Influence of Backbone Substituents on the Ethylene (Co)polymerization Properties of α‑diimine Pd(II) and Ni(II) Catalysts

A series of α-diimine ligands
with different substituents on the acenaphthyl backbone were synthesized
and characterized. The corresponding Ni­(II) and Pd­(II) complexes were
prepared and used in ethylene polymerization and copolymerization
with methyl acrylate. In ethylene polymerization, these Ni­(II) complexes
showed activities of up to 1.6 × 107 g/((mol of Ni)
h), generating polyethylene with a molecular weight (Mn) of up to 4.2 × 105. Interestingly,
these Ni­(II) complexes behave very similarly in ethylene polymerization
except for the complex with two methoxy substituents on the ortho position of the acenaphthyl backbone, in which case
about 3 times higher polyethylene molecular weight and much lower
branching density were observed. The ligand substituent effect is
much more dramatic for the Pd­(II) complexes. In ethylene polymerization,
activities of up to 1.7 × 105 g/((mol of Pd) h) and
a polyethylene molecular weight (Mn) of
up to 4.7 × 104 could be obtained. The Pd­(II)
complex with two methoxy substituents on the ortho position of the
acenaphthyl backbone demonstrated much higher activity and generated
polyethylene with about 3 times higher molecular weight than that
for the classic Pd­(II) complex. A similar trend was maintained in
ethylene–methyl acrylate copolymerization.