Catalytic polymerization in dense CO₂ to controlled Microstructure polyethylens

A series of partially novel, (K²-N,O) Ni(II) methyl complexes coordinated by pyridine or N, N, N′, N′-tetramethylethylenediamine (tmeda), based on different K²-N,O-chelating salicylaldimines, 2,6-diisopropylanilinotropone, 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 N,OH ligands over a range of CO₂ densities (p = 0.2-0.9 g mL^-1) at 25 and 50°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_w = 10⁶ 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 % < 20% and T_m < 70° C.