Homo- and co-polymerization of styrene with ethylene by novel nickel catalysts

The homopolymerization of styrene has been studied by using catalysts based on bis(α-nitroacetophenonate)Nickel(II) [Ni(naph)2] and bis(hexafluoroacetylacetonate)Nickel(II) [Ni(hfacac)2] precursors and methylalumoxane (MAO) as co-catalyst. The former system displayed at room temperature a higher activity as compared with other previously reported nickel catalysts, giving rise to a substantially atactic polystyrene with a quite low molecular weight. The use of the basic and bulky tricyclohexylphosphine (PCy3) as ancillary ligand allowed to increase both molecular weight (∼20,000 Da) and isotacticity degree (∼50%) of the polymer. On the contrary, the use of the less basic triphenylphosphine (PPh3) caused a reduction of isotacticity degree. On increasing the reaction temperature at 70°C, even in absence of the phosphine ligand, both catalytic systems gave a polymer characterized by about 50% isotacticity degree. A significant further increase of isotacticity degree (65–70%) was obtained when a AlMe3 free MAO was adopted in combination with the above nickel precursors. The GC/MS analysis of the oligomeric products allowed us to conclude that styrene insertion is mainly of secondary type. The above catalytic systems were found also to copolymerize ethylene with styrene, giving rise mainly to oligomeric products.