A series of partially novel, (kappa 2-N,O) Ni(II) methyl complexes coordinated by pyridine or N, N, N-t, N-t-tetramethylethylenedramine (tmeda), based on different kappa(2)-N,O-chelating sahcylaldimines, 2,6-dnso-propylamhnotropone, and a ketoenamine, were studied as precatalysts for olefin polymerization in dense carbon dioxide Bis(trifluoromethyl)phenyl substituents promote solubility in the CO2 reaction medium, as quantified by solubility studies of the free NtOH ligands over a range of CO2 densities (rho = 0.2-0.9 g mL(-1)) at 25 and 50 degrees C and thus enhance catalyst activities. By appropriate choice of the catalyst precursor and polymerization reaction conditions (CO2 density, ethylene concentration, and temperature), strictly linear polyethylene with high molecular weight is obtained (<1 branch/1000 carbon atoms, M-n = 10(6) g mol(-1), M-w/M-n = 2) which displays typical thermal characteristics and density of ultrahigh molecular weight polyethylene. Copolymerization with 1-hexene, and particularly with norbornene, allows for controlling polymer crystallinity and melting temperature down to chi < 20% and T-m < 70 degrees C.