An optical brightening preparation in the form either of a dried powder or of a dispersion in water or in an organic solvent comprises a solid polymeric carrier material capable of forming fibres and having a particle size of less than 50m , and fixed on the carrier material 0.01-5% by weight of an optical brightening substance having at least one azole ring. The term "fixed" implies that the brightening agent cannot be removed from the fibre-forming material with water. Such preparations are used for the optical brightening of cellulosic materials, e.g. paper pulp, paper, cardboard or papier mach<\>ae; spinning compositions of viscose or cellulose triacetate; nitrocellulose lacquers; or films of acetyl- or nitro-cellulose. Dyeing or pigmenting, e.g. with titanium dioxide, cadmium yellow or azo dyes, or bleaching of the cellulosic materials, may be carried out at the same time. The fixation is carried out in an aqueous bath or in organic solution, the polymeric carrier being already in a finely divided form. The final state of subdivision, preferably of 1-5m , is achieved by wet grinding in a colloid mill, with or without the aid of dispersing agents such as the condensation product of octadecyl alcohol with 30-40 mols. of ethylene oxide, the condensation product of p-nonyl phenol with 9 mols. of ethylene oxide, sodium dinaphthyl methane sulphonate or sodium dodecyl benzene sulphonate. The earlier state of subdivision may be achieved either (a) by dissolving the polymer in conc. sulphuric acid or dimethyl formamide and reprecipitating, or (b) by dissolving the polymer (especially nylon or acetyl cellulose) at high temperature and pressure in aqueous (m)ethanol, acetone, or methylene chloride/ethanol, and precipitating either by cooling or by introducing the solution into water or ethanol, or (c) by grinding the polymer into a paste. In examples: (1) polyethylene terephthalate is treated in a water bath with formic acid and a paste of <FORM:0862645/IV (a)/1> and the condensation product of octadecyl alcohol with 30-40 mols. of ethylene oxide. (2) The ground product of Example (1) is further ground with nitrocellulose, butanol and butyl acetate. (3) Polyethylene terephthalate is treated in a bleaching water bath with sodium chlorite, formic acid and a paste of <FORM:0862645/IV (a)/2> with the condensation product of octadecyl <FORM:0862645/IV (a)/3> or else with aqueous ammonia and the brightener of Example (6). In the subsequent examples, the products of Examples (1)-(7) which have been wet ground with one of the dispersing agents mentioned above are used as follows: (8) The products of (1) or (3) are stirred into an a -cellulose viscose spinning mass. (9) The products of (1), (3) or (7) are mixed with chemically bleached sulphite cellulose, resin size and aluminium sulphate, ground and made into paper. (10) An aqueous mixture of the products of (1), (5) or (6), resin size, aluminium sulphate and starch is used to treat a paper web produced alcohol and 30-40 mols. of ethylene oxide. (4) Polyacrylonitrile is treated in a water bath with sodium chlorite, oxalic acid and hydrogen peroxide; the filtrate is then treated in a further water bath with sodium bisulphite and oxalic acid; the resultant bleached powder is dissolved in dimethyl formamide with <FORM:0862645/IV (a)/4> in a kneading apparatus and water is added, the resultant granulate being freed of solvent, dried and ground. (5) Polyacrylonitrile is treated in a water bath with sodium chlorite, oxalic acid and hydrogen peroxide, and there is added <FORM:0862645/IV (a)/5> dissolved in ethanol with the aid of caustic soda; the resultant ground paste can be spray dried. (6) A solution of cellulose diacetate in acetone is mixed with a solution of <FORM:0862645/IV (a)/6> in ethanol and is precipitated into water; alternatively, the mixed solutions may be spray-dried. (7) 6,6- or 6-nylon is treated in a water-bath with sodium chlorite and acetic acid: the resultant product is then treated with concentrated formic acid and from chemically bleached sulphite cellulose. (11) A nitrocellulose lacquer is mixed with the product of (2) and coated on to paper. (12) An unbleached viscose crepe is treated on a foulard with an aqueous mixture of the product of (4) or (6) and a paste of the condensation product of 2 mols. of stearic acid with 1 mol. of diethylene triamine in glacial acetic acid and water. (13) An undyed cotton cloth is treated on a foulard with an aqueous mixture of starch and the product of (6) or (7). Specifications 611,510, 712,764, 746,046, 813,093 and 827,745 are referred to. 65 - 96 an average R/Si ratio of from 1 to 3, each R being a hydrogen atom or monovalent hydro carbon or halo-hydrocarbon radical, and con taining at least one SiOH group per molecule, any remaining valencies being satisfied by oxygen atoms, is polymerized by contacting it with a compound of the formula:- <FORM:0862645/IV (a)/7> where