Reformed polysilazane and method of producing same

Abstract: A novel, reformed polysilazane obtained by reacting a polysilazane with a compound selected from ammonia, primary and secondary amines, hydrazine and mono-, di- and tri-substituted hydrazines to cross-link the polysilazane with the compound serving as a cross-linking agent or to link the compound to the polysilazane.

Description: This invention relates to a novel polysilazane and to a method of preparing same by polycondensation. There are known polysilazanes which are proposed to be used as a precursor material for the production of silicon nitride-containing ceramics. Known polysilazanes are liquid or solid and the solid polysilazanes are insoluble in an organic solvent such as toluene. For example, A. Stock discloses an inorganic polysilazane of the formula (SiH₂NH) n prepared by reacting dichlorosilane with ammonia using benzene as a solvent (Ber. 54 , 740 (1921). This inorganic polysilazane is an oligomer (n = 7 to 8) and is a viscous liquid at room temperature. D. Seyferth et al suggest an inorganic polysilazane obtained by reacting dichlorosilane with ammonia using dichloromethane as a solvent (US-A-4,397,828). This polysilazane is an oily liquid having a proton ratio Si-H/N-H of about 3.3 and becomes solidified when heated at about 200 °C or allowed to stand for 3-5 days. The solidified polysilazane is insoluble in an organic solvent such as o-xylene. Japanese Published Unexamined Patent Application (Tokkyo Kokai) No. 60-145,903 discloses an inorganic polysilazane having a molecular weight of 690-2,000 and obtained by reacting a dihalosilane adduct such as a dichlorosilane-pyridine adduct with ammonia. The resultant polysilazane upon removal of the solvent therefrom is a viscous liquid or a resinous solid. This solid, however, is insoluble in an organic solvent such as o-xylene. In utilizing an inorganic polysilazane as raw materials for ceramic fibers, binders, coating agents or the like, it is highly desirable that the polysilazane be soluble in an organic solvent, have a high molecular weight and exhibit good spinnability, solidifiability and moldability. In these respects, the above-described known inorganic polysilazanes are not fully satisfactory. The present invention has been made with the foregoing problems of the conventional inorganic polysilazane in view and provides a polycondensed polysilazane having a number average molecular weight of 1,500-500,000, which (a) is derived from a starting polysilazane having a number average molecular weight of 100-1,400 and containing recurring units of the formula: wherein R₁ and R₂, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, (b) is free of NH₂ groups and (c) contains one or more skeletal groups of the formulae (I), (II), (III) or (IV); wherein formula (I) is: wherein R₁ and R₂ are, independently from each other, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, A stands for a direct bond or -N(R₄)- where R₄ stands for hydrogen, alkyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, and R₃ stands for (i) hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or -N(R₅)(R₆) where R₅ and R₆, independently from each other, stand for alkyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, when the group A is a direct bond or (ii) hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, when the group A is -N(R₄)-; wherein formula (II) is: wherein R₂ is alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, and A and R₃ are as defined in relation to formula (I); wherein formula (III) is: wherein R₁ is alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, and A and R₃ are as defined in relation to formula I; and wherein formula (IV) is: wherein A and R₃ are as defined in relation to formula (I), and wherein said number average molecular weights are measured by gel permeation chromatografy. In another aspect, the present invention provides a method of forming a novel polycondensed polysilazane as defined above, comprising reacting a starting polysilazane having a number average molecular weight of 100-1,400 and containing recurring units of the formula: wherein R₁ and R₂, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, in an organic base-containing solvent with a compound of the formula I: wherein A stands for a direct bond or -N(R₄)- where R₄ stands for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, and R₃ and R₇, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, with the proviso that at least one of R₃ and R₇ is hydrogen. The present invention will now be described in greater detail below. The raw matsrial inorganic polysilazane to be used as the starting polysilazane in the method according to the present invention has as its main skeletal structure recurring units represented by the following formula: wherein R₁ and R₂, independently from each other, stand for hydrogen, alkyl such as methyl, ethyl, propyl, butyl, octyl or decyl; alkenyl such as vinyl, allyl, butenyl, octenyl or decenyl; cycloalkyl such as cyclohexyl or methylcyclohexyl; alkylamino such as methylamino or ethylamino; aryl such as phenyl, tolyl, xylyl or naphthyl; aralkyl such as benzyl; or alkylsilyl such as methylsilyl, ethylsilyl, propylsilyl, butylsilyl, octylsilyl or decylsilyl. These alkyl, alkenyl, cycloalkyl, aryl, alkyl amino, aralkyl and alkylsilyl groups may each optionally be substituted by one or more substituents, which are inert to the protons bonded to the silicon atoms of the polysilazane skeleton, such as an alkyl, an aryl, an alkoxy and an alkoxycarbonyl. A raw material polysilazane having a number average molecular weight of 100-1,400, preferably 600-1,400, is used. The polysilazane may have a cyclic or linear form. Polysilazanes having both cyclic and linear portions may also be suitably used. Examples of suitable raw material polysilazanes to be used for the purpose of the present invention are those obtained by the methods disclosed in: Japanese Tokkyo Kokai No. 60-145903; U. S. Patent No. 4,397,828 (Seyferth et al); Ber. 54, (1921), p740 (Stock); Inorg. Chem. (1972), 11 (Scantlin et al); U. S. Patent No. 3,318,823 (Aylett); U. S. Patent No. 4,482,669 (Seyferth et al); Japanese Tokkyo Kokai No. 61-89230. The above polysilazane is reacted with a compound of the following general formula (I) in an organic base-containing solvent: wherein A stands for a direct bond or -N(R₄)- where R₄ stands for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or heterocyclic group, and R₃ and R₇, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, with the proviso that at least one of R₃ and R₇ is hydrogen. The alkyl, alkenyl, cycloalkyl, aryl, aralkyl and heterocyclic groups represented by the symbols R₃, R₄ and R₇ may optionally be substituted by one or more substituents, which are inert to the protons on the silicon atoms of the polysilazane skeleton, such as alkyl, aryl, alkoxy and alkoxycarbonyl groups. The compounds expressed by the general formula (I) are selected from : 1. ammonia; 2. hydrazine; 3. primary amines of the formula R₃NH₂; 4. 