Lateral group cross-linked polyimide with triphenylpyridine structure and preparation method of lateral group cross-linked polyimide

Abstract

The invention belongs to the technical field of electronic packaging materials, and particularly relates to cross-linked polyimide with a triphenylpyridine structure side group and a preparation method of the cross-linked polyimide. The triphenylpyridine with a large-volume rigid structure is introduced into polyimide, so that the stacking density can be reduced, polarization can be reduced, the dielectric constant and the dielectric loss are effectively reduced, and meanwhile, the heat resistance and the mechanical property are maintained or improved. Crosslinkable polyimide is prepared in a solution membrane casting mode, and crosslinked polyimide is obtained through a thermal crosslinking reaction. Compared with a corresponding original PI film, more free volume can be obtained through the formation of the cross-linked network, so that the Dk of polyimide is further reduced. The cross-linked network can also limit deflection of polar imide groups in the PI chain and reduce Df. In addition, excellent thermal performance, mechanical performance and low moisture absorption performance of PI are effectively maintained.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic packaging materials, and particularly relates to a crosslinked polyimide and a preparation method thereof. Background Art

[0002] With the development of electronic components towards miniaturization, thinness, lightness, multifunctionality, and high performance, higher requirements are put forward for the circuit boards and substrates that carry electronic components. This brings high-frequency and high-speed signal transmission. Therefore, the interlayer dielectric layer material should have a low dielectric constant and dielectric loss at high frequencies. Polyimide (PI) has become an indispensable insulating material in the microelectronics industry due to its excellent comprehensive properties. However, due to the strong intermolecular charge transfer (CT) effect of PI and the presence of polar imide rings, the dielectric constant (D k ) and dielectric loss (D f ) of PI are relatively high, significantly higher than those of non-polar polymers such as polytetrafluoroethylene. Therefore, there is an urgent need to prepare a new type of PI with both low D k and D f to meet the requirements of modern integrated circuits, high-speed and high-frequency communication technologies. Based on the Clausius-Mossotti equation, a large number of studies have been devoted to reducing the D k of polyimide. As for reducing D f , it is necessary to increase the rigidity of the polyimide molecular chain to limit the deflection of polar imide bonds. It can be seen that the mechanisms for reducing D k and D f of PI are different. Therefore, synergistically reducing D k and D f of PI is a major challenge. If the polarizability is reduced or the free volume is increased simultaneously, and the dipole orientation is restricted, it is expected to prepare a PI with low D k and low D f . Summary of the Invention

[0003] The object of the present invention is to overcome the above deficiencies in the prior art and provide a crosslinked polyimide film with a triphenylpyridine structure side group having a low dielectric constant and low dielectric loss and a preparation method thereof.

[0004] The present invention designs a polyimide with a triphenylpyridine structure and a crosslinked structure in the side chain. Triphenylpyridine has a large-volume rigid structure. Introducing the triphenylpyridine structure into the polyimide can reduce the packing density and polarization, thereby effectively reducing the dielectric constant and dielectric loss, while maintaining or enhancing the heat resistance and mechanical properties. A crosslinkable polyimide is prepared by solution casting, and a crosslinked polyimide is obtained through a thermal crosslinking reaction. Compared with the corresponding original PI film, the formation of a crosslinked network can obtain more free volume, thereby further reducing the D of the polyimide. k The crosslinked network can also restrict the deflection of polar imide groups in the PI chain and reduce D. f In addition, this design strategy effectively maintains the excellent thermal properties, mechanical properties and low moisture absorption properties of the PI.

[0005] The present invention first provides a crosslinked polyimide with a triphenylpyridine structure side chain, the main chain of which has a triphenylpyridine structure with a large free volume and the side chain has a crosslinkable group, and its chemical structural general formula is as shown in (Ⅰ):

[0006]

[0007] Wherein, X a is one of the structures of formula (X a-1 ), formula (X a-2 ), formula (X a-3 ):

[0008]

[0009] X b , X c , X d , X e is one of H and trifluoromethyl;

[0010] In addition, Xa and X b can also represent a methylene (CH 2 ) z bridging structure, z is 2, and the specific molecular structure is:

[0011]

[0012] m and n represent the degree of polymerization of two chain segments.

