Polymers for High Technology - American Chemical Society

Chapter 38 Cocyclotrimerization of Aryl Acetylenes: Substituent Effects on Reaction Rate Daniel J. Dawson, Janice D. Frazier, Phillip J. Brock, and Robert J. Twieg

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Almaden Research Center, IBM, San Jose, CA 95120-6099

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cy clotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants ΔδH and ΔδC for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. ß

Since its inception, microelectronics has been evolving towards denser configurations of smaller circuit elements. Critical to these devices is the concurrent development of insulating materials capable of isolating these elements to allow the construction of multi-level circuitry. The chemical and physical semiconductor processing requirements are aggressive enough to eliminate most polymers from this application. Due to their high thermal stability, polyimides have been widely used for this purpose but have two major drawbacks: 1) their polar imide structure permits water absorption and release, which are potential sources of corrosion of nearby metals; and 2) the high melt viscosity of most polyimides limits the degree of planarization that can be achieved. One class of materials that minimizes both of these problems without compromising thermal stability is poly (aryl acetylenes). Over the past several years, two types of these polymers have been investigated in our laboratories: diethynyl oligomers and cyclotrimerized multi-ethynyl aromatics (1). Examples of these two types of oligomers (1 and 2, respectively) are illustrated below.

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1

The highly aromatic structure of these materials provides good thermooxidative stability, extremely low water absorption, and a low dielectric constant. The branched structure of these oligomers, together with their low polarity, affords a low melt viscosity (which leads to good planarization) and high solubility in most organic solvents. Because the cyclotrimerized materials (2) undergo a slower, more easily controlled thermal cure, they have been the target of our recent work. Due to the branching inherent in the cyclotrimerization reaction, use of a multi-ethynyl aromatic monomer alone inevitably leads to gel formation if the reaction is driven to completion. Stopping the reaction before the onset of gelation gives low yields and difficulty in molecular weight reproduction. To circumvent these problems, co-cyclotrimerizations can be conducted with a mono-ethynyl aromatic co-monomer, such as phenylacetylene, that terminates the growth of many of the polymer branches. This strategy results in a more controlled reaction, which can be pushed to high conversion with good reproducibility. Key to the success of this co-cyclotrimerization procedure is the selection of the appropriate monomers. A co-cyclotrimerization in which one monomer reacts much more rapidly than the other will result in a heterogeneous product as the monomer ratio changes. Moreover, if the mono-ethynyl capping agent reacts much more slowly than the multi-ethynyl monomer, gel formation can occur early in the reaction. Alternatively, if the mono-ethynyl material reacts much more rapidly, it can be exhausted early in the reaction, having produced a non-reactive trimer, and the multi-ethynyl monomers will gel later in the reaction. These problems can be avoided by using monomers of comparable reactivity or by adjusting the feed ratio to compensate for unequal reactivities. With either approach, it is necessary to determine the relative cyclotrimerization rates of each ethynyl monomer. In this paper, we report the initial results of our measurements of these rates. Discussion In order to avoid complications in kinetics and analysis due to polymerization, multi-ethynyl monomers were not used during the initial rate studies. This study focussed instead on /rara-substituted phenylacetylenes. The substituted phenylacetylenes (6) were all prepared by a modified (2-6) Stephens-Castro coupling (7) of an aryl halide (3) with a monoprotected acetylene [2-methyl-3-butyn-2-ol (4, R' = -C(CH ) OH) or trimethylsilylacetylene (4, R' * -Si(CH ) )] in a refluxing dialkylamine solvent, followed by a deprotection step (Scheme I). 3

3

2

3


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The catalyst system for the coupling reaction was a Pd(II)-triphenylphosphine complex, usually prepared in situ, with excess triphenylphosphine and either cuprous iodide or cupric acetate as a co-catalyst. A l ternatively, a preformed catalyst mixture prepared from these reagents may be utilized (see Experimental Section). When 2-methyl-3-butyn-2-ol was used as the protected acetylene, the intermediates 5a-d were converted to the corresponding aryl acetylenes 6a-d by a retro-Favorskii-Babayan (8) reaction utilizing potassium f-butoxide in toluene under conditions of slow distillation. In the case of /^iododimethylaniline (3e), trimethylsilylacetylene was used as the ethynyl source. The intermediate (5e) was treated with hydroxide to generate the free aryl acetylene 6e. The syntheses of 6d and 6e are described in the Experimental section below. Scheme I

