecular weights of 18-27 on monomer concentration. Typical of condensation polymerizations, an increase in monomer concentration results in an increase in molecular weight and polydispersity (PD) of the polymers. In general the molecular weight of the imide copolymers were lower than those prepared in our earlier study from biscyclopentadienones and diethynylaromatics (13). In several instances (18b,19) the polymers precipitated from solution during the polymerization, resulting in lower molecular weights. 1 3

NMR spectra of polymers. Analysis of *H and C NMR spectra of the poly(phenylene-imide)s was relatively straightforward, with the ratio of aliphatic to aromatic protons generally agreeing well with the structures of the constituent monomer units. In all instances, the resonance for the carbonyl carbon of the cyclopentadienone monomers at -200 ppm disappeared, to be replaced by an increase in the complexity of the aromatic region due to the formation of the new (Diels-Alder produced) phenyl rings. *H NMR analysis of the polymers derived from 9 showed the loss of the acetylene proton at 3.13 ppm. The higher molecular ( M - > 40,000) polymers formed thick (1-7 μπι) coherent films on a variety of substrates (silicon, quartz, aluminum oxide). Table 2 summarizes the UV data for polymers 18-27; most polymers show a maximum in absorbance in the UV spectrum at ~ 250 nm. Of most interest to us was the low absorbances of the materials at 364 nm (typically -0.015-0.025 absorbance units/μπι) suggesting that photodefinition of the polymers may be possible. n

Initial measurements of the dielectric properties of the polymers as 7 μπι thick films on aluminum confirmed that the polymers have dielectric constant values in the range of 2.6-2.9, comparing favorably with typical values of 3.0-3.5 for polyimides. Thermal Properties. The thermal properties of the polymers are summarized in Table 2. All polymers showed a weak glass transition (Tg) temperature in the range of 225-270°C., which is in the range typical of soluble polyimides (18). The temperatures reported in columns 3 and 4 of Table 2 are the onset temperatures for weight loss (except for 23 and 27) measured by thermogravimetric analysis. Poly(phenylene-imide) 23 shows a weight gain of 2% in argon and air and polymer 27 a 1% increase in weight in air due to oxidation of the methyl groups. On further heating 23 and 27 start losing weight at around 450°C and followed a pattern similar to other poly(phenylene-imide)s.


526

MICEOMfiCraOMCS TECHNOLOGY

Conclusion. The Diels-Alder polymerization of biscyclopentadienones and ethynyl terminated bis-imides yields soluble polymers of moderate molecular weight Hie glass transition temperature of the polymers aie in the range of 250-275°C, and the materials have low absorbance in the UV at 364 nm. Since the monomers contained preimidized groups, these poly(phenylene-imides) need no curing step. These polymers may find use as novel coatings or as organic dielectrics. We are continuing to explore these possibilities.


Acknowledgements. We thank Timothy M . Miller for helpful discussions and Wai Tai for the GPC measurements. Literature Cited 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

Satou, H.; Suzuki, H.; Makino, D. In Polyimides Wilson, D.; Stenzenberger, H. D.; Hergenrother, P. M. Eds.; Chapman and Hall: NY, 1990, Chapter 8. Stone, D, S.; Martynenko, Z. Polymers in Electronics, Fundamentals and Applications; Elsevier: New York, 1989. Hougham, G.; Tesoro, G.; Shaw, J. Macromolecules 1994, 27, 3642. Goff, D, L.; Yuan, E. L.; Long, H.; Neuhaus, H. J. Organic Dielectric Materials with Reduced Moisture Absorption and Improved Electrical Properties. In Polymeric Materials for Electronics Packaging and Interconnection; Lupinski, J. H.; Moore, R. S. Eds.; American Chemical Society: Washington, DC. 1989; pp 93-100 . St Clair, T. L. In Polyimides Wilson, D.; Stenzenberger, H. d.; Hergenrother, P. M. Eds.; Chapman and Hall: NY, 1990, Chapter 3. Stille, J. K. Noren, G. K. Macromolecules 1972, 5, 49. Stille, J. K.; Gilliams, Y. Macromolecules 1971, 4, 515. Stille, J. K.; Noren, G. K. J. Polym.Sci., Part Β 1969, 7, 525. Stille, J. K. J. Macromol.Sci.Chem. 1969, 3, 1043. Stille, J. K.; Rakutis, R. O.; Mukamal, H.; Harris, F. W. Macromolecules 1968, 1, 431. Mukamal, H.; Hairis, F. W.; Stille, J. K. J. Polym. Sci., Part Α-1 1967, 5, 2721. Stille, J. K.; Harris, F. W.; Rakutis, R. O.; Mukamal, H. J. Polym. Sci., Part Β 1966, 4, 791. Kumar, U.; Neenan, T. X. Macromolecules 1995, 28, 124. (a) Unroe, M . R . ; Reihhardt, B. A. J. Poly.Sci.Part. A, Polymer Chem. 1990, 28, 2207. Melissaris, A. P; Litt, M. H. Macromolecules 1994, 27, 2675. Hergenrother, P. M . In Polyimides Wilson, D.; Stenzenberger, H . D.; Hergenrother, P. M . Eds.; Chapman and Hall: NY, 1990, Chapter 6. The earlier polymerization procedure reported by Stille and Harris involved the Diels-Alder condensation between biscyclopentadienones and diacetylenes in a Parr reactor at - 200°C and at pressures of - 200 psi. Bell, V. L.; Stump, B. L.; Gager, H. J. Polym.Sci.Chem. Ed. 1976, 14, 2275.

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