WO1992011646A1 - Thermoplastic norbornene dicarboximide polymers useful for dielectric devices - Google Patents

Abstract

Electrical devices, e.g. molded circuit boards, connectors and capacitors, fabricated from electrical insulating component, comprising norbornene dicarboximide thermoplastic polymer exhibiting a glass transition temperature greater than about 200 °C, moisture absorption after equilibrating in boiling water of less than about 3 weight percent and an electrical permittivity at 1000 hertz measured at 25 °C and 50 percent relative humidity of less than 2.8. Preferred polymers comprise N-methyl norbornene dicarboximide, N-ethyl norbornene dicarboximide, N-propyl norbornene dicarboximide, N-butyl norbornene dicarboximide, N-trifluoroethyl norbornene dicarboximide, N-phenyl norbornene dicarboximide, N-trifluoromethylphenyl norbornene dicarboximide, N-cyclohexyl norbornene dicarboximide, N-cycloheptyl norbornene dicarboximide, N-cyclooctyl norbornene dicarboximide. Especially preferred are devices prepared from polymers of N-cyclohexyl norbornene dicarboximide having a Tg of 208 °C, dielectric constant of about 2.5 and boiling water equilibrated weight gain of only about 2 %. Polymers are advantageously prepared from monomers having a melting point less than 150 °C by melt polymerization with conventional metathesis ring-opening polymerization catalysts.

Description

THERMOPLASTIC NORBORNENE DICARBOXIMIDE POLYMERS USEFUL

FOR DIELECTRIC DEVICES Disclosed herein are electrical devices having dielectric components fabricated from thermoplastic norbornene dicarboximide polymers exhibiting advantageously high heat resistance, low dielectric permittivity and low moisture absorption. Preferred devices are made from novel polymers of N-cycloal yl norbornene dicarboximide. Also disclosed are methods of making and using such polymers. BACKGROUND OF THE INVENTION

The speed and power requirements of micro¬ electronic devices, e.g. capacitors, supporting sub- strates and connectors, and the like are critically dependent on the dielectric properties of the materi¬ als of construction of such devices. For instance, the net speed of a microelectronic device is equiva¬ lent to the sum of all the circuit delays. A major source of circuit delay is the time required for an electrical pulse to travel between gates. For a bipo¬ lar circuit the propagation velocity is inversely proportional to the square root of the dielectric permittivity of the substrate, which is often a poly- meric material. Thus, reducing the permittivity of the substrate results in an increased device speed or an expansion of the circuit dimensions without a loss in speed. The reduced capacitance associated with circuit traces allows a closer spacing of circuit elements without excessive levels of crosstalk. Re¬ duction of line capacitance also directly reduces power consumption, which in turn reduces the heat output of the device.

High temperature resistant ther oset polymers are commonly used for fabricating dielectric devices for electronic and electrical applications.- For instance, printed circuit interconnections can be fabricated from thermoplastics such as polyimides, polyesters, fluoropolymers or thermosetε such as reinforced epoxy laminates. Such polymeric materials typically have dielectric permittivities of about 3 or higher. Owing to their ease of fabrication, e.g. by conventional thermoplastic molding methods, high performance temperature thermoplastic polymers such as polyetherimides, polyetherketones, polysulfones and polyamideimides may be preferred over thermosets for a variety of high value, high performance applications. Temperature performance of thermoplastic polymers is commonly characterized in terms of heat distortion temperature and/or glass transition temperature (Tg) . The Tg of the polymers of this invention, as determined by differential scanning calorimeter (DSC) , is preferably at least 180 "C.

Despite their advantages, high performance temperature thermoplastic polymers are typically characterized by one or more disadvantages such as high cost arising from complex preparation methods, higher dielectric permittivity or high moisture absorption. For instance, although fluorocarbons have relatively low permittivity, e.g., about 3.1, they exhibit high moisture absorption, e.g. more than 6% weight gain after equilibrating in boiling water for 7 hours. The fluorocarbons also suffer from low melting point (typically less than 260 ^βC) and poor dimen¬ sional stability. In the case of conventional polyimides, i.e. polymers having i ide linkage in the polymer backbone, the dielectric permittivity can be as low as about 2.8 when dry; however, due to the tendency to absorb moisture the permittivity will rise to about 3.2 with moisture absorption.

