The 2-phosphinophenolate nickel catalyzed oligo- or polymerization of ethylene in the presence of olefins usually leads to improved conversion or copolymerization. The outcome depends strongly on the nature of the P-substituents and olefins. The diphenylphosphinophenolate ligand (PPHH) favors improved yields of waxy linear ethylene oligomers, whereas the more P-basic dicyclohexylphosphinophenolate (CCHH) induces incorporation of alkyl-, aryl-, and ester-substituted alpha-olefins into a growing ethylene polymer chain and larger molar masses of the polymers. Depending on the nature of the olefin, the reaction rate and the conversion are lowered by olefin incorporation. Ester groups are tolerated if distant to the C=C bond but block the catalyst if close to the olefinic group. A remarkable increase of activity accompanied by small branching was observed in the copolymerization with ethyl undecenoate by the nickel diphenylphosphinophenolate catalyst. C-13 and H-1 NMR spectra give evidence that ethylene copolymers with isolated side groups, imbedded into linear polyethylene blocks with vinyl end groups, are formed.