Patent GB-653468-A (GB)
Title: Improvements in or relating to the preparation of vinylidene cyanide
Filed: 1948-07-15
Abstract: In the production of vinylidene cyanide by pyrolysing 1 - acetoxy - 1,1 - dicyanoethane (see Group IV (b)), the vinylidene cyanide may be separated from the product mixture by allowing the vinylidene cyanide to polymerize as it forms or by admixing it with water, e.g. by pouring the liquid product into water, and then separating the solid polymer which is then depolymerized by pyrolysis, preferably at 170 DEG  to 250 DEG  C. to obtain the monomer. This method is employed in two of the examples and an example is also given in which the liquid product obtained in the pyrolysis of 1-acetoxy-1,1-dicyano ethane in the presence of zinc chloride is heated to polymerize the monomer. Further heating of the solid polymer at 200 DEG  C. in the presence of phosphorus pentoxide causes it to depolymerize.  The polymeric vinylidene cyanide and its copolymers with other materials polymerizable therewith are stated to be suitable as synthetic rubbers and may be spun into synthetic filaments.ALSO:Vinylidene cyanide is obtained by pyrolysing 1 - acetoxy - 1,1 - dicyanoethane at 400 DEG  to 750 DEG  C. and separating vinylidene cyanide from the reaction mixture.  The pyrolysis may be effected under reduced pressure and in the presence of a metallic chloride, especially a chloride of a metallic element occurring in Group II of the Periodic Table.  Such chlorides include those of beryllium, magnesium, calcium, zinc, strontium, cadmium and barium, zinc chloride being preferred.  The preferred temperature range is from 600 DEG  to 650 DEG  C. and the pyrolysis is preferably effected at from 2 to 50 mm. pressure, but pressures up to and even above atmospheric pressure are operative.  The mixture resulting from the pyrolysis contains unreacted 1-acetoxy-1,1-dicyano ethane, acetic acid and vinylidene cyanide.  The latter is separated from the mixture by distillation or by one of the following methods: (a) the vinylidene cyanide is allowed to polymerize as it forms or the liquid product is poured into water to hasten polymerization. The solid polymer is then separated and the polymerized vinylidene cyanide is then depolymerized by pyrolysis, preferably at 170 DEG  to 250 DEG  C., to obtain monomeric vinylidene cyanide; (b) the vinylidene cyanide is extracted from the liquid mixture by a solvent, preferably one from which the vinylidene cyanide will crystallize, e.g. toluene or similar aromatic hydrocarbon, the mixture being then cooled to allow the vinylidene cyanide to crystallize; (c) the liquid product is treated with a conjugated diolefin such as butadiene or cyclopentadiene which reacts with the vinylidene cyanide to form a solid substituted cyclohexene which is separated and pyrolysed at above 400 DEG  C. to give monomeric vinylidene cyanide.  When these three methods are used a polymerization inhibitor is preferably added and this may be mixed with the 1-acetoxy-1,1-dicyano ethane prior to pyrolysis or placed in the receiver for the liquid pyrolysis product. The polymerization inhibitor should also be preferably present in the receiver when the vinylidene cyanide is separated by distillation or when the polymer or the conjugated diolefin product is pyrolysed. Suitable polymerization inhibitors include antimony pentoxide, concentrated sulphuric acid and especially phosphorus pentoxide and phosphorus pentasulphide.  In examples: 1-acetoxy-1,1-dicyano ethane is pyrolysed at reduced pressure (1) in the presence of zinc chloride, the mixture being vaporized and passed at reduced pressure through a brass tube heated to 560 DEG  C. The product is separated by distillation and is obtained partly in the polymeric form; the example is repeated using beryllium chloride as catalyst and also in the absence of a catalyst; (2) pyrolysis is conducted at 675 DEG  C. using zinc chloride as in (1) and the product is poured into water when it polymerizes.  It is then depolymerized by heating to 200 DEG C.; (3) as in (1) but the brass tube contains brass chain packing and the temperature is 670 DEG  C.; (4) as in (1) except that the pyrolysis is over a red-hot nickel wire at 600 DEG  C., the liquid product is collected in a cold receiver and is then heated when the product polymerizes.  The solid polymer is separated and heated at 200 DEG  C. in the presence of phosphorus pentoxide to form monomeric vinylidene cyanide; (5) pyrolysis is conducted in the presence of zinc chloride and phosphorus pentoxide at 460 DEG  C., the product is then treated with butadiene in a sealed tube at room temperature and the contents then distilled at 450 DEG  C. into a receiver lined with phosphorus pentoxide, the monomer being obtained; (6) as in (2) except that a glass tube filled with ceramic packing is used and the temperature is 510 DEG  C.; (7) as in (3) the product being separated by pouring into water and then distilling to separate acetic acid and unreacted starting material; (8) the product obtained in (3) is distilled to yield an intermediate fraction which is mixed with dry toluene and cooled to -25 DEG  C. to form crystals of the monomer. 1-acetoxy-1,1-dicyano ethane is prepared by the dimerization of acetyl cyanide which can be made from acetic anhydride and sodium cyanide or by the reaction of ketene and hydrogen cyanide.