Unsaturated thermoplastic polymers of norbornene dicarboximides as disclosed in French

Brevet d¹invention 1,594,934 and U.S. Patent 3,959,234 exhibit a wide range of heat resistance depending on the hydrocarbon radical forming the imide group, ranging from poly(N-cyclohexyl norbornene dicarboximide) with a Tg of -172 °C to poly(N-methyl norbornene dicarboximide) with a Tg of 189 °C. It has been discovered that high performance temperature thermoplastic polymers of certain norbornene dicarboximides can, in fact, exhibit a favorable balance of properties for electrical devices such as low weight, high temperature resistance, high solvent resistance, high tensile strength, low moisture absorption and low dielectric permittivity. For instance, polymers of N-phenyl norbornene dicarboximides as disclosed in U. S. Patent 4,965,330 having a Tg greater than 200 °C and exhibit low moisture absorption, e.g. about 3% after equilibrating in boiling water. Unfortunately, polymers of N-phenyl norbornene dicarboximides are not simple to prepare, e.g. the precursor monomer is not amenable to melt polymerization because it has a high melt tempera- ture. Commercial preparation of the polymer by solution polymerization, e.g. in chlorinated hydrocarbon solvents, is undesirable because of cost and environmental burdens.

An object of this invention is to provide a novel high temperature performance polymers that can be prepared by simple polymerization methods, e.g-. melt polymerization, and that exhibit desirably low dielectric permittivity low moisture absorption and high heat resistance. Another object is to provide dielectric devices of such thermoplastic polymers which exhibit low dielectric permittivity. SUMMARY OF THE INVENTION

This invention provides electronic and elec¬ trical devices which comprise an electrical conducting component and a polymeric electrical insulating com¬ ponent fabricated from a polymer comprising norbornene dicarboximide units. Such polymeric electrical in¬ sulating components exhibit high heat resistance, e.g. a Tg greater than about 200 "C, low moisture absorption and low increase in permittivity with moisture absorption. For instance, after equilibrating in boiling water certain preferred polymers of norbornene dicarboximides exhibit less than about 3 percent weight increase and an electrical permittivity of less than 2.8 measured in 25 °C air at 50 percent relative humidity, e.g. over a frequency range of 10 to 100,000 hertz. Preferred devices are fabricated from novel insulating polymers comprising units of N-cycloalkyl norbornene dicarboximide, e.g. where the cycloalkyl radical ranges from 3-8 carbon atoms, i.e. cyclopropyl to cyclooctyl. Especially preferred are polymers comprising N-cyclohexyl norbornene dicarboximide units. Because the N-cycloalkyl monomers have a melt point less than 150 "C, the polymers of this invention are advantageously prepared by melt polymerization using conventional metathesis polymerization catalysts.

The polymers used in this invention exhibit desirably high heat resistance, e.g. Tg's of at least 180 'C, low dielectric permittivity, e.g. less than 3, and low moisture absorption, e.g. less than 2.5 % weight increase after equilibrating in boiling water. Preferred devices include circuit boards, e.g. molded circuit boards, and other substrates for mounting electronic circuits or microelectronic de¬ vices, connectors and capacitors. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The electrically insulating polymer used in the electrical/electronic devices can be a saturated or unsaturated homopolymer or copolymer comprising norbornene dicarboximide units selected from the group consisting of N-methyl, N-ethyl, N-propyl, N-butyl, N-trifluoroethyl, N-phenyl, N-trifluoromethylphenyl, N-cyclopropyl, N-cyclobutyl, N-cyclopentyl, N-cyclo¬ hexyl, N-cycloheptyl and N-cyclooctyl norbornene dicarboximide. Preferred devices are prepared from N-cycloalkyl norbornene dicarboximides polymer. Depending on the cycloalkyl radical of the imide group the Tg of homopolymers of N-cycloalkyl norbornene dicarboximides can range from about 180 °C for homopolymers of cyclobutyl and cyclopentyl imides, to about 190 "C for homopolymers of the cyclopropyl imide and greater than 205 "C for the homopolymers of cyclohexyl, cycloheptyl and cyclooctyl imides. Because of their higher Tg's the homopolymers of the 6-8 carbon atom cycloalkyl norbornene dicarboximides are preferred; most preferred is the homopoly er of the cyclohexyl imide.