R

CH

a b c d e

X

-CN -CF -C H -OC H -N(CH )

3

3

6

5

6

5

3

-C(CH ) OH -C(CH ) OH -C(CH ) OH -C(CH ) OH -Si(CH ) 2

3

2

3

2

3

2

2

3

3

Br Br Br Br I

The cyclotrimerization reactions were conducted in dioxane, using a nickel acetylacetonate/triphenylphosphine catalyst system at 90°C. This catalyst system produces cyclotrimerized products in preference to linear polyenes (9-11). An effort was made to minimize the rate-affecting variables in the kinetics runs. Use of an oil bath and a stirred reactor provided good heat transfer and the assurance that the desired temperature was maintained despite the exothermic nature of cyclotrimerizations. This reaction is quite sensitive to catalyst concentration and temperature; it is drastically inhibited or terminated by oxygen contamination. Accordingly, care was taken to use the same catalyst formulation in each kinetics run and the reaction was thoroughly deoxygenated and conducted under argon. To compensate for any unforeseen variables that might have affected the overall reaction rate, each aryl acetylene 6a-e was cyclotrimerized with an equimolar amount of phenylacetylene, allowing a direct ranking of reaction rates. HPLC was used to monitor the concentration of each of the starting materials. An inert compound (naphthalene) was included as an internal HPLC standard.

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448

As a rule, each phenylacetylene derivative was evaluated at an initial concentration of 125 mM; reactions using phenylacetylene alone were conducted at concentrations of 125, 250, and 500 mM in order to establish the effect of ethynyl concentration on the measured rate. Because more than one material is produced in even the simplest cyclotrimerization reaction, all reactions were followed only by measuring the disappearance of the starting material(s). Although attempts were made to fit the resulting data into the expected second- or third-order kinetics plots, it was finally concluded that the reactions were better described as zero-order. Accordingly, data were plotted on linear concentration and time scales. Figures 1 to 3 illustrate the disappearance of phenylacetylene, starting at three different concentrations: 125, 250, and 500 mM. From these plots, it is clear that there were two different reactions taking place. At high concentrations (>200 mM ethynyl), the reaction was rapid; below 150 mM, a second reaction took over with a rate approximately an order of magnitude less. The fact that both rates appeared to be zero-order suggests that catalyst turnover was rate-limiting, with the effective catalytic species altered by the ethynyl concentration. In order to obtain information on both the fast and slow reactions, the kinetics runs on the substituted aryl acetylenes were conducted at an initial total ethynyl concentration of 250 mM. The resulting plots are shown in Figures 4 to 8. Several conclusions were drawn from these plots: 1) The fast/slow pair of reactions occurred with each of the five aryl acetylene mixtures. 2) With one exception (6b, R * p-CF ), the rate of disappearance of the aryl acetylenes shifted from fast to slow at a total ethynyl concentration of 120-140 mM. The mixture of /^-trifluoromethylphenylacetylene and phenylacetylene changed rates at a total concentration of -195 mM. 3) There was a definite substituent effect on the high-concentration (fast) reaction rates. The low-concentration (slow) rates were almost always identical for each pair of reactants. 3

Because preparative cyclotrimerization reactions are usually conducted at high concentration, the initial, faster rates in this study were considered more important. For each run, the rates of disappearance of the substituted aryl acetylene and phenylacetylene, along with the reaction ratio, are listed in Table I. T A B L E I. Cyclotrimerization Rates for /^-Substituted Phenylacetylenes (Sub) and Phenylacetylene (PA) Substituent CN* CF C H OC H N(CH ) H 3

6

5

6

5

3

2

Reaction Rates (xlO" motel/ sec" ) Sub PA 4.5 2.0 0.98 0.69 4.2 3.6 2.2 2.1 0.48 1.1 1.7 6

1

1

Rate Ratio Sub/PA 2.25 1.4 1.2 1.05 0.43


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of Aryl

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Acetylenes

ο Ε ο|


= «I

§ PL ο

1000

2000 Time (minutes)

3000

F i g u r e 1. C o n c e n t r a t i o n o f P h e n y l a c e t y l e n e (125 m M ) v s . T i m e .

13 ·*- ο c ο 4)

«-

2000 Time (minutes)

3000

F i g u r e 2. C o n c e n t r a t i o n o f P h e n y l a c e t y l e n e (250 m M ) v s . T i m e .