1-mono-substituted hydrazines of the formula R₃NH-NH₂; 5. 1,2-di-substituted hydrazines of the formula R₃NH-NHR₄; and 6. 1,1-di-substituted hydrazines of the formula R₃R₇N-NH₂. Illustrative of suitable primary amines are methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, amylamine, hexylamine, heptylamine, octylamine, allylamine, crotylamine, cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, 2-methylcyclohexylamine, 2-amino-1-cyclopentylpropane, aniline, toluidine, benzylamine and naphthylamine. Illustrative of suitable 1-mono-substituted hydrazines are methylhydrazine, ethylhydrazine, isopropylhydrazine, propylhydrazine, phenylhydrazine, benzylhydrazine, 2-methylphenylhydrazine, 3-methylphenylhydrazine, 4-methylphenylhydrazine, 4-ethylphenylhydrazine, 1-phenylethylhydrazine, 2-phenylethylhydrazine, 1-naphthylhydrazine, 2-naphthylhydrazine, 2-hydrzinobiphenyl, 3-hydrazinobiphenyl, 4-hydrazinobiphenyl, 1-hydrazinophthalazine, 2-hydrazinoquinoline, 3-hydrazinoquinoline, 4-hydrazinoquinoline and 8-hydrazinoquinoline. Illustrative of suitable 1,2-di-substituted hydrazines are 1,2-dimethylhydrazine, 1,2-diethylhydrazine, 1-isopropyl-2-methylhydrazine, 1-methyl-2-phenylhydrazine, 1-ethyl-2-phenylhydrazine, 1,2-dibenzylhydrazine, 2,2'-hydrazobiphenyl, 4,4'-hydrazobiphenyl, 2,2'-diemthylhydrazobenzene, 2,4-dimethylhydrazobenzene, 3,3'-dimethylhydrazobenzene, 3,4'-dimethylhydrazobenzene, 3,5-dimethylhydrazobenzene, 4,4'-dimethylhydrazobenzene, 2-aminohydrazobenzene, 3-aminohydrazobenene, 4-aminohydrazobenzene, 1,2-di(1-naphtyl)hydrazine, 1,2-di(2-naphthyl)hydrazine, 2,2',3,3'-tetramethylhydrazobenzene, 2,2',4,4'-tetramethylhydrazobenzene, 2,2',5,5'-tetramethylhydrazobenzene, 3,3',4,4'-tetramethylhydrazobenzene and 3,3',5,5'-tetramethylhydrazobenzene. Illustrative of suitable 1,1-di-substituted hydrazines are 1,1-dimethylhydrazine, 1,1-diethylhydrazine, 1-methyl-1-phenylhydrazine, 1-ethyl-1-phenylhydrazlne, 1,1-diphenylhydrazine, 1,1-dibenzylhydrazine, 1,1-di(1-naphthyl)hydrazine and 1,1-di(2-naphthyl)hydrazine. In carrying out the polycondensation of the polysilazane with the compound (I), these materials are dissolved in an organic base-containing solvent and the solution is subjected to dehydrogenative polycondensation conditions. The organic base-containing solvent may be either a liquid organic base or a non-basic organic solvent having dissolved therein an organic base. Any liquid organic base which does not react with the raw material polysilazane may be used as the organic base-containing solvent. Illustrative of suitable liquid organic bases are tertiary amines such as trimethylamine, dimethylamine, dimethylethylamine, diethylmethylamine, triethylamine, pyridine and a substituted pyridine, dimethylaniline and a substituted dimethylaniline, pyrazine and a substituted pyrazine, pyrimidine and a subsituted pyrimidine, pyridazine and a substituted pyridazine, pyrrole, 3-pyrroline, pyrazole, 2-pyrazoline and mixtures thereof. The organic base to be dissolved in the non-basic organic solvent may be tertiary amines such as described immediately above, secondary amines having bulky groups and phosphines. The non-basic organic solvent may be, for example, aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, aliphatic ethers and alicyclic ethers. Illustrative of suitable non-basic organic solvents are methylene chloride, chloroform, carbon tetrachloride, bromoform, ethylene chloride, ethylidene chloride, trichloroethane, tetrachloroethane, ethyl ether, isopropyl ether, ethyl butyl ether, butyl ether, 1,2-dioxyethane, dioxane, dimethyl dioxane, tetrahydrofuran, tetrahydropyrane, pentane, hexane, isohexane, methylpentane, heptane, isoheptane, octane, isooctane, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, benzene, toluene, xylene, ethylbenzene and mixtures thereof. The amount of the organic base is at least 5 parts by weight, preferably at least 20 % by weight per 100 parts by weight of the non-basic organic solvent. The solution of the raw material polysilazane in the organic base-containing solvent generally has a concentration of 0.1-50 % by weight. Too high a concentration of the polysilazane in excess of 50 % by weight causes a difficulty in controlling the reaction. On the other hand, too low a concentration below 0.1 % by weight is undesirable because the reaction proceeds too slowly. The concentration of the raw material polysilazane is preferably 1-12 % by weight. The compound of the formula (I) may be used generally in an amount of 0.001-2000 mols, preferably 0.1-1000 mols per mole of the raw material polysilazane when the compound (I) is ammonia or hydrazine and in an amount of 0.01-5000 mols, preferably 0.5-1000 mols per mole of the polysilazane when the compound (I) is a primary or secondary amine or a substituted hydrazine. The reaction is performed at a temperature of generally -78 to 300 °C, preferably -48 to 200 °C. A temperature below -78 °C is insufficient to effect the polycondensation reaction while too high a temperature in excess of 300 °C causes difficulties in homogeneously proceeding the reaction. Preferably the reaction is carried out in an atmosphere of dry nitrogen, dry argon or the like inert atmosphere. When the reaction is performed in a closed reactor such as autoclave, the reaction pressure increases as the reaction proceeds because of the in situ production of hydrogen gas. It is not necessary, however, to carry out the polycondensation in a pressurized condition. The reaction may be performed under ambient pressure. The reaction time varies with the kinds and concentrations of the raw material polysilazane and the organic base, the reaction temperature adopted and the intended properties of the polycondensed polysilazane product but generaly in the range of about 0.5-20 hours. The optimum reaction conditions vary with the average molecular weight and molecular weight distribution of the raw material polysilazane. More severe conditions are generally adopted as the molecular weight of the raw material polysilazane becomes low. The reaction mixture after the completion of the polycondensation reaction is generally a solution containing the polycondensed polysilazane and the organic base-containing solvent. It is desirable to reduce the concentration of the organic base in the reaction product, since otherwise a gellation of the reaction mixture will result when it is allowed to stand for a long time at room temperature. The reduction of the concentration of the organic base may be effected by removing at least a portion of the organic base by distillation and substituting therefor a suitable amount of a non-basic organic solvent such as a hydrocarbon, a halogenated hydrocarbon or an ether. The non-basic organic solvents exemplified previously are suitably used. Such a replacement