[0013] Furthermore, the number average molecular weight of the polymer is 30 - 40 kDa, the weight average molecular weight is 70 - 115 kDa, and the dispersity is 2 - 3.5.

[0014] The present invention also provides a method for preparing the above crosslinked polyimide having a triphenylpyridine structural side group. The specific steps are as follows: Under a nitrogen atmosphere, a crosslinkable diamine monomer represented by formula (II), other diamine monomers represented by formula (III), and an acid anhydride represented by formula (IV) are subjected to ternary copolymerization to obtain a crosslinkable polyimide including the structure of formula (I).

[0015]

[0016] Among them, the structure of the crosslinkable diamine monomer represented by formula (II) is selected from one of the following formulas:

[0017]

[0018] The structure of the diamine monomer represented by formula (III) is selected from one of the following formulas:

[0019]

[0020] The structure of the dianhydride monomer represented by formula (IV) is selected from one of the following:

[0021]

[0022] Furthermore, the method for synthesizing polyimide can adopt the high-temperature one-step method or the chemical imidization method;

[0023] (1) High-temperature one-step method: Under N 2 protection, add a diamine monomer containing a crosslinkable group represented by formula (II), other diamine monomers represented by formula (III), and a dianhydride monomer represented by formula (IV) to the reaction vessel. In the presence of a catalyst and a high-boiling solvent, carry out polycondensation reaction at a temperature of 150 - 250 °C for 12 - 24 hours to obtain a crosslinkable polyimide represented by the structure of formula (I).

[0024] (2) Chemical imidization method: Under N 2 protection, add a diamine monomer containing a crosslinkable group represented by formula (II), other diamine monomers represented by formula (III), and a dianhydride monomer represented by formula (IV) to the reaction vessel. Carry out ternary copolymerization in a solvent, reaction temperature: 10 - 25 °C, time 12 - 24 hours, to obtain polyamic acid (PAA). Then mix the PAA solution with a solution composed of an imidization reagent and a dehydrating agent, and carry out cyclization dehydration at a temperature of 50 - 80 °C for 12 - 24 hours to obtain a crosslinkable polyimide represented by the structure of formula (I).

[0025] The molar ratio of the sum of the feed molar amounts of the diamine monomer (II) containing crosslinkable groups and other diamine monomers (III) to the feed molar amount of the dianhydride monomer (IV) is 1:(1 - 1.02); in order to maintain excellent mechanical properties of the polyimide film after crosslinking, the feed molar amount of the diamine monomer (II) with crosslinkable groups is less than the molar amount of other diamine monomers (III). Among them, the preferred molar ratios of the structural monomers of formula (II), formula (III), and formula (IV) are 1:9:10, 2:8:10, 3:7:10, and 4:6:10.

[0026] In the high-temperature one-step method, the reaction solvents are cresol, p-chlorophenol, m-cresol, o-dichlorobenzene, 1,2,4-trichlorobenzene, etc.; the catalysts are quinoline, tertiary amine, alkali metal salt or zinc salt of carboxylic acid;

[0027] In the chemical imidization method, the reaction solvents are one or more of N,N-dimethylacetamide, N-methylpyrrolidone, m-cresol, and dimethyl sulfoxide, and the dehydrating agents are acetic anhydride, propionic anhydride, butyric anhydride, acetyl chloride, phosphoryl chloride, trifluoroacetic anhydride, phthalic anhydride, and triethylamine, etc.; the imidization reagents are isoquinoline, quinoline, methylpyridine, triethylamine, and N,N-dimethylaniline. The above dehydrating agents and imidization reagents can be used alone or in combination, and the solid content is 10% - 15%.

[0028] The crosslinked polyimide is used to obtain a crosslinkable polyimide film by solution casting. The specific preparation method is as follows:

[0029] Dissolve the crosslinkable polyimide powder in DMAc and continuously stir to prepare a polyimide solution with a concentration of 5 - 10 mg / mL. Then filter the solution and cast it onto a clean glass plate. Dry it in a vacuum oven at 60 - 80 °C for 4 - 6 hours, 100 - 120 °C for 4 - 6 hours, and 140 - 160 °C for 4 - 6 hours to remove the solvent and obtain a crosslinkable polyimide film; perform thermal crosslinking on it to obtain a crosslinked polyimide film.

[0030] The main purpose of stepwise drying is to control the solvent removal rate, prevent defect formation, reduce the influence of thermal stress, and ensure the thermal stability and mechanical properties of the final material.

