JP2003002632A - Ultrafine silica particle having uniform diameter, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ultrafine silica particles having 1-100 nm average particle diameter distribution of which is simple dispersing type. SOLUTION: The ultrafine silica particles are manufactured by the following processes (a), (b), and (c); the process (a): a dendorimer having a reactive functional group, which reacts with isocyanates, at a branch terminal and an alkoxysilane as a reaction reagent having isocyanate group are reacted together to obtain a dendorimer having an alkoxysilyl group at the branch terminal (silyl dendorimer), a process (b): an alkoxysilane at the branch terminal (surface layer) of the dendorimer having the alkoxysilyl group at the branch terminal (silyl dendorimer) is hydrolyzed and dehydrated and condensated to obtain a precursor of the ultrafine silica particles, a process (c): the precursor is added with an alkoxysilane and reacted to laminate a silica layer on the surface layer.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to ultrafine silica particles and a method for producing the same. More specifically, the present invention relates to a high performance catalyst, an abrasive, an optical material, a semiconductor material, or silica particles coated with silica, which are suitable for use in the field of nanotechnology, and a method for producing the same.

[0002]

2. Description of the Related Art Fine particles having a particle size of approximately 0.1 to 10 μm have been used as a constituent material for cement, cosmetics, toner for electronic copying, etc. for a long time. Particle size is 0.1 μm
Fine particles of 100 nm or less (hereinafter, also referred to as ultrafine particles) are also known, and these are used in the fields of ceramics, magnetic tapes, VLSI elements, and the like.

As a method for producing ultrafine particles of 0.1 μm (100 nm) or less, a metal alkoxide hydrolysis method,
Coprecipitation method, inorganic salt hydrolysis method, spray drying method, plasma method,
A method such as a laser method is known, and it is possible to obtain ultrafine particles of nm order. Further, among the above-mentioned production methods, the metal alkoxide hydrolysis method is a widely used method. When alkoxysilane is used as the metal alkoxide, the alkoxysilane is converted to an alkoxysilane (by an acid or base catalyst).
It is a method of utilizing hydrolysis and subsequent dehydration condensation reaction (edited by the Chemical Society of Japan, ultrafine particles-science and application-, Chemistry Review No. 48, Academic Publishing Center (1985); Shimohira, Ishijima: Nikkashi. , 1981, 1503-1505).

[0004]

However, although the above metal alkoxide hydrolysis method can produce ultrafine particles on the order of nm, the obtained ultrafine particles generally have a particle size distribution.
It shows a polydisperse type (two or more peaks in the particle size distribution). The object of the present invention is to provide a method for producing silica ultrafine particles having a monodisperse type particle size distribution (there is one peak in the particle size distribution), and a silica particle having a uniform particle size obtained by the production method. It is to provide ultrafine particles.

[0005]

Means for Solving the Problems The present inventors have conducted various studies to obtain ultrafine silica particles having a monodispersed particle size distribution (one peak in the particle size distribution) and a narrow particle size distribution. In the process, a method for easily introducing an alkoxysilyl group at the terminal of a dentrimer of an organic component (having no alkoxysilyl group) was found, and this method was developed to complete the present invention.

That is, the present invention is a method for producing ultrafine silica particles, which comprises the following steps (b) and (c). Step (b): a step of hydrolyzing the alkoxysilane at the branched end (surface layer) of the "dendrimer having an alkoxysilyl group at the branched end" (silylated dendrimer), dehydration-condensation to give a silica ultrafine particle precursor; Step (c): a step of newly adding an alkoxysilane to the precursor and causing the precursor to react, and laminating a silica layer on the surface layer thereof.

Here, in the above manufacturing method, step (b)
The following step (a) can be carried out before. Step (a): A dendrimer having a functional group capable of reacting with an isocyanate at a branched end is reacted with an alkoxysilane having an isocyanate group (reaction reagent) to give a “dendrimer having an alkoxysilyl group at a branched end”. (Silylated dendrimer);

The present invention is also ultrafine silica particles obtained by the above production method. The particle size distribution of the ultrafine particles is monodisperse type, and the average particle size is silica ultrafine particles in the range of 1 nm to 100 nm. The average particle diameter of the ultrafine silica particles is usually 1 nm to 100 nm (preferably 5 nm to
100 nm, more preferably 5 nm to 50 nm), the particle size distribution shows a monodisperse type, and the particles are even. The particle size of the ultrafine silica particles can be appropriately adjusted by changing the production conditions.