450

0 F i g u r e 4.

400

800 Time (minutes)

1200

C o n c e n t r a t i o n of />-Cyanophenylacetylene

1600 (A)

and Phenyl-

acetylene ( · ) vs. T i m e .



of Aryl

Acetylenes


DAWSON ET A L .

2000 Time (minutes)

Figure 6. Concentration of />-Phenylphenylacetylene (A) and Phenylacetylene (·) vs. Time.



452


0

1000

2000

3000

Time (minutes) F i g u r e 7. C o n c e n t r a t i o n o f />-Phenoxyphenylacetylene (A) a n d P h e n y l acetylene ( · ) v s . T i m e .

0

400

800

1200

1600

Time (minutes) F i g u r e 8. C o n c e n t r a t i o n o f />-Dimethylaminophenylacetylene (A) a n d Phenylacetylene ( · ) vs. T i m e .


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Acetylenes

) As a control experiment, benzonitrile and phenylacetylene were subjected •O the standard cyclotrimerization conditions. Benzonitrile did not react. Two different interpretations of this data are possible. If the direct re action rates of the substituted aryl acetylenes are compared, they would be ranked as follows: CN>C H >OC H >H>CF >N(CH )


6

5

6

5

3

3

2

However, because the reaction rate for phenylacetylene in these same runs varied by a factor of five, this type of ranking is suspect. If, as originally in tended, the rate ratios are compared, a different ranking is produced: CN>CF >C H >H~OC H >N(CH ) 3

6

5

6

5

3

2

which is very close to the order of the Hammett values for the /rara-substituents: CN(.66)>CF (.54)>C H (.01)~H(0)>OC H (-.32)>N(CH ) (-.83) 3

6

5

6

5

3

2

It is interesting to note that the rate ratios follow the spectroscopic Δ Ο Η (chemical shift difference of the acetylene hydrogen on the substituted vs. parent phenylacetylene) and the ΔδΟβ (chemical shift difference of the ter minal acetylene carbon in the substituted vs. parent phenylacetylene) (12). Δδ : Η

CN(.251)>CF (.159)>C H (.0515)>H(0)>OC H (-.0453)>N(CH ) (-.147) 3

6

5

6

5

3

2

ΔδΟ«: CNC4.02)>CF (2.43)>C H (0.56)>H(0)>OC H (-0.65)>N(CH ) (-2.28) Because the exact mechanism of the cyclotrimerization reaction is not adequately understood, it is useless to conjecture on exactly how the substi tuent influences the reaction rate. However, it is useful to know that spec troscopic data correlates with the observed rates and this may prove advan tageous in the prediction of cyclotrimerization rates for other substituted phenylacetylenes. 3

6

5

6

5

3

2

Conclusion The results of this study suggest that diethynyl monomers such as diethynylbiphenyl, diethynylterphenyl, and diethynyldiphenylether could be cyclo trimerized with equimolar amounts of phenylacetylene to give homogeneous polymers in high yield. Bis(p-ethynylphenyl)X monomers where X = -SiR and -P(R)- with Hammett constants close to 0 would be expected to behave similarly. In contrast, compounds in which X = -S0 -, -C(=0)-, -CF -, and -N=N- would be expected to cyclotrimerize two to three times faster than phenylacetylene. Use of these materials to prepare analogous homogeneous polymers in high yield might well require constant adjustment of reaction stoichiometry or the use of an electron-deficient mono-ethynyl component such as the />-cyano or p-CF materials described above. 2