[0031] The thermal crosslinking process is to place the pre-prepared crosslinkable polyimide in a tube furnace and heat-treat it at 260 - 280 °C for 2 - 4 hours to obtain a crosslinked polyimide film.

[0032] The present invention also provides a crosslinked porous polyimide film. By introducing unstable components such as polyethylene glycol, polystyrene microspheres, diphenyl sulfone or introducing porous fillers such as zeolite, silica microspheres, cage-shaped polyhedral oligomeric silsesquioxane, a porous crosslinked polyimide film is prepared, which has an ultra-low dielectric constant and dielectric loss. Specifically: 0.5 - 1 g of crosslinkable polyimide is dissolved in 10 mL of N,N-dimethylacetamide, 0.1 - 1 g of diphenyl sulfone is added, and the mixture is stirred overnight on a magnetic stirrer until dissolved. Then the polymer solution containing diphenyl sulfone is cast on a clean glass plate through a 0.22 μm filter head, and then the temperature is raised. First, it is dried in a vacuum oven at 60 - 80 °C for 4 - 6 hours, 100 - 120 °C for 4 - 6 hours, and 140 - 160 °C for 4 - 6 hours to remove the solvent, obtaining a crosslinkable polyimide film. A crosslinked polyimide film is obtained by thermal crosslinking it. The obtained polymer film is placed in a vacuum oven and thermally crosslinked at 260 - 280 °C for 2 - 4 hours to obtain a crosslinked polymer film. Then the crosslinked polymer film is placed in hot ethanol and washed for 24 - 48 hours to remove diphenyl sulfone, obtaining a low-dielectric crosslinked porous polyimide film.

[0033] In the present invention, the diamine monomer (Ⅱ) containing a crosslinkable group is a compound specially designed by the present invention. The preparation of monomer (Ⅱ) mainly undergoes aldol condensation, Michael addition reaction, condensation reaction, dehydrogenation reaction and nitro reduction to construct a diamine monomer with a crosslinkable group having a triphenylpyridine structure.

[0034] Due to the large-volume rigid structure of triphenylpyridine, introducing the triphenylpyridine structure into polyimide in the present invention can reduce the packing density and polarization, thereby effectively reducing the dielectric constant and dielectric loss, while maintaining or improving the heat resistance and mechanical properties, making it have broad application prospects in the fields of microelectronic packaging, flexible electronics, 5G high-frequency substrates, etc.

[0035] The advantages of preparing this polyimide by the high-temperature one-step method or chemical imidization method are that, compared with the thermal imidization method, the imidization temperature is lower, the required imidization time is shorter, the production efficiency can be greatly improved, and the prepared PI has excellent comprehensive properties. Secondly, it can avoid premature crosslinking of the crosslinkable polyimide, making it impossible to characterize its nuclear magnetic resonance hydrogen spectrum and molecular weight.

[0036] The material is tested and characterized and found that the crosslinked network can increase the free volume of the film. In addition, the crosslinked network can also limit the shift of polar imide groups in the polymer chain. This design strategy is beneficial to reducing the D k and D f . At the same time, maintaining the overall advantageous properties of PI, such as thermal properties and mechanical properties. This makes the crosslinked PI promising to meet the requirements of the semiconductor and communication industries. Description of the Drawings

[0037] Figure 1 1H NMR spectrum of monomer Ⅱ-2

[0038] Figure 2 1H NMR spectrum of monomer Ⅱ-6

[0039] Figure 3 FT-IR spectrum of monomer Ⅱ-2

[0040] Figure 4 FT-IR spectrum of monomer Ⅱ-6 Detailed Description of the Invention

[0041] Example 1, Synthesis of Crosslinkable Diamine Monomer Ⅱ-2

[0042] Under N 2 atmosphere, 4-(2,6-bis(4-aminophenyl)pyridin-4-yl)phenol (3.53 g, 10 mmol, 1 eq), 2,3,4,5,6-pentafluorostyrene (2.13 g, 11 mmol, 1.1 eq), CaH 2 (1.68 g, 40 mmol, 4 eq), CsF (7.60 g, 50 mmol, 5 eq) and 60 mL of DMAc were added to a 250 mL three-necked flask. The reaction was stirred and refluxed at 80 °C for 8 hours and monitored by TLC spotting. After completion of the above steps, CaH 2 and insolubles were removed by filtration. The filtrate was poured into deionized water to precipitate a white solid. The crude product was further purified by a chromatographic column (ethyl acetate:petroleum ether = 1:5 v / v% as the eluent). The product was a white solid. (3.74 g, yield: 71%).

