Abstract Our research directed to obtaining new rubbers based on the copolymerization of ethylene with propylene started some years ago. We soon succeeded in establishing that it was possible to prepare copolymers of these two monomers showing interesting elastomeric properties. From the beginning, the problem arose of how to vulcanize these copolymers: while certain interesting methods for direct vulcanization were investigated and identified, we also investigated the possibility of terpolymers containing unsaturation and therefore vulcanizable with the ingredients normally employed in the rubber industry, particularly for the low unsaturated rubbers. The problem involved some difficulties, due to the following three reasons. (1) In order to maintain the peculiar characteristics of resistance to aging and chemicals, typical of ethylene-propylene copolymers, it was necessary to obtain terpolymers in which the unsaturation was present in a very small amount. (2) To obtain valuable properties in the vulcanizates, the unsaturation had to be well distributed in all the macromolecules. (3) In order to avoid branching or crosslinks, the residual double bond of the diene entering the polymer chain must not be able to react further during polymerization. Our research led to a number of monomers which may copolymerize with ethylene and propylene and which, under suitable conditions, gave terpolymers having a sufficiently homogeneous composition. Investigation of the behavior, in anionic coordinated polymerization of hydrocarbons containing internal double bonds led to an interesting solution to the problem of producing terpolymers.