2

3


2


454


Experimental 4-Fhenoxyphenvlacetvlene (6d). A 250-mL round-bottomed flask equipped with a heating mantle, magnetic stir bar, thermometer, and condenser with a nitrogen/vacuum source, was charged successively with 6.27 g (74.5 mmol, 1.626 equiv per Ar-Br) of 2-methyl-3-butyn-2-ol, 11.42 g (45.8 mmol) of 4bromodiphenylether, 0.35 g (1.33 mmol, 0.029 equiv) of triphenylphosphine, 84.2 mg (0.27 mmol, 0.0059 equiv) of cupric acetate monohydrate, 38.6 mg (0.218 mmol, 0.0048 equiv) of palladium(II) chloride, and 100 mL (73.8 g, 729 mmol, 15.9 equiv) of di-H-propylamine. The light purple suspension was deoxygenated and then heated to reflux. By the time the internal temperature had reached 80°C the reaction mixture had become a clear, light yellow sol ution. Di-«-propylamine hydrobromide began to precipitate before the re action mixture reached reflux temperature (115°C). After 2 hours at reflux, T L C (silica gel, 1:1 ether/hexane) indicated that the starting material had been consumed. After a further 10 minutes at reflux, the heating mantle was removed, and the reaction mixture was cooled to room temperature with a water bath. The thick crystalline slurry was vacuum filtered and the cake was carefully washed with two 16-mL portions of toluene. The combined filtrate was filtered through 16 g of silica gel (40-140 mesh) prepared in toluene. The column was then rinsed with two 16-mL portions of toluene and blown dry with nitrogen. The resultant clear yellow solution weighed 103 g and was used immediately in the next step. A 250-mL round-bottomed flask equipped with a heating mantle, mag netic stir bar, thermometer, and a Claisen head/distillation system, was charged with the yellow solution prepared above. Potassium /-butoxide (0.85 g, 7.87 mmol, 0.164 equiv) was added to the stirred solution, which was then immediately deoxygenated, left at 250 mm Hg vacuum, and heated to boiling in order to drive off the acetone formed in the reaction. The internal tem perature during distillation was initially 94°C, dropped to 92° over a 12-minute period, and then slowly rose to 96°C after 22 minutes of distillation time. T L C (silica gel, 10% ether/hexane) indicated that the intermediate had been consumed. The brown reaction mixture was transferred to a 1-liter flask and the solvents stripped off at reduced pressure. Hexane (150 mL) was added to the residue, followed by 0.8 mL of acetic acid in 10 mL of hexane. Norite (4 grams) was introduced next. This suspension was stirred and warmed to 60°C over a 20 minute period, 2.1 g of Celite was added, and the hot mixture was filtered through G F / A filter paper and then through a column containing 32 g of 40-140 mesh silica gel in hexane. The eluant was stripped to dryness to afford 3.06 g (34%) of 4-phenoxyphenylacetylene (6d) as a pale violet oil: T L C (silica gel, 10% ether/hexane) R 0.66; Ή NMR (CDC1 ) δ 2.95 (s, 1H, C - C H ) , 6.9-7.3 (m, 9H, ArH). f

3

Preformed Ethynylation Catalyst. Into a 500-mL round-bottomed flask equipped with a magnetic stir bar, reflux condenser and nitrogen bubbler was placed palladium chloride (1.77 g, 10 mmol), di-n-propylamine (100 mL) and triphenylphosphine (15.74 g, 60 mmol). The resulting slurry was boiled for two hr after which time the brown palladium chloride had been consumed and the yellow bis(triphenylphosphine) palladium(II) chloride had formed. The slurry was cooled and cupric acetate monohydrate (1.99 g, 10 mmol) was ad ded in one portion and the slurry boiled for one hr longer. After cooling, the solvent was removed by rotary evaporation and then finally under high vac-


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Acetylenes

uum to give the ethynylation catalyst mixture as a yellow-brown solid in quantitative yield. Each 19.5 mg of the solid catalyst mixture contained about 0.01 mmol of palladium. Samples of this mixture lost little if any activity over a one-year period although the material gradually darkened with age. 4-N N-IMmethylamino(trimethylsilylethvnyl)benzene (5e). A 500-mL round-bottomed flask equipped with an oil bath, magnetic stir bar, condenser and nitrogen bubbler was charged successively with 4-iodo-N,N-dimethylaniline (13) (24.8 g, 100 mmol), diisopropylamine (100 mL, 72.2 g, 0.714 mole, 7.14 equiv), ethynyltrimethylsilane (12.30 g, 125 mmol, 1.25 equiv) and preformed catalyst mixture (1.0 g). The resulting slurry was heated at a gentle reflux with stirring for 16 hr after which time T L C analysis indicated that the starting iodide had been consumed. The slurry was concentrated by rotary evaporation to remove excess solvent and then taken up in ethyl acetate and water and transferred to a separatory funnel. The organic phase was washed repeatedly with water, then dried (MgS0 ), filtered through a short column of silica gel in ethyl acetate and concentrated by rotary evaporation to a brown solid. Two recrystallizations from ethanol afforded pure product, 15.01 g (69%) of 5e, mp 88-9°C: Ή NMR δ 0.24 (s, 9H, Si(CH ) ), 2.94 (s, 6H, N(CH ) ), 6.50 (d, J=9, 2H, A r H ο to N), 7.26 (d, J=9, 2H, A r H m to N). Additional crude product, 2.30 g (10%), could be obtained from the mother liquors.