[0043] Example 2, Synthesis of Crosslinkable Diamine Monomer Ⅱ-6

[0044] Under N 2 atmosphere, 4-(bicyclo[4.2.0]octa-1,3,5-triene-3-yl)-2,6-bis(4-nitrophenyl)pyridine (4.23 g, 10 mmol, 1 eq), Pd / C (0.400 g) and 30 mL of DMF were added to a 250 mL three-necked flask and heated at 90 °C. After 30 minutes, 3 mL of N 2 H 4 ·H 2O (30.0 mmol). The reaction raw materials were detected to have completely reacted and the product was formed by TLC thin-layer chromatography plate. The Pd / C was removed by hot filtration. The filtrate was poured into deionized water, and a white solid was precipitated. The crude product was further purified by a chromatographic column (ethyl acetate: petroleum ether = 1:1 v / v% as the eluent). The product was a white solid (2.51 g, yield: 69%).

[0045] Example 3, Synthesis of Crosslinkable Polyimide with Tetrafluorostyrene Side Group

[0046] Under an N 2 atmosphere, the above-prepared monomer II-2 (1.0550 g, 2 mmol, 2 eq), 2,2'-bis(trifluoromethyl)benzidine (structure (III)) (2.5619 g, 8 mmol, 8 eq), hexafluorodiacid anhydride (structure (IV)) (4.4424 g, 10 mmol, 10 eq) and 78 mL of m-cresol were added to a 250 mL three-necked flask equipped with a stirring device and refluxed under a nitrogen atmosphere. The mixture was stirred at 60 °C for 1 hour and at 120 °C for 6 hours. 1.5 mL of isoquinoline was added, and then the reaction system was heated to 180 °C for 12 hours. After that, the temperature was lowered to 100 °C, and the reaction mixture was poured into 300 mL of ethanol. The precipitate was collected by filtration and extracted with ethanol in a Soxhlet extractor for 48 hours. Finally, it was dried in a vacuum oven at 120 °C for 24 hours to obtain a crosslinkable polyimide with tetrafluorostyrene side group, denoted as FPI (7.08 g, yield: 92%).

[0047] Example 4, Preparation of Crosslinked Polyimide with Benzocyclobutene Structure:

[0048] Under an N 2 atmosphere, the above-prepared monomer II-6 (0.7263 g, 2 mmol, 2 eq), 2,2'-bis(trifluoromethyl)benzidine (2.5619 g, 8 mmol, 8 eq), hexafluorodiacid anhydride (4.4424 g, 10 mmol, 10 eq) and 79 mL of DMAc were added to a 250 mL three-necked flask equipped with a stirring device and refluxed under a nitrogen atmosphere. The mixture was stirred at 25 °C for 12 hours, then 1.5 mL of acetic anhydride and 0.5 mL of pyridine were added, and stirring was continued at 80 °C for 12 hours. The reaction mixture was poured into 300 mL of ethanol. The precipitate was collected by filtration and extracted with ethanol in a Soxhlet extractor for 48 hours. Finally, it was dried in a vacuum oven at 120 °C for 24 hours to obtain a crosslinkable polyimide with benzocyclobutene structure, denoted as BCB-PI (7.02 g, yield: 92%).

[0049] Example 5, Preparation of Crosslinkable Polyimide Film:

[0050] The crosslinkable polyimide prepared in Example 3 or 4 was used to prepare a crosslinkable polyimide film by a solution casting process. The method for preparing the film was as follows: 10 mg of the crosslinkable polyimide was dissolved in 1 mL of DMAc to prepare a polyimide solution with a concentration of 10 mg / mL, and the solution was stirred until a homogeneous solution was obtained. Then the solution was filtered and cast onto a clean glass plate. The temperature was set at 80 °C for 6 hours, 100 °C for 6 hours, and 150 °C for 6 hours in a vacuum oven to remove the solvent and obtain the crosslinkable polyimide films: FPI and BCB-PI.

