Process for producing a polyborosilazane

Abstract: Disclosed is a polyborosilazane having a B/Si atomic ratio of 0.01 to 3 and a number-average molecular weight of about 200 to about 500,000 which can be pyrolyzed to give ceramics. The polyborosilazane is produced by reacting a polysilazane having a number-average molecular weight of about 100 to about 50,000 with a boron compound.

Description: This invention relates to a process for producing polyborosilazanes. Si-N-B series ceramics produced from a precursor polyborosilazane are useful as reinforcing materials for composite materials to be used at elevated temperatures because of the high heat resistance and high hardness thereof. They are expected to find wide applications in the space-aircraft and automobile industries. Borosiloxanes, organoborosilanes and boron-containing carbosilanes obtained by the reaction between a boron compound such as boric acid (including derivative thereof), a metal borate or a boron halogenide and an organosilicon compound are well known. The reported organosilicon compounds have a low molecular weight, i.e. at most are oligomers (JP-A-57-26608). EP-A-0278734 discloses, inter alia, preceramic borazine polymers, which can be converted to boron nitride ceramics, of the formula: wherein R is C₁-C₄ alkyl and x is a relatively small integer, e.g. about 5. EP-A-0175382 teaches that the ceramic yield of a ceramic material obtained by firing a R₃SiNH-containing silazane polymer (R is hydrogen, vinyl, C₁-C₃ alkyl or phenyl) may be increased by adding a suitable boron compound to the silazane polymer prior to firing. Organoboron compounds of the formula BR''₃ wherein R'' represents e.g. C₁-C₅ alkyl, phenyl etc may be used for this purpose. It is postulated that the boron compound additive interacts with the R₃SiNH-containing silazane during the early stages of the pyrolysis reaction, possibly to form or promote crosslinkages. US-A-4535007 discloses a process for preparing a silicon nitride-containing ceramic material by heating an R₃SiNH-containing silazane polymer (R is as defined in relation to EP-A-0175382) which has been rendered infusible by treatment with certain reactive metal halides, including boron tribromide, BBr₃, possibly through crosslinking reactions. US-A-4482689 teaches R₃SiNH-containing metallosilazane polymers (R is as previously defined in relation to EP-A-0175382) useful as precursors for ceramic materials and obtained by reacting chlorine-containing disilanes and, e.g. boron halides, with a disilazane of the formula (R₃Si)₂NH. The present invention is aimed at the provision of an improved process for making polyborosilazanes from which may be obtained by pyrolysis ceramics having a marked heat resistance. In accordance with the present invention there is provided a process for producing a polyborosilazane having a boron/silicon atomic ratio of 0.01 to 3 and a number-average molecular weight of up to 500,000, which comprises reacting, in the presence of a basic compound selected from trialkylamines, tertiary amines, pyrrole, 3-pyrroline, pyrazole, 2-pyrazolyl, dimethylsulfide and mixtures thereof, a polysilazane having a number-average molecular weight of up to 50,000 and containing a skeletal structure of at least three consecutive units represented by the general formula (I): wherein R¹, R² and R³, independently from each other, represent hydrogen, a hydrocarbyl group, a substituted hydrocarbyl group, an alkylsilyl group, an alkylamino group or an alkoxy group, with the proviso that at least one of R¹, R² and R³ is hydrogen, with a boron compound represented by the following general formula (II), (III), (IV) or (V): B(R⁴)₃ (II) (R⁴BO)₃ (III) B(R⁴)₃ : L (V) wherein R⁴ may be the same or different and represents hydrogen, halogen, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an alkoxy group, an alkylamino group, a hydroxy group or an amino group, and L represents a compound capable of forming a complex with B(R⁴)₃. The present invention will now be described in detail with reference to the accompanying drawings, in which Figs. 1 to 3 are X-ray diffraction patterns of ceramics obtained by pyrolysis of polyborosilazanes made by the process of this invention, and Figs. 4 and 5 are X-ray diffraction patterns of ceramics obtained by pyrolysis of polyzirconosilazane and polytitanosilazane, respectively. The reaction between the polysilazane and the boron compound, and the structure of the polymer obtained by the reaction, vary with the kind of the boron compound which is used. In the case of using, for example, a boron alkoxide as the boron compound, the polyborosilazane obtained has a structure wherein side chains, cyclization and/or crosslinkages are formed by the reaction between hydrogen atoms bound to at least some of the silicon atoms and/or nitrogen atoms of the polysilazane main skeleton and the boron alkoxide to effect condensation between the silicon atom and/or nitrogen atom and the boron alkoxide. For instance, in the reaction between a Si-H bond of a polysilazane and a boron alkoxide [B(OR)₃], the organic group (R⁴) of the boron alkoxide pulls the hydrogen atom off the Si-H bond to form R⁴H and a Si-O-B bond as follows. Again, in the reaction between a N-H bond of a polysilazane and a boron alkoxide, the boron alkoxide pulls the hydrogen atom off the N-H bond to thereby form N-O-B bond or N-B bond (hereinafter these are represented as "N-Y-B" bond) as follows. Since the boron alkoxide can be trifunctional at the most, the resulting polyborosilazane can be obtained by reacting one, two or three of the reactive groups of the boron compound depending upon the kind of starting boron alkoxide or the reaction conditions employed. A polymer obtained by reaction with a single reactive group has the following structure wherein pendant groups are bound to Si and/or N of the main chain of the polysilazane: In a polymer formed by reaction with two or all three reactive groups of the boron alkoxide, a cyclic and/or crosslinked structure is formed in or between the polysilazane skeletons through B atoms as shown below. The cyclic structures include a structure wherein two reactive groups within one molecule of the boron alkoxide are condensed with a silicon atom and a nitrogen atom located adjacent to each other on the polysilazane. A crosslinked structure is formed when two or more reactive groups of the boron alkoxide are condensed with two or more molecules of the polysilazane. The polymers formed by reaction with all three reactive groups of the boron compound may possess both the cyclic structure and the crosslinked structure described above at the same time. The reaction between the polysilazane and a boron alkoxide usually yields a polymer represented by (VI) or (VII). Thus, the structural changes involved when the polysilazane is converted to the polyborosilazane includes the formation of pendant groups, cyclic structures and/or crosslinked structure on the polysilazane fundamental skeletons. In the reaction between a Si-H bond of a polysilazane and a halogen-containing boron compound of the formula B(R⁴)₃, a halogen atom of the boron compound BX m (R⁴) 3-m (wherein X represents a halogen atom, and m = 1, 2 or 3) pulls off the hydrogen atom of an Si-H bond to form HX and a Si-B bond as follows. In the reaction between a N-H bond of a polysilazane and a halogen-containing boron compound, a halogen atom of the boron compound pulls off the hydrogen atom of an N-H bond to form HX and an H-B bond as follows. Since the boron compound BX m (R⁴) 3-m can be trifunctional at the most, depending upon the number of halogen atoms, the resulting polyborosilazane can be obtained by reacting one, two or all three of the reactive groups of the boron compound. A polymer obtained by reaction with a single reactive group has the following structure wherein pendant groups are bound to Si and/or Ni atoms of the main chain of polysilazane: In a polymer formed by reaction with two or all three reactive groups of the boron compound a cyclic and/or crosslinked structure is formed in or between the polysilazane skeletons