The preferred polymers used in this invention will exhibit desirably low dielectric constant, e.g. less than 3 when measured over a wide range of frequency, e.g. 0.1-100 kilohertz, at 25 °C and 50% relative humidity. More preferably the permittivity will be less than 2.8. Especially preferred polymers of the N-cyclohexyl imide exhibit a dielectric permittivity of about 2.5; e.g. about 2.4 as molded and 2.6 after equilibration in room temperature air 50% relative humidity. Although other properties are also important, selection of norbornene dicarboximide polymers and copolymers for the devices of this invention can be facilitated on the basis of dielectric permittivity as indicated in the following Table 1. TABLE 1 Imide Group

Homopolymers Permittivity*

-CH₃ 2 . 4 -CH₃ 3 . 1 ( 50% RH)

-C_βHn 2. 4

-CJltt. 2 . 6 (50% RH)

-p-Cl-C₆H₄ 2 . 3 copolymers -CH₃/o-CF₃CH₂-C₆H₄- 2 . 3

-CH₃/p-CF₃CH₂-C₆H₄- 2 . 5

-CH₃/CF₃CH₂-^' 2 . 4

-C₆K.₁/m-CF₃CH₂-C₆H₄- 2 . 4

-C^n/o-CF_jCHg-C^- 3 . 1 -C₆H_{1 l}/p-CF₃CH₂-C₆H₄- 2 . 5

— CgH'_{] 'j}/CF CH₂— 2 . 2

Permittivity was measured at 1000 hertz on a dry molded sample in a dry nitrogen atmosphere at 100 °C, except where for measurements indicated by "50% RH" which were measured on a sample equilibrated in a 50% relative humidity air at 25 ^βC.

The polymers of this invention exhibit low moisture absorption, e.g. less than 2.5% weight increase after equilibrating in boiling water. Preferred homopolymers of the N-cyclohexyl imide exhibit about 2% weight increase after equilibrating in boiling water.

Another aspect of this invention comprises copolymers of an N-cycloalkyl norbornene dicarboximide monomer and one or more other norbornene derivative monomers. In the case of copolymers the monomer units are selected by routine experimentation to provide a desired balance of properties. An especially preferred copolymer of N-cyclohexyl and N-trifluoro- ethyl norbornene dicarboximide (monomer units in the ratio of 9:1) exhibits an especially advantageous per¬ mittivity of about 2.2. Since polymers of norbornene nitrile have a Tg of about 124 "C, copolymers of norbornene nitrile with an N-cycloalkyl norbornene dicarboximide will typically exhibit lower Tg in proportion to the amount of norbornene nitrile units. For instance, a polymer containing as little as 20% norbornene nitrile units and 80% N-cyclohexyl norbornene dicarboximide units has a Tg of 173 "C. A preferred copolymer comprises monomer units of N-methyl norbornene dicarboximide or N-trifluoroethyl norbornene dicarboximide. Because homopolymers of

N-methyl norbornene dicarboximide exhibit high Tg of about 210 ^βC and dielectric permittivity of about 3.1, copolymers of N-methyl and N-cyclohexyl norbornene dicarboximides will have a Tg of at least 205 °C and a dielectric permittivity between 2.5 and 3 depending on the proportion of monomer units. Copolymers of N-methyl and N-cycloalkyl norbornene dicarboximide are advantageously prepared by melt polymerization at temperatures lower than the melting point of either monomer, i.e. about 115 ^βC for N-methyl norbornene dicarboximide and about 140 ° C for N-cyclohexyl norbornene dicarboximide. For instance, monomer mix¬ tures of about 25-70% N-cyclohexyl norbornene dicar¬ boximide melt at temperatures of 85-100 °C and monomer mixtures of about 50-60% N-cyclohexyl norbornene dicarboximide melt at eutectic temperatures of 85- 90 °C. Such melt polymerization conducted at lower temperatures reduces the polymer's exposure to the potentially deleterious effects of higher temperature polymerization.

