Preparation of high molecular weight organo-polysiloxane

Abstract: The process for preparing a high molecular weight organopolysiloxane comprises subjecting a hydroxyl-containing organosiloxane, as a starting material, to a dehydration condensation reaction in the presence of an acidic compound having an acid dissociation constant at 20°C of not more than 3. The process is advantageous over the conventional methods in that compounds having toxicity or the like are neither used nor by-produced. The process also has the merit of easy control of the molecular weight of the intended organopolysiloxane.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for preparing a high molecular weight organopolysiloxane by use of a hydroxyl-containing organosiloxane as a starting material. 2. Description of the Prior Art Methods of lengthening the molecular chain of hydroxyl-containing organosiloxanes by use of various silane compounds have been known. For instance, U.S. Pat. No. 3,776,934 discloses a method in which methylvinylbis(N-methylacetamido)silane is reacted with an organopolysiloxane blocked by hydroxyl group at both ends thereof to form a silicone elastomer composition which is curable at room temperature. Also, Japanese Patent Publication No. 55-49623 (1980) discloses a method of obtaining a hydroxyl-containing organosiloxane with an increased molecular weight by mixing a hydroxyl-containing organosiloxane with methylvinylbis(ε-caprolactamo)silane. Furthermore, Japanese Patent Publication No. 61-12932 (1986) discloses a method in which a hydroxyl-containing organosiloxane, a cyclosiloxane having two vinyloxy groups bonded thereto and a guanidine derivative are mixed together to form a hydroxyl-containing organosiloxane with an increased molecular weight. According to the above methods of the prior art, however, an amide or caprolactam having toxicity or odor is by-produced or there is need to use a cyclosiloxane which is difficult to synthesize. Therefore, the methods of the prior art are not so high in practicability. FR-A-1 275 384 discloses the preparation of organopolysiloxanes in the presence of fluor containing carboxylic acids such as trifluoroacetic acid. SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a process for preparing a high molecular weight organopolysiloxane by which the drawbacks of the prior art as mentioned above are overcome and which is extremely high in practicability. According to this invention, there is provided a process for preparing a high molecular weight organopolysiloxane which comprises subjecting an organosiloxane having at least one Si-bonded hydroxyl group in one molecule thereof to a condensation reaction in the presence of at least one acidic compound selected from the group consisting of in an amount of 0.001 to 10 parts by weight per 100 parts by weight of the organosiloxane. That is, in the process of this invention, a dehydration condensation reaction represented by the following formula: ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H₂O proceeds in the presence of the aforementioned acidic compound as a catalyst, whereby the molecular chain of the organosiloxane is lengthened, resulting in the formation of an organopolysiloxane with an increased molecular weight. According to the process of this invention, the acidic compound used as a catalyst can be removed easily, and it is possible to control, as desired, the viscosity (equivalent or corresponding to molecular weight) of the intended hydroxyl-containing organopolysiloxane by regulating the reaction time or the reaction temperature. In the process of the invention, furthermore, compounds having toxicity or odor are neither used nor by-produced. Therefore, the process is extremely high in practicability. DETAILED DESCRIPTION OF THE INVENTION Hydroxyl-containing organosiloxane As the starting material in this invention, an organosiloxane having at least one Si-bonded hydroxyl group in one molecule thereof is used. The organosiloxane is not particularly limited in structure, so far as it has at least one hydroxyl group as mentioned above and the aforementioned dehydration condensation reaction is performed effectively. For instance, the organosiloxane may be a homopolymer or a copolymer, and may also be a mixture of various polymers. Further, the structural unit of the organosiloxane may be a monoorganosiloxane unit, a diorganosiloxane unit, a triorganosiloxy unit or a combination of these units. Organic groups bonded to silicon atoms may be any of monovalent hydrocarbon groups, monovalent halogenated hydrocarbon groups and monovalent cyanohydrocarbon groups. Generally, the monovalent hydrocarbon groups which can be used include, for example, methyl, ethyl, propyl, hexyl, octadecyl, vinyl, allyl, cyclohexyl, cyclohexenyl, or phenyl, whereas the monovalent halogenated hydrocarbon groups usable include, for example, chloromethyl, 3-chloropropyl, 3,3,3-trifluoropropyl, α,α,α-trifluorotolyl, chlorobenzyl, or bromopropyl, and the monovalent cyanohydrocarbon groups usable include, for example, cyanoethyl or cyanopropyl. Of these groups, particularly preferred for use in this invention are methyl, vinyl, phenyl, and 3,3,3-trifluoropropyl. In this invention, furthermore, it is desirable that the Si-bonded hydroxyl group is present at an end of the molecular chain of the organosiloxane, and it is particularly preferable that the organosiloxane is blocked by hydroxyl group at both ends of the molecular chain thereof, in view of rapid progress of the dehydration condensation reaction. Acidic compound The acidic compound to be mixed with the hydroxylcontaining organosiloxane in this invention has a pKa value at 20°C of not more than 3, and is The parenthesised numerical values represent the pKa at 20°C of the relevant