through B atoms as shown below. The cyclic structures include a structure wherein two reactive groups within one molecule of the boron compound are condensed with a silicon atom and a nitrogen atom located adjacent to each other on the polysilazane. A crosslinked structure is formed when two or more reactive groups of the boron compound are condensed with two or more molecules of polysilazane: A polymer formed by reaction with all three reactive groups of the boron compound may possess both the cyclic structure and the crosslinked structure described above at the same time. In the reaction between a Si-H bond of a polysilazane and a hydrogen-containing boron compound of the formula B(R⁴)₃, a hydrogen atom of the boron compound BH m (R⁴) 3-m pulls off the hydrogen atom of the Si-H bond to form H₂ and an Si-B bond as follows: In the reaction between an N-H bond of a polysilazane and a hydrogen-containing boron compound, a hydrogen atom of the boron compound pulls off the hydrogen atom of N-H bond to form H₂ and a N-B bond as follows: Since the boron compound BH m (R⁴) 3-m can be trifunctional at the most depending upon the number of hydrogen atoms, the resulting polyborosilazane can be obtained by reacting one, two or all three of the reactive groups of the boron compound. Hence, there can be obtained polysilazanes of the same structure as described with respect to the halogen-containing boron compound BX m (R⁴) 3-m . In the reaction between a Si-H bond of a polysilazane and the boron compounds B(R⁴)₃ containing an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group or an alkylamino group, such an organic group of the boron compound B(R⁴)₃ pulls off the hydrogen atom of Si-H bond to form R⁴H and a Si-B bond as follows: However, when one of the two moieties bound to the nitrogen atom of the alkylamino group is a hydrogen atom, a dehydrogenation reaction takes place to form an Si-N-B bond according to the following reaction: In the reaction between a N-H bond of a polysilazane and such a boron compound having an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group or an alkylamino group, such an organic group of the boron compound pulls off the hydrogen atom of the N-H bond to form R⁴H and an N-B bond as follows: Since the boron compound B(R⁴)₃ can be trifunctional at the most, the resulting polyborosilazane has the same structure as shown above with respect to the case of using the boron compounds BX m (R⁴) 3-m or BH m (R⁴) 3-m . In the reaction between an Si-H bond of a polysilazane and a boron compound B(R⁴)₃ containing an amino group or a hydroxyl group, a hydrogen atom in Z of the boron compound BZ m (R⁴) 3-m (where Z is an amino group or a hydroxyl group) pulls off the hydrogen atom of the Si-H bond to form H₂ and a Si-N-B bond or a Si-O-B bond as follows: In the reaction between an N-H bond of a polysilazane and such a boron compound having an amino group or hydroxyl group, a hydrogen atom of the N-H bond is pulled off to form H₂ and a N-N-B bond or a N-O-B bond as follows: Since the boron compound BZ m (R⁴) 3-m can be trifunctional at the most depending upon the number of amino groups or hydroxyl groups, the resulting polyborosilazane can be obtained by reacting one, two or all three of the reactive groups of the boron compound. A polymer obtained by reaction with a single such reactive group has a structure wherein pendant groups are bound to Si and/or N atoms of the main chain of polysilazane as shown below: In a polymer formed by reaction with two or three reactive groups of the boron compound, a cyclic and/or crosslinked structure is formed in or between the polysilazane skeletons through B atoms as shown below. The cyclic structures include a structure wherein two reactive groups within one molecule of the boron compound are condensed with a silicon atom and a nitrogen atom adjacent to each other on the polysilazane. A crosslinked structure is formed when two or more reactive groups of the boron compound are condensed with two or more molecules of the polysilazane. The polymers obtained by reaction of all three reactive groups of the boron compound may possess both the cyclic structure and the crosslinked structure described above at the same time. In the reaction between an Si-H bond of a polysilazane and a boron compound (R⁴OB)₃, R⁴ of the boron compound (R⁴OB)₃ pulls the hydrogen atom off the Si-H bond to form R⁴H and a Si-B bond as follows. In the reaction between an N-H bond of a polysilazane and a boron compound (R⁴OB)₃, R⁴ of the boron compound (R⁴OB)₃ pulls the hydrogen atom off the N-H bond to form R⁴H and a N-B bond as follows. Since the boron compound (R⁴OB)₃ can be trifunctional at the most, the resulting polyborosilazane can be obtained by reaction of one, two or three of the reactive groups of the boron compound, depending upon the kind of the starting boron compound or upon the reaction conditions employed. A polymer obtained by reaction with a single reactive group has a structure wherein pendant groups are bound to Si and/or N of the main chain of polysilazane as shown below: In a polymer formed by reaction with two or all three reactive groups of the boron compound, a cyclic and/or crosslinked structure is formed in or between the polysilazane skeletons through B atoms as shown below. The cyclic structures include a structure wherein two reactive groups within one molecule of the boron compound are condensed with a silicon atom and/or a nitrogen atom of a single molecule of the polysilazane. A crosslinked structure is formed when two or more reactive groups of the boron compound are condensed with two or more molecules of polysilazane: The polymers obtained by reaction with all three reactive groups of the boron compound may possess both the cyclic structure and the crosslinked structure described above at the same time. In the reaction between a Si-H bond of a polysilazane and a boron compound R⁴ of the boron compound pulls off the hydrogen atom of the Si-H bond to form R⁴H and a Si-B bond or a SiN bond as follows: In the reaction between a N-H bond of a polysilazane and a boron compound R⁴ of the boron compound pulls off the hydrogen atom of the N-H bond to form R⁴H and a N-B bond or a N-N bond as follows. Since the boron compound can be hexafunctional at the most, the resulting polyborosilazane can be obtained by reacting from one up to all six of the reactive groups of the boron compound. A polymer obtained by reaction with a single such reactive group has a structure wherein pendant groups are bound to Si and/or N of the main chain of polysilazane as shown below: In a polymer obtained by reacting from two up to six of the reactive groups, a cyclic and/or crosslinked structure is formed in or between the polysilazane skeletons through B atoms as shown below. The cyclic structures include a structure wherein two functional groups within one molecule of the boron compound are respectively condensed with a silicon atom and/or a nitrogen atom of a single molecule of the polysilazane. A crosslinked structure is formed when two or more functional groups of the boron compound are condensed with two or more molecules of the polysilazane. The polymers obtained by reaction with from 3 to 6 reactive groups may possess both of the cyclic structure and the crosslinked structure described above at the same time. The reaction between a polysilazane and a boron compound B(R⁴)₃:L is similar to that between a polysilazane and B(R⁴)₃ with respect to the reaction mechanism and the structure of the product, since the complex-forming ligand L does not react with the polysilazane. The polysilazane used in the present invention has Si-H bonds and N-H bonds within the molecule. Copolymers or mixtures of polysilazanes with other polymers or compounds may also be used for the purpose of the present invention. The polysilazane to be used as a starting material in the present invention includes those which have a linear, cyclic and/or crosslinked structure within the molecule thereof. These may be used independently or as a mixture thereof. Typical examples of the polysilazane