Nickel Complexes Bearing 2-pyridinemethanamine Ligands: Syntheses And Catalytic Properties For Living Ethylene Polymerization

A series of novel nickel complexes (1-9) bearing 2-pyridinemethanamine ligands[(Ar1-NH-C(R1R2)-Ar2)NiBr2 (Ar1=2,6-diisopropylphenyl, Ar2=2-pyridinyl, R1=H, R2=methyl, 1; Ar1=2,6-diisopropylphenyl, Ar2=2-pyridinyl, R1=H ,R2=2,4,6-trimethylphenyl, 2; Ar1=2,6-diisopropylphenyl, Ar2=2-pyridinyl, R1=H,R2=phenyl, 3; Ar1=2,6-diisopropylphenyl, Ar2=2-pyridinyl, R1= methyl, R2= phenyl, 4; Ar1=2,6-diisopropylphenyl, Ar2=6-phenyl-2-pyridinyl, R1=H, R2=methyl, 5; Ar1=2,6-diisopropylphenyl, Ar2=6-α-naphthyl-2-pyridinyl, R1=H, R2=methyl, 6; Ar1=2,6-dimethylphenyl, Ar2=2-pyridinyl, R1=H , R2=2,4,6-trimethylphenyl, 7; Ar1=2,6-dimethyl-4-fluorophenyl, Ar2=2-pyridinyl, R1=H,R2=2,4,6-trimethylphenyl, 8; Ar1=2,4,6-trimethylphenyl, Ar2=2-pyridinyl, R1=H,R2=2,4,6-trimethylphenyl, 9)] have been first synthesized and characterized. The complexes 1-9 initiated by activator were used for ethylene polymerization, and the resulting polymers were characterized. Furthermore, the polymerization mechanism and the relationship between the catalyst structure and the catalytic property were also discussed.1. Comparisons of polymerizations catalyzed by complexes 1-4/MAO reveal substituents on the backbone of the complexes have a great influence on the performance of the catalysts. With bigger substituents, the complexes produce PEs with higher molecular weights and narrower polydispersitis. Complex with sufficiently bulky substituent can produce PE with very narrow polydispersity (<1.3), suggesting living ethylene polymerization. The results provide a clear demonstration that the steric hindrance of the substituent on the bridge backbone plays a dominant role in resisting the rotation of the CAr-N bond to eliminate chain transfer. Using complexes 5 or 6 as catalyst precursor causes dramatic decrease in activity and molecular weight of the PEs, suggesting substituents on the 6-position of pyridinyl may block the insertion of monomer. The crystal graphs of complexes 2 and 4 show ortho substituents of the aniline are positioned above and below the coordination planes blocking the the axial sites, which is believed to be the key to get high molecular weight and narrow polydispersity polymer. With methyl, 2,6-diisopropyl or phenyl on the ortho sites of anilines, and bulky enough substituent on the bridging carbon, complexes can produce PEs with narrow polydispersity (<1.2). Only oligomers were got when the polymerization catalyzed by complex without substituents on the ortho sites of anilines. Compared to complex 7, with F on the para site of aniline, complex 8 produced PE with higher molecular weight and narrower polydispersity, but had reduced activity. While complex 9 with methyl had higher activity.2. Number average molecular weights (Mn) of PEs produced by complexes 2 and 4 increase linearly with polymerization time and the polydispersity can be maintained in 1.15-1.3 for hours. The stable linear increase of Mn value indicates a long-lifetime living polymerization. To the best of our knowledge, this is the first report of nickel complex to catalyze longstanding living ethylene polymerization. Longstanding living ethylene polymerization catalyzed by other transition metals has been achieved only in rare instances. Catalyst 2 can remain living for 10h without ethylene monomer, suggesting high potential for synthesis of ethylene-based block copolymers.3. A-B diblock polymers of ethylene and 1-hexen were successfully synthesized by catalyst 2. Capped the chain end of PE with H2C––CH(CH2)9–OSiMe3 results the end functionalized PE.