Components of catalysts for preparation of elastomeric copolymers of ethylene with alpha-olefins, or of ethylene with alpha-olefins and at least one diene, comprising:(a) a magnesium compound selected from the group consisting of anhydrous magnesium halides and compounds having general formula XMgOR, or XMgR, wherein X is a halogen and R is a C1 -C20 alkyl, cycloalkyl or aryl radical; (b) a halo-a