Polymers of N-methylnorbornene dicarboximide exhibit a favorable dielectric permittivity of about 2.6 when dry as molded; however, the polymer is suffi¬ ciently hygroscopic that it can absorb about 7 percent water. Due to a high affinity for water, the permit¬ tivity^*of homopolymers of N-methyl norbornene dicar¬ boximide increases to about 3.1 after equilibrating in room temperature air at 50% relative humidity. Such adverse effects caused by moisture absorption can be avoided by maintaining the electronic device in a dry environment or, more preferably, by incorporating hydrophobic monomer units into the polymer. For instance, copolymers of N-methyl norbornene dicarboximide with units of N-cyclohexyl norbornene dicarboximide or N-trifluoroethyl norbor¬ nene dicarboximide have increased hydrophobicity, resulting on lower water absorption and low permit¬ tivity that is not adversely affected by humid envi- ronments.

PREPARATION OF MONOMERS

Norbornene dicarboximide monomers are advantageously derived from monomers of norbornene dicarboxylic acids. Such monomers are readily prepared through Diels-Alder reaction of cyclopentadiene and maleic anhydride by methods that are well known, e.g. as disclosed in U.S. Patent 4,022,954. In general the Diels-Alder reaction product of cyclopentadiene and maleic anhydride is the endo stereoisomer which can be converted to a predominately exo stereoisomer by heating and recrystallization, e.g. by well-known procedures such as disclosed by Castner et al. in Journal of Molecular Catalysis 15, (1982) 47-59. For instance, in the case of norbornene dicarboxylic anhydride heating at about 198 °C for about two hours provides a molten equilibrium mixture of about 45 percent endo stereoisomer and about 55 percent exo stereoisomer. Predominately exo stereoisomer of NDA can be recovered by selective crystallization from a solvent such as toluene. Through multiple recrystallizations substantially high levels of the exo stereoisoirer can be recovered, e.g. at least about 85 percent or higher. The dicarboximides can be prepared by reacting a primary amine, e.g. cyclohexyla ine, with the norbornene dicarboxylic anhydride providing an amic acid which can be readily i idized. PREPARATION OF POLYMERS

The polymers useful in the devices of this invention can be advantageously prepared using well known metathesis ring-opening polymerization techniques, including melt polymerization in an extruder as well as by solution polymerization where the monomer is dissolved in solvent such as toluene or dichloroethane. Polymerization is effected using conventional metathesis polymerization catalysts such as tungsten hexachloride catalyst with aluminum alkyl initiator or ruthenium chloride catalyst. Preferred catalyst systems are the non-pyrophoric catalysts, e.g. systems comprising ruthenium initiator and tungsten catalyst, as disclosed in United States application Serial No.07/531,663.

Because the N-cycloalkyl norbornene dicarboximide monomers have a melt point less than 150° C, preferred polymers useful in the devices of this invention are advantageously prepared by melt polymerization, e.g. in an extruder, using conventional metathesis polymerization catalyst systems. Certain of the norbornene dicarboximide monomers form eutectic mixtures allowing low temperature melt polymerization. For instance, especially useful copolymers can be prepared by melt polymerization of a mixture of N-methyl and N-cyclohexyl norbornene dicarboximide monomers which has a depressed melting point of less than 100 °C; these copolymers exhibit the advantageous qualities of high heat resistance (i.e. Tg of at least 205 °C) and dielectric permittivity less than 3 and moisture absorption less than that of homopolymers of the N-methyl imide.