The particle size distribution is a value obtained by a dynamic light scattering measurement method or a photon correlation spectroscopy method (specifically, Coulter counter). Instead of the above dynamic light scattering measurement method or photon correlation spectroscopy, other methods such as electron microscope, sedimentation method, X-ray diffraction method and the like may be used. The “silica ultrafine particles” of the present invention are those whose main constituent component is represented by the composition formula SiO ₂ , and strictly speaking, it may be more appropriate to call them “silica-coated ultrafine particles”. However, the properties of ultrafine particles are not so much influenced by the properties inside the particles, and depend on the properties near the particle surface.
The “silica-coated ultrafine particles” will be simply referred to as “silica ultrafine particles”. Further, the “dendrimer” means a three-dimensionally branched multibranched compound, which means a multibranched compound having high regularity and a narrow molecular weight distribution. FIG. 1 shows a schematic diagram of a dendrimer (an example of a third generation dendrimer).

[0010]

The present invention will be described in more detail below. An outline of the method for producing ultrafine silica particles of the present invention is shown in FIG. Hereinafter, the steps will be described in order. <Step (a)> A dendrimer having a functional group capable of reacting with an isocyanate at a branched end (substrate dendrimer) is reacted with an alkoxysilane having an isocyanate group (reaction reagent) to produce “alkoxysilyl at a branched end”. To create a dendrimer with groups "(silylated dendrimer):

(Dendrimer) The functional group at the branched end of the dendrimer is a functional group capable of reacting with an isocyanate, and examples of such a group include a hydroxyl group (--OH; phenolic or alcoholic), a primary amino group (-). NH ₂ ),
A secondary amino group, a carboxyl group (—COOH), a thiol group (—SH), and the like, all of which are groups containing active hydrogen. Of these, primary amino groups or hydroxyl groups are
It is preferable because it has excellent reactivity with the isocyanate group of the reaction reagent and can introduce many alkoxysilyl groups. The number of terminal branches or the number of active hydrogens of the dendrimer is usually 4 or more, and the upper limit thereof is determined by the number of generations of the dendrimer.

Specific examples of the dendrimer include polyamidoamine-based dendrimers, propyleneimine-based dendrimers (Japanese Patent Publication No. 7-330631), and Che.
m. Rev. , 97, 1681 (1997) and Poly
m. Prep. , 40 (2), 978 (1999)),
Alternatively, JP-A-2000-73055 and JP-A-200
There is a dendrimer disclosed in JP-A No. 0-26597. Some of these are commercially available. The structural formula of an example of the dendrimer is shown in FIG. The size of the dendrimer used may be determined in consideration of the size of the target ultrafine silica particles. Generally expressed in terms of the number of generations
You will have to choose from 10 generations.

(Reaction Reagent) As the "alkoxysilane having an isocyanate group" (reaction reagent) which is reacted with the dendrimer, a compound represented by the following formula (1) [Chemical Formula 1] is preferably used.

[0014]

[Chemical 1] (In the formula, n is 0 or 1, G is a divalent organic group, R ¹ is an optionally substituted alkyl group having 1 to 8 carbon atoms or a phenyl group, and R ² is substituted. Which is an optionally substituted alkyl group having 1 to 8 carbon atoms or a phenyl group.) More specifically, G, which is a divalent organic group, is an optionally substituted saturated or unsaturated group having 1 to 6 carbon atoms. It is a saturated alkylene group or an arylene group.

As such a reaction reagent, for example, 3
There are (ω-isocyanatoalkyl) triethoxysilanes, including-(triethoxysilyl) propylisocyanate, which are commercially available.

The amount of the reaction reagent used is 0.1 mol equivalent to 2.0 mol equivalent, preferably 0.2 mol equivalent to 1.8 mol equivalent to 1 mol equivalent of active hydrogen of the dendrimer. Amount, more preferably 1.0 molar equivalent to 1.5 molar equivalents.

