Dimerization process

A process for the dimerisation of acrylonitrile to obtain a product which is predominantly 1,4-dicyano-butenes, comprising:

• dissolving acrylonitrile in an organic solvent capable of donating protons but substantially unreactive with respect to acrylonitrile and organic phosphorus (III) compounds, and
• contacting acrylonitrile with an organic phosphorus (III) compound having the formula:
• ##STR5## where R.sub.1 is hydrocarbyl as such or substituted with halogen, cyanide or alkoxy, R.sub.2 is alkoxy or cycloalkoxy, R.sub.3 is hydrocarbyl as such or substituted with halogen, cyanide or alkoxy, alkoxy or cycloalkoxy, and R.sub.4 is divalent hydrocarbyl as such or substituted with halogen, cyanide or alkoxy, or hydrocarbyloxy, the carbon content of each of the groups R.sub.1 to R.sub.4 being from 1 to 10, at a temperature from 0.degree. to 120.degree. C., the concentrations of acrylonitrile and said organic phosphorus (III) compound being in the ranges 5 to 75% and 0.01% to 5% by volume, respectively, and the acrylonitrile and solvent being essentially dry and free of phenolic acrylonitrile stabilisers, the product thus obtained containing at least 58% 1,4-dicyanobutenes.