Polymeric Materials for Electronics Packaging and Interconnection

Chapter 9

Synthesis and Characterization of the f-Butyl Ester of the Oxydianiline—Pyromellitic Dianhydride Polyamic Acid F. M . Houlihan, B. J . Bachman, C. W. Wilkins, Jr., and Coralie A. Pryde

Downloaded by UNIV OF PITTSBURGH on March 10, 2015 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch009

AT&T Bell Laboratories, Murray Hill, NJ 07974

Novel t-butyl esters of oxydianiline/pyromellitic dianhydride polyamic acid were prepared in good yield. The polymers were prepared with either meta or para repeating units. The cure behavior of these t-butyl esters was studied by IR, MS and TG analysis and was compared to that of both the parent polyamic acid and its methyl ester. It was found that a rapid deprotection of the t-butyl group occurs at around 200ºC with liberation of free polyamic acid. Consequently, the cure behavior at 200ºC of the t-butyl ester approaches that of the parent polyamic acid. Furthermore, the isomerism of the repeating units does not appear to have any detectable effect on the cure behavior of the polymer, although, meta isomerism appears to enhance solubility of the polymer in organic solvents.

Linear alkyl esters of polyamic acids have been prepared in the past. Such esters are of interest because they are more stable in solution than the parent acids " and show little if any change in molecular weight with time. Unfortunately, the curing rates of linear alkyl esters are much slower than those of the corresponding acids (up to 60 times slower for the η-butyl ester). In our work we sought to avoid this difficulty by synthesizing a polyamic ester capable of thermally liberating the free polyamic acid. Moreover, it was hoped that this type of approach would yield novel polyimide precursors with thermally removable 4

7

4

bulky solubilizing groups. This could enable us to take advantage of the solubilizing effect of such groups during processing, giving access to low-boilingpointfilm-castingsolvents, while ensuring that the final cured polyimide retains valuable physical properties. This work reports the synthesis of the i-butyl esters of the ODA/PMDA polyamic acids, and describes some preliminary physical properties of these materials. The ί-butyl group (carbonate or ester) has been reported to thermally deprotect the hydroxyl group in styrene and polycarbonate0097-6156/89/0407-0101$06.00/0 ο 1989 American Chemical Society

In Polymeric Materials for Electronics Packaging and Interconnection; Lupinski, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

102

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

based polymer systems. This precedent encouraged us to choose this type of protecting group for our application. " 9

10

EXPERIMENTAL


Materials and Characterization. Elemental analyses were obtained from Galbraith Laboratories Inc. Polymer molecular weights were determined using a Waters Model 590 gel permeation chromatograph equipped with a Zorbax column, a Waters 410 differential refractometer, and a Waters Lambda-Max LC spectrophotometer. Molecular weights are given relative to polystyrene standards. NMR spectra were obtained on a Jeol 90Q FTNMR spectrometer, IR spectra were obtained on a Digilab FTS60 FTIR spectrometer and mass spectra were obtained using an HP 5995C mass spectrometer with a direct insertion probe (DIP). Preparation of the di-r-butyl Ester of Pyromellitic Acid. Potassium r-butoxide, 20.60 g (184 mmoles), was added, while stirring under dry argon, to 20.00 g (91.69 mmoles) pyromellitic dianhydride dissolved in 400 mL of THF. After stirring overnight the resulting suspension was filtered-off, washed with THF and ether, and dried to give the potassium salt of the desired product. The salt was neutralized in aqueous solution with dilute HC1 while keeping cold (10 C). The product was filtered, washed with distilled water, dried, and recrystallized several times from ethanol water. A total of 22.08 g (66% yield) of di-i-butylester was recovered in three crops of crystals. These crops consisted of 10.50 g of pure para isomer, 7.43 g of pure meta isomer, and a 50% mixture of the two isomers which constituted the remainder. e

meta isomer: Elemental analysis (C,H,0): found (58.98, 6.10, 35.29); calc (59.01, 6.05, 34.93). IR (KBr, cm" ): 3415, 2987, 2941, 1704, 1502, 1442, 1412, 1394, 1310, 1276, 1260, 1163, 1137, 1112, 927, 848, 796, 778. 1

H - l NMR (ppm, DMSO D-6)*: 13.64 (broad s, 2H,OH acid); 8.05 (s, 1H, CH#6); 7.81 (s, 1H, CH#3); 1.65 (s, 18H, C H i-butyl). *(For assignment see Scheme 1). 3

para isomer: Elemental analysis (C,H,0): found (58.87, 6.04, 34.79); calc (59.01, 6.05, 34.93). 1