(Solvent, etc.) In addition, the reaction in the present invention can be carried out without using a solvent if the dendrimer as a raw material and the reaction reagent are liquids each having a low viscosity. However, in order to maintain the fluidity of the reaction solution,
Preferably, a proper amount of solvent is used. As the solvent, a solvent that does not inhibit the reaction or promote the side reaction is selected. For example, pentane, cyclopentane, hexane, cyclohexane, heptane, octane, benzene, toluene, xylene, dichloromethane, dichloroethane, chloroform, carbon tetrachloride, dichlorobenzene, acetone, methyl ethyl ketone, ethyl acetate, butyl acetate, diethyl ether, tetrahydrofuran, dioxane. , Ethyl cellosolve acetate, diethylene glycol dimethyl ether, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dimethyl sulfone and the like. You may use these in combination of 2 or more types.

(Other Reaction Conditions) The dendrimer, the reaction reagent, the solvent and the like for producing the "dendrimer having an alkoxysilyl group at the branched end" (silylated dendrimer) of the present invention may be added in any order. The reaction is usually carried out at atmospheric pressure with stirring and mixing. In order to suppress side reactions derived from water, the reaction is preferably carried out under a dry air flow, more preferably under dry nitrogen or dry argon. Also,
It may be carried out under pressure or under reduced pressure to accelerate the reaction.

The reaction temperature is -1 in order to allow the reaction to proceed.
00 ° C to 200 ° C (or solvent reflux temperature), preferably-
The temperature is 30 ° C to 150 ° C, more preferably 0 ° C to 100 ° C. If the temperature exceeds 200 ° C, by-products are likely to occur. 0 ℃ ~
If it is in the range of 100 ° C., it can be heated or cooled with water, is less dangerous, can be synthesized by general-purpose equipment, and energy consumption can be reduced.

Considering a sufficient reaction rate, the reaction time is
It is usually 5 minutes to 72 hours, preferably 1 hour to 24 hours, and more preferably 1 hour to 12 hours. If it is less than 5 minutes, it may not react sufficiently.

The end point of the reaction is gas chromatography,
It can be confirmed by high performance liquid chromatography, thin phase chromatography, nuclear magnetic resonance spectrum, infrared absorption spectrum and the like.

After completion of the reaction, the product may be taken out from the reaction vessel and used as it is as a target dendrimer (crude product). By further removing the solvent and the reaction reagent used in the reaction by a purification operation such as distillation or recrystallization, a silylated dendrimer with higher purity can be obtained. The reaction solution is dispersed in water and extracted with an organic solvent such as pentane, hexane, heptane, benzene, toluene, diethyl ether, ethyl acetate, dichloromethane, dichloroethane, chloroform, butanol, and the extract solution is sodium sulfate, magnesium sulfate, calcium chloride. It is also possible to obtain a highly pure silylated dendrimer by removing the reaction reagent by concentrating the solvent and the like by distilling it off after drying with etc. The silylated dendrimer obtained by the above operation may be further purified by combining distillation, recrystallization, reprecipitation, column chromatography, gel permeation chromatography (GPC), preparative HPLC and the like.

The terminal portion (alkoxysilyl group) of the silylated dendrimer thus obtained has the following formula (2).
It is represented by [Chemical Formula 2].

[Chemical 2] (In the formula, n, G, R ¹ and R ² have the same meaning as in the formula (1)).

It should be noted that, without carrying out the step (a),
Known silylated dendrimers may be used. As such a silylated dendrimer, JP-A 2000-4 is known.
Carbosiloxane dendrimers described in Japanese Patent No. 4579 and Macromolecules, 33, 356.
6-3578 (2000) or JP-A-11-26383.
There are poly (amidoamine-organosilicon) dendrimers described in Japanese Patent No.

<Step (b)> The alkoxysilane at the branched end (surface layer) of the silylated dendrimer is hydrolyzed,
Step of dehydration condensation to obtain “silica ultrafine particle precursor”: First, the silylated dendrimer is dissolved in a solvent, water and a catalyst are added, and the terminal alkoxysilyl group of the silylated dendrimer is hydrolyzed. The solvent (solvent, water, catalyst, etc.) is used to maintain the fluidity of the reaction solution or to prevent aggregation of silylated dendrimers. As the solvent, a solvent having a high solubility of the silylated dendrimer and having no influence on the hydrolysis / condensation of the alkoxysilyl group is selected. For example, pentane, cyclopentane, hexane, cyclohexane, heptane,
Octane, benzene, toluene, xylene, dichloromethane, dichloroethane, chloroform, carbon tetrachloride, dichlorobenzene, acetone, methyl ethyl ketone, ethyl acetate, butyl acetate, diethyl ether, tetrahydrofuran, dioxane, ethyl cellosolve acetate, diethylene glycol dimethyl ether, N, N-dimethyl. Formamide, N, N-dimethylacetamide, N-
Examples include methylpyrrolidone, dimethyl sulfoxide, dimethyl sulfone and propylene glycol monopropyl ether. You may use these in combination of 2 or more types.