R1 is a monovalent aliphatic hydrocarbon radical of 5 to 30 C atoms, R11 is an aliphatic hydrocarbon acyl group of 5 to 30 C atoms and Y is H or CH3, until the desired degree of poly merization is obtained. The reaction, which can be used for making siloxane homo- and co polymers, can be effected at room or elevated temperature, and water is preferably removed from the reaction mixture. Catalysts specified are pentyl-, pentenyl-, undecyl-, undecenyl-, heptadecyl-, heptadecenyl-, C24 H42- and C21H41 -b -aminobutyric acids and lactams, N-caproyl glycine, N-caproyl sarcosine, N-palmityl sar cosine, N-oleyl glycine, N-behenyl glycine and N-linoleyl glycine. In typical examples (3) a liquid hydroxyl-terminated dimethylpolysiloxane of 4,300 cs. viscosity at 25 DEG C. was mixed with a mixture of various b -aminolactams and heated under vacuum at 80 DEG to 160 DEG C. to give a siloxane polymer of 2,800,000 cs. viscosity at 25 DEG C.; (9) a hydroxylated co-polymer of monomethyl-, mono xenyl-, monooctadecyl-, monovinyl-, chloro phenylmethyl- and cyclohexyl-siloxane units was cured by heating with the catalyst of (3) at 150 DEG C.; (10) hexamethyldisiloxane was formed when trimethylsilanol was mixed with <FORM:0862645/IV (a)/8> and allowed to stand at room temperature. Uses: in making high polymers for making silicone rubbers; curing siloxane resins which may be applied as coatings for electrical con ductors or as protective coatings for wood or metal surfaces.ALSO:An optical brightening preparation in the form either of a dried powder or of a dispersion in water or in an organic solvent comprises a solid polymeric carrier material capable of forming fibres and having a particle size of less than 50m , and fixed on the carrier material 0,01-5% by weight of an optical brightening substance having at least one azole ring. The term "fixed" implies that the brightening agent cannot be removed from the fibre-forming material with water. Such preparations are used for the optical brightening of cellulosic materials e.g. paper pulp, paper, cardboard or papier mache. Dyeing or pigmenting, e.g. with titanium dioxide, cadmium yellow or azo dyes, or bleaching of the cellulosic materials, may be carried out at the same time. The fixation is carried out in an aqueous bath or in organic solution, the polymeric carrier being already in a finely divided form. The final state of subdivision, preferably of 1-5m , is achieved by wet grinding in a colloid mill, with or without the aid of dispersing agents such as the condensation product of octadecyl alcohol with 30-40 mols of ethylene oxide, the condensation product of p-nonyl phenol with 9 mols of ethylene oxide, sodium dinaphthyl methane sulphonate or sodium dodecyl benzene sulphonate. The earlier state of subdivision may be achieved either (a) by dissolving the polymer in conc. sulphuric acid or dimethyl formamide and reprecipitating, or (b) by dissolving the polymer (especially nylon or acetyl cellulose) at high temperature and pressure in aqueous (m)ethanol, acetone, or methylene chloride/ethanol, and precipitating either by cooling or by introducing the solution into water or ethanol, or (c) by grinding the polymer into a paste. Examples:-(1) polyethylene terephthalate is treated in a water bath with formic acid and a paste of <FORM:0862645/IV(c)/1> and the condensation product of octadecyl alcohol with 30-40 mols of ethylene oxide. (2) The ground product of Example (1) is further ground with nitrocellulose, butanol and butyl acetate. (3) Polyethylene terephthalate is treated in a bleaching water bath with sodium chlorite, formic acid and a paste of <FORM:0862645/IV(c)/2> with the condensation product of octadecyl alcohol and 30-40 mols of ethylene oxide. (4) Polyacronlonitrile is treated in a water bath with sodium chlorite, oxalic acid and hydrogen peroxide; the filtrate is then treated in a further water bath with sodium bisulphite and oxalic acid; the resultant bleached powder is dissolved in dimethyl formamide with <FORM:0862645/IV(c)/3> in a kneading apparatus and water is added, the resultant granulate being freed of solvent, dried and ground. (5) Polyacrylonitrile is treated in a water bath with sodium chlorite, oxalic acid and hydrogen peroxide, and there is added <FORM:0862645/IV(c)/4> dissolved in ethanol with the aid of caustic soda; the resultant ground paste can be spray dried. (6) A solution of cellulose diacetate in acetone is mixed with a solution of <FORM:0862645/IV(c)/5> in ethanol and is precipitated into water; alternatively, the mixed solutions may be spray-dried. (7) 6, 6- or 6-nylon is treated in a water-bath with sodium chlorite and acetic acid: the resultant product is then treated with concentrated formic acid and <FORM:0862645/IV(c)/6> or else with aqueous ammonia and the brightener of Example (6). In the subsequent examples the products of Examples (1)-(7) which have been wet ground with one of the