expressed by the above general formula (I) include the following ones which, however, are not limitative at all. In general, R¹, R² and R³ of the general formula (I) may be hydrogen, a hydrocarbyl group, a substituted hydrocarbyl group, an alkylsilyl group, an alkylamino group and an alkoxy group. Examples of substituents for the hydrocarbyl group include amino, alkylamino, nitro, nitroso, nitrosoamino, nitroamino, hydrazino, hydroxyl, alkylsilyl, siloxy, alkoxy, cyano, isocyano, cyanate, formyl, carboxyl, formamide, acetyl, acetoxy, acetoamide, acetoimidoyl, acetoacetyl, glycyl, acetonitrile and halogen. It is preferred that R¹, R² and R³ be selected from hydrogen, an alkyl group containing 1 to 5 carbon atoms, an alkenyl group containing 2 to 6 carbon atoms, a cycloalkyl group containing 5 to 7 carbon atoms, an aryl group, an aralkyl group, an alkylsilyl group containing 1 to 4 carbon atoms, an alkylamino group containing 1 to 5 carbon atoms and an alkoxy group containing 1 to 5 carbon atoms, since these groups have a less steric hindrance. More preferable examples of R¹, R² and R³ include hydrogen, a methyl group, an ethyl group, a vinyl group, an allyl group, a methylamino group, an ethylamino group, a methoxy group and an ethoxy group. Those polysilazanes represented by the general formula (I) wherein R¹, R² and R³ are all hydrogen atoms are perhydropolysilazanes. Process for their preparation are reported in, for example, Japanese Unexamined Patent Publication No. 60-145903 and D. Seyferth et al; "Communication of Am. Cer. Soc.", C-13, January 1983. These processes give a mixture of polymers having different structures which, however, fundamentally contain linear portions and cyclic portions and are represented by the following chemical formula: -(SiH₂NH) a - -(SiH₂N) b < -(SiH₃) c (a + b + c = 1) One example of the structure of a perhydropolysilazane is shown below: A process for preparing a polysilazane of the general formula (I) wherein R¹ and R² are hydrogen atoms and R³ is a methyl group is reported by D. Seyferth et al in "Polym. Prepr." Am. Chem. Soc., Div. Polym. Chem., 25 , 10 (1984). The polysilazane obtained by this process is a linear polymer and a cyclic polymer having recurring units of -(SiH₂NCH₃)- and is free of crosslinks. Processes for preparing a polysilazane of the general formula (I) wherein R¹ and R³ are hydrogen atoms and R² is an organic group are reported by D. Seyferth et al in "Polym. Prepr." Am. Chem. Soc., Div. Polym. Chem., 25 , 10 (1984) and Japanese Unexamined Patent Publication 61-89230. The polysilazanes obtained by these processes are those which have a cyclic structure having recurring units of -(R²SiHNH)- with a polymerization degree of mainly 3 to 5 and those which have both a linear structure and a cyclic structure at the same time and which are represented by the chemical formula of (R²SiHNHH) x [(R²SiH) 1.5 N] 1-x (0.4 < x < 1). Polysilazanes of the formula (I) wherein R¹ is a hydrogen atom and R² and R³ are organic groups and polysilazanes of (I) wherein R¹ and R² are organic groups and R³ is a hydrogen atom have a cyclic structure having recurring units of -(R¹R²SiNR³)- with a polymerization degree of mainly 3 to 5. The polysilazane of the formula (I) further include polyorgano(hydro)silazanes which have a crosslinked structure within the molecule as are reported by D. Seyferth et al; "Communication of Am. Cer. Soc.", C-132, July 1984. One example is shown below: In addition, polysilazanes (R¹Si(NH) x ) obtained by ammonolysis of R¹SiX₃ (where X is halogen) and having a crosslinked structure as reported by Japanese Unexamined Patent Publication No. 49-69717 or polysilazanes of the following structure : obtained by coammonolysis of R¹SiX₃ and R 2 / 2SiX₂ may also be used as the starting materials. As described above, polysilazanes used in the present invention have main skeletons comprising at least three consecutive units represented by the general formula (I). The units of (I) in some cases are cyclized as is apparent from the above description and, in such cases, the cyclized portion forms a terminal group. In other cases where such cyclized portions are not formed, the main skeletons can be terminated by the same group as R¹, R² or R³ or by hydrogen. As the starting polysilazanes, organic solvent-insoluble ones as shown below may be used as well as organic solvent-soluble ones as described above. However, since these organic solvent-insoluble polysilazanes provide organic solvent-insoluble reaction products, they are limited as to the application of the product. The polysilazanes used in the present invention have a number-average molecular weight of up to 50,000 and include cyclic polysilazanes, linear polysilazanes or a mixture thereof. Preferred polysilazanes have a number-average molecular weight of up to 20,000 more preferably 500 to 10,000. However, if the molecular weight is low, there results a low-molecular weight reaction product with a boron compound. Such a product is a viscous liquid and is, therefore, limited as to its application. In addition, it gives a ceramic in only a low yield, thus not being preferable. If the molecular weight exceeds 50,000, on the other hand the resulting reaction product with a boron compound becomes solvent-insoluble or liable to form a gel. As previously stated, the boron compound to be used in the present invention may be a compound represented by one of the following general formulae: B(R⁴)₃ (II) (R⁴BO)₃ (III) B(R⁴)₃:L (V) wherein R⁴ and L are the same as defined hereinbefore. The group R⁴ is preferably hydrogen, halogen, an alkyl group having 1 - 20, more preferably 1 - 10, most preferably 1 - 4 carbon atoms, an alkoxy group having 1 - 20, more preferably 1 - 10, most preferably 1 - 4 carbon atoms, an aryl group having up to 10 carbon atoms or an aralkyl group up to 10 carbon atoms. Illustrative of suitable groups R⁴ are methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, phenyl, benzyl, tolyl, methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, t-butoxy and phenoxy. Of B(R⁴)₃, examples of those which have a halogen atom or atoms as R⁴ include fluoroborane, tribromoborane, trifluoroborane, trichloroborane, difluoroborane, diiodoborane, iodoborane, dibromomethylborane, dichloromethylborane, difluoromethylborane, difluoromethoxyborane, diiodomethylborane, ethynyldifluoroborane, difluorovinylborane, dibromoethylborane, dichloroethylborane, dichloroethoxyborane, ethyldifluoroborane, ethyldiiodoborane, bromodimethylborane, dibromo(dimethylamino)borane, chlorodimethylborane, chlorodimethoxyborane, fluorodimethylborane, fluorodimethoxyborane, dichloroisopropylborane, dichloropropylborane, difluoropropoxyborane, bromo(dimethylamino)methylborane, chlorodivinylborane, dibromobutylborane, butyldichloroborane, butyldifluoroborane, butoxydifluoroborane, bromodiethylborane, dibromo(diethylamino)borane, chlorodiethylborane, chlorodiethoxyborane, dichloro(pentafluorophenyl)borane, dichloro(diethylamino)borane, (diethylamino)difluoroborane, bromobis(dimethylamino)borane, chlorobis(dimethylamino)borane, bis(dimethylamino)fluoroborane, dibromophenylborane, dichlorophenylborane, difluorophenylborane, difluorophenoxyborane, diiodophenylborane, dibromo(1,3-dimethyl-1-butenyl)borane, dibromo(3,3-dimethyl-1-butenyl)borane, dibromo(1-ethyl-1-butenyl)borane, dibromo-1-hexenylborane, dibromo(2-methyl-cyclopentyl)borane, 2-methylcyclopentyl-dichloroborane, dibromohexylborane, dibromo(2-methylpentyl)borane, difluoroborane, dibromo(dipropylamino)borane, chlorodipropylborane, chloro(1,1,2-trimethylpropyl)borane, dichloro(diisopropylamino)borane, butyl(dimethylamino)fluoroborane, dichloro(4-methylphenyl)borane, dichloro(methylphenylamino)borane, bromo(dimethylamino)phenylborane, chloro(dimethylamino)phenylborane, 9-bromo-9-borabicyclo[3,3,1]nonane, 9-chloro-9-borabicyclo[3,3,1]nonane, diethylaminochloro-(1-butenyloxy)borane, dichlorooctylborane, bromobis(1-methylpropyl)borane, bromodibutylborane, dibromo(dibutylamino)borane, chlorobis(2-methylpropyl)borane, dibutylchloroborane, dichloro(dibutylamino)borane, dibutylfluoroborane, bromobis(biethylamino)borane, chlorobis(diethylamino)borane, dichloro(2,4,6-trimethylphenyl)borane, 