This invention relates generally to polymers suitable for making elastomeric films, membranes or sheets. More specifically, this invention is directed to polymers of the ethylene, alpha-olefin, diene terpolymer type that are reactor blends of two components, an amorphous major component and semicrystalline minor component, wherein the viscosity of the major component is less than one fourth the viscosity of minor component. These polymers are particularly suitable for elastomeric membranes such as roofing membranes.