IR (KBr, cm" ): 3570, 3001, 2982, 2935, 1728, 1704, 1502, 1442, 1422, 1369, 1306 1280, 1266, 1259, 1177, 1142, 917, 850, 803. H - l NMR (ppm, DMSO D-6)*: 13.64 (broad s, 2H, OH acid); 7.93 (s, 2H, CH#6,3); 1.65 (s, 18H, C H r-butyl).* (for assignments see Scheme 1). 3


9. HOULIHAN ET AL.

UButyl Ester of a Polyamic Acid

103

Formation of the Polyamic Acid Ester Derived From the para-di-f-butyl Ester of Pyromellitic Acid and ODA in NMP. A solution was prepared under argon consisting of 0.7327 g (2 mmole) of paradi-f-butyl ester of PMDA, 0.4005 g (2 mmole) ODA, and 0.56 mL methyl amine in solution in 2 mL NMP. The solution was cooled to -10 C and 1.6976 g (4 mmole) of N,N'-phenylphosphinobis[2(3H)-benzothiazolone] was added slowly. The reaction was kept at -10 C for 12 min and then allowed to return to room temperature, and stirred for 2 more hours. After diluting with 8 mL of NMP, the polymer solution was precipitated in 200 mL of methanol, filtered, washed with methanol and dried under vacuum for several days. In this way 0.83 g (79% yield) of tan polymer was isolated. e


e

Molecular weight: Mw=18,000, Mn=l 1,000, D=1.6 Elemental analysis (C,H,N,0): found (66.56, 5.88, 5.30, 21.50); calc (67.91, 5.69, 5.27, 21.10). 1

IR: (KBr, cm" ): 3320, 1722, 1679, 1607, 1512, 1407,1391, 1220, 1249, 1162, 1135, 1106. H - l NMR (DMSO D6, ppm)*: 10.63 (broad s, 2H, amide); 7.93 (s, 2H, H#6); 7.78 (d, 4H, H#2); 7.04 (d, 4H, H#3); 1.49 (s, H#ll).* (for assignments see Figure lb). Formation of the Polyamic Acid Ester Derived From the meta-di-r-butyl Ester of Pyromellitic Acid and ODA in NMP. The procedure was the same as in 2.1.2 except that the meta di-i-butyl ester of PMDA was used as a starting material. The workup differed in that the polymer was precipitated in a 50/50 mixture of methanol and water. In this way 0.87 g (82% yield) of tan polymer was recovered. Molecular weight: Mw=20,000 Mn=7,000 D=2.8 Elemental analysis (C,H,N,0): found (66.63, 5.55, 5.22, 21.65); calc (67.91, 5.69, 5.27, 21.10). 1

IR: (KBr, cm" ): 3320, 1722, 1685, 1605, 1407,1392, 1497, 1229, 1251, 1159, 1133, 1109. H - l NMR (DMSO D6, ppm)*: 10.62 (broad s, 2H amide); 8.13 (s, 1H, H#8); 7.78 (d, 4H, H#2); 7.60 (s, 1H, H#6); 7.04 (d, 4H, H#3); 1.43 (s, 18H, H#12).* (for assignments see Figure la). Studies of Polymer Cure. The Mass spectral curing studies were done by heating each sample of polymer in the DIP consecutively at 100°C, 200°C, and 350°C for 1 hour each. The extent of cure at 200°C was estimated by taking the ratio of the area for the peak at 200°C in the TIC (or the ion chromatogram of water or isobutene) to the combined area of the peaks at 200 C and 350°C. The samples for the IR curing study were spun from dimethylacetamide solutions of the polyamic acid or its esters onto NaCl e


104


plates. The spun samples were dried at 110°C for about one hour, heated slowly to the cure temperature, allowed to remain at that temperature for one hour and then cooled. The degree of imidization then was calculated from the 1375 cm" imide peak (C-N stretch) using the 1501 cm" band (CH aromatic ring breathing) as an internal standard. Thermogravimetry (TG) curing studies were performed on a Perkin-Elmer TGS-2 thermogravimetric analyzer interfaced with a System 4 microprocessor for temperature programming and a TADS Model 3700 data station for data acquisition and operational control. All measurements were obtained in N with a gas flow rate of 30 cc per minute. Heating rates used for the dynamic study were 5 and 25°C/minute. 1

1

11


2

RESULTS AND DISCUSSION Materials. The synthetic pathway to arrive at the di-r-butyl esters of pyromellitic acid is shown in Scheme 1. These esters could not be made by the classical pathway used for the diester derived from linear alcohols. The synthesis of the polyamic acid esters was accomplished by a procedure involving a direct coupling of a diester diacid monomer with an aromatic diamine (Scheme 2). A peptide coupling agent (Ν,Ν'-Phenylphosphino-bis [2(3H)-benzothiazolone], reported useful in the synthesis of aromatic polyamides was used in this reaction. This approach not only had the advantage of avoiding conditions that would cleave the i-butyl ester, but also decreased the number of steps needed to prepare the polymer. The polymerization with ODA was done with both the meta and the para isomer of the diacid diester and the solvents used were either NMP or THF. The yield of polymer ranged from 78 to 81%. 8

