Mechanistic Studies on Nickel-Catalyzed Ethylene Polymerization: Ligand Effects and Quantitative Structure-Activity Relationship Model

The polymerization mechanisms of ethylene cata-lyzed by two cationic nickel catalysts, viz., diimine-type Ni complex A [(ArN=C(Me)-C(Me)=NAr)NiMe](+) (Ar = 2,6-diisopropyl-phenyl) and alpha-imino-ketone Ni complex B [(ArN=C(Me)-C(Me)=O)NiMe](+) (Ar = 2,6-diisopropylphenyl), have been comparatively studied through density functional theory (DFT) calculations. It has been found that the calculated activity trend toward ethylene polymerization is consistent with experimental results. In comparison with species A, the higher activity of complex B is originated from the stronger interaction between the monomer and the catalyst in the chain growth process. The stronger interaction can be explained by the more positive NBO charge of the metal center and the smaller energy gap between the HOMO of ethylene and the LUMO of active intermediate (B)P1. We also report a combination of multivariate linear regression (MLR) and DFT calculations to analyze and verify the factors affecting the ethylene polymerization activity of catalysts A and B. The trained MLR model features an R-2 value of 0.85 and a leave-one-out Q(2) value of 0.78. Strong correlations were found between the energy barrier of ethylene polymerization (Delta G(double dagger)) and the distance of the nickel-carbon bond (d(Ni-C3)) as well as the NBO charge (Q(N)) on the coordinating N atom in the transition state of the chain growth.