3-bromo-7-methyl-3-borabicyclo[3,3,1]nonane, (diethylamino)chloro(cyclopentenyloxy)borane, dichloro(1,2,3,4,5-pentamethyl-2,4-cyclopentadien-1-yl)borane, dibromo(1,2,3,4,5-pentamethyl-2,4-cyclopentadien-1-yl)borane, diiodo(1,2,3,4,5-pentamethyl-2,4-cyclopentadien-1-yl)borane, chlorodicyclopentylborane, chloro(diethylamino)phenylborane, bromodicyclopentylborane, (1-butyl -1-hexenyl)dichloroborane, bromodipentylborane, chlorodiphenylborane,bromodiphenylborane, dichloro(diphenylamino)borane, chloro(diisopropylamino)phenylborane, chloro(dipropylamino)phenylborane, bromobis(2-bromo-1-hexenyl)borane, chlorobis(2-chloro-1-hexenyl)borane, chlorobis(2-chloro-1-hexenyl)borane, chlorodicyclohexylborane, chlorodi-1-hexenylborane, chloro(1-ethyl-1-butenyl)(1,1,2-trimethylpropyl)borane, chloro-1-hexenyl(1,1,2-trimethylpropyl)borane, [methyl(4-bromophenyl)amino]chloro(phenyl)borane, chloro-(2-phenylethynyl)(1,1,2-trimethylpropyl)borane, chloro(dibutylamino)phenylborane, chlorooctyl(1,1,2-trimethylpropyl)borane, chlorobis(dibutylamino)borane, fluorobis(2,4,6-trimethylphenyl)borane, (1-bromo-1-hexenyl)-bis(2-methylpentyl)borane, (1-bromo-1-hexenyl)dihexylborane, bis(1-butyl-1-hexenyl)chloroborane, and (5-chloro-1-pentenyl)bis(1,2-dimethylpropyl)borane. Examples of the boron compounds wherein R⁴ is an alkoxy group include dihydroxymethoxyborane, dimethoxyborane, methoxydimethylborane, methyldimethoxyborane, trimethoxyborane, ethyldimethoxyborane, dimethylaminomethoxymethylborane, (dimethylamino)dimethoxyborane, diethylmethoxyborane, dimethoxypropylborane, bis(dimethylamino)methoxyborane, ethoxydiethylborane, butyldimethoxyborane, diethoxyethylborane, triethoxyborane, cyclopentyldimethoxyborane, methoxydipropylborane, dimethoxyphenylborane, (2-methylcyclopentyl)dimethoxyborane, butoxydiethylborane, ethoxydipropylborane, hexyldimethoxyborane, 3-methoxy-3-borabicyclo[3,3,1]nonane, 9-methoxy-9-borabicyclo[3,3,1]nonane, di-butylmethoxyborane, methoxybis(1-methylpropyl)borane, methoxybis(2-methylpropyl)borane, propoxydipropylborane, triisopropoxyborane, tripropoxyborane, butoxydipropylborane, dibutylethoxyborane, diethyl(hexyloxy)borane, dibutoxyethylborane, di-tert-butoxyethylborane, dicyclopentylmethoxyborane, dibutylpropoxyborane, ethoxydipentylborane, (hexyloxy)dipropylborane, tributoxyborane, tri-tert-butoxyborane, tris(2-butoxy)borane, tris(2-methylpropoxy)borane, methoxydiphenylborane, dicyclohexyl(methoxy)boran, dibutyl(2-penten-3-yloxy)borane, dibutoxypentylborane, ethoxydiphenylborane, (2-aminoethoxy)diphenoxyborane, dibutyl(1-cyclo-hexenyloxy)borane, butoxydipentylborane, dibutyl(hexyloxy)borane, dibutoxyhexylborane, dihexyloxypropylborane, tripentyloxyborane, butoxydiphenylborane, (2-methylpropoxy)diphenylborane, diphenoxyphenylborane, triphenoxyborane, tricyclohexyloxyborane, methoxybis(2,4,6-trimethylphenyl)borane, tribenzyloxyborane, tris(3-methylphenoxy)borane, trioctyloxyoxyborane, trinonyloxyborane and trioctadecyloxyborane. Examples of the boron compounds wherein R⁴ is an alkenyl group include ethynylborane, vinylborane, dihydroxyvinylborane, 2-propenylborane, ethynyldimethoxyborane, methyldivinylborane, trivinylborane, 1-hexenyldihydroxyborane, dimethoxy(3-methyl-1,2-butadienyl)borane, diethyl-2-propenylborane, dihydroxy(2-phenylethenyl)borane, (diethylamino)diethynylborane, diethylaminodi-1-propynylborane, 2-butenyldiethylborane, diethyl(2-methyl-2-propenyl)borane, bis(dimethylamino)(1-methyl-2-propenyl)borane, 2-butenylbis(dimethylamino)borane, tri-2-propenylborane, tri(2-propenyloxy)borane, diethyl(3-methyl-2-butenyl)borane, 2-propenyldipropylborane, (diethylamino)di-1-propynylborane, butyldi-2-propenylborane, 2-butenyldipropylborane, diethyl(1-ethyl-2- butenyl)borane, (2-methyl-2-propenyl)dipropylborane, diethyl(1,1-dimethyl-3-buten-1-yloxy)borane, diethyl(1-hexen-4-yloxy)borane, 9-(2-propenyl)-9-borabicyclo[3,3,1]nonane, dibutyl-2-propenylboran, (3-methyl-2-butenyl)dipropylborane, 9-(2-buteny)-9-borabicyclo[3,3,1]nonane, tri-2-butenylborane, tris(2-methyl-2-propenyl)borane, hexyldi-2-propenylborane, 2-butenyldibutylborane, bis(1,2-dimethylpropyl)(2-phenylethenyl)borane, and bis(1,2-di-methylpropyl)-1-octenylborane. Examples of the boron compounds wherein R⁴ represents an alkylamino group or an amino group include aminoborane, diaminoborane, aminodimethylborane, (dimethylamino)borane, dimethyl(methylamino)borane, methylbis(methylamino)borane, tris(methylamino)borane, (dimethylamino)dimethylborane, bis(dimethylamino)borane, bis(dimethylamino)methylborane, aminodipropylboran, (diethylamino)dimethylborane, (dimethylamino)diethylborane, tris(dimethylamino)borane, isopropylbis(dimethylamino)borane, dimethyl(phenylamino)borane, bis(methylamino)phenylborane, bis(dimethylamino)-1-pyrrolylborane, aminodibutylborane, diethylaminodiethylborane, dimethylaminodipropylborane, bis(dimethylamino)phenylborane, dibutyl(dimethylamino)borane, di-tert-butyl(dimethylamino)borane, dibutyl(diethylamino)borane, tris(diethylamino)borane, tris(ethylamino)borane,dimethylaminodiphenylborane and aminobis(2,4,6-trimethylphenyl)borane. Examples of the boron compounds wherein R⁴ represents a hydroxy group include boric acid, hydroxyborane, dihydroxy(methyl)borane, hydroxydimethylborane, ethyldidroxyborane, dihydroxypropylborane, 2-furanyldihydroxyborane, diethylhydroxyborane, butyldihydroxyborane, cyclopentyldihydroxyborane, pentyldihydroxyborane, (3-aminophenyl)dihydroxyborane, phenyldihydroxyborane, heptyldihydroxyborane, dihydroxy(2-phenylethyl)borane, dihydroxy(1-naphthalenyl)borane, hydroxybis(2,4,6-trimethylphenyl)borane and hydroxydiphenylborane. Examples of the boron compounds wherein R⁴ represents an alkyl group include methylborane, dimethylborane, ethylborane, trimethylborane, diethylborane, ethyldimethylborane, diethylmethylborane, 3-methyl-2-butylborane, triethylborane, (1,1,2-trimethylpropyl)borane, dibutylborane, triisopropylborane, tripropylborane, bis(3-methyl-2-butyl)borane, bis(1,1,2-tri-methylpropyl)borane, tri-tert-butylborane, tributylborane, tris(1-methylpropyl)borane, tris(2-methylpropyl)borane, tripentylborane, tris(1,2-dimethylpropyl)borane, trihexylborane and trioctylborane. Examples of the boron compounds wherein R⁴ represents a cycloalkyl group include cyclopentylborane, cyclohexylborane, dicyclohexylborane, cyclohexyl(1,1,2-trimethylpropyl)borane, tricyclopentylborane, and tricyclohexylborane. Examples of the boron compounds wherein R⁴ represents an aryl group include tri-1-naphthylborane, tris(2,4,6-trimethylphenyl)borane, tribenzylborane, tris(4-methylphenyl)borane, triphenylborane, phenylborane and ethyldiphenylborane. An example of the boron compound wherein R⁴ represents a hydrogen atom is borane. Examples of (R⁴BO)₃ include boroxine, trifluoroboroxine, trimethylforoxine, trimethoxyboroxine, triethylboroxine, triethoxyboroxine, triphenylboroxine, triphenoxyboroxine, tris(4-ethenylphenyl)boroxine, tris(dimethylamino)boroxine, tributylboroxine, tributoxyboroxine, and tricyclohexylboroxine. Examples of include 2,4,6-trichloroborazine, 2,4,6-tribromoborazine, 2,4,6-trifluoroborazine, borazine, 1-methlborazine, 2,4,6-trichloro-1,3,5-trimethylborazine, 2,4,6-trifluoro-1,3,5-trimethylborazin, 1,3,5-trimethylborazine, 2,4,6-trimethylborazine, 2,4,6-trimethoxyborazine, 2,4-dichloro-1,3,5,6-tetramethylborazine, 2-chloro-1,3,4,5,6-pentamethylborazine, 2,4,6-trichloro1,3,5-triethylborazine, hexamethylborazine, 1,3,5-triethylborazine, ,4,6-triethylborazine, 1,3,5-tripropylborazine, 2,4,6-triethyl-1,3,5-trimethylborazine, 1,3,5-tributyl-2,4,6-trichloroborazine, hexaethylborazine, 2,4,6-trichloro-1,3,5-triphenylborazine, 2,4,6-triphenylborazine, 2,4,6-tri(diethylamino)borazine, 2,4,6-tri(bis(trimethylsilyl)amino)borazine, 2,4,6-tris(dimethylamino)-1,3,5-trimethylborazine, and 1,3,5-trimethyl-2,4,6-triphenylborazine. Examples of B(R⁴)₃:L include borane-phosphine, borane-hydrazine, trifluoroborane-methanol, cyanoborane-ammonia, difluoroborane-methylamine, borane-methylamine, tribromoborane-dimethylsulfide, trichloroborane-dimethylsulfide, trifluoroborane-dimethyl