[0051] Example 6, Preparation of Crosslinked Polyimide Film:

[0052] The pre-prepared crosslinkable polyimide film was placed in a tube furnace and heat-treated at 260 °C for 2 hours to obtain the crosslinked films FPI-CL and BCB-PI-CL.

[0053] Example 7, Thermal Property Testing of Polyimide Film

[0054] For the thermal property testing of the polyimide film, thermal performance analysis was carried out using TGA, DMA, and TMA respectively.

[0055]

[0057] Example 8, Mechanical Property Testing of Polyimide Film

[0058] The thickness of the film for the tensile property testing of the polyimide film was 25 - 40 μm, and the testing standard was GB / T 16421 - 1996.

[0059]

[0060] Example 9, Dielectric Property Testing of Polyimide Film

[0061] The dielectric properties of the PI film were measured using a Keysight E4980AL precision LCR meter at 25 °C with frequencies ranging from 1 kHz to 1 MHz. First, capacitance measurement was carried out, and then the dielectric constant of the film was calculated using the formula The dielectric constant of the crosslinked polyimide film at high frequencies was tested using a split-post dielectric resonator (SPDR).

[0062]

[0063] Example 10, Moisture Absorption Testing of Polyimide Film

[0064] The moisture absorption of the polyimide film was studied using the water absorption rate (W a ) and the contact angle (C a)Evaluate the moisture absorption behavior of the polyimide film. The moisture absorption rate measures the hygroscopicity using a high-precision electronic balance. The film sample is dried in an oven at 110 °C for 24 hours and then its weight (m 1 ) is measured. Then the sample film is soaked in water at 25 °C for 48 hours and its weight (m 2 ) is measured. The formula for calculating the moisture absorption rate is as follows (m 2 - m 1 / m 1 ) × 100%. The water contact angle is measured using a contact angle measuring instrument. The test liquid is deionized water, the injection volume of each droplet is 3 μL, and the injection speed is 1 μL / s.

[0065]

[0067] Example 11, Optical Property Test of Polyimide Film

[0068] The ultraviolet-visible (UV-vis) spectrum of the polyimide film (about 25 μm) is tested and characterized using a UV / Vis / NIR spectrophotometer. The optical property analysis of the polyimide film is evaluated using the cut-off wavelength (λ cutoff ), the transmittance at 450 nm is (T 450 nm ), and the transmittance at 550 nm is (T 550 nm ).

[0069]

Claims

1. A cross-linked polyimide having a triphenylpyridine structure side group, characterized in that: The main chain has a triphenylpyridine structure with a large free volume, and the side group has a cross-linkable group. The general chemical structure is shown in (I): Among them, X a Formula (X a-1 ), formula (X a-2 ), formula (X a-3 ) structure: X b , X c , X d , X e is one of H and trifluoromethyl; In addition, Xa and X b Can also represent methylene (CH2) z Bridge structure, z is 2, the specific molecular structure is: m and n represent the degree of polymerization of the two segments.

2. The cross-linked polyimide having a triphenylpyridine structure side group according to claim 1, characterized in that: The number average molecular weight is 30-40 kDa, the weight average molecular weight is 70-115 kDa, and the dispersity is 2-3.

5.

3. A method for preparing a cross-linked polyimide having a triphenylpyridine structure side group as claimed in claim 1, characterized in that: Under a nitrogen atmosphere, a crosslinkable diamine monomer as shown in formula (II), another diamine monomer as shown in formula (III) and an acid anhydride as shown in formula (IV) are ternary copolymerized to prepare a crosslinkable polyimide having a structure of formula (I); The structure of the crosslinkable diamine monomer shown in formula (II) is selected from one of the following formulas: The structure of the diamine monomer shown in formula (III) is selected from one of the following formulas: The dianhydride monomer structure shown in formula (IV) is selected from the following:

4. The preparation method according to claim 3, characterized in that: Specifically, a high temperature one-step method or a chemical imidization method is used; (1) High temperature one-step method: under N2 protection, a diamine monomer containing a crosslinkable group as shown in formula (II), other diamine monomers as shown in formula (III), and a dianhydride monomer as shown in formula (IV) are added to a reaction vessel, and a condensation reaction is carried out at a temperature of 150-250°C for 12-24 hours in a high boiling point solvent in the presence of a catalyst to obtain a crosslinkable polyimide as shown in formula (I); (2) Chemical imidization method: under N2 protection, a diamine monomer containing a crosslinkable group as shown in formula (II), other diamine monomers as shown in formula (III), and a dianhydride monomer as shown in formula (IV) are added to a reaction vessel, and terpolymerized in a solvent at a reaction temperature of 10-25°C for 12-24 hours to obtain polyimide acid (PAA), and then the PAA solution is mixed with a solution consisting of an imidization agent and a dehydrating agent, and cyclodehydrated at a temperature of 50-80°C for 12-24 hours to obtain a crosslinkable polyimide as shown in formula (I).