operation may be repeated twice or more, if desired, to obtain a stable solution of the reformed polysilazane. In particular, it is preferred that the concentration of the organic base in the product polysilazane solution be 30 % or less, more preferably 5 % or less based on the total weight of the organic base and the non-basic organic solvent contained in the solution. When the compound (I) is ammonia, hydrazine a primary amine, a mono-or di-substituted hydrazine, the polycondensation reaction involves dehydrogenative cross-linking reactions with the compound (I) serving as a cross-linking agent. Additionally, when the group R₂ of the raw material polysilazane is hydrogen, direct cross-linking between the polysilazane can occur. These reactions may be schematically shown as follows: (A) When both R₁ and R₂ are organic groups: (B) When R₁ is hydrogen and R₂ is an organic group: (C) When R₁ is an organic group and R₂ is hydrogen: (D) When both R₁ and R₂ are hydrogen: The polysilazane according to the present invention is a polymer obtained by the polycondensation reaction of the raw material polysilazane and the compound of the formula (I) and has newly introduced bonding of -A-NR₃- which cross-links the polysilazane molecules to each other, as shown above. Thus, the polycondensed polysilazane has a larger molecular weight than the raw material polysilazane. The polycondensed polysilazane has a molecular weight of 1,500-5000,000, preferably 1,500-10,000. In the case of polycondensed polysilazanes obtained using ammonia as compound (I), the molecular weight thereof is preferably more than 2,000. In addition, the polycondensed polysilazane has a higher content of nitrogen atoms than the raw material polysilazane. For instance, the atomic ratios N/Si of perhydropolysilazane, methylhydrosilazane, N-methylsilazane, N-(triethylsilyl)allylsilazane, N-(dimethylamino)cyclohexylsilazane and phenylpolysilazane are increased upon being converted into the polycondensed polysilazanes, typically from 0.60-0.75 to 0.80 or more, from 0.90-0.97 to 0.98 or more, from 0.67-1.50 to 1.6 or more, from 0.55-0.70 to 0.87 or more, from 1.1-2.0 to 2.2 or more, and from 0.85-0.96 to 0.98 or more, respectively. The N/Si ratio of the polycondensed polysilazane may be increased to any extent as long as the solubility of the polycondensed polysilazane in an organic solvent such as o-xylene is not adversely affected. Generally, the N/Si atomic ratio of the polycondensed polysilazane is not greater than 2.5, preferably not greater than 2.0. The polycondensed polysilazane is also distinct from the raw material polysilazane in an increased number of branched side chains. For this reason, even when the polycondensation results in the formation of a polycondensed polysilazane which becomes solidified upon removal of the solvent, this solid is soluble in an organic solvent such as o-xylene, notwithstanding its high molecular weight. The branched chains are considered to result from the following reaction: When the group R₁ of the raw material polysilazane is hydrogen, the formation of the branched chains may be confirmed by comparing proton NMR spectra of the polycondensed polysilazane and the raw material polysilazane. Namely, the integration (S 2H ) of the peak at δ4.8 ppm (attributed to the SiH₂ group) and that (S 3H ) of the peak at δ4.4 ppm (attributed to the SiH₃ group) gives a SiH₂/SiH₃ molar ratio (= 3S 2H /2S 3H ). Typically, while the SiH₂/SiH₃ molar ratio is 5.0-19.0 in the case of the raw material polysilazane, the polycondensed product gives a reduced SiH₂/SiH₃ molar ratio of 2.5-8.4. Since the polycondensed polysilazane of the present invention is soluble in various organic solvents and since it is able to be converted into silicon nitride or silicon nitride-containing ceramics upon calcination, shaped ceramic bodies such as continuous fibers, films and coatings which have good properties such as mechanical strengths at elevated temperatures, heat resistance, corrosion resistance, oxidation resistance and resistance to thermal shock may be obtained from the reformed polysilazane. The high ceramic yield of the polycondensed polysilazane also permits the use thereof as binders and impregnating materials. Moreover, the polycondensed polysilazane, which is free of undesirable impurities such as metal catalysts, catalysts causing decomposition of the polysilazane, or the like impurities, is stable and easy to handle, withstands a long term storage, and gives ceramics of a high purity with a high ceramic yield. Additionally, because of the high molecular weight and the increased crosslinkage of the polycondensed polysilazane, the solidifiability, moldability and the spinnability are improved. Further, the polycondensed polysilazane can be easily obtained by the method of the present invention. The following examples will further illustrate the present invention. Example 1 To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 490 ml of dry pyridine (deaerated), the flask was cooled in an ice bath. Then 51.6 g of dichlorosilane were added into the flask to form a white precipitate of adduct (SiH₂Cl₂·2C₅H₅N). Subsequently, with stirring and cooling in the ice bath, 51.0 g of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube was bubbled through the reaction mixture within the flask. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry pyridine, followed by filtration in a nitrogen atmosphere to give 850 ml of a filtrate containing perhydropolysilazane. When the solvent was removed from the filtrate (5 ml) by evaporation in vacuo, 0.102 g of resinous solid was obtained. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 980. The IR spectrum of the polysilazane (solvent: dry o-xylene, concentration: 10.2 g/liter) showed absorptions at 3350 cm⁻¹ (absorbance index ε= 0.557 liter·g⁻ ¹·cm⁻¹) and 1175 cm⁻¹ (based on NH), at 2170 cm⁻¹ (ε= 3.14, based on SiH) and at 1020-820 cm⁻¹ (based on SiH and SiNSi). The 60 MHz proton NMR spectrum (solvent: CDCl₃, reference: TMS) showed broad signals at 4.8 (SiH₂) and 4.4 (SiH₃) and at 1.5 (NH). This polysilazane will be hereinafter referred to as polysilazane (I). Polysilazane (I) was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.04 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 2.8 g (0.165 mol) of purified anhydrous ammonia were added. The mixture was then reacted at 100 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 1.2 kg/cm² as a result of the reaction. After cooling to room temperature, dry o-xylene (200 ml) was mixed with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 5.22 g of white powder. This powder (polycondensed product) was found to be soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that the polycondensed product had a number-average molecular weight of 3810. The IR spectrum of the polycondensed product (solvent: o-xylene) showed absorptions at 3350 cm⁻¹ and 1175 cm⁻¹ (based on NH), at 2170 cm⁻¹ (based