ether, trifluoroborane-ethanol, borane-isocyanomethane, dibromoborane-dimethylsulfide, dichloroborane-dimethylsulfide, trichloroborane-dimethylamine, trifluoroborane-ethylamine, cyanoborane-methylamine, bromoborane-dimethylsulfide, chloroborane-dimethylsulfide, difluoroborane-dimethylamine, iodoborane-dimethylsulfide, chloroborane-dimethylamine, borane-dimethylamine, borane-dimethylphosphine, tribromoborane-trimethylphosphine, tribromoborane-trimethylamine, trichloroborane-trimethylamine, trichloroborane-trimethylphosphine, trifluoroborane-trimethylamine, trifluoroborane-trimethylphosphine, triiodoborane-trimethylphosphine, cyanoborane-dimethylamine, difluoroborane-trimethylamine, bromoborane-trimethylphosphine, chloroborane-trimethylphosphine, fluoroborane-trimethylamine, iodoborane-trimethylamine, iodoborane-trimethylphosphine, borane-trimethylamine, trimethylborane-ammonia, trimethoxyborane-ammonia,borane-trimethylphosphite, borane-trimethylphosphine, trifluoroborane-2-methylimidazole, trifluoroborane-tetrahydrofuran, chloroborane-tetrahydrofuran, trichloroborane-diethyl ether, trifluoroborane-diethyl ether, dibromoborane-diethyl ether, dichloroborane-diethyl ether, cyanoborane-trimethylamine, bromoborane-diethyl ether, dibromoborane-trimethylamine, dibromomethylborane-trimethylphosphine, chloroborane-diethyl ether, borane-tert-butylamine, borane-diethylamine, tribromoborane-pyridine, trichloroborane-pyridine, trifluoroborane-pyridine, borane-pyridine, borane-4-aminopyridine, bromodimethylborane-trimethylphosphine, dichlorocyanoborane-pyridine, trifluoroborane-phenol, cyanoborane-pyridine, dibromomethylborane-pyridine, borane-4-methylpyridine, trifluoroborane-1-hexanol, tribromoborane-triethylamine, trichloroborane-triethylamine, chloroborane-triethylamine, borane-triethylamine, trimethylborane-trimethylamine, borane-tris(dimethylamino)phosphine, trifluoroborane-methoxybenzene, trifluoroborane-4-methylaniline, borane-2,6-dimethylpyridine, trifluoroborane-dibutyl ether, phenyldichloroborane-triethylamine, tribromoborane-triphenylphosphine, trichloroborane-triphenylphosphine, trifluoroborane-triphenylphosphine, borane-triphenylamine, borane-triphenylphosphine, trimethylborane-triphenylamine, triphenylborane-trimethylamine, triphenylborane-pyridine, and triphenylborane-triethylamine. In addition to the above-described compounds, tetraborane (10), pentaborane (9), pentaborane (11), hexaborane (10), hexaborane (12), octaborane (12), octaborane (18), isononaborane (15), nonaborane (15), decaborane (14), 1,1'-bipentaborane (9), 1,2'-bipentaborane (9), 2,2'-bipentaborane (9), 1-carbahexaborane (7), 2-carbahexaborane (9), 1,2-dicarbahexaborane (6), 1,2-dicarbapentaborane (7), 2,4-dicarbaheptaborane (7), 2,3-dicarbahexaborane (8), 1,7-dicarbaoctaborane (8), 1,2-dicarbadodecaborane (12), 1,7-dicarbadodecaborane (12) and 1,12-dicarbadodecaborane (12) can offer good results. Most of these boron compounds are commercially available, and those which are not commercially available can generally be prepared in the same manner as the commercially available ones. A method of preparing the polyborosilazane will now be described in detail below. The polysilazane and the boron compound are used in such aamounts that the mixing ratio of the polysilazane to the boron compound is 0.001 to 60, preferably 0.01 to 5, more preferably 0.05 to 2.5, in terms of B/Si atomic ratio. An amount of the boron compound in excess of the above range does not give any advantage since the excess boron compound is merely recovered as an unreacted reactant without enhancing the reactivity of the polysilazane. If, on the other hand, the boron compound is used in an amount below the above-specified range, high molecular weight product cannot be obtained. The reaction may be conducted in the absence of a solvent. However, in comparison with the case of conducting the reaction in the presence of an organic solvent, the reaction control is then difficult and, in some cases, a gel product is formed. Hence, the use of an organic solvent is generally preferable. As the solvent, there may be used hydrocarbyl solvents such as aromatic hydrocarbons, aliphatic hydrocarbons or alicyclic hydrocarbons; halogenated hydrocarbons; aliphatic ethers and alicyclic ethers. Preferred solvents include halogenated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, bromoform, ethylene chloride, ethylidene chloride, trichloroethane and tetrachloroethane, ethers such as ethyl ether, isopropyl ether, ethyl butyl ether, butyl ether, 1,2-dihydroxyethane, dioxane, dimethyldioxane, tetrahydrofuran and tetrahydropyrane, and hydrocarbons such as pentane, hexane, isohexane, methylpentane, heptane, isoheptane, octane, isooctane, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, benzene, toluene, xylene and ethylbenzene. For the purpose of obtaining high molecular weight polyborosilazanes, the reaction of the polysilazane withe the boron compound is performed in the presence of a basic compound selected from trialkylamines, tertiary amines, pyrrole, 3-pyrroline, pyrazole, 2-pyrazolyl, dimethylsulfide and mixtures thereof. In some cases the basic compound can be used also as a reaction solvent, either alone or in a mixture of a basic solvent and a non-basic solvent mentioned above. The basic compound is used in an amount of at least 5 parts by weight, preferably at lest 20 parts by weight, per 100 parts by weight of the reaction solvent. If the amount of the basic compound is less 5 parts by weight, a markedly high molecular weight product can not be obtained. Examples of basic compounds which may also serve as a solvent for the reaction include trialkylamines such as trimethylamine, dimethylethylamine, diethylmethylamine and triethylamine, tertiary amines such as pyridine, picoline, dimethylaniline, pyrazine, pyrimidine, pyridazine and derivatives thereof, pyrrole, 3-pyrroline, pyrazole, 2-pyrazolyl, and a mixture thereof. The reaction temperature is selected so as to maintain the reaction system in a liquid state. The reaction is generally conducted at a temperature higher than the boiling boint of a solvent used so as to increase the molecular weight of the resulting polyborosilazane. In particular the reaction is conducted at 400°C or lower, preferably -78°C to 300°C for the purpose of preventing gelation due to thermal decomposition of the polyborosilazane. As to reaction pressure, an ambient pressure is favored. Application of a pressure to the reaction system is not forbidden, but a reduced pressure is not favorable since low-boiling components are distilled off to decrease the yield of the intended product. The reaction time is generally 30 minutes to about one day but, in order to enhance polymerization degree of polyborosilazane, a prolonged reaction time is preferred. The reaction atmosphere is preferably a dry inert atmosphere such as dry nitrogen or dry argon for the purpose of preventing oxidation or hydrolysis of the starting material and the reaction product. The process according to the present invention is advantageous because expensive, noble metal catalysts need not be used. The product polyborosilazane is separated from the reaction mixture by distilling off the unreacted boron compound under reduced pressure or by gel permeation chromatography or high-speed liquid chromatography. The thus obtained polyborosilazanes have an increased number of cross-linkages and a higher number average molecular weight (up to 500,000, preferably up to 200,000) than the starting polysilazanes. Hence, they have improved solidifying properties and permit rapid shaping at an ambient temperature. The high molecular weight of the polyborosilazanes serves to minimize evaporation loss upon pyrolysis at elevated temperatures, thus the yield of ceramics therefrom being improved. For example, the polyborosilazanes obtainable by the present invention can give ceramics in a yield of 70% or more, particularly 80% or more. Notwithstanding their high molecular weight, the polyborosilazanes are soluble in organic