NEW POLYSILAZANE AND METHOD FOR SYNTHESIS THEREOF

Abstract: There is provided a new polysilazane product from a specific precursor. Desired molecular weight of the polysilazane can be obtained by selecting proper molar ratio of R₁SiHX₂ and R₂R₃SiX₂ as raw materials. The polysilazane is stable during storage without gelation. The polymerization of the precursor can be carried out without gelation even at a high temperature and hence can be completed in a short time. The polysilazane can be made into ceramics fiber by spinning from a solution and subsequent firing.

Description: The present invention relates to a new polysilazane useful for the production of Si₃N₄/SiC ceramics. Nowadays, silicon nitride, silicon carbide, and a mixture thereof are attracting considerable attention as ceramics materials. Researches are being carried out to produce such ceramics materials, especially ceramic fiber, by pyrolyzing a polysilazane whose main chain is composed of nitrogen and silicon. According to a known technology disclosed in Japanese Patent Laid-open No. 226890/1985, an organic silazane polymer is produced by reacting anhydrous ammonia with RSiHX₂ in a solution, thereby forming a cyclic or linear precursor, and reacting this precursor in the presence of a basic catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom, thereby forming the Si₂N₂ bridge. This procedure is accomplished by, for example, blowing ammonia gas into methyldichlorosilane cooled in diethyl ether to bring about ammonolysis, filtering off the resulting ammonium chloride precipitates, and distilling away ether from the filtrate under reduced pressure, thereby obtaining a precursor having the skeletal repeating unit of the formula: Having a number-average molecular weight of about 300, this precursor is considered to be a cyclic compound composed of about 5 skeletal repeating units. This precursor polymerizes, giving off hydrogen gas, when added dropwise to a tetrahydrofuran solution in which is suspended potassium hydride (KH) with stirring. This is elucidated as follows by the inventors, Seyferth et al. At first, the following reaction forms the amide functional group. A pair of such functional groups join to each other to form a four-membered Si₂N₂ ring, permitting the metal hydride to be regenerated. Upon the addition of methyl iodide (CH₃I) to the reaction system after the completion of the following reaction: the amide functional group reacts with methyl iodide, forming potassium iodide precipitates. By centrifugally removing the potassium iodide precipitates and distilling away the solvent, there is obtained a polymer in the form of white powder. This polymer is found to have, for example, the following composition according to proton NMR and elemental analysis. (CH₃SiHNH) 0.39 (CH₃SiHNCH₃) 0.04 (CH₃SiN) 0.57 This composition may be represented by the following model. The structure enclosed by the broken line denotes the residue unit of the precursor. Unfortunately, the above-mentioned process has some disadvantages. That is, when the precursor is polymerized in the presence of a metal hydride catalyst, the crosslinking reaction proceeds excessively to such an extent that the polymer gels into an insoluble jelly. This makes it difficult to control the molecular weight and also makes the reaction less reproducible. Thus the resulting polymer is unstable and becomes insoluble as the result of crosslinking which proceeds with time. Therefore, it is poor in storage properties. In addition, the polymerization reaction has to be carried out carefully at a low temperature (below 30°C) because gelation takes place in a very short time if it is carried out at, say, 66°C (the reflux temperature of tetrahydrofuran). The polymerization reaction takes a long time before a desired molecular weight is reached. In order to overcome the above-mentioned disadvantages, the present inventors carried out a series of researches, which led to the findings that the disadvantages in the prior art arise from the fact that the precursor contains excessive crosslinking sites and that the number of crosslinking sites can be limited by proper means. The present invention was completed on the basis of these findings. Polysilazane obtained by the present invention has a skeleton composed of repeating units of the formula: in which polysilazane, the precursor residues, each composed of the repeating units of the formula: are connected to one another through the structural unit of the formula: characterized in that said precursor is composed of the units of the formulas: The present ivention is directed to a polysilazane, obtainable by a process which comprises the first step