on SiH) and at 1020-820 cm⁻¹ (based on SiH and SiNSi). The proton NMR spectrum (solvent: CDCl₃, reference: TMS) showed broad signals at δ4.8 (SiH₂), at δ4.4 (SiH₃) and at δ1.5 (NH). The integrated ratio of (SiH₂)/(SiH₃) was 4.1. Example 2 Polysilazane (I) obtained in Example 1 was dissolved in pyridine to give 80 ml of a pyridine solution containing 10.3 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 4.1 g of purified anhydrous ammonia were added. The mixture was then reacted at 50 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 0.8 kg/cm² as a result of the reaction. The gas product was identified as being hdyrogen upon gas chromatographic analysis. After cooling to room temperature, dry o-xylene (200 ml) was mixed with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 8.1 g of white powder which was soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 2290. The elementary analysis of the polycondensed product gave (% by weight): Si: 59.9, N: 28.0, O: 1.42, C: 3.55, H:5.21 The proton NMR spectrum revealed that the integrated ratio (SiH₂)/(SiH₃) was 4.3. The polycondensed polysilazane was so stable that no changes were observed with respect to the IR spectrum, proton NMR spectrum and number average molecular weight after storage for 1 months at room temperature in a nitrogen atmosphere. Example 3 The polycondensed polysilazane obtained in Example 1 was heated to 1000 °C in the atmosphere of nitrogen at a heat-up rate of 10 °C/min to obtain a calcined product which was brown ceramic powder. The yield was 86.0 % by weight. The powder X-ray diffraction analysis of this ceramic powder showed that this powder was amorphous. The powder was then heated for calcination to 1500 °C at a heating rate of 3 °C/min to give light brown solids. The powder X-ray diffraction analysis of this ceramic powder gave the following diffraction lines: 2ϑ (°) Crystal face Substance 20.5 (101) α-Si₃N₄ 22.9 (110) 26.4 (200) 30.9 (201) 31.7 (002) 34.5 (102) 35.2 (210) 38.8 (211) 39.4 (112) 40.1 (300) 41.8 (202) 43.4 (301) 46.9 (220) 48.2 (212) 48.8 (310) 23.3 (110) β-Si₃N₄ 26.9 (200) 33.6 (101) 36.0 (210) 41.4 (201) 49.9 (310) 28.4 (111) Si 47.3 (220) From the X-ray diffraction pattern, the calcined product is identified as being crystalline silicon nitride. The elementary analysis of the calcined product gave (% by weight): Si: 58.1, N: 36.7, O: 1.42, C: 1.45 Example 4 Polysilazane (I) obtained in Example 1 was dissolved in a 50:50 (wt/wt) pyridine/o-xylene mixed solvent to give 100 ml of a solution containing 5.64 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 3.0 g of purified anhydrous ammonia were added. The mixture was then reacted at 120 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced. After cooling to room temperature, o-xylene was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 5.2 g of white powder soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 4080. Examples 5-7 Example 1 was repeated in the same manner as described except that the pyridine into which polysilazane (I) was dissolved was replaced with the solvent shown in Table 1 below, that polysilazane (I) was used in the concentration shown in Table 1 below and that the amount of ammonia was changed as shown in Table 1. The resultant polycondensed polysilazane had the properties shown in Table 1. Example 5 Example 6 Example 7 Solvent 4-picoline trimethylamine pyrrol Concentration of polysilazane (I), wt % 5.14 10.9 6.26 Amount of ammonia (molar ratio to polysilazane (I)) 0.933 0.484 0.375 Number average molecular weight 3610 1350 1690 Form of polycondensed polysilazane white powder rubber-like solid rubber-like solid Example 8 To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 500 ml of dry dichloromethane, the flask was cooled in an ice bath. Then 48.6 g of dichlorosilane were added into the flask. Subsequently, with stirring and cooling in the ice bath, 42.5 g of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube was, as a mixture with dry nitrogen gas, bubbled through the reaction mixture within the flask. During the course of the reaction, it was necessary to occasionally beat the gas passage downstream of the flask in order to remove a powdery substance which deposited on and accumulated in the inside surface thereof and to prevent the clogging thereof. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry pyridine. After filtration, the solvent was removed from the filtrate by evaporation in vacuo to leave 9.6 g of viscous oily polysilazane. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 640. This polysilazane will be hereinafter referred to as polysilazane (II). Polysilazane (II) thus obtained was dissolved in pyridine to give 90 ml of a pyridine solution containing 3.87 % by weight of polysilazane (II). The solution was charged in an autoclave with a 300 ml inside volume, to which 2.0 g of purified anhydrous ammonia were added. The mixture was then reacted at 110 °C under autogeneous pressure for 5 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 1.2 kg/cm² as a result of the reaction. After cooling to room temperature, o-xylene was mixed with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 3.7 g of white powder soluble in toluene, tetrahydrofuran and chloroform. The gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 3270. The proton NMR spectrum showed that the integrated ratio (SiH₂)/(SiH₃) of 5.8. Example 9 Polysilazane (I) obtained in Example 1 was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.16 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 1.5 ml of anhydrous hydrazine was added while cooling the autoclave in an ice bath. A gas was generated as soon as the hydrazine was added. The mixture was reacted at room temperature for 20 hours with stirring. As a result, the pressure within the autoclave was increased by 0.8 kg/cm². After cooling to room temperature, dry o-xylene (200 ml) was mixed with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give white powder. The gel permeation chromatography revealed that the polycondensed product had a number-average molecular weight of 5690. Example 10 To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 450 ml of deaerated dry tetrahydrofuran, the flask was cooled in a dry ice-methanol bath. Then 46.2 g of dichlorosilane were added into the flask. Subsequently, with stirring and cooling, 44.2 g of anhydrous methylamine was, as a mixture with dry nitrogen gas, bubbled through the reaction mixture within the flask. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry tetrahydrofuran, followed by filtration in a nitrogen atmosphere to obtain 820 ml of a filtrate. The solvent was removed from the filtrate by evaporation in vacuo to leave 8.4 g of viscous oily N-methylsilazane. The gel permeation chromatography revealed that the N-methylsilazane had a number-average molecular weight of 1100. This N-methylsilazane will be hereinafter referred to as polysilazane (III). Polysilazane (III) thus obtained was dissolved in pyridine to give 100 ml of a pyridine solution containing 4.56 % by weight of polysilazane (III). The solution was charged in an autoclave with a 300 ml inside volume, to which 3.8 g of purified anhydrous ammonia were added. The mixture was then reacted at 120 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced so that the inside pressure was increased by 0.7 kg/cm² as a result of the reaction. After cooling to room temperature, o-xylene was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. Gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1350. Example 11 (Comparative) To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 300 ml of dry dichloromethane and 24.3 g (0.211 mol) of methyldichlorosilane, the flask was cooled in an ice bath. Then, with stirring and cooling in the ice bath, 18.1 g of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube for purification was bubbled through the reaction mixture within the flask. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry dichloromethane, followed by filtration in a nitrogen atmosphere. The solvent was removed from the filtrate by evaporation in vacuo to leave 8.81 g of polymethylsilazane in the form of a colorless, transparent liquid. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 380. This polysilazane will be hereinafter referred to as polysilazane (IV). Polysilazane (IV) thus obtained was dissolved in pyridine to give 35 ml of a pyridine solution containing 4.50 % by weight of polysilazane (IV). The solution was charged in an autoclave with a 300 ml inside volume, to which 1.7 g of purified anhydrous ammonia were added. The mixture was then reacted at 140 °C under autogeneous pressure for 3 hours with stirring. An amount of a gas was found to be produced so that the inside pressure was increased by 0.4 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, viscous liquid. Gel permeation chromatography revealed that this liquid (polycondensed polysilazane) had a number-average molecular weight of 600. Example 12 To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 500 ml of dry benzene, the flask was cooled in a water bath. Then 40.6 g of dichlorosilane were added into the flask. Subsequently, with stirring and cooling in the water bath, 42.0 g of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube for purification was, as a mixture with dry nitrogen gas, bubbled through the reaction mixture within the flask. During the course of the reaction, it was necessary to occasionally beat the gas passage downstream of the flask in order to remove a powdery substance which deposited on and accumulated in the inside surface thereof and to prevent the clogging thereof. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry o-xylene. After filtration, the solvent was removed from the filtrate by evaporation in vacuo to leave 5.2 g of viscous oily perhydropolysilazane. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 320. This polysilazane is referred to as polysilazane (V) hereinafter. Polysilazane (V) thus obtained was dissolved in pyridine to give 100 ml of a pyridine solution containing 6.16 % by weight of polysilazane (V). The solution was charged in an autoclave with a 300 ml inside volume, to which 2.1 g of purified anhydrous ammonia were added. The mixture was then reacted at 100 °C under autogeneous pressure for 8 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 1.1 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give 5.3 g of white powder soluble in toluene, tetrahydrofuran and chloroform. The gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 2470. The proton NMR spectrum showed that the integrated ratio (SiH₂)/(SiH₃) of 6.8. Example 13 The polycondensed polysilazane obtained in Example 2 was dissolved in o-xylene to give a solution containing 78 % by weight of the polycondensed polysilazane. This solution was ejected from a nozzle into a high temperature atmosphere and the resultant spun fibers were successively wound around a roll. Colorless, transparent, continuous fibers of the reformed polysilazane were obtained. Example 14 (Comparative) To a four-necked 1000 ml flask equipped with a gas feed conduit, a dropping funnel, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. Then, 400 ml of dry benzene and 64.5 g of allyldichlorosilane which had been obtained by the conventional method (J. Am. Chem. Soc., Vol. 67, 1813 (1945)) were added into the flask. While stirring the mixture in the flask, a solution of 42.5 g of triethylaminosilane in 50 ml dry benzene was added dropwise from the dropping funnel. The triethylaminosilane used was obtained by the known method (J. Am. Chem. Soc. Vol. 70, 435 (1948)). Thereafter, the reaction mixture was refluxed in an oil bath with stirring to complete the reaction. The reaction mixture was centrifuged and the supernatant was washed with dry benzene. After filtration in a nitrogen atmosphere, the solvent was removed from the filtrate by evaporation in vacuo to leave 19.2 g of N-(triethylsilyl)allylsilazane in the form of liquid. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 360. This polysilazane is referred to as polysilazane (VI) hereinafter. Polysilazane (VI) thus obtained was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.64 % by weight of polysilazane (VI). The solution was charged in an autoclave with a 300 ml inside volume, to which 0.8 g of purified anhydrous ammonia were added. The mixture was then reacted at 100 °C under autogeneous pressure for 5 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.9 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. The gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 940. Example 15 (Comparative) To 110g of trichlorosilane, 62.0 g of Grignard reagent obtained from cyclohexyl bromide were gradually added. The distillation in vacuo of the mixture gave 16.4 g of cyclohexyldichlorosilane. To a four-necked 1000 ml flask equipped with a gas feed conduit, a dropping funnel, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. Then, 420 ml of dry benzene and 12.5 g of allyldichlorosilane obtained above were added into the flask. While stirring the mixture in the flask, a solution of 15.6 g of 1,1-dimethylhydrazine in 40 ml of dry benzene was added dropwise from the dropping funnel. Thereafter, the reaction mixture was stirred at room temperature to complete the reaction. The reaction mixture was centrifuged and the supernatant was washed with dry benzene. After filtration in a nitrogen atmosphere, the solvent was removed from the filtrate (730 ml) by evaporation in vacuo to leave 3.2 g of N-(dimethylamino)cyclohexylsilazane in the form of an oil. Gel permeation chromatography of the polysilazane revealed that the polysilazane had a number-average molecular weight of 390. This polysilazane is referred to as polysilazane (VII) hereinafter. Polysilazane (VII) thus obtained was dissolved in pyridine to give 