The water used is water from which impurities have been removed (distilled water,
Ion exchange water, ultrafiltered water, etc.) is used. If the amount used is too small, hydrolysis will not proceed, and if it is too large, side reactions will easily occur in the subsequent manufacturing process. About 1 to 5 mol is preferable for 1 mol of the silyl group of the silylated dendrimer.

The catalyst is used for hydrolysis of alkoxysilanes.
Used to accelerate the condensation reaction (generally slower than other metal alkoxide reactions). As the catalyst, bases such as aqueous ammonia, amines, alkali metal hydroxides and alkali metal carbonates, or hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, maleic acid, p-toluenesulfonic acid,
Acids such as trifluoromethanesulfonic acid are used. The amount of the catalyst used is preferably about 0.01 to 1 mol with respect to 1 mol of the silyl group of the silylated dendrimer. It should be noted that it is preferable to add a mixed solution of water and the catalyst, rather than adding the water and the catalyst separately. This is because side reactions such as aggregation can be suppressed during the hydrolysis / condensation reaction of the terminal alkoxysilyl group.

The reaction is usually carried out at atmospheric pressure with stirring and mixing. Alternatively, it may be performed under nitrogen or argon. Further, it may be carried out under pressure or under reduced pressure to accelerate the reaction. The reaction temperature is −100 ° C. to 200 ° C. for the reaction to proceed.
C (or solvent reflux temperature), preferably -30 to 150
C., more preferably 0 to 100.degree. If it exceeds 200 ° C., a polydisperse type particle size distribution tends to be obtained. 0 ℃ ~ 10
Heating or cooling with water is possible within the range of 0 ° C,
It is less dangerous, can be synthesized with general-purpose equipment, and can reduce energy consumption.

Considering a sufficient reaction rate, the reaction time is
It is usually 5 minutes to 72 hours, preferably 1 hour to 24 hours, and more preferably 1 hour to 12 hours. If it is less than 5 minutes, it may not react sufficiently.

The end point of the reaction is gas chromatography,
It can be confirmed by high performance liquid chromatography, thin phase chromatography, nuclear magnetic resonance spectrum, infrared absorption spectrum or the like.

By the above operation, a "silica ultrafine particle precursor" having a silica thin film derived from an alkoxysilyl group formed on the surface side of the dendrimer initially used as a nucleus is obtained. In this case, the reaction liquid is the ultrafine particle precursor, a solvent,
A mixture containing a catalyst and excess water. The ultrafine particle precursor can be directly transferred to the next step without being separated.

<Step (c)>"Silica ultrafine particle precursor"
Then, a step of newly adding and reacting the alkoxysilane and laminating a silica layer on the surface layer thereof: subsequently, further laminating a silica layer on the surface of the silica ultrafine particle precursor. First,
An alkoxysilane represented by the formula (3) [Chemical Formula 3] is added to the above-mentioned mixed liquid.

[0033]

[Chemical 3] (In the formula, R ³ , R ⁴ , R ⁵ , and R ⁶ are each independently
It is selected from a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a phenyl group, a vinyl group and an alkoxyl group having 1 to 6 carbon atoms, and at least one is an alkoxyl group having 1 to 6 carbon atoms. )

Specific examples of such alkoxysilanes include tetramethoxysilane, trimethoxymethylsilane, dimethoxydimethylsilane, methoxytrimethylsilane, tetraethoxysilane, triethoxymethylsilane, diethoxydimethylsilane and ethoxytrimethylsilane. There is. You may use these in combination of 2 or more types.

If the amount of alkoxysilane used is too small, silica will not be laminated on the surface of the silica ultrafine particle precursor,
If the amount is too large, the excess alkoxysilane alone forms particles, and a polydispersed particle size distribution tends to occur. About 1 to 10 mol is preferable for 1 mol of the silyl group of the silylated dendrimer.