acidic compound. These compounds may be used singly or in combinatin of two or more. As contrasted to the above, the pKa of acetic acid at 20°C is 4.56. If acetic acid with pKa of 4.56 is used, as for instance shown in the example below, the dehydration condensation reaction between the molecules of the hydroxyl-containing organosiloxane does not proceed effectively, and it is difficult to achieve formation of an organopolysiloxane with an increased molecular weight. The acidic compound as mentioned above is used in an amount of 0.001 to 10 parts by weight, preferably 0.1 to 1.0 part by weight, per 100 parts by weight of the hydroxyl-containing organosiloxane. If the amount of the acidic compound used is less than 0.001 part by weight, the intended dehydration condensation reaction progresses little. On the other hand, use of the acidic compound in an amount of more than 10 parts by weight does not produces any additional effect and is, rather, disadvantageous on an economical basis. Dehydration condensation reaction In this invention, the dehydration condensation reaction between the molecules of the hydroxyl-containing organosiloxane is caused to proceed rapidly by heating or maintaining the mixture of the organosiloxane with the acidic compound to or at a temperature of 20 to 100°C, preferably 20 to 80°C. If the temperature is below 20°C, the reaction rate is so low that the practicability is spoiled. If the temperature is above 100°C, on the other hand, a siloxane chain cutting reaction occurs with the undesirable result of formation of lower molecular weight siloxane. The reaction generally takes place without addition of solvent, but a solvent may be used, as required. However, most uses of the organopolysiloxane obtained according to the process of this invention do not require the use of solvent, and, accordingly, it is desirable to use no solvent in the process, thereby avoiding waste of energy and loss of time in removal of solvent. The reaction is carried out for a time ranging generally from several hours to one day, whereby an organopolysiloxane with the desired viscosity (molecular weight) can be obtained. The control of the reaction time, or the termination of the reaction, can be achieved easily by various methods. For example, where the starting material is a low boiling point compound, this purpose can be accomplished by distilling off the compound under a reduced pressure. Where the starting material is a high boiling point compound which is difficult to distill off under a reduced pressure, the purpose can be accomplished by adding a diluted alkali such as sodium carbonate, etc. to the reaction system, and agitating the reaction system to neutralize the acidic compound, followed by filtration. After the reaction is finished, distillation under a reduced pressure is carried out to remove the acidic compound and by-produced water, whereby an organopolysiloxane with an increased molecular weight is obtained. EXAMPLES Examples 1 to 4 (Examples 1 and 2 are for comparison). Admixtures of 100.0 g of a dimethylpolysiloxane blocked by hydroxyl group at both ends of its molecular chain and having a viscosity at 25°C of 760 cSt (mm²s⁻¹) with 0.0015 mol each of the respective acidic compounds set forth in Table 1 were left to stand at 20°C. As a result, the respective reaction products exhibited variations in viscosity with time, as shown in Table 1. The values of viscosity shown in Table 1, and in the subsequent tables, are all in centistokes (cSt) (mm²s⁻¹). Examples 5 to 8 Admixtures of 100.0 g of a dimethylpolysiloxane blocked by hydroxyl group at both ends of its molecular chain and having a viscosity at 25°C of 760 cSt (mm²s⁻¹) with 0.1 g, 0.5 g, 1.0 g and 3.0 g, respectively, of a fluorine-containing carboxylic acid represented by the formula: were left to stand at 20°C. As a result, the reaction products exhibited variations in viscosity with time, as shown in Table 2. Examples 9 to 11 Admixtures of 100.0 g each of dimethylorganopolysiloxanes blocked by hydroxyl group at both ends of the molecular chain thereof and having respective viscosities at 25°C as shown in Table 3 with 0.5 g of a fluorine-containing carboxylic acid represented by the formula: were left to stand at 20°C. Consequently, the reaction products exhibited variations in viscosity with time, as shown in Table 3. Examples 12 to 15, for Comparison Admixtures of 100.0 g of a dimethylpolysiloxane blocked by hydroxyl group at both ends of its molecular chain and having a viscosity at 25°C of 760 cSt (mm²s⁻¹) with 0.0015 mol each of the respective acidic compounds set forth in Table 4 were left to stand at 25°C. As a result, the respective reaction products exhibited variations in viscosity with time, as shown in Table 4.

Claims: A process for preparing a high molecular weight organopolysiloxane, which comprises subjecting an organosiloxane having at least one Si-bonded hydroxyl group in one molecule thereof to a condensation reaction in the presence of at least one acidic compound selected from the group consisting of in an amount of 0.001 to 10 parts by weight per 100 parts by weight of the organosiloxane. A process according to claim 1, wherein the acidic compound is used in an amount of 0.1 to 1.0 part by weight per 100 parts by weight of the hydroxyl-containing organosiloxane. A process according to claim 1 or 2, wherein the condensation reaction is carried out at a temperature of 20 to 100°C. A process according to claim 3, wherein the condensation reaction is stopped by addition of diluted alkali to the reaction system.

Inventors: MATSUDA TAKASHI, SATO SHINICHI, TAKAGO TOSHIO, MATSUDA, TAKASHI, SATO, SHINICHI, TAKAGO, TOSHIO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1990
Main classification: C08G77
IPC codes: C08G77