5. The preparation method according to claim 4, characterized in that: The ratio of the sum of the feeding molar amounts of the diamine monomer (II) containing a crosslinkable group and other diamine monomers (III) to the feeding molar amount of the dianhydride monomer (IV) is 1:(1-1.02), and the feeding molar amount of the diamine monomer (II) containing a crosslinkable group is less than the feeding molar number of other diamine monomers (III).

6. The preparation method according to claim 5, characterized in that: In the high-temperature one-step method, the reaction solvent is selected from cresol, p-chlorophenol, m-cresol, o-dichlorobenzene, 1,2,4-trichlorobenzene, etc.; the catalyst is selected from quinoline, tertiary amine, alkali metal salt or zinc salt of carboxylic acid; In the chemical imidization method, the reaction solvent is selected from one or more of N,N-dimethylacetamide, N-methylpyrrolidone, m-cresol, and dimethyl sulfoxide; the dehydrating agent is selected from acetic anhydride, propionic anhydride, butyric anhydride, acetyl chloride, phosphorus oxychloride, trifluoroacetic anhydride, phthalic anhydride and triethylamine; the imidization reagent is selected from isoquinoline, quinoline, picoline, triethylamine and N,N-dimethylaniline; the dehydrating agent and imidization reagent are used alone or in combination, and the solid content is 10%-15%.

7. The preparation method according to any one of claims 3 to 6, characterized in that: Furthermore, the obtained cross-linkable polyimide is used to obtain a cross-linkable polyimide film by solution casting, and the specific method is as follows: The cross-linkable polyimide powder is dissolved in DMAc, and the solution is continuously stirred to prepare a polyimide solution with a concentration of 10 mg / mL; the solution is then filtered and cast onto a clean glass plate; the solution is dried in a vacuum oven at 60-80°C for 4-6 hours, 100-120°C for 4-6 hours, and 140-160°C for 4-6 hours to remove the solvent to obtain a cross-linkable polyimide film; the cross-linking is then thermally cross-linked to obtain a cross-linked polyimide film; The thermal crosslinking is to put the pre-prepared crosslinkable polyimide film into a tubular furnace and perform heat treatment at 260-280° C. for 2-4 hours to obtain a crosslinked polyimide film.

8. The preparation method according to claim 7, characterized in that: Furthermore, a porous cross-linked polyimide film is prepared by introducing unstable components, so that it has ultra-low dielectric constant and dielectric loss; the unstable components are selected from one of polyethylene glycol microspheres, polystyrene microspheres, diphenyl sulfone, zeolite, silica microspheres, and cage-shaped polysilsesquioxane.

9. The preparation method according to claim 8, characterized in that: Specifically, 0.5-1g of cross-linkable polyimide is dissolved in 10mL of N,N-dimethylacetamide, 0.1-1g of diphenyl sulfone is added, and the mixture is stirred overnight on a magnetic stirrer until dissolved; then the polymer solution containing diphenyl sulfone is cast on a clean glass plate through a 0.22μm filter head, and then the temperature is increased, firstly, the solution is dried in a vacuum oven at 60-80°C for 4-6 hours, then at 100-120°C for 4-6 hours, and then at 140-160°C for 4-6 hours, and the solvent is removed to obtain a cross-linkable polyimide film; the solution is thermally cross-linked to obtain a cross-linked polyimide film; the obtained polymer film is placed in a vacuum oven, and thermally cross-linked at 260-280°C for 2-4 hours to obtain a cross-linked polymer film; then the cross-linked polymer film is placed in hot ethanol for washing for 24-48 hours to remove diphenyl sulfone, and a low dielectric cross-linked porous polyimide film is obtained.

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