While adding the alkoxysilane, stirring, and if necessary diluting with a solvent (selected from the above),
Stir for about 0.5 to 6 hours. The temperature of the reaction liquid at the time of stirring is generally -100 ° C to 200 ° C (or solvent reflux temperature).
Is.

Next, water and a catalyst (preferably a mixed solution of water and a catalyst) are added to carry out hydrolysis / condensation of alkoxysilane on the surface of the silica ultrafine particle precursor. The amount of water added is preferably about 1 to 5 mol per mol of alkoxysilane, and the amount of the catalyst is preferably about 0.01 to 1 mol per mol of alkoxysilane.

In the hydrolysis / condensation of this alkoxysilane, the reaction conditions such as water, catalyst, reaction temperature and reaction time are the same as the reaction conditions of "hydrolysis and condensation of silylated dendrimer" in the above step (b). Is.

In order to obtain ultrafine silica particles having a larger particle size, addition of alkoxysilane and its hydrolysis / condensation may be repeated.

(Termination) In order to prevent the obtained ultrafine silica particles from aggregating with each other, preferably, silanol residues present on the particle surface are protected with chlorosilane (termination). Examples of such chlorosilanes include trimethylchlorosilane, triethylchlorosilane, and tert-butyldimethylchlorosilane. The addition amount of chlorosilane is preferably about 10 to 1000 parts by weight with respect to the total amount of alkoxysilane (100 parts by weight). The time required for termination is about 0.5 to 24 hours. In addition, the temperature of the reaction liquid at the time of termination is approximately −100 ° C. to 20 ° C.
0 ° C. (or solvent reflux temperature).

Thereafter, the particle size distribution of the ultrafine silica particles is measured by a dynamic light scattering measuring method or a photon correlation spectroscopy (Coulter counter). Ultrafine silica particles of uniform particle size are obtained. When the silica ultrafine particles are applied to a substrate or the like, they may be applied while containing the solvent. Alternatively, the solvent may be gently distilled off to give a dry powder, and then the powder may be applied (after resuspending with a suitable solvent).

The ultrafine silica particles thus obtained show a monodisperse particle size distribution, and the average particle size is 1 nm.
To 100 nm (preferably 5 nm to 100 nm, more preferably 5 nm to 50 nm). In addition, the particle size distribution of the ultrafine silica particles is generally a normal distribution,
The variation is about 5 nm or less when the average particle diameter is 10 nm (50% or less if expressed by a coefficient of variation).

[0043]

EXAMPLES Example 1 (I) Synthesis of terminal silylated dendrimer (step a) Fourth generation polyamidoamine type dendrimer (PAMAM-OH, molecular weight 14,279, containing 64 terminal hydroxyl groups) having the structural formula shown in FIG. (Purchased from Aldrich)) 1
To a solution of 08 mg (7.6 × 10 ⁻³ mmol) in dimethyl sulfoxide (5 ml), 143 mg (0.58 m) of 3- (triethoxysilyl) propyl isocyanate
A dimethylsulfoxide (1 ml) solution of (mol) was added, and the mixture was stirred at 50 ° C. for 2 hours. After completion of the reaction, the solvent and a small excess of 3- (triethoxysilyl) propyl isocyanate were distilled off under reduced pressure to obtain 225 mg of terminal silylated dendrimer (yield: 100%).

IR: 3400 (br), 2970
(W), 1650 (s), 1570 (s), 1260
(W), 1080 (m) , 950 (w) cm -1 1 H NMR: δ0.41-0.61 (m, 128
H), 1.14 (t, J = 7.0 Hz, 576H),
1.30-1.55 (m, 128H), 1.90-3.
25 (m, 996H), 3.31 to 3.47 (br, 1
28H), 3.73 (q, J = 7.0Hz, 384)
H), 5.76-6.10 + 7.65-8.10 (b
r, 188H) ¹³ C NMR: δ7,18,23,33,37,4
2,50,52,58,158,171,172

The particle size distribution (dimethyl sulfoxide solution) of the obtained terminal silylated dendrimer was measured by Coulter.
It was measured by Counter N-4-SD (Fig. 4). The particle size distribution was monodisperse, and the average particle size was 7.8 nm.

