Method of polymerization of conjugated diolefins using iron catalysts and nitrogen ligands

A process for polymerizing conjugated diolefinic monomers containing from 4 to about 12 carbon atoms by bringing said monomers into contact with a catalytically effective amount of a catalyst consisting essentially of (1) an iron containing compound, (2) an organometallic reducing agent from Groups I and III of the Periodic Table, and (3) a nitrogen-containing ligand characterized in that said ligand has at least two functional groups or atoms wherein the first functional group is a cyano group (--C.tbd.N) and the second functional group is selected from the group consisting of (a) cyano group, (b) azo group, (c) imine group, (d) vinylene group, (e) mercapto group, (f) hydroxyl group, and (g) thio group, and there must not be more than two additional carbon atoms separating the first functional group and the second functional group.