Patent GB-632954-A (GB)
Title: Improvements in and relating to polysiloxanes
IPC: C08G77/12
Filed: 1947-03-20
Abstract: Polysiloxanes of the general formula <FORM:0632954/IV (a)/1> where R represents a monovalent hydrocarbon radical and R1 a lower alkyl radical (i.e. methyl, ethyl, propyl and butyl) and n is an integer which is at least 1 and not more than 4 are obtained by mixing together in the presence of concentrated sulphuric acid a disiloxane of the formula <FORM:0632954/IV (a)/2> and the oily product of hydrolysis of a halogenosilane having the formula R1SiHX2, where X is chlorine or bromine, separating the acid phase from the polymer phase, washing the latter and distilling it to separate the polysiloxane(s).  The hydrocarbon radical R may be aliphatic, cycloaliphatic, aryl, aliphatic-substituted aryl or aryl-substituted aliphatic.  The polysiloxanes of the present invention differ from those previously described, e.g. in Specification 586,956 in that instead of two monovalent radicals being attached directedly to each Si atom intermediate of the terminal Si atoms there are attached to each intermediate Si atom one lower alkyl radical and one hydrogen atom.  In consequence, while these siloxanes are stable to non-oxidising acids, e.g. the HCl produced in the reaction by the hydrolysis of the halogenosilane, with alkaline reagents the silicon-hydrogen bonds are cleaved to yield silanols which may be dehydrated to give polysiloxanes.  The Si-H bonds of different compounds show different degrees of activity, thus while the siloxane of formula (CH3)3SiOSiH(CH3)OSi(CH3)3 i not hydrolysed by treatment with concentrated potassium hydroxide, concentrated ammonium hydroxide, a weaker base, displaces hydrogen from the next higher homologue (octamethyltetrasiloxane).  The mixed reactants may be dissolved in solvents, e.g. iso or normal hexanes, pentanes, octanes. 17.4 mols. methyldichlorosilane are added slowly to a vigourously stirred solution of hexamethyldichlorosilane (28.6 mols.) in mixed hexanes in a of slurry ice which is constantly replenished.  The oil layer was washed, filtered and reacted vigorously with 95 per cent sulphuric acid for 2 hours.  The acid layer was separated and the polymer phase washed and distilled, careful fractionation yielding 1390 gs. heptamethyltrisiloxane, 328 gs. octamethyl tetrasiloxane and 2370 gs. hexamethyldisiloxane, the latter with the distillation residue being again reacted with sulphuric acid to give further yields of heptamethyltrisiloxane, and octamethyl tetrasiloxane together with heptamethyl - 3 - trimethylsiloxytrisiloxane, nonamethylpentasiloxane and decamethylhexasiloxane.  Lists of siloxanes obtainable according to the invention are given in the Specification.  Another method of obtaining the siloxanes of formula (III) comprises hydrolysing a mixture of organohalogenosilanes, one member of which has the formula (R)3SiX and the other member the formula R1SiHX2.  The mixture is hydrolysed and the product of the cohydrolysis separated from the aqueous halogen acid, washed free of acid and then agitated with the concentrated sulphuric acid with subsequent washing and distillation as described above.  The Specification as open to inspection under Sect. 91 described the condensation in presence of concentrated hydrochloric acid. This subject-matter does not appear in the Specification as accepted.ALSO:Polysiloxanes of the general formula (I) <FORM:0632954/IV (b)/1> where R represents a monovalent hydrocarbon radical and R1 a lower alkyl radical (i.e. methyl, ethyl, propyl butyl) and n is an integer which is at least 1 and not more than 4, are obtained by mixing together in the presence of concentrated sulphuric acid a disiloxane of the formula (II) <FORM:0632954/IV (b)/2> and the oily product of hydrolysis of a halogensilane having the formula (III) R1SiH2, where X is Cl or Br, separating the acid phase from the polymer phase, washing the latter and distilling it to separate the polysiloxanes.  The radical R may be aliphatic, cycloaliphaticaryl, or arylaliphatic.  Disiloxanes of formula (II) are hexamethyl, hexaethyl, hexaphenyl, 1,1,1-trimethyl, 3,3,3-triethyl, hexabenzyl disiloxanes, and are prepared by replacing 3 of the 4 Cl atoms in SiCl4 by monovalent hydrocarbon radicals by employing the appropriate Grignard reagent to obtain the groupings desired about each Si atom in the siloxane.  The tri-mono-chlorosilane is then hydrolised and condensed to the desired disiloxane.  If an unsymmetrically-substituted disiloxane is required, it is obtained by carrying out the process in two stages, using in each stage the appropriate Grignard reagent.  The halogenosilanes of formula (III) are obtained by the interaction of a lower alkyl magnesium halide, e.g. ethyl magnesium chloride or bromide, with trichlorosilane, approximately one mole, of the appropriate Grignard reagent dissolved in ether being used for each mole. of the trihalogensilane.  Examples of the trihalogenosilanes are methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, dichloro or dibromosilanes.