The polymers and copolymers of this invention are advantageously useful for preparing injection molded parts intended for high service temperature applications e.g. above 150 °C. Owing to the low dielectric permittivity of these materials, an especially advantageous application is in electronic devices such as molded circuit boards, integrated circuit substrates, connectors, capacitors and the like. The disclosure in the following examples illustrate specific embodiments and aspects of this invention but is not intended to imply any limitation of the scope of this invention. EXAMPLE 1

This example illustrates the preparation of a polymer of a N-cyclohexyl norbornene dicarboximide. 2 g of substantially exo-stereoisomeric N-cyclohexyl norbornene dicarboximide, melting point of 135-140 °C, was dissolved in toluene at 65 °C providing a solution containing 30 wt.% monomer. The solution was mixed with 0.1 ml of a catalyst solution comprising 0.5 M tungsten hexachloride in toluene and 0.2 ml of an activator solution comprising 2 M diethyl aluminum chloride in heptane. After mixing for about 1.5 hours the polymerization reaction was stopped by admixture of methanol. The recovered reaction product of pol (N-cyclohexyl norbornene dicarboximide) had a molecular weight of 58.5 K (number average) and 421 K (weight average) , a Tg of 208 "C (by DSC) , a dielectric permittivity of 2.55 and equilibrated boiling water uptake of 2%.

EXAMPLE 2 This example illustrates the preparation of a polymer of a N-cyclohexyl norbornene dicarboximide. The procedure of example 1 was essentially repeated except the toluene solvent was replaced with 1,2-dichloroethane. The resulting polymer exhibited a molecular weight of 36 K (number average) and 90 K (weight average) , a Tg of 207 °C (by DSC) and equilibrated boiling water uptake of 2%.

EXAMPLE 3 This example illustrates the preparation of a polymer of a N-cyclohexyl norbornene dicarboximide. 2 g of substantially exo-stereoisomeric N-cyclohexyl norbornene dicarboximide was mixed in a melt at 140 °C with 0.1 ml of a catalyst solution of 0.5 M tungsten hexachloride and 0.2 ml of an activator solution of 2 M diethyl aluminum chloride for about 1.5 hours produ¬ cing a polymer recovered in methanol having molecular weights of 62 K (number average) and 1290 K (weight average) , a Tg of 209 'C (by DSC) , a dielectric permittivity of 2.55 and equilibrated boiling water uptake of 2%.

EXAMPLE 4 This example illustrates the preparation of polymers of N-cyclohexyl norbornene dicarboximide. Endo stereoisomeric N-cyclohexyl norbornene dicar¬ boximide was added to the exo-stereoisomeric monomer in the amount indicated in Table 2 and the mixture was polymerized essentially in the manner of Example 1.

Polymerization yield and Tg for the resulting polymers is indicated in Table 2.

TABLE 2

EXAMPLE 5 This example illustrates the preparation of copolymer of N-cyclohexyl and N-methyl norbornene dicarboximide by melt polymerization and solution polymerization. A 50/50 mixture by weight of

N-cyclohexyl and N-methyl norbornene dicarboximides has a eutectic melting point of about 85 °C. Polymers prepared by melt polymerization essentially in the manner of Example 3 provided copolymers typically having a molecular weight of about 40 K (number average) and 200 K (weight average) and a Tg of about 208 °C. Mixtures of N-cyclohexyl and N-methyl norbornene dicarboximides were polymerized in toluene solutions using catalyst and initiator solutions essentially according to the procedure of example 1. Copolymers were prepared having 20, 40, 50, 60 and 80 mole percent N-cyclo hexyl norbornene dicarboximide units having a Tg of about 209 "C and molecular weights in the range of 36-50 (number average) and 150-420 (weight average) .

EXAMPLE 6 This example illustrates polymers of a variety of N-cyclo alkyl norbornene dicarboximides. N-cyclo- propyl, N-cyclobutyl, N-cyclopentyl, N-cycloheptyl and N-cyclooctyl norbornene dicarboximides were polymerized. essentially in the manner of Example 1 providing polymers having Tg (by DSC) of 191, 182, 180, 204 and 208 °C, respectively.

