Ethylene Polymerization by Niobium(V) N,N-Dialkylcarbamates Activated with Aluminum Co-catalysts

The niobium N,N-dialkylcarbamates Nb(O2CNR2)n [n = 5, R = Me, 1, Et, 2; n = 4, R = Et, 3; n = 3, R = Et, 4], in combination with organoaluminum cocatalysts, were found to be active in the polymerization of ethylene. The effects of the main reaction parameters (solvent, ethylene pressure, time, temperature, cocatalyst, reactivator) on the polymerization reaction were investigated in detail. When 2 was used as precursor in combination with trimethylaluminum (TMA)-depleted MAO and 1,2-dichloroethane as reactivator, activity up to 110 kgpolyethylene molNb−1 h−1 bar−1 was ascertained and linear, high molecular weight polyethylene was produced. The catalytic activities related to the different precursors increase along the series 2 > 1 > 3 > 4. The 1:1 reaction of 2 with Al2Et3Cl3 proceeds via ligand exchange, giving the X-ray characterized mixed alkyl-carbamato derivative [Al3(μ-O2CNEt2)6Et2]Cl (6), in admixture with presumable species containing niobium in lower oxidation state. The latter and not the former seem to be responsible for directing the polymerization process, according to the evidence that 6, produced by ligand exchange between Al(O2CNEt2)3 and AlEt2Cl, is not active under analogous experimental conditions