(II) Preparation of ultrafine silica particles (step b)
And step c) 376 mg (1.3%) of the obtained terminal silylated dendrimer.
× 10 ^-2 mmol, containing about 0.8 mmol silyl group)
32m of water in dimethylsulfoxide (2.5ml) solution of
g (1.8 mmol) and aqueous ammonia (29%) 7 mg
A mixed solution of (0.12 mmol) was added, and the mixture was stirred at 50 ° C. for 3 hours. Next, 304 mg of tetramethoxysilane (2.
0 mmol) was added and the mixture was stirred at 50 ° C. for 1 hour. 81 ml of water after diluting the reaction solution with 2 ml of dimethyl sulfoxide
g (4.5 mmol) and ammonia water (29%) 18 m
A mixed solution of g (0.3 mmol) was added, and the mixture was stirred at 50 ° C. for 3 hours. Furthermore, 304 mg of tetramethoxysilane
(2.0 mmol) was added, and the mixture was stirred at 50 ° C. for 1 hr.
The reaction solution was diluted with 2 ml of dimethyl sulfoxide, and then 81 mg (4.5 mmol) of water and aqueous ammonia (29%)
A mixed solution of 18 mg (0.3 mmol) was added, and the mixture was stirred at 50 ° C. for 3 hours. After the reaction, chlorotrimethylsilane 17
4 mg (1.6 mmol) was added, and the mixture was stirred at room temperature overnight.

The particle size distribution (dimethylsulfoxide solution) of the obtained silica ultrafine particles was measured by Coulter Count.
er N-4-SD (FIG. 5). The particle size distribution is monodisperse, the average particle size is 11.7 nm, and the standard deviation is 4.8.
was nm.

[0048]

Industrial Applicability With the remarkable development and progress of electronics technology in recent years, application of ultrafine particles to the field of electronics technology (for example, insulating materials for nano-sized devices related to electronics, optical waveguides, photonic crystals, light emitting materials, color Applications to filters and solar cell materials) and expectations are increasing. The silica ultrafine particles of the present invention are monodisperse type and uniform particles, and such ultrafine particles have not been obtained hitherto. The silica ultrafine particles of the present invention are insulating materials for nano-sized devices related to electronics, optical waveguides, photonic crystals, light emitting materials,
It can be suitably used for color filters and solar cell materials, and can also be suitably used for high performance catalysts, abrasives and the like. By the production method of the present invention, it is possible to easily produce monodisperse type, uniform-particle silica ultrafine particles.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a dendrimer (third generation dendrimer).

FIG. 2 is an outline of a method for producing ultrafine silica particles.

FIG. 3 is an example of a dendrimer used (PAMAM-O
H, molecular weight 14,279).

FIG. 4 is a particle size distribution of the obtained silylated dendrimer.

FIG. 5 is a particle size distribution of the obtained ultrafine silica particles.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kenzo Susa             48 Wadai, Tsukuba, Ibaraki Prefecture Hitachi Chemical Co., Ltd.             Company Research Institute F term (reference) 4G072 AA25 BB05 DD06 DD07 GG01                       GG03 HH30 MM01 RR05 TT01                       TT02 UU01 UU15 UU30                 4J031 CA07 CD26

Claims (3)

[Claims] 1. A method for producing ultrafine silica particles comprising the following steps (b) and (c): (b) Branching of a "dendrimer having an alkoxysilyl group at a branched end" (silylated dendrimer) A step of hydrolyzing the terminal alkoxysilane and dehydrating and condensing it to obtain a silica ultrafine particle precursor; (c) A step of adding a new alkoxysilane to the precursor and reacting it, and laminating a silica layer on the surface layer. 2. The production method according to claim 1, wherein a silylated dendrimer obtained by the following step (a) is used: (a) a dendrimer having a functional group capable of reacting with an isocyanate at a branched end, and an isocyanate. A step of reacting with an alkoxysilane having a nato group (reaction reagent) to form a "dendrimer having an alkoxysilyl group at a branched end" (silylated dendrimer); 3. Ultrafine silica particles obtained by the method according to claim 1 or 2, wherein the ultrafine particles have a monodisperse particle size distribution and the average particle size is in the range of 1 nm to 100 nm. Ultra fine particles.