12

8

The t-butyl esters of ODA/PMDA polyamic acid retained the meta or para isomerism of the parent di-i-butyl ester diacid even on prolonged storage in DMSO solution. The identification of the isomers was based on the data provided by C-13 and H-l NMR spectra. Figures la,b shows the two distinct C-13 NMR spectra given by the meta and para isomers. The polymer made up of meta units is soluble in low boiling solvents such as THF or glyme. In contrast, the polymer consisting of para units is soluble only in high boiling, polar solvents such as DMF and NMP. This difference in solubility cannot be accounted for by differences in molecular weight since the two polymers have similar molecular weights. This difference in solubility between para and meta polymers is probably related to the different shapes of the polymer molecules, which depend on the isomerism of the repeating units, and are accentuated by the steric bulk of the tbutyl group and its ability to disrupt interchain interactions. It has also been found that only partial meta isomerism (70%) is enough to give solubility of the polymer in low boiling point film casting solvents. The same procedure was also extended to the η-butyl and methyl ester of the ODA/PMDA polyamic acid giving 79% and 84% yield respectively.



HOULIHAN ET A L

t-Butyl Ester of a Polyamic Acid

0

0

t-Bu

SCHEME 2


106


A)

m e t a - t e r t - B u ESTER OF POLYAMIC ACID OF ODA/PMDA

12


o=c^r^^c=o I C(CH )

t-Bu

11 12

1 7

3

3

23

10

200

180 160 140 120 100 8 0

60

40

20

0

ppm B)

p a r a - t e r t - B u ESTER OF POLYAMIC ACID OF ODA/PMDA

it*-

0

t-Bu

10 ?

4

15

2

w-K I

200

Figure 1.

L

_l

I

I

180 160 140 120 100 8 0 ppm

I

60

L.

1

1

1

40

20

0

Carbon-13 NMR spectra of the two isomers of the t-butyl

ester of the ODA/PMDA polyamic acid


9. HOULIHAN ET A L

107

t-Butyl Ester of a Polyamic Acid

Curing Behavior of Polymers. Thermogravimetric Analysis. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves obtained using a heating rate of 25 C per minute for ODA/PMDA polyamic acid and its esterified meta and para isomeric products are shown in Figure 2. It can be seen that the ODA/PMDA polyamic acid TG curve (Figure 2) has a broader region for weight loss. Figure 2 also shows the asymmetric shaped DTG curves for the isomeric esters compared with the symmetric curves obtained for the polyamic acid. Additionally, the maximum rate of reaction can been seen to occur earlier for the isomeric esters than for the polyamic acid. The asymmetric shaped DTG curves for the i-butyl ester could be explained by the presence of two simultaneously occurring reactions, namely, the deprotection of the carboxylic acid ί-butyl group, which gives isobutene, and the imidization reaction, which generates water.


e

Table 1 summarizes the maximum rate of reaction, onset temperature, and weight loss percent at 495 C for the ODA/PMDA polyamic acid as well as the para and meta isomers. Additionally, a comparison of data obtained with and without solvent removal is provided. e

Table 1. TG Analyses of 0DA-PDA Polyamic Acid and I t s E s t e r i f i e d Products Polymer

Heating

Condition

Rate

Onset

Maximum Rate

Weight

Weight

Temperature

of Reaction

Loss (Theory)

Loss (Actual)

CQ

CQ

%

%

ODA-PMDA

25

Solvent

153

202

8.61

23.9

ρ-ί-Bu

25

Solvent

183

197

27.9

25

Solvent

173

190

27.9

30.0

p-f-Bu Ester

5

Solvent

164

178

27.9

28.6

m-/-Bu Ester

5

Solvent

155

167

27.9

30.0

ODA-PMDA

5

Solvent Removed

150

187

p-/-Bu Ester

5

Solvent Removed

163

178

27.9

27.2

m-/-Bu Ester

5

Solvent Removed

164

202

27.9

28.3

Methyl Ester

5

Solvent Removed

176

217

15.3

16.9

η-Butyl Ester

5

Solvent Removed

193

247

27.9

29.0

('C per min)

Ester

m-/-Bu Ester

8.61

28.3

12.1

The theoretical weight loss for the elimination of water, shown in Table 1, for ODA/PMDA polyamic acid is 8.61%. Contrast this with the actual weight loss obtained before and after solvent removal/drying of 23.9% and 13.1%, respectively. Although there are heating rate differences, the ODA/PMDA polyamic acid still exhibited a larger weight loss value before and after extraction/drying than predicted from the theoretical value. Solvation of the polyamic acid by NMP could account for these larger weight losses. Conversely, both isomeric polyamic acid esters approached their theoretical weight loss of 27.9% regardless of solvent removal procedures. e

The para-methyl diester of ODA/PMDA had a weight loss of 16.9% at 495 C


108

POLYMERS FOR ELECTRONICS PACKAGING AND

100


(a)

^^DERIVATIVE RATE

INTERCONNECTION

2 5 . 0 0 DEG./MIN.