of reacting anhydrous ammonia with a mixture of organohalosilanes represented by R₁SiHX₂ and R₂R₃SiX₂ [where R₁ R₂, and R₃ each represents hydrogen (except for R₂ and R₃), a lower alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted vinyl group, a substituted or unsubstituted allyl group, a substituted or unsubstituted lower aryl group having 6 to 10 carbon atoms, and tri(lower)alkyl- or di(lower)alkylsilyl group, wherein (lower)alkyl means an alkyl group, having 1 to 6 carbon atoms or di(lower)alkyl-amino group, with R₁, R₂, and R₃ being the same or different from one another and X is a halogen atom] in a solution, thereby forming a cyclic or linear silazane precursor, and the second step of deprotonating, cyclizing, and crosslinking said precursor in the presence of a metal or metal hydride catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom. The invention encompasses further a process for producing a polysilazane as described above, wherein the molar ratio of R₁SiHX₂ to R₂R₃SiX₂ is 2:3 to 100:1 or wherein R₁, R₂, and R₃ in said each formula are all methyl groups. In the above formulae, R₁, R₂, and R₃ each represents hydrogen (except for R₂ and R₃); a lower alkyl group having 1 to 6 carbon atoms such as methyl group, ethyl group, n -propyl group, and isopropyl group; a substituted or unsubstituted vinyl group; a substituted or unsubstituted allyl group; a substituted or unsubstituted lower aryl group having 6 to 10 carbon atoms such as phenyl group, tolyl group, and xylyl group; tri(lower)alkyl- or di(lower)alkylsilyl such as trimethyl-, dimethyl-, methyl- ethyl-, and triethyl-silyl groups; and di(lower)alkylamino group such as dimethyl-, diethyl-, methylethyl-, and diisopropyl-amino groups; with R₁, R₂, and R₃, being the same or different from one another; and X denotes a halogen such as chlorine and bromine. (The same shall apply hereinafter.) The metal or metal hydride catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom in the precursor includes metal hydrides such as potassium hydride, sodium hydride, and lithium aluminum hydride (LiAlH₄), and metals such as lithium, sodium, and potassium. The present invention produces the following effects. (1) It is possible to produce the polymer having a desired molecular weight almost invariably by properly selecting the molar ratio of the raw materials R₁SiHX₂ and R₂R₃SiX₂. (2) The finished polymer changes very little with time, and hence it can be stored for a long time without gelation. (3) The polymerization can be carried out without gelation even at the reflux temperature of tetrahydrofuran, and hence the polymerization time can be shortened. The reason why the present invention produces the above-mentioned effects may be explained as follows: The precursor produced by ammonolysis of methyldichlorosilane according to the prior art is a cyclic compound which is composed of about 5 structural units of the formula: and has five crosslink reaction sites. If all of them take part in reaction, the reaction product is a crosslinked polymer having a three-dimensional network. Apparently, it is a insoluble gel. The compound should ideally be one which has two reaction sites, so that it forms a linear polymer through linkage. Such a polymer should be soluble and capable of spinning. The essential requirement for the reaction site is that a hydrogen atom be connected to each of adjoining silicon atom and nitrogen atom. In the case where a mixture of methyldichlorosilane and dimethyldichlorosilane is subjected to ammonolysis according to the present invention, the resulting precursor contains a unit represented by the formula: which can be the reaction site, and a unit represented by the formula: which can not be the reaction site. It follows, therefore, that the number of reaction sites can be controlled by adjusting the amount of dimethyldichlorosilane to be added. The invention is now described with reference to the following examples and comparative examples. EXAMPLES First step: In a 1-liter four-necked flask equipped with a stirrer, gas inlet tube, dry ice condenser, and thermometer, with the system completely dried and the atmosphere replaced with nitrogen, was placed 500 ml of tetrahydrofuran (THF) which had freshly been purified by distillation in the presence of lithium aluminum hydride (LiAlH₄). To the flask were added methyldichlorosilane and dimethyldichlorosilane in a prescribed amount as shown in Table 1. After dissolution, the solution was bubbled with ammonia gas (dried by passing through a sodium hydroxide column) at a flow rate of 3 ml/min. During bubbling, the solution was stirred and kept at 35°C. When the solution became saturated