Abstract: The monomer 5-methyl-1,4-hexadiene (5-MHD), uncontaminated with 4-methyl-1,4-hexadiene (4-MHD), or other hydrocarbons, is obtained by reacting 1-chloro-2-methylpropene with allylmagnesium bromide in equimolar amounts under gentle reflux at atmospheric pressure in an organic solvent medium (diethyl ether) and in the presence of a catalytic amount of a dichloro-[1,2-bis(dimethylphosphino)ethane]nickel (II) in an inert atmosphere. The allyl magnesium bromide is added dropwise with stirring with a mixture of 1-chloro-2-methylpropene and catalyst in the solvent at room temperature. The solvent is separated from the product by distillation, giving substantially pure product. Absence of other hydrocarbons in either the crude product or the pure product may be shown by .sup.1 H NMR, .sup.13 C NMR and GC analysis. Other nonconjugated diolefins can be prepared in an analogous manner using an alkenyl halide and an alkenyl Grignard reagent which upon cross coupling will give the desired nonconjugated diene.