A method of preparing amorphous copolymers of ethylene and at least one norbornene (NB)-type comonomer is disclosed. These polymers may be random or alternating depending on the choice of catalyst and/or the relative ratio of the monomers used. This method comprises polymerizing said monomers in a diluent or in bulk in the presence of a neutral nickel catalyst which may be represented by the formula ##STR1## wherein Y may be a saturated or unsaturated hydrocarbyl chain, X may be oxygen or sulfur, E may be phosphorus, arsenic, antimony, oxygen or nitrogen, R and R' independently each is hydrogen or a hydrocarbyl group, L is a ligand containing a heteroatom P, N or O or L and R together with L may form part of a chelating structure in which case L is an ethylenic double bond.