with ammonia and the heat generation came to a halt, the blowing of ammonia gas was suspended. The solution was stirred overnight to expel excess ammonia. Ammonium chloride precipitates formed by the reaction were filtered off and THF was distilled away from the filtrate. Thus there was obtained a silazane oligomer precursor in a 90% yield. It was found to have a number-average molecular weight (Mn) of about 300 (n ≈ 4.7) according to GPC on the assumption that it is represented by the formula: {(CH₃HSiNH) 1-z [(CH₃)₂SiNH] z } n where z denotes the molar fraction of the [(CH₃)₂SiNH] unit. No. CH₃SiHCl₂ g (X mole) (CH₃)₂SiCl₂ g (Y mole) Molar ratio X/Y (z x 100%) Mn n 1 81.3 (0.707) 132.4 (1.026) 0.69 (59) 319 4.75 2 108.7 (0.945) 94.4 (0.732) 1.29 (44) 324 4.97 3 81.4 (0.707) 36.2 (0.281) 2.52 (28) 270 4.29 4 135.6 (1.179) 40.5 (0.314) 3.75 (21) 282 4.55 5 83.8 (0.729) 15.7 (0.122) 5.98 (14) 304 4.99 6 102.1 (0.888) 16.1 (0.125) 7.10 (12) - - Second step: In a 500-ml three-necked flask equipped with a stirrer, dropping funnel, and condenser, was placed potassium hydride (KH) in an amount as shown in Table 2. With the system completely dried and the atmosphere replaced with nitrogen, 200 ml of tetrahydrofuran (THF) which had freshly been purified by distillation in the presence of lithium aluminum hydride was placed in the flask so that KH was suspended by stirring. To the flask was added dropwise from the dropping funnel 100 ml of THF containing a prescribed amount of the silazane oligomer precursor obtained in the first step. The dropwise addition was continued over about 30 minutes, with the solution temperature kept at 30°C. The solution temperature was raised to 66°C (the boiling point of THF) and reaction was carried out for 20 hours under refluxing. The results are shown in Table 2. The precursor was stirred for a long time to see if gelation takes place. No gelation took place in experiment Nos. 1∼4; however, a small amount of insoluble matter occurred in experiment No. 5 and gelation took place in experiment No. 6. The precursor which remained free of gelation was quenched with methyl iodide and then stored for 30 days to see if any change occurs in average molecular weight (Mn). It was confirmed that no change occurred with time. No. Precursor (g) KH (mg) Mn Polymer form After stirring for a long time 1 20 110 695 Oily No gelation 2 20 204 1153 Waxy No gelation 3 15 134 1855 Powdery No gelation 4 50 241 2860 Powdery No gelation 5 45 252 6600 Powdery No gelation (some insolubles) 6 50 284 5700 Powdery Gelation COMPARATIVE EXAMPLE CH₃SiHCl₂ alone was subjected to ammonolysis according to the procedure of the first step in Example. The resulting precursor was polymerized according to the procedure of the second step in Example. Gelation took place after polymerization at 30°C for 16 hours. Also, gelation took place within 10 minutes when the polymerization temperature raised to 66°C. EXAMPLE OF SPINNING The polymer (having a number-average molecular weight of 2860) indicated by No. 4 in Table 2 was dissolved in toluene, and the toluene solution was made into green fiber (unfired fiber) by dry spinning. Two hundred pieces of the green fiber in bundle, with a 2-g weight attached, were fired in a vertical tubular electric furnace. There was obtained ceramics fiber.

Claims: A polysilazane, obtainable by a process which comprises the first step of reacting anhydrous ammonia with a mixture of organohalosilanes represented by R₁SiHX₂ and R₂R₃SiX₂ [where R₁, R₂, and R₃ each represents hydrogen (except for R₂ and R₃), a lower alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted vinyl group, a substituted or unsubstituted allyl group, a substituted or unsubstituted lower aryl group having 6 to 10 carbon atoms, and tri(lower)alkyl- or di(lower)alkylsilyl group or a di(lower)alkyl-amino group; wherein (lower)alkyl means an alkyl group, having 1 to 6 carbon atoms with R₁, R₂, and R₃ being the same or different from one another and X is a halogen atom] in a solution, thereby forming a cyclic or linear silazane precursor, and the second step of deprotonating, cyclizing, and crosslinking said precursor in the presence of a metal or metal hydride catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom. A process for producing a polysilazane as claimed in Claim 1, wherein the molar ratio of R₁SiHX₂, to R₂R₃SiX₂ is 2:3 to 100:1. A process for producing a polysilazane as claimed in Claim 1, wherein R₁, R₂, and R₃ in said each formula are all methyl groups.

Inventors: NOAKE, KANEO, SERITA, TAMIO, TAKEUCHI, HIROYUKI
Applicants: CHISSO CORPORATION
Application year: 1988
Main classification: C08G77
IPC codes: C04B35, C08G77