100 ml of a pyridine solution containing 4.12 % by weight of polysilazane (VII). The solution was charged in an autoclave with a 300 ml inside volume, to which 0.6 g of purified anhydrous ammonia was added. The mixture was then reacted at 80 °C under autogeneous pressure for 6 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.8 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. The gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1080. Example 16 The polycondensed polysilazane obtained in Example 2 was heated to 1450 °C in the atmosphere of a 50:50 (vol/vol) nitrogen/ammonia mixed gas at a heating rate of 10 °C/min and calcined at that temperature for 3 hours to obtain a calcined product in the form of light brown solids with a yield of 87.2 % by weight. Upon the powder X-ray diffraction analysis, the calcined product is identified as being crystalline silicon nitride of α-Si₃N₄ and β-Si₃N₄. Example 17 The polycondensed polysilazane obtained in Example 4 was heated to 1500 °C in the atmosphere of hydrogen at a heating rate of 5 °C/min and calcined at that temperature for 5 hours to obtain a calcined product in the form of light brown solids with a yield of 85.4 % by weight. Upon the powder X-ray diffraction analysis, the calcined product is identified as being crystalline silicon nitride of α-Si₃N₄ and β-Si₃N₄. Example 18 The polycondensed polysilazane obtained in Example 10 was heated to 1600 °C in vacuum (3-4 mmHg) at a heating rate of 3 °C/min and calcined at that temperature for 6 hours to obtain a calcined product in the form of dark brown solids with a yield of 77.3 % by weight. Upon the powder X-ray diffraction analysis, the calcined product is identified as being α-Si₃N₄, β-Si₃N₄ and SiC (8F type). Example 19 (Comparative) To a four-necked 1000 ml flask equipped with a gas feed conduit, a mechanical stirrer and a Dewar condenser, an oxygen-free, dry nitrogen gas was fed to replace the inside of the flask therewith. After charging 500 ml of dry toluene , the flask was cooled in an ice bath. Then 52.1 g of phenyldichlorosilane were added into the flask. Subsequently, with stirring and cooling in the ice bath, 30.0 g of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube was bubbled through the reaction mixture within the flask. After completion of the reaction, the reaction mixture was centrifuged and the supernatant was washed with dry pyridine, followed by filtration in a nitrogen atmosphere. The solvent was removed from the filtrate by evaporation in vacuo to leave 6.8 g of oily phenylpolysilazane. Gel permeation chromatography revealed that the polysilazane had a number-average molecular weight of 380. This polysilazane will be hereinafter referred to as polysilazane (VIII). Polysilazane (VIII) was dissolved in pyridine to give 100 ml of a pyridine solution containing 6.04 % by weight of polysilazane (VIII). The solution was charged in an autoclave with a 300 ml inside volume, to which 0.6 g of purified anhydrous ammonia was added. The mixture was then reacted at 120 °C under autogeneous pressure for 6 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 0.5 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give colorless transparent, rubber-like solids. These solids (polycondensed product) was found to have a number-average molecular weight of 1090 upon analysis by gel permeation chromatography. Example 20 Polysilazane (I) obtained in Example 1 was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.26 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 2 ml (20.2 mmol) of dehydrated n-butylamine used as a cross-linking agent were added. The mixture was then reacted at 120 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 2.0 kg/cm² as a result of the reaction. After cooling to room temperature, dry o-xylene (200 ml) was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 5.1 g of white powder. This powder (polycondensed product) was found to be soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that the polycondensed product had a number-average molecular weight of 3820. The IR spectrum of the reformed product (solvent: toluene) showed absorptions at 3350 cm⁻¹ and 1175 cm⁻¹ (based on NH), at 2170 cm⁻¹ (based on SiH), at 1020-820 cm⁻¹ (based on SiH and SiNSi) at 2690, 2930, 2880, 1460 and 1380 cm⁻¹ (based on CH) and at 1090 cm⁻¹ (based on CN). The proton NMR spectrum (solvent: CDCl₃, reference: TMS) showed broad signals at δ4.8 (SiH₂) at δ4.4 (SiH₃), at δ2.7 (CH₂), at δ1.4 (NH) and at 0.9 (CH₃). The elementary analysis of the reformed product gave (% by weight): Si: 57.4, N: 26.8, O: 3.63, C: 9.30 Example 21 Polysilazane (I) obtained in Example 1 was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.12 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 15 ml of aniline purified by distillation in vacuo were added. The mixture was then reacted at 120 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 1.7 kg/cm² as a result of the reaction. The gas product was identified as being hdyrogen upon gas chromatographic analysis. After cooling to room temperature, dry o-xylene (200 ml) was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 4.9 g of white powder which was soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 1720. The IR spectrum of the polycondensed product (solvent: o-xylene) showed absorptions at 3350 cm⁻¹ and 1175 cm⁻ ¹ (based on NH), at 2170 cm⁻¹ (based on SiH), at 1020-820 cm⁻¹ (based on SiH and SiNSi) at 3050, 755 and 695 cm⁻¹ (based on CH), at 1610 and 1495 cm⁻¹ (based on CC) and at 1290 cm⁻¹ (based on CN). The proton NMR spectrum (solvent: CDCl₃, reference: TMS) showed broad signals at δ7.2 (CH), at δ6.7 (CH), at δ4.8 (SiH₂) at δ4.4 (SiH₃) and at δ1.5 (NH). Example 22 Polysilazane (I) obtained in Example 1 was dissolved in pyridine to give 50 ml of a solution containing 5.26 % by weight of polysilazane (I). The solution was charged in an autoclave with a 300 ml inside volume, to which 15 ml of methylhydrazine were added. The mixture was then reacted at 25 °C under autogeneous pressure for 1 hour with stirring. A large amount of a gas was found to be produced so that the inside pressure was increased by 1.2 kg/cm². After cooling to room temperature, o-xylene was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 2.4 g of white powder which was soluble in toluene, tetrahydrofuran and chloroform. Gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 2250. The IR spectrum of the polycondensed product (solvent: o-xylene) showed absorptions at 3350 cm⁻¹ and 1175 cm⁻¹ (based on NH), at 2170 cm⁻¹ (based on SiH), at 1020-820 cm⁻¹ (based on SiH and SiNSi) and at 2950, 2850, 2800 and 1440 cm⁻¹ (based on CH). The proton NMR spectrum (solvent: CDCl₃, reference: TMS) showed broad signals at δ4.8 (SiH₂), at δ4.4 (SiH₃), at δ3.3 (CH₃) and at δ1.5 (NH). Examples 23-26 Example 20 was repeated in the same manner as described except that polysilazane (I) was used in the concentration shown in Table 