CD

Ld

™* ~**

75 h"

_

^Ν \

/ V/

/ ODA/PMDA

50

I . I . I

I

ι

I

-DERIVATIVE

100

ι

I

(b)

RATE: 25.00 DEG./MIN. χ CD UJ

75 h

p - t - B u ESTER —i

50 -DERIVATIVE

100

L

(c)

RATE: 25.00 DEG./MIN. »X CD LU

75 k

m - t - B u ESTER 50

J 40

140

240

340

L 440

540

TEMP. (C) Figure 2. TG and DTG curves for the ODA/PMDA polyamic acid and the two isomers of the t-butyl ester.


9. HOULIHAN ET A L

UButyl Ester of a Polyamic Acid

109

compared to the theoretical value of 15.3%. Onset of weight loss for this polymer at a heating rate of 5 C/min was 177 C with a maximum rate of reaction at 217 C. As was the case for the polyamic acid samples the DTG for this polymer is symmetric. The methyl ester has a higher onset and maximum rate of reaction when compared to ODA/PMDA polyamic acid and its ί-butyl esters, indicating that the ί-butyl ester falls intermediate between ODA/PMDA polyamic acid and its methyl ester in ease of imidization. Since earlier work had indicated that the nbutyl ester was far less susceptible to imidization than the methyl ester, this is a strong indication that the reaction does not proceed by the elimination of i-butanol since this process should be even harder to achieve than the elimination of nbutanol. Indeed, a TGA (5 C/min) of a sample of the η-butyl ester, confirmed that this material had a higher onset (193 C), and maximum rate of reaction than either the methyl or ί-butyl ester. Figure 3 shows an isothermal TGA study which illustrates the drastic difference in the imidization rates for the t-butyl ester and the η-butyl ester. In this study a sample of polymer was heated at three temperatures (100, 200, 400 C) for an hour at each temperature. By comparing the weight loss at 200 C to that at the final cure temperature we could calculate the % imidization. Thus the η-butyl ester was found to have a much lower % imidization (46%) than was found for the ί-butyl ester (93%). e

e

e

5


e

e

e

e

IR Spectroscopy. A IR study was made of the curing behavior of the parent ODA/PMDA polyamic acid compared to both the meta and para ί-butyl esters. Samples heated to 200°C for 1 hour showed no significant differences in cure for either of the two esters as compared to PMDA/ODA polyamic acid: Analysis of all three samples showed 87 ±2% imidization, based on results for sample given a final cure of 350°C. As can be seen in Figure 4a samples heated for 1 hour at 200°C give an IR spectrum very close to that of the parent polyamic acid cured under the same conditions (Figure 4b). Mass Spectroscopic Analysis. Mass spectroscopy showed that the products given off during the curing cycle were water and isobutene. As can be seen in Figure 5, the curing of the para-ibutyl ester can be followed by the total ion chromatogram (TIC) or, alternatively, the evolution of water and isobutene can be monitored separately by looking at their respective molecular ion chromatograms. Figure 5b,c shows that the liberation of water and isobutene from the polymer occurred simultaneously at 200 C and that isobutene was still being given off at 350°C. The extent of cure found by using either the total ion chromatograph or the ion chromatograph for the molecular ion of isobutene (87% and 88% respectively) is in good agreement with the values found in the FTIR curing study (88%). This suggests that, although the removal of the i-butyl group may be the rate determining step, this process is still rapid enough not to hinder significantly the amount of imidization occurring in an hour at 200 C. The meta isomer gives identical curing behavior consistent with the IR results. In contrast the η-butyl ester undergoes much less curing (43%) at 200°C (Figure 6a). Even the methyl ester, (Figure 6a), which is expected to cure more rapidly is much slower than the t-butyl ester (52%). As e

e



TIME (min)

Figure 3. Isothermal TG study of the imidization of the t-butyl and η-butyl esters of the ODA/PMDA polyamic acid.

p a r a - d i - t - B U T Y L ESTER OF ODA/PMDA POLYAMIC ACID BAKE 110°C CURE 2 0 0 ° C

^

0.2

(a)

I

Polymeric Materials for Electronics Packaging and Interconnection

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