t

4

3

3

3

2

4-N N-Dimethylaminophenvlacetylene (6e). A 250-mL round-bottomed flask equipped with a stir bar and nitrogen bubbler was charged successively with 4-N,N-dimethylamino(trimethylsilylethynyl)benzene (10.86 g, 50 mmol), tetrahydrofuran (THF) (50 mL), methanol (25 mL), and 45% K O H solution (6.25 g). After 2.0 hr of stirring at ambient temperature, the reaction was checked by T L C and found to be complete. The T H F and methanol were stripped off by rotary evaporation and the residue was taken up in hexane and water and transferred to a separatory funnel. The phases were separated and the hexane phase washed with water, dried (MgSÔ ) and filtered through a pad of silica gel. After concentration by rotary evaporation, the crude product was crystallized from methanol to afford 6.35g (87%) of 6e: mp 51-2°C [lit. (14) mp 52-3°C]. t

4

Kinetics Experiments. A 100-mL three-necked, round bottomed flask equipped with a magnetic stir bar, serum cap, thermocouple temperature sensor, and reflux condenser with an argon/vacuum source, was positioned in an oil bath which was carefully controlled to provide a reaction temperature of 90.0 ± 0.3 °C. The flask was charged with 50 mL of a stock solution of p-dioxane (OmniSolv, E M Science) containing nickel acetylacetonate (15 mM), triphenylphosphine (45 mM), and naphthalene (0.5 wt%), deoxygenated, and allowed to warm to 90°C under argon. At time t « 0, a solution of 6.5 mmol (664 mg) of phenylacetylene and 6.5 mmol of one of the substituted phenylacetylenes in 2 mL of dioxane was added by syringe. Samples (0.5-mL) were removed for analysis by syringe over a period of 1 to 3 days; samples not analyzed immediately were stored at 2°C. Sample analysis was performed on a Waters HPLC (Model 6000 pump, Model 660 solvent programmer) using a C reverse-phase column (12-inch, Analytical Sciences, Inc.) and a solvent program running from 40% acetonitrile/water to 100% acetonitrile with a flow rate of 1.0 mL/min. A Waters (Model 450) U V detector (254 nm) was 1 8


456


connected to an H P Model 87 minicomputer for peak integration. The areas of each of the aryl acetylene peaks were divided by that of the naphthalene peak, then normalized to 0.125 M (the starting concentration) at t - 0 for plotting purposes. Acknowledgments The authors would like to thank M s . Heidi Bauer for her many invaluable contributions to the work described above.


Literature Cited 1. Dawson, D. J.; Fleming, W. W.; Lyerla, J. R.; Economy, J. In Reactive Oligomers; Harris, F. W.; Spinelli, H. J., Eds.; ACS Symposium Series No. 282; American Chemical Society: Washington, DC, 1985; pp 63-79. 2. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467. 3. Sabourin, E. T.; Selwitz, C. M. U.S. Patent 4 223 172, 1980. 4. Austin, W. B.; Bilow, N.; Kelleghan, W. J.; Lau, K.S.Y. J. Org. Chem. 1981, 46, 2280. 5. Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627. 6. Ames, D. E.; Bull, D.; Takundwa, C. Synthesis 1981, 364. 7. Stephens R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 2163, 3313. 8. Shchelkunov, Α. V.; Muldakhmetov, Ζ. M.; Rakhimzhanova, Ν. Α.; Favorskaya, T. A. Zhurnal Organicheskoi Khimii 1970, 6, 930. 9. Jabloner, H. Ger. Offen. 2 235 429, 1973. 10. Cessna, L. C. U.S. Patent 3 926 897, 1975. 11. Jabloner, H.; Cessna, L. C. Polym.Prepr.,Am. Chem. Soc. Div. Polym. Chem. 1976, 17(1), 169. 12. Dawson, D. Α.; Reynolds, W. F. Can. J. Chem. 1975, 53, 373. 13. Reade, T. H.; Sim, S. A. J. Chem. Soc. 1924, 157. 14. Barbieri, P. Compt. Rend. 1950, 231, 57. RECEIVED May 1, 1987


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