2 below, that the reaction temperature was changed as shown in Table 2 and that the amount and the kind of the cross-linking agent was changed as shown in Table 2. The resultant polycondensed polysilazane had the properties shown in Table 2. Example 23 Example 24 Example 25 Example 26 Cross-linking agent DMH MPH CHA allylamine Amount of cross-linking agent 10 20 2 5 Concentration of polysilazane (I), wt % 5.12 2.56 6.72 8.84 Reaction temperature, °C 80 80 100 80 Number average molecular weight 2150 2270 2870 2220 Form of polycondensed polysilazane white powder white powder white powder white powder Example 27 Polysilazane (II) obtained in Example 8 was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.83 % by weight of polysilazane (II). The solution was charged in an autoclave with a 300 ml inside volume, to which 5.5 g of p-toluidine were added. The mixture was then reacted at 120 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.8 kg/cm² as a result of the reaction. After cooling to room temperature, o-xylene was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give 5.5 g of white powder soluble in toluene, tetrahydrofuran and chloroform. The gel permeation chromatography revealed that this powder (polycondensed polysilazane) had a number-average molecular weight of 2480. Example 28 Polysilazane (III) obtained in Example 10 was dissolved in pyridine to give 100 ml of a pyridine solution containing 9.56 % by weight of polysilazane (III). The solution was charged in an autoclave with a 300 ml inside volume, to which 10 ml of benzylhydrazine were added. The mixture was then reacted at 80 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced so that the inside pressure was increased by 0.7 kg/cm² as a result of the reaction. After cooling to room temperature, o-xylene was mixted with the reaction mixture. The solvent was then removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. Gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1890. Example 29 Polysilazane (IV) obtained in Example 11 was dissolved in γ-picoline to give 80 ml of a solution containing 10.4 % by weight of polysilazane (III). The solution was charged in an autoclave with a 300 ml inside volume, to which 30 ml of aniline were added. The mixture was then reacted at 150 °C under autogeneous pressure for 6 hours with stirring. An amount of a gas was found to be produced so that the inside pressure was increased by 0.5 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. Gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1680. Example 30 Polysilazane (V) obtained in Example 11 was dissolved in pyridine to give 100 ml of a pyridine solution containing 8.53 % by weight of polysilazane (V). The solution was charged in an autoclave with a 300 ml inside volume, to which 15 ml of phenylhydrazine were added. The mixture was then reacted at 80 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.6 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give yellowish brown solids. The gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 2770. Example 31 Polysilazane (VI) obtained in Example 14 was dissolved in pyridine to give 100 ml of a pyridine solution containing 7.27 % by weight of polysilazane (VI). The solution was charged in an autoclave with a 300 ml inside volume, to which 20 ml of isopropylamine were added. The mixture was then reacted at 100 °C under autogeneous pressure for 5 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.5 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. The gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1650. Example 32 Polysilazane (VII) obtained in Example 15 was dissolved in pyridine to give 50 ml of a pyridine solution containing 5.53 % by weight of polysilazane (VII). The solution was charged in an autoclave with a 300 ml inside volume, to which 15 ml of 1,2-dimethylhydrazine were added. The mixture was then reacted at 120 °C under autogeneous pressure for 6 hours with stirring. A large amount of a gas was found to be produced so that the pressure within the autoclave was increased by 0.3 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by distillation at 50-70 °C under 3-5 mmHg to give light yellow, rubber-like solids. The gel permeation chromatography revealed that these solids (polycondensed polysilazane) had a number-average molecular weight of 1530. Example 33 Polysilazane (VIII) obtained in Example 19 was dissolved in pyridine to give 100 ml of a pyridine solution containing 5.15 % by weight of polysilazane (VIII). The solution was charged in an autoclave with a 300 ml inside volume, to which 10 ml of 1-isopropyl-2-methylhydrazine were added. The mixture was then reacted at 100 °C under autogeneous pressure for 3 hours with stirring. A large amount of a gas was found to be produced and the pressure within the autoclave was increased by 1.0 kg/cm² as a result of the reaction. After cooling to room temperature, the solvent was removed by `distillation at 50-70 °C under 3-5 mmHg to give white powder. This powder (polycondensed product) was found to have a number-average molecular weight of 2210 upon analysis by gel permeation chromatography. Example 34 The polycondensed polysilazane (number-average molecular weight: 3810) obtained in Example 1 was dissolved in o-xylene, to which silicon carbide (average particle size: about 10 µ) was mixed, thereby to obtain a dispersion containing 30 % by weight of reformed polysilazane, 50 % by weight of silicon carbide and 2o % by weight of o-xylene. The dispersion was coated on a SUS 304 stainless steel plate by spray coating. The coated substrate was then heated to 400 °C at a heating rate of 3 °C/min in an oven under a nitrogen atmosphere for drying. The dried coat had a thickness of about 160 µm and a pencil hardness of 9H or more. The coat exhibited excellent adhesion to the substrate and was free of cracks. Example 35 A dispersion containing 25 % by weight of the polycondensed polysilazane, 69.5 % by weight of silicon carbide, 0.5 % by weight of hexamethyldisilazane (adjuvant) and 5 % by weight of o-xylene was prepared in the same manner as that in Example 34. The dispersion was applied over the surface of a SUS 304 stainless steel plate by brush coating. The coat was dried and calcined at 1000 °C for 1 hour in the nitrogen atmosphere. The coated layer thus obtained had a pencil hardness of 9H or more and exhibited excellent adhesion to the substrate. No cracks were observed. Example 36 A dispersion containing 50 % by weight of the polycondensed polysilazane, 30 % by weight of silicon carbide and 20 % by weight of o-xylene was prepared in the same manner as that in Example 34. The dispersion was applied over the surface of a SUS 304 stainless steel plate by immersion coating. The coat was dried and calcined at 200 °C for 1 hour in air. The coated layer thus obtained had a pencil hardness of 7H and exhibited excellent adhesion to the substrate. No cracks were observed.

