Dinuclear nickel(II) complexes with 2,5-diamino-1,4-benzoquinonediimine ligands as precatalysts for the polymerization of styrene: electronic and steric substituent effects.

Catalytic polymerization of styrene with a series of dinuclear nickel(II) complexes [{Ni(acac)}2{μ-C6H2-(N-Ph-R)4}] (R = 4-Et (1a), 4-OEt (1b), 2-OEt (1c), 2-Et (1d), and 2,6-Et2 (1e)) in the presence of methylaluminoxane was studied under various conditions to evaluate the substituent effect. The activity of 1a-1e, except 1c, increased with an increase in the reaction temperature, and the highest activity (7.46 × 10(5) g PS mol(-1) Ni h(-1)) was obtained using 1e at 70 °C. The electronic and steric properties of the ligand influenced the activity at room temperature, 50 °C, and 60 °C, steric factors barely, whereas electronic factors slightly dominated the activity at 70 °C. The activity with 1c remained constant at all temperatures, probably due to the masking of the active center by the formation of inactive N,N',O-chelate species. The obtained polymers were atactic polystyrenes with molecular weights and molecular weight distributions in the range of 15,000-43,400 and 1.71-2.14, respectively. While a clear dependence of the molecular weight and molecular weight distribution on the temperature was observed, no significant dependence on the substituent was noted.