solvents such as o-xylene because of the formation of a number of pendant groups and side chains and, therefore, can be used as precursors for ceramic fibers, as binders for the preparation of composite materials, etc. The polyborosilazanes obtainable by the present process can be easily converted to ceramics by pyrolysis under an atmospheric gas or in vacuo. As the atmospheric gas, nitrogen is advantageous, but argon or ammonia may be used as well. A mixed gas of, for example, nitrogen, ammonia, argon and hydrogen may also be utilized. The pyrolysis temperature is generally in the range of from 700 to 1900°C. If the pyrolysis temperature is too low, the pyrolysis requires a prolonged period of time, whereas a too high temperature is disadvantageous in view of energy cost. The heating rate is generally in the range of from 0.1°C/min to 300°C/min. If the heating rate is too slow, the pyrolysis requires a prolonged period of time, whereas too fast a rate causes rapid pyrolysis and shrinkage which can cause cracks in the resulting ceramics. Favorable results can be obtained by controlling the heating rate to 0.5°C/min to 50°C/min in the temperature range of up to 600°C wherein pyrolysis of the polyborosilazane mainly takes place. Polymetallosilazanes previously proposed by the inventors provide amorphous ceramics having an excellent high-temperature strength and modulus. For example, the ceramics retain the amorphous structure even when kept at 1200 to 1300°C for about one hour. However, ceramics obtained by pyrolysis of the polyborosilazanes obtained by the process of the present invention have a better heat resistance. That is, they retain the amouphous structure at 1500°C or above, some of them at 1700°C or above. In general, polycrystalline substances have a poorer mechanical strength than amorphous substances since a grain boundary can act as the origin of fractures. Ceramics obtained by pyrolysis of the polyborosilazane obtained by the present invention retain the amorphous structure thereof even at a temperature as high as 1700°C and, therefore, exhibit an excellent high-temperature mechanical strength. The amorphous structure at 1700°C is theoretically considered almost maximum with Si-N series ceramics. In consideration that 1700°C is near the upper limit of heat resistance with respect to crystalline Si-N series ceramics, the heat resistance of the ceramics obtained by pyrolysis of the polyborosilazanes will be understood to be excellent. The polyborosilazanes obtainably by means of the present invention provide the following advantages. (1) Ceramics obtained from metallosilazanes containing Al, Ti or Zr are known to have a better heat resistance than ceramics obtained from a starting polysilazane. Ceramics obtained from the polyborosilazane have a still better heat resistance than the above-described ceramics obtained from metallosilazanes, thus an improvement in high-temperature mechanical strength being attained. (2) Since the polyborosilazanes are soluble in organic solvents and can be converted to Si-N-B ceramics by calcination,they can be used for the production of composite ceramic moldings with high performance. (3) The presence of boron in a ceramic obtained by pyrolysis of the polyborosilazane serves to improve the hardness of the ceramic. (4) Since formulation of the ceramic can be controlled, electric conductivity thereof can be selected from a wide range. The following examples will further illustrate the present invention. Preparation 1 To a four-necked 1 liter 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 air within the flask therewith. After charging 490 ml of deaerated, dry pyridine, the flask was cooled in an ice bath. Then, 51.6 g of dichlorosilane were added into the flask to form a white, solid precipitate of an adduct (SiH₂Cl₂ 2C₅H₅N). Subsequently, with stirring and cooling the reaction mixture 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 were 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, perhydropolysilazane was obtained. Gel permeation chromatography of the thus obtained polymer revealed that the polysilazane had a number-average molecular weight of 980. An infrared spectrum of this polymer (solvent: dry xylene; concentration of the perhydroolyslazane: 10.2 g/liter) indicated peaks at a wavenumber of 3350 (cm-¹)(apparent absorptivity coefficient ε = 0.557 g⁻¹cm⁻¹) and at 1175 cm⁻¹ based on NH, a peak at 2170 (ε = 3.14) based on SiH, a broad peak at 1020 - 820 based on SiH and SiNSi. A ¹HNMR (proton nuclear magnetic resonance) spectrum of this polymer (60 MHz; solvent:CDCl₃; reference substance: TMS) indicated broad peaks at δ of 4.8 ,4.4 (br, SiH) and 1.5 (br, NH). Preparation 2 The reaction was conducted using the same apparatus used in Preparation 1 as follows. In the four-necked flask used in Preparation 1, 450 ml of dry tetrahydrofuran was charged, and 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 in the bath, 44.2 g of anhydrous methylamine were bubbled as a mixture gas with nitrogen 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 give 820 ml of a filtrate containing N-methylsilazane. When the solvent was removed from the filtrate by evaporation in vacuo, 8.4 g of viscous oil was obtaned. Gel permeation chromatography of the N-methylsilazane revealed that the N-methylsilazane had a number-average molecular weight of 1100. Preparation 3 To a four-necked 1 liter 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 air within 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, 18.1 g (1.06 mol) of ammonia which had been passed successively through a sodium hydroxide-containing tube and an active carbon-containing tube were 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. When the solvent was removed from the filtrate by evaporation in vacuo, 8.81 g of colorless, transparent methyl(hydro)silazane was obtained. Gel permeation chromatography of the methyl(hydro)silazane revealed that the methyl(hydro)silazane had a number-average molecular weight of 380. Preparation 4 The reaction was conducted using the same apparatus used in Preparation 1 as follows. In the four-necked flask used in Reference Example 1, 500 ml of dry toluene was charged, and 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 were bubbled through the reaction mixture within the flask. After completion of the reaction, the reaction mixture was treated in the same manner as in Preparation 1 to obtain 6.8 g of oily phenylpolysilazane. Gel permeation chromatography of the phenylpolysilazane revealed that the phenylpolysilazane had a number-average molecular weight of 380. Example 1 100 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in pyridine (concentration of perhydropolysilazane: 5.10 wt %) were charged in a 300-ml autoclave for reaction, and 4.0 cc (0.035 mol) of trimethyl borate were added thereto. The mixture was stirred for 3 hours at 160°C in a closed system. A rise in pressure of 1.0 kg/cm² was observed after completion of the reaction. Gas chromatography of generated gas revealed that the gas was composed of hydrogen and methane. After cooling the reaction mixture to room temperature, 100 ml of dry xylene was added thereto. When the solvent was removed at a temperature of 50 to 70°C and under a pressure of 3 to 5 mmHg, 5.45 g of white powder was obtained. This powder was soluble in organic solvents such as toluene, tetrahydrofuran and chloroform. Gel permeation chromatography of the polymer powder revealed that the polymer powder had a number-average molecular weight of 2100. IR spectrum of the polymer indicated peaks at wavenumbers (cm⁻¹) of 3350 and 1175 based on NH, a peak at 2170 based on SiH, a broad peak at 1020 to 820 based on SiH and SiNSi, peaks at 2960, 2940 and 2840 based on CH, a peak at 1090 based on SiO, and a broad peak at 1550 to 1300 based on BO. ¹HNMR spectrum (CDCl₃, TMS) of the polymer powder indicated peaks at δ 4.8 (br, SiH₂), δ 4.7 (br, OSiH₂), δ 4.4 (br, SiH₃), δ 3.6 (br, CH₃O) and δ 1.4 (br, NH). The results of elemental analysis of the polymer were as follows by weight. Elemental analysis (wt %): Si: 