Synthesis and characterization of α-diamine palladium complexes and insight into hybridization effects of nitrogen donor atoms on norbornene (co)polymerizations

Abstract

Ligand donor atoms play an important role in catalytic olefin polymerization. The nitrogen atom is easily ligated to the metal center with different hybridizations, which is widely applied in the synthesis of palladium complexes. We have herein reported the synthesis and characterization of a series of α-diamine palladium complexes featuring two sp³ hybridized nitrogen donor atoms and their catalytic properties toward norbornene (NBE) (co)polymerizations. α-Diamine palladium catalysts showed a very high activity up to 7.02 × 10⁷ g PNBE per mol Pd per h for norbornene polymerization. The hybridization effects of nitrogen donor atoms on homo- and copolymerizations of norbornene and polar norbornene monomers with palladium complexes bearing α-diamine, amine-imine, and α-diimine ligands activated by methylaluminoxane (MAO) were studied systematically. With variation of the two nitrogen donor atoms from the amine nitrogen with sp³ hybridization to the imine nitrogen with sp² hybridization, homo- and copolymerization activities of the palladium catalysts decreased consistently. The molecular structure analysis of the palladium complexes showed that the ligand redistribution reaction and more electrophilic palladium center led to high activity for norbornene polymerization using α-diamine palladium catalysts.