EXAMPLE 7 This example illustrates the effect of molecular weight on Tg of polymers of N-cyclohexyl norbornene dicarboximide. N-cyclohexyl norbornene dicarboximide was polymerized essentially in the manner of Example 1 except that varying amounts of 1-hexene was added as a polymer chain terminator. The results presented in

Table 3 indicate a correlation between number average molecular weight and Tg.

EXAMPLE 8 This example illustrates the preparation of a copolymer of N-cyclohexyl norbornene dicarboximide and N-trifluoroethyl norbornene dicarboximide. 9 parts of N-cyclohexyl norbornene dicarboximide and 1 part N-trifluoroethyl norbornene dicarboximide were polymerized essentially in the manner of Example 3 providing a copolymer having a permittivity of 2.2

While specific embodiments have been described, it should be apparent to those skilled in the art that various modifications thereof can be made without departing from the true spirit and scope of the invention. Accordingly, it is intended that the following claims cover all such modifications within the full inventive concept.

Claims

WHAT IS CLAIMED IS:

1.- A device comprising an electrical conducting component and an electrical insulating component, wherein said electrical insulating component comprises a thermoplastic polymer exhibiting a glass transition temperature greater than about 200 °C, moisture absorption after equilibrating in boiling water of less than about 3 weight percent and an electrical permittivity at 1000 hertz measured at 25 °C and 50 percent relative humidity of less than 2.8, wherein said polymer comprises units of norbornene dicarboximide.

2. A device according to claim 1 comprising a molded circuit board, a connector or a capacitor.

3. A device according to claim 1 wherein said norbornene dicarboximide units comprise N-cycloalkyl norbornene dicarboximide units wherein said cycloalkyl units range from cyclopropyl to cyclooctyl.

4. A device according to claim 1 wherein said polymer is a saturated or unsaturated homopolymer or copolymer comprising units selected from the group consisting of N-methyl norbornene dicarboximide, N-ethyl norbornene dicarboximide, N-propyl norbornene dicarboximide, N-butyl norbornene dicarboximide, N-trifluoroethyl norbornene dicarboximide, N-phenyl norbornene dicarboximide, N-trifluoromethylphenyl norbornene dicarboximide, N-cyclopropyl norbornene dicarboximide, N-cyclobutyl norbornene dicarboximide, N-cyclopentyl norbornene dicarboximide, N-cyclohexyl norbornene dicarboximide, N-cycloheptyl norbornene dicarboximide, N-cyclooctyl norbornene dicarboximide.

5. A device according to claim 1 wherein said polymer exhibits electrical permittivity of less than 2.8 when measured at 25 °C and 50 percent relative humidity over a frequency range of 10 to 100,000 hertz.

6. A polymer comprising N-cycloalkyl norbornene dicarboximide units wherein said cycloalkyl ranges from cyclopropyl to cyclooctyl, wherein said polymer has a glass transition temperature of at least 180 °C.

7. A polymer according to claim 6 consisting of poly(N-cyclohexyl norbornene dicarboximide) .

8. A polymer according to claim 6 wherein said polymer exhibits a dielectric permittivity less than 2.8 at 1000 hertz in air at 25 ^βC and 50% relative humidity and moisture absorption after equilibrating in boiling water of less than 2.5%.

9. A polymer according to claim 8 consisting of poly (N-cyclohexyl norbornene dicarboximide) .

10. A polymer according to claim 6 prepared by melt polymerization wherein molten N-cycloalkyl norbornene dicarboximide monomer is mixed in an extruder with metathesis ring-opening polymerization catalyst at a temperature less than 150 °C.

11. A method of preparing a polymer comprising N-cycloalkyl norbornene dicarboximide units wherein said cycloalkyl ranges from cyclopropyl to cyclooctyl and wherein said polymer has a glass transition temperature of at least 180 °C, said process com¬ prising mixing in an extruder molten N-cycloalkyl norbornene dicarboximide and metathesis ring-opening polymerization catalyst at a temperature less than 150 °C.