Claims: A polycondensed polysilazane having a number average molecular weight of 1,500-500,000, which (a) is derived from a starting polysilazane having a number average molecular weight of 100-1,400 and containing recurring units of the formula: wherein R₁ and R₂, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, (b) is free of NH₂ groups and (c) contains one or more skeletal groups of the formulae (I), (II), (III) or (IV); wherein formula (I) is: wherein R₁ and R₂ are, independently from each other, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, A stands for a direct bond or -N(R₄)- where R₄ stands for hydrogen, alkyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, and R₃ stands for (i) hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or -N(R₅)(R₆) where R₅ and R₆, independently from each other, stand for alkyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, when the group A is a direct bond or (ii) hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, when the group A is -N(R₄)-; wherein formula (II) is: wherein R₂ is alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, and A and R₃ are as defined in relation to formula (I); wherein formula (III) is: wherein R₁ is alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, and A and R₃ are as defined in relation to formula I; and wherein formula (IV) is: wherein A and R₃ are as defined in relation to formula (I), and wherein said number average molecular weights are measured by gel permeation chromatografy. A polysilazane according to Claim 1, wherein the number-average molecular weight is 1,500-10,000. A method of forming a polycondensed polysilazane as defined in Claim 1, comprising reacting a starting polysilazane having a number average molecular weight of 100-1,400 and containing recurring units of the formula: wherein R₁ and R₂, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, alkylamino, aryl or alkylsilyl, in an organic base-containing solvent with a compound of the formula I: wherein A stands for a direct bond or -N(R₄)- where R₄ stands for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, and R₃ and R₇, independently from each other, stand for hydrogen, alkyl, alkenyl, cycloalkyl, aryl, aralkyl or a heterocyclic group, with the proviso that at least one of R₃ and R₇ is hydrogen. A method according to Claim 3, wherein the organic base-containing solvent consists of a liquid organic base. A method according to Claim 4, wherein the liquid organic base is a member selected from the group consisting of trimethylamine, dimethylamine, dimethylethylamine, diethylmethylamine, triethylamine, pyridine and a substituted pyridine, dimethylaniline and a substituted dimethylaniline, pyrazine and a substituted pyrazine, pyrimidine and a substituted pyrimidine, pyridazine and a substituted pyridazine, pyrrole, 3-pyrroline, pyrazole, 2-pyrazoline and mixtures thereof. A method according to Claim 3, wherein the organic base-containing solvent is a non-basic organic solvent having dissolved therein an organic base. A method according to Claim 6, wherein said organic base is a member selected from the group consisting of tertiary amines, secondary amines, phosphines and mixtures thereof and said non-basic organic solvent is a member selected from the group consisting of methylene chloride, chloroform, carbon tetrachloride, bromoform, ethylene chloride, ethylidene chloride, trichloroethane, tetrachloroethane, ethyl ether, isopropyl ether, ethyl butyl ether, butyl ether, 1,2-dioxyethane, dioxane, dimethyl dioxane, tetrahydrofuran, tetrahydropyrane, pentane, hexane, isohexane, methylpentane, heptane, isoheptane, octane, isooctane, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, benzene, toluene, xylene, ethylbenzene and mixtures thereof. A method according to any one of Claims 3-7, wherein said polysilazane solution has a concentration of 0.1-50% by weight. A method according to any one of Claims 3-8, wherein said compound of the formula (I) is used in an amount of 0.001-2000 mols per mole of said polysilazane when the compound of the formula (I) is ammonia or hydrazine and in an amount of 0.01-5000 mols per mole of said polysilazane when said compound of the formula (I) is an amine or a substituted hydrazine. A method according to any one of Claims 3-9, wherein said starting polysilazane has a number-average molecular weight of 600-1,400 and said reaction is performed so that the resulting polysilazane has a number-average molecular weight of 1,500-10,000. A method according to any one of Claims 3-10, wherein said reaction is performed so as to increase the N/Si molar ratio of said polysilazane.

Inventors: ARAI MIKIRO, FUNAYAMA OSAMU, ISODA TAKESHI, ARAI, MIKIRO, FUNAYAMA, OSAMU, ISODA, TAKESHI
Applicants: PETROLEUM ENERGY CENTER FOUND, TOA NENRYO KOGYO KK, PETROLEUM ENERGY CENTER, TOA NENRYO KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: C08G77
IPC codes: C04B35, C08G77