42.4, N: 25.9, C: 8.8, O: 12.7, B: 7.0, H: 3.8 Example 2 100 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in o-xylene (concentration of perhydropolysilazane: 5.84 wt %) were charged in a 300-ml autoclave for reaction, and 4.0 cc (0.0396 mol) of pyridine-borane complex were added thereto. The mixture was stirred for 3 hours at 80°C in a closed system. A rise in pressure of 0.2 kg/cm² was observed after completion of the reaction. Gas chromatography of generated gas revealed that the gas was composed of hydrogen. When the solvent was removed in the same manner as in Example 1, 4.98 g of red solid was obtained. Gel permeation chromatography of the solid product revealed that the solid product had a number-average molecular weight of 170,000. Example 3 100 ml of a solution of the N-methylsilazane obtained in Preparation 2 in γ-picoline (concentration of N-methylsilazane: 4.95 wt %) were charged in a 300-ml autoclave for reaction, and 4.0 cc (0.0148 mol) of tributyl borate was added thereto. The mixture was stirred for 3 hours at 120°C in a closed system. A rise in pressure of 0.8 kg/cm² was observed after completion of the reaction. The reaction mixture was cooled to room temperature. When the solvent was removed from the reaction mixture in the same manner as in Example 1 in vacuo, 5.03 g of pale yellow rubbery solid were obtained. Gel permeation chromatography of the solid product revealed that the solid product had a number-average molecular weight of 1880. Example 4 80 ml of a solution of the phenylpolysilazane obtained in Preparation 4 in pyridine (concentration of phenylpolysilazane: 3.65 wt %) were charged in a 300-ml autoclave for reaction, and 1.26 g (0.0204 mol) of boric acid was added thereto. The mixture was stirred for 1 hour at 60°C in a closed system. A rise in pressure of 0.6 kg/cm² was observed after completion of the reaction. The reaction mixture was cooled to room temperature. When the solvent was removed from the reaction mixture in the same manner as in Example 1 in vacuo, 2.46 g of white solid were obtained. Gel permeation chromatography of the solid product revealed that the solid product had a number-average molecular weight of 3.2040. Example 5 100 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in pyridine (concentration of perhydropolysilazane: 5.37 wt %) were charged in a 300-ml pressure vessel for reaction, and 3.2 ml (0.0376 mol) of 1,3,5-trimethylborazine were added thereto. The mixture was stirred for 3 hours at 120°C in a closed system. A rise in pressure of 0.3 kg/cm² was observed after completion of the reaction. Gas chromatography of generated gas revealed that the gas was composed of hydrogen. The reaction mixture was cooled to room temperature. When the solvent was removed in vacuo in the same manner as in Example 1, 4.86 g of white powder were obtained. This powder was soluble in organic solvents such as toluene, tetrahydrofuran and chloroform. Gel permeation chromatography of the solid product revealed that the powder had a number-average molecular weight of 2430. Example 6 100 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in pyridine (concentration of perhydropolysilazane: 6.32 wt %) were charged in a 300-ml autoclave for reaction, and 5.5 ml (0.0379 mol) of trimethoxyboroxine were added thereto. The mixture was stirred for 3 hours at 140°C in a closed system. A rise in pressure of 0.2 kg/cm² was observed after completion of the reaction. Gas chromatography of generated gas revealed that the gas was composed of hydrogen and methane. The reaction mixture was cooled to room temperature. When the solvent was removed in vacuo in the same manner as in Example 1, 5.77 g of white powder were obtained. This powder was soluble in organic slvents such as toluene, tetrahydrofuran and chloroform. Gel permeation chromatography of the solid product revealed that the powder had a number-average molecular weight of 1940. Example 7 The polyborosilazane obtained in Example 1 was pyrolyzed by heating in ammonia up to 1000°C at a heating rate of 3°C/min to obtain white solid in a yield of 88.0 wt %. X-ray diffraction patterns of the thus obtained ceramic revealed that the ceramic was amorphous. Elemental analysis of this solid based on weight gave the following results. Elemental analysis (wt %): Si: 40.7, N: 33.5, C: 1.57, O: 12.0, B: 7.00 Subsequently, the solid was further heated up to 1700°C in a nitrogen atmosphere at a heating rate of 10°C/min to effect calcination, thereby obtaining a gray solid. X-ray diffraction patterns of this solid revealed that it remained amorphous as shown in Fig. 1. Example 8 The polyborosilazane obtained in Example 2 was pyrolyzed in an ammoniacal atmosphere up to 1000°C at a heating rate of 3 °C/min to give a pale brown solid in a yield of 82.0 wt %. X-ray diffraction patterns of the thus obtained ceramic powder revealed that the ceramic was amorphous. Elemental analysis of this solid based on weight gave the following results. Elemental analysis (wt %): Si: 43.5, N: 38.7, C: 0.70, O: 8.40, B: 5.60 Subsequently, the solid was further heated up to 1500°C in a nitrogen atmosphere at a heating rate of 10°C/min to effect calcination, thereby obtaining blackish gray solid. X-ray diffractiometry of this solid revealed that it remained amorphous as shown in Fig. 2. Then, this solid was still further heated up to 1700°C in a nitrogen atmosphere at a heating rate of 10°C/min to effect calcination, thereby obtaining blackish gray solid. X-ray diffractiometry of this solid indicated, as shown in Fig. 3, a (101) diffraction peak of α-Si₃N₄ at 2ϑ = 20.5°, a (110) diffraction peak of α-Si₃N₄ at 2ϑ = 22.9°, a (200) diffraction peak of α-Si₃N₄ at 2ϑ = 26.4°, a (201) diffraction peak of α-Si₃N₄ at 2ϑ = 30.9, a (002) diffraction peak of α-Si₃N₄ at 2ϑ = 31.7°, a (102) diffraction peak of α-Si₃N₄ at 2ϑ = 34.5°, a (210) diffraction peak of α-Si₃N₄ at 2ϑ = 35.2°, a (211) diffraction peak of α-Si₃N₄ at 2ϑ = 38.8°, a (112) diffraction peak of α-Si₃N₄ at 2ϑ = 39.4°, a (300) diffraction peak of α-Si₃N₄ at 2ϑ = 40.1°, a (202) diffraction peak of α-Si₃N₄ at 2ϑ = 41.8°, a (301) diffraction peak of α-Si₃N₄ at 2ϑ = 43.4°, a (220) diffraction peak of α-Si₃N₄ at 2ϑ = 46.9°, a (212) diffraction peak of α-Si₃N₄ at 2ϑ = 48.2°, a (212) diffraction peak of β-Si₃N₄ at 2ϑ = 23.3°, a (200) diffraction peak of β-Si₃N₄ at 2ϑ = 26.9°, a (101) diffraction peak of β-Si₃N₄ at 2ϑ = 33.6°, a (210) diffraction peak of β-Si₃N₄ at 2ϑ = 36.0°, a (201) diffraction peak of β-Si₃N₄ at 2ϑ = 41.4°, and a (310) diffraction peak of β-Si₃N₄ at 2ϑ = 49.9°, thus the solid product being found to be crystalline silicon nitride. Comparative Example 1 To a four-necked 100 ml flask equipped with a condenser, a seal cap, a thermometer and a magnetic stirrer, a dry nitrogen gas was fed to replace the air within the flask therewith. After charging 63.4 g of a solution in benzene of the perhydropolysilazane obtained in Preparation 1 (concentration of the perhydropolysilazane: 4.45 wt %), a solution of 4.00 g (12.2 mmol) of zirconium tetraisopropoxide in 6.0 ml of dry benzene was fed with stirring to the flask using a syringe, followed by refluxing the resulting solution to react. After completion of the reaction, the reaction solution was subjected to gel pertition chromatography to obtain pale yellow solid polyhydrozirconosilazane. Cryoscopic method (solvent: dry benzene) revealed that the thus obtained polymer had a number-average molecular weight of 2100. Elemental analysis of the polymer revealed that the polymer had a formulation of Si: 34.0, Zr: 18.6, N: 13.0, O: 13.2, C: 14.4 and H: 5.1 (wt %). When the thus obtained polymer was calcined at 1350°C in a nitrogen atmosphere, a black solid was obtained in a yield of 78 wt %. X-ray diffractiometry of this solid indicated a diffraction pattern of amorphous ZrO₂ phase as shown in Fig. 4. Calcination of perhydropolysilazane under the same conditions was confirmed by X-ray diffractiometry to produce silicon nitride crystalline, whereas calcination of polyhydrozirconosilazane involved generation of an amorphous ZrO₂ phase which served to keep the amorphous state of the silicon nitride to a higher temperature level. Comparative Example 2 To a four-necked 200 ml flask equipped with a condenser, a seal cap, a thermometer and a magnetic stirrer, a dry nitrogen gas was fed to replace the air within the flask therewith. After charging 110 g of a solution in benzene of the perhydropolysilazane obtained in Preparation 1 (concentration of the perhydropolysilazane: 4.57 wt %), a solution of 6.30 g (22.2 mmol) of titanium tetraisopropoxide in 6.5 ml of dry benzene was fed with stirring to the flask using a syringe. The color of the reaction solution changed from colorless to pale brown, pale brown to violet, and violet to black. After completion of the reaction, the solvent was removed in vacuo to obtain dark brown solid polyhydrotitanosilazane in a yield of 84 wt %.. Cryoscopic method (solvent: dry benzene) revealed that the thus obtained polymer had a number-average molecular weight of 1840. The thus obtained polymer was not a mere mixture of perphydropolysilazane and titanium alkoxide but was a condensation reaction product between the two materials which had therefore a higher molecular weight than the two individual compounds. Elemental analysis of the resulting polymer revealed that the polymer had a formulation of Si: 33.0, Ti: 9.8, N: 14.0, O: 11.8, C: 23.4 and H: 6.6 (wt %). When the thus obtained polymer was calcined at 1350°C for one hour in a nitrogen atmosphere, a black solid was obtained in a yield of 728 wt %. X-ray diffractiometry of this solid indicated a diffraction pattern of amorphous TiN phase alone as shown in Fig. 5. Calcination of perhydropolysilazane under the same conditions was confirmed by X-ray diffractiometry to produce silicon nitride crystalline, whereas calcination of polyhydrotitanosilazane involved generation of an amorphous TiN phase which served to keep the amorphous state of the silicon nitride to a higher temperature level. Elemental analysis of the thus obtained ceramic gave the following results (wt %). Si: 41.3, Ti: 12.9, N: 20.5, O: 19.9, C: 4.5 Comparative Example 3 120 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in pyridine (concentration of perhydropolysilazane: 5.25 wt %) were charged in a 300-ml pressure vessel for reaction, and 13.0 g (0.0637 mol) of aluminum triisopropoxide was added thereto. The mixture was stirred for 3 hours at 1240°C in a closed system. A rise in pressure of 1.8 kg/cm² was observed after completion of the reaction. The reaction mixture was cooled to room temperature. When the solvent was removed in vacuo in the same manner as in Example 1, 12.3 g of pale yellow powder was obtained. The thus obtained polymer was heated up to 1500°C in a nitrogen atmosphere at a temperature-increasing rate of 10°C/min, thereby obtaining grayish white solid. X-ray diffractiometry of this solid indicated a (100) diffraction peak of β-Sialon at 2ϑ = 13 3°, a (110) diffraction peak of β-Sialon at 2ϑ = 23.1°, a (200) diffraction peak of β-Sialon at 2ϑ = 26.8°, a (101) diffraction peak of β-Sialon at 2ϑ = 33.2°, a (210) diffraction peak of β-Sialon at 2ϑ = 35.8°, a (111) diffraction peak of β-Sialon at 2ϑ = 38.4°, a (201) diffraction peak of β-Sialon at 2ϑ = 40.8°, (220) and (211) diffraction peaks of β-Sialon at 2ϑ = 47.5°, a (310) diffraction peak of β-Sialon at 2ϑ = 49.5°, a (301) diffraction peak of β-Sialon at 2ϑ = 51.6°, a (221) diffraction peak of β-Sialon at 2ϑ = 57.2°, and a (311) diffraction peak of β-Sialon at 2ϑ = 59.1°, thus the solid product being found to be crystalline β-Sialon. Elemental analysis of the thus obtained ceramic gave the following results (wt %). Si: 38.9, N: 25.8, Al: 21.9, C: 3.2, O: 12.3 Example 9 100 ml of a solution of the perhydropolysilazane obtained in Preparation 1 in o-xylene (concentration of perhydropolysilazane: 4.53 wt %) were charged in a 300 ml autoclave for reaction, and 3.34 g (0.044 mol) of borane-dimethylsulfide complex were slowly added thereto. The mixture was stirred for 3 hours at 20 °C in a closed system. A rise in pressure of 1.2 kg/cm² was observed after completion of the reaction. Gas chromatography revealed that the generated gas was hydrogen. When the solvent was removed at a temperature of 50 to 70 °C under a pressure of 3 to 5 mmHg, 4.15 g of white powder was obtained. This powder was soluble in organic solvents such as toluene, tetrahydrofuran and chloroform. Gel permeation chromatography of the polymer powder revealed that the polymer powder had a number-average molecular weight of 4400. IR spectrum of the polymer indicated peaks at wavenumbers (cm⁻¹) of 3350 and 1175 based on NH, a peak at 2170 based on SiH, a broad peak at 1020 to 820 based on SiH and SiNSi and a peak at 1550 to 1300 based on BN. Proton NMR spectrum (CDCl₃, TMS) of the polymer powder indicated peaks at 4.8 (br, SiH₂), 4.4 (br, SiH₃) and 1.4 (br, NH). The polymer powder was then gradually heated to 1000 °C in ammonia at a heating rate of 5 °C/min to effect pyrolysis, thereby obtaining gray solids with a yield of 83.0 wt %. Powder X-ray diffraction patterns of the thus obtained ceramic revealed that the ceramic was amorphous. This ceramic powder was further heated up to 1700 °C in a nitrogen atmosphere at a heating rate of 10 °C/min for calcination, thereby obtaining dark gray solids which were found to be amorphous upon analysis by powder X-ray diffraction patterns. Elemental analysis (wt %): Si: 47.5, N: 36.8, C: 1.30, O: 2.23, B: 11.42

Claims: A process for producing a polyborosilazane having a boron/silicon atomic ratio of 0.01 to 3 and a number-average molecular weight of up to 500,000, which comprises reacting, in the presence of a basic compound selected from trialkylamines, tertiary amines, pyrrole, 3-pyrroline, pyrazole, 2-pyrazolyl, dimethylsulfide and mixtures thereof, a polysilazane having a number-average molecular weight of up to 50,000 and containing a skeletal structure of at least three consecutive units represented by the general formula (I): wherein R¹, R² and R³, independently from each other, represent hydrogen, a hydrocarbyl group, a substituted hydrocarbyl group, an alkylsilyl group, an alkylamino group or an alkoxy group, with the proviso that at least one of R¹, R² and R³ is hydrogen, with a boron compound represented by the following general formula (II), (III), (IV) or (V): B(R⁴)₃ (II) (R⁴BO)₃ (III) B(R⁴)₃ : L (V) wherein R⁴ may be the same or different and represents hydrogen, halogen, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an alkoxy group, an alkylamino group, a hydroxy group or an amino group, and L represents a compound capable of forming a complex with B(R⁴)₃. A process according to Claim 1, wherein said basic compound also serves as a solvent for said reaction. A process according to Claim 2, wherein said basic compound is pyridine. A process according to any preceding claim, wherein R¹, R² and R³ are independently selected from hydrogen, C₁-C₅ alkyl, C₂-C₆ alkenyl, C₅-C₇ cycloalkyl, aryl, aralkyl, C₁-C₄ alkylsilyl, C₁-C₅ alkylamino and C₁-C₅ alkoxy groups. A process according to Claim 4, wherein R¹, R² and R³ are independently selected from hydrogen, methyl, ethyl, vinyl, allyl, methylamino, ethylamino, methoxy and ethoxy groups. A process according to any preceding claim, wherein R⁴ is hydrogen, halogen, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, an aryl group containing up to 10 carbon atoms or an aralkyl group having up to 10 carbon atoms. A process according to any preceding claim, wherein the mixing ratio of said polysilazane and said boron compound is such that the B/Si atomic ratio is from 0.05 to 2.5. A process for making a ceramic material comprising the steps of: (a) producing a polyborosilazane by a process according to any one of claims 1-7, and (b) pyrolysing the polyborosilazane thus produced at from 700° to 1900°C.

Inventors: ARAI MIKIRO, FUNAYAMA OSAMU, ISODA TAKESHI, SATO KIYOSHI, TASHIRO YUUJI, ARAI, MIKIRO, FUNAYAMA, OSAMU, ISODA, TAKESHI, SATO, KIYOSHI, TASHIRO, YUUJI
Applicants: TONEN CORP, TONEN CORPORATION
Application year: 1